Altering oxygen binding by redox-inactive metal substitution to control catalytic activity: oxygen reduction on manganese oxide nanoparticles as a model system
Electrocatalytic transformations of oxygen, i.e., the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER), are key processes in renewable energy conversion, defining to a large extent, the efficiency of numerous energy conversion technologies, such as fuel cells, metal-air batteries or water electrolyzers. However, the development of highly effective, stable and inexpensive materials for such conversion processes is a bottleneck. Hence, establishing generic catalyst design principles by identifying structural features of catalysts that influence their performance would constitute a major step towards the rational engineering of advanced electrocatalysts. In this study, by investigating a series of metal-substituted manganese oxide (spinel), Mn3O4:M (M = Sr, Ca, Mg, Zn, Cu), nanoparticles as a model system, we demonstrate experimentally and rationalized the dependence of the activity of Mn3O4:M for ORR and the oxygen binding strength in Mn3O4:M oxides on the properties of the M substituent, viz. the enthalpy of formation of the binary MO oxide and the Lewis acidity of the M 2+ substituent. Incorporation of elements M that have a low enthalpy of formation of MO (i.e., highly exothermic oxides featuring relatively strong M-O bonding) enhances the oxygen binding strength in Mn3O4:M, which increases its activity in ORR due to the established correlation between ORR activity and the binding energy of *O/*OH/*OOH species on the catalyst surface. Our work provides a new perspective on the design of new compositions for oxygen electrocatalysis relying on the substitution by redox-inactive elements affecting the binding energy of oxygen to the surface of the complex metal oxide catalysts (ORR/OER activity descriptor). We speculate that this concept is general and transferrable to a broad selection of materials and processes involving oxygen adsorption and redox, beyond electrocatalysis.
INTRODUCTION<!>Synthesis and characterization of Mn3O4:M nanoparticles<!>Oxygen reduction activity of the Mn3O4:M series<!>Reactivity trends: theoretical considerations<!>General implications for catalysts design<!>CONCLUSIONS
<p>Increasing the penetration of renewable energy production and storage technologies in energy systems is one of the major global challenges. [1][2][3] One of the most crucial elements towards this goal is the development of efficient catalysts for electrochemical energy conversion processes such as the oxygen reduction reaction (ORR) and the reverse oxygen evolution reaction (OER), which are defining the efficiencies of water-splitting electrolyzers, fuel cells and metal-air batteries. [4][5][6][7][8][9][10][11][12] Yet, despite decades of research in this area, even the best known catalysts operate at relatively high overpotentials. The origin of this limited efficiency stems from scaling relations between the free energies of key ORR/OER intermediates and transition states that causes them to change in a concerted manner in response to variations in the catalyst structure. Hence, even an optimum balance between adsorption energies inevitably yields a reaction overpotential. [13][14][15][16][17] Importantly, these scaling relations are fundamental properties of the reaction intermediates sharing the same binding site and are independent of the nature of the catalyst surface. 18 Extending the descriptor approach beyond correlations based on the adsorption energies of the reaction intermediates (which are derived from theoretical calculations and are difficult to assess experimentally), relationships between the bulk properties of the material and its surface reactivity have also been proposed. This has led to the introduction of a series of descriptors correlating electronic band structure features, [19][20][21] electron count, 22 molecular orbital occupancy, 23,24 redox and thermochemical properties [25][26][27][28][29] to the surface reactivity of the electrocatalyst.