Ten-percent solar-to-fuel conversion with nonprecious materials

Direct solar-to-fuels conversion can be achieved by coupling a photovoltaic device with water-splitting catalysts. We demonstrate that a solar-to-fuels efficiency (SFE) > 10% can be achieved with nonprecious, low-cost, and commercially ready materials. We present a systems design of a modular photovoltaic (PV)–electrochemical device comprising a crystalline silicon PV minimodule and low-cost hydrogen-evolution reaction and oxygen-evolution reaction catalysts, without power electronics. This approach allows for facile optimization en route to addressing lower-cost devices relying on crystalline silicon at high SFEs for direct solar-to-fuels conversion.Distributed and grid-scale adoption of nondispatchable, intermittent, renewable-energy sources requires new technologies that simultaneously address energy conversion and storage challenges (1, 2). Coupling photovoltaics to drive catalytic fuel-forming reaction, such as water splitting to generate H₂, allows for direct solar-to-fuels conversion. The solar-generated H₂ can effectively be harnessed to electricity by fuel cell devices (3, 4) or converted to liquid fuels upon its combination with CO or CO₂ (5–7). For this technology to be effectively implemented, a solar-to-fuels conversion efficiency (SFE) of 10% or higher is desirable (8, 9).Direct photoelectrochemical (PEC) water splitting by a single absorber material has attracted a vast amount of attention (10, 11), and recent progress indicates improvements in the field (12, 13); but after decades of research, direct PEC faces three challenges to increase conversion efficiency: (i) Direct absorber band alignment is required to provide carriers with appropriate potential to both half reactions. Although such an alignment is difficult to achieve in a single material initially, any change in band alignment due to changing surface conditions can result in further efficiency degradation. This makes it challenging to design devices that maintain robust, high efficiencies in actual operation. (ii) The wide absorber bandgap (>1.23 eV; typically >1.6 eV) needed to drive the water-splitting reaction is not optimized for the solar spectrum, which results in a maximum SFE of only 7% (14–16). (iii) The absorbers are poor catalysts, and they are incapable of efficiently performing the four proton-coupled electron transfer chemistry (17–22) that is needed for water splitting.These deficiencies can be overcome by substituting a PEC device with a buried-junction photovoltaic (PV) device and an electrochemical catalyst (EC) system, forming a PV–EC tandem (23–27). In a buried-junction device, the electric field is generated at an internal junction within the semiconductor and is then coupled with water-splitting catalysts through ohmic contacts, which can either be conductive coatings directly deposited onto the PV or connected through wires to the electrodes. The buried junction relaxes the constraints imposed by a PEC device because it separates light absorption from catalysis, and does not require that the absorber be stable in aqueous electrolytes in which the pH regime for the absorber and best water-splitting catalyst may not be compatible. However, PV–EC devices have been viewed historically as too expensive to be economically viable, primarily because of the use of noble-metal water-splitting catalysts and expensive and/or low-efficiency PV devices. Indeed, the solar splitting of water by nonprecious materials and under simple conditions has long been identified to be a “holy grail” of solar energy conversion (28). We have pursued this goal by using a buried-junction device coated with transparent conducting oxides, overlaid with self-healing catalysts that self-assemble upon oxidation of Co²⁺ (29–36), Ni²⁺ (37–39), and Mn²⁺ (40, 41) ions in phosphate or borate electrolytes. These catalysts have shown great fidelity for interfacing with a variety of buried junctions (42–46) to deliver what is more commonly known as the artificial leaf (47). This approach is generally being adopted by others (48, 49).Efficiencies are predicted to be as high as 18% for PV–EC devices comprising series-connected single-junction PV devices and higher for multijunction PV devices (14–16). Of significant consequence to the design of PV–EC devices is the quickly changing landscape of silicon as a PV material. In the past 7 y, the price of crystalline silicon (c-Si) solar cells has decreased by 86%, and the price of PV modules has dropped 77%. In the meantime, average commercial c-Si solar-cell efficiencies have increased to 17.5% for multicrystalline silicon and 19.5% for monocrystalline silicon (50, 51). We now report an approach that leverages c-Si solar cells and our nonprecious metal catalysts to furnish a solar-to-fuels device with an efficiency of 10%. Because a single c-Si solar cell is unable to provide enough voltage to drive the water-splitting reaction, we use multiple single-junction solar cells series connected into minimodules. Although this approach does not result in a monolithic structure in which catalysts are directly deposited on the PV device in a buried-junction configuration (e.g., an artificial leaf), the equivalent circuit for both constructs is identical (52). This approach allows for modular independent optimization, after which the components could be integrated into a monolithic design. Our device bears resemblance to recently reported copper indium gallium diselenide-based devices (53), but incorporates low-cost and nonprecious crystalline silicon solar cells and oxygen-evolution and hydrogen-evolution catalysts.