Crystal-facet-dependent surface transformation dictates the oxygen evolution reaction activity in lanthanum nickelate

Surface Transformation in Lanthanum Nickelate for Enhanced Oxygen Evolution Catalysis

Nickel-based perovskite oxides are identified as promising candidates for oxygen evolution reaction (OER) catalysts in view of their low cost, highly tunable structure, and potential high activity. However, the performance and catalyst design are hindered by their sluggish surface reconstruction kinetics. We introduce a ferric ion pre-etching strategy to enhance the surface reconstruction of typical LaNiO3. The hydrolysis of ferric ions generates hydrated protons that corrode the La-O terminal sites, inducing lattice distortion and lowering the energy barrier for reconstruction. Concurrently, ferric ion substitution for Ni creates crucial active sites after OER reconstruction, and enables the low-activity LaNiO3 to become highly active and superior to the benchmark RuO2 and NiFe layered double hydroxides (LDHs). In situ X-ray absorption spectroscopy (XAS) and in situ Raman spectroscopy reveal substantial surface transformation from corner-sharing to edge-sharing NiO6 at 1.43 V versus reversible hydrogen electrode (RHE) in the surface pre-etched sample (LNFeIII-spe). This reconstruction is initiated by the lattice oxygen mechanism (LOM) and transitions to the adsorbate evolution mechanism (AEM), underscoring the transformation of distinct OER mechanisms.

Quantifying Dynamic Changes of Oxygen Nonstoichiometry and Formation of Surface Phases of SrCoOx Electrocatalysts by Operando Characterizations

Perovskite electrocatalysts like strontium cobaltite (SrCoOx, denoted as SCO) experience dynamic changes in both surface and bulk during the oxygen evolution reaction (OER), rather than remaining static. This dynamic, electrochemically driven evolution in composition, structure, and ionic defects (e.g., oxygen vacancies) can strongly impact the OER activity and stability. Yet, the current lack of quantitative information on these processes impedes a precise and predictive evaluation of the individual and combined effect of both bulk and surface transformations. Here, using epitaxial SCO thin films as a model system, we demonstrate that SCO is a bulk and surface redox-active OER electrocatalyst that undergoes a bulk phase transition via electrochemically induced oxygen intercalation, as well as a surface phase transition toward Co (oxy-)hydroxide. Specifically, applying a suite of operando and ex situ characterization we established a reliable relationship between oxygen nonstoichiometry, optical density, and conductivity as a function of applied potentials. We further accurately quantify the evolution of oxygen stoichiometry in the SCO bulk and the thickness of the formed surface secondary phase. Our work provides a reliable and generalizable workflow and operando characterization toolbox for quantitative assessment of surface and bulk transformations in oxygen-deficient perovskite electrocatalysts.

Revealing the Intrinsic Oxygen Evolution Reaction Activity of Perovskite Oxides across Conductivity Ranges Using Thin Film Model Systems

The development of efficient electrocatalysts in water electrolysis is essential to decrease the high overpotentials, especially at the anode where the oxygen evolution reaction (OER) takes place. However, establishing catalyst design rules to find the optimal electrocatalysts is a substantial challenge. Complex oxides, which are often considered as suitable OER catalysts, can exhibit vastly different conductivity values, making it challenging to separate intrinsic catalytic activities from internal transport limitations. Here, we systematically decouple the limitations arising from electrical bulk resistivity, contact resistances to the catalyst support, and intrinsic OER catalytic properties using a systematic epitaxial thin film model catalyst approach. We investigate the influence of the resistivity of the three perovskite oxides LaNiO3-δ (3.7 × 10-4 Ω cm), La0.67Sr0.33MnO3-δ (2.7 × 10-3 Ω cm), and La0.6Ca0.4FeO3-δ (0.57 Ω·cm) on the observed catalytic activity. We tuned the electron pathway through the catalyst bulk by comparing insulating and conductive substrates. The conducting substrate reduces the electron pathway through the catalyst bulk from the millimeter to nanometer length scale. As we show, for the large electron pathways, the observed catalytic activity scales with resistivity because of a highly inhomogeneous lateral current density distribution. At the same time, even on the conducting substrate (Nb-doped SrTiO3), large contact resistances occur that limit the determination of intrinsic catalytic properties. By inserting interfacial dipole layers (in this case, LaAlO3) we lifted these interface resistances, allowing us to reveal the intrinsic catalytic properties of all examined catalysts. We find that La0.6Ca0.4FeO3-δ and LaNiO3-δ exhibit a similar intrinsic overpotential of 0.36 V at 0.1 mA/cm2, while their resistivities differ by 3 orders of magnitude. This finding shows that optimizing the electron pathway of the OER catalyst can lead the way to find new structure-activity relationships and to identify high-activity catalysts even if the electronic resistance is high.

