First-row Transition Metal Oxides Research Articles

Dynamic Activity and Stability of Transition Metal (oxy)Hydroxide Oxygen Evolution Electrocatalysts Under Steady and Intermittent Operation

The oxygen evolution reaction (OER) plays a critical role in several energy conversion technologies, requiring catalysts that combine high activity with stability for successful commercialization. Catalysts from first-row transition metal oxides have garnered considerable attention in this context. Their noteworthy OER activity and stability in alkaline environments position them as a more economical option than their counterparts in acidic media. However, recent studies have shown that these materials tend to experience significant dissolution issues and undergo changes in their chemical composition, a phenomenon observed even during steady-state operation.1–3 Addressing these challenges is essential for advancing commercial energy conversion technologies, which often face intermittent loads and reverse currents. A deeper understanding of the dynamic reconstruction under fluctuating conditions is crucial for developing more durable and efficient electrocatalytic materials.4 In this work, we investigate the dynamic performance of nickel cobalt (oxy)hydroxide electrocatalytic films containing iron active sites in alkaline media under steady and intermittent operation. First, we systematically examine the changes in OER catalytic activity in a typical three-electrode configuration, focusing on the incorporation and redissolution of iron and cerium during different electrochemical conditioning methods. As shown in Figure 1a, conditioning using chronopotentiometry decreases the OER overpotential without significantly shifting the redox peak. In contrast, conditioning using cyclic voltammetry significantly shifts the redox peak position (Figure 1b). These differences are attributed to the (oxy)hydroxide film thickness, which forms in situ during conditioning and affects the Fe dissolution and incorporation rates based on the conditioning method.5 We use different analytical techniques to probe chemical transformations during conditioning. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) and X-ray photoelectron spectroscopy (XPS) are utilized to analyze compositional changes in the electrocatalytic films. Moreover, inductively coupled plasma mass spectrometry (ICP-MS) tracks metal dissolution.Next, we utilize an electrochemical flow electrolyzer with a zero-gap configuration, integrated with online ICP-MS and post-experimental TOF-SIMS and XPS, to study the reconstruction of these electrocatalysts under intermittent conditions. A NiFe anode operating in 6 M KOH with 1 ppm of Fe3+ shows a 40-mV change after 120 cycles of alternating low and high currents, including a simulated shutdown step (Figure 1c). This cell potential change, becoming more negative with continued cycling (Figure 1d), is attributed to the shutdown step. Our findings underscore how operating conditions affect the dynamic reconstruction of electrocatalytic materials, offering critical insights for developing more robust and resilient materials suitable for commercial applications.

(Invited) Enhanced Material Durability of Transition Metal-Antimony X-Ide Nanoparticles in Oxygen Reduction Electrocatalysis

