Regulating proton-coupled electron transfer for efficient water splitting by manganese oxides at neutral pH

Abstract

Manganese oxides have been extensively investigated as model systems for the oxygen-evolving complex of photosystem II. However, most bioinspired catalysts are inefficient at neutral pH and functional similarity to the oxygen-evolving complex has been rarely achieved with manganese. Here we report the regulation of proton-coupled electron transfer involved in water oxidation by manganese oxides. Pyridine and its derivatives, which have pKa values intermediate to the water ligand bound to manganese(II) and manganese(III), are used as proton-coupled electron transfer induction reagents. The induction of concerted proton-coupled electron transfer is demonstrated by the detection of deuterium kinetic isotope effects and compliance of the reactions with the libido rule. Although proton-coupled electron transfer regulation is essential for the facial redox change of manganese in photosystem II, most manganese oxides impair these regulatory mechanisms. Thus, the present findings may provide a new design rationale for functional analogues of the oxygen-evolving complex for efficient water splitting at neutral pH.

Towards Versatile and Sustainable Hydrogen Production through Electrocatalytic Water Splitting: Electrolyte Engineering

“…A systematic investigation was also reported for Mn electrodes 165, 166, 167. When an unbuffered solution was used as a supporting electrolyte (0.5 mol L −1 Na 2 SO 4 ), the onset potential for the OER was found to be independent of the solution pH on the SHE scale (Figure 13 a),166 which interestingly coincides with the observation for the HER (see Section 3.2).…”

“…reported a strong correlation between the OER performance and p K a of the buffering equilibrium: even at similar pH, the performance was significantly dependent on the p K a of the buffering action. Particularly, the larger p K a resulted in an improved OER onset potential (Figure 13 b and c) 167. This observation indicates the involvement of the deprotonation/protonation process in the rate‐determining step,167 which was also observed for the Co OER catalysts 162…”

“…Particularly, the larger p K a resulted in an improved OER onset potential (Figure 13 b and c) 167. This observation indicates the involvement of the deprotonation/protonation process in the rate‐determining step,167 which was also observed for the Co OER catalysts 162…”

“…The observed pH independence in their study was ascribed to the formation of Mn 3+ from Mn 2+ 165. The OER–pH relationship was drastically altered when buffering action was introduced into the system 166, 167. Particularly, Nocera and co‐workers claimed that the self‐healing mechanism was also present in the Mn–phosphate system, which results in the high stability of the Mn OER catalysts 166.…”

“…However, the high‐cost and scarcity of such noble metals is considered a huge issue, which has driven the exploration of non‐noble metal‐based OER electrodes at near‐neutral pH. Among the more cost‐effective elements, Co,158, 159, 160, 161, 162, 163, 164 Mn,165, 166, 167 and Ni168, 169 have been the subject of research studies in this decade.…”