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2 production and storage as formate with efficiency > 10%

An artificial leaf device built with earth-abundant materials for combined Hproduction and storage as formate with efficiency > 10% &#;

Claudio Ampelli *a, Daniele Giusi a, Matteo Miceli a, Tsvetelina Merdzhanova b, Vladimir Smirnov b, Ugochi Chime b, Oleksandr Astakhov b, Antonio José Martín c, Florentine Louise Petronella Veenstra c, Felipe Andrés Garcés Pineda d, Jesús González-Cobos&#; d, Miguel García-Tecedor§ e, Sixto Giménez e, Wolfram Jaegermann f, Gabriele Centi a, Javier Pérez-Ramírez c, José Ramón Galán-Mascarós d and Siglinda Perathoner a
aDepartment of Chemical, Biological, Pharmaceutical and Environmental Sciences (ChiBioFarAm), University of Messina, ERIC aisbl and CASPE/INSTM, Messina, Italy. :
bIEK5-Photovoltaik, Forschungszentrum Jülich GmbH, Jülich, Germany
cInstitute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir-Prelog-Weg 1, Zürich, Switzerland
dInstitute of Chemical Research of Catalonia (ICIQ), The Barcelona Institute of Science and Technology (BIST), Av. Paisos Catalans 16, Tarragona, Spain
eInstitute of Advanced Materials (INAM), Universitat Jaume I, Castelló, Spain
fTechnical University of Darmstadt, Darmstadt, Germany

Received 4th October

, Accepted 14th February

First published on 15th February

Abstract

A major challenge for achieving energy transition and transforming the current energy model into distributed production is the development of efficient artificial leaf-type devices capable of directly converting carbon dioxide (CO2), water and sunlight into sustainable fuels and chemicals under ambient conditions. These devices should avoid using critical raw materials to be sustainable and cost-competitive. We report top-level results for the first time in converting CO2 and H2O to fuels (formate and H2) using sunlight and electrodes based solely on earth-abundant materials. The cell provides a solar-to-fuel efficiency of >10% combined with world-record current densities to comparable devices operating at room temperature, without adding sacrificial donors or electrical bias. In addition, we present the novel concept of producing at the same time H2 and an H2-storage element (formate), the latter used to produce H2 when light is absent. This solution allows continuous (24 h) hydrogen production using an artificial-leaf device. For the first time, we show the feasibility of this solution. The experimental results were obtained in an optimised, compact electrochemical flow cell, with electrodes based on Cu&#;S and Ni&#;Fe&#;Zn oxide (for CO2 reduction and oxygen evolution reactions, respectively) supported on gas-diffusion substrates, integrated with a low-cost Si-based photovoltaic module. The cell design allows for easy scale-up and low manufacturing and operating costs. The cell operates at a current density of about 17 mA cm&#;2 and a full-cell voltage of 2.5 V (stable for at least ten hours and in on&#;off operations), providing formate productivity of 193 μmol h&#;1 cm&#;2, paving the way towards the implementation of affordable artificial-leaf type systems in the future energy scenario.

Broader context

Developing artificial leaves to directly convert CO2, H2O and sunlight into valuable fuels and chemicals (by mimicking the behaviour of plants in nature) is one of the fascinating challenges to face in the energy transition. However, artificial leaves or photosynthesis technology is still considered a long-term objective due to limitations such as expensive materials, low robustness and poor performance. We have demonstrated in this study that it is possible to construct artificial-leaf devices that reach a solar-to-fuel efficiency of 10% (still considered a long-term target in the area) using easily scalable systems with no critical raw materials, operating under mild conditions and without adding sacrificial donors or electrical biases. This study proves the concept of manufacturing on a lab scale an artificial leaf made with only earth-abundant materials with stable operations until at least 10 h of light irradiation and with performance among the highest values reported in the literature in terms of current density and solar-to-fuel efficiency. At the same time, we demonstrate the proof of concept of another challenge: how to produce with continuity (all day) hydrogen in a device using sunlight. We have developed an artificial-leaf device that integrates the chemical hydrogen storage as formate, which can produce H2 again during dark periods. The new artificial leaf produces formate with high C-selectivity and hydrogen. Formate is one of the most promising liquid energy carriers working under ambient conditions, showing potential as a hydrogen carrier for the new hydrogen economy. The combination of formate and H2 production is thus proposed here for the first time as a new solution for the continuous daily production of hydrogen.

1 Introduction

2) and water directly into value-added chemicals is one of today's challenges to meet growing energy demands and curb climate changes due to the accumulation of CO2 in the environment.

i.e.

, water, carbon dioxide, dinitrogen, and methane) using renewable sources such as solar power.

Artificial photosynthesis to convert carbon dioxide (CO) and water directly into value-added chemicals is one of today's challenges to meet growing energy demands and curb climate changes due to the accumulation of COin the environment. 1&#;3 Artificial leaves (ALs) are small-scale reactors involving the electrocatalytic transformation of abundant small molecules (, water, carbon dioxide, dinitrogen, and methane) using renewable sources such as solar power. 4 ALs can have different configurations: (i) a wired or wireless configuration, 5 with the photoactive element in contact with the electrolyte or (ii) a configuration where the photoactive element (a photovoltaic &#; PV &#; cell) is not in contact with the electrolyte, although integrated into the device. This second configuration is preferable for stability and performance. 3 To be sustainable, ALs should be manufactured using earth-abundant materials. 5,6 It is also crucial to adopt cell engineering and design suitable for easy scale-up and mass production, 3 avoiding using sacrificial donors or electrical bias. 7 Stability is another critical requirement, which implies using stable materials, and cell design and operating conditions enabling long-term operations.

Hydrogen (H2) is considered the simplest fuel generated from an AL. Khaselev and Turner already studied direct water electrolysis in . They reported an integrated-monolithic photoelectrochemical-photovoltaic device with 12.4% hydrogen efficiency at 11 Suns of light irradiation using rare earth elements to prepare electrodes (i.e., GaInP2/GaAs).8 Another pioneering device for artificial photosynthesis was the cell introduced by Nocera's group over ten years later, unveiling the first practical artificial leaf (working at one SUN under near-neutral pH conditions) for water splitting with 5% solar efficiency.9 This cell was made of cheap materials but can only be used for hydrogen production. In addition, severe stability issues were present due to photo corrosion of the integrated PV cell in contact with the electrolyte.

ALs producing higher energy-density fuels by converting CO2 into C-containing products (allowing closing of the cycle of CO2 formation/consumption) are an alternative and more attractive solution than water-splitting devices that produce H2, with the benefits of generating &#;drop-in&#; carbon fuels ready to be introduced into the current energy infrastructure.10 The most common carbon fuels/chemicals formed by (photo-) electrocatalytic (PEC) reduction of CO2 include carbon monoxide (CO), formate (HCOO&#;), methanol (CH3OH), ethylene (C2H4), and other C2+ alcohols, hydrocarbons, and acids (where C2+ refers to products with two or more carbon atoms). However, carbon monoxide11 and formate,12 formed by two electron reduction of CO2, have so far been the only carbon products generated with high Faradaic efficiency (FE) at relevant current densities (>10 mA cm&#;2). Carbon monoxide requires further processing with H2 to produce fuels or chemicals. Formate is instead the base material for several industries and is a highly desirable product.13,14 It can also act as an energy carrier for direct formate fuel cells, for example, or as an H2-carrier. Formate has a high volumetric H2 storage capacity of 53 gH2 L&#;1,15 and low toxicity and flammability for low-cost hydrogen storage and transportation.

From this perspective, the combined production of formate and H2 in an efficient AL-type device is an attractive solution because it can produce at the same time green H2, and a hydrogen carrier easy to store. Such a device can be integrated into distributed networks of green H2, with fluctuations in H2 production due to sunlight variability mitigated by the use of the hydrogen carrier. CO2 can then be closed-up by recirculating it.

This solution presents advantages over the combinations of PV, wind, and electrical energy storage (batteries), eventually coupled with water electrolysers to produce H2 continuously. The chemical energy storage has a volumetric energy density about three orders of magnitude greater. Realising a distributed approach of many parallel devices (artificial trees) would require enough compact artificial leaf devices and thus explore chemical storage as an alternative to electrochemical storage. It is possible to imagine an artificial tree that produces H2 and integrates the storage of H2 as formate to produce on-demand H2 during dark periods.

However, it is a largely unexplored option, still lacking available devices with suitable characteristics for this technology. In this sense, the combined and balanced production of formate and H2 with high solar-to-fuel efficiency is the target, rather than obtaining a high FE to formate. However, the C-selectivity to formate, i.e., the selectivity of converting CO2 to formate rather than other C-products such as CO, methanol and higher alcohols, hydrocarbons, etc., is required.

One of the most recent PEC approaches for selective formate generation was developed by Domen's group, reporting a scalable CO2 reduction process using a molecular-based hybrid photocatalyst with a remarkable selectivity to formate (97 ± 3%).16 However, critical raw materials were used for the preparation of electrodes (i.e., SrTiO3:La, Rh) and a very low solar-to-formate efficiency was obtained (0.08 ± 0.01%).

