Oxygen Evolution Reaction Pathway Research Articles

Surface self-reconstruction of telluride induced by in-situ cathodic electrochemical activation for enhanced water oxidation performance

Metal tellurides attract recent attention because of their promising applications as effective catalysts for the oxygen evolution reaction (OER). However, inappropriate adsorption energy between OER intermediates and telluride leads to an unsatisfactory electrocatalytic intrinsic activity. Herein, we adopt a unique in-situ cathodic electrochemical activation process to facilitate the surface self-reconstruction to form oxygen vacancy (OV)-rich TeO2 layer onto Fe-doped NiTe (OV@Fe-NiTe). Characterizations and theoretical calculation demonstrate that the presence of the OV-rich TeO2 layer realizes the adjustment of d-band center of the active site that translates into an enhancement of the adsorption of *OOH intermediate and thus the optimization of the OER pathway. Consequently, the OV@Fe-NiTe only requires an ultralow overpotential of 245 mV to drive 100 mA cm-2 in 1 M KOH, 95 mV lower than that of Fe-NiTe, and hence becoming the best water oxidation electrocatalysts amongst recently reported telluride electrocatalysts. This study presents a unique strategy to exploit telluride-based catalysts through electrochemical surface engineering.

Structural Design Strategy and Active Site Regulation of High-Efficient Bifunctional Oxygen Reaction Electrocatalysts for Zn-Air Battery.

Zinc-air batteries (ZABs) exhibit high energy density as well as flexibility, safety, and portability, thereby fulfilling the requirements of power batteries and consumer batteries. However, the limited efficiency and stability are still the significant challenge. Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are two crucial cathode reactions in ZABs. Development of bifunctional ORR/OER catalysts with high efficiency and well stability is critical to improve the performance of ZABs. In this review, the ORR and OER mechanisms are first explained. Further, the design principles of ORR/OER electrocatalysts are discussed in terms of atomic adjustment mechanism and structural design in conjunction with the latest reported in situ characterization techniques, which provide useful insights on the ORR/OER mechanisms of the catalyst. The improvement in the energy efficiency, stability, and environmental adaptability of the new hybrid ZAB by the inclusion of additional reaction, including the introduction of transition-metal redox couples in the cathode and the addition of modifiers in the electrolyte to change the OER pathway, is also summarized. Finally, current challenges and future research directions are presented.

Phosphate-induced interfacial electronic engineering in VPO4-Ni2P heterostructure for improved electrochemical water oxidation

Anodic oxygen evolution reaction (OER) is the key bottleneck for water electrolysis technique owing to its sluggish reaction kinetics. Interfacial engineering on the rationally designed heterostructure can regulate the electronic states efficiently for intrinsic activity improvement. Here, we report a co-phosphorization approach to construct a VPO4-Ni2P heterostructure on nickel foam with strongly chemical binding, wherein phosphate acts as electronic modifier for Ni2P electrocatalyst. Profiting from the interfacial interaction, it is uncovered that electron shifts from Ni2P to VPO4 to render valence increment in Ni species. Such an electronic manipulation rationalizes the chemical affinities of various oxygen intermediates in OER pathway, giving a substantially reduced energy barrier. As a result, the advanced VPO4-Ni2P heterostructure only requires an overpotential of 289 mV to deliver a high current density of 350 mA/cm2 for OER in alkaline electrolyte, together with a Tafel slope as low as 28 mV/dec. This work brings fresh insights into interfacial engineering for advanced electrocatalyst design.

A cobalt hydroxide coated metal-organic framework for enhanced water oxidation electrocatalysis

Although metal–organic frameworks (MOFs) with coordinatively unsaturated metal centers have been regarded as promising electrocatalysts towards oxygen evolution reaction (OER), few reports focus on MOFs with inactive coordinative-saturated metal centers. Herein, we report a universal strategy to generate a layer of Co(OH)2 coating on the surface of Co-MOFs (Co(OH)2/Co-MOF) through an in situ cathodic electro- transformation approach. Experimental analysis and density functional theory (DFT) calculations reveal that electrons transfer from Co(OH)2 to Co-MOF could synergistically tailor the adsorption free energy of oxygenic intermediates in the OER pathways, which together with the unique structure of Co-MOF, contribute to the outstanding OER performance and long-term stability, with overpotential of 196 mV to achieve the current density of 10 mA cm−2.

Facet-Independent Oxygen Evolution Activity of Pure β-NiOOH: Different Chemