Modulating metal oxides: The more difficult step in fuel cells and water electrolysis is the oxygen evolution reaction. The search for earth-abundant materials to replace noble metals for this reaction often turns to oxides of three-dimensional metals such as iron. Zhang et al. show that the applied voltages needed to drive this reaction are reduced for iron-cobalt oxides by the addition of tungsten. The addition of tungsten favorably modulates the electronic structure of the oxyhydroxide. A key development is to keep the metals well mixed and avoid the formation of separate phases. Science , this issue p. 333

The development of high-performance catalysts for the oxygen-evolution reaction (OER) is paramount for cost-effective conversion of renewable electricity to fuels and chemicals. Here we report the significant enhancement of the OER activity of electrodeposited NiO_x films resulting from the combined effects of using cerium as a dopant and gold as a metal support. This NiCeO_x–Au catalyst delivers high OER activity in alkaline media, and is among the most active OER electrocatalysts yet reported. On the basis of experimental observations and theoretical modelling, we ascribe the activity to a combination of electronic, geometric and support effects, where highly active under-coordinated sites at the oxide support interface are modified by the local chemical binding environment and by doping the host Ni oxide with Ce. The NiCeO_x–Au catalyst is further demonstrated in a device context by pairing it with a nickel–molybdenum hydrogen evolution catalyst in a water electrolyser, which delivers 50 mA consistently at 1.5 V over 24 h of continuous operation.

A major roadblock in realizing large-scale production of hydrogen via electrochemical water splitting is the cost and inefficiency of current catalysts for the oxygen evolution reaction (OER). Computational research has driven important developments in understanding and designing heterogeneous OER catalysts using linear scaling relationships derived from computed binding energies. Herein, we interrogate 17 of the most active molecular OER catalysts, based on different transition metals (Ru, Mn, Fe, Co, Ni, and Cu), and show they obey similar scaling relations to those established for heterogeneous systems. However, we find that the conventional OER descriptor underestimates the activity for very active OER complexes as the standard approach neglects a crucial one-electron oxidation that many molecular catalysts undergo prior to O–O bond formation. Importantly, this additional step allows certain molecular catalysts to circumvent the “overpotential wall”, leading to enhanced performance. With this knowledge, we establish fundamental principles for the design of ideal molecular OER catalysts.

Mechanisms for H₂ dissociation on metal oxides have been typically inferred from the infrared spectra of reaction products on the basis of the presence or lack of M–H fingerprints. Here, we demonstrate by means of density functional theory that oxides with polar M–O bonds might favor heterolytic dissociation. Moreover, we report that the resulting heterolytic product can further evolve to the homolytic one provided that metal ions are reducible. Hence, it follows that the redox capacity of the metal determines the reaction outcome. This finding sheds light on why both M–H and O–H bands appear in the infrared spectra of nonreducible oxides such as MgO or γ-Al₂O₃, while only O–H bands are observed for reducible oxides like CeO₂. It results in a unified mechanism for polar oxides that can be generalized to other materials exhibiting significant charge separation. Importantly, we also show that the low activity of CeO₂ toward H₂ can be improved by enhancing the basicity of surface O atoms upon lattice expansion. This may pave the way for the efficient use of CeO₂ in selective hydrogenation reactions and for the further advance on processes involving dissociation of nonpolar bonds like C–H.