In , Zhou et al. reported a high 10% energy-conversion efficiency with a current density of about 9 mA cm&#;2 but using a costly InGaP/GaAs PV component, a noble metal-based cathode with a very small electrode surface area (0.03 cm2), i.e. not suited to analyse scalability to large-scale application.17 Furthermore, Piao et al. reported an overall solar conversion efficiency of about 8.5% with a current density of about 10 mA cm&#;2 but using expensive electrodes (i.e., IrO2 as the anode) and a chlorite-promoted electrolyte (with corrosion and safety issues for the possible formation of Cl2).18 Recently, Kato et al.19 reported a solar-driven electrochemical reduction of CO2 to formate with a conversion efficiency of 7.2% using a Si-based PV cell, then scaled up to 1 m2 eight-stacked electrodes with an improved efficiency of 10.5%,20 but also, in this case, costly noble-metal based electrodes (anodes based on IrOx and cathodes on a molecular Ru-complex polymer) were used. In summary, examples of efficient artificial-leaf-type devices free of noble metals or expensive materials have not been reported so far for formate production at a relatively high current density, nor has the balanced coproduction of formate and H2 been investigated. Coproducing H2 and an H2-storage element, which can produce H2 during dark periods, is a novel possibility opening breakthrough research directions.

In this study, we present the realisation of a lab-scale bias-free PV/EC flow cell for converting CO2 and water into formate and H2 (and O2) under sunlight irradiation (one SUN), using only earth-abundant materials for device construction. The PV/EC architecture generally indicates a photovoltaic (PV) module integrated within the electrochemical (EC) cell but not in contact with the electrolyte to enable high stability of the PV component. In PEC (photo-electro-catalytic) cells, instead, the PV unit is in direct contact with the electrolyte or a semiconductor-based photoelectrode is used. A survey of state-of-the-art results indicates that the PV/EC architecture is currently preferable.21

Our PV/EC cell, working as an AL-type device at room temperature and atmospheric pressure, has a highly compact design, amenable to scale-up, and is integrated with gas diffusion electrodes based on Cu&#;S for CO2 reduction and Ni&#;Fe&#;Zn oxide for water oxidation. The electrochemical (EC) part is coupled with a low-cost PV four-cell module with shingled interconnection based on the silicon heterojunction (SHJ) technology, leading to a full lab-scale prototype that delivers outstanding performance by converting over 6% of sunlight into carbon fuels (with high selectivity to formate), and delivers an overall 10% solar-to-fuel efficiency also including hydrogen production. Formate is accumulated in the liquid phase located at the EC part and separated from the gas outlet rich in H2. Formate can then be recovered from the electrolyte without using energy-intensive separation solutions,22 or stored and then decomposed (to produce H2) without the need to be separated from the electrolyte.

The realisation of the combined PV/EC device is the result of optimisation of many aspects in relation to:

(i) operating close to the maximum power point of the PV device;

(ii) reaching a cell voltage in the EC device high enough to drive the reduction/oxidation reactions;

(iii) minimising energy losses due to overpotential;

(iv) controlling potentials within the stability limits of the catalysts.

This artificial leaf-type system was developed within a European project (A-LEAF) dedicated to realising a prototype from earth-abundant materials for sustainable solar production of CO2-based chemicals and fuels.23 In this project, the integrated development of the photoactive element, the electrocatalysts, the electrodes and the cell was studied. The results of this study show that the optimal behaviour does not derive from the independent optimisation of the separated elements. Rather, an integrated (combined) development is necessary. This manuscript reports the results of this integrated effort combining scientific advances in materials with the identification of the limiting aspects that determine the overall behaviour and the way to overcome them.

2 Description of the device

Fig. 1 schematically illustrates our AL-type device and its components. The device is composed of two main parts: (i) a two-compartment electrochemical (EC) cell and (ii) a photovoltaic (PV) module, combined in a single PV/EC unit to minimise energy losses and operate at maximum performance.
Fig. 1

Visual description of the artificial leaf and its elements. (a) Exploded view of the electrochemical (EC) &#; photovoltaic (PV) device and related electrodes, including O-rings and rubbers; (b) representative scheme of the working principle of the artificial leaf (a side picture is also provided on the left). The CO2 Reduction Reaction (CO2RR) occurs in a liquid electrolyte (catholyte) at the cathode side. A gas chamber enhances the local concentration of gaseous CO2 (directly on the electrode surface) flowing through a Gas Diffusion Layer (GDL). The counter-reaction is the Oxygen Evolution Reaction (OER) by water oxidation. &#;cat.&#; represents catalyst; (c) picture of the full PV/EC device; and (d) scheme and picture of the silicon heterojunction (SHJ) four-cell module used to absorb sunlight and provide electricity.

The following subsections describe the EC cell and PV module.

2.1 Electrochemical cell

The EC cell is a lab-scale device made of Plexiglas (a transparent material for an easy visual inspection of what happens inside), working as an electrochemical flow cell with optimised pathways for gas and liquid flows. An exploded view of the artificial leaf shows details of the main elements of the EC cell (see Fig. 1a ).

A gas diffusion layer (GDL) separates a gas chamber (directly fed with pure CO2) from the liquid catholyte (0.1 M KHCO3 aqueous solution saturated with CO2, pH 6.8). The active catalytic material for CO2 reduction (i.e., Cu&#;S) faces the electrode side towards the liquid part. The gas diffusion electrode &#;GDE &#; (in combination with the gas chamber) facilitates an efficient gas&#;liquid contact in the proximity of the electrode, improving the transport of gaseous species through the carbon fibre support due to hydrophobic repulsion with the liquid phase.24 The counter anodic process is the Oxygen Evolution Reaction (OER) occurring over a Ni&#;Fe&#;Zn oxide-based electrode in 1 M KOH aqueous solution (pH &#; 14).

The use of two different electrolytes in the cathode and anode compartments is due to the individual optimisation of the electrocatalytic performance of Cu&#;S and Ni&#;Fe&#;Zn oxide, respectively, occurring at different pHs, as well as due to the stability of the electrodes under these conditions to the working potential (see Sections S1 and S2 in the ESI&#;). Using a commercial proton exchange membrane (i.e., PTFE-reinforced Nafion) allowed keeping this pH difference during the electrocatalytic tests, providing an optimal charge transfer between catholyte and anolyte liquids and limiting crossover of intermediate/product species. The choice of the best membrane was accomplished by selecting among a series of commercial cationic, anionic and bipolar membranes with different characteristics (see Section S3 in the ESI&#; for further details).

The EC cell was designed in a highly compact configuration following general criteria to minimise overpotentials and resistances (e.g., short distance between the electrodes; optimisation of electrolyte volumes), supported by preliminary experimental tests to verify the origin of internal energy losses (see Section S4 in the ESI&#;).

Careful optimisation of gas and liquid flows was demanded to guarantee a proper diffusion of CO2 through the GDL and avoid liquid leaks in the gas chamber. Catholyte and anolyte were continuously recirculated between each half-cell and two reservoirs. Fig. 1b reports a representative scheme showing the working principle of the artificial leaf, while Fig. 1c shows a picture of the full prototype. We also included a short video in the ESI&#; to show the artificial leaf during its regular operation.

The half-reactions occurring at the cathode side refer to CO2 reduction to formate and CO (eqn (1) and (2)), and proton reduction to produce H2 (eqn (3)), reported as follows:

 

CO2 + 2e&#; + H+ &#; HCOO&#;

(1) 

CO2 + 2e&#; + 2H+ &#; CO + H2O

(2) 

2H+ + 2e&#; &#; H2

(3)while at the anode side, the OER takes place: 

2OH&#; &#; 1/2O2 + 2e&#; + H2O

(4)Note that the gas products obtained from 2, respectively) are collected in full from the outlet of the gas chamber (diffusing back through the GDL), and do not dissolve in the liquid catholyte, as the gas chamber operates at atmospheric pressure.

2.2 Photovoltaic module

2, interconnected in a shingled configuration

We used a mini-PV device composed of four silicon heterojunction solar cells, 26 with an area of 12.71 cm, interconnected in a shingled configuration 27 and encapsulated using a glass/thermoplastic polyolefin (TPO)/module/TPO/back sheet structure 28 (see Fig. 1d ). The PV module is located externally close to the anode side of the EC cell (not in contact with the liquid anolyte) and connected with short electrical wires to the EC part. The solar PV module absorbs sunlight and provides electricity needed for the electrocatalytic processes.

3. Results and discussion

3.1 Electrocatalysts and electrodes

et al.

,2 reduction reaction. However, the synthesis of Cu&#;S was up-scaled for the preparation of technical electrodes to be used in an integrated PV/EC device under relevant conditions.

The cathode is made of sulphur-modified copper oxide (Cu&#;S), synthesised by the solvothermal method. The synthesis and optimisation of Cu&#;S were earlier reported by Shinagawa 29 along with its performance in the electrocatalytic COreduction reaction. However, the synthesis of Cu&#;S was up-scaled for the preparation of technical electrodes to be used in an integrated PV/EC device under relevant conditions.

while at the anode side, the OER takes place:Note that the gas products obtained from reactions (2) and (3) (CO and H, respectively) are collected in full from the outlet of the gas chamber (diffusing back through the GDL), and do not dissolve in the liquid catholyte, as the gas chamber operates at atmospheric pressure.

25

Copper-based systems are affordable and earth-abundant catalysts for the (photo-)electro-reduction of CO2 into various compounds ranging from C1 to C3 with appreciably Faradaic efficiencies.30,31 To deliver a single liquid carbon product and thus avoid downstream separation needs, sulphur-modified copper oxide (Cu&#;S) was the catalyst of choice because of its scalable preparation (see details in the experimental part section) and total selectivity towards formate among carbon products.29 The surface modification of copper systems with small amounts of sulphur (0.45 at% content in this work) suppresses routes initiated by CO adsorption, thus avoiding its formation and that of more complex compounds.