Hydrogen fuel cells (FCs) are key energy devices that can accelerate our transition away from non-renewable, carbon-intensive fuels especially in the grid-scale storage and transportation sectors. FC cathodes perform the oxygen reduction reaction (ORR), and despite decades of progress towards higher performance electrocatalysts, the ORR remains kinetically sluggish and limits the efficiency of assembled FC devices. Modern, commercially available FC cathodes typically contain platinum (Pt)-based catalysts that can cause the overall FC to become prohibitively expensive for some applications so identifying alternate non-platinum group ORR catalyst formulations could increase deployment of FCs. Physics-based calculations with density functional theory (DFT) have predicted many first-row transition metal oxides (MOx, M = Mn, Fe, Cr, Ni, Co) as having favorable ORR performance trends. Additionally, there is experimental validation of nanoscale ternary and quaternary transition metal oxide systems with a mixture of transition metals used for catalyzing the ORR where relatively modest activity enhancements are reported. Despite this progress, an important limitation is present in the low material stability of non-precious transition metal oxides especially in acidic environments such as those encountered in proton exchange membrane FCs. Using synthetic material strategies to overcome such stability limitations in a variety of FC operating conditions will provide a deeper understanding of the relationship between structure, activity, and degradation.In this work, we employ two simultaneous strategies to work towards enhancing the activity and stability of transition metal oxides: (1) addition of antimony (Sb) as a ligand and (2) functionalization with nitrogen or sulfur. We utilized a previously reported colloidal synthesis for uniform ~ 50 nm oxide nanoparticles for manganese, nickel, cobalt, and iron to produce four sets of nanoparticles based on each of these metals. By using a colloidal synthesis that concurrently introduces Sb with the secondary metal and drying the particles afterwards, we incorporate Sb evenly throughout the polycrystalline structure of the of these nanoparticle catalysts. A high temperature surface modification reaction with reactive gas such as air, ammonia, or hydrogen sulfide was used to convert any remaining transition metal antimonate into an oxide, nitride, or sulfide, respectively. In total, twelve materials are used in this study: the oxide, nitride, and sulfide (collectively “X-ides”) of each of the four transition metal containing, antimony incorporated nanoparticles. We optimize this material synthesis and evaluate the nanoparticle catalyst’s ability to enhance electrochemical ORR activity in both acidic (pH 1) and alkaline (pH 13) conditions which are simulating operating conditions of proton-exchange membrane and anion-exchange membrane FCs, respectively. In addition to activity modulation, we measure the ORR selectivity change towards hydrogen peroxide formation for each material and note the role of nitrogen and sulfur modification in suppressing hydrogen peroxide formation in alkaline environments. In addition to electrochemical characterization, we employed a vast array of bulk and surface characterization techniques to understand catalyst dynamics and contextualize observed activity trends. Transmission electron microscopy (TEM) enabled determination of particle sizes and morphologies alongside electron diffraction patterns to validate the catalyst synthesis. X-ray diffraction (XRD) then provided further information about crystallinity that connected bulk-level insights to the local information from TEM. X-ray photoelectron spectroscopy (XPS) enabled determination of oxidation state changes in the transition metal and in Sb that serve as important descriptors for estimating the activity of these catalysts. We also employed in situ/operando techniques to probe catalyst dynamics and stability under various conditions. Using manganese K-edge synchrotron x-ray absorption spectroscopy (XAS) on the manganese antimony X-ides, we find significant changes to oxidation state and coordination of manganese in the nitride and sulfide as a function of potential indicating processes that displace nitrogen and sulfur in the lattice under certain conditions. Lastly, we use an in situ electrochemical flow cell coupled to inductively coupled plasma-mass spectrometry (ICP-MS) to measure corrosion as a function of ORR current density, potential sweeping, and electrolyte gas saturation to determine degradation patterns. Insights from all these techniques provide a fascinating perspective on what factors in a material and its environment are responsible for enhanced activity, selectivity, and stability. We perform DFT modeling of these systems to further elucidate material properties that are principally responsible for achieving key metrics in activity and stability especially as non-precious catalysts are evaluated for their performance in assembled device conditions. Combining electrochemical experiments, physics-based modeling, and material characterization to better understand structure-performance relationships is a promising path towards accelerating the process of materials discovery for eventual deployment in critical energy applications for the future.

In-situ Raman unveiled Nb-O-bond-dependency selectivity for methanol electro-oxidation at high current density

Selective methanol oxidation reaction (MOR) to value-added formate can maximize the energy utilization by replacing the high onset-potential and sluggish oxygen evolution reaction. However, to date, only first-row 3d transition metal oxides exhibit the capacity of selective MOR, motivating the design of new kinds of electrocatalysts to further enhance the performance. Here we develop amorphous Nb2O5 as an efficient electrocatalyst, for the first time, for selective MOR to formate, with a low operating potential of 1.47 V vs. RHE to achieve the industry-relevant current densities (>100 mA cm−2) with a formate Faradaic efficiency of ∼100 %. We demonstrate, via operando Raman measurements, that this superior electrochemical MOR capacity originates from the in-situ induced short Nb-O bonds from the amorphous structure. Besides, the interface between amorphous Nb2O5 and nickel foam could also faciltate the MOR. This work may provide insights for designing novel transition metal oxides as efficient catalysts for selective MOR.

