Catalyst–Support Surface Charge Effects on Structure and Activity of IrNi-Based Oxygen Evolution Reaction Catalysts Deposited on Tin-Oxide Supports

Ir-based nanoparticles supported on conductive oxide supports show high water oxidation (oxygen evolution reaction, OER) activity and represent a promising alternative to state-of-art anode catalysts in water electrolyzers. Physicochemical interactions between the Ir-based catalytic nanoparticles and the oxide supports can critically affect the weight loading, surface area, activity, and stability of the Ir-based catalysts under electrochemical OER conditions. However, systematic insight on the influences of surface charge on deposition yield and dispersion of the nanoparticles on oxide supports and the influence of this interaction on the catalytic performance of supported Ir-based alloys is missing. In this work, the impact of electrostatic interactions between catalyst–support surface charges during catalyst synthesis on the structure and performance of Ir-based OER electrocatalysts is studied. Supported IrNi NPs were synthesized comparing a direct and a stepwise deposition technique onto selected doped tin oxide supports including antimony tin oxide (ATO), In-rich indium tin oxide (ITO), and fluorine tin oxide (FTO), with commercial ATO and unsupported particles as references. Data suggest that electrostatic attractions between particles and supports majorly impact the deposition yield of IrNi NPs. Photoemission spectra, XPS, of supports and supported catalysts show declines in the doping elements concomitant to the variation of the oxide oxidation state. We demonstrate how controlled pretreatments and alterations of repulsive forces between supports and nanoparticles resulted in great improvements in nanoparticle deposition and thus enhanced OER activity. Our findings can be transferred to other nanoparticles/support couples to help improve the distribution and adhesion of the nanoparticles and therefore improve their catalytic performances

Anion-Tuned Layered Double Hydroxide Anodes for Anion Exchange Membrane Water Electrolyzers: From Catalyst Screening to Single-Cell Performance

Anion exchange membrane water electrolysis (AEMWE) is an attractive emerging green hydrogen technology. However, the scaling of trends in activity of anode catalysts for the oxygen evolution reaction (OER) from a liquid-electrolyte, three-electrode environment to the two-electrode single-cell format has remained poorly considered. Herein, we critically investigate the scaling of kinetic and catalytic properties of a family of highly active Ni foam (NF) supported, anion (A–)-tuned NiFe(-A–)-OER catalysts. Trends in catalytic activity suggest impressive improvements of up to 91-fold in three-electrode setups (3LC) compared to uncoated NF. While we demonstrate the successful qualitative structure–performance tunability in a 5 cm2 AEMWE single cell, we also find serious limitations in the quantitative predictability of three-electrode setups for single-cell performance trends. Cell environments appear to equalize the cell performances of designer catalysts, which has important ramifications for electrode development. We succeed in analyzing and discussing some of these translation limitations in terms of previously overlooked effects summarized in the activity improvement factor f

Morphology and mechanism of highly selective Cu II oxide nanosheet catalysts for carbon dioxide electroreduction

Cu oxides catalyze the electrochemical carbon dioxide reduction reaction (CO2RR) to hydrocarbons and oxygenates with favorable selectivity. Among them, the shape-controlled Cu oxide cubes have been most widely studied. In contrast, we report on novel 2-dimensional (2D) Cu(II) oxide nanosheet (CuO NS) catalysts with high C2+ products, selectivities (> 400mAcm(-2)) in gas diffusion electrodes (GDE) at industrially relevant currents and neutral pH. Under applied bias, the (001)-orientated CuO NS slowly evolve into highly branched, metallic Cu-0 dendrites that appear as a general dominant morphology under electrolyte flow conditions, as attested by operando X-ray absorption spectroscopy and in situ electrochemical transmission electron microscopy (TEM). Millisecond-resolved differential electrochemical mass spectrometry (DEMS) track a previously unavailable set of product onset potentials. While the close mechanistic relation between CO and C2H4 was thereby confirmed, the DEMS data help uncover an unexpected mechanistic link between CH4 and ethanol. We demonstrate evidence that adsorbed methyl species, *CH3, serve as common intermediates of both CH3H and CH3CH2OH and possibly of other CH3-R products via a previously overlooked pathway at (110) steps adjacent to (100) terraces at larger overpotentials. Our mechanistic conclusions challenge and refine our current mechanistic understanding of the CO2 electrolysis on Cu catalysts. Copper oxides (CuO) can selectively catalyze the electrochemical reduction of CO2 to hydrocarbons and oxygenates. Here, the authors study the activity and morphological evolution of 2D CuO nanosheets under applied electrode potentials to conclude the primacy of dendritic shapes and involvement of a new coupling pathway

The Role of Surface Hydroxylation, Lattice Vacancies and Bond Covalency in the Electrochemical Oxidation of Water (OER) on Ni-Depleted Iridium Oxide Catalysts

The usage of iridium as an oxygen-evolution-reaction (OER) electrocatalyst requires very high atom efficiencies paired with high activity and stability. Our efforts during the past 6 years in the Priority Program 1613 funded by the Deutsche Forschungsgemeinschaft (DFG) were focused to mitigate the molecular origin of kinetic overpotentials of Ir-based OER catalysts and to design new materials to achieve that Ir-based catalysts are more atom and energy efficient, as well as stable. Approaches involved are: (1) use of bimetallic mixed metal oxide materials where Ir is combined with cheaper transition metals as starting materials, (2) use of dealloying concepts of nanometer sized core-shell particle with a thin noble metal oxide shell combined with a hollow or cheap transition metal-rich alloy core, and (3) use of corrosion-resistant high-surface-area oxide support materials. In this mini review, we have highlighted selected advances in our understanding of Ir–Ni bimetallic oxide electrocatalysts for the OER in acidic environments

Key role of chemistry versus bias in electrocatalytic oxygen evolution

The oxygen evolution reaction has an important role in many alternative-energy schemes because it supplies the protons and electrons required for converting renewable electricity into chemical fuels. Electrocatalysts accelerate the reaction by facilitating the required electron transfer, as well as the formation and rupture of chemical bonds5. This involvement in fundamentally different processes results in complex electrochemical kinetics that can be challenging to understand and control, and that typically depends exponentially on overpotential. Such behaviour emerges when the applied bias drives the reaction in line with the phenomenological Butler–Volmer theory, which focuses on electron transfer, enabling the use of Tafel analysis to gain mechanistic insight under quasi-equilibrium or steady-state assumptions. However, the charging of catalyst surfaces under bias also affects bond formation and rupture, the effect of which on the electrocatalytic rate is not accounted for by the phenomenological Tafel analysis8 and is often unknown. Here we report pulse voltammetry and operando X-ray absorption spectroscopy measurements on iridium oxide to show that the applied bias does not act directly on the reaction coordinate, but affects the electrocatalytically generated current through charge accumulation in the catalyst. We find that the activation free energy decreases linearly with the amount of oxidative charge stored, and show that this relationship underlies electrocatalytic performance and can be evaluated using measurement and computation. We anticipate that these findings and our methodology will help to better understand other electrocatalytic materials and design systems with improved performance