# Noble Metal-Based Heterogeneous Catalysts for Electrochemical Hydrogen Evolution Reaction
## 2 Factors That Affect HER Performance
### Adsorption and Desorption Process  
Due to the low conductivity of pure water, adding a strong acid, strong base or salt into the water can form a high-conductivity electrolyte for the HER process. Therefore, different electrolyte states (different pH values) produce different catalytic mechanisms of adsorption and desorption.  
Under acidic conditions, H\({}^{+}\) or H\({}_{3}\)O\({}^{+}\) is adsorbed at the active site after the mass transfer process, that is, the Volmer reaction, as shown in Equation (1) (* represents adsorption site):  
\[\mathrm{H}_{3}\mathrm{O}^{+}+\mathrm{e}^{-}\rightarrow\mathrm{H}^{*}+\mathrm{ H}_{2}\mathrm{O} \tag{1}\]  
There are two possible desorption steps, which are the electrochemical desorption step, namely the Heyrovsky reaction, as shown in Equation (2), or the chemical desorption step, namely the Tafel reaction, as shown in Equation (3):  
\[\mathrm{H}^{*}+\mathrm{H}_{3}\mathrm{O}+\mathrm{e}^{-}\rightarrow\mathrm{H}_{ 2}+\mathrm{H}_{2}\mathrm{O} \tag{2}\]\[\mathrm{H^{*}+H^{*}\to H_{2}} \tag{3}\]  
Under alkaline conditions, the electrochemical adsorption step (the Volmer reaction) occurs first, as shown in Equation (4):  
\[\mathrm{H_{2}O+e^{-}\to H^{*}+OH^{-}} \tag{4}\]  
There are also two possible desorption steps, namely electrochemical desorption (the Heyrovsky reaction), as shown in Equation (5), or chemical desorption (the Tafel reaction), as shown in Equation (3):  
\[\mathrm{H^{*}+H_{2}O+e^{-}\to H_{2}+OH^{-}} \tag{5}\]  
The rate of adsorption and desorption affects the overall HER kinetics. Generally, Tafel slopes of 120, 40 and 30 mV dec\({}^{-1}\) were observed for the Volmer, Heyrovsky and Tafel determining rate steps, respectively. Additionally, for the Volmer rate-determining step, a Tafel slope of 120 mV dec\({}^{-1}\) was observed in the higher coverage region (\(\mathrm{\theta_{H}>0.6}\)). Therefore, the rate-determining step of the HER can be judged from the Tafel slope. Tafel formula is shown in Equation (6):  
\[\mathrm{\eta=b\log{\left|j\right|}+a} \tag{6}\]  
where \(\mathrm{\eta}\) is the hydrogen evolution overpotential, \(j\) is the current density, \(\mathrm{b}\) is the Tafel slope and \(\mathrm{a}\) is the overpotential value when the current density is 1 A cm\({}^{-2}\). According to the value of \(\mathrm{a}\), the metals can be divided into the following three categories: high overpotential metals (\(\mathrm{a}\approx\) 1.0-1.5 V), mainly including Pb, Cd, Hg, Tl, Zn, Ga, Bi, Sn, etc.; medium overpotential metals (\(\mathrm{a}\approx\) 0.5-0.7 V), mostly including Fe, Co, Ni, Cu, W, Au, etc.; low overpotential metals (\(\mathrm{a}\approx\) 0.1-0.3 V), usually including some platinum group metals such as Pt, Pd and Ru [27]. The Tafel slope can be obtained with the linear part of the Tafel curve. If the Tafel slope value is about 120 mV dec\({}^{-1}\), the Volmer reaction is the rate-determining step. If the Tafel slope value is about 40 mV dec\({}^{-1}\), the electrochemical desorption process is the rate-determining step, corresponding to the Heyrovsky reaction. If the Tafel slope value is about 30 mV dec\({}^{-1}\), the rate-determining step is a chemical desorption reaction, corresponding to the Tafel reaction [28]. A smaller value of \(\mathrm{b}\) means a smaller requirement of \(\mathrm{\eta}\) to achieve the same current density and a faster transfer kinetic of the electrons. The surface electronic structure of the catalyst is a key factor affecting the adsorption and desorption of hydrogen. Therefore, designing an excellent HER catalyst by modulating the surface electronic structure is important to improve the overall kinetics of the HER.