Orientation-Dependent Oxygen Evolution Catalytic Performance and Mechanistic Insights of Epitaxial Co9S8 Thin Films

Cobalt sulfide (Co9S8) nanomaterials exhibit an efficient electrochemical catalytic performance due to their unique properties and electronic structure. The preparation of epitaxial Co9S8 thin films with varying crystal orientations and the study of their catalytic kinetics and mechanisms remain significant gaps. This study addresses the preparation of epitaxial Co9S8 thin films with orientations of (100), (110), and (111) on yttrium-doped zirconia (YSZ) substrates using pulsed laser deposition. Characterization confirmed their single-crystalline nature and consistent thickness. Electrochemical measurements revealed a similar hydrogen evolution reaction (HER) performance across all films but significant differences in the oxygen evolution reaction (OER) performance. The (111) orientation showed the best OER activity, with a current density of 24.2 mA cm-2 at 1.8 V vs RHE, outperforming the (100) and (110) orientations, which achieved 14.5 and 6.7 mA cm-2, respectively. Density functional theory (DFT) calculations indicated that the (100) orientation favored the traditional four-electron transfer mechanism, with a lower theoretical overpotential (0.37 V). In contrast, the (110) and (111) orientations demonstrated more complex adsorption behaviors, resulting in a higher overpotential of 0.49 V and a lower overpotential of 0.29 V, respectively. These results highlight the unique reactivity of different Co9S8 crystal orientations and provide valuable insights for optimizing the catalyst design to enhance the OER performance.

Iron-Based Layered Perovskite Oxyfluoride Electrocatalyst for Oxygen Evolution: Insights from Crystal Facets with Heteroanionic Coordination

Mixed-anion compounds have recently attracted attention as solid-state materials that exhibit properties unattainable with those of their single-anion counterparts. However, the use of mixed-anion compounds to control the morphology and engineer the crystal facets of electrocatalysts has been limited because their synthesis method is still immature. This study explored the electrocatalytic properties of a Pb-Fe oxyfluoride, Pb3Fe2O5F2, with a layered perovskite structure for oxygen evolution reaction (OER) and compared its properties in detail with those of a bulk-type cubic three-dimensional (3D) perovskite, PbFeO2F. A Pb3Fe2O5F2 electrode prepared with carbon nanotubes and a graphite sheet as a conductive support and a substrate, respectively, demonstrated better OER performance than a PbFeO2F electrode. The role of specific crystal facets of Pb3Fe2O5F2 in enhancing the OER activity was elucidated through electrochemical analysis. Density functional theory calculations indicated that the Pb3Fe2O5F2 (060) facet with Fe sites exhibited a lower theoretical overpotential for the OER, which was attributed to a moderately strong interaction between the active sites and the reaction intermediates; this interaction was reinforced by the strong electron-withdrawing behavior of fluoride ions. This finding offers new insights for developing efficient electrocatalysts based on oxyfluorides, leveraging the high electronegativity of fluorine to optimize the electronic states at active sites for the OER, without relying on precious metals.

