Electrocatalytic Site Activity Enhancement via Orbital Overlap in A₂MnRuO₇ (A = Dy³⁺, Ho³⁺, and Er³⁺) Pyrochlore Nanostructures

Abstract

Oxygen electrocatalysis at transition metal oxides is one of the key challenges underpinning electrochemical energy conversion systems, involving a delicate interplay of bulk electronic structure and surface coordination of the active sites. In this work, we investigate for the first time the structure-activity relationship of A2RuMnO7 (A = Dy 3+ , Ho 3+ , Er 3+ ) nanoparticles, demonstrating how orbital mixing of Ru, Mn, and O promotes high density of states (DOS) at the appropriate energy range for oxygen electrocatalysis. The bulk and surface structure of these multicomponent pyrochlores are investigated by high-resolution transmission electron microscopy, X-ray diffraction, X-ray absorption (XAS), X-ray emission (XES) and X-ray photoemission (XPS) spectroscopies. The materials exhibit high phase purity (cubic fcc with a space group Fd3 � m), in which variations in M-O bonds length are less than 1% upon replacing the A-site lanthanide. XES and XPS show that the mean oxidation state at the Mn-site as well as the nanoparticle surface composition were slightly affected by the lanthanide. The pyrochlore nanoparticles are significantly more active than the binary RuO2 and MnO2 towards the 4-electron oxygen reduction reaction (ORR) in alkaline solutions. Interestingly, normalization of kinetic parameters by the number density of electroactive sites concludes that Dy2RuMnO7 shows twice higher activity than benchmark materials such as LaMnO3. Analysis of the electrochemical profiles supported by DFT calculations reveals that the origin of the enhanced catalytic activity is linked to the mixing of Ru and Mn d-orbitals and O p-orbitals at the conduction band which strongly overlap with the formal redox energy of O2 in solution. The activity enhancement strongly manifests in the case of Dy2RuMnO7 where Ru/Mn ratio is closer to 1 in comparison with the Ho 3+ and Er 3+ analogs. These electronic effects are discussed in the context of the Gerischer formalism for electron transfer at the semiconductor/electrolyte junctions.

Relationship between Mn Oxidation State Changes and Oxygen Reduction Activity in (La,Ca)MnO3 as Probed by In Situ XAS and XES

“…Previous XES studies have shown that such spectra can be fitted as a linear combination of the Kβ of the constituent cation species. − The normalized Mn-Kβ spectra for the reference samples (Figure S9) collected in this study were thus linearly combined to fit the Mn-Kβ spectra for LaMnO 3 and CaMnO 3 . Figure c,d shows the normalized Mn Kβ spectra for the LaMnO 3 and CaMnO 3 electrodes at the open-circuit potential (OCP, corresponding to 1.9 V) as a combination of the reference standards (fits for the different potentials can be found in Figure S10 and Table S5, as well as statistics of the fits in Table S6).…”

Correlating Orbital Composition and Activity of LaMnxNi1–xO3 Nanostructures toward Oxygen Electrocatalysis

“…Therefore, a more fundamental approach to probing the electrocatalytic activity of transition metal oxides is to establish the link between the observed electrocatalytic activity and the element-projected density of states (p-DOS) of these strongly correlated materials across the energy (potential) range relevant to oxygen electrocatalysis. This approach has been used to rationalize the electrocatalytic activity of perovskites and pyrochlores, identifying Mn-based oxides as a key active element in ORR electrocatalysis. , …”

“…This approach has been used to rationalize the electrocatalytic activity of perovskites and pyrochlores, identifying Mn-based oxides as a key active element in ORR electrocatalysis. 24,25 LaMn x Ni 1−x O 3 (LMNO) provides an interesting test case to probe correlations between electronic structure and activity, given the contrasting electrocatalytic properties of Mn and Ni states toward ORR and OER. Bradley and co-workers performed a detailed high-throughput combinatorial analysis of the La−Mn−Ni composition space, concluding that Ni richcompositions provide optimum oxygen electrocatalysis, although without a clear correlation with the structure or electronic properties.…”

Synergy of Oxygen-Deficient LaFeO3−δ and N-Doped Reduced Graphene Oxide in Oxygen Reduction Reaction in Alkaline Solutions

“…Celorrio et al linked the electrocatalytic activity to the presence of density of states (DOS) in the perovskite structure at potentials close to the ORR . Meanwhile, they reported the connection DOS in the region close to 1 V vs RHE. , Sun et al have shown that DOS can be promoted in the region close to 1 V by changing the oxidation state of Fe sites . However, unfortunately, the reduction peak is almost not observed in Figure S11, and the response between 1.1 and 1.2 cannot be determined to be the oxidation peak of Fe 2+ to Fe 3+ or/and the peak of pseudocapacitance within the potential investigated in the absence of oxygen.…”