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Research drives development of sustainable electrocatalytic technologies, but efforts are hindered by inconsistent reporting of advances in catalytic performance. Iridium-based oxide catalysts are widely studied for electrocatalytic technologies, particularly for the oxygen evolution reaction (OER) for proton exchange membrane water electrolysis, but insufficient techniques for quantifying electrochemically accessible iridium active sites impede accurate assessment of intrinsic activity improvements. We develop mercury underpotential deposition and stripping as a reversible electrochemical adsorption process to robustly quantify iridium sites and consistently normalize OER performance of benchmark IrOx electrodes to a single intrinsic activity curve, where other commonly used normalization methods cannot. Through rigorous deconvolution of mercury redox and reproportionation reactions, we extract net monolayer deposition and stripping of mercury on iridium sites throughout testing using a rotating ring disk electrode. This technique is a transformative method to standardize OER performance across a wide range of iridium-based materials and quantify electrochemical iridium active sites. 

Earth-abundant manganese-based oxides have emerged as promising alternatives to noble-metal-based catalysts for the oxygen evolution reaction (OER) in acidic conditions; however, their inferior activity and stability present critical challenges for the sustainable production of hydrogen via water electrolysis. Moving beyond oxides, heteroanionic materials, which incorporate anions with lower electronegativity than oxygen, have shown potential for improving the OER performance, but a detailed understanding of the underlying mechanisms is lacking. Here, we investigate manganese based oxychlorides (Mn8O10Cl3 and FeMn7O10Cl3) that exhibit excellent activity and stability for acidic OER to elucidate material property dynamics and correlate them with OER behaviors. Our rigorous electrochemical stability testing reveals that the high operating potential mitigates Mn dissolution over prolonged exposure to the OER conditions. Through a combination of ex situ and in situ surface and bulk-sensitive X-ray spectroscopy analyses, we observe a trade-off between increasing Mn valence and maintaining structural integrity, which results in dynamic bond length changes within the [MnCl6] octahedra during the activation and degradation processes of these oxychloride catalysts. This study provides insights into the fundamental relationships between the chemical, electronic, and geometric properties of the catalysts and their electrocatalytic outcomes. 

The interaction between catalyst surfaces and adsorbed oxygen intermediates is critical to catalytic performance for electrochemical water oxidation to oxygen. However, the relationship between adsorption energetics and electrocatalytic activity is primarily assessed for pristine catalyst materials, which leaves much unknown about the dynamics of these properties in relationship to catalyst performance during long-term operation. In this work, we experimentally assess OH and O adsorption on Ca2IrO4 nanoparticles and monitor their evolution during extensive chronoamperometry tests at highly oxidizing potentials in a range of low pH electrolytes. In situ x-ray absorption spectroscopy reveals changes for surface adsorbate energetics and local iridium structures with applied potentials. Increasingly unfavorable adsorption of OH and formation of O intermediates after long-term operation is correlated with severe metal dissolution, distorted [IrO6] octahedral linkages, and a decreased average Ir valence. This work establishes connections between surface adsorption energetics, Ir structure, OER kinetics, and material stability outcomes. 

Water electrolysis using proton exchange membrane technology offers an ideal process for green hydrogen production, but widespread deployment is inhibited by insufficient catalyst activity, stability and affordability. Iridium-based oxides provide the best overall performance for acidic water oxidation, the limiting reaction for this process, but further improvements are impeded by poor understanding of the restructured active catalyst surface that forms under reaction conditions. Here we present a combination of X-ray and electron scattering data that reveals direct evidence for three paracrystalline structural motifs at the restructured surfaces of highly active catalysts (including rutile IrO2 and perovskite SrIrO3) that have previously been described as amorphous. These insights enable the design of a paracrystalline IrOx catalyst that is independent of the bulk crystalline support and maintains higher activity, longer stability and more effective use of iridium to promote the production of green hydrogen. 

The development of active and acid-stable iridium-based catalysts is crucial to meet the requirements of proton exchange membrane technologies for the sustainable production of hydrogen via water electrolysis. However, long-term stability remains a critical challenge. In this work, we focus on a Ca2IrO4 catalyst to develop a holistic picture of catalyst electronic and geometric structure evolution under various applied potentials by probing electrochemically active surface area, metal dissolution, Ir valence, and surface morphology. We observe an initial activity increase in parallel with increasing capacitance and minor iridium dissolution. Extensive chronoamperometry tests at oxidizing potentials lead to significant activity loss that occurs simultaneously with a dramatic drop in capacitance and a change in impedance. Using a combination of electrochemical and spectroscopic tools, we provide fundamental insights to these material degradation processes to enable future catalyst design with balanced activity and long-term stability. 

Abstract An improved understanding of catalyst dynamics for the oxygen evolution reaction (OER) in acid is critical for informing the development of highly efficient, stable, and cost‐effective OER catalysts for proton exchange membrane water electrolysis applications. Herein highly tunable, active, and dynamic Ir 5+ materials are studied, Ln₃IrO₇(Ln = Pr, Nd, Sm, and Eu). Leveraging a combination of in situ and ex situ characterization, as well as an advanced mercury underpotential deposition technique for Ir surface site quantification, the dynamic nature of Ln₃IrO₇materials throughout electrochemical activation under OER conditions is characterized. The trends are elucidated between intrinsic OER activity, surface Ir site quantity, and metal site dissolution behavior as tuned by the Ln site's atomic number. A critical relationship is uncovered to show that maintenance of excellent OER activity throughout performance testing is correlated with a catalysts’ ability to preserve a high degree of Ir enrichment, where heightened stability of Ir sites interestingly parallels reduced stability of Ln sites throughout testing. It is found that as the Ln site's atomic number is decreased, the materials’ intrinsic OER performance improves, due to an increased thermodynamic driving force for Ln dissolution, which is hypothesized to enable the maintenance of highly active Ir‐based surface motifs.