Abstract
The performance of four polymorphs of manganese (Mn) dioxides as the catalyst for the oxygen evolution reaction (OER) in proton exchange membrane (PEM) electrolysers was examined. The comparison of the activity between Mn oxides/carbon (Mn/C), iridium oxide/carbon (Ir/C) and platinum/carbon (Pt/C) under the same condition in PEM electrolysers showed that the γ-MnO2/C exhibited a voltage efficiency for water electrolysis comparable to the case with Pt/C, while lower than the case with the benchmark Ir/C OER catalyst. The rapid decrease in the voltage efficiency was observed for a PEM electrolyser with the Mn/C, as indicated by the voltage shift from 1.7 to 1.9 V under the galvanostatic condition. The rapid deactivation was also observed when Pt/C was used, indicating that the instability of PEM electrolysis with Mn/C is probably due to the oxidative decomposition of carbon supports. The OER activity of the four types of Mn oxides was also evaluated at acidic pH in a three-electrode system. It was found that the OER activity trends of the Mn oxides evaluated in an acidic aqueous electrolyte were distinct from those in PEM electrolysers, demonstrating the importance of the evaluation of OER catalysts in a real device condition for future development of noble-metal-free PEM electrolysers.
Highlights
The performance of four polymorphs of manganese (Mn) dioxides as the catalyst for the oxygen evolution reaction (OER) in proton exchange membrane (PEM) electrolysers was examined
The comparison of the activity between Mn oxides/ carbon (Mn/C), iridium oxide/carbon (Ir/C) and platinum/ carbon (Pt/C) under the same condition in PEM electrolysers showed that the g-MnO2/C exhibited a voltage efficiency for water electrolysis comparable to the case with Pt/C, while lower than the case with the benchmark Ir/C OER catalyst
The X-ray diffraction (XRD) pattern of the g-MnO2 sample showed the existence of extensive microtwinning in the ramsdellite 021/061 planes, which is the typical structure of electrolytic Mn dioxides (EMD) [50] prepared at high current densities [51]
Summary
The performance of four polymorphs of manganese (Mn) dioxides as the catalyst for the oxygen evolution reaction (OER) in proton exchange membrane (PEM) electrolysers was examined. PEM electrolysers possess great advantages for the energy conversion systems from renewable to chemical energy, due to the high voltage efficiency, rapid response to input changes, high current density, usage of non-corrosive liquid and relatively low working temperature (less than 1008C) [20]. These advantages are directly or indirectly related to current standard PEMs with low gas permeation rate and high proton conductivity, typically composed of perfluorinated sulfonic acid polymers [20,23]. The lower mobility of OH2 than that of Hþ is an intrinsic problem for AEMs [23,28]
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