# Noble Metal-Based Heterogeneous Catalysts for Electrochemical Hydrogen Evolution Reaction
## 2 Factors That Affect HER Performance
### Mass Transfer Process  
Many research works have shown that the mass transfer of nanocatalysis is very important to the whole catalytic process. Favorable mass transfer can improve the HER kinetics to a large extent [17; 18]. For example, the mass transfer of the HER in an acidic electrolyte is H\({}_{3}\)O\({}^{+}\) (solution body) \(\rightarrow\) H\({}_{3}\)O\({}^{+}\) (liquid layer near the electrode surface). The influence of ion migration can be minimized by increasing agitation, increasing the concentration of hydrogen ions in the electrolyte and changing the distance between the two electrodes [19]. As for the electrode, the surface structure of the electrode attached to the catalytic material and its loading amount (thickness) also have an important influence on mass transfer.  
Some unique structures of catalysts have been shown to play a crucial role in mass transfer, such as ordered porous structures [20; 21; 22]. [PERSON] et al. [23] prepared a three-dimensional ordered macroporous/mesoporous nickel using the wet chemical method. Combining ordered bi-continuous mesoporous and three-dimensional connected periodic microporous structures can realize highly efficient mass transfer. [PERSON] et al. [21] prepared an ultrathin palladium network (Pd NMs) with a thickness of about 3 nm using solution oxidation etching, whose unique ordered mesoporous structure could realize rapid mass transfer. This catalyst embedded Pt nanoparticles into Pd NMs, and the resulting Pd NMs/Pt had extremely high HER activity and durability in acidic conditions. In addition, noble metal catalysts can be combined with other porous structures [24]. The porous shell structure facilitates the reactant to reach the active center on the inner surface of the shell through diffusion and mass transport [24]. However, the constructed porous structure is often hard to accurately regulate and appears disorderly, resulting in insufficient pore utilization. Irregular pore arrangement and/or tortuous pore channels are not conducive to rapid mass transfer in catalytic reactions [22]. Therefore, ordered porous structures are more advantageous for energy storage and conversion due to their efficient mass transfer.  
In terms of basic research, the influence of the catalytic material loaded on the working electrode in mass transmission should not be ignored [14]. If the loading amount is too low, the coating slurry cannot completely cover the electrode surface and there will not be enough catalytic active sites. If the loading amount is too high, the aggregation of catalytic materials will affect the contact between the active components and the electrolyte, further influencing the mass transfer process. The thickness of the coating paste is usually controlled to be a few microns thick, and the thickness can be determined with cross-section measurements [25] using a scanning electron microscope (SEM) or numerical estimation. In addition, too much Nafion (binder) in the coating slurry will form a thicker Nafion film, resulting in additional mass transfer resistance within the Nafion layer [26]. Therefore, to ensure that the diffusion resistance of the Nafion film is negligible, it is very important to optimize the additive amount of Nafion [26].