Decoding structure-spectrum relationships with physically organized latent spaces

A new semi-supervised machine learning method for the discovery of structure-spectrum relationships is developed and demonstrated using the specific example of interpreting X-ray absorption near-edge structure (XANES) spectra. This method constructs a one-to-one mapping between individual structure descriptors and spectral trends. Specifically, an adversarial autoencoder is augmented with a novel rank constraint (RankAAE). The RankAAE methodology produces a continuous and interpretable latent space, where each dimension can track an individual structure descriptor. As a part of this process, the model provides a robust and quantitative measure of the structure-spectrum relationship by decoupling intertwined spectral contributions from multiple structural characteristics. This makes it ideal for spectral interpretation and the discovery of new descriptors. The capability of this procedure is showcased by considering five local structure descriptors and a database of over fifty thousand simulated XANES spectra across eight first-row transition metal oxide families. The resulting structure-spectrum relationships not only reproduce known trends in the literature, but also reveal unintuitive ones that are visually indiscernible in large data sets. The results suggest that the RankAAE methodology has great potential to assist researchers to interpret complex scientific data, test physical hypotheses, and reveal new patterns that extend scientific insight.

First-row transition-metal hybrid oxide nanoparticles (MFe2O4) as UV-Fenton-like catalysts for Rhodamine B treatment: A comparative study

First-row transition-metal hybrid oxide nanoparticles (CoFe2O4, NiFe2O4, ZnFe2O4, CuFe2O4 and Fe3O4) were fabricated by co-precipitation. The obtained nanomaterials were investigated by X-ray diffraction (XRD) and scanning electron microscope (SEM). The UV-Fenton-like catalytic reactivity of those metal oxide nanoparticles was assessed via the removal of Rhodamine B (RhB) at a concentration of 20 ppm from an aqueous solution. In general, in a strong acidic environment, iron-hybrid transition-metal oxides (MFe2O4) as UV-Fenton-like catalysts showed higher catalytic activity than magnetite (Fe3O4) as they could generate more OH• radicals by increasing Fe3+/Fe2+ catalytic cycles and photocatalysis as well. In addition, ferrites were easily separated by an external magnet and thus reused for further cycles preserving their catalyst activities. This result highlighted the efficient utilization of iron-hybrid oxides as UV-Fenton-like catalysts for the RhB removal from an aqueous solution.

Adsorbed CO2-Mediated CO2 Photoconversion Cycle into Solar Fuel at the O Vacancy Site of Zirconium Oxide

ZrO2 photoreduces 13CO2 under ultraviolet–visible light to 13C-product(s) negligibly affected by adventitious carbon. The dual-site reaction pathway was theoretically clarified from CO2 to CO using monoclinic ZrO2, followed by multiple hydrogenation steps to methane over Ni nanoparticles. The oxygen vacancy (VO••) played essential roles, including the M-shaped CO2 adsorption over the VO•• site and the direct occupation of the VO•• site by the dissociated O and/or hydroxy group from the hydroxycarbonyl species favorably on the ZrO2(111) surface. The rate-limiting step was for the regeneration of the VO•• site with an activation energy (Eact) of 2.6 eV, but the water desorption energy was greatly compensated by the CO2 adsorption energy at the VO•• site, in contrast to the first-row transition-metal oxides. The COH and/or CO species transfer from ZrO2 to Ni in a concerted mechanism was energetically favorable, and the apparent Eact value from hydroxycarbonyl species to methane was reduced to 0.67 eV.

Ab Initio Calculations of XUV Ground and Excited States for First-Row Transition Metal Oxides

Transient X-ray spectroscopies have become ubiquitous in studying photoexcited dynamics in solar energy materials due to their sensitivity to carrier occupations and local chemical or structural dynamics. The interpretation of solid-state photoexcited dynamics, however, is complicated by the core–hole perturbation and the resulting many-body dynamics. Here, an ab initio, Bethe–Salpeter equation (BSE) approach is developed that can incorporate photoexcited state effects for solid-state materials. The extreme ultraviolet (XUV) absorption spectra for the ground, photoexcited, and thermally expanded states of first row transition metal oxides─TiO2, α-Cr2O3, β-MnO2, α-Fe2O3, Co3O4, NiO, CuO, and ZnO─are calculated to demonstrate the accuracy of this approach. The theory is used to decompose the core–valence excitons into the separate components of the X-ray transition Hamiltonian for each of the transition metal oxides investigated. The decomposition provides a physical intuition about the origins of XUV spectral features as well as how the spectra will change following photoexcitation. The method is easily generalized to other K, L, M, and N edges to provide a general approach for analyzing transient X-ray absorption or reflection data.

