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XANES characterization of pristine Co-based mixed metal oxides. (a) Co K-edge normalized XANES spectra of LSCoO-75, La 0.25 Sr 0.75 CoO 3 , and SrCoO 3 (in their as-synthesized, pristine states), along with the Co standards. The inset zooms in at the 0.5 edge step to show the difference in edge positions between the samples and the standards. (b) Linear correlation plot generated for the determination of the Co oxidation state, based on the XANES region using a half-edge step method. The Co standards with known Co oxidation states are plotted in black squares and the linear trend line is represented using a black line. The pristine states of LSCoO-75, La 0.25 Sr 0.75 CoO 3 , and SrCoO 3 are plotted in red, blue, and green circles, respectively.
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Compositionally versatile, nonstoichiometric, mixed ionic–electronic conducting metal oxides of the form An+1BnO3n+1 (n = 1 → ∞; A = rare-earth-/alkaline-earth-metal cation; B = transition-metal (TM) cation) remain a highly attractive class of electrocatalysts for catalyzing the energy-intensive oxygen evolution reaction (OER). The current design s...
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Context 1
... observed uniform distribution of all ions is in line with the appropriate stochiometric atomic ratios of La (16.2 ± 1.4 atom %), Sr (51.1 ± 1.1 atom %), and Co (32.6 ± 1.6 atom %) cations in LSCoO-75 obtained via energy dispersive X-ray spectroscopy (EDS) ( Figure S4). Atomic resolution HAADF-STEM imaging of the near-surface region of the oxide particles (Figure 1d and Figure S5) showed continuous lattice fringes with a d spacing of 3.6 Å along the [001] zone axis, consistent with that of the bulk. EELS mapping of the same surface region of LSCoO-75 (Figure 1e,f) measured a homogeneous distribution of the metal cations, indicating the absence of surface phase segregation of any of the metal cations. ...
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... determine the nature of the emerged surface phase in these A n+1 Co n O 3n+1 (n = 1 and n = ∞) oxides upon electrochemical cycling, XAS studies were employed. The absorption edge energy at the Co K-edge of normalized X-ray absorption near edge structure (XANES) spectra of LSCoO-75, La 0.25 Sr 0.75 CoO 3 , and SrCoO 3 suggested that all of these oxides, in their as-synthesized, pristine state, had an average Co oxidation state that was higher than 3+, as indicated by the comparison to the oxidic Co standards (Figure 5a). To estimate the average oxidation state of the Co cations, the energy position at half of the edge step was used to develop a correlation based on Co oxides of known oxidation state. ...
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... estimate the average oxidation state of the Co cations, the energy position at half of the edge step was used to develop a correlation based on Co oxides of known oxidation state. 56−58 The resulting linear correlation using this method is shown in Figure 5b, and the estimated average Co oxidation states calculated are summarized in Table S2. These results indicate that Co cations in La 0.25 Sr 0.75 CoO 3 have the highest oxidation state, followed by SrCoO 3 and LSCoO-75 in descending order. ...
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... The difference between EXAFS spectra of (i) pretreated and pristine and (ii) spent and pristine for LSCoO-75 resembled each other in the overall shape (Figure 7), suggesting that a similar structural change occurred in both states, but to a different degree, in line with the XANES studies. The second peak centered at 2.5 Å in the Fourier-transformed difference spectra for both pretreated and spent samples (Figure 7b) was described by a Co−Co scattering path at 2.88 ± 0.01 Å, as indicated by a good-quality fit in both magnitude and imaginary portions of the EXAFS ( Figure S25 and Table S3). This Co−Co path at a radial distance of 2.88 ± 0.01 Å can be assigned to edge-sharing Co− O octahedra, as this distance was significantly different from that of corner-sharing octahedra, evidenced by a Co−Co scattering path at ∼3.9 Å. 38,39 The latter scattering path at a distance of ∼3.9 Å was present in the pristine LSCoO-75 (Table S6), validating the presence of corner-sharing octahedra in the A 2 BO 4 type R-P phase consistent with its crystal symmetry ( Figure S1). ...
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Challenges in rational designing dual‐atom catalysts (DACs) give a strong motivation to construct coordination‐activity correlations. Here, thorough coordination‐activity correlations of DACs based on how the changes in coordination shells (CSs) of dual‐atom Cu,Co centers influence their electrocatalytic activity in oxygen reduction reaction(ORR),...
Citations
... To date, the majority of reports that are concerned with the phenomenon of surface reconstruction under OER conditions focus on alkali-or alkaline-earth-metal-containing (primarily Li, Ca, and Sr) metal oxides, in which the alkali or alkaline-earth elements are typically rapidly dissolved (with subsequent loss of transition metals) under alkaline OER conditions. 12,17,18,25 This results in an uncontrolled transformation of the catalyst, often leading to its degradation. In contrast, La-containing oxides (e.g., perovskites, LaMO 3 ) featuring less ionic La−O bonds (as compared to Li−O/Ca−O/Sr−O) are relatively stable making them promising targets to study the phenomenon of surface reconstruction not associated with severe degradation of the electrocatalyst. ...
