Co(OH)2 in the form of hexagonal nanoplates synthesized by a simple hydrothermal reaction has shown even greater activity than cobalt oxides (CoO and Co3O4) in oxygen reduction and oxygen evolution reactions (ORR and OER) under alkaline conditions. The bifunctionality for oxygen electrocatalysis as shown by the OER-ORR potential difference (ΔE) could be reduced to as low as 0.87 V, comparable to the state-of-the-art non-noble bifunctional catalysts, when the Co(OH)2 nanoplates were compounded with nitrogen-doped reduced graphene oxide (N-rGO). The good performance was attributed to the nanosizing of Co(OH)2 and the synergistic interaction between Co(OH)2 and N-rGO. A zinc-air cell assembled with a Co(OH)2-air electrode also showed a performance comparable to that of the state-of-the-art zinc-air cells. The combination of bifunctional activity and operational stability establishes Co(OH)2 as an effective low-cost alternative to the platinum group metal catalysts.

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Although transition‐metal oxides are common non‐platinum group metal catalysts for the industrially important oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), the performance gap between transition‐metal oxides and platinum group metal catalysts is still substantial and there is a continuing need to search for alternatives. In this study, transition‐metal (Mn, Fe, Co, and Ni) phosphates prepared by a solution chemistry method under ambient conditions are found to display interesting electrocatalytic properties for the ORR and OER in alkaline solution. Among them, manganese phosphate is more active than most state‐of‐the‐art manganese oxides for the ORR, and nickel phosphate is as active as the best Ni‐based catalysts for the OER. Hence these phosphates can be used as tandem catalysts for rechargeable metal–air batteries in which both the ORR and OER take place. The good performance may be attributed to the stabilization of the catalytic centers by the phosphate framework. This study establishes phosphates as yet another class of highly active low‐cost non‐platinum group metal alternatives for oxygen electrocatalysis in alkaline solution.

Nickel(II) phosphate (NPO) can be an effective catalyst for oxygen evolution reaction (OER) in alkaline solution. Unlike other transition metal phosphates, the OER is preceded by a prominent reversible redox reaction. The current study is aimed at understanding the origin of NPO‐catalyzed OER, in particular the role of the pre‐OER redox reaction. We detected the progressive formation of Ni(OH)2/NiOOH on the NPO surface during potential cycling in the anodic region similar to previous observations from Ni oxides, and found that it is associated with the change in the Ni/phosphate stoichiometry from 1.5 to 1.0, resulting in the loss of phosphate to the alkaline solution. Despite the conversion, NPO still has a good and sustained OER performance due to the activity similarity between NiPO4 (and possibly NPO) and NiOOH (the activated form of Ni oxide and hydroxide catalysts). NPO is however an easier catalyst to use than Ni oxides and hydroxides, which require extensive electrode conditioning to reach a stable performance. We also mixed NPO with Fe in view of recent findings that Ni–Fe double hydroxides are more active than simple Ni oxides in oxygen catalysis. The experimental results corroborate the positive outcome of this modification.

Although the successful deployment of lithium-ion batteries (LIBs) in various fields such as consumer electronics, electric vehicles and electric grid, the efforts are still ongoing to pursue the next-generation battery systems with higher energy densities. Interest has been increasing in the batteries relying on the multivalent-ions such as Mg 2+ , Zn 2+ , and Al 3+ , because of the higher volumetric energy densities than those of monovalent-ion batteries including LIBs and Na-ion batteries. Among them, magnesium batteries have attracted much attention due to the promising characteristics of Mg anode: a low redox potential (−2.356 V vs. SHE), a high volumetric energy density (3,833 mAh cm −3), atmospheric stability and the earth-abundance. However, the development of Mg batteries has progressed little since the first Mg-ion rechargeable battery was reported in 2000. A severe technological bottleneck concerns the organic electrolytes, which have limited compatibility with Mg anode and form an Mg-ion insulating passivation layer on the anode surface. Consequently, beneficial to the good chemical and mechanical stability, Mg-ion solid electrolyte should be a promising alternative to the liquid electrolyte. Herein, a mini review is presented to focus on the recent development of Mg-ion solid conductor. The performances and the limitations were also discussed in the review. We hope that the mini review could provide a quick grasp of the challenges in the area and inspire researchers to develop applicable solid electrolyte candidates for Mg batteries.