Stability Design Principles of Manganese-Based Oxides in Acid

The use of non-precious catalysts (e.g., transition metal oxides) in electrochemical energy technologies in acid (e.g., proton exchange membrane fuel cells and electrolyzers) has been significantly hampered by the instability of such catalysts at low pHs. While first-row late transition metal oxides based on Fe, Co, and/or Ni have been reported with comparable or higher catalytic activity for the alkaline oxygen reduction and oxygen evolution reactions than their precious-metal-based counterparts (e.g., Pt and IrO2),[1] these oxide catalysts are not stable in acid.[2] Moreover, Mn-based oxides are more acid-stable than those based on late transition metals[1] and can be potentially stabilized by reaching dissolution-deposition equilibrium at moderately acidic pHs,[3] but they still corrode and deactivate in strong acid and after long operation.[4] To tackle this challenge, extensive efforts over the past decades have been focused on combining active, yet unstable metal oxides with corrosion-resistant oxides.[5-7] Nevertheless, having a higher fraction of corrosion-resistant elements results in more acid-stable, yet less active catalysts, indicating an activity-stability trade-off.[2,5,6] To design transition metal oxide catalysts (such as Mn-based oxides) with an optimal trade-off or bypass this limitation, it is critical to establish their stability descriptors in acid. These descriptors can offer a fundamental understanding of oxide dissolution and provide guiding principles to enhance their intrinsic stability. In this work, we employed a library of Mn-based oxides with diverse structures and Mn oxidation states to identify oxide stability descriptors in acid based on intrinsic electronic structure and energetic parameters. Using time-dependent dissolution experiments and density functional theory calculations, greater amounts and faster kinetics of oxide dissolution in acid were correlated with decreased Mn oxidation states, which are accompanied by lowered Mn-O covalency, weakened Mn-O bonds, and reduced barriers for key reaction steps (such as protonation, vacancy formation, and metal ion solvation). Such design principles were shown broadly with a computational screening across a vast chemical space of ~1,000 Mn-based oxides, where an Mn oxidation state of 4+ was found to give rise to the lowest energetic driving force for the dissolution of Mn-based oxides in acid. Moreover, limiting the percentage of ionic metal substituents in oxides and using more acidic substituents were shown to stabilize Mn-based oxides in acid. These findings can provide novel insights into designing acid-stable Mn-based oxides for renewable energy storage and conversion.

Sustainable oxygen evolution electrocatalysis in aqueous 1 M H2SO4 with earth abundant nanostructured Co3O4

Earth-abundant electrocatalysts for the oxygen evolution reaction (OER) able to work in acidic working conditions are elusive. While many first-row transition metal oxides are competitive in alkaline media, most of them just dissolve or become inactive at high proton concentrations where hydrogen evolution is preferred. Only noble-metal catalysts, such as IrO2, are fast and stable enough in acidic media. Herein, we report the excellent activity and long-term stability of Co3O4-based anodes in 1 M H2SO4 (pH 0.1) when processed in a partially hydrophobic carbon-based protecting matrix. These Co3O4@C composites reliably drive O2 evolution a 10 mA cm–2 current density for >40 h without appearance of performance fatigue, successfully passing benchmarking protocols without incorporating noble metals. Our strategy opens an alternative venue towards fast, energy efficient acid-media water oxidation electrodes.

Electrically Tunable Reactivity of Substrate-Supported Cobalt Oxide Nanocrystals.