... 52,53 It is important to highlight that the surface amorphization of La 2 NiO 4+δ during alkaline treatment observed here is different from previous reports which suggested that the surface reconstruction is triggered by the applied electric bias initiating e.g., lattice oxygen oxidation followed by the dissolution of alkali or alkaline-earth ions. 14,17,54 In contrast, we observed that the surface change in La 2 NiO 4+δ is a spontaneous, non-electrochemical reaction with alkaline electrolyte species. In a recent related study, 55 it was proposed that surface reconstruction can indeed occur via a non-electrochemical reaction in oxides with a high density of O 2p states near the Fermi level (E F ). ...
... In addition to the the Tafel slope the ECSA of the A-NiO-LCO-NF sample were further improved. Similarly, Samira et al. [97] discussed the stability and catalytic activity of the hydroxyl oxide CoOOH generated in situ with its associated oxides by surface reconstruction under OER cycling conditions. The surface reconstruction process taking ReP phase (A 2 BO 4 ) as an example is also shown (Fig. 7C). ...
... Such a measurement allows for the catalyst to reach a 'steady state' within the electrochemical environment, as would be expected of a mixed-metal oxide in an acidic environment undergoing surface reconstruction upon initial electrification. 42 , 43 Automation complexities associated with more traditional benchmarking methods that better isolate intrinsic catalyst activity, such as RDE with tafel analysis, do prevent such methods from being performed and utilise 'true' catalyst performance as a benchmark of optimization. However, chronopotentiometry is a quantitative metric that can be used to evaluate catalyst performance, and within the confines of our optimization, this will allow the optimizer to make informed decisions regarding relative catalyst performance, and optimize the synthesis conditions as such. ...
This work highlights the potential of earth-abundant mixed-metal oxide catalysts for the acid-based oxygen evolution reaction. These catalysts offer numerous combinations of metal-centre compositions, which can enhance catalytic activity and stability compared to precious-metal-based catalysts commonly used today. Despite substantial research in this field, there is a need for new methods and approaches to accelerate the exploration of these materials. In this study, we present a comprehensive approach to designing, developing, and implementing a self-driving laboratory to optimize the electrodeposition synthesis of amorphous mixed-metal oxide catalysts for the acidic oxygen evolution reaction. We particularly emphasize the development of methodologies to address experimental variability. We investigate crucial parameters and considerations when transitioning from manual bench-top synthesis and evaluation to automation and machine learning guided optimization. We address both experimental and optimization algorithm considerations in the presence of experimental variability. To illustrate our approach, we demonstrate the optimization of CoFeMnPbOx electrodeposited catalyst materials through multiple campaigns. Our results highlight considerations for optimizing overpotential and stability based on the outcomes of our experiments.
... Unfortunately, the accurate accounting for these contributions even in the simplest case of pure spinel materials is a daunting task, requiring to make strong assumptions and imposing constraints on the possible structures, as demonstrated, for example, in the rigorous work by Calvin et al. 26 In the simple cases where only two different species coexist, differential analysis approaches and linear algebra-based methods can be a viable solution. 27 However, they do not constitute a general answer for the challenging problem of EXAFS spectra interpretation for the mixtures of different oxide phases evolving under reaction conditions. ...
Bimetallic transition-metal oxides, such as spinel-like CoxFe3-xO4 materials, are known as attractive catalysts for the oxygen evolution reaction (OER) in alkaline electrolytes. Nonetheless, unveiling the real active species and active states in these catalysts remains a challenge. The coexistence of metal ions in different chemical states and in different chemical environments, including disordered X-ray amorphous phases that all evolve under reaction conditions, hinders the application of common operando techniques. Here, we address this issue by relying on operando quick X-ray absorption fine structure spectroscopy, coupled with unsupervised and supervised machine learning methods. We use principal component analysis to understand the subtle changes in the X-ray absorption near-edge structure spectra and develop an artificial neural network to decipher the extended X-ray absorption fine structure spectra. This allows us to separately track the evolution of tetrahedrally and octahedrally coordinated species and to disentangle the chemical changes and several phase transitions taking place in CoxFe3-xO4 catalysts and on their active surface, related to the conversion of disordered oxides into spinel-like structures, transformation of spinels into active oxyhydroxides, and changes in the degree of spinel inversion in the course of the activation treatment and under OER conditions. By correlating the revealed structural changes with the distinct catalytic activity for a series of CoxFe3-xO4 samples, we elucidate the active species and OER mechanism.
... 13 Similar surface leaching of lanthanide and alkaline earth elements (A-sites in the ABO 3 perovskite family) is observed in some complex oxides employed as OER catalysts in alkaline media, 44 leading to surfaces rich in first row transition metal oxides. 45 Open questions remain as to the extent the resulting oxide surface is configurationally influenced by the subsurface, which likely depends not only on the thickness of the Asite depleted layer, but also the dynamic nature of the redox-active surface. Together, the breath of configurational interactions resultant in both thin films 43 and particles 46 highlight the importance of considering the subsurface layer in catalyst design. ...
