HER Mechanisms on Low Overvoltage Cathodes

The reaction pathways involved in the course of the HER on smooth metals are described in Section 4.2. The generally accepted mechanism of the HER in alkaline solutions involves water molecules as a source of protons, which discharge on the cathode surface, M, to form an M-H bond, as in Eq. (2) (Table 4.6.1). The adsorbed H can either combine with another H to form H2 (Eq. 3) or react with a water molecule to form H2 (Eq. 4). 

Whenever adsorbed intermediates are involved in the slow step of the HER, the Tafel slopes assume low values, as shown theoretically and depicted in Table 4.6.1. However, mechanisms that give rise to low Tafel slopes, because of the dependence of coverage by the adsorbed intermediates on the potential, may have a disadvantage on metals, as hydrogen embrittlement [5] can occur because of the diffusion of adsorbed H into the metal. Thus, while the low Tafel slopes are advantageous from a voltage viewpoint, they can have a serious adverse influence on long-term performance. 

A comparison of polarization data on high surface area preparations, with the same metal in the bulk form, shows that as the surface area increases, the Tafel slope decreases [10,187]. Thus, the Tafel slope on smooth Ni in alkaline solutions decreases from about 100 mV to 30-40 mV with Raney Ni and various other compositions (Fig. 4.6.15). This behavior is not unique to Ni, but is also observed on Pt during the course of the HER in alkaline solutions (Fig. 4.6.16). 

As the HER occured on high-surface area matrices, the effect of the porous structure on the Tafel slope behavior was examined [ 188] by assuming a homogeneous elementary flooded pore model and deducing the current-voltage relationships, taking into account the backward rate and the coverage dependence on potential. This analysis showed that 

FIGURE 4.6.15. Polarization data for the HER on various Ni-based compositions in 15% NaOH-l-17% NaCl solutions at 95 C [10]. (With permission from the Indian Academy of Sciences.) 

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