First-row transition metal oxides are promising materials for catalyzing the oxygen evolution reaction. Surface sensitive techniques provide a unique perspective allowing the study of the structure, adsorption sites, and reactivity of catalysts at the atomic scale, which furnishes rationalization and improves the design of highly efficient catalytic materials. Here, a scanning probe microscopy study complemented by density functional theory on the structural and electronic properties of CoO nanoislands grown on Au(111) is reported. Two distinct phases are observed: The most extended displays a Moiré pattern (α-region), while the less abundant is 1Co:1Au coincidental (β-region). As a result of the surface registry, in the β-region the oxide adlayer is compressed by 9%, increasing the unoccupied local density of states and enhancing the selective water adsorption at low temperature through a cobalt inversion mechanism. Tip-induced voltage pulses irreversibly transform α- into β-regions, thus opening avenues to modify the structure and reactivity of transition metal oxides by external stimuli like electric fields.

Oxygen Electrocatalysis on Mixed-Metal Oxides/Oxyhydroxides: From Fundamentals to Membrane Electrolyzer Technology

ConspectusCatalyzing the oxygen evolution reaction (OER) is important for key energy-storage technologies, particularly water electrolysis and photoelectrolysis for hydrogen fuel production. Under neutral-to-alkaline conditions, first-row transition-metal oxides/(oxy)hydroxides are the fastest-known OER catalysts and have been the subject of intense study for the past decade. Critical to their high performance is the intentional or accidental addition of Fe to Ni/Co oxides that convert to layered (oxy)hydroxide structures during the OER. Unraveling the role that Fe plays in the catalysis and the molecular identity of the true “active site” has proved challenging, however, due to the dynamics of the host structure and absorbed Fe sites as well as the diversity of local structures in these disordered active phases.In this Account, we highlight our work to understand the role of Fe in Ni/Co (oxy)hydroxide OER catalysts. We first discuss how we characterize the intrinsic activity of the first-row transition-metal (oxy)hydroxide catalysts as thin films by accounting for the contributions of the catalyst-layer thickness (mass loading) and electrical conductivity as well as the underlying substrate’s chemical interactions with the catalyst and the presence of Fe species in the electrolyte. We show how Fe-doped Ni/Co (oxy)hydroxides restructure during catalysis, absorb/desorb Fe, and in some cases degrade or regenerate their activity during electrochemical testing. We highlight the relevant techniques and procedures that allowed us to better understand the role of Fe in activating other first-row transition metals for OER. We find several modes of Fe incorporation in Ni/Co (oxy)hydroxides and show how those modes correlate with activity and durability. We also discuss how this understanding informs the incorporation of earth-abundant transition-metal OER catalysts in anion-exchange-membrane water electrolyzers (AEMWE) that provide a locally basic anode environment but run on pure water and have advantages over the more-developed proton-exchange-membrane water electrolyzers (PEMWE) that use platinum-group-metal (PGM) catalysts. We outline the key issues of introducing Fe-doped Ni/Co (oxy)hydroxide catalysts at the anode of the AEMWE, such as the oxidative processes triggered by Fe species traveling through the polymer membrane, pH-gradient effects on the catalyst stability, and possibly limited catalyst utilization in the compressed stack configuration. We also suggest possible mitigation strategies for these issues. Finally, we summarize remaining challenges including the long-term stability of Fe-doped Ni/Co (oxy)hydroxides under OER conditions and the lack of accurate models of the dynamic active surface that hinder our understanding of, and thus ability to design, these catalysts.

Thermally Activated Optical Absorption into Polaronic States in Hematite.

Polaron formation, whereby an electron or hole strongly couples to a lattice distortion, inhibits the carrier mobility of many first-row transition metal oxide semiconductors. Recently reported XUV transient absorption measurements of hematite (α-Fe2O3) demonstrate formation of electron small polarons upon photoexcitation into an undistorted charge-transfer state followed by subpicosecond lattice reorganization. Here, we show that polaronic states of hematite can be accessed directly via optical transitions from the ground state in a thermally activated lattice. Thermal difference spectra collected from 30 to 573 K combined with Stokes resonance Raman spectra indicate strong coupling between optical transitions near the band-edge (2.1-2.3 eV) and zone-center a1g and longitudinal (LO) optical phonons. Density functional theory calculations of the electronic and vibrational structures of pristine and polaron-distorted hematite lattices confirm that the geometric distortion corresponding to electron small polaron formation lies along the 28-meV a1g and 81-meV LO phonon coordinates and reproduce the features observed in the experimental thermal difference and resonance Raman spectra.

