Transition metal perovskite (ABO3) is an emerging type of oxygen evolution reaction (OER) electrocatalyst that shows reasonably good activity and moderate stability. Although efforts have been made to improve perovskite' OER performance by various element substitution at A/B-site, the influence of ion, particularly non-metallic ion, substitutions on the OER mechanism are rarely studied. More and more evidence has shown that the metal-center theory has failed to explain lots of OER-related phenomena. Therefore, it is urgent to understand how the cation and anion sites in perovskite determine OER performance. Here, we used a Fe and P co-doped LaCoO3 as a model system to explore the influence of substitution in perovskite by combinng operando/ex-situ X-ray characterization and density functional theroy (DFT). We observed enhanced OER catalytic activities in co-doped materials, which are attributed to the stronger transition-metal-oxygen-bonding-covalency (TMOBC). The detailed analyses by O K-edge XAS, electrochemical performance, and DFT suggest that the hybridization between O 2p and transition metal 3d eg orbitals could be a more credible descriptor of perovskite for OER, which is the combination of eg orbital theory and TMOBC theory. The finding in our work provides insights into the OER catalysis mechanism on metal oxides, which could guide new design of cost-effective oxide electrocatalysts.

We combine state-of-the-art oxide epitaxial growth by hybrid molecular beam epitaxy with transport, x-ray photoemission, and surface diffraction, along with classical and first-principles quantum mechanical modeling to investigate the nuances of insulating layer formation in otherwise high-mobility homoepitaxial n-SrTiO3(001) films. Our analysis points to charge immobilization at the buried n-SrTiO3/undoped SrTiO3(001) interface as well as within the surface contamination layer resulting from air exposure as the drivers of electronic dead-layer formation. As Fermi level equilibration occurs at the surface and the buried interface, charge trapping reduces the sheet carrier density (n2D) and renders the n-STO film insulating if n2D falls below the critical value for the metal-to-insulator transition.

The capacity decay of layered cathodes in high-voltage applications underscores the need to utilize accurate and precise techniques to understand the underlying mechanisms. Here, we use well-defined epitaxial LiCoO2 (LCO) films on SrRuO3/SrTiO3 (SRO/STO) with controlled orientations and defect structures along with in situ electrochemical atomic force microscopy to probe the structural and morphological evolutions during the charge and overcharge processes. We quantitatively show the morphological changes in both the reversible delithiation regime and the irreversible over-delithiation regime and correlate the overall electrochemical behaviors to atomic scale defect evolutions in the films. We also observe a significantly lower charging capacity for LCO/SRO/STO(111) compared to that of LCO/SRO/STO(001) films of the same thickness, which is ascribed to different types of atomic scale defects formed during the film growth process. Our high- resolution scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) studies reveal that the antiphase boundaries in LCO/SRO/STO(111) act as viable channels for Li migration but are more susceptible to irreversible phase transitions, which then block subsequent Li diffusion. The failure mechanisms developed here may provide insight into the design of future cathode materials.

We show how transient electron scavengers can be utilized to control the carrier concentration at polar/nonpolar perovskite interfaces. By combining quantitative synchrotron x-ray-based interface structure determination with ab initio modeling, we demonstrate that Nd vacancy formation and the resulting formation of Nd adatoms, stabilized by oxygen scavengers at the growth front, can quantitatively account for the decreased carrier concentration at the SrTiO3/n NdTiO3/SrTiO3 (001) heterojunction for n = 1 unit cell. This study yields insight into growth mechanisms and the effect of transient species and defects on the electronic properties of oxide heterojunctions.

Epitaxial growth is a powerful tool for synthesizing heterostructures and integrating multiple functionalities. However, interfacial mixing can readily occur and significantly modify the properties of layered structures, particularly for those containing energy storage materials with smaller cations. Here, we show a two-step sequence involving the growth of an epitaxial LiCoO2 cathode layer followed by the deposition of a binary transition metal oxide. Orientation-controlled epitaxial synthesis of the model solid-state-electrolyte Li2WO4 and anode material Li4Ti5O12 occurs as WO3 and TiO2 nucleate and react with Li ions from the underlying cathode. We demonstrate that this lithiation-assisted epitaxy approach can be used for energy materials discovery and exploring different combinations of epitaxial interfaces that can serve as well-defined model systems for mechanistic studies of energy storage and conversion processes.

