Toward Understanding Carboxylate Group Contributions to Mn Oxide Catalysts in the Oxygen-Evolution Reaction

In contrast to the previous assumption that manganese (hydr)oxides, in the absence of other metal ions, exhibit high overpotentials for catalyzing the oxygen-evolution reaction (OER) under neutral conditions, this study uncovers a more nuanced behavior. We demonstrate that layered manganese oxides, when treated with carboxylate groups, exhibit OER activity at the Mn(III) to Mn(IV) oxidation peak following charge accumulation. Upon the addition of poly(acrylic acid) (PAA), the Mn(III) to Mn(IV) transition occurs at a lower potential. While the current density remains modest, this activity is observed at an extraordinarily low overpotential of just 20 mV in a phosphate buffer solution. We present a detailed mechanistic proposal for OER in this low-overpotential regime, focusing on the Mn(III) to Mn(IV) transition and the surrounding OER environment. Oxygen measurements reveal that at an applied potential of 1.25 V, the turnover frequency (TOF) increases from 2.6 × 10^-2 s^-1 prior to PAA treatment to 4.7 × 10^-2 s^-1 post-treatment. However, the Tafel slope increases from 384.76 mV/decade before PAA treatment to 414.30 mV/decade after treatment. The observed reduction in overpotential is attributed to the complex interaction between the OER process and charge accumulation, mirroring key mechanisms in natural systems such as the OEC in photosystem II (PSII). This interplay likely facilitates the low overpotential observed in this system, highlighting the relevance of these bioinspired processes in designing efficient electrocatalysts for OER. These findings provide important insights for the development of highly efficient and robust electrocatalysts for water splitting, with significant implications for the future of energy conversion and storage technologies. By emulating the natural Mn redox processes observed in PSII, our work paves the way for the design of more effective catalysts that operate with minimal energy loss, advancing the potential for sustainable energy solutions.