Film Fabrication and Composition Control of Transition Metal Oxide Nanoparticles for Oxygen Evolution Catalysts

Development of highly efficient oxygen evolution reaction (OER) catalysts is necessary for commercial implementation of water electrolysis due to overcome slow kinetics of OER. Noble metal based electrocatalysts such as Ir and Ru demonstrate high performance for OER in acidic media. In contrast, the first-row transition metal oxides such as Ni, Co, and Fe oxides show the high catalytic activity and stability in alkaline media. In this work, the composition of bimetallic metal oxide NPs was tuned and their electrochemical performance was analyzed. Furthermore, the electrophoretic deposition (EPD) technique was utilized for the efficient fabrication of NP thin film electrode. The bimetallic transition metal oxide NPs were synthesized by using Ni(acac)2 as Ni precursor and Fe(acac)3 as Fe precursor, leading to the phase control of bimetallic Ni-Fe oxide NPs. As Fe precursor amount was gradually increased compared to that of Ni, the particle size of Ni-Fe oxide NPs was increased, and the shape of NPs was transformed from spherical to irregular shape. The magnetic property of Ni-Fe oxide NPs was also analyzed by vibrating sample magnetometer. The saturation magnetization (Ms) of bimetallic metal oxide was increased and the magnetic coercivity was decreased when the Fe composition was increased. For the electrochemical analysis for OER, the overpotential of bimetallic metal oxide NPs catalysts was highly influenced by the composition of bimetallic metal oxide NPs. Higher Fe contents than Ni in the NPs led to high performance as OER catalysts. The EPD technique was introduced to manufacture the uniform NP films with improved catalytic activity. The catalyst film formed through EPD technique was prepared under the applied voltage of 500 V for 10 minutes. The film of Ni-Fe oxide NPs was well covered on the substrate of the Ti foil. The overpotential of the bimetallic metal oxide NP film fabricated by EPD was decreased from 510 mV to 400 mV at 10 mA/cm2 compared to the films fabricated by drop casting. These results provide useful insights not only for understanding the catalytic activity of bimetallic metal oxide NPs in OER but also for suggesting the film fabrication method for enhancing the catalytic activity.

Quasi-1D Mn2O3 Nanostructures Functionalized with First-Row Transition-Metal Oxides as Oxygen Evolution Catalysts

The development of cheap and efficient catalysts for the oxygen evolution reaction (OER) plays a critical role for sustainable energy conversion and storage. Herein, we report on Mn2O3-based system...

(Invited) Water-Splitting Electrocatalysts Made By Pulsed-Laser in Liquids Synthesis

Conversion of solar energy into storable fuels is urgently needed to combat climate change. Efficient, robust materials that are exclusively made of non-precious elements are imperative for tomorrow's global sustainable energy economy and necessitate the development of new nanostructured multi-metallic electrocatalysts that turn our most abundant feedstocks, water or carbon dioxide, into carbon-neutral fuels. The sheer number of possible elemental compositions in multi-metallic nanostructures requires a deep understanding of electrocatalytic mechanisms, including identification of reactive intermediates, to rationally design new materials and guide their optimization.We used pulsed-laser in liquids synthesis to realize series of earth-abundant multi-metallic first-row transition metal oxide and hydroxide nanostructures. The pulsed-laser in liquids technique is a flexible synthetic strategy for surfactant-free nanomaterials with independently controlled compositional, morphological, and structural properties, and defect densities. It permits rapid preparation or modification of tailored, complex nanostructures in sufficiently large quantities to study them in bulk. Importantly, metastable materials become accessible as nanoparticles form from a laser-induced plasma that is confined by the liquid and characterized by very high temperatures and pressures, thus enabling access to extreme regions of the material's phase diagram.Our approach enabled unprecedented atomistic-level structural and mechanistic insights into highly active and robust nickel–iron layered double hydroxide nanocatalysts for water oxidation in base; this understanding allowed rapid optimization. Operando spectroscopy data allowed us to identify a cis-dioxo iron(VI) reactive intermediate as the lowest-energy species before O–O bond formation during water oxidation electrocatalysis. Further, we capitalized on the unique advantages of the laser process for integration of our water-splitting electrocatalysts with photoanodes.

