Spin Effect on Oxygen Electrocatalysis

including and play a key role in Recent calculations and experimental investigations reveal that the energy loss in ORR/OER is closely related to the triplet O 2 generation/conversion step. the spin-related have long been neglected in understanding of the ORR/OER mechanism. This review highlights recent in understanding and application of the spin-related effect in oxygen electrocatalysis. It is demonstrated that the exchange interaction in magnetic catalysts can build a spin-selective channel to ﬁ lter the electron spins with appropriate orientation in the O 2 gen-eration/conversion step during ORR/OER. It is believed that the introduction of spin effect can establish a more comprehensive understanding of oxygen electrocatalysis and assist designing more reactive oxygen electrocatalysts. 2) Regulating the spin-orbit coupling in heavy metal oxides, such as iridium oxide, [54] to achieve spin-selectivity. Similar to chiral induced spin selectivity, which is essentially the interacting between electron momentum and its spin, the spin-orbit coupling in heavy metal oxides can cause d-band splitting, thus generate spin-polarized currents. 3) In addition to magnetic ﬁ elds, external ﬁ elds such as light, electricity, and heat can be combined to regulate the charge, spin, and orbital coupling of the catalyst and/or the spin arrangement of the reaction intermediates. 4) Spin-polarized density functional theory calculations should be used to accurately describe the spin-related catalytic phenomena and provide more reliable predictions for catalyst designing. [58]

DOI: 10.1002/aesr.202100034 Oxygen involved reactions, including oxygen reduction (ORR) and oxygen evolution reactions (OERs), play a key role in electrochemical energy devices, such as fuel cells and metal-air batteries. Recent theoretical calculations and experimental investigations reveal that the energy loss in ORR/OER is closely related to the triplet O 2 generation/conversion step. However, the spin-related phenomena have long been neglected in understanding of the ORR/OER mechanism. This review highlights recent advances in understanding and application of the spin-related effect in oxygen electrocatalysis. It is demonstrated that the exchange interaction in magnetic catalysts can build a spin-selective channel to filter the electron spins with appropriate orientation in the O 2 generation/conversion step during ORR/OER. It is believed that the introduction of spin effect can establish a more comprehensive understanding of oxygen electrocatalysis and assist designing more reactive oxygen electrocatalysts.
described in Equation (1)-(4), whereas OER proceeds in the reverse direction where * denotes a surface catalytic active site. It is worth noticing that although there are also some ORR/OER mechanisms [80] that propose sequential proton-electron transfer pathways, the basic approach is similar and will not be discussed in detail here. The reaction free energy (ΔG 0 ) for each step can be calculated using where ΔH 0 and ΔS 0 are the enthalpy and entropy changes for each step, respectively. In principle, the step with the most positive adsorption free energy changes determines the overpotential of the overall reaction ( Figure 3a). [39,40,81] During the past decades, many research efforts have been devoted to uncovering the correlation of adsorption energies of the ORR/OER intermediates (Figure 3b), including OOH*, O*, and OH*, [29,39,43,79] which provides a guideline for the exploration and rational design of catalysts to replace the precious metal-based catalysts.
To date, transition metal oxides are considered as the most potential and economical ORR/OER catalysts. [17,32,36,37,44,45,[82][83][84][85][86][87][88] Most of these oxides are magnetics with net spins (unpaired electrons), in which spin-dependent interactions, such as exchange interaction and/or strong spin-orbit coupling, determine the electronic structure of the oxides, [54] thus the ORR/ OER thermodynamics and kinetics. [56] However, the spin effect on the thermodynamics and kinetics of ORR/OER has long been ignored in understanding of the ORR/OER mechanism.

Understanding Spin Effect from Reaction Enthalpy
Recent research efforts demonstrated that the spin effect resulted from the magnetic exchange interaction in catalysts can regulate the electron transfer between the orbitals in the catalysts and the chemisorbed reactants. [56,77] For instance, during the reduction of the triplet state O 2 , the Pauli exclusion principle restricts the transfer of an electron from the orbital of the catalyst to the O 2 orbitals if there is already an electron of the same spin occupying the O 2 2p orbitals ( Figure 4). [56,61,67] In contrast, the exchange interaction in ferromagnetic catalysts enhances the electron transferring with antiparallel spin to the O 2 orbitals. Therefore, the single-oriented electrons of the ferromagnetic catalyst will reduce the reaction barrier of electron transfer between   the catalysts and the reactants by reducing the Coulomb repulsion. [54,55,70] In this case, the reaction enthalpy (ΔH) can be expressed as where ΔH spin is the spin-dependent change of enthalpy. This formula indicates that ferromagnetic ordering in metal oxides helps moderate the binding energies of the reactants through exchange interactions, and correspondingly their catalytic efficiencies improve. [58,89] This is in accordance with Sabatier's principle. [43,90]