The fresh catalyst contains polycrystalline Cu2O displaying a homogeneous sulphur distribution (Fig. 2a), where CuS and CuS2 domains could be identified by selected area electron diffraction (SAED) analysis (Fig. 2b). Cu&#;S systems undergo reconstruction under operating conditions leading to a stable sulphur-enriched Cu surface responsible for the preference towards formate of these systems, as reported elsewhere.29

Fig. 2 0.9Zn0.1Fe2O4 cubic phase (space group =

FD

3

M

) with

a

=

b

=

c

= 8. Å. From the crystalline domain in the inset, the Ni0.9Zn0.1Fe2O4 lattice fringe distances are measured to be 0.294 nm, 0.292 nm and 0.295 nm, at 60.64° and 121.06°, which could be interpreted as the cubic Ni0.9Zn0.1Fe2O4 phase, visualised along its [11-1] zone axis. The characterization of the as-prepared NiFeZn oxide catalyst (before electrolysis) is reported in Section S5 in the ESI.

Characterisation of electrocatalytic materials. For the cathodic Cu&#;S catalyst: (a) representative scanning transmission electron microscopy (STEM) image in the high-angle annular dark field (HAADF) and corresponding elemental maps obtained by energy-dispersive X-ray spectroscopy (EDX); and (b) selected area electron diffraction (SAED) showing the presence of sulphide phases. A more detailed characterisation can be found in ref. 29 . For the anodic NiFeZn oxide catalyst (after electrolysis): (c) HAADF STEM micrograph of the Ni&#;Fe&#;Zn nanoblock sample; (d) EELS chemical composition maps obtained of the STEM micrograph, showing individual Ni L2,3-edges at 855 eV (red), Fe L2,3-edges at 708 eV (green), Zn L2,3-edges at eV (blue) and O K-edge at 532 eV (yellow) as well as composites of NiFeZn and NiFeZn-O; and (e) high-resolution transmission electron microscopy (HRTEM) image taken from an agglomerate nanoparticle squared in orange. Details of the orange squared region and its corresponding power spectrum reveal that this nanoparticle has a crystal phase in agreement with the NiZnFecubic phase (space group =) with= 8. Å. From the crystalline domain in the inset, the Ni0.9Zn0.1Felattice fringe distances are measured to be 0.294 nm, 0.292 nm and 0.295 nm, at 60.64° and 121.06°, which could be interpreted as the cubic NiZnFephase, visualised along its [11-1] zone axis. The characterization of the as-prepared NiFeZn oxide catalyst (before electrolysis) is reported in Section S5 in the ESI. &#;

Cu&#;S was finally deposited on a commercial GDL by spray coating (1 mg cm&#;2 loading) to facilitate gas transport of reagents (CO2) and gaseous products (CO and H2).

The anodic material is Ni&#;Fe&#;Zn oxide, obtained by hydrothermal methods in water (more details are provided in the experimental section), then deposited on a commercial GDL to prepare the electrode (1 mg cm&#;2 loading) and facilitate O2 desorption.

Ni&#;Fe&#;Zn oxide shows outstanding performance during the OER in alkaline media. This oxide interestingly improved its electrocatalytic behaviour during accelerated stability tests compared to the as-prepared oxide (see Section S5 in the ESI&#;), as also confirmed and reported earlier.32

Nanoparticles with narrow-distribution sizes (8&#;10 nm) were obtained, identified as a spinel system with a space group of Fdm. This mixed metallic oxide exhibited high stability, retaining its structure after long-term periods of polarisation at a potential higher than 1.5 V vs. RHE with only minimal Zn2+ leaching (see Section S5 in the ESI&#;), responsible for the increased electrochemically active surface area during operation. The electron microscopy analyses confirmed the robustness of this oxide. The images shown in Fig. 2c&#;e refer to the Ni&#;Fe&#;Zn oxide after the electrocatalytic tests, evidencing no substantial differences from the as-prepared oxide (before electrolysis) as reported in Section S5 in the ESI.&#;

3.2 Electrocatalytic performance in a three-electrode configuration

The electrochemical testing (without solar irradiation) was first carried out in the compact EC device described above by adopting a three-electrode configuration and applying an electrical bias to the cathode from &#;0.4 to &#;1.0 V

vs.

RHE. A very thin capillary Ag/AgCl (3 M KCl) electrode was used as the reference electrode and placed very close to the Cu&#;S/GDL.

The adopted EC design is quite different from the conventional electrochemical setups (e.g., the H-type configuration) in which an electrocatalytic substrate is immersed in a liquid electrolyte. Our compact prototype shows many advantages, such as (i) low energy loss (due to the compact cell configuration); (ii) high local concentration of CO2 on the electrode surface (due to the combined use of a GDE with a gas chamber for a direct feed of CO2 through the GDL); and (iii) easy scale-up to higher geometrical surface areas. While liquid products from CO2 reduction (i.e., formate) are separately collected in the catholyte and sampled from the external cathode reservoir for the analysis, the gas products diffuse back through the GDE and exit from the gas-chamber outlet (see Fig. 1b). To confirm the pathway of gas products, we checked out the absence of CO and H2 in the headspace of the cathode reservoir.

An accurate, standardised protocol, consisting of a series of Cyclic Voltammetry (CV) analyses to stabilise the electrodes and to measure the capacitance before and after each test, was strictly followed (more details are reported in Section S6 in the ESI&#;).

A series of preliminary blank tests were also performed to verify that formate derives from the CO2 feed to the PV/EC cell. A standard procedure was to feed an inert gas rather than CO2 and monitor whether carbon products were formed during these tests.

Fig. 3a shows the results obtained from the electrochemical testing. The FE to formate (see the green bars) increases with the applied voltage reaching a maximum value of 62.2% at &#;1.0 V and 15.1 mA cm&#;2. In contrast, the FE to hydrogen decreases by increasing the voltage. At &#;1.0 V, carbon monoxide also forms (FE to CO is 4.2%). Thus, the voltage at the working electrode (cathode) should not overcome &#;1.0 V to maximise the carbon selectivity towards formate and avoid costs in downstream gas separation. Fig. 3b shows the results of a further validation test performed in a galvanostatic mode by applying a constant current density (10 mA cm&#;2) and measuring the working and cell voltages. A steady-state value of 2.5 V was reached for the cell voltage (difference between working and counter voltages), evidencing quite good stabilisation in 1 hour and a half. These electrochemical tests provided relevant information to set the operating conditions of the artificial leaf and select the PV module suitable for coupling.

Fig. 3

Performances of the electrochemical device. (a) Faradaic efficiency (FE) from electrochemical tests at a different applied voltage (

vs.

RHE) in a three-electrode configuration. The measured current density profile is also reported. (b) Working-counter (cell voltage) and working-reference (

vs.

RHE) voltage profiles

versus

time obtained by an electrochemical test at 10 mA cm&#;2.

Table 1 summarises the specifications of the main components of the EC device.

Table 1

Specifications of the main components of the electrochemical EC device

Cathode Cu&#;S/GDL (1.0 mg cm&#;2) Anode Ni&#;Fe&#;Zn oxide/GDL(1.0 mg cm&#;2) Catholyte 0.1 M KHCO3 aqueous solution saturated with CO2 (pH = 6.8) Anolyte 1 M KOH aqueous solution (pH > 13) Membrane Nafion&#; N324, Teflon&#; Fabric Reinforced Reference electrode 3 M KCl Ag/AgCl (1 mm dia.)

3.3 PV module and coupling

As indicated in the introduction, rather than decorating PV cells with electrocatalytic materials in a &#;wireless&#; configuration 33&#;35 (with all the intrinsic limits of stability and photo corrosion), we opted to a PV/EC configuration, with the PV module not in contact with the electrolyte, to enhance stability of operation but also to flexibly change the PV module. Our device incorporates an external PV module wired to the EC cell to reach the required photo potential for both half-cell reactions, thus avoiding the inevitable decrease in performance due to charge recombination effects at the interface.

The 4-cell SHJ module, described in Section 2.2, was designed and realised to match the EC cell's characteristics and size. The current&#;voltage (I&#;V) characteristics and the PV parameters of the 4-cell SHJ module are reported in Section S9 in the ESI.&#; Under these conditions, the PV module provides 20.3% conversion efficiency at the maximum power point (MPP).

The coupling procedure between EC and PV was performed using a Xe arc-lamp of 300 W equipped with an Air-Mass 1.5 Global filter. The PV module was calibrated to operate at 1 SUN (100 mW cm&#;2) by accurately setting the distance from the lamp and the opening of the diaphragm (see Section S10 in the ESI&#;). Under real operating conditions (without cooling of the PV module), the measured temperature was slightly higher than 25 °C (i.e., 31.5 °C), reducing the conversion efficiency of the PV module from 20.3 to 19.74%.

After the preliminary tests on the whole integrated PV/EC system, we refined the design of the EC cell. We manufactured a more compact cell (further reducing the volume of both liquid compartments and the distance between the electrodes) to reach a cell voltage of 2.5 V at the operating point and stay as close as possible to the MPP of the PV module. The resulting combination of the two polarisation curves of EC and PV is shown in Fig. 4.

