Itinerant Spins and Bond-Lengths in Oxide Electrocatalysts for the

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Itinerant Spins and Bond-Lengths in Oxide Electrocatalysts for the Oxygen Evolution and Reduction Reactions Jose Gracia J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 26 Mar 2019 Downloaded from http://pubs.acs.org on March 26, 2019

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The Journal of Physical Chemistry

Itinerant Spins and Bond-Lengths in Oxide Electrocatalysts for the Oxygen Evolution and Reduction Reactions Jose Gracia* MagnetoCat SL, General Polavieja 9 3I, 03012 Alicante (Spain); *[email protected] Abstract: Thorough analyses of structural factors in catalysis are interesting because allow the massive pre-screening of potential optimum compositions. Overall, this article shows how the orbital physics of magnetic compositions relates with spin-lattice interactions, and then band-gaps and bond-lengths together become relevant descriptors in catalytic oxygen technologies. Active electrocatalysts for the oxygen evolution reaction (OER) include magnetic oxides with metals at relatively high oxidation states, so chemisorbed molecular O2 is not very stable. On the other hand, ideal compositions for the oxygen reduction reaction (ORR) have metals in a comparatively lower oxidation state, which can supply electrons to activate O2 molecules towards electron-richer oxygen atoms. Spin-lattice interactions in these strongly correlated oxides relate the orbital configurations, oxidation state, with distinctive metal-oxygen bond distances: indicating localized or itinerant electronic behaviour and selectivity in oxygen electrochemistry. OER at low overpotentials coincide with anti-Jahn-Teller contractions in ferromagnetic (FM) metal-oxygen (M-O) bonds; however, active oxides for ORR have longer FM M-O bonds, electron-richer. In both cases OER and ORR, dominant FM couplings moderate the binding energies of the reactants, because of the stabilizing quantum spin exchange interactions (QSEI) associated with the openshell orbital configurations; and correspondingly their catalytic efficiency improves in accordance with the Sabatier’s principle. The presence of FM holes in the M-O bonds also enhances spinselective charge transport, the other crucial enthalpic contribution in electrocatalysis. These specific effects of spin-dependent potentials in heterogeneous catalysis define the explicit field of spintro-catalysis, needed to allow the inclusion of strongly correlated electrons in theoretical models; and as we show here also with the advantage of the recognising structural descriptors ligated to spin-lattice interactions.

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Introduction. Not much has been said about the role of strongly correlated electrons in heterogeneous catalysis; however, significant advances let’s start understanding why the activity and selectivity in oxygen electrocatalysis relates with the electronic and magnetic structure of quantum materials.1,2,3,4 Essentially, cooperative quantum spin exchange interactions in openshells (QSEIopen shells) are a differentiated part of the stabilizing potentials in the orbital physics of magnetic materials,5,6,7 and then a distinguished contribution to their catalytic properties: spintrocatalysis.3 The orbital engineering of QSEIopen shells in (correlated) magnetic catalysts is a promising strategy to reduce and improve the use of expensive noble metals in oxygen and hydrogen technologies, by optimizing the activity introducing abundant metals as nature does.8 For instance, Pt metal9 or the non-magnetic conductive oxide LaNiO3 (Ni3+-3d7)10 are active compositions for the oxygen evolution and reduction reactions (OER and ORR), good charge transport and moderate binding energies justify the activity, however not optimum. Higher activities appear after the formation of dominant spin-polarized ferromagnetic (FM) interactions, in this case for Pt3(TM)1 alloys with TM = Fe, Co or Ni9 and for LaNi1-XFeXO3 oxides.10 Ruderman-KittelKasuya-Yosida (RKKY) type interactions cause the FM ordering of the conduction spins,5,2 where the additional stabilizing inter-atomic FM QSEIopen shells at the frontier orbitals help to improve the OER and ORR activities. Once intra-atomic (on-site) QSEI stabilize open-shell configurations, we can differentiate between leading antiferromagnetic (AFM) orbital orderings, where inter-atomic Coulomb potentials maximize the localization of the valence electrons (in the lower Hubbard bands);11 or dominant FM orderings where charge transport is spin selectivity,12,4 and settled via exchange hopping.3 Overall, in most cases QSEIopen shells enhance the activity of catalysts based on magnetic elements, because they are more noble-like and form less stable chemisorption states. In particular, leading inter-atomic FM interactions in mixed-valence compounds induce spin conductivity and decrease the rate limiting bonding energies to values more optimum. The specific effects of QSEIopen shells yield to Eq. 1 and 2, that justify the oscillations of the catalytic activity with the orbital filling (𝒇𝒅), unexpected for standard models, due to the change of the properties of strongly correlated electrons. Eq. 1 and 2 can explain or account for the general experimental multipeak activity trends versus 𝒇𝒅 observed in heterogeneous catalysts based on magnetic configurations, see for instance the literature.13,14,15,16,17,18 2 ACS Paragon Plus Environment