</p><p>Despite these advances in the fundamental understanding of structure-activity relationships in oxygen electrocatalysis, the above descriptors are typically applicable to only a restricted set of material families, and/or correlate the catalytic activity with properties (e.g., O 2p band centre, charge transfer energy, adsorption energy of O2, etc.), which are complex, convoluted functions of the bulk structure of a catalyst. Therefore, establishing a structural feature of a material, which can be tailored in a straightforward manner to directly impact the catalyst performance, would constitute a major step towards the rational engineering of advanced catalysts for oxygen electrocatalysis.</p><p>In this study, we set out to rationalize and generalize the effect of the incorporation of redox-inactive elements in the structure of metal oxides on their activity in oxygen electrocatalysis. Mn3O4 was selected as a model catalyst system due to its high activity in oxygen reduction and because the Mn3O4 stoichiometry represents the actual chemical state operating at ORR potentials 30 (justifying 2 the choice of Mn3O4 as a model for the theoretical studies discussed below). We established that the activity of M-substituted Mn3O4:M (M = Sr, Ca, Mg, Zn, Cu) nanoparticles in ORR can be varied by nearly one order of magnitude by incorporation of as little as ca. 2.5 at.% of M into the structure of Mn3O4; the substitution of Mn by Sr, Ca or Mg increases the ORR activity, while the incorporation of Zn or Cu results in a decrease of the ORR activity with respect to unsubstituted Mn3O4. The observed trends with regard to Mn substitution by M are due to the correlation between the enthalpy of formation of the binary oxide MO (ΔfH°(MO)) and the binding energies of ORR intermediates (*O/*OH/*OOH species) on the catalyst surface, which have been established in previous studies as a descriptor for the catalytic activity of metal oxides in oxygen redox reactions. 13,14,17 Our finding provides an effective and universal tool for tailoring the catalytic properties of catalysts for ORR/OER: the introduction of redox-inactive metal substituents/dopants into the host metal oxide catalyst enables a fine tuning of the binding energies of reaction intermediates, which define the position of the catalyst in the ORR/OER activity volcano diagram. 31 We believe this principle is general and applicable to various oxide/hydroxide families and processes concerning oxygen adsorption and/or redox reactions involving oxygen.</p><!><p>To study the effect of the incorporation of redox-inactive metal substituents into oxides on their activity for ORR, we have selected the Mn3O4 oxide family as a model system owing to the ability of the spinel structure to readily accommodate different elemental substitutions, 32 the high activity of Mn3O4 for ORR, 30,33 and the fact that the Mn3O4 stoichiometry represents the actual chemical state of MnOx catalysts at ORR potentials. 30 The metal substituents for the Mn3O4:M (Fig. 1a) series (M 2+ = Sr 2+ , Ca 2+ , Mg 2+ , Zn 2+ , Cu 2+ ) were selected to cover elements with pKa([M(H2O)n] 2+ ) and enthalpies of MO oxide formation (ΔfH°) (see Table S1 in the Electronic Supplementary Information, ESI) that exceed and fall below the respective values of Mn 2+ (i.e., pKa([Mn(H2O)n] 2+ ) and ΔfH°(MnO)). The Mn3O4:M nanoparticles were prepared by colloidal synthesis following a literature procedure 34 using a nominal mass loading of 2.5 at.% of the M substituent with respect to the manganese content. For comparison, Mn3O4:Sr nanoparticles with a higher loading (i.e., 5 at.% of Sr) were also synthesized. Higher concentrations of the substituent were not explored in Mn3O4:M series, as in this case the synthesis yielded particles of irregular size and shape distribution. Below, we focus on a detailed structural analysis of Mn3O4:Sr and Mn3O4:Cu, as these compositions demonstrate the highest and lowest ORR activity, respectively, within the Mn3O4:M series.