Fig. 4

Coupling of the photovoltaic module with the electrochemical device. The

IV

curve of the SHJ photovoltaic (PV) module and the polarisation curve of the electrochemical (EC) cells are plotted together. The working point (WP) of the artificial leaf and the maximum power point (MPP) of the PV module are also indicated on the plot (at a cell voltage of 2.5 V and 2.3 V, respectively). The blue triangles indicate the evolution of the WPs (in the direction indicated by the arrow) during the 10 h validation test.

The EC polarisation curve reported on the graph was constructed by varying the applied voltage at the working electrode and measuring both current and cell voltage after about 30 min of operation (needed to stabilise the signal). The point of intersection between the two curves (i.e., 2.6 V) should indicate the artificial leaf's working point (WP). However, the electrochemical behaviour of the EC cell is quite dynamic, mainly due to the evolution of the electrocatalysts, especially in the first 2&#;3 h of the reaction. During the stability validation test of 10 h, we thus investigated the evolution of the WP, which stabilised at a cell voltage of 2.5 V (in line with the electrochemical tests performed with only the EC device), as will be discussed in the next section.

3.4 Validation of the device

The coupled PV/EC device was validated under certified standard conditions of solar irradiation (AM1.5 G, 1 SUN, 100 mW cm&#;2), working in a two-electrode configuration (without a reference electrode).

Note that the electrodes were made with earth-abundant materials (Cu, Ni, Fe, Zn, and C), and neither electrical bias nor sacrificial donors were added. The latter means that the energy needed for running the electrochemical reactions (CO2 reduction at the cathode and water oxidation at the anode) only originates from the solar simulator.

The AL-type device was validated in two series of 5 h photo-electrochemical tests (10 h in total). These tests indicate the robustness and stability of the device, as well as the performance recovery after on&#;off operations. After the first five hours, light irradiation was stopped, and the anolyte and catholyte (the latter containing the CO2 reduction products, i.e., formate) were replaced with new solutions. This procedure was used to verify the reproducibility of the initial performance after five hours of continuous tests.

The cumulative production of formate, carbon monoxide and hydrogen during these two cycles of 5 h is shown in Fig. 5. The current density profiles and cell voltages during irradiation time are also reported. Note that formate accumulates during the experiment, increasing the electrolyte conductivity. This fact reduces the total overpotential of the EC cell, which in turn shifts the WP to lower voltages. Since the WP is located in the steepest part of the IV curve, the reduction in voltage leads to a considerable increase in the measured current of the PV/EC device.

Fig. 5

Performances of the electrochemical device after coupling with the photovoltaic module. (a) Cumulative production of formate (HCOO&#;), carbon monoxide (CO) and hydrogen (H2) in 5 h of solar irradiation and (b) in the subsequent 5 h of irradiation. (c) Profiles of current density and cell voltage during the first 5 h and (d) during the next 5 h.

After 2 h, stable production of formate is attained, with no appearance of deactivation or loss of performance of the catalysts. Note the excellent reproducibility of the two consecutive experiments, evidencing that no relevant transformation to the electrodes occurs on these tests.

Previous electrocatalytic studies performed in the same aqueous electrolyte (0.1 M KHCO3) showed that the solvothermally prepared Cu&#;S electrocatalysts exhibited high FEs to formate (ca. 80% at &#;0.8 V vs. RHE) and sustained their high selectivity over 12 h.29 The high stability obtained in our photo-electrocatalytic tests suggests that the working potential at the cathode does not exceed &#;0.8 V (vs. RHE), as also expected from the polarisation studies. On the other hand, the NiFeZn electrode provided excellent performance towards the oxygen evolution reaction and exceptional stability in 1 M KOH, as shown by supplementary experimentation reported in Section S5 in the ESI.&#;

Similarly, the current density increases in the first 2&#;3 h of each cycle, reaching stable and constant values of around 16&#;17 mA cm&#;2 (cell voltage: 2.5 V). The FE to formate was close to 60%, similar to the values obtained in the electrochemical validation tests.

Using the data obtained from this validation test, we can describe the evolution of the working points of the device, starting from a cell voltage of around 2.7 V and finally stabilising at 2.5 V, as described by the blue triangles on the plot in Fig. 4.

3.5 Solar-to-fuel (STF) efficiency

i.e.

, 1 Sun, 100 mW cm&#;2). The artificial leaf works at a current density

j

= 17.3 ± 0.5 mA cm&#;2, with a cell potential of 2.5 V (

V

WP) and an electrode area of

A

e = 5.31 ± 0.05 (cm2). With these parameters, we can estimate the ratio of solar to electrical power conversion (

i.e.

, the photovoltaic conversion efficiency) as: 

(5)where

P

is the incident irradiation power (in W cm&#;2), and

A

PV is the irradiated surface area of the PV module (in cm2), accounting for the solar irradiation power. Thus, the PV/EC setup is only losing around 2% of energy, mainly due to the slight deviation of the WP from the MPP (2.5

vs.

2.3 V) and partially to electrical connections and formation of interfaces since 18% of the solar energy is driving the electrochemical reaction. The deviation of the WP from the MPP is typically quantified as a coupling factor or coupling efficiency, which is a ratio of the WP power to the power at the MPP.

We calculated the device's photoconversion efficiency to evaluate the system's effective capability to store solar energy. Before testing the device, the solar simulator was carefully calibrated to provide a collimated output beam simulating the solar standard terrestrial irradiation (, 1 Sun, 100 mW cm). The artificial leaf works at a current density= 17.3 ± 0.5 mA cm, with a cell potential of 2.5 V () and an electrode area of= 5.31 ± 0.05 (cm). With these parameters, we can estimate the ratio of solar to electrical power conversion (, the photovoltaic conversion efficiency) as:whereis the incident irradiation power (in W cm), andis the irradiated surface area of the PV module (in cm), accounting for the solar irradiation power. Thus, the PV/EC setup is only losing around 2% of energy, mainly due to the slight deviation of the WP from the MPP (2.52.3 V) and partially to electrical connections and formation of interfaces since 18% of the solar energy is driving the electrochemical reaction. The deviation of the WP from the MPP is typically quantified as a coupling factor or coupling efficiency, which is a ratio of the WP power to the power at the MPP.

Fig. 6 shows the coupling factor versus time for the 10 h validation test, evidencing the evolution of the coupling factor until reaching a value of approximately 0.914 in both the two 5 h experimental runs. This increase in the coupling factor is related to the shift of the working voltage in the direction of the MPP discussed in the previous section. By multiplying the efficiency of the PV module at the MPP (19.74%) and the coupling factor (0.914), we can easily calculate the solar-to-electricity efficiency (18.04%), which is consistent with the calculation made in eqn (5), confirming that the major losses refer to the coupling loss.

Fig. 6

Coupling factor by integrating the electrochemical device and the photovoltaic module. Coupling factor for the two 5 h experimental runs of the validation test.

Taking into account the fuel production, we can further estimate the solar-to-fuel (STF) energy efficiency with the equation:

where

E

i

corresponds to the energy stored in each product

i

:

E

formate = 1.43 V and

E

hydrogen = 1.23 V, and FE

i

is the Faradaic efficiency to the

i

-reduction product.

wherecorresponds to the energy stored in each product= 1.43 V and= 1.23 V, and FEis the Faradaic efficiency to the-reduction product.

The STF efficiency to formate is calculated as 6.2 ± 0.4%, representing a record for a wired stand-alone artificial leaf using only non-critical raw materials and without applying external bias or adding sacrificial donors. The additional STF efficiency to hydrogen, as collected from the gas chamber, is 3.9 ± 0.3%. The overall STF efficiency combining these two major products is 10.1 ± 0.5, accounting for about 99.5% of the Faradaic efficiency. Other minor products are CO (around 0.3%), formaldehyde and C2+, which appeared as traces and could not be adequately quantified.

A performance comparison of different artificial leaf-type systems reported in the literature is presented in Fig. 7. The plot reports the main results of the current density as a function of the STF efficiencies in CO2 reduction processes to formate.

Fig. 7

Comparison of the results of artificial leaves in the literature. Current density

vs.

STF efficiency in CO2 reduction processes to formate obtained in artificial leaf-type systems. &#;FA&#; refers to the formate, while the green squares refer to artificial leaves not using critical raw materials.

The data used to build the plot in Fig. 7 are summarised in Table 2, reporting selected literature results16&#;19,36&#;43 for comparison with the data presented here.

Table 2

Summary of the main artificial leaf-type systems reported in the literature for CO2 reduction, including catalysts, operating conditions and performance in terms of solar-to-fuel (STF) efficiency

Catalysts Critical raw materials? Cell configuration Electrolytes Electrode area (cm2) Current density (mA cm&#;2) Operating voltage Faradaic selectivity to formate (%) Formate production rate (μmol h&#;1 cm&#;2) STF (%) Ref. Cathode: Cu&#;S/GDL; anode NiFeZn oxide on GDL No Wired CO2-saturated 0.1 M KHCO3 aqueous solution (pH = 6.6) as catholyte; 1 M KOH aqueous solution (pH > 13) 5.3 17.3 Cell voltage = 2.5 V 59.7 192.68 6.2 (10.1 cumulative formate + H2) This work RuO2 catalysts onto a Au layer, with SrTiO3:La,Rh and BiVO4:Mo light absorbers modified by phosphonated Co(ii) bis(terpyridine) Yes Wireless CO2-saturated KHCO3 aqueous solution (0.1 M, pH = 6.7) &#;1 0.11 +0.45 V

vs.

RHE 83.9 ± 5.8% 1.088 0.08 ± 0.01 Wang

et al.

