δ-MnO 2 was proposed as a promising cathode material for aqueous Zn-ion batteries. Nonetheless, its potential application has been impeded by its unstable crystal structure and sluggish reaction kinetics with Zn ions. Herein, we utilized Cr to modify δ-MnO 2 by a simple hydrothermal method to develop cathode materials for aqueous Zn-ion batteries with better stability and electron transport properties. The introduction of Cr enhanced the specific capacity and cycling stability of δ-MnO 2 , in particular, the 3% Cr doped δ-MnO 2 electrode exhibited a large specific capacity of 229.2 mAh·g −1 at 0.1 A·g −1 and an excellent capacitance retention of 86.0% after performing 300 cycles. The density functional theory (DFT) calculations indicate that the introduction of Cr decreases the band gap of MnO 2 and enhances electron migration in the electrode material. Meanwhile, the introduction of Cr substantially improved the Zn ion adsorption and enhanced the binding energy of δ-MnO 2 , indicating that the addition of Cr promoted the interaction between the Cr doping δ-MnO 2 (Cr-MnO 2 ) electrode materials with Zn ions, while making the structure of the electrode materials more stable. As a result, this suppresses the Jahn-Teller distortion and reduces the solvation and phase transition of Mn.
For the electrocatalytic oxygen evolution reaction (OER) in alkaline media, there is an urgent need to optimize the adsorption strength of OH*. Here, a flower-like hybrid of Cr-doped FeOOH/Ni(OH)2 was used as an OER catalyst with a low overpotential of 291 mV at 50 mA cm−2. The results showed that faster charge transfer was achieved at the electrode/solution interface during the OER process after the FeOOH/Ni(OH)2 was modified by Cr, which facilitates the rate-determining step of the Volmer reaction. Furthermore, the results of faradaic efficiency and X-ray photoelectron spectroscopy (XPS) measurements confirmed that the synergistic effect between the ternary metal and oxygen vacancies led to excellent OER performance. This work provides a new strategy for the preparation of high-efficiency and low-cost oxygen evolution electrocatalysts.
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