Primary Oxide Latent Storage and Spillover Enabling Electrocatalysts with Reversible Oxygen Electrode Properties and the Alterpolar Revertible (PEMFC versus WE) Cell

• Published in: Journal of physical chemistry. C 2014-05, Vol.118 (17), p.8723-8746
• Main Authors: Jaksic, Milan M, Botton, Gianluigi A, Papakonstantinou, Georgios D, Nan, Feihong, Jaksic, Jelena M
• Format: Article
• Language: English
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/jp412292w

Surface in and ex situ analysis have shown that in the course of cathodic oxygen reduction (ORR), all along the reversible potential range (the low slope Tafel plots, about 30 mVs/dec), nanostructured Pt electrocatalyst is covered by the interfering primary (Pt–OH) and surface (PtO) oxide mixture, while the higher polarization (120 mVs/dec) characterizes electrocatalytic surface deprived from these oxides and, consequently, the reaction mechanism of direct electron exchange on clean electrode surface. The substantial difference between the standard RHE (reversible hydrogen electrode) and ROE (reversible oxygen electrode), is that the former implies spontaneous hydrogen adsorption, fast H-adatoms (Pt–H) effusion and reversible electrode behavior (Pt­(H2)/Pt–H/H3O+), while the latter features the strong irreversible PtO adsorptive strength, and which is more significant, missing the Pt–OH spillover within the critical potential range between the primary oxide adsorption/desorption peaks position and oxygen evolving limits in both potentiodynamic scan directions (or the imposed polarization energy barrier of about 600 mVs). Since the Pt–OH presence and spillover are unavoidable decisive and indispensable for establishing the ROE properties, and thermodynamic electrode equilibrium (Pt­(O2)/Pt–OH/PtO/OH–), within the pronounced high polarization broad potential range, such spillover species has the same meaning and significance for the ROE as Pt–H plays for the RHE. Thence, to fill such a high polarization gap, the guiding concept implies homogeneous nanostructured distribution and selective grafting while interactive hypo-hyper-d-d-interelectronic bonding of Pt nanoclusters upon various mixed valence hypo-d-oxide supports, primarily Nb2O5,TiO2 (or Ta2O5,TiO2), because of their much thermally advanced electronic conductivity and extra high stability. In such a constellation, nanoparticles of Pt and solid oxides establish the so-called SMSIs (strong metal–support interactions), the strongest ones in all of chemistry, together with advanced electron conductive transfer, while the exposed surface of the latter undergoes spontaneous dissociative adsorption of water molecules (Nb2O5 → 2 Nb­(OH)5), and thereby becomes, along with continuous further water vapor supply, the undisturbed and almost unlimited, (alike electrons in metals) renewable latent storage and spillover source of the Pt–OH all along the potential axis between oxygen and hydrogen evolving limits,