(Invited) Mitigating Layer Collapse at High Voltages

Layered lithium first-row transition metal oxide is widely used as cathode materials in lithium-ion batteries and promises a high theoretical capacity. However, only 70% of its theoretical capacity is utilized in practice due to irreversible changes in the structure as well as the microstructure of the electrode when cycled to high voltages. Previous studies have identified intergranular cracking as a dominant cause of the long-term capacity fading induced by the anisotropic volume change upon lithium de-intercalation. Such anisotropic volume change can be mitigated by preventing the layer collapse at high voltages. One approach to such an effect is through cation substitution. This work presents a structure-function relationship study to evaluate the efficacy of such an approach.

Evaluating optimal U for 3d transition-metal oxides within the SCAN+ U framework

Redox-active $3d$ transition-metal oxides (TMOs) are crucial ingredients for multiple sustainable energy applications, including solar cells, batteries, catalysis, and solar thermochemical water splitting. However, any predictive modeling, such as that employing density functional theory, needs to describe accurately the energetics of redox reactions involving transition metals, if new candidate materials are to be identified in a reliable fashion. Recently, we demonstrated that the state-of-the-art, strongly constrained and appropriately normed (SCAN) exchange-correlation functional requires a Hubbard $U$ correction (determined, e.g., from experimental oxidation enthalpies) to reproduce the ground-state structure, lattice parameters, magnetic moments, and electronic properties of Ce-, Mn-, and Fe-based oxides. In the present work, we extend our approach to identify optimal $U$ values for other $3d$ TMOs, specifically Ti, V, Cr, Co, Ni, and Cu, within the $\mathrm{SCAN}+U$ framework. We determine optimal $U$ values of 2.5, 1.0, 3.0, and 2.5 eV for Ti, V, Co, and Ni oxides, respectively, while Cr and Cu oxides best reproduce redox thermodynamics without any $U$ correction at all. While the $U$ values required for Ti, V, Co, and Ni are lower than those needed within the generalized gradient approximation (GGA) + $U$ or local density approximation (LDA) + $U$ approaches, inclusion of $U$ makes non-negligible improvements in ground-state property evaluations of these oxides. Here we also validate our optimal $U$ values by performing a number of transferability checks for each $3d$ material. A $\mathrm{SCAN}+U$ framework (with an appropriately determined $U$) therefore is needed to assess accurately the ground-state energies and qualitatively consistent electronic structures for (most) first-row TMOs.

First-row transition metal oxide oxygen evolution electrocatalysts: regulation strategies and mechanistic understandings

Regulation strategies and mechanistic understandings of first-row transition metal oxide-based oxygen evolution electrocatalysts.

(Invited) Electronic Structures of Metal Centers in OER Catalyst Models and Electron/Energy Relays in the Excited State Supramolecular Dinuclear Transition Metal Complexes

