Moreover, the H-upd is commonly analyzed thermodynamically by investigating its peak positions at low scan rates but very little is known about its kinetics. Overview of the CV patterns of the H-upd First, let us consider a typical cyclic voltammogram of Pt(pc) reported for acidic and alkaline conditions (Fig. ). The common understanding is that in the region at about 450 mV vs RHE the Pt surface is free of adsorbed oxygen and hydrogen species and a rearrangement of the electrochemical double layer (EDL) is responsible for the double layer capacitance. In a negative potential sweep (cathodic direction), protons are donated to the surface (either from hydronium ions or water) and form adsorbed hydrogen atoms (H-upd) consuming electric charge in a reaction resembling the Volmer step (Eq. 27c). Close to the RHE potential, molecular hydrogen is formed (HER) and the potential sweep is reversed. The H-upd is oxidized involving proton acceptors (either water or hydroxyl ions). Passing through the double layer region, electric charge is transferred to the surface through oxidizing water/hydroxyl ions forming oxygen species on the surface. Reversing the potential sweep to its initial direction, these oxygen species are reduced in a characteristic peak and one potential cycle is complete. One common usage of the H-upd is for the determination of the electrochemical surface area (ECSA) of platinum-based catalysts. Experimentally, one integrates the H-upd between the double layer region and a local minimum in the magnitude of the current, caused by the increasing HER and decreasing H-upd current. As the H-upd is a pseudocapacitive process, while the HER is ideally a steady-state process, the local minimum (of the absolute current) will then depend on the scan rate, moving to more negative potentials with increasing scan rate. Furthermore, it has been demonstrated that the H-upd extends to more negative potentials than the position of the local minimum, by potential step experiments . These complications are generally acknowledged, leading to CO-stripping as the preferred method for ECSA determination of Pt surfaces.