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𝒐𝒑𝒆𝒏

― )𝑚𝑎𝑔𝑛𝑒𝑡𝑖𝑐 𝒔𝒉𝒆𝒍𝒍𝒔 Eq. 1) ∆𝐻(𝑛𝑜𝑛 ; = 𝑎(𝒇𝒅 ― ℎ)𝟐 +𝑘 ± ∆𝑱𝑸𝑺𝑬𝑰 𝑎𝑐𝑡𝑖𝑐𝑎𝑡𝑖𝑜𝑛 𝒂𝒄𝒕𝒊𝒗𝒂𝒕𝒊𝒐𝒏(𝒇𝒅) ⋅ 𝑺𝒄𝒂𝒕.(𝒇𝒅)

𝒐𝒑𝒆𝒏

― )𝑚𝑎𝑔𝑛𝑒𝑡𝑖𝑐 ― 𝑠ℎ𝑒𝑙𝑙 𝒔𝒉𝒆𝒍𝒍𝒔 Eq. 2) ∆𝐻(𝑛𝑜𝑛 . = ∆𝐻𝑐𝑙𝑜𝑠𝑒𝑑 ― ∆𝑱𝑸𝑺𝑬𝑰 𝑎𝑑𝑠𝑜𝑟𝑝𝑡𝑖𝑜𝑛 𝑎𝑑𝑠𝑜𝑟𝑝𝑡𝑖𝑜𝑛 𝒂𝒅𝒔𝒐𝒓𝒑𝒕𝒊𝒐𝒏(𝒇𝒅) ⋅ 𝑺𝒄𝒂𝒕.(𝒇𝒅)

The success of the d-band model restricts to closed-shells, and Eq. 1 and 2 introduce the basis of a more complete QSECI-model for understanding the electronic interactions in heterogeneous catalysis including cooperative 𝑄𝑆𝐸𝐼𝑜𝑝𝑒𝑛 𝑠ℎ𝑒𝑙𝑙𝑠. For nonmagnetic materials, 𝑺𝒄𝒂𝒕. = 𝟎, a parabolic regime for the activation energies (∆𝑯𝒏𝒐𝒏𝒎𝒂𝒈𝒏𝒆𝒕𝒊𝒄 ) versus the orbital filling, 𝒇𝒅, 𝒂𝒄𝒕. reasonably appears for the rupture of covalent bonds. At low 𝒇𝒅 values 𝒉 represents limitations because of the strong adsorption of the reactants, while 𝒇𝒅 increases the electronic repulsions with the atomic number; and 𝒂 and 𝒌 are proportional constants for the reaction and family of catalysts. 𝒐𝒑𝒆𝒏

𝒔𝒉𝒆𝒍𝒍𝒔 In magnetic compositions, the additional spin-dependent ± ∆𝑱𝑸𝑬𝑰 (𝒇𝒅) ⋅ 𝑺𝒄𝒂𝒕.(𝒇𝒅) term 𝒂𝒄𝒕.

