Hydrogen evolution reaction in alkaline solution - Catalytic influence of Pt supported on graphite vs Pt inclusions in graphite

Graphite powders with supported Pt [5 and 20 weight percent (w/o) Pt] and Pt inclusions (13 and 17 w/o) in graphite have been bonded with an inorganic polymer LaPO4 and used as cathodes for the hydrogen evolution reaction (HER) in 1 M KOH solution. Pt-supported electrodes [Pt(s)/C/LaPO4] are more active than the electrodes containing Pt inclusions in graphite [Pt(i)/C/LaPO4]. However, in high-current-density conditions (0.25 A/cm(2)), Pt(i)/C/LaPO4 is stable while Pt(s)/C/LaPO4 disintegrates after 24 h. The ac impedance diagrams display two semicircles on the complex-plane plot for both Pt(s)/C/LaPO4 and Pt(i)/C/LaPO4 electrodes but only one for pure graphite. The same equivalent circuit with two constant-phase elements has been used to explain the behavior of these electrodes. For both materials, the HER proceeds via the Volmer-Heyrovsky mechanism. The difference in the electroactivity of these two materials is related to the effective amount of Pt available.