Understandings Spin Effect from Reaction Entropy
In addition to reaction enthalpy (ΔH), the spin effect in ferromagnetic catalysts can also affect the reaction entropy (ΔS) in oxygen electrocatalysis. Note that the entropy of each transferred electron in ORR/OER can be described as where the total number of the possible states (g) of the electron depends on the spin degeneracy (g spin ) and the configuration degeneracy (g c ) mainly originating from orbital degrees of freedom, that is, g ¼ g spin ⋅ g c . [67,91] To explain the spin effect on reaction entropy, we compare the first electron-hopping step in ORR from a ferromagnetic oxide (with ordered spins) with that from a paramagnetic oxide (with disordered spins, that is, there are both spin" and spin# states) ( Figure 5). This step involves the electron transfer from the metal d orbital to the triplet O 2 p orbital. Assuming the triplet O 2 are in #O¼O# configuration, according to the Pauli repulsion principle, the electron transferred from the catalyst should be spin", which is antiparallel to the spin state of the triplet #O¼O# molecule. [61] For the ferromagnetic ordering state, electron transfer occurs through extended long-range ferromagnetic exchange interactions, indicating that g spin ¼ 1 before the electron transfer. Therefore, the reaction entropy contributed by spin degeneracy is ΔS spin ¼ 0. On the contrary, for the paramagnetic state, due to the existence of both spin" and spin# electrons, ΔS spin < 0, indicating an entropy reduction process, which is thermodynamically and kinetically unfavorable. Therefore, the ferromagnetic ordering in electrocatalysts is beneficial to accelerate the first electron transfer step in ORR by optimizing the reaction entropy.
It is recently revealed that the overpotential in OER is related to restrictions on the e-spins in generating a ground state triplet O 2 molecule. [50] Undoubtedly, as discussed earlier, magnetic exchange interactions at the catalytic interface can reduce the energy loss in the O 2 generation step by optimizing the entropy and enthalpy through selecting appropriate e-spins. Given that there is a linear relationship between the activation barrier and the reaction free energy (enthalpy and entropy), [78] the spin effect can affect the activation barrier of e-spin steps, thus the kinetics of oxygen electrocatalysis. [54,56,67]   . Schematic diagram of electron transfer process from a ferromagnetic catalyst to an adsorbed triplet-state O 2 molecule with antiparallel spin orientation. Reproduced with permission. [56] Copyright 2018, Elsevier Inc.

Physical Origin of Spin Effect on Oxygen Electrocatalysis
As mentioned earlier, the magnetic exchange interaction will change the enthalpy and entropy of the electron transfer at the catalytic interface, thus accelerate the electron transition process. In essence, electron transfer is a quantum transition in which electron delocalizes from one stationary state (donor) and localizes in another stationary state (acceptor) through thermal activation or tunneling. [92] For thermal activation, electron exhibits incoherent hopping, lose phase information, and needs to cross the energy barrier. [56,69] For tunneling, electron moves from one state to another, maintains a definite phase, and loses negligible energy, guaranteeing a rapid electron transition. [93] The probability of an electron tunneling depends on the relative direction of spins between adjacent atoms, and is proportional to cos(θ/2) (θ is the angle between two spins) (Figure 6a). [94] Therefore, the antiferromagnetic interaction results in zero probability of electron tunneling, whereas the ferromagnetic interaction (all spins are aligned in parallel) gives the greatest probability of electron tunneling, facilitating fast tunneling process. In ferromagnetic materials, subbands with different spins are exchanged and split, and the spin" and spin# subbands are relatively displaced (Figure 6b), resulting in different electron density at the Fermi level, thus generating spin polarization. [93,95,96] In this case, the spin" and spin# electrons experience different tunneling barriers, with the spin" electrons encountering a relatively lower tunneling barrier than the spin# electrons. [97] Therefore, the spin" electrons will be screened out through the ferromagnetic coupling between the metal ions, thereby forming a spin-polarized channel for electron transfer (Figure 6c). [56] At the catalytic interface, the filtered spin" electron will be transferred to the triplet O 2 (#O¼O#) molecule. Therefore, the ferromagnetic exchange interaction in catalysts creates a spinselective channel to filter the "correct" e-spins in ORR/OER (Figure 6d).

Manipulating Spin Selectivity in Oxygen Electrocatalysis
As discussed earlier, possessing a spin-selective channel to filter the "correct" e-spins toward catalytic interface is a unique advantage of electrocatalysts with magnetic ordering structure. In this section, some recent examples of manipulating the spin-selective channel to facilitate charge transfer in electrocatalysis are presented.

