Large-scale electrolysis of water for hydrogen generation requires better catalysts to lower the kinetic barriers associated with the oxygen evolution reaction (OER). Although most OER catalysts are based on crystalline mixed-metal oxides, high activities can also be achieved with amorphous phases. Methods for producing amorphous materials, however, are not typically amenable to mixed-metal compositions. We demonstrate that a low-temperature process, photochemical metal-organic deposition, can produce amorphous (mixed) metal oxide films for OER catalysis. The films contain a homogeneous distribution of metals with compositions that can be accurately controlled. The catalytic properties of amorphous iron oxide prepared with this technique are superior to those of hematite, whereas the catalytic properties of a-Fe(100-y-z)Co(y)Ni(z)O(x) are comparable to those of noble metal oxide catalysts currently used in commercial electrolyzers.

Photochemical metal-organic deposition (PMOD) was used to prepare amorphous metal oxide films containing specific concentrations of iron, cobalt, and nickel to study how metal composition affects heterogeneous electrocatalytic water oxidation. Characterization of the films by energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy confirmed excellent stoichiometric control of each of the 21 complex metal oxide films investigated. In studying the electrochemical oxidation of water catalyzed by the respective films, it was found that small concentrations of iron produced a significant improvement in Tafel slopes and that cobalt or nickel were critical in lowering the voltage at which catalysis commences. The best catalytic parameters of the series were obtained for the film of composition a-Fe20Ni80. An extrapolation of the electrochemical and XPS data indicates the optimal behavior of this binary film to be a manifestation of iron stabilizing nickel in a higher oxidation level. This work represents the first mechanistic study of amorphous phases of binary and ternary metal oxides for use as water oxidation catalysts, and provides the foundation for the broad exploration of other mixed-metal oxide combinations.

In order to be economically competitive with simple “brute force” (i.e., PV + electrolyzer) strategies or the production of promising solar fuels, like H2, from fossil fuels, a practical photoelectrochemical device must optimize cost, longevity, and performance. A promising approach that meets these requirements is the combination of stable and inexpensive oxide semiconductor electrodes in a tandem photoelectrochemical device. In this article, we give an overview of the field including an examination of the potential solar-to-fuel conversion efficiency expected in a device with realistic losses. We next discuss recent advances with increasing the performance of promising semiconductor electrode materials and highlight how these advances have led to state-of-the-art solar-to-chemical efficiencies in the 2–3% range in real devices. Challenges for further optimization are further outlined.

Sorbent-assisted water harvesting from air represents an attractive way to address water scarcity in arid climates. Hitherto, sorbents developed for this technology have exclusively been designed to perform one water harvesting cycle (WHC) per day, but the productivities attained with this approach cannot reasonably meet the rising demand for drinking water. This work shows that a microporous aluminum-based metal-organic framework, MOF-303, can perform an adsorption–desorption cycle within minutes under a mild temperature swing, which opens the way for high-productivity water harvesting through rapid, continuous WHCs. Additionally, the favorable dynamic water sorption properties of MOF-303 allow it to outperform other commercial sorbents displaying excellent steady-state characteristics under similar experimental conditions. Finally, these findings are implemented in a new water harvester capable of generating 1.3 L kgMOF–1 day–1 in an indoor arid environment (32% relative humidity, 27 °C) and 0.7 L kgMOF–1 day–1 in the Mojave Desert (in conditions as extreme as 10% RH, 27 °C), representing an improvement by 1 order of magnitude over previously reported devices. This study demonstrates that creating sorbents capable of rapid water sorption dynamics, rather than merely focusing on high water capacities, is crucial to reach water production on a scale matching human consumption.