Keywords: high-valence Ni 3+ , hierarchical porous structure, Ni3S4, oxygen evolution reaction, durability 2 Electrochemical water splitting is a common way to produce hydrogen gas, but the sluggish kinetics of oxygen evolution reaction (OER) significantly limits the overall energy conversion efficiency of the water splitting. In this work, a highly active and stable, meso-macro hierarchical porous Ni3S4 architecture, enriched in Ni 3+ was designed as an advanced electrocatalyst for OER. The obtained Ni3S4 architectures exhibit a relatively low overpotential of 257 mV at 10 mA cm −2 , and 300 mV at 50 mA cm −2 . Additionally, this Ni3S4 catalyst has excellent long-term stability (no degradation after 300 h at 50 mA cm −2 ). The outstanding OER performance is due to the high concentration of Ni 3+ and the meso-macro hierarchical porous structure. The presence of Ni 3+ enhances the chemisorption of OH − which facilitates the electron transfer to the surface during OER. The hierarchical porosity increases the number of exposed active sites, and facilitates mass transport. A water-splitting electrolyzer using the prepared Ni3S4 as the anode catalyst and Pt/C as the cathode catalyst achieved a low cell voltage of 1.51 V at 10 mA cm −2 . Therefore, this work provides a new strategy for the rational design of highly active OER electrocatalysts with high valence Ni 3+ and hierarchical porous architectures.

Imidazolium methanesulfonate (1) has been studied as a model proton conductor for high temperature polymer electrolyte membrane fuel cells (PEMFCs). It is found that 1 undergoes transformation from crystalline to plastic crystalline and then molten states successively from ambient temperature to 200 C.The solid-solid phase transition of 1 at 174 C has been preliminarily verified by differential scanning calorimetry (DSC) and temperature-dependent X-ray diffraction (XRD). At the melting point of 188 C, 1 displays a low entropy of fusion of around 24 J mol À1 K À1 . In particular, a high ionic conductivity of 1.0 Â 10 À2 S cm À1 is reached at 185 C in the plastic phase. The activation energy for ionic conduction decreases as 1 is heated from the crystal phase to the melt phase. In the molten state, the contribution of protons to the ionic conductivity of 1 was corroborated electrochemically. In addition, 1 is electrochemically active for H 2 oxidation and O 2 reduction at a Pt electrode while it shows a high electrochemical window of 2.0 V. Furthermore, a NafionÒ membrane has been successfully doped with 1, as identified by infrared spectroscopy, powder XRD, grazing incidence XRD and thermogravimetric analysis. To the best of our knowledge, this may be the first report on a protic organic ionic plastic crystal (OIPC) consisting of protonated imidazole (C 3 H 5 N 2 + ) and an organic anion. The good thermal stability, high ionic conductivity, wide electrochemical window, favorable plastic crystal behavior and simple synthesis make 1 a highly interesting model proton conductor for high temperature PEMFCs.