appears because of the additional reduction of the adsorption energies of the intermediates, as Eq. 2 indicates, and enhanced (FM) or more limited charge transport (AFM). Magnetic factors make that catalysts with dominant FM interactions have more nobleness and enhanced spinconductivity; what is usually an optimum enthalpic advantage to accelerate reaction rates via the use of abundant metals. In the case of compositions with leading AFM orbital ordering, the 19 can originate higher activation barriers, because of nonlocalizing inter-atomic 𝑄𝑆𝐸𝐼𝑜𝑝𝑒𝑛 𝑠ℎ𝑒𝑙𝑙𝑠

moderate chemisorption enthalpies, characteristically very weak chemisorption for electron donors, and more limited charge conductivity. Overall, Eq. 1 and 2 agree with the experiments and emphasize that the kinetics of charge transfer reactions can be in part rate-controlled by cooperative spin-dependent interactions. The reaction parameters 𝒂, 𝒉 and 𝒌 can be used to represent the overall tendency of a series of catalysts on the same period, at similar reaction conditions. If the activity versus the atomic number, or 𝒇𝒅, almost shows a parabolic shape, with a single activity peak, then the magnetic oscillations are not relevant. However, if the overall activity trend oscillates significantly with 𝒇𝒅, sudden changes in activity with the orbital filling, it is an indication that spin-potentials are playing a significant role, and such changes are introduced 𝒐𝒑𝒆𝒏

𝒔𝒉𝒆𝒍𝒍𝒔 by the correction factor ± ∆𝑱𝑸𝑺𝑬𝑰 . Theory says that the magnetic factor 𝒂𝒄𝒕𝒊𝒗𝒂𝒕𝒊𝒐𝒏(𝒇𝒅) ⋅ 𝑺𝒄𝒂𝒕.(𝒇𝒅)



𝒐𝒑𝒆𝒏

𝒔𝒉𝒆𝒍𝒍𝒔 typically enhances the activity, ― ∆𝑱𝑸𝑺𝑬𝑰 , for dominant FM 𝑄𝑆𝐸𝐼𝑜𝑝𝑒𝑛 𝑠ℎ𝑒𝑙𝑙𝑠 𝒂𝒄𝒕𝒊𝒗𝒂𝒕𝒊𝒐𝒏(𝒇𝒅) ⋅ 𝑺𝒄𝒂𝒕.(𝒇𝒅)

interactions; while the activation barriers might increase for leading AFM localization, + 𝒐𝒑𝒆𝒏

𝒔𝒉𝒆𝒍𝒍𝒔 . ∆𝑱𝑸𝑺𝑬𝑰 𝒂𝒄𝒕𝒊𝒗𝒂𝒕𝒊𝒐𝒏(𝒇𝒅) ⋅ 𝑺𝒄𝒂𝒕.(𝒇𝒅)

After briefly revising the most fundamental and advanced aspects in catalysis associated with strongly correlated electrons. In this work, we analyse the band gap and the specific strong couplings between the electrons and the lattice in magnetic oxides in relation with their intrinsic OER and ORR activities. To advance in the identification of optimal descriptors is an actual relevant topic in oxygen technologies,15 but empathizing also the need of better physical understanding. Results and Discussion. Anti-Jahn-Teller contractions (aJTc) in oxides with outstanding OER activity. Close to their highest stable oxidation states during OER at basic conditions, 𝐹𝑒~4 + , 𝐶𝑜~4 + and 𝑁𝑖~3 + oxides are especially active catalysts;20 in particular for mixed-valence FM structures with good electronic conductivity ligated to spin coherent exchange mechanisms.3 In such cases, due to the partial occupation of the antibonding 3d-orbitals, the spin-oriented electrons delocalize in a degenerate space with a significant impact of 𝑄𝑆𝐸𝐼𝑜𝑝𝑒𝑛 𝑠ℎ𝑒𝑙𝑙𝑠, that reduce the intra- and inter-atomic electronic repulsions. Chemisorbed molecular oxygen (O2*) is not very stable on these oxidized electrophile surfaces. 𝑄𝑆𝐸𝐼𝑜𝑝𝑒𝑛 𝑠ℎ𝑒𝑙𝑙𝑠 help to stabilize the 3d-orbitals and to accumulate FM holes also in the oxygen ligands;4 consequently these FM oxides can rather without difficulty insert lattice oxygens towards the formation of spin-polarized adsorbed O2*.21,22 Correspondingly, the coupling of charge, spin and lattice, typical of strongly correlated materials,23,24 induces a representative response of the crystal structure associated with the electrophile FM configurations: the reduction of the electronic repulsions, due to the excess of inter-atomic Fermi holes, causes an additional shortening of the FM metal-oxygen bonds. Then, the specific selectivity towards OER can be related with extended anti-Jahn-Teller contractions (aJTc), because of the attractive electrostatic potentials as Kamimura et al. showed,25 plus the 4 inter-atomic FM 𝑄𝑆𝐸𝐼𝑜𝑝𝑒𝑛 𝑠ℎ𝑒𝑙𝑙𝑠. These facts let us derive/propose Eq. 3 that uses the experimental