Chiral Induced Spin Selectivity
In addition to intermolecular interaction and orbital overlap, [98] spin effect also plays a crucial role in the electron transfer of chiral molecules, which are geniuses to create spin-selective channels to filter e-spins toward catalytic interface. [50,52,53,59,96,[99][100][101][102][103] Noteworthy is that as electrons move along a chiral molecule, they experience the electrostatic potential of the molecule and the inherent magnetic field produced by themselves, which helps screen spins and produces a spin-polarized current. [101] In particular, right-handed and left-handed molecules generate spin" and spin# currents, respectively.
Göhler and co-workers [59] reported the spin-selective transport of electrons through self-assembled monolayers of doublestranded DNA on gold (Figure 7a). By measuring the spin of the transmitted electrons with a Mott polarimeter, they found that the spin polarizations of DNA molecules exceeds 60% at room temperature, and the spin filtration efficiency of the  DNA depends on the length of DNA molecule and its organization structure. This work demonstrates the potential of applying self-assembled monolayers of chiral molecules as very effective spin filters in spintronic applications at room temperature. Naaman and co-workers [50] reported filtering the e-spins by coating the photoelectrode with chiral organic semiconductors. Their magnetic conducting atomic force microscopy measurements showed that the chiral semiconductor has a strong spin-selective ability, so the electrons transferred to the electrode are spin polarized. In this case, the formation of H 2 O 2 , a byproduct in OER, is symmetrically forbidden. Notably, during water splitting, two OH À species must combine to form a triplet oxygen molecule, in which an electron from each OH À is transferred to the electrode, leaving OH· with one unpaired electron. The spinselectivity by chiral molecules causes the spins of two unpaired electrons in OH· to align parallel to produce a triplet O 2 , while prohibiting the generation of the singlet H 2 O 2 (Figure 7b). This results in more efficient O 2 production and longer electrode durability. In addition, Naaman and co-workers [100] assembled chiral molecules on Fe 3 O 4 nanoparticles with large surface area to promote water splitting at high current densities (Figure 7c).

Magnetic Ordering Structure Induced Spin Selectivity
We emphasize earlier that the spin-selective channel generated by ferromagnetic interactions in catalysts plays a key role in the triplet O 2 generation and conversion steps in ORR/OER. It should be noted that not only ferromagnets but also materials with ferromagnetic ordering structure can be ideal spin-selective catalysts. [57,60,61,69,104] Xu and co-workers [57] reported a layered antiferromagnetic inverse spinel oxide LiCoVO 4 (Figure 8a). In each Co-O layer, the spins of Co 2þ are arranged in parallel, whereas the spins of Co 2þ in the adjacent two layers are arranged in reverse, forming the spin" and spin# conduction channels through spin polarization (Figure 8b,c). They claimed that the specific spin-selective channel can help the extraction of electrons with specific spin orientation, facilitating the generation the triplet O 2 molecules (Figure 8d). Figure 6. a) Schematic diagram of the spin orientations of adjacent ions. b) Schematic diagram of band splitting of ferromagnetic catalyst due to exchange interaction. c) Electron-tunneling process in spin-polarized channel for ORR and OER. Reproduced with permission. [56] Copyright 2018, Elsevier Inc. d) Schematic diagram of spin filtering effect.

Magnetic Field Introduced Spin Selectivity
In addition to the intrinsic magnetism of the catalysts, the introducing of an external magnetic field can also induce the spin selectivity in ORR/OER. This is because the magnetic field can cause the spin flip of the magnetic ions in catalysts and the reaction intermediates with net spins, therefore optimizing the reaction path and improving the reaction efficiency. [105][106][107][108] Galán-Mascarós and co-workers [109] investigated the effect of external magnetic fields on the OER performance of nonmagnetic, ferromagnetic, and antiferromagnetic catalysts (Figure 9a-d). The magnetic field enhancement was observed in the ferromagnetic and antiferromagnetic catalysts, which should be attributed to the influence of the external magnetic field on the O 2 generation step. That is, the magnetic field promotes the parallel alignment of net spins in intermediates and Figure 7. a) Schematic diagram of using monolayer of DNA as a spin filter. Reproduced with permission. [59] Copyright 2011, American Association for the Advancement of Science. b) Schematic diagram of production of H 2 O 2 and O 2 on achiral and chiral molecules, respectively. Reproduced with permission. [50] Copyright 2017, American Chemical Society. c) Schematic diagram of chiral molecules chemisorbed on Fe 3 O 4 nanoparticles to facilitate photoelectrochemical water splitting. Reproduced with permission. [100] Copyright 2018, American Chemical Society.  [57] Copyright 2020, Wiley-VCH GmbH.
www.advancedsciencenews.com www.advenergysustres.com products, such as #O…O# and #O¼O#. We take NiO, an antiferromagnetic catalyst, as an example (Figure 9e). Notably, OER at high pH occurs through the interaction of two O atoms adsorbed on two surface metal sites of oxides, during which the spins of the two O atoms would be either in parallel or antiparallel. When the spins of the two Ni atoms are in antiparallel, the intermediate "O…O# and the product "O¼O# will be in singlet-state configurations with high energy. Under the external magnetic field, the spins of Ni atoms will be aligned in a parallel fashion, thus the intermediate #O…O# and the product #O¼O# will turn to be triplets with low energy. Their theoretical computation verified that this spin-selected reaction pathway induced by magnetic field is indeed thermodynamically favored.