... In other cases, such as oxides for OER, reports are mixed as to the extent that oxide surfaces might change under reaction conditions. [44][45]51 Here we highlight that even in the case where oxides might yield similar active surfaces during OER-for example transition metal oxide/oxyhydroxide phases 45 -interplay with the subsurface can still impact activity. The established utility of volume-averaged (bulk) descriptors to accurately describe reactivity trends further highlights this fact. ...
... after 100 OER cycling up to 1.7 V. The ICP-MS studies and EDX analysis corroborates the Sr-deficient and Co-rich nature of the restructured surface shell, due to irreversible dissolution of alkaline-earth-metal cations[137]. This surface restructuring was observed in the Fe-free (<1 ppb Fe) electrolyte (Figure 10), yet also led to an increase in the electrocatalytic activity. ...
... It may seem that this contradicts the conclusions from Refs.[119] (Figure 7b), where no activity changes were observed for La1-xSrxCoO3 (x = 0, 0.3) samples in the Fe-free electrolyte. However, these two studies:[119],[137] used different Sr-substitution in the perovskite, i.e x = 0.3 and 0.75, and activity was reported after a different number of OER cycles in the Fe-free electrolyte (50 and 100). Thus, from these two works, it can be emphasized that when studying the surface of the OER electrocatalyst, it is important to consider the influence of the doping level on the rate of surface restructuring, as well as the influence of the presence or absence of Fe ions on the activity and stability of the restructured layer. ...
... The thickness of the amorphous shell was found to be 6.5±0.5 nm. Reprinted from[137]. Copyright 2021 The Authors. ...
Electrochemical water splitting is considered as a sustainable and ecologically
friendly energy storage technology providing clean energy for the future. The materials
containing 3d transition metals, such as Ni-Fe-based compounds, are a worthy alternative
to precious metal-based electrocatalysts, showing promising catalytic activity for the
oxygen evolution reaction (OER) in alkaline media. In this work, Ca-doped Ni/Fe-mixed
perovskites were synthesized using ultrasonic spray pyrolysis technique. A modified
approach of synthesis with sorbitol as fuel and ozone as oxidizer results in chemically
homogeneous hollow spheres with specific surface area as high as ~15 m² g-1. The crystal
structure and the chemical composition were determined with powder X-ray diffraction,
electron diffraction, aberration-corrected scanning transmission electron microscopy,
energy-dispersive X-ray mapping, 57Fe Mössbauer spectroscopy, iodometric titration and
X-ray photoelectron spectroscopy. Being employed as a catalyst for OER in 1M NaOH,
the La0.6Ca0.4Fe0.7Ni0.3O2.9 perovskite demonstrates a superior specific and mass
activities. The enhanced activity of La0.6Ca0.4Fe0.7Ni0.3O2.9 compared to that of undoped
LaFe0.7Ni0.3O3 was rationalized from a comparison of DFT-calculated electronic
structures. The Ca doping increases Ni and Fe oxidation states, enhances covalency of the
Ni/Fe-O bonds, shifts the center of O 2p band closer to the Fermi level thus decreasing
formation energy of the oxygen vacancies. This provokes faster leaching of A-site cations
and restructuring of the surface layers of perovskite into Ni-Fe (oxy)hydroxides, which
strongly interact with Fe ions in the electrolyte. As a result, the electrocatalytic activity of
the perovskite materials is greatly enhanced. In order to assess the effect of the most4
common impurity in alkaline electrolytes on the stability and activity of perovskite
catalyst, OER experiments were conducted with the deliberate addition of Fe ions to the
electrolyte followed by ex situ studies using transmission electron microscopy. The
restructuring of the perovskite surface layer into Ni0.5Fe0.5Ox(OH)2-x was discovered, as a
result of interaction of Fe ions in electrolyte with the perovskite surface after the OER.
... At the same time, recent studies have shown that near-surface phase transitions may play a key role in the catalytic process. 8,12,28 Hence, it is apparent that surface phase transformations can trigger catalytic activity and are also involved in the aging of catalysts. The link between these processes however remains unresolved to date. ...
The stability of perovskite oxide catalysts for the oxygen evolution reaction (OER) plays a critical role in their applicability in water splitting concepts. Decomposition of perovskite oxides under applied potential is typically linked to cation leaching and amorphization of the material. However, structural changes and phase transformations at the catalyst surface were also shown to govern the activity of several perovskite electrocatalysts under applied potential. Hence, it is crucial for the rational design of durable perovskite catalysts to understand the interplay between the formation of active surface phases and stability limitations under OER conditions. In the present study, we reveal a surface-dominated activation and deactivation mechanism of the prominent electrocatalyst La0.6Sr0.4CoO3-δ under steady-state OER conditions. Using a multiscale microscopy and spectroscopy approach, we identify the evolving Co-oxyhydroxide as catalytically active surface species and La-hydroxide as inactive species involved in the transient degradation behavior of the catalyst. While the leaching of Sr results in the formation of mixed surface phases, which can be considered as a part of the active surface, the gradual depletion of Co from a self-assembled active CoO(OH) phase and the relative enrichment of passivating La(OH)3 at the electrode surface result in the failure of the perovskite catalyst under applied potential.