band-gap energies and metal-oxygen bond distances as correlation to compare the relative intrinsic OER overpotentials between 3d-metal oxides. The comparison should be done at similar 4 ACS Paragon Plus Environment

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experimental conditions taking one sample as reference. Eq. 1 and 2 agglutinate all type of compositions, but most importantly Eq. 3 incorporates the observable consequences that the FM Coulomb-Exchange (optimum 𝒇𝒅) offers for outstanding OER electrocatalytic activity in oxides: 𝒄𝒂𝒕𝒂𝒍𝒚𝒔𝒕 charge-spin delocalization (∆𝑯𝒄𝒂𝒕𝒂𝒍𝒚𝒔𝒕 ). 𝒃𝒂𝒏𝒅.𝒈𝒂𝒑) and aJTC (𝑴𝑶𝒃𝒐𝒏𝒅

𝒄𝒂𝒕𝒂𝒍𝒚𝒔𝒕 𝑟𝑒𝑓𝑒𝑟𝑒𝑛𝑐𝑒 𝒄𝒂𝒕𝒂𝒍𝒚𝒔𝒕 Eq. 3) ∆𝐻𝑂𝐸𝑅 ― 𝑀𝑂𝑟𝑒𝑓𝑒𝑟𝑒𝑛𝑐𝑒 ) 𝑟𝑒𝑙.𝑜𝑣𝑒𝑟𝑝𝑜𝑡. ∝ 𝜏 ⋅ (∆𝑯𝒃𝒂𝒏𝒅.𝒈𝒂𝒑 ― ∆𝐻𝑏𝑎𝑛𝑑.𝑔𝑎𝑝 ) + 𝜁 ⋅ (𝑴𝑶𝒃𝒐𝒏𝒅 𝑏𝑜𝑛𝑑

Fig. 1 includes an ample collection of 3d-metal oxides, selected to understand trends in OER, like: LaMO3 (M = Mn, Fe, Co, Ni),15,26,27,28 La1-XSrXCoO3,21,29,30 LaNi0.8Fe0.2O3,31 LaCo0.8Ni0.2O3,32 SrCoO3-δ,33 Co2FeO4,34,35 CaCu3Fe4O1236 and Li0.35CoO2-δ.37 These compositions have been chosen because of their relevance as OER catalysts based on 3d-metals in basic media, so we find activity data at similar reaction conditions, and not because they fit Eq. 3. We obtain that for the ranges 0.1 < 𝝉 (𝑒𝑉) > 0.2 and 3 < 𝜻 (𝑒𝑉 ⋅ Å ―1) > 4, correlations via Eq. 3 are almost inside the experimental error. Eq. 3 correctly places FM compositions as the most active, while in the presence of dominant AFM orderings (red labels in Fig. 1) the OER activity sharply decreases as it occurs for: LaFeO3, LaMnO3, Co2FeO4, CoO, NiO or La3CuFe4O12.2,3 The experimental JahnTeller elongations and band gaps are indicators of the localization of the electron pairs, that in Eq. 3 detect the AFM inter-atomic 𝑄𝑆𝐸𝐼𝑜𝑝𝑒𝑛 𝑠ℎ𝑒𝑙𝑙𝑠.

