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CeO2 greatly enhances the electrocatalytic oxygen evolution reaction (OER) activity of CoOx, though the enhancement mechanism beyond this synergy is yet to be understood. Here, operando hard X‐ray absorption spectroscopy (hXAS) is applied to monitor the Co K edge and Ce L3 edge in CoOx/CeO2 to shed light on the evolution of the Co and Ce oxidation states during OER. In addition, ex situ soft XAS (sXAS) characterizations provide information on the irreversible surface‐specific transformations of the Co L3 edge as well as of the O K edge. Combining the operando and ex situ spectroscopic characterizations with comprehensive electrochemical analyses, it is confirmed that CeO2 is not the active center for the OER. However, coupling CeO2 with CoOx introduces significant modifications in the Co and O species at the CoOx surface and alters the flat band potential (Efb), leading to more favorable Co oxidation state transformations during OER and possibly modifying the preferential reaction pathway. This work establishes the connections between electronic structures, Co oxidation state and the OER reaction mechanism for CoOx/CeO2 composites electrodes. [ABSTRACT FROM AUTHOR]

Surface reconstruction of cobalt-based oxides is recognized as a key to efficiently electrocatalyze the oxygen evolution reaction (OER) in alkaline environment. Identifying material features that promote surface reconstruction is crucial to rationally improve OER electrocatalysts. Here, the Fe-content effects on the surface reconstruction of flame-spray synthesized Ba0.50Sr0.50Co1−xFexO3−δ (BSCo1−xFex) is systematically investigated by gradually substituting Co with Fe (0 < x < 1). The electrochemical characterization reveals a volcano-shaped trend of the OER activity and stability as a function of the Fe-content, and identifies BSCo0.80Fe0.20 as the best performing electrocatalyst. This Fe-content dependent performance trend directly correlates with the extent of surface reconstruction, as unveiled by combining ex situ surface and operando bulk X-ray absorption spectroscopy. More specifically, the increasing electrocatalytic performance from x = 0.01 to 0.20 is explained by the ability of Fe to stabilize surface Co2+-atoms in the pristine material. This enhances the electrochemically triggered irreversible surface Co oxidation, leading to a more extensive formation of a Co- and Fe-based (oxyhydr)oxide layer that reaches deep into the electrochemically metastable bulk. The decreasing performance trend for x > 0.20 is related to the increasing oxygen content in the pristine material, leading to a stabilization of the bulk structure and preventing the (oxyhydr)oxide from growing into the bulk. Moreover, a high Fe-content (x > 0.40) stabilizes the surface Co2+-atoms in such an extent that the irreversible surface Co oxidation is increasingly suppressed, limiting the reconstruction process even on the surface. Overall, this study provides a fundamental understanding of the Fe-content effects on surface reconstruction in BSCo1−xFex and deciphers the highest electrocatalytic performance of BSCo0.80Fe0.20 as a combination of optimally, neither too weakly nor too strongly, stabilized surface Co2+-atoms and bulk structure, leading to the most extensive surface reconstruction. [ABSTRACT FROM AUTHOR]

Cobalt‐based oxides have attracted attention as active electrocatalysts for the oxygen evolution reaction (OER) in alkaline electrolytes. However, highly OER active catalysts at near‐neutral pHs are also desired for practical applications. Herein, a dry phosphate functionalization process is presented to enhance the OER activity of different cobalt‐based catalysts at near‐neutral pHs. Electrochemical evaluations show that the P‐functionalization can effectively improve the OER activity at near‐neutral pHs for La0.2Sr0.8CoO3–δ, La0.2Sr0.8Co0.8Fe0.2O3–δ, and CoOx catalysts, but not for La0.5Sr1.5CoO4–δ. Bulk and surface sensitive X‐ray absorption spectroscopy and X‐ray photoelectron spectroscopy unveil the influence of P incorporated in the order of ppm on the electronic state, local structure, and surface composition of the investigated catalysts. The P‐functionalization reduces the Co oxidation state in La0.2Sr0.8CoO3–δ and La0.5Sr1.5CoO4–δ, but the latter also presents significant Sr‐based segregations on the surface‐inhibiting OER activity at near‐neutral pHs. Differently, La0.2Sr0.8CoO3–δ, and to a lesser extent La0.2Sr0.8Co0.8Fe0.2O3–δ and CoOx, shows improved OER activity at neutral pH after the P‐functionalization. The findings disclose that P‐functionalization successfully enhances OER activity at near‐neutral pHs and that both phosphate ion assistance in the OER mechanism and catalyst Co oxidation state can play a role in the enhanced OER activity. [ABSTRACT FROM AUTHOR]

Water electrolysis technologies for hydrogen production from renewable energy play an important role in a carbon‐free energy scenario. Anion exchange membrane water electrolysis (AEMWE) is one of the most cost‐effective and promising water electrolysis technologies due to the application of efficient, non‐noble metal electrocatalysts and the fast development of anion exchange membranes (AEMs). In this review, the most recent and significant achievements in AEMWE were summarized, including the progress in enhancing AEM conductivity and understanding their degradation mechanism, and the hot topics concerning the electrocatalyst design. The key factors that affect AEMWE performance were pointed out, further challenges and opportunities have also been discussed. The expectations for future development of AEMWE devices were also addressed. Finally, this review provided valuable guidelines for developing highly active and durable electrocatalysts and advancing the fabrication of AEMWE devices. [ABSTRACT FROM AUTHOR]

Understanding the mechanism of the oxygen evolution reaction (OER) on perovskite materials is of great interest for the development of more active catalysts. Despite a lot of literature reports, the complexity of catalytic systems and scarce in situ and operando surface sensitive spectroscopic tools render the detection of active sites and the understanding of reaction mechanisms challenging. Here, we carried out and compared in situ and ex situ ambient pressure X-ray photoelectron spectroscopy experiments on a La0.2Sr0.8CoO3−δ perovskite OER catalyst. The experimental results show that segregated surface strontium, which is present in the as prepared sample, is leached into the electrolyte after immersion, leading to surface cobalt active site enrichment. Such a cobalt-enriched oxide surface evolves into a new phase, whose spectral feature is detected in situ and after the OER. With the help of theoretical simulations, such a species is assigned to cobalt oxyhydroxide, providing direct evidence of its formation and surface segregation during the oxygen evolution reaction. [ABSTRACT FROM AUTHOR]