16

Pd/C nanoparticle-coated Ti mesh cathode; tandem GaAs/InGaP/TiO2/Ni photoanode Yes Wireless 2.8 M KHCO3(aq) (pH = 8.0) as catholyte; 1.0 MKOH(aq) (pH = 13.7) as anolyte 0.03&#;0.04 8.5 Cathode exhibited <100 mV overpotential; cell voltage = 2.04 V >94% 149.1 &#;10% Zhou

et al.

17

Porous Bi dendrite electrodes synthesised on Cu substrates as cathode, IrO2 anode Yes Wired CO2-saturated 1 M KHCO3 aqueous solution as electrolyte (only the catholyte promoted with 0.1 M CsCl) 1.2&#;1.6 10.46 Cell voltage = 2.7 V &#;95% 125 8.7 Piao

et al.

18

Cathode: Ti/graphite/CS/MWCNTs/RuCP, anode: FTO/Ag/IrO

x

Yes Wired 0.4 M KPi (single compartment) 973 6.44 ± 0.04 Cell voltage = 1.85 V 80 96.1 7.2± 0.18 Kato

et al.

19

Cathode: Ti/graphite/CS/MWCNTs/RuCP, anode: Ti/IrO

x

Yes Wired 0.4 M KPi (single compartment) 7.42 Cell voltage = 1.65&#;1.69 V 96.0 133.2 10.5 Kato

et al.

20

BiOI&#;Bi (BOI&#;Bi) cathode; argon-treated TiO2 (TiO2-Ar) photoanode No Wired CO2-saturated 0.5 M KHCO3 aqueous solution (pH = 7.3) as catholyte; Ar-saturated 0.1 M KOH aqueous solution (pH = 12.9) as anolyte 1 1.26 Cell voltage = 1.24 V &#;96.5 22.68 8.3 Zhao

et al.

36

BiVO4||Perovskite|IO-TiO2|FDH tandem No Wireless 86 mM MOPS [3-(

N

-morpholino)-propane sulfonic acid], 50 mM NaHCO3, 50 mM CsCl 0.19 5 +0.4 V

vs.

RHE 83 ± 5 7.0 0.8 Moore

et al.

37

InP/[RuCP] semiconductor/metal-complex hybrid photocathode and reduced SrTiO3 photoanode Yes Wireless 5 mL of 0.1 M NaHCO3 aqueous solution mixed with phosphoric acid (pH 7.7) 0.25 0.1 &#;0.75 V (

vs.

Ag/AgCl) 71.6 1.93 0.08 Arai

et al.

38

CuFeO2 and CuO mixed p-type catalysts No Wireless 0.1 M bicarbonate solution purged with CO2 0.25 0.8&#;1.2 +0.9 V

vs.

RHE, cell voltage = 0.22 V >90 10.0 0.7&#;1.2 Kang

et al.

39

Cathode: N,Zn&#;Fe2O3 on TiO2 overlayer and a Cr2O3 underlayer, with Ru complex (Ru(MeCN)CO2C3Py) deposited; photoanode: n-SrTiO3 Yes Wireless CO2-saturated 0.1 M KHCO3 electrolyte (pH 6.6) 1 0.15 Application of 0.1 V

vs.

RHE &#;80 0.04 0.15 Sekizawa

et al.

40

3D TiN nanoshells ClFDH Bioelectrode No Wireless CO2-saturated phosphate buffer (pH 6.5, 50 mM bicarbonate purged with CO2) 0.5 0. &#;0.8 V

vs.

Ag/AgCl) 83.1 2.12 0.08 Kuk

et al.

41

Cathode: indium-based electrocatalyst coating deposited on a 3D Cu mesh; anode: metal Ti folded screen with an iridium oxide electrocatalyst coating Yes Wired Catholyte: 1 M potassium bicarbonate; anolyte:1 M H2SO4 (95&#;98%) 2.5&#;3.1 Cell voltage = 3.7 V 67 0. 1.8 White

et al.

42

An amorphous Si&#;Ge triple junction (3jn-SiGe) as a light absorber, a Ru polymer (RuCP) coated on a carbon cloth as cathode, and IrO

x

as anode Yes Wireless CO2-saturated aqueous phosphate buffer (0.1 M K2HPO4 + KH2PO4), pH = 6.4 4 No current reported &#;0.18 V

vs.

RHE No FE reported &#;3.8 4.6 Morikawa

et al.

43

Among the works not using critical raw materials and operating under relevant conditions, we obtained the maximum STF efficiency to formate (i.e., 6.2%) or to combined formate + H2 (i.e., 10.1%). Note that Zhao et al.36 reported a higher efficiency. Still, they worked under conditions very far from those that should be adopted for practical implementation (current density of about 1.3 mA cm&#;2, geometrical electrode surface area = 1 cm2). Compared to state of the art, it is also relevant to mention that the remarkable STF performance of our device is measured at a very high current density, i.e., 17.3 mA cm&#;2, which is the maximum value ever reported, not only considering the works involving earth-abundant materials. The formate productivity reaches about 0.19 mmol h&#;1 cm&#;2, which is 45% higher than the world-record results recently obtained by Toyota20,44 using noble metal-based electrodes (&#;0.13 mmol h&#;1 cm&#;2, FE to formate = &#;96%).

3.6 Technology assessment

2. The flowsheet reports the quantitative data (in grams per day, assuming 1 m2 of the PV panel and 7.4 h of solar illumination per day, which is the annual average value in Sicily &#; Italy). By balancing the direct H2 production and that deriving from formate decomposition (when sunlight is not present) and using a small intermediate vessel to store H2, a continuous (24 h) production of H2 of about 2 g h&#;1 per m2 of the PV panel can be obtained. In this simplified scheme, 100% FE to formate + H2 has been assumed. Although we experimentally observed the formation of low amounts of CO (as well as of other minor by-products), we believe that further optimisation of the system (acting both on the electrocatalyst formulation and operating conditions) can reduce these by-products to a non-critical level. Eventually, CO can be removed by the established PROX (Preferential Oxidation) method, which can efficiently be performed over metal catalysts of the platinum group (modified with promoters like alkali metals or reducible metal oxides) operating at low temperatures in the presence of water and CO2,+ active sites hindering H2 dissociation.Fig. 8 reports the simplified scheme of the technology of combined production of formate (used as a hydrogen storage molecule) and H. The flowsheet reports the quantitative data (in grams per day, assuming 1 mof the PV panel and 7.4 h of solar illumination per day, which is the annual average value in Sicily &#; Italy). By balancing the direct Hproduction and that deriving from formate decomposition (when sunlight is not present) and using a small intermediate vessel to store H, a continuous (24 h) production of Hof about 2 g hper mof the PV panel can be obtained. In this simplified scheme, 100% FE to formate + Hhas been assumed. Although we experimentally observed the formation of low amounts of CO (as well as of other minor by-products), we believe that further optimisation of the system (acting both on the electrocatalyst formulation and operating conditions) can reduce these by-products to a non-critical level. Eventually, CO can be removed by the established PROX (Preferential Oxidation) method, which can efficiently be performed over metal catalysts of the platinum group (modified with promoters like alkali metals or reducible metal oxides) operating at low temperatures in the presence of water and CO 45 or also over earth-abundant material-based catalysts like copper oxide&#;cerium oxide, in which the high selectivity can be related to the preferential adsorption of CO on Cuactive sites hindering Hdissociation. 46
Fig. 8

Simplified quantitative flowsheet of the technology for the combined formate and H2 production on the PV/EC cell, with quantitative data in grams per m2 of the PV panel and day (assuming 7.4 h a day of sunlight). &#;dec.&#; indicates a stage of catalytic decomposition of formate (FA) to CO2 and H2.

Based on this quantitative scheme, some assessment considerations can be made. The first question is with regard to the often-raised issue of the impact of green H2 production on water. The amount by weight of water feed to H2 produced is around 9, corresponding to the ratio between the molecular weights. The difference is the produced O2 because the atom efficiency of the reaction is 100%. The use of H2 will form H2O again. Thus, it is not essentially water consumption but a transfer of solar energy to the form of chemical energy (i.e., H2) that can be used as a clean fuel or reactant. While a more comprehensive life-cycle assessment (LCA) would be required, we do not see the impact of this technology on water as a critical issue. In contrast, the production of fuels and chemicals from fossil resources often has a higher impact on water on LCA bases.

The other point emerging from Fig. 8 is that in the presented concept of formate + H2 combined production, CO2 will be fully recycled, except for the tiniest part going to other C-based products (however, the C-selectivity to formate can be further improved as discussed above). Thus, this technology is not designed to use CO2 to reduce its emission directly but to produce ultra-low carbon (green) H2, bypassing the discontinuity in solar light availability. The key element is a technology integrating the production of H2 and an H2-storage compound that can form hydrogen during dark periods. This is a novel concept in the area of green H2 production.

The proposed technology will overcome the downstream separation process to obtain pure FA (usually performed by distillation or extraction methods).22 In fact, separating FA from the electrolyte is unnecessary, as it can be directly used in catalytic decomposition to produce H2 without separation. The decomposition of FA is an already established process, usually performed in reactors using both homogeneous and heterogeneous catalysts.47 The fixed bed reactor is the most common and more studied reactor for FA decomposition. For example, Koós and Solymosi48 reported the catalytic activity of Mo2C catalysts in the vapour-phase using a continuous flow fixed bed reactor. Under the conditions adopted for the experimental tests, Mo2C is an effective catalyst for the dehydrogenation of FA to yield H2 and CO2. Still, it competes with the FA dehydration reaction to yield H2O and CO. However, the authors observed that adding water to FA eliminated CO formation giving a CO-free H2 stream. Thus, the presence of water can even be beneficial for this process step, making the separation of FA from the electrolyte unnecessary.