Deciphering Water Oxidation Catalysts: The Dominant Role of Surface Chemistry over Reconstruction Degree in Activity Promotion

Water splitting hinges crucially on the availability of electrocatalysts for the oxygen evolution reaction. The surface reconstruction has been widely observed in perovskite catalysts, and the reconstruction degree has been often correlated with the activity enhancement. Here, a systematic study on the roles of Fe substitution in activation of perovskite LaNiO3 is reported. The substituting Fe content influences both current change tendency and surface reconstruction degree. LaNi0.9Fe0.1O3 is found exhibiting a volcano-peak intrinsic activity in both pristine and reconstructed among all substituted perovskites in the LaNi1-xFexO3 (x = 0.00, 0.10, 0.25, 0.50, 0.75, 1.00) series. The reconstructed LaNi0.9Fe0.1O3 shows a higher intrinsic activity than most reported NiFe-based catalysts. Besides, density functional theory calculations reveal that Fe substitution can lower the O 2p level, which thus stabilize lattice oxygen in LaNi0.9Fe0.1O3 and ensure its long-term stability. Furthermore, it is vital interesting that activity of the reconstructed catalysts relied more on the surface chemistry rather than the reconstruction degree. The effect of Fe on the degree of surface reconstruction of the perovskite is decoupled from that on its activity enhancement after surface reconstruction. This finding showcases the importance to customize the surface chemistry of reconstructed catalysts for water oxidation.

Influence of Annealing Temperature on the OER Activity of NiO(111) Nanosheets Prepared via Microwave and Solvothermal Synthesis Approaches

Earth-abundant transition metal oxides are promising alternatives to precious metal oxides as electrocatalysts for the oxygen evolution reaction (OER) and are intensively investigated for alkaline water electrolysis. OER electrocatalysis, like most other catalytic reactions, is surface-initiated, and the catalyst performance is fundamentally determined by the surface properties. Most transition metal oxide catalysts show OER activities that depend on the predominantly exposed crystal facets/surface structure. Therefore, the design of synthetic strategies to obtain the most active crystal facets is of significant research interest. In this work, rock salt NiO OER catalysts with (111) predominantly exposed facets were synthesized by a solvothermal (ST) method either heated under supercritical or microwave-assisted (MW) conditions. Particular emphasis was placed on the influence of the post annealing temperature on the structural configuration and OER activity to compare their catalytic performances. The as-prepared electrocatalysts are pure α-Ni hydroxides which were converted to rock salt NiO (111) nanosheets with hexagonal pores after heat treatment at different temperatures. The OER activity of the electrodes has been evaluated in 0.1 M KOH using geometric and intrinsic current densities via normalization by the disk area and BET area, respectively. The lowest overpotential at a geometric current density of 10 mA/cm2 is found for samples pretreated by heating between 400 and 500 °C with a catalyst loading of 115 μg/cm2. Despite the very similar nature of the catalysts obtained from the two methods, the ST electrodes show a higher geometric and intrinsic current density for 500 °C pretreatment. The MW electrodes, however, achieve an optimal geometric current density for 400 °C pretreatment, while their intrinsic current density requires pretreatment over 600 °C. Interestingly, pretreated electrodes show consistently higher OER activity as compared to the poorly crystalline/less ordered hydroxide as-prepared electrocatalysts. Thus, our study highlights the importance of the synthesis method and pretreatment at an optimal temperature.

Facet-Engineered BiVO4 Photocatalysts for Water Oxidation: Lifetime Gain Versus Energetic Loss

