Our civilization is facing one of the biggest challenges in its entire history: the energy transition. The growing demand for energy, the scarcity of resources and climate change are the three main factors driving the change. Firstly, the world population is expected to keep on growing, and many poor countries are projected to experience rapid economic development in the near feature, which calls for the continued growth of global energy supply. Secondly, fossil fuels, the back-bone of the current energy-infrastructure, while still sufficient in quantity for the upcoming decades, are finite. In addition, the combustion of these fossil fuels is leading to global climate change, which is a great threat to humanity as we know it, in environmental, economical, political and sociological terms. Thus, a move away from relying on fossil fuels and towards renewable, clean and sustainable energy resources like the Sun, is both necessary and inevitable… @en

Solar-assisted water splitting with bismuth vanadate (BiVO₄) photoanodes has progressed significantly with many efforts devoted to improving charge separation and surface charge injection through synthetic methods, including dopants and catalytic layers. In contrast, postsynthetic treatments occur after the synthesis of electrodes. Recently, such postsynthetic treatments based upon illumination, chemistry, electrochemistry, or combinations thereof have led to dramatic improvements in the performance and efficiency of BiVO₄ photoanodes. This Perspective summarizes recent BiVO₄ postsynthetic treatments with mechanistic details and highlights important future directions. One broad challenge is that multiple interpretations of defect changes may be consistent with routine X-ray photoelectron spectroscopy data. Further experiments are suggested to better differentiate between the proposed defect changes. Also, performance changes are considered separately with respect to charge separation and charge injection efficiencies as well as within the context of known synthetic modifications. The emergence of postsynthetic treatments highlights new opportunities to understand and improve photoelectrodes. Similar mechanisms may be of further utility as researchers turn more focus toward the development of novel multinary metal oxide photoabsorbers for the production of solar fuels. Lastly, postsynthetic treatments also elucidate possible electrode changes under extended service and can provide new strategies to enable extended device performance.

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Metal-insulator-semiconductor (MIS) photoelectrodes offer a simple alternative to the traditional semiconductor-liquid junction and the conventional p-n junction electrode. Highly efficient MIS photoanodes require interfacial surface passivating oxides and high workfunction metals to produce a high photovoltage. Herein, we investigate and analyze the effect of interfacial oxides and metal workfunctions on the barrier height and the photovoltage of a c-Si photoanode. We use two metal components in a bimetal contact configuration and observe the modulation of the effective barrier height and the resulting photovoltage as a function of the secondary outer metal. The photovoltage shows a strong linear dependence by increasing the inner metal workfunction, with the highest photovoltage achieved by a MIS photoanode using a platinum inner metal. We also found that coupling a thin aluminium oxide with an interfacial silicon oxide and controlling the oxide thickness can significantly improve the photovoltage of an MIS junction photoanode.

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Bismuth vanadate (BiVO₄) is one of the most efficient light absorbing metal oxides for solar water splitting. BiVO₄ photoanodes immersed in an electrolyte in an open circuit configuration and exposed to simulated solar illumination for prolonged time achieve superior photoelectrochemical (PEC) activity. This photocharging (PC) effect is capable of almost completely overcoming the surface and bulk limitations of BiVO₄. Herein we show that alkaline conditions favor the PC effect; specifically BiVO₄ photoanodes subjected to PC treatment at pH 10 achieve a record high photocurrent for undoped and uncatalyzed BiVO₄ of 4.3 mA cm^-2 @ 1.23 V_RHE, an outstandingly low onset potential of 0.25 V_RHE, and a very steep photocurrent onset. Alkaline conditions also facilitate excellent external and internal quantum efficiencies of 75 and 95% respectively (average in the 440 nm > λ > 330 nm range). Moreover, impedance spectroscopy and in situ XAS study suggest that electronic, structural and chemical properties of the bulk of these films remain unchanged during the PC treatment. However, appreciable changes in the surface-related properties take place. Ultimately, our results indicate that the improved activity of PC-BiVO₄ is enhanced by surface reaction pathways of the semiconductor-liquid junction, which is directly correlated with the electrochemical environment in which it is modified.