Moreover, the authors obtained the best results by working with an aqueous solution of FA (5&#;6%). Considering that our artificial leaf-type system allows the production of FA at a rate of &#;8 mg cm&#;2 h&#;1, it is easy to reveal that we can reach the desired FA concentration in about 8&#;10 h, which precisely refers to the period time of our validation tests (and also approximately to the time of sunlight irradiation per day). Note that a careful techno-economic assessment (TEA) would be required to validate this technology.

Our preliminary calculations, based on experimentation with our lab-scale device, show that the combined production of formate and H2 can provide a continuous and constant H2 productivity that, although relatively low, could be used for decentralised production of H2 to cover domestic needs. Further improvements in the performance of the PV/EC device are possible, i.e. the solar-to-fuel efficiency may be enhanced to 15%, or the FE to formate to 80%, but this will not essentially increase the daily productivity of H2. The main parameter affecting this value is the current density. The methods to improve it at the level necessary for exploitability are (i) integrating the PV module with other external electrical energy sources (wind-powered, for example) or (ii) using solar concentrators to enhance SUN irradiation (tests here are made with one SUN).

There are various relatively cheap solutions to have a SUN concentration in the 5&#;100 SUN range (one SUN indicates the irradiance of one solar constant), which could be integrated into the device. A SUN concentration in the 10&#;20 range could improve the H2 productivity while limiting overheating of the device. Note also that with the availability of higher current densities, the design of the EC part (including synthesis of the electrocatalytic materials, electrode engineering and optimisation of the operating conditions) should be adapted to be coupled with PV modules working at higher light irradiation/current density (to operate close to the MPP).

We also remark that our AL presents many advantages concerning analogous systems that instead produce CO. As discussed above, we realised a device based on only earth-abundant materials with top-level performance (as reported in Fig. 7). In contrast, most other literature results are based on using noble and costly metals for CO formation. For example, Cheng et al.49 valuably reported the performance of a solar-driven gas diffusion electrode flow cell with a solar-to-fuel conversion efficiency of 19.1%. They used a gas diffusion electrode employing an Ag nanoparticle catalyst layer for CO2 reduction in their PEC cell wired with a GaInP/GaInAs/Ge triple-junction photovoltaic cell. The availability of all these materials is considered to be at serious risk within 100 years.50 We demonstrate that it is possible to achieve high performance even without these elements. In addition, they used very small electrodes and PV cells (with a geometrical area of &#;0.3 cm2). The size of our electrodes and PV cell is much larger (almost 6 cm2 and 13 cm2, respectively), thus more than one order of magnitude higher. Although small demonstrations are of fundamental importance, especially in the first step of the investigation, there are aspects related to mass transfer and charge diffusion not taken into account with low electrode areas. The size of the electrodes/devices is an issue of paramount importance in determining the needed cell voltage for additional overpotential phenomena along the surface of the electrodes that increase with the geometrical area. Moreover, it is more challenging to prepare uniformly larger electrodes, which should be taken into strong consideration in scaling up the fabrication of the electrodes. Lab results obtained with very low geometrical surface areas are insufficient to prove the concept and analyse the system's scalability.

Furthermore, the thermodynamic potential needed for the reduction of CO2 to CO is more favourable than the reduction potential to FA (&#;0.53 V against &#;0.61 V, respectively).51 Our AL produces FA and no CO (CO formation was very low and may be easily reduced at zero, as discussed above). Apart from the importance of making selectively FA (which is a liquid product that can be completely separated from the hydrogen gas produced), FA is a more valuable product than CO. Carbon monoxide might be considered as a fuel, but when downstream converted in combination with H2 (with the further issue to control the CO:H2 ratio depending on the process). In contrast, FA is a base material for several industries and is considered a promising hydrogen carrier or energy carrier for direct fuel cell power. It is more challenging to obtain FA than CO, as the reaction mechanism for FA formation involves the introduction of H atoms in the CO2 molecule after selective activation of the latter, although the mechanistic aspects still need to be clarified. Thus, the selective formation of FA is not only a matter of catalyst, but other aspects, such as the diffusion of H species towards the electrode surface, can play a fundamental role. Recently, some of us studied the influence of proton diffusion in the liquid- and gas-phase, highlighting the importance of cell/electrode design and operating conditions in addressing the selectivity of CO2 reduction beyond the properties of the electrocatalysts themselves.52 The design of our AL considers all these phenomena, which can favour the production of FA and H2 in place of CO.

Our AL device uses different electrolytes in cathode and anode compartments because the two electrodes (Cu&#;S and Ni&#;Fe&#;Zn oxide) are stable and work efficiently at different pHs. In general, the pH in the electrolyte solutions can vary for (i) crossover of ions and/or (ii) consumption of H+ and OH&#; needed for the half-reactions (CO2RR and OER, respectively). In our case, we observed that, in the period of tests (i.e. ten hours), the PTFE-reinforced Nafion allowed maintaining the initial pH difference during the electrocatalytic tests (pH variation to only 0.2 points, with an analytical error of ±0.07), also limiting the crossover of ions (as usually observed from standard thin Nafion or anion exchange membranes).

However, the pH variation may become important for more extended operations, and recycling the catholyte and anolyte solutions in an auxiliary process is demanding if no countermeasures are taken. The most helpful strategy is the adoption of a bipolar exchange membrane, which offers stable local pH values by hindering cations and anions from passing through the membrane and supplying H+ to the cathode and OH&#; to the anode for water dissociation at the interface layer of the membrane11 (see also Section S3 in the ESI&#;). Of course, in the case of bipolar membranes, acid&#;base neutralisation should be considered in the economy of the process.53

Regarding energy efficiency, the solar-to-fuel value of the proposed device is about 10%, which can be only slightly reduced from the downstream units illustrated in Fig. 8. A direct comparison of this efficiency with a conventional electrolyser (with green electrical energy supplied by a PV field and a storage unit for electrical energy storage to allow 24 h continuous operations) may only have limited meaning. This technology aims to develop AL devices, which may lead to artificial trees that produce H2 for domestic uses. Thus, the integration of the elements is a must, despite the absolute efficiency. Solar light itself is not a reactant having a cost. Nevertheless, we can indicate that the energy efficiency of the integrated system reported in Fig. 8 can be a cost-effective and efficient technology advantageous to the current state of the art54&#;56 when further developed.

4. Conclusions

i.e.

, 6.2% to formate or 10.1% to combined H2 + formate) and for storing solar energy as formate with productivity >8 mg cm&#;2 h&#;1 by CO2 electroreduction. This achievement was realised by optimised electrode development and adjusted and accurate integration of all parts and components to minimise losses. Furthermore, the PV module and the EC cell were designed to offer a correct matching to operate at the best working point.2 reduction and oxygen evolution reactions) will lead to even higher performance values.

In summary, we have demonstrated that the combination of an electrochemical (EC) cell and a photovoltaic (PV) module in a wired configuration, exclusively built with earth-abundant and non-critical raw materials, is a suitable option for reducing energy losses, reaching a high efficiency (, 6.2% to formate or 10.1% to combined H+ formate) and for storing solar energy as formate with productivity >8 mg cmby COelectroreduction. This achievement was realised by optimised electrode development and adjusted and accurate integration of all parts and components to minimise losses. Furthermore, the PV module and the EC cell were designed to offer a correct matching to operate at the best working point. 57 Additional reductions of the overvoltage of the two half-cell reactions (COreduction and oxygen evolution reactions) will lead to even higher performance values.

Our device was validated for over ten hours without observing any deactivation of the electrocatalytic materials (in both the cathode and anode), providing high stability during the solar irradiation tests.

The device is proposed to produce ultra-low carbon (green) H2 with an integrated system to store hydrogen as formate, which can then decompose to produce H2, providing a continuous daily H2 production of 2 g h&#;1 per m2 of the PV panel. This solution realises better-integrated AL devices with advantages over non-integrated systems, particularly for an objective to move to future artificial-tree systems for domestic production of H2. Although the H2 productivity is relatively low, its constant continuous output can be used for decentralised energy production, but a careful TEA analysis is necessary to validate this technology. Alternatively, the device could produce chemicals (formate) by CO2 electro-conversion; in this case, further improvements are needed to increase the FE to formate.

In both cases, scaling up the electrode surface area (e.g., to &#;500 cm2) will be required to implement this device. Furthermore, additional efforts are needed in the manufacturing processes for integration, such as (i) lower cathodic and anodic overpotential, (ii) more extended stability tests, (iii) lower cathodic pH to produce formic acid instead of formate, to enable the implementation of affordable artificial leaves.

In addition, the technology proposed here overcomes the problem of discontinuity in the production of H2 by devices using (only) renewable electrical energy because it integrates a solution for efficient intermediate hydrogen storage. Only the AL-type device was tested here because the other elements of the overall scheme presented in Fig. 8 are already established. At the same time, the discussion evidences some directions to explore for moving to application.

