# Noble Metal-Based Heterogeneous Catalysts for Electrochemical Hydrogen Evolution Reaction
## 5 Conclusions and Prospects  
The research progress of noble metal electrocatalysts for hydrogen production using water electrolysis in recent years is reviewed in this paper. For noble metals, great progress has been made in improving the utilization efficiency by reducing particle size to increase surface area, designing a core-shell structure to expose atoms to the shell and binding with conductive substrate to increase the dispersion and stability of the noble metal atoms. We highlighted several methods for designing novel nanostructured catalysts to enhance the active center and optimize the electronic and geometric structure, including the construction of noble metal alloys and core-shell structures with secondary metallic elements to adjust the electronic structure and optimize the thermodynamic hydrogen adsorption/desorption on the catalyst surface. Furthermore, the noble metal and the carrier with a favorable electrical conductivity are combined to form a heterostructure or heterointerface to reduce the load of the noble metal and accelerate the transmission of electrons and ions, so as to reduce the kinetic reaction barrier. In addition, we also mentioned the preparation of working electrodes and the choice of the electrolyte on the performance of aquatic hydrogen electrolysis. Despite the remarkable progress made to date, many challenges remain in this area. 1. Noble metals are expensive, and the preparation process of catalysts is complex. Although various effective noble metal-based catalysts with low metal loading capacity and small size have been developed, the high surface energy leads to the easy aggregation of metal atoms in the catalytic process, resulting in reduced catalytic activity, which makes it difficult to meet the commercial demand. Therefore, it is urgent to  
Figure 11: (**a**) Preparation process of Ru SAs@PN. (**b**) Polarization curves at 10 mV s\({}^{-1}\). (**c**) The calculated free-energy diagram of the HER at the equilibrium potential for Pt/C, Ru/C, Ru SAs@C, Ru SAs@C\({}_{3}\)N\({}_{4}\) and Ru SAs@PN [91]. Copyright 2018 John Wiley and Sons. develop a simple synthesis method to prepare small monodisperse catalysts with favorable dispersion and high loading capacity. 2. A thorough understanding of the HER mechanism is of great significance to adjusting catalytic activity and guiding the structural design of prospective catalysts. Although the DFT calculations have been used to predict reaction intermediates and active centers of catalysts and to design efficient HER catalysts, theoretical models are mostly simplifications of actual catalytic conditions. Especially in an alkaline medium, there is no consensus on the HER activity descriptor and action mechanism. At present, most of the catalyst characterization techniques used are situ techniques, which can only provide information before and after the measurement of the catalyst, while ignoring the evolution of the microstructure of the atomic layer on the catalyst surface and the reaction intermediates adsorbed on the catalyst surface during the process. Therefore, there is an urgent need to develop advanced in situ characterization techniques and theoretical simulations to accurately elucidate the reaction mechanism at the molecular level. 3. The development of HER and OER bifunctional catalysts with high activity, selectivity and stability are necessary, which can reduce the costs and simplify setups. The efficiency of hydrogen production depends not only on the cathode catalytic efficiency but also on the anode catalytic efficiency in practical water electrolysis hydrogen production devices and applications. Most HER catalysts (e.g., Pt-based catalysts, chalcogenides, and phosphides) have better catalytic effects in acidic media, while most OER catalysts (e.g., transition metal oxides and (oxy)hydroxides) have better catalytic activity in alkaline media. Under the same experimental conditions, few catalysts can effectively catalyze both the HER and OER.