Figure 1. Relative overpotentials with respect to the intrinsic activity of LaNiO3 in mV according to Eq. 3 versus the experimental M-O bond distances for OER at similar alkaline conditions, if available the experimental activity range is shown with dotted vertical lines. Reaction conditions are like in the excellent study by Hong et al.15 5 ACS Paragon Plus Environment


Eq. 3 allows the fast screening of oxides for OER from experimental values; and it is a mayor advance to rationally progress in the reduction and understanding of the vast number of possible descriptors.15 This approach and achievement of spintro-catalysis and the QSECI-model, unequivocally essential for the universal understanding of all type of electronic structures, should not be a surprise; the activity of magnetic catalysts is not explained by the band theory and the dband model, limited to closed-shells. Goodenough in his outstanding work indicated that in magnetic transition-metal oxides, cooperative spin-potentials can be linked to bond-length and band-gap fluctuations.23 Overall, for any transition metal oxide, 3d- 4d- or 5d-, in any oxidation state or structure, we do not expect that the simple Eq. 3 will be perfect, but let us understand some fundamental tendencies and physical principles. Conductive lattice and longer bonds in outstanding ORR magnetic catalysts. Optimum oxides for ORR should have FM M-O conductive bonds, creating itinerant spin-phases; and in addition a high concentration of centers which can supply electrons to activate adsorbed O2 towards more ionic 𝑂 ― ions, and as in the total oxidation of molecules.38 The most promising oxides are mainly based on mixed-valent 𝑀𝑛~3 + , 𝐶𝑜~3 + , 𝑁𝑖~3 + and 𝑅𝑢~ > 4 + cations;39,40,41,31 since about these low-intermediate oxidation states, they create covalent oxides with open-shell configurations able of acting as electron donors. The relative higher occupation of the frontier antibonding orbitals in the FM conduction bonds in excellent ORR catalysts,1 comparatively versus highly active oxides for OER, naturally rises the electronic repulsions. ORR lattices do not present minimal metal-oxygen bond distances nor Jahn-Teller (JT) distortions: the initial condition for optimal ORR activity is maximum M-O distances in FM bonds, indication of charge-spin conductivity and nucleophilicity. Good electrocatalysts always avoid AFM JT localization in Mott insulators,3 a characteristic added again in Eq. 4 via the experimental band-gap energy. In essence in Eq. 4, we are once more coupling the electronic factors for ORR activity, optimum Coulomb potentials, exchange interactions and kinetic exchange (all together as double exchange) with the experimental lattice and band-gap response.5,42 𝒄𝒂𝒕𝒂𝒍𝒚𝒔𝒕 𝑟𝑒𝑓𝑒𝑟𝑒𝑛𝑐𝑒 𝑟𝑒𝑓𝑒𝑟𝑒𝑛𝑐𝑒 Eq. 4) ∆𝐻𝑂𝑅𝑅 ― 𝑴𝑶𝒄𝒂𝒕𝒂𝒍𝒚𝒔𝒕 ) 𝑟𝑒𝑙.𝑜𝑣𝑒𝑟𝑝𝑜𝑡. ∝ 𝜏 ⋅ (∆𝑯𝒃𝒂𝒏𝒅.𝒈𝒂𝒑 ― ∆𝐻𝑏𝑎𝑛𝑑.𝑔𝑎𝑝 ) + 𝜁 ⋅ (𝑀𝑂𝑏𝑜𝑛𝑑 𝒃𝒐𝒏𝒅