In conclusion, it is helpful to remark on the breakthrough advances presented in this work:

1. We report world top-level results regarding solar-to-fuel efficiency and current density, as shown in Fig. 7. The combined solar-to-fuel efficiency is 10% (to FA and H2), a value still often presented at conferences as a target for future research. Although recent papers reported good results in terms of STF efficiency higher or close to 10% at reasonable current density, it is essential to remark that indicating only the solar-to-fuel efficiency, as often reported in the literature, is misleading because high STF efficiency values can be obtained under conditions that are not relevant in terms of productivity (current density). From this perspective, this paper represents a breakthrough in achieving top-level results considering current density and solar-to-fuel efficiency.

2. We demonstrate for the first time the feasibility of producing with a PV-EC device a continuous production of H2 (24 h) via the coproduction of an H2-storage molecule (formate &#; FA) which can be decomposed during dark operations (with recycling of CO2) to allow the continuous production of H2. This concept is entirely original and presented for the first time, representing another breakthrough element opening new directions and overcoming the issues related to using PEC devices only when solar light is present, thus a fraction of the day.

3. We realised a device based on earth-abundant materials with top-level performance, while most of the other literature results are based on using noble and costly metals, such as Ir, Ru, etc. We demonstrate that high performance can be achieved even without these elements.

Many efforts have been devoted to integrating all the elements constituting the artificial leaf (electrocatalytic materials, substrates, membranes, PV module, etc.). Although their main characteristics and individual behaviours were already known, the resulting performance in a full device could be very low without proper integration of the main pieces. An optimal WP was obtained from coupling the EC device with the PV cell (see Fig. 4), which was very close to the maximum power point and defined suitable working conditions (in terms of the current&#;potential couple) to provide high FEs to the target products.

Other systems in the literature reported high STF efficiency but working at relatively low current density, as the latter depends on integrating the EC device with the PV cell. We would also like to remark that the current density (specific to the target products) is the main parameter to consider from the implementation point of view, as it is directly proportional to the process yield/productivity.

The realisation of the artificial leaf was made possible thanks to multidisciplinary scientific work involving top-level research institutions and universities participating in this work within the funded European A-LEAF Project,23 each focusing on optimising a specific aspect/component of the artificial leaf.

The work has introduced many novel aspects of the production of solar fuels, which have been critically discussed here. The paper has also highlighted the main issues of these types of systems (such as the need to maintain the pH difference between the two half-cells of the EC device), which can stimulate further research to find new technological solutions and make the implementation of artificial leaves economically affordable.

5. Experimental

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5.1 Synthesis of Cu&#;S

3)2·3H2O and 0.067 mmol of elemental sulfur (Sigma-Aldrich, >95%) were mixed in 40 mL of ethylene glycol (Sigma-Aldrich, 99%) under stirring at room temperature for 30 min. The mixture, transferred into a 50 mL Teflon-lined autoclave, was heat-treated at 140 °C for ten hours (with a heating rate of 5 °C min&#;1), and finally cooled at room temperature. The resulting mixture was washed with ultrapure water three times by centrifugation ( rpm, 10 min), and the obtained powder was dried under vacuum overnight at 80 °C.

Copper sulfide (Cu&#;S) was prepared by adapting a solvothermal method reported elsewhere. 58,59 Briefly, four mmol of Cu(NO·3HO and 0.067 mmol of elemental sulfur (Sigma-Aldrich, >95%) were mixed in 40 mL of ethylene glycol (Sigma-Aldrich, 99%) under stirring at room temperature for 30 min. The mixture, transferred into a 50 mL Teflon-lined autoclave, was heat-treated at 140 °C for ten hours (with a heating rate of 5 °C min), and finally cooled at room temperature. The resulting mixture was washed with ultrapure water three times by centrifugation ( rpm, 10 min), and the obtained powder was dried under vacuum overnight at 80 °C.

5.2 Synthesis of Ni&#;Zn&#;Fe oxide

The metal oxide NiZnFeO

x

was obtained

via

a hydrothermal method. Equimolecular amounts of metal nitrates were dissolved in water (metal concentration: 50 mM), and the solution was hydrolysed with diluted aqueous ammonia until pH = 8.5 was achieved. The solution was introduced into a Teflon cup and mounted in an autoclave at 140 °C for 2 h. After the hydrothermal treatment, the pressure vessel was cooled in air, the product was washed with H2O and CH3CH2OH, and the nanoparticles were collected by centrifugation (final particle size: &#;8 nm).

5.3 Electrode preparation

Cu&#;S (or Ni&#;Fe&#;Zn oxide) powders were deposited on carbon-based substrates to prepare the electrodes. In detail, a weighted amount of catalyst (to obtain a final loading of 1.0 mg cm&#;2) was mixed with 4 mL of water and 4 mL of isopropanol and 50 μL of 10% Nafion® perfluorate and sonicated until the formation of a stable suspension (15 min). The resulting ink was deposited by spray coating onto a pre-heated porous carbon-based gas-diffusion layer (GDL, Sigracet® 39BCE, supplied by Ion Power), acting as the support, allowing the evaporation of the solvent and fixing the powder uniformly on the electrode surface.

5.4 Electrochemical device

2. A 3 M Ag/AgCl capillary electrode (1 mm dia., supplied by Alvatek) was used as the reference electrode and located very close to the working electrode to minimise overpotential. The potential values were then translated into RHE (Reversible Hydrogen Electrode) voltages by using the following equation: 

E

(RHE) =

E

Ag/AgCl + 0.059 pH + 0.21

(7)Continuous recirculation of the catholyte and anolyte between each half-cell (cathode and anode, respectively) and two independent reservoir containers was performed using a peristaltic pump working at a liquid flow rate of 50 mL min&#;1.

The electrochemical setup is described in Section 2. The Plexiglas cell has a highly compact configuration. The two GDEs (the cathode with Cu&#;S and the anode with Ni&#;Fe&#;Zn oxide) were located within the electrochemical cell, with the catalyst facing the liquid phase. The geometrical surface area of both electrodes is about 5.3 cm. A 3 M Ag/AgCl capillary electrode (1 mm dia., supplied by Alvatek) was used as the reference electrode and located very close to the working electrode to minimise overpotential. The potential values were then translated into RHE (Reversible Hydrogen Electrode) voltages by using the following equation:Continuous recirculation of the catholyte and anolyte between each half-cell (cathode and anode, respectively) and two independent reservoir containers was performed using a peristaltic pump working at a liquid flow rate of 50 mL min

20 mL min&#;1 of pure CO2 was flowed into the catholyte reservoir (in connection with the cathode compartment of the cell) to saturate the KHCO3 solution. At the equilibrium, a pH of 6.78 was reached in the catholyte before starting the electrochemical tests. Other 20 mL min&#;1 of pure CO2 also flowed into the gas chamber.

A potentiostat/galvanostat (Autolab PGSTAT204 with FRA32M Module, supplied by Metrohm) was used to perform the electrochemical characterisation.

Gaseous products from the cathode outlet stream (H2, CH4, CO, C2H4, and C2H6) were detected using a Gas Chromatograph (MicroGC GCX Pollution Analytic Equipment), having a sensitivity of 1&#;2 ppm, by sampling from the cathodic reservoir every 20 min. The liquid catholyte was analysed by Ion Chromatography (IC Metrohm 940 Professional, column Metrohm Organic Acids) to detect the presence of liquid products such as formate (and eventually acetate and oxalate). The presence of other liquid products (i.e., methanol, ethanol, methyl formate, and acetone) was checked by Gas Chromatography-Mass Spectrometry (GC-MS Thermo -Tsq Evo, column Stabilwax).

Author contributions

This paper derives from a joint EU project (A-LEAF) where the partners (a) developed the cell, the partners (b) the PV module, the partners (c) and (d) the electrocatalytic components and the partners (e) and (f) to the analysis of the results. The manuscript was designed and written by CA in collaboration with GC, with the contribution of all other authors.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The European Union funded this work through the H-FETPROACT Project Number titled &#;An Artificial Leaf: a photo-electro-catalytic cell from earth-abundant materials for sustainable solar production of CO2-based chemicals and fuels (A-LEAF)&#;, which is gratefully acknowledged. TM, VS, UC and OA would also like to thank the silicon heterojunction solar cell baseline; Alain Doumit for the wafer texture and cleaning; Silke Lynen, Volker Lauterbach, and Andreas Mück for PECVD deposition; Hildegard Siekmann for the ITO sputtering; Sunny Peris for module encapsulation; and Dr Kaining Ding, Dr Andreas Lambertz and Dr Weiyuan Duan for scientific support. Additional sources of funding were the European Union through the DECADE H project (ID: ) and the MIUR (Italy) through the PRIN Project CO2 ONLY (No. WR2LRS), acknowledged by CA, DG, MM, GC and SP, and ETH Zurich through an ETH Research Grant (ETH-47 19-1), acknowledged by FLPV. SG also acknowledges support from the project PID-RB-C41 funded by MCIN/AEI/10./.

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Footnotes

Electronic supplementary information (ESI) available. See DOI: https://doi.org/10./d2eee

&#;

Present address: Institut de Recherches sur la Catalyse et l&#;Environnement de Lyon, UMR , CNRS, Université Claude Bernard Lyon 1, 2 Avenue A. Einstein, Villeurbanne, France.

§

Present address: Photoactivated Processes Unit, IMDEA Energy, Avda. Ramón de la Sagra, 3, Móstoles, Spain.

This journal is © The Royal Society of Chemistry

The artificial leaf &#; a silicon solar cell with different catalytic materials bonded onto its two sides &#; needs no external wires or control circuits to operate. Simply placed in a container of water and exposed to sunlight, it quickly begins to generate streams of bubbles: oxygen bubbles from one side and hydrogen bubbles from the other. If placed in a container that has a barrier to separate the two sides, the two streams of bubbles can be collected and stored, and used later to deliver power: for example, by feeding them into a fuel cell that combines them once again into water while delivering an electric current.

