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

Abstract

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 (Li2MnP2O7), which has not been utilized for water oxidation catalysis to date, as a model system. Due to the monophasic behavior of Li2MnP2O7 with delithiation, the Mn valency of Li(2-x)MnP2O7 (x = 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(2-x)MnP2O7 (x = 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(2-x)MnP2O7 increases from 2 to 3, the catalytic performance is enhanced in the series Li2MnP2O7 < Li(1.7)MnP2O7 < Li(1.5)MnP2O7 < LiMnP2O7. Moreover, Li2MnP2O7 itself exhibits superior catalytic performance compared with MnO or MnO2. Our study provides valuable guidelines for developing an efficient Mn-based catalyst under neutral conditions with controlled Mn valency and atomic arrangement.

In Situ Formation of Efficient Cobalt‐Based Water Oxidation Catalysts from Co2+‐Containing Tungstate and Molybdate Solutions

“…For a catalytic current of 1 mA cm −2 , the required overpotentials are η =420 mV for Co−W and η =500 mV for Co−Mo. Compared with the other electrocatalysts based on non‐noble‐metal oxides under benign conditions reported in Table , Co−W showed a lower onset potential and higher catalytic activity for water oxidation . Tafel slopes of Co−W and Co−Mo were determined to be 133 and 79 mV dec −1 , respectively (Figure a).…”

A New Member of Electrocatalysts Based on Nickel Metaphosphate Nanocrystals for Efficient Water Oxidation

“…Cobalt‐based phosphate, the earliest phosphate member discovered for OER, has been regarded as highly active water oxidation catalyst in electrolysis and photoelectrochemistry . Nam and co‐workers demonstrated Li 2 MnP 2 O 7 , Mn 3 (PO 4 ) 2 ·3H 2 O, and Na 2 CoP 2 O 7 as efficient water oxidation catalysts with understanding of the interplay between atomic structure and catalytic activity . Li and Zhao synthesized NiFe/NiFe:Pi catalyst with enhanced surface wettability and catalytic activity .…”

Carbonate Ions Induce Highly Efficient Electrocatalytic Water Oxidation by Cobalt Oxyhydroxide Nanoparticles

“…[7,8] In the last decade, many efforts have been devoted to develop water oxidation catalysts as OEC models, especially efficient, earth-abundantc atalysts with low overpotential and sufficient robustness to fabricate artificial photosynthetic deviceso fp racticalu se. [8][9][10][11][12][13] Such laudable studies have yielded varioust ypes of complex and oxide catalysts based on earth-abundant metals,s uch as cobalt, [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29] manganese, [30][31][32] copper, [33][34][35] nickel, [36] and iron. [28,37,38] However,a ttention in most of these studies has been paido nly to the composition and structure of active sites in the OEC models, even though the cofactors are also essential for water oxidation at the photosynthetic OEC.…”