Underpotential deposition hydrogen

The effective electroactive surface may be determined directly by the voltammogram in evaluating the quantity of electricity involved in the adsorption-desorption of hydrogen (region I). The quantity of electricity is obtained by the integration of the current-potential curves, because the potential varies linearly with time. The active surface area (S = 0.56 cm2) is then calculated from the charge under the hydrogen underpotential-deposited (UPD) peaks, which needs 210 pC cm 2, as follows ... 

On several noble metals (Pt, Rh, Ru, Ir, and Pd) hydrogen adsorption takes place at the potentials positive to the equilibrium potential for the hydrogen evolution reaction. This is a so-called hydrogen underpotential deposition reaction (HUPD) and indicates a strong adsorptive interaction between atomic hydrogen and the surface metal atoms. Similar UPD processes are observed for the deposition of metals on metals [237]. Certain reactions, like Cu UPD at Pt, Ru, or Rh, are used as diagnostic tools to determine the real surface area of electrocatalytic materials. 

The research on hydrogen underpotential deposition (UPD) at electrodes has identified the Gibbs energy of H-binding on the surface () and the half-bond... 

Schneider lA, Kramer D, Wokaun A et al (2007) Effect of inert gas flow on hydrogen underpotential deposition measurements in polymer electrolyte fuel cells. Electrochem Commun 9 1607-1612... 

The break-in process of 3M NSTF films involves voltage cycling to create a smooth polycrystalline Pt surface on the whiskers. The surface area enhancement factors of resulting structures are from 10 to 25, determined from cyclic voltammo-grams in the potential region of hydrogen underpotential deposition. The surface area enhancement factor (or roughness factor or real-to-apparent surface area ratio) of a... 

Similar measurements for the cases of supported nanosized catalysts require preparation of the electrode in a specific way. Usually, catalyst is dispersed in suitably chosen media and desired amount is transfer to solid electrode serving as electrical contact [11]. This issue will be elaborated in more details later on. When stable thin catalyst layer is prepared cyclic voltammetry may be used to investigate surface electrochemical processes (Figure 4). In contrast to single-crystal and polycrystalline surface additional factor arises - namely, these processes depend also on the particle size. For example, in the case of supported Pt nanocatalysts it was observed that both hydrogen underpotential deposition and adsorption of oxygen species are clearly dependent on the particle size [12]. In specific, it was observed that smaller particles are more oxophilic and that surface oxidation is more irreversible for smaller particles. 

Figure 4. Cyclic voltammograms of Pt-poly disk electrode and Pt/C catalysts (40 wt.% Pt, Alfa Aesar), recorded at 50 mV s in de-aeiated 0.1 mol dm HCIO4 solution. Cyclic voltammograms are normalized by real surface area of Pt assessed by investigating hydrogen underpotential deposition.

Two semicircles are followed by a finite diffusion line in the case of hydrogen underpotential deposition (UPD), coupled or not with hydrogen adsorption. In the case of underpotential deposition the ad-... 

---