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With the increasing demand for diversification of renewable energy sources, the high activity and stability of electrocatalysts in all pH ranges have been highlighted as one of the future directions in electrochemical energy generation and storage systems. Noble-metal-free nitrogen-doped carbon (M-N/C) has been explored as a substitute for the expensive platinum-based oxygen reduction reaction (ORR) electrocatalysts. However, its ORR activity remains limited to alkaline electrolytes. In acidic medium, its low activity and stability correspond to metallic site dissolution and protonation of N-functional groups, which are even worse in neutral electrolytes because of low ionic conductivity and low H+concentration. This review summarizes strategies to improve the stability and activity of M-N/C as pH-universal ORR electrocatalysts. First, the ORR mechanism focusing on active site identification for each pH condition is discussed. Four strategies, including engineering pore structure, adding carbon shell wrapping, and introducing multiple nonmetal dopants and dual metallic active sites on the carbon substrate, are then evaluated to design pH-universal ORR electrocatalysts with distinguished activity and stability. Lastly, future perspectives are given to show the viewpoint of further development and potential applications of M-N/C electrocatalysts.

The hydrogen evolution reaction (HER) is being widely researched and developed because it is a favorable alternative to fossil fuels to produce renewable energy. The prominent development in HER is to construct an efficient electrocatalyst aside from Pt-based catalysts, with low economic cost. In this study, gold nanoparticles (Au NPs) supported MXene (Ti3C2Tx) was designed to enhance the performance of a HER electrocatalyst. The preparation of Au NPs-supported MXene was confirmed by XRD, FTIR, Raman spectroscopy, and UV-Vis DRS. The addition of Au NPs in the MXene structure is believed to improve its catalytic performance in the hydrogen evolution reaction. The catalytic performance data of Au/MXene exhibits a better result than non-modified MXene, with a lower overpotential (179.9 mV), onset potential (87.6 mV), Tafel slope (91 mV/dec), and a higher ECSA value (54 cm2), with excellent electrocatalyst stability after ten hours of reaction time. Furthermore, the Tafel slope data of Au/MXene indicated the reaction was using the Volmer-Heyrovsky path, with the rate-determining stage being that of the Heyrovsky mechanism. The increased activity as an electrocatalyst in HER was assigned to the synergetic effect between Au nanoparticles and MXene structures, which results in high-conductivity materials with enhanced ion diffusion and charge transfer.