Stabilization of a Mn-Co Oxide During Oxygen Evolution in Alkaline Media

Influence of Co and Mn Doping on the Surface Reconstruction of Faceted NiO(111) Nanosheets after the Oxygen Evolution Reaction

Understanding dynamic surface reconstruction processes on transition metal oxides for the oxygen evolution reaction (OER) in alkaline electrolytes is crucial to the development of more active catalysts in water electrolysis technologies. Effective strategies in material development for activity enhancement include doping with additional transition metals and surface structuring through controlled exposure of defined surface facets. Here, a microwave-assisted synthesis route was used, that resulted in phase-pure Co- and Mn-doped NiO with various doping levels while maintaining the rock salt crystal structure of the pure, faceted NiO(111) nanosheets. X-ray diffraction and transmission electron microscopy showed an unaltered structure and morphology up to doping levels of 10 mol %. The impact of doping levels between 2 and 10% on the electrochemistry and OER overpotential was studied using the rotating disc electrode technique. A modest overpotential reduction of 34 mV was achieved for 5% Co-doping, being the most active material in the comparison, and an increase in overpotential of 56 mV for 10% Mn-doping, being the least active material, compared to the undoped NiO(111) material. Associated changes in the physical surface area and charges associated with surface redox reactions were aligned with detailed X-ray absorption spectroscopy and X-ray photoelectron spectroscopy analysis before and after electrochemical measurements, which showed different extents of surface reconstruction depending on the dopant and doping level. Thus, transformation of the less active rock salt structure to more active NiOOH functionalities was hampered by a low extent of surface reconstruction, explaining the modest activity enhancement after potentiodynamic cycling for 350 scans. The results demonstrate the effective synthesis of facet-controlled doped NiO-based model catalysts to scrutinize the impact of individual dopants on the electrochemical behavior and, thus the OER electrode activity.

A Review of Surface Reconstruction and Transformation of 3d Transition-Metal (oxy)Hydroxides and Spinel-Type Oxides during the Oxygen Evolution Reaction

Developing efficient and sustainable electrocatalysts for the oxygen evolution reaction (OER) is crucial for advancing energy conversion and storage technologies. 3d transition-metal (oxy)hydroxides and spinel-type oxides have emerged as promising candidates due to their structural flexibility, oxygen redox activity, and abundance in earth's crust. However, their OER performance can be changed dynamically during the reaction due to surface reconstruction and transformation. Essentially, multiple elementary processes occur simultaneously, whereby the electrocatalyst surfaces undergo substantial changes during OER. A better understanding of these elementary processes and how they affect the electrocatalytic performance is essential for the OER electrocatalyst design. This review aims to critically assess these processes, including oxidation, surface amorphization, transformation, cation dissolution, redeposition, and facet and electrolyte effects on the OER performance. The review begins with an overview of the electrocatalysts' structure, redox couples, and common issues associated with electrochemical measurements of 3d transition-metal (oxy)hydroxides and spinels, followed by recent advancements in understanding the elementary processes involved in OER. The challenges and new perspectives are presented at last, potentially shedding light on advancing the rational design of next-generation OER electrocatalysts for sustainable energy conversion and storage applications.

Tafel Slope Plot as a Tool to Analyze Electrocatalytic Reactions

Kinetic and nonkinetic contributions to the Tafel slope value can be separated using a Tafel slope plot, where a constant Tafel slope region indicates kinetic meaningfulness. Here, we compare the Tafel slope values obtained from linear sweep voltammetry to the values obtained from chronoamperometry and impedance spectroscopy, and we apply the Tafel slope plot to various electrocatalytic reactions. We show that similar Tafel slope values are observed from the different techniques under high-mass-transport conditions for the oxygen evolution reaction on NiFeOOH in 0.2 M KOH. However, for the alkaline hydrogen evolution reaction and the CO2 reduction reaction, no horizontal Tafel slope regions were observed. In contrast, we obtained the expected Tafel slope of 30 mV/dec for the HER on Pt in 1 M HClO4. We argue that widespread application of the Tafel slope plot, or similar numerical differentiation techniques, would result in an improved comparison of kinetic data for many electrocatalytic reactions when the traditional Tafel plot analysis is ambiguous.