Projecting a cost‐effective and highly efficient electrocatalyst for the oxygen reaction reduction (ORR) counts a great deal for Zn‐air batteries. Herein, a hierarchical core–shell ORR catalyst (Co2N/CoP@PNCNTs) is developed by embedding cobalt phosphides and/or cobalt nitrides as the core into N, P‐doped carbon nanotubes (PNCNTs) as the shell via one‐step carbonization, nitridation, and phosphorization of pyrolyzing Co‐MOF precursor. The globally N, P‐doped structure of Co2N/CoP@PNCNTs demonstrates an outstanding electrocatalytic activity in the alkaline solution with the onset and half‐wave potentials of 1.07 and 0.85 V respectively. Moreover, a Zn‐air battery assembled from Co2N/CoP@PNCNTs as the air cathode delivers an open circuit potential of 1.49 V, a maximum power density of 151.1 mW cm−2 and a specific capacity of 823.8 mAh kg−1. It is reflected that Co2N/CoP@PNCNTs provides a long‐term durability with a slight decline of 15 h in the chronoamperometry measurement and an excellent charge–discharge stability with negligible voltage decay for 150 h at 10 mA cm−2 in Zn‐air batteries. The results reveal that Co2N/CoP@PNCNTs has superiority over most Co‐Nx‐C or CoxP@C catalysts reported so far. The excellent catalytic properties and stability of Co2N/CoP@PNCNTs derive from synergistic effects between Co2N/CoP and mesoporous N, P‐doped carbon nanotubes.
The reduced chromite ore processing residue (rCOPR) deposited in environments is susceptible to surrounding factors and causes reoccurrence of Cr(VI). However, the impact of natural sunlight on the stability of rCOPR is still unexplored. Herein, we investigated the dissolution and transformation behaviors of Cr(III)−Fe(III) hydroxide, a typical Cr(III)-containing component in rCOPR, under visible light. At acidic conditions, the release rate of Cr(III) under illumination markedly increased, up to 7 times higher than that in the dark, yet no Cr(VI) was produced. While at basic conditions, only Cr(VI) was obtained by photooxidation, with an oxidation rate of ∼7 times higher than that by δ-MnO 2 under dark conditions at pH 10, but no reactive oxygen species was generated. X-ray absorption near-edge structure and density functional theory analyses reveal that coexisting Fe in the solid plays a critical role in the pH-dependent release and transformation of Cr(III), where photogenerated Fe(II) accelerates Cr(III) produced at acidic conditions. Meanwhile, at basic conditions, the production of intermediate Cr(III)−Fe(III) clusters by light leads to the oxidation of Cr(III) into Cr(VI) through the nonradical "metal-to-metal charge transfer" mechanism. Our study provides a new insight into Cr(VI) reoccurrence in rCOPR and helps in predicting its environmental risk in nature.
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