</p><p>Annular dark-field scanning transmission electron microscopy (ADF-STEM) and high-resolution transmission electron microscopy (HR-TEM) imaging of the as-synthesized nanoparticles (Fig. 1b-d) revealed their high monodispersity with an average particle size of the different M substituted nanoparticles in the range 3-5 nm (Fig. S1, Table S2). Fig. 1d (left panel) shows an atomic resolution ADF-STEM micrograph of a Mn3O4:Sr nanocrystal oriented along the <100> zone axis. The bright spots correspond to atomic columns of Mn and Sr, whereas the oxygen sub-lattice was not registered by the ADF detector due to its low scattering power and the different angular distribution of the scattered electrons. Direct identification of the Sr sites proved to be challenging, as we could not observe any noticeable contrast change in the local atomic rows corresponding to the M 2+ sites due to the low amount of Sr incorporated and its high dispersion. The atomic arrangement is consistent with a spinel model for the cation sub-lattice as visualized by the overlay of the ADF-STEM image with the respective structural model (Fig. 1d, right panel). Along the <100> zone axis, the spinel structure contains close-packed planes with [MnO6] octahedral (Mnoct) layers alternating with mixed [MnO6] and [MnO4] tetrahedral (Mntet) layers. The detailed ADF-STEM analysis of the particle surface (Fig. 1d, S2) revealed that the majority of the terminating layers were composed exclusively of Mnoct. Similar terminations of closed packed layers with cations in octahedral positions have been observed previously for several isostructural systems, such as γ-Ga2O3, γ-Fe3O4 and γ-Al2O3. [35][36][37] In line with experimental observations, DFT calculations (see discussion below) indicate that the surface terminated by Mnoct is thermodynamically the most stable surface.</p><p>X-ray diffraction (XRD, Fig. S3) and electron diffraction (Fig. 1e) patterns also confirm the presence of a nanocrystalline spinel structure and the absence of any detectable amounts of secondary phases. The low content of M incorporated into the spinel structure (ca. 2.5 at.% from inductively coupled plasma (ICP) spectroscopy analysis, see Table S2) precludes a noticeable shift of the diffraction peaks. In addition, energy dispersive X-ray spectroscopy (EDX) of the sample Mn3O4:Sr evidenced a homogeneous distribution of the elements in the synthesized material (Fig. S4), providing further evidence for the successful incorporation of M 2+ ions into the Mn3O4 spinel structure. Additionally, Raman spectroscopy that is sensitive to variations of the manganese oxides composition 38 confirmed the formation of the Mn3O4 spinel structure (Fig. S5).</p><p>A comparison of the pair distribution functions (PDF) obtained from electron diffraction data is shown in Fig. 1f. The PDF peaks located at 1.44 Å and 2.48 Å are due to the presence of an amorphous carbon film in the TEM sample grid. The first pair correlation peak at ca. 2 Å is due to Mn-O distances, the second PDF peak at ca. 3 Å is due to edge-sharing Mn 3+ -Mn 3+ octahedra, while the third and the fourth PDF peaks (3.45 Å and 3.75 Å) are mainly due to the contribution from Mn-Mn corner-sharing polyhedra. Notably, the PDF peak positions and their intensities ratio are the same for Mn3O4, Mn3O4:Sr, Mn3O4:Cu indicating that the local atomic arrangement remained unperturbed by the substitution of Mn by M. X-ray absorption spectroscopy (XAS) on the Mn K-edge was applied to probe the average oxidation state and local environment of the manganese sites in Mn3O4:M. The shape of the white line in the X-ray absorption near edge structure (XANES) spectrum of Mn3O4:M nanoparticles showed only slight differences when compared to that of the commercial bulk Mn3O4 reference (Fig. S6). We observed a small yet distinguishable (ca. 0.2 eV) shift of the position of the absorption edge towards high energies for all Msubstituted Mn3O4:M materials with respect to the unsubstituted Mn3O4 nanoparticles, as well as the rise of the white line upon substitution (Fig. 1g), consistent with the replacement of Mn 2+ sites in the spinel structure with M 2+ ions and an associated increase of the average oxidation state of manganese (owing to an increased ratio of Mn 3+ :Mn 2+ sites). In line with this argument, the smallest change in the edge position and increase in the intensity of the white line was observed for the Mn3O4:Cu sample that contained the smallest quantity of the substituent in Mn3O4:M series (1.0 at.