A New Water Oxidation Catalyst: Lithium Manganese Pyrophosphate with Tunable Mn Valency

The development of a water oxidation catalyst has been a demanding challenge for the realization of overall water-splitting systems. Although intensive studies have explored the role of Mn element in water oxidation catalysis, it has been difficult to understand whether the catalytic capability originates mainly from either the Mn arrangement or the Mn valency. In this study, to decouple these two factors and to investigate the role of Mn valency on catalysis, we selected a new pyrophosphate-based Mn compound (Li<sub>2</sub>MnP<sub>2</sub>O<sub>7</sub>), which has not been utilized for water oxidation catalysis to date, as a model system. Due to the monophasic behavior of Li<sub>2</sub>MnP<sub>2</sub>O<sub>7</sub> with delithiation, the Mn valency of Li<sub>2‑<i>x</i></sub>MnP<sub>2</sub>O<sub>7</sub> (<i>x</i> = 0.3, 0.5, 1) can be controlled with negligible change in the crystal framework (e.g., volume change ∼1%). Moreover, inductively coupled plasma mass spectrometry, X-ray photoelectron spectroscopy, ex-situ X-ray absorption near-edge structure, galvanostatic charging–discharging, and cyclic voltammetry analysis indicate that Li<sub>2‑<i>x</i></sub>MnP<sub>2</sub>O<sub>7</sub> (<i>x</i> = 0.3, 0.5, 1) exhibits high catalytic stability without additional delithiation or phase transformation. Notably, we observed that, as the averaged oxidation state of Mn in Li<sub>2‑<i>x</i></sub>MnP<sub>2</sub>O<sub>7</sub> increases from 2 to 3, the catalytic performance is enhanced in the series Li<sub>2</sub>MnP<sub>2</sub>O<sub>7</sub> < Li<sub>1.7</sub>MnP<sub>2</sub>O<sub>7</sub> < Li<sub>1.5</sub>MnP<sub>2</sub>O<sub>7</sub> < LiMnP<sub>2</sub>O<sub>7</sub>. Moreover, Li<sub>2</sub>MnP<sub>2</sub>O<sub>7</sub> itself exhibits superior catalytic performance compared with MnO or MnO<sub>2</sub>. Our study provides valuable guidelines for developing an efficient Mn-based catalyst under neutral conditions with controlled Mn valency and atomic arrangement.