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Figure 2. Relative overpotentials with respect to the intrinsic activity of LaNiO3 in mV according to Eq. 4 versus the experimental M-O bond distances for ORR at alkaline conditions, if available the experimental activity range is shown with dotted vertical lines. Reaction conditions are like in the excellent study by Stoerzinger et al.39 The most relevant oxides specially needed to understand trends for the ORR activity are (Fig. 2): Pb2Ru2O6+δ,43,44 Pb2Ru1.5Mn0.5O6-7,40 LaMnO3+δ,45 LaMO3 (M = Mn, Fe, Co, Ni),15,26,27,28 and LaNi0.8Fe0.2O3.31 We maintain once more 0.1 < 𝝉 (𝑒𝑉) > 0.2 and 3 < 𝜻 (𝑒𝑉 ⋅ Å ―1) > 4 to keep the theoretical correlation almost inside the experimental error. But we cannot quantitatively add the experimental activity ranges for all the samples, at similar basic conditions (vertical dotted lines), qualitatively the ORR relative activity of all the oxides seems correct with the literature. Eq. 4 places FM compositions as the most active, while in the presence of dominant AFM orderings (red labels in Fig. 2) the ORR activity sharply decreases as occurs for: LaFeO3 and LaMnO3.2,3 The experimental JT elongations and band gaps are the indicators of the localization of the electron-pairs.23 Spin-exchange mechanism associated to active oxide electrocatalysis for OER and ORR. During OER or ORR real electron transfers occur between the overlapping orbitals of the reactants and the catalyst; where inter-atomic QSEI in the open-shells determine the responsible FM double exchange mechanism, Fig. 3. In a full analysis via spintro-catalysis, we have the advantage of adapting the Goodenough-Kanamori rules to describe details of the electronic mechanism:3 a) The spin angular momentum is conserved in the real electron transfer. 7 ACS Paragon Plus Environment


b) The Pauli exclusion principle only allows electron transfer to an empty spin-orbital. c) The inter-atomic QSEI is ferromagnetic. d) The expectation value of the electron transfer is spin-dependent. The spin-dependent part 𝒐𝒑𝒆𝒏

of the energy is represented by: ∆𝑱𝑸𝑺𝑬𝑰𝒔𝒉𝒆𝒍𝒍𝒔 ⋅ 𝑺𝒄𝒂𝒕.; so that a specific catalytic exchangeenergy factor appears.

Figure 3. Spin-exchange mechanism in FM bonds on conductive oxides associated to active magnetic electrocatalysis for left) OER and right) ORR. The ligands orbitals (lattice oxygen) participate in the exchange mechanism; while in optimum oxide catalysts for ORR the lattice/surface oxygen 2p-orbitals are full, ideal OER catalysts accumulate holes also in the 2p-orbitals. If we guarantee always a leading FM conductivity for the same metal in the same structure, the higher (lower) the oxidation state better OER (ORR) activity. Also, correspondingly, we will find shorter or longer M-O bonds; we can also understand Fig. 1 and 2 from the radii of the metal cations since, it diminishes with increasing oxidation state. Conclusions. Eq. 3 and 4 offer a reasonable description of the relative OER and ORR electrocatalytic activity for oxides; and they are between the most judicious attempts to derive design factors in catalysis able of providing a link between theoretical and experimental factors. Their key relevance is that will allow the high-throughput screening of materials. Such a vision was possible because the explanation of advanced aspects of the properties of strongly correlated materials from the influence of cooperative 𝑄𝑆𝐸𝐼𝑜𝑝𝑒𝑛 𝑠ℎ𝑒𝑙𝑙𝑠. We insist that with the consideration of spin-potentials, the classical concepts in catalysis remain unchanged, but are better linked with the actual quantum orbital physics.


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Overall, non-classical inter-atomic 𝑄𝑆𝐸𝐼𝑜𝑝𝑒𝑛 𝑠ℎ𝑒𝑙𝑙𝑠 can alter significantly the electronic repulsions, resulting in strong coupling of charge, spin and lattice dynamics in highly active oxygen electrocatalysts. The cooperative spin interactions also introduce mechanistic insights in catalytic electron transfer reactions, only comprehensible from quantum mechanics. The overall description of the explicit influence of the spin of the electron in heterogeneous catalysis is a distinctive on-going research field: spintro-catalysis; that it incorporates strongly correlated materials to the electronic models in catalysis, novel concepts emerge that help in the understanding and design of optimum catalysts. Acknowledgment. MagnetoCat SL gratefully acknowledge significant funding from Syngaschem BV, Eindhoven (The Netherlands) References. (1)

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Table of Contents Graphic.

Spin-exchange mechanism in FM bonds on conductive oxides associated to active magnetic electrocatalysis for left) oxygen evolution reaction and right) oxygen reduction reaction.


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Itinerant Spins and Bond-Lengths in Oxide Electrocatalysts for the

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