The creation of the device is Science. Nocera, the Henry Dreyfus Professor of Energy and professor of chemistry at MIT, is the senior author; the paper was co-authored by his former student Steven Reece PhD &#;07 (who now works at Sun Catalytix, a company started by Nocera to commercialize his solar-energy inventions), along with five other researchers from Sun Catalytix and MIT.

The device, Nocera explains, is made entirely of earth-abundant, inexpensive materials &#; mostly silicon, cobalt and nickel &#; and works in ordinary water. Other attempts to produce devices that could use sunlight to split water have relied on corrosive solutions or on relatively rare and expensive materials such as platinum.

The artificial leaf is a thin sheet of semiconducting silicon &#; the material most solar cells are made of &#; which turns the energy of sunlight into a flow of wireless electricity within the sheet. Bound onto the silicon is a layer of a cobalt-based catalyst, which releases oxygen, a material whose potential for generating fuel from sunlight was discovered by Nocera and his co-authors in . The other side of the silicon sheet is coated with a layer of a nickel-molybdenum-zinc alloy, which releases hydrogen from the water molecules.



An 'artificial leaf' made by Daniel Nocera and his team, using a silicon solar cell with novel catalyst materials bonded to its two sides, is shown in a container of water with light (simulating sunlight) shining on it. The light generates a flow of electricity that causes the water molecules, with the help of the catalysts, to split into oxygen and hydrogen, which bubble up from the two surfaces.
Video courtesy of the Nocera Lab/Sun Catalytix


&#;I think there&#;s going to be real opportunities for this idea,&#; Nocera says. &#;You can&#;t get more portable &#; you don&#;t need wires, it&#;s lightweight,&#; and it doesn&#;t require much in the way of additional equipment, other than a way of catching and storing the gases that bubble off. &#;You just drop it in a glass of water, and it starts splitting it,&#; he says.

Now that the &#;leaf&#; has been demonstrated, Nocera suggests one possible further development: tiny particles made of these materials that can split water molecules when placed in sunlight &#; making them more like photosynthetic algae than leaves. The advantage of that, he says, is that the small particles would have much more surface area exposed to sunlight and the water, allowing them to harness the sun&#;s energy more efficiently. (On the other hand, engineering a system to separate and collect the two gases would be more complicated in such a setup.)

The new device is not yet ready for commercial production, since systems to collect, store and use the gases remain to be developed. &#;It&#;s a step,&#; Nocera says. &#;It&#;s heading in the right direction.&#;

Ultimately, he sees a future in which individual homes could be equipped with solar-collection systems based on this principle: Panels on the roof could use sunlight to produce hydrogen and oxygen that would be stored in tanks, and then fed to a fuel cell whenever electricity is needed. Such systems, Nocera hopes, could be made simple and inexpensive enough so that they could be widely adopted throughout the world, including many areas that do not presently have access to reliable sources of electricity.

Professor James Barber, a biochemist from Imperial College London who was not involved in this research, says Nocera&#;s finding of the cobalt-based catalyst was a &#;major discovery,&#; and these latest findings &#;are equally as important, since now the water-splitting reaction is powered entirely by visible light using tightly coupled systems comparable with that used in natural photosynthesis. This is a major achievement, which is one more step toward developing cheap and robust technology to harvest solar energy as chemical fuel.&#;

Barber cautions that &#;there will be much work required to optimize the system, particularly in relation to the basic problem of efficiently using protons generated from the water-splitting reaction for hydrogen production.&#; But, he says, &#;there is no doubt that their achievement is a major breakthrough which will have a significant impact on the work of others dedicated to constructing light-driven catalytic systems to produce hydrogen and other solar fuels from water. This technology will advance side by side with new initiatives to improve and lower the cost of photovoltaics.&#;

Nocera&#;s ongoing research with the artificial leaf is directed toward &#;driving costs lower and lower,&#; he says, and looking at ways of improving the system&#;s efficiency. At present, the leaf can redirect about 2.5 percent of the energy of sunlight into hydrogen production in its wireless form; a variation using wires to connect the catalysts to the solar cell rather than bonding them together has attained 4.7 percent efficiency. (Typical commercial solar cells today have efficiencies of more than 10 percent). One question Nocera and his colleagues will be addressing is which of these configurations will be more efficient and cost-effective in the long run.

Another line of research is to explore the use of photovoltaic (solar cell) materials other than silicon &#; such as iron oxide, which might be even cheaper to produce. &#;It&#;s all about providing options for how you go about this,&#; Nocera says.

Researchers led by MIT professor Daniel Nocera have produced something they&#;re calling an &#;artificial leaf&#;: Like living leaves, the device can turn the energy of sunlight directly into a chemical fuel that can be stored and used later as an energy source.The artificial leaf &#; a silicon solar cell with different catalytic materials bonded onto its two sides &#; needs no external wires or control circuits to operate. Simply placed in a container of water and exposed to sunlight, it quickly begins to generate streams of bubbles: oxygen bubbles from one side and hydrogen bubbles from the other. If placed in a container that has a barrier to separate the two sides, the two streams of bubbles can be collected and stored, and used later to deliver power: for example, by feeding them into a fuel cell that combines them once again into water while delivering an electric current.The creation of the device is described in a paper published Sept. 30 in the journal. Nocera, the Henry Dreyfus Professor of Energy and professor of chemistry at MIT, is the senior author; the paper was co-authored by his former student Steven Reece PhD &#;07 (who now works at Sun Catalytix, a company started by Nocera to commercialize his solar-energy inventions), along with five other researchers from Sun Catalytix and MIT.The device, Nocera explains, is made entirely of earth-abundant, inexpensive materials &#; mostly silicon, cobalt and nickel &#; and works in ordinary water. Other attempts to produce devices that could use sunlight to split water have relied on corrosive solutions or on relatively rare and expensive materials such as platinum.The artificial leaf is a thin sheet of semiconducting silicon &#; the material most solar cells are made of &#; which turns the energy of sunlight into a flow of wireless electricity within the sheet. Bound onto the silicon is a layer of a cobalt-based catalyst, which releases oxygen, a material whose potential for generating fuel from sunlight was discovered by Nocera and his co-authors in . The other side of the silicon sheet is coated with a layer of a nickel-molybdenum-zinc alloy, which releases hydrogen from the water molecules.&#;I think there&#;s going to be real opportunities for this idea,&#; Nocera says. &#;You can&#;t get more portable &#; you don&#;t need wires, it&#;s lightweight,&#; and it doesn&#;t require much in the way of additional equipment, other than a way of catching and storing the gases that bubble off. &#;You just drop it in a glass of water, and it starts splitting it,&#; he says.Now that the &#;leaf&#; has been demonstrated, Nocera suggests one possible further development: tiny particles made of these materials that can split water molecules when placed in sunlight &#; making them more like photosynthetic algae than leaves. The advantage of that, he says, is that the small particles would have much more surface area exposed to sunlight and the water, allowing them to harness the sun&#;s energy more efficiently. (On the other hand, engineering a system to separate and collect the two gases would be more complicated in such a setup.)The new device is not yet ready for commercial production, since systems to collect, store and use the gases remain to be developed. &#;It&#;s a step,&#; Nocera says. &#;It&#;s heading in the right direction.&#;Ultimately, he sees a future in which individual homes could be equipped with solar-collection systems based on this principle: Panels on the roof could use sunlight to produce hydrogen and oxygen that would be stored in tanks, and then fed to a fuel cell whenever electricity is needed. Such systems, Nocera hopes, could be made simple and inexpensive enough so that they could be widely adopted throughout the world, including many areas that do not presently have access to reliable sources of electricity.Professor James Barber, a biochemist from Imperial College London who was not involved in this research, says Nocera&#;s finding of the cobalt-based catalyst was a &#;major discovery,&#; and these latest findings &#;are equally as important, since now the water-splitting reaction is powered entirely by visible light using tightly coupled systems comparable with that used in natural photosynthesis. This is a major achievement, which is one more step toward developing cheap and robust technology to harvest solar energy as chemical fuel.&#;Barber cautions that &#;there will be much work required to optimize the system, particularly in relation to the basic problem of efficiently using protons generated from the water-splitting reaction for hydrogen production.&#; But, he says, &#;there is no doubt that their achievement is a major breakthrough which will have a significant impact on the work of others dedicated to constructing light-driven catalytic systems to produce hydrogen and other solar fuels from water. This technology will advance side by side with new initiatives to improve and lower the cost of photovoltaics.&#;Nocera&#;s ongoing research with the artificial leaf is directed toward &#;driving costs lower and lower,&#; he says, and looking at ways of improving the system&#;s efficiency. At present, the leaf can redirect about 2.5 percent of the energy of sunlight into hydrogen production in its wireless form; a variation using wires to connect the catalysts to the solar cell rather than bonding them together has attained 4.7 percent efficiency. (Typical commercial solar cells today have efficiencies of more than 10 percent). One question Nocera and his colleagues will be addressing is which of these configurations will be more efficient and cost-effective in the long run.Another line of research is to explore the use of photovoltaic (solar cell) materials other than silicon &#; such as iron oxide, which might be even cheaper to produce. &#;It&#;s all about providing options for how you go about this,&#; Nocera says.

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