Abstract
Interactions of the calcium-birnessite catalyst with phosphate buffer anions under water-oxidation conditions.
Highlights
The study presented here tries to provide answers to the following two questions concerning oxygen evolution reaction (OER) electrocatalysis by manganese oxide (MnOx) in near-neutral conditions: (1) why is the use of a buffering electrolyte generally bene cial and (2) why are phosphate containing media apparently especially wellsuited?11 To investigate this, it has to be noted that the number of possible aqueous electrolytes for OER experiments is generally limited as it is of particular importance in OER electrocatalysis that the species making up the electrolyte are themselves redox-inert at strongly oxidizing conditions in water
Both features can show a splitting of the current-density traces between the forward and the return scans. These splits are very different between the three solvent systems: in phosphate, forward and return scans mainly differ in the prewave region, in imidazolium the splits are substantial for both parts, while in sulphate, the splits are clearly visible over the entire scanned potential range
While few studies on water-oxidation catalysis by manganese oxides emphasize the critical role of the electrolyte for effective water oxidation catalysis, our results clearly show the potential bene ts of a rational, concerted development of catalyst and electrolyte in parallel
Summary
One major bottleneck for the production of hydrogen by electrochemical water splitting is the sluggish kinetics of the oxygen evolution reaction (OER).[1,2] Here, a rational design of improved electrocatalysts requires a better understanding of the mechanisms controlling OER activity and stability.Inspired by the oxygen-evolving complex (OEC) of photosystem II – a Mn4CaO5 cluster which constitutes the OER active site in biological photosynthesis – a large variety of manganese oxide (MnOx) catalysts has been studied as potential synthetic OER catalyst materials.[3,4,5,6,7,8,9,10,11] Beside some crystalline binary or ternary manganese oxides, disordered MnOx materials with a low degree of long-range order o en show a high intrinsic catalytic activity.[11,12,13,14,15,16,17,18,19] For some crystalline systems, the formation of catalytically much more active nanocrystals or disordered surface layers during the OER process has been observed.[5,6,20,21,22]Among the different less-ordered manganese oxides, birnessites have been identi ed as especially promising OER catalysts showing both high activity and stability.[13,16,23] The birnessite structure is built up from edge-sharing [MnO6]octahedra forming extended layers.
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