%, Table S2). A slight decrease in the pre-edge peak for the Msubstituted materials suggests an increase in the average symmetry of the sites, possibly this was due to the increased fraction of Mntet sites (that are less distorted than the Mnoct sites).</p><p>Extended X-ray absorption fine structure (EXAFS) analysis was used to probe the local environment of the manganese sites in the Mn3O4:M materials (Fig. 1h). The fitting of the experimental EXAFS data (Fig. 1h) for the Mn3O4:M series are in good agreement with the crystallographic model of the Mn3O4 spinel 39 (Fig. 1a) and our PDF analysis (Fig. 1e). The structural model consists of two Mn sites, in tetrahedral (Mntet) and octahedral (Mnoct) coordination with O, which were used to generate two series of scattering paths. Fixing the coordination numbers, allowed us to determine the Mn-O (within the polyhedra) and Mn-Mn (between the polyhedra) distances. The first coordination shell is composed of three Mn-O scattering paths corresponding to Mntet-O and two non-equivalent Mnoct-O subshells that are due to the highly distorted octahedra, while the second sphere is dominated by Mnoct-Mnoct and Mnoct-Mntetra paths (Table S3, Fig. S7). Although, the element substitution is expected to affect the covalency of the Mn-O bond (hence also the bond length) due to the inductive effect of M, 40 structural changes associated with the substitution of Mn by M are likely too subtle to be traced by EXAFS analysis: indeed, for the entire Mn3O4:M series, all Mn-O bond lengths are virtually undistinguishable within experimental uncertainty (0.01-0.03 Å). Similarly, small changes in the XANES caused by the substitution of Mn 2+ by M 2+ (i.e., shift of the edge position) make it challenging to quantify the effect of differences in the electron withdrawing strength of M on the electronic state of manganese. In line with XAS analysis, Mn 2p core level X-ray photoelectron spectroscopy (XPS) (Fig. S8) does not demonstrate any pronounced effect of substituent M in Mn3O4:M on the Mn states as well.</p><p>In contrast, XPS measurements of the O 1s core levels allowed us to confirm the inductive effect of M as evidenced by a shift of the binding energy of the lattice oxygen component (by ca. 0.2 eV) to lower binding energies when comparing Mn3O4:Sr to Mn3O4:Cu (Fig. S9). This shift is indicative of a larger negative charge on the oxygen sites in Mn3O4:Sr compared to Mn3O4:Cu, which is in line with an increased electron donating strength (i.e., a reduced Lewis acidity) of strontium with respect to copper (the trend holds well for the entire substitution series, Fig. S10).</p><p>Overall, our detailed structural characterizations of the series of Mn3O4:M nanoparticles demonstrate the formation of phase-pure, M-substituted Mn3O4 oxides. The incorporation of M had a negligible effect on the crystal structure, the nanoparticle morphology and the manganese valence states, making this series of materials a reliable model system to decouple and investigate the individual effect of the different M substituents on the electrocatalytic activity of the material.</p><!><p>The ORR activity of the series of Mn3O4:M nanoparticles was assessed in a conventional three-electrode electrochemical setup using 0.1 M KOH as the electrolyte, whereas the H2O2/H2O selectivity was estimated using rotating ring-disk electrode measurements (RRDE, see ESI for the experimental details). Intrinsic ORR activities were assessed using steady-state galvanostatic measurements (to eliminate the contribution of redox processes within the bulk of the electrode material, which can interfere with the measured catalytic currents in conventional CV studies, 41 Fig. S11). The currents were normalized to the geometric surface area of the nanoparticles estimated by TEM (see Fig. S1, Table S2). As the CV curves for Mn3O4:M samples do not feature a well-defined limiting current plateau, our analysis focused on the current range that was much smaller than the mass transport limit, where the mass transport correction can be neglected (for Tafel plots for extended current range, see Fig. S12).</p><p>The Mn3O4:M nanoparticle series studied here feature mass and electrode surface area normalized activities (Fig. S13) that are comparable to previously reported activities of MnOx catalysts. 42 The specific activities (per oxide surface area) are similar to other high surface area MnOx oxides (ca. 1 µA cmoxide -2 at 0.8 VRHE), 43,44 but below the activities typically reported for non-porous highly crystalline materials (> 10 µA cmoxide -2 at 0.8 VRHE). 42 Remarkably, we found that the measured ORR activity depends strongly on the type of the M substituent. Indeed, incorporation of 2 at.% of Sr increased the ORR activity by almost an order of magnitude compared to Mn3O4:Cu (Fig. 2a).</p><p>Importantly, as described above, this was achieved without changing the crystal structure, particle morphology, nor the valence state of manganese. This suggests that there exist direct correlations between certain intrinsic properties of the substituting elements and the catalytic performance of Mn3O4:M. In fact, the influence of the incorporation of redox-inactive substituents on the redox properties of biological systems 45 and molecular complexes has been reported [46][47][48][49][50] and the observed trends have been rationalized typically by an inductive effect triggered by the substituent/dopant altering the covalency of the bonds between the parent metal centers and adjacent oxygen . 40 More recently, the concept of the inductive effect was extended further to explain the effect of substituents on the redox properties (and in turn the catalytic performance) of complex oxide materials. 40 Although many studies have explored the effect of metal substitution in oxides on their catalytic performance, changes in the catalytic performance have been attributed typically to changes in the valence state of the active site (for aliovalent substitution), 51,52 the oxide crystal structure, 53 or the introduction of crystallographic strain, 54 i.e., effects originating from the mismatch of the size or the formal charge of the substituent and the substituted site (note that in this context, we do not consider systems for which the substitution results in a change of the reaction site, e.g., from Fe to Mn in MnxFe3-xO4 55 ). On the other hand, there are very few experimental works that aim to leverage electronic, or inductive, effects arising from the incorporation of substituents to rationally tailor the electrocatalytic performance of oxides. [56][57][58] Hence, establishing a unified description of the effect conferred by the introduction of redox-inactive metals into a host oxide on their catalytic ORR/OER activity (irrespective of their structure) would provide a new perspective (and experimental tool) for the design of materials for targeted electrochemical transformations. O2-saturated 0.1 M KOH, 1600 rpm, oxide loading 0.1 mg cm -2 ; no mass transport correction was applied. (b) Scaling between the ORR activity expressed as potential to achieve a current density of 1 µA cm -2 (normalized to the oxide surface area estimated by TEM measurements, see Table S2) and pKa ([M(H2O)n] 2+ ) 59 (measure of Lewis acidity) and enthalpy of formation of the binary oxide, MO, ΔfH°(MO). 60 (c) ORR selectivity towards H2O2 production in terms of Faradic efficiency and ORR electron transfer number (ne-) obtained from rotating ring-disk electrode (RRDE) measurements as a function of the electrode potential. Data are obtained from galvanostatic measurements at 900 rpm. Labels on the plots refer to the specific M in the Mn3O4:M series.</p><!><p>Next, we aim to assess the correlation(s) between the ORR activity of the Mn3O4:M nanoparticles (Fig. 2a) and the properties of the substituting element M. First, we observed a strong correlation between the ORR activity of Mn3O4:M and the Lewis acidity of the substituting M 2+ ions (expressed as pKa of the corresponding hydrated ion, [M(H2O)n] 2+ , Fig. 2b), implying that the Mn-O bond covalency (tuned by the inductive effect of M) controls the catalytic activity of the material. As discussed above, XPS O 1s core level measurements confirmed the inductive effect of M reflected by a larger negative charge on the oxygen sites in Mn3O4:Sr compared to Mn3O4:Cu owing to the greater electron donating strength (i.e., a reduced Lewis acidity) of strontium with respect to copper. This robust correlation between the ORR activity and the pKa([M(H2O)n] 2+ ) provides clear evidence for the relevance of the Lewis acidity of the redox-inactive substituent on the electrocatalytic activity of Mn3O4 (in a related case, the oxygen exchange kinetics of Pr0.1Ce0.9O2-δ were also shown to depend on the acidity [on the Smith scale] of the surface-infiltrated oxides species, 61 yet the catalytic activities in ORR/OER reported for a wide row of oxides did not demonstrate a correlation to the Smith acidities of the respective oxides 62 ). However, to construct a clear and direct link between the properties of M and the catalytic activity of an oxide in terms related to the mechanism of oxygen electrocatalysis, we further analyzed the activity and selectivity (Fig. 2c) trends. As the ORR/OER mechanism includes the formation/cleavage of metal-oxygen bonds (whereby their binding strength controls the catalytic activity), 13 it is expected that the energy of oxygen vacancy formation [E(Ovac)] also scales with catalytic activity. 29 Indeed, the calculated values for E(Ovac) (for O atoms bound to octahedrally coordinated Mn atoms serving as the active sites in spinel oxides 63 ) in the vicinity of M in a Mn3O4:M model slab (Fig. 3a) demonstrate a good correlation with the experimentally measured activities (Fig. 3b), hence confirming a relationship between the catalytic activity and the oxygen binding strength on the catalyst surface. As a consequence, we observe an excellent correlation between the ORR activity of Mn3O4:M nanoparticles and the enthalpy of formation of the binary oxide, MO (ΔfH°(MO), Fig. 2b), which is rooted in the dependence of the binding strength of the oxygen species on the oxide surface on ΔfH°(MO) (Fig. 3c): incorporation of an element with a more negative ΔfH°(MO) (i.e., a thermodynamically more stable oxide) increases the binding strength of oxygen species (*O/*OH/*OOH) on the surface of the multimetallic compound (Mn3O4:M). It is important to note here, that although the ORR activity of Mn3O4:M features a nearly linear correlation with both ΔfH°(MO) and pKa([M(H2O)n] 2+ ) (due to the strong scaling between ΔfH°(MO) and pKa([M(H2O)n] 2+ ) values for a given set of M studied here), we believe ΔfH°(MO) provides a more accurate estimation of the changes of the oxygen binding energy/reactivity upon the substitution of M into the oxide lattice. This rationale follows from our previous study 57 , in which the OER activity of the Y1.8M0.2Ru2O7 pyrochlore family revealed a strong correlation with ΔfH°(MOx) (which can also be related to the change of the binding energies of oxygen species, similar to the current study), but not with pKa([M(H2O)z] n+ .</p><p>The correlations established here demonstrate that elemental substitution/doping can be used as a highly versatile tool to rationally tune the activity of oxide/hydroxide catalysts for ORR and OER, whereby the most appropriate choice of the substituent/dopant depends on the rate-limiting step of the catalytic reaction. Specifically for the commonly quoted associative mechanism of the oxygen reduction reaction, 13 the reaction rate is limited by the adsorption of either *OH (in case of strong oxygen binding) or *OOH (weak oxygen binding). 64 Therefore, in the latter scenario by increasing the oxygen binding strength by incorporating elements with a highly negative ΔfH°(MOx) an activity enhancement would be expected. As shown previously, ORR on a Mn3O4 surface is indeed limited by *OOH adsorption (the least exothermic step at U = 0), 65 implying that an increase of the oxygen adsorption strength would improve its activity. Our experimental results for the Mn3O4:M system (with M = Sr, Ca, Mg) confirm this rationale. Note that as little as 2 at.% of the substituent was sufficient to yield an increase in ORR activity by an order of magnitude (Mn3O4:Sr compared to Mn3O4:Cu); increasing the concentration of Sr substitution from 2 at.% to 5 at.% improved the activity of Mn3O4:Sr further (Fig. S14). (As mentioned above, materials with higher concentrations of M were not studied, as in this case the synthesis yielded particles of irregular shape and size distribution.)</p><p>Interestingly, the substitution of Mn by M does not only affect the ORR activity, but also the H2O2/H2O (2e -/4e -) selectivity. Indeed, the Faradaic efficiency towards the 2ereduction (H2O2 synthesis) increased from ca. 5% for M = Sr, Ca, Mg, Mn (catalytically more active oxides in the Mn3O4:M series) to ca. 8% for M = Zn, and ca. 25% for M = Cu at high overpotentials (Fig. 2c). Such a selectivity trend was predicted for ORR catalysts characterized by a weak oxygen binding strength (i.e., the reaction is limited by O2 adsorption to form *OOH, as in the case of Mn3O4 65 ), whereas a further decrease of the adsorption energy was proposed to enhance the 2ereduction selectivity. 66 This experimental observation provides additional evidence for the interplay between the properties of the redox-inactive substituent (e.g., ΔfH°(MOx)), the adsorption strength of catalytic intermediates (*O/*OH/*OOH), and the ORR activity and selectivity.</p><!><p>Despite its simplicity, the concept described here has not been studied systematically or considered as a general strategy for rational catalyst design. While previous studies probing correlations between thermochemical properties of bulk oxides and their activity in ORR/OER electrocatalysis 25,26,29 attempted to explain the reactivity trends for certain oxide families, our findings can be used to instruct the design of catalyst compositions across different material families by the judicious choice of a substituent/dopant. It should be noted here that the incorporation of substituents/dopants into the lattice of the host oxide may introduce multiple effects, such as the generation of crystal lattice strain and distortion, modification of the crystal structure, change of spin or the valence state of the active site, etc., which can have counteracting effects on the catalytic activity, and therefore have to be considered as well. Yet, for a large variety of oxide families that are capable of accommodating a broad range of cationic species without leading to noticeable structural changes, and particularly for structurally non-rigid (amorphous) metal (oxyhydr)oxides, the concept introduced here offers a potent tool for the design of highly active materials for oxygen electrocatalysis, and can be applied to engineer compositions with benchmark performances starting from current state-of-the-art materials (e.g., RuO2/IrO2/Ni-Fe oxyhydroxides), however, this is yet to be demonstrated. We believe this principle is general and can be transferred to a broad selection of materials and processes concerning oxygen adsorption and redox reactions involving oxygen, reaching beyond electrocatalysis, such as metal-ion batteries, 67 chemical looping, 68 or heterogeneous catalysis. 69</p><!><p>By studying a series of model, metal-substituted, manganese oxide spinel nanoparticles, Mn3O4:M (M = Sr, Ca, Mg, Zn, Cu), we established a direct correlation between the enthalpy of formation of the binary MO oxide (ΔfH°(MO)), the Lewis acidity of the M 2+ substituent (expressed as pKa of [M(H2O)n] 2+ ion) and the activity (and selectivity) of the Mn3O4:M family in the oxygen reduction reaction. Incorporation of elements with a low (highly negative) ΔfH°(MO) enhances the oxygen binding strength in Mn3O4:M, as confirmed by XPS and DFT calculations (revealing a strong correlation between the experimental ΔfH°(MO) values and the energy of oxygen vacancy formation E(Ovac) on the Mn3O4:M surfaces). This implies that there is also a correlation between ΔfH°(MO) and the binding energies of intermediate oxygen species (*O/*OH/*OOH) on the catalyst surface, which serve as key descriptors for ORR activity. Our observations concerning the effect of M substitution of Mn3O4 on its ORR activity, as well as previous studies on Y1.8M0.2Ru2O7 pyrochlores in OER catalysis, 57 demonstrate that the introduction of redox-inactive metal substituents/dopants into the host structure of a metal oxide is a highly versatile approach to tune the binding strength of reaction intermediates (determining the position of the catalyst on the ORR/OER activity volcano), and in turn improve the ORR/OER activity and indeed also its selectivity. We believe this principle is general and can be employed to tailor the properties of a broad variety of oxide materials for diverse applications that reach beyond electrocatalysis associated with oxygen adsorption and/or redox reactions involving oxygen.</p>