Electronic Structures of Metal Centers in an OER Catalyst Model. One of the challenges in solar fuels generation is generating multiple redox equivalents for the oxygen evolution reaction (OER) (i.e., 2H2O→O2+4H++4e-) that is a four-electron, four-proton coupled process. Earth-abundant first-row transition metal oxides of cobalt, nickel, and their mixed-metal forms can drive this half-reaction at relatively low overpotentials. In collaboration with Nocera Group, we used a combination of in situ and ex situ X-ray absorption and emission spectroscopies as well as quantum mechanical calculations on OER thin films and their molecular and heterogeneous inorganic analogs. These studies provided insights into the electronic structures of the high-valent states involved in the mechanisms of O-O bond formation and for the high-valent states to drive the bond-forming and bond-breaking steps of OER reactions. Using a Co4O4 cluster as a model for the Co-based OER anode, CoPi, which has a similar topology and can host a stable, but electronically delocalized, mixed-valent Co(IV) state, we employed complementary X-ray spectroscopies, X-ray absorption (XAS) and 1s3p resonant inelastic X-ray scattering (Kβ RIXS), to effectively extract Co(IV) contributions within a spectroscopically active background (Figure 1). Co K- and L-edge X-ray absorption directly probe the 3d-manifold of effectively localized Co(IV), providing a handle on the covalency of the dπ-based redox active molecular orbital (MO) of Co4O4. Kβ RIXS is shown to be highly sensitive to metal cluster oxidation state and oxo-mediated metal-metal coupling in high-valent, delocalized mixed-valent species. We also probed the doubly oxidized Co(III)2(IV)2 state of the Co4O4 cluster, which features a cofacial Co(IV)2 site and is thus a molecular model of the active site in Co-OEC. Electron and Energy Relays in the Excited State Supramolecular Dinuclear Transition Metal Complexes. The rational design of multinuclear transition metal complexes (TMCs) for photochemical catalysis of homogeneous and/or heterogeneous multi-electron reactions (e.g. for producing solar fuels) requires a detailed understanding of the often unique and convoluted excited state charge and energy transfer dynamics in this class of compounds. Using combined optical and X-ray transient spectroscopic measurements, excited state electron electron relays in tetrapyridophenazine-bridged heteroleptic dinuclear Cu(I) bis(phenanthroline) complexes have been investigated as an exemplary system.

Bismuth Substituted Strontium Cobalt Perovskites for Catalyzing Oxygen Evolution

Late first-row transition metal oxides, based on cobalt, nickel and iron [1] are reported to be the most active non-precious catalysts for oxygen evolution reaction (OER) in basic solution. Experimental and computational studies in the past decade have been focusing on elucidating OER mechanisms [2-3] and identifying activity and stability descriptors [3-6], where perovskites family (ABO3-δ) with immense structural, chemical and electronic flexibility associated with vast selections of A-site and B-site metal ions and oxygen deficiency [7] has been used to develop design principles of OER activity and stability. Recent works [6-8] have shown that lowering charge-transfer gap or increasing metal-oxygen covalency in perovskites can enhance the OER kinetics, by reducing the energetic barriers associated with electron transfer on the surface of metal oxides including the most active catalysts. However, reducing the charge-transfer gap also lowers the Fermi level on the absolute energy scale, making it below the OER redox potential in the basic solution for the most active catalysts and rendering weaker surface hydroxide affinity [6]. Consequently, the OER kinetics on highly covalent/active metal oxides are limited by proton transfer. Moreover, increasing the covalency typically moves the O 2p band closer to the Fermi level, rendering less energy penalty for the creation of oxygen vacancies and thus leading to surface or bulk instability at OER potentials [5]. Here we explored the substitution of A-site ions with high electronegativity or Lewis acidity in the cobalt perovskites to maintain high Co-O covalency by the inductive effect [9], and tune the surface acid-base chemistry by introducing highly Lewis acidic A-site ions to facilitate OER kinetics. Bismuth-substituted strontium cobalt perovskite, Bi0.2Sr0.8CoO3-δ, was shown to exhibit record OER specific activity in alkaline solution, exceeding those of other Co-based perovskite oxides reported to date, including SrCoO3-δ [8], at high current densities (> 1 mA cm-2 oxide). In addition, neither structural or chemical changes have been found for Bi0.2Sr0.8CoO3-δ, indicative of greater structural stability than other highly covalent oxide catalysts, e.g. Ba0.5Sr0.5Co0.8Fe0.2O3-δ [4]. The enhanced OER kinetics and high surface stability can be attributed to the stronger affinity towards hydroxide ions to facilitate surface deprotonation due to the presence of strong Lewis acidic surface Bi3+ ions, and the lowered O 2p-band center relative to the Fermi level upon bismuth substitution into the perovskite structure, respectively. This work exemplifies a novel strategy to facilitate the OER kinetics of highly active oxide catalysts by leveraging the inductive effect associated with rational metal substitution to maintain high metal-oxygen covalency and strengthen hydroxide affinity without the expense of surface stability.