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New striking prospects both in low and medium temperature polymer electrolyte membrane fuel cell (PEMFC) and in water electrolysis (WE) have been opened by the interactive supported individual (Pt) or prevailing hyper-d-electronic nanostructured metal clusters (WPt3, NbPt3, HfPd3, ZrNi3), grafted upon and within high altervalent capacity hypo-d-oxides (WO3, NbO2, TaO2, TiO2) and their proper mixed valence compounds, to create a novel type of alternating polarity (alterpolar) interchangeable composite electrocatalysts for hydrogen and oxygen electrode reactions. Whereas in aqueous media Pt (Pt/C) features either chemisorbed catalytic surface properties of Pt−H or PtO, missing any effusion of other interacting species, a new generation of composite SMSI (strong metal−support interaction) electrocatalysts in condensed wet state primarily characterizes interchangeable extremely fast reversible spillover of either H-adatoms or the primary oxides (Pt−OH, Au−OH) or the invertible bronze type behavior of these significant interactive electrocatalytic ingredients. Altervalent hypo-d-oxides impose spontaneous dissociative adsorption of water molecules and pronounced membrane spillover transferring properties instantaneously resulting with corresponding bronze type (Pt/H x WO3) 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/H0.35WO3 ⇔ Pt/W(OH)6) and substantially advanced electrocatalytic properties of these composite interactive electrocatalysts. Such nanostructured type electrocatalysts, even of mixed hypo-d-oxide structure (Pt/H0.35WO3/TiO2/C, Pt/H x NbO3/TiO2/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 substantiation of the revertible cell assembly for spontaneous reversible alterpolar interchanges between PEMFC and WE. The underpotential spillover double layer charging and discharging properties of the primary oxide (M−OH), interrelated with the interactive self-catalytic effect of dipole-oriented water molecules, has also been pointed out.

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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).

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(H 2 )/Pt−H/H 3 O + ), 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(O 2 )/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 Nb 2 O 5 ,TiO 2 (or Ta 2 O 5 ,TiO 2 ), 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 (Nb 2 O 5 → 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, with inexhaustible abilities of further optimizations. The reversible alterpolar changes instantaneously result by the spillover of H-adatoms with corresponding bronze type (Pt/H x NbO 5 , x ≈ 0.3) electrocatalysts under cathodic, and/or its hydrated state (Pt/Nb(OH) 5 ), responsible for Pt−OH effusion, under anodic polarization. This way there establishes the reversibly revertible alterpolar bronze features (Pt/H x NbO 5 ↔ Pt/Nb(OH) 5 ), as the thermodynamic equilibrium, and thereby substantially advanced electrocatalytic properties of th...