Restructuring is an important yet less understood phenomenon in the catalysis community. Recent studies have shown that a group of transition metal sulfide catalysts can completely or partially restructure during electrochemical reactions which then exhibit high activity even better than the best commercial standards. However, such restructuring processes and the final structures of the new catalysts are elusive, mainly due to the difficulty from the reaction-induced changes that cannot be captured by ex situ characterizations. To establish the true structure-property relationship in these in situ generated catalysts, we use multimodel operando characterizations including Raman spectroscopy, X-ray absorption spectroscopy, and X-ray reflectivity to investigate the restructuring of a representative catalyst, Co9S8, that shows better activity compared to the commercial standard RuO2 during the oxygen evolution reaction (OER), a key half reaction in water-splitting for hydrogen generation. We find that Co9S8 ultimately converts to oxide cluster (CoO x ) containing six oxygen coordinated Co octahedra as the basic unit which is the true catalytic center to promote high OER activity. The density functional theory calculations verify the in situ generated CoO x consisting of edge-sharing CoO6 octahedral clusters as the actual active sites. Our results also provide insights to design other transition-metal-based materials as efficient electrocatalysts that experience a similar restructuring in OER.

Aqueous sodium-ion batteries represent a promising approach for stationary energy storage; however, the lack of appropriate anode materials has substantially retarded their development. Herein, we demonstrated an iron-based phosphate material of Na3Fe3(PO4)4 as an inexpensive and efficacious anode alternative. While the Fe3+/Fe2+ redox couple renders a two-Na-insertion reaction with desirable potentials, its unique layered structure further facilitates the Na-insertion kinetics and reversibility. Consequently, this electrode exhibits an appealing Na-insertion performance, with a reversible capacity of ∼83 mAh g–1, suitable anode potential of −0.4 V vs Ag/AgCl, excellent rate capability of 200 C, and outstanding cycling of 6000 cycles. Utilizing operando synchrotron X-ray diffraction and X-ray absorption spectroscopy, we revealed the structural evolution of the Na3Fe3(PO4)4 anode during the two-electron reaction, where the extremely small volume expansion (∼3%) enables its fast-charging and long-cycling capability. Our work suggests new considerations of developing versatile iron phosphate compounds as appealing anode materials for energy storage in aqueous electrolytes.

Rare earth nickelates including LaNiO3 are promising catalysts for water electrolysis to produce oxygen gas. Recent studies report that Fe substitution for Ni can significantly enhance the oxygen evolution reaction (OER) activity of LaNiO3. However, the role of Fe in increasing the activity remains ambiguous, with potential origins that are both structural and electronic in nature. On the basis of a series of epitaxial LaNi1-xFexO3 thin films synthesized by molecular beam epitaxy, we report that Fe substitution tunes the Ni oxidation state in LaNi1-xFexO3 and a volcano-like OER trend is observed, with x = 0.375 being the most active. Spectroscopy and ab initio modeling reveal that high-valent Fe3+δ cationic species strongly increase the transition-metal (TM) 3d bandwidth via Ni-O-Fe bridges and enhance TM 3d-O 2p hybridization, boosting the OER activity. These studies deepen our understanding of structural and electronic contributions that give rise to enhanced OER activity in perovskite oxides.