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Solar-assisted water splitting can potentially provide an efficient route for large-scale renewable energy conversion and storage. It is essential for such a system to provide a sufficiently high photocurrent and photovoltage to drive the water oxidation reaction. Here we demonstrate a photoanode that is capable of achieving a high photovoltage by engineering the interfacial energetics of metal-insulator-semiconductor junctions. We evaluate the importance of using two metals to decouple the functionalities for a Schottky contact and a highly efficient catalyst. We also illustrate the improvement of the photovoltage upon incidental oxidation of the metallic surface layer in KOH solution. Additionally, we analyse the role of the thin insulating layer to the pinning and depinning of Fermi level that is responsible to the resulting photovoltage. Finally, we report the advantage of using dual metal overlayers as a simple protection route for highly efficient metal-insulator-semiconductor photoanodes by showing over 200 h of operational stability.

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In this work, the selective electrocatalytic reduction of carbon dioxide to carbon monoxide on oxide-derived silver electrocatalysts is presented. By a simple synthesis technique, the overall high faradaic efficiency for CO production on the oxide-derived Ag was shifted by more than 400 mV towards a lower overpotential compared to that of untreated Ag. Notably, the Ag resulting from Ag oxide is capable of electrochemically reducing CO₂to CO with approximately 80 % catalytic selectivity at a moderate overpotential of 0.49 V, which is much higher than that (ca. 4 %) of untreated Ag under identical conditions. Electrokinetic studies show that the improved catalytic activity is ascribed to the enhanced stabilization of COOH^.intermediate. Furthermore, highly nanostructured Ag is likely able to create a high local pH near the catalyst surface, which may also facilitate the catalytic activity for the reduction of CO₂with suppressed H₂evolution.

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Bismuth vanadate (BiVO4) is a promising semiconductor material for the production of solar fuels via photoelectrochemical water splitting, however, it suffers from substantial recombination losses that limit its performance to well below its theoretical maximum. Here we demonstrate for the first time that the photoelectrochemical (PEC) performance of BiVO4 photoanodes can be dramatically improved by prolonged exposure to AM 1.5 illumination in the open circuit (OC) configuration. Photoanodes subjected to such light treatment achieve a record photocurrent for undoped and uncatalysed BiVO4 of 3.3 mA cm−2 at 1.23 VRHE. Moreover, photoelectrochemical tests with a sacrificial agent yield significantly enhanced catalytic efficiency over the whole operating potential range, suggesting elimination of major losses at the semiconductor–electrolyte interface. Finally, we demonstrate that this so-called ‘photocharging’ technique induces a considerable cathodic shift in the photocurrent onset potential and increases the photovoltage extracted from BiVO4 photoanodes.@en

Ni-based oxygen evolution catalysts (OECs) are cost-effective and very active materials that can be potentially used for efficient solar-to-fuel conversion process toward sustainable energy generation. We present a systematic spectroelectrochemical characterization of two Fe-containing Ni-based OECs, namely nickel borate (Ni(Fe)−Bi) and nickel oxyhydroxide (Ni(Fe)OOH). Our Raman and X-ray absorption spectroscopy results show that both OECs are chemically similar, and that the borate anions do not play an apparent role in the catalytic process at pH 13. Furthermore, we show spectroscopic evidence for the generation of negatively charged sites in both OECs (NiOO–), which can be described as adsorbed “active oxygen”. Our data conclusively links the OER activity of the Ni-based OECs with the generation of those sites on the surface of the OECs. The OER activity of both OECs is strongly pH dependent, which can be attributed to a deprotonation process of the Ni-based OECs, leading to the formation of the negatively charged surface sites that act as OER precursors. This work emphasizes the relevance of the electrolyte effect to obtain catalytically active phases in Ni-based OECs, in addition to the key role of the Fe impurities. This effect should be carefully considered in the development of Ni-based compounds meant to catalyze the OER at moderate pHs. Complementarily, UV–vis spectroscopy measurements show strong darkening of those catalysts in the catalytically active state. This coloration effect is directly related to the oxidation of nickel and can be an important factor limiting the efficiency of solar-driven devices utilizing Ni-based OECs.@en