A Bifunctional Electrocatalyst for Full Water Splitting: CoNi@BSCF Encapsulated in N-Doped Carbon

Oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) are two half reactions of full electrochemical water splitting, which is a promising solution to produce hydrogen, a clean and renewable energy carrier to replace fossil fuels. Up to now, the benchmark catalysts for OER and HER are noble metal based materials, such as IrO 2 and Pt/C. However, the scarcity, extremely high cost, and poor stability of these noble metals highly limit their large-scale commercialization. Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3−δ (BSCF), a perovskite material with cubic structure, has been recognized as one of the most promising OER catalysts to replace noble metals. The excellent OER activity of BSCF is partially due to its high oxygen vacancy content, which, however, leads to a relatively poor operation stability. As to HER, BSCF also shows superior performance in comparison to other oxide catalysts, but still far from comparable to Pt/C. Herein, we report a facile way to enhance the operation stability as well as the HER and OER activity of BSCF. Through hydrothermal preparation and mechanical ball milling, we obtained a three-layer composite containing BSCF particles, CoNi nanoparticles, and N-doped carbon. With the protection of carbon shell, this composite, CoNi@BSCF/NC, displays a significant enhanced stability in both OER and HER in comparison to BSCF in alkaline media. The OER activity of CoNi@BSCF/NC is much higher than that of BSCF, including a smaller onset potential (by 30 mV) and a smaller overpotential at 10 mA/cm 2 (by 72 mV). More interestingly, this composite displays a self-improvement in OER activity during CV test, while BSCF shows obvious activity decay upon CV cycle numbers. This is mostly due to (i) the gradually oxidation of Ni in CV process, (ii) the protection of carbon shell, and (iii) the synergistic effect of BSCF and CoNi nanoparticles. Moreover, the HER activity of CoNi@BSCF/NC is also greatly improved with a small onset potential, which is 180 mV smaller than BSCF. Furthermore, CoNi@BSCF/NC also shows an excellent performance in full water splitting with our homemade electrolyzer, with a small overpotential at 10 mA/cm and good operation stability. Our study shows that CoNi@BSCF/NC is one of the best OER catalysts reported so far and among the best perovskite-based HER catalysts. This work may open new opportunity to further enhance the activity and operation stability of some state-of-the-art catalysts, such as BSCF.