Ammonia (NH3) electrosynthesis gains significant attention as NH3 is essentially important for fertilizer production and fuel utilization. However, electrochemical nitrogen reduction reaction (NRR) remains a great challenge because of low activity and poor selectivity. Herein, a new class of atomically dispersed Ni site electrocatalyst is reported, which exhibits the optimal NH3 yield of 115 mu g cm(-2) h(-1) at -0.8 V versus reversible hydrogen electrode (RHE) under neutral conditions. High faradic efficiency of 21 +/- 1.9% is achieved at -0.2 V versus RHE under alkaline conditions, although the ammonia yield is lower. The Ni sites are stabilized with nitrogen, which is verified by advanced X-ray absorption spectroscopy and electron microscopy. Density functional theory calculations provide insightful understanding on the possible structure of active sites, relevant reaction pathways, and confirm that the Ni-N-3 sites are responsible for the experimentally observed activity and selectivity. Extensive controls strongly suggest that the atomically dispersed NiN3 site-rich catalyst provides more intrinsically active sites than those in N-doped carbon, instead of possible environmental contamination. This work further indicates that single-metal site catalysts with optimal nitrogen coordination is very promising for NRR and indeed improves the scaling relationship of transition metals.

Atomically dispersed FeN4 active sites have exhibited exceptional catalytic activity and selectivity for the electrochemical CO2 reduction reaction (CO2RR) to CO. However, the understanding behind the intrinsic and morphological factors contributing to the catalytic properties of FeN4 sites is still lacking. By using a Fe-N-C model catalyst derived from the ZIF-8, we deconvoluted three key morphological and structural elements of FeN4 sites, including particle sizes of catalysts, Fe content, and Fe-N bond structures. Their respective impacts on the CO2RR were comprehensively elucidated. Engineering the particle size and Fe doping is critical to control extrinsic morphological factors of FeN4 sites for optimal porosity, electrochemically active surface areas, and the graphitization of the carbon support. In contrast, the intrinsic activity of FeN4 sites was only tunable by varying thermal activation temperatures during the formation of FeN4 sites, which impacted the length of the Fe-N bonds and the local strains. The structural evolution of Fe-N bonds was examined at the atomic level. First-principles calculations further elucidated the origin of intrinsic activity improvement associated with the optimal local strain of the Fe-N bond.

A novel strategy is designed to stabilize atomic Pt catalysts in alloyed platinum cobalt nanosheets with trapped interstitial fluorine (SA-PtCoF) for zinc-air batteries.

Creating new functionality in materials containing transition metals is predicated on the ability to control the associated charge states. For a given transition metal, there is an upper limit on valence that is not exceeded under normal conditions. Here, it is demonstrated that this limit of 3+ for Ni and Fe can be exceeded via synthesis of (SrNiO3 )m /(LaFeO3 )n superlattices by tuning n and m. The Goldschmidt tolerance constraints are lifted, and SrNi4+ O3 with holes on adjacent O anions is stabilized as a perovskite at the single-unit-cell level (m = 1). Holding m = 1, spectroscopy reveals that the n = 1 superlattice contains Ni3+ and Fe4+ , whereas Ni4+ and Fe3+ are observed in the n = 5 superlattice. It is revealed that the B-site cation valences can be tuned by controlling the magnitude of the FeO6 octahedral rotations, which, in turn, determine the energy balance between Ni3+ /Fe4+ and Ni4+ /Fe3+ , thus controlling emergent electrical properties such as the band alignment and resulting hole confinement. This approach can be extended to other systems for synthesizing novel, metastable layered structures with new functionalities.

A carbon support with favorable balance between graphitization and hierarchical porosity is promising to address carbon corrosion issue in cathode catalysts for proton exchange membrane fuel cells (PEMFCs).

