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.

Revved‐up rust! Light‐induced water splitting over iron oxide (hematite) has been achieved by using a particle‐assisted deposition technique and IrO2‐based surface catalysis. Photocurrents in excess of 3 mA cm−2 were obtained at +1.23 V versus the reversible hydrogen electrode under AM 1.5 G 100 mW cm−2 simulated sunlight. These photocurrents are unmatched by any other oxide‐based photoanode. FTO=fluorine‐doped tin oxide.

Water oxidation is a key chemical reaction, central to both biological photosynthesis and artificial solar fuel synthesis strategies. Despite recent progress on the structure of the natural catalytic site, and on inorganic catalyst function, determining the mechanistic details of this multiredox reaction remains a significant challenge. We report herein a rate law analysis of the order of water oxidation as a function of surface hole density on a hematite photoanode employing photoinduced absorption spectroscopy. Our study reveals a transition from a slow, first order reaction at low accumulated hole density to a faster, third order mechanism once the surface hole density is sufficient to enable the oxidation of nearest neighbor metal atoms. This study thus provides direct evidence for the multihole catalysis of water oxidation by hematite, and demonstrates the hole accumulation level required to achieve this, leading to key insights both for reaction mechanism and strategies to enhance function.

The kinetic competition between electron-hole recombination and water oxidation is a key consideration for the development of efficient photoanodes for solar driven water splitting. In this study, we employed three complementary techniques, transient absorption spectroscopy (TAS), transient photocurrent spectroscopy (TPC), and electrochemical impedance spectroscopy (EIS), to address this issue for one of the most widely studied photoanode systems: nanostructured hematite thin films. For the first time, we show a quantitative agreement between all three techniques. In particular, all three methods show the presence of a recombination process on the 10 ms to 1 s time scale, with the time scale and yield of this loss process being dependent upon applied bias. From comparison of data between these techniques, we are able to assign this recombination phase to recombination of bulk hematite electrons with long-lived holes accumulated at the semiconductor/electrolyte interface. The data from all three techniques are shown to be consistent with a simple kinetic model based on competition between this, bias dependent, recombination pathway and water oxidation by these long-lived holes. Contrary to most existing models, this simple model does not require the consideration of surface states located energetically inside the band gap. These data suggest two distinct roles for the space charge layer developed at the semiconductor/electrolyte interface under anodic bias. Under modest anodic bias (just anodic of flatband), this space charge layer enables the spatial separation of initially generated electrons and holes following photon absorption, generating relatively long-lived holes (milliseconds) at the semiconductor surface. However, under such modest bias conditions, the energetic barrier generated by the space charge layer field is insufficient to prevent the subsequent recombination of these holes with electrons in the semiconductor bulk on a time scale faster than water oxidation. Preventing this back electron-hole recombination requires the application of stronger anodic bias, and is a key reason why the onset potential for photocurrent generation in hematite photoanodes is typically ~500 mV anodic of flat band and therefore needs to be accounted for in electrode design for PEC water splitting.

The utilisation of Cu 2 O photocathodes for photoelectrochemical water splitting requires their stabilisation due to photocorrosion in aqueous electrolytes. Ultrathin films of wide band gap semiconducting oxides deposited by atomic layer deposition (ALD) on top of cuprous oxide can perform the dual function of both facilitating charge extraction (through the creation of a p-n junction) and protecting the absorber material from the aqueous electrolyte, thereby suppressing corrosion in favor of hydrogen generation. The factors that determine the photocurrent performance as well as the stability of these photoelectrodes are examined. Specifically, the influence of ALD deposition temperature, electrolyte pH, electrolyte composition as well as post-deposition annealing treatments was studied. The successful development of protective overlayers must fulfil the dual requirements of favourable band alignments as well as chemical stability. At long time scales, the deactivation of the photocathodes proceeds through etching of the amorphous overlayer, accompanied by the loss of the platinum catalyst particles. Through the deposition of a semi-crystalline TiO 2 overlayer, 62% stability over 10 hours of testing has been demonstrated without re-platinization.