Modulating the Ruthenium-Cobalt Active Pair with Moderate Spacing for Enhanced Acidic Water Oxidation

Ruthenium (Ru)-based catalysts have emerged as promising alternatives to Iridium (Ir) catalysts in proton exchange membrane water electrolysis cells due to their lower price and excellent oxygen evolution reaction (OER) activity. However, their stability is compromised by generation of unstable high-valence Ru sites and oxygen vacancy in a lattice oxygen-mediated (LOM) pathway. Here, a low-load Ru site on a Barium (Ba)-doped Co3O4 (RuBaxCo3-xO4) catalyst is developed with abundant Ruthenium─Cobalt (Ru─Co) pairs for enhanced acidic OER activity. The incorporation of Ba can efficiently modulate the lattice of Co3O4, creating Ru─Co active pairs with optimized spacing through compression stress. In situ characterizations exhibit contractive Ru─Co pairs that promote the rapid and direct coupling of *O─O* radicals, bypassing the sluggish *OOH species and avoiding the oxygen vacancies, which can trigger the oxide path mechanism (OPM) for an efficient and stable OER process. As a result, the designed catalyst delivers a low overpotential of 219 mV to achieve a current density of 10 mA cm-2, and also demonstrates excellent stability, maintaining performance over 50 h of continuous operation at a larger current density of 50 mA cm-2. These findings highlight the potential of the RuBaxCo3-xO4 catalysts for durable and efficient OER applications.

Operando Identification of the Reversible Skin Layer on Co3O4 as a Three-Dimensional Reaction Zone for Oxygen Evolution

Co oxides and oxyhydroxides have been studied extensively in the past as promising electrocatalysts for the oxygen evolution reaction (OER) in neutral to alkaline media. Earlier studies showed the formation of an ultrathin CoO_x(OH)_y skin layer on Co₃O₄ at potentials above 1.15 V vs reversible hydrogen electrode (RHE), but the precise influence of this skin layer on the OER reactivity is still under debate. We present here a systematic study of epitaxial spinel-type Co₃O₄ films with defined (111) orientation, prepared on different substrates by electrodeposition or physical vapor deposition. The OER overpotential of these samples may vary up to 120 mV, corresponding to two orders of magnitude differences in current density, which cannot be accounted for by differences in the electrochemically active surface area. We demonstrate by a careful analysis of operando surface X-ray diffraction measurements that these differences are clearly correlated with the average thickness of the skin layer. The OER reactivity increases with the amount of formed skin layer, indicating that the entire three-dimensional skin layer is an OER-active interphase. Furthermore, a scaling relationship between the reaction centers in the skin layer and the OER activity is established. It suggests that two lattice sites are involved in the OER mechanism.

Improving the intrinsic activity of electrocatalysts for sustainable energy conversion: where are we and where can we go?

As we are in the midst of a climate crisis, there is an urgent need to transition to the sustainable production of fuels and chemicals. A promising strategy towards this transition is to use renewable energy for the electrochemical conversion of abundant molecules present in the earth's atmosphere such as H₂O, O₂, N₂ and CO₂, to synthetic fuels and chemicals. A cornerstone to this strategy is the development of earth abundant electrocatalysts with high intrinsic activity towards the desired products. In this perspective, we discuss the importance and challenges involved in the estimation of intrinsic activity both from the experimental and theoretical front. Through a thorough analysis of published data, we find that only modest improvements in intrinsic activity of electrocatalysts have been achieved in the past two decades which necessitates the need for a paradigm shift in electrocatalyst design. To this end, we highlight opportunities offered by tuning three components of the electrochemical environment: cations, buffering anions and the electrolyte pH. These components can significantly alter catalytic activity as demonstrated using several examples, and bring us a step closer towards complete system level optimization of electrochemical routes to sustainable energy conversion.

Combining Co3S4 and Ni:Co3S4 nanowires as efficient catalysts for overall water splitting: an experimental and theoretical study

In the quest for mass production of hydrogen from water electrolysis, to develop highly efficient, stable and low-cost catalysts is still the central challenge. When designing a novel catalyst, it is necessary to optimize the exposure and accessibility of its active sites as well as the reaction kinetics. This can be realized by combining an appropriate chemical composition of the material, including doping with metal elements, and a properly nanostructured morphology offering a high surface contact. We report here on the design and performances of cobalt-based oxide and sulfide nanowires as catalysts that can be used for both hydrogen and oxygen evolution reactions (denoted HER and OER respectively) in the same compatible electrolyte. Following a sulfuration process, Co3O4 nanowires are entirely converted into Co3S4 nanowires showing greatly improved OER catalytic performances with an overpotential of 283 mV (instead of 371 mV for Co3O4) to deliver a current density of 10 mA cm-2. Besides, when doping the surface of these Co3S4 nanowires with small amounts of nickel, the resulting Ni:Co3S4 nanowires exhibit an HER overpotential of 199 mV to reach 10 mA cm-2. But most importantly, two-electrode electrolyzer cells combining Co3S4 and Ni:Co3S4 electrodes show operating voltages as low as 1.70 V at 10 mA cm-2 over 40 hours, a value that competes advantageously with the best reported catalysts in 1.0 M KOH. Meanwhile, density functional theory (DFT) calculations show that the free energy of the intermediates has been reduced after the introduction of sulfur and nickel atoms, which have smaller overpotentials and corresponding enhanced electrocatalytic performance. Our results open a new avenue in the quest for overall water splitting using electrochemical systems.

Addressing global uncertainty and sensitivity in first-principles based microkinetic models by an adaptive sparse grid approach

In the last decade, first-principles-based microkinetic modeling has been developed into an important tool for a mechanistic understanding of heterogeneous catalysis. A commonly known, but hitherto barely analyzed issue in this kind of modeling is the presence of sizable errors from the use of approximate Density Functional Theory (DFT). We here address the propagation of these errors to the catalytic turnover frequency (TOF) by global sensitivity and uncertainty analysis. Both analyses require the numerical quadrature of high-dimensional integrals. To achieve this efficiently, we utilize and extend an adaptive sparse grid approach and exploit the confinement of the strongly non-linear behavior of the TOF to local regions of the parameter space. We demonstrate the methodology on a model of the oxygen evolution reaction at the Co₃O₄ (110)-A surface, using a maximum entropy error model that imposes nothing but reasonable bounds on the errors. For this setting, the DFT errors lead to an absolute uncertainty of several orders of magnitude in the TOF. We nevertheless find that it is still possible to draw conclusions from such uncertain models about the atomistic aspects controlling the reactivity. A comparison with derivative-based local sensitivity analysis instead reveals that this more established approach provides incomplete information. Since the adaptive sparse grids allow for the evaluation of the integrals with only a modest number of function evaluations, this approach opens the way for a global sensitivity analysis of more complex models, for instance, models based on kinetic Monte Carlo simulations.