Zhi Qiaoa,1, Sooyeon Hwangb,1, Xing Lib,h,1, Chenyu Wangc,1, Widitha Samarakoond, Stavros Karakalose, Dongguo Lic, Mengjie Chena, Yanghua Hea, Maoyu Wangd, Zhenyu Liug, Guofeng Wangg, Hua Zhouf, Zhenxing Feng *d, Dong Su*b, Jacob S. Spendelow*c and Gang Wu*a a, Department of Chemical and Biological Engineering, University at Buffalo, The State University of New York, Buffalo, New York 14260, United States b, Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States c, Materials Physics and Applications Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States d, School of Chemical, Biological, and Environmental Engineering, Oregon State University, Corvallis, Oregon 97333, United States e, Department of Chemical Engineering, University of South Carolina, Columbia, South Carolina 29208, United States f X-ray Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States g, Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, PA 15261, United States h, Department of Physics and Engineering, Key Laboratory of Material Physics, Zhengzhou University, Zhengzhou, 450052, China

FeN4 moieties embedded in partially graphitized carbon are the most efficient platinum group metal free active sites for the oxygen reduction reaction in acidic proton-exchange membrane fuel cells. However, their formation mechanisms have remained elusive for decades because the Fe-N bond formation process always convolutes with uncontrolled carbonization and nitrogen doping during high-temperature treatment. Here, we elucidate the FeN4 site formation mechanisms through hosting Fe ions into a nitrogen-doped carbon followed by a controlled thermal activation. Among the studied hosts, the ZIF-8-derived nitrogen-doped carbon is an ideal model with well-defined nitrogen doping and porosity. This approach is able to deconvolute Fe-N bond formation from complex carbonization and nitrogen doping, which correlates Fe-N bond properties with the activity and stability of FeN4 sites as a function of the thermal activation temperature.

Vanadium redox-flow battery (VRB) as a promising electrochemical power source for large-scale energy storage, suffers from various polarization losses despite that it has been extensively studied in the past decades. Among these losses, the sluggish interfacial charge transfer of the vanadium species on the respective electrode renders large overpotentials giving rise to inevitable hydrogen and oxygen evolutions during the charging process. In this study, we report an unprecedented method based on the redox targeting concept to tackle the above issues. Prussian blue (PB) and a Prussian blue analogues (PBA) with identical redox potentials to VO2+/VO2+ and V2+/V3+ are grafted on cathode and anode, respectively. Upon operation, the reversible proton-coupled redox targeting reactions between PB and VO2+/VO2+ on cathode, PBA and V2+/V3+ on anode facilitate the interfacial charge transfer of the vanadium species and concomitantly inhibit the hydrogen and oxygen evolutions, which improves the selectivity of the redox reactions and considerably enhances the round-trip energy efficiency and cycling performance of VRB in a wide range of current densities. The above redox-assisted catalytic reactions were scrutinized and the mechanisms are unequivocally manifested with various electrochemical and spectroscopic measurements. We anticipate the surface immobilized redox catalysis approach demonstrated here would generically provide a paradigm for improving the sluggish kinetic processes in a variety of electrochemical devices.

Waste management practices such as open dumping and composting are widely used for the management of municipal solid waste (MSW) in Sri Lanka. Severe socio-economic and environmental problems occur due to open dumping of major fraction of MSW. Anaerobic digestion is a sustainable waste management strategy to manage organic fraction of MSW (OFMSW). In order to evaluate the feasibility of this strategy, a mechanistic model was developed using a steady state anaerobic digestion model which was modified using improved Chen-Hashimoto kinetic equations. It was applied to treat OFMSW generated in open dumping site at Karadiyana in Sri Lanka. The daily MSW arrival to the site is 550 tons/day and input OFMSW slurry to the anaerobic bioreactor contained of total solids 70 g/l, volatile solids 55.6 g/l, carbohydrates 58.5%, proteins 13% and lipids 3%. According to the mechanistic model, minimum payback period of 1.79 years was achieved when anaerobic bioreactor operated in continuously fed mode at operating temperature of 40°C and hydraulic retention time of 11.28 days. The volatile solid reduction efficiency of 76.62%, methane yield of 0.26 m 3 per kg of volatile solids added, electricity generation of 10.31 W per kg of OFMSW per day and carbon dioxide generation of 0.21 kg per kg of OFMSW were achieved at these optimized process conditions. The model was limited to apply for continuously fed anaerobic bioreactors operated at temperature ranging from 30°C to 40°C.