A limiting factor to the efficiency of water Oxygen Evolution Reaction (OER) in metal oxide nanoparticle photocatalysts is the rapid recombination of holes and electrons. Facet-engineering can effectively improve charge separation and, consequently, OER efficiency. However, the kinetics behind this improvement remain poorly understood. This study utilizes photoinduced absorption spectroscopy to investigate the charge yield and kinetics in facet-engineered BiVO4 (F-BiVO4) compared to a non-faceted sample (NF-BiVO4) under operando conditions. A significant influence of preillumination on hole accumulation is observed, linked to the saturation and, thus, passivation of deep and inactive hole traps on the BiVO4 surface. In DI-water, F-BiVO4 shows a 10-fold increase in charge accumulation (∼5 mΔOD) compared to NF-BiVO4 (∼0.5 mΔOD), indicating improved charge separation and stabilization. With the addition of Fe(NO3)3, an efficient electron acceptor, F-BiVO4 demonstrates a 30-fold increase in the accumulation of long-lived holes (∼45 mΔOD), compared to NF-BiVO4 (∼1.5 mΔOD) and an increased half-time, from 2 to 10 s. Based on a simple kinetic model, this increase in hole accumulation suggests that facet-engineering causes at least a 50-100 meV increase in band bending in BiVO4 particles, thereby stabilizing surface holes. This energetic stabilization/loss results in a retardation of OER relative to NF-BiVO4. This slower catalysis is, however, offset by the observed increase in density and lifetime of photoaccumulated holes. Overall, this work quantifies how surface faceting can impact the kinetics of long-lived charge accumulation on metal oxide photocatalysts, highlighting the trade-off between lifetime gain and energetic loss critical to optimizing photocatalytic efficiency.

Crystal Facets-Activity Correlation for Oxygen Evolution Reaction in Compositional Complex Alloys

Compositional complex alloys, including high and medium-entropy alloys (HEAs/MEAs) have displayed significant potential as efficient electrocatalysts for the oxygen evolution reaction (OER), but their structure-activity relationship remains unclear. In particular, the basic question of which crystal facets are more active, especially considering the surface reconstructions, has yet to be answered. This study demonstrates that the lowest index {100} facets of FeCoNiCr MEAs exhibit the highest activity. The underlying mechanism associated with the {100} facet's low in-plane density, making it easier to surface reconstruction and form amorphous structures containing the true active species is uncovered. These results are validated by experiments on single crystals and polycrystal MEAs, as well as DFT calculations. The discoveries contribute to a fundamental comprehension of MEAs in electrocatalysis and offer physics-based strategies for developing electrocatalysts.

Facet Dependence of the Oxygen Evolution Reaction on Co3O4, CoFe2O4, and Fe3O4 Epitaxial Film Electrocatalysts

The main obstacle for the electrocatalytic production of "green hydrogen" is finding suitable electrocatalysts which operate highly efficiently over extended periods of time. The topic of this study is the oxygen evolution reaction (OER), one of the half-reactions of water splitting. It is complex and has intricate kinetics, which impairs the reaction efficiency. Transition metal oxides have shown potential as electrocatalysts for this reaction, but much remains unknown about the atomic scale processes. We have investigated structure-composition-reactivity correlations for Co3O4, CoFe2O4, and Fe3O4 epitaxial thin-film electrocatalysts exposing either the (001) or (111) surface facets. We found that for Co3O4, the (001) facet is more reactive, while for the other oxides, the (111) facet is more active. A Tafel-like evaluation reveals systematically smaller "Tafel" slopes for the (001) facets. Furthermore, the slopes are smaller for the iron-containing films. Additionally, we found that the oxyhydroxide skin layer which forms under OER reaction conditions is thicker on the cobalt oxides than on the other oxides, which we attribute to either a different density of surface defects or to iron hindering the growth of the skin layers. All studied skin layers were thinner than 1 nm.

Crystal-facet-dependent surface transformation dictates the oxygen evolution reaction activity in lanthanum nickelate

Electrocatalysts are the cornerstone in the transition to sustainable energy technologies and chemical processes. Surface transformations under operation conditions dictate the activity and stability. However, the dependence of the surface structure and transformation on the exposed crystallographic facet remains elusive, impeding rational catalyst design. We investigate the (001), (110) and (111) facets of a LaNiO3-δ electrocatalyst for water oxidation using electrochemical measurements, X-ray spectroscopy, and density functional theory calculations with a Hubbard U term. We reveal that the (111) overpotential is ≈ 30-60 mV lower than for the other facets. While a surface transformation into oxyhydroxide-like NiOO(H) may occur for all three orientations, it is more pronounced for (111). A structural mismatch of the transformed layer with the underlying perovskite for (001) and (110) influences the ratio of Ni2+ and Ni3+ to Ni4+ sites during the reaction and thereby the binding energy of reaction intermediates, resulting in the distinct catalytic activities of the transformed facets.