Although sunlight-driven water splitting is a promising route to sustainable hydrogen fuel production, widespread implementation is hampered by the expense of the necessary photovoltaic and photoelectrochemical apparatus. Here, we describe a highly efficient and low-cost water-splitting cell combining a state-of-the-art solution-processed perovskite tandem solar cell and a bifunctional Earth-abundant catalyst. The catalyst electrode, a NiFe layered double hydroxide, exhibits high activity toward both the oxygen and hydrogen evolution reactions in alkaline electrolyte. The combination of the two yields a water-splitting photocurrent density of around 10 milliamperes per square centimeter, corresponding to a solar-to-hydrogen efficiency of 12.3%. Currently, the perovskite instability limits the cell lifetime.

Due to its abundance, scalability, and nontoxicity, Cu2O has attracted extensive attention toward solar energy conversion, and it is the best performing metal oxide material. Until now, the high efficiency devices are all planar in structure, and their photocurrent densities still fall well below the theoretical value of 14.5 mA cm(-2) due to the incompatible light absorption and charge carrier diffusion lengths. Nanowire structures have been considered as a rational and promising approach to solve this issue, but due to various challenges, performance improvements through the use of nanowires have rarely been achieved. In this work, we develop a new synthetic method to grow Cu2O nanowire arrays on conductive fluorine-doped tin oxide substrates with well-controlled phase and excellent electronic and photonic properties. Also, we introduce an innovative blocking layer strategy to enable high performance. Further, through material engineering by combining a conformal nanoscale p-n junction, durable protective overlayer, and uniform catalyst decoration, we have successfully fabricated Cu2O nanowire array photocathodes for hydrogen generation from solar water splitting delivering unprecedentedly high photocurrent densities of 10 mA cm(-2) and stable operation beyond 50 h, establishing a new benchmark for metal oxide based photoelectrodes.

The solar-driven electrochemical reduction of CO 2 to fuels and chemicals provides a promising way for closing the anthropogenic carbon cycle. However, the lack of selective and Earth-abundant catalysts able to achieve the desired transformation reactions in an aqueous matrix presents a substantial impediment as of today. Here we introduce atomic layer deposition of SnO 2 on CuO nanowires as a means for changing the wide product distribution of CuO-derived CO 2 reduction electrocatalysts to yield predominantly CO. The activity of this catalyst towards oxygen evolution enables us to use it both as the cathode and anode for complete CO 2 electrolysis. In the resulting device, the electrodes are separated by a bipolar membrane, allowing each half-reaction to run in its optimal electrolyte environment. Using a GaInP/GaInAs/Ge photovoltaic we achieve the solar-driven splitting of CO 2 into CO and oxygen with a bifunctional, sustainable and all Earth-abundant system at an e ciency of 13.4%.T he electrochemical reduction of CO 2 to fuels and chemicals has the promise to provide a versatile way of storing renewable electrical energy in chemical bonds while simultaneously closing the anthropogenic carbon cycle. A number of products have been successfully synthesized by this process, most notably carbon monoxide (CO) 1-3 , formic acid (HCOOH), methane (CH 4 ) 4 , ethylene (C 2 H 4 ) 5 and ethanol (CH 3 CH 2 OH) 6 , as well as other compounds 7,8 . Due to the numerous possible reaction pathways, selectively targeting one specific product at high yield has remained a challenge, which, to the present day, has been achieved only for CO and formic acid in aqueous electrolytes. Unfortunately, selective electroreduction of CO 2 to these products relies on the use of precious metals (Au, Ag, Pd) [9][10][11][12] , requires operation at considerable overpotentials 13 , or requires the use of electrolyte additives, such as ionic liquids 14 . Developing inexpensive, selective and stable catalysts operating at low overpotentials is therefore a crucial requirement.Recently, substantial progress toward decreasing the overpotential of copper-based electrodes was made by employing catalysts derived from copper oxides 15 . However, the insufficient selectivity remained an issue, with the catalyst producing CO, H 2 and formic acid at comparable selectivities. Following up on this work, it was demonstrated that by electrochemically reducing copper oxide in the presence of indium ions, the selectivity toward producing CO could be substantially enhanced 16,17 . More recently, the same group demonstrated tin to have a similar effect 18 . Although adding sources of metal ions during the catalyst reduction process is effective in tuning the selectivity, it is difficult to control and may not guarantee uniform coating.Here, we demonstrate the surface modification of CuO nanowire electrodes with SnO 2 using atomic layer deposition (ALD), leading to a highly selective catalyst for the electrochemical reduction of CO 2 to CO. By using SnO 2 -modified...

Photocathodes based on cuprous oxide (Cu2O) are promising materials for large scale and widespread solar fuel generation due to the abundance of copper, suitable bandgap, and favorable band alignments for reducing water and carbon dioxide. A protective overlayer is required to stabilize the Cu2O in aqueous media under illumination, and the interface between this overlayer and the catalyst nanoparticles was previously identified as a key source of instability. Here, the properties of the protective titanium dioxide overlayer of composite cuprous oxide photocathodes are further investigated, as well as an oxide‐based hydrogen evolution catalyst, ruthenium oxide (RuO2). The RuO2‐catalyzed photoelectrodes exhibit much improved stability versus platinum nanoparticles, with 94% stability after 8 h of light‐chopping chronoamperometry. Faradaic efficiencies of ∼100% are obtained as determined by measurement of the evolved hydrogen gas. The sustained photocurrents of close to 5 mA cm−2 obtained with this electrode during the chronoamperometry measurement (at 0 V vs. the reversible hydrogen electrode, pH 5, and simulated 1 sun illumination) would correspond to greater than 6% solar‐to‐hydrogen conversion efficiency in a tandem photoelectrochemical cell, where the bias is provided by a photovoltaic device such as a dye‐sensitized solar cell.

Artificial photosynthesis, mimicking nature in its efforts to store solar energy, has received considerable attention from the research community. Most of these attempts target the production of H2 as a fuel and our group recently demonstrated solar-to-hydrogen conversion at 12.3% efficiency. Here, in an effort to take this approach closer to real photosynthesis, which is based on the conversion of CO2, we demonstrate the efficient reduction of CO2 to carbon monoxide driven solely by simulated sunlight using water as the electron source. Employing series-connected perovskite photovoltaics and high-performance catalyst electrodes, we reach a solar-to-CO efficiency exceeding 6.5%, which represents a new benchmark in sunlight-driven CO2 conversion. Considering hydrogen as a secondary product, an efficiency exceeding 7% is observed. Furthermore, this study represents one of the first demonstrations of extended, stable operation of perovskite photovoltaics, whose large open-circuit voltage is shown to be particularly suited for this process.