Rational Design Oxygen Electrocatalysts based on Spin Effect
Aforementioned examples highlight the role of manipulating the spin-selective channels in promoting ORR/OER. However, strong magnetism of the magnetic catalyst does not guarantee high reactivity because both spin and space conditions are required for electron transfer at the catalytic interface between the catalyst and the reactants/intermediates. For example, in ZnMn 2 O 4 , although the spin selection is satisfied, the orbitals of Mn 3d above the Fermi level are the 3d xy and 3d yz orbitals, which have weak πÀπ interactions with the adsorbed OH À , thus restricts the interfacial electron transfer. [69] In the past few decades, considerable research efforts have been made to propose appropriate descriptors to describe the binding energy of ORR/OER intermediates on the metal and oxide surfaces, and various volcano-shaped relationships have been established to predict the most active ORR/OER catalysts ( Figure 10). [39,40,43,79] In fact, all the catalysts at the top of the volcano plots possess ideal space condition for electron transfer at the catalytic interface. For example, the pioneer work by Shao-Horn and co-workers [44,45] suggest that the e g orbitals of transition metal in perovskite interact with the 2p orbitals of the oxygen to stabilize the ORR/OER intermediates, and e g filling % 1 is optimum for the best activity (Figure 10b).
We would like to point out here that for the catalysts at the top activity of the volcano plots, if the reaction enthalpy and entropy are further optimized by spin effect, the catalyst activity is highly expected to be improved beyond the volcano peak, surpassing precious metal-based catalysts. [56,67]

Conclusions and Perspectives
This review highlights recent advances in understanding and application of the spin effect in oxygen electrocatalysis. It demonstrates that spin effect can build a spin-selective channel to filter the "correct" e-spins for O 2 generation/conversion step in ORR/OER, thus optimize the entropy and enthalpy of electron transfer at the catalytic interface between the catalyst and the reactants/intermediates. Undoubtedly, incorporating spin effect can establish a more comprehensive understanding of oxygen electrocatalysis, and help design more active oxygen electrocatalysts. Future research directions in this field include but are not limited to the following: 1) simple and operable methodology, such as heteroatom doping, strain control, or size limitation, should be developed to tailor the magnetic ordering structure of electrocatalysts, thus effectively adjust the spin-selective channel to filter the "correct" e-spins toward the catalytic interface. Figure 9. a) Bar diagram with the maximum magnetocurrent observed for the various magnetic OER catalysts. b) Correlation between the maximum relative magnetocurrent and bulk magnetization. c,d) Linear sweep voltammetry curves and current density-time (I-t) of NiZnFe 4 O x particles decorated Ni-foam electrode recorded with and without an applied magnetic field. e) OER mechanism on NiO surface with the spin antiparallel and parallel pathways. (a-e) Reproduced with permission. [109] Copyright 2019, Springer Nature.
www.advancedsciencenews.com www.advenergysustres.com 2) Regulating the spin-orbit coupling in heavy metal oxides, such as iridium oxide, [54] to achieve spin-selectivity. Similar to chiral induced spin selectivity, which is essentially the interacting between electron momentum and its spin, the spin-orbit coupling in heavy metal oxides can cause d-band splitting, thus generate spin-polarized currents. 3) In addition to magnetic fields, external fields such as light, electricity, and heat can be combined to regulate the charge, spin, and orbital coupling of the catalyst and/or the spin arrangement of the reaction intermediates. 4) Spin-polarized density functional theory calculations should be used to accurately describe the spin-related catalytic phenomena and provide more reliable predictions for catalyst designing. [58] Figure 10. a) OER activity trends as a function of ΔG O* -ΔG OH* in metal oxides. Reproduced with permission. [43] Copyright 2011, Wiley-VCH GmbH. b) ORR activity trends as a function of e g orbital in perovskite-based oxides. Reproduced with permission. [45] Copyright 2011, Springer Nature.