Spillover Phenomena and Its Striking Impacts in Electrocatalysis for Hydrogen and Oxygen Electrode Reactions

Abstract

The core subject of the present paper represents the interrelated spillover (effusion) phenomena both of the primary oxide and the H-adatoms, their theory and practice, causes, appearances and consequences, and evidences of existence, their specific properties, and their alterpolar equilibria and kinetic behavior, structural, and resulting catalytic, and double layer charging features. The aim is to introduce electron conductive and d-d interactive individual and composite (mixed valence) hypo-doxide compounds, of increased altervalent capacity, or their suboxides (Magnéli phases), as the interactive catalytic supports and therefrom provide (i) the strong metal-support interaction (SMSI) catalytic effect and (ii) dynamic spillover interactive transfer of primary oxides (M-OH) and free effusional H-adatoms for further electrode reactions and thereby advance the overall electrocatalytic activity. Since hypo-d-oxides feature the exchange membrane properties, the higher the altervalent capacity, the higher the spillover effect. In fact, altervalent hypo-d-oxides impose spontaneous dissociative adsorption of water molecules and then spontaneously pronounced membrane spillover transferring properties instantaneously resulting with corresponding bronze type (Pt/H x WO 3 ) under cathodic and/or its hydrated state (Pt/W(OH) 6 ), responsible for Pt-OH effusion, under anodic polarization, this way establishing instantaneous reversibly revertible alterpolar bronze features (Pt/H 0.35 WO 3 ⇔ Pt/W(OH) 6 ) and substantially advanced electrocatalytic properties of these composite interactive electrocatalysts. Such nanostructured-type electrocatalysts, even of mixed-valence hypo-d-oxide structures (Pt/H 0.35 WO 3 /TiO 2 /C, Pt/H x NbO 3 /TiO 2 /C), have for the first time been synthesized by the sol-gel methods and shown rather high stability, electron conductivity, and nonexchanged initial pure monobronze spillover and catalytic properties. Such a unique electrocatalytic system, as the striking target issue of the present paper, has been shown to be the superior for substantiation of the revertible cell assembly for spontaneous reversible alterpolar interchanges between PEMFC and WE. The main target of the present thorough review study has been to throw some specific insight light on the overall spillover phenomena and their effects in electrocatalysis of oxygen and hydrogen electrode reactions from diverse angles of view and broad contemporary experimental methods and approaches (XPS, FTIR, DRIFT, XRD, potentiodynamic spectra, UHRTEM).

Primary oxide spillover DL pseudo-capacitance charging and discharging in the light of the first principle theory

“…Thus, from thermodynamic definition, no one metal in the Periodic In fact, in the absence of the Pt-OH, therefrom imposes such a critical and typical, very pronounced the reaction polarization range (Fig. 1), all along until oxygen potential evolving limits, and in both scan directions, back and forth [4][5][6][7]. Such highly reversible (Eq.…”

“…2), behave as unlimited latent storage and spillover sources of the primary oxide [4][5][6][7]20,21], continuously being renewed and recovered simply by water vapor supply and instantaneously proceeding with spontaneous dissociative adsorption of water molecules.…”

“…The overall summation (Σ) for the ion exchange properties of hypo-d-oxide hydrated membrane transferring matrix, under directional electric field effect or polarization, yields the reversible Pt-OH adsorptive and/or desorptive spillover upon, or from metal electrocatalyst reactive surface, resulting that way with the characteristic pronounced reversible effusion peaks of extremely fast electrode reaction [2][3][4][5][6][7], (Eq. 1-a, 1-b and 1-c, yielding by such a summation the same former Eq.…”

“…1 and 2, once when missing Pt-OH spillover (Pt-OH → 0, Pt=O → 1), and when enriched spillover enables enormous primary oxide adsorptive deposition and reverse desorption, exactly as big capacitors do, and when present all over potentiodynamic cycluses [4][5][6][7].…”

“…3), and both together assemble the reversible hydrogen fueled L&MT (Low amd Medium Temperature) PEMFCs. In other words, the Pt-OH (or Au-OH) plays the same thermodynamic constitutional role for establishing the equilibrium for the ROE, as Pt-H does for the RHE, and thence, unavoidably imposes continuous need and requires a permanent source of the former to overcome the reaction polarization by the latter, and enables the reversible electrocatalytic properties of the ROE [6,7]. The substance is that Pt-OH (Au-OH) is insoluble in aqueous media and represents surface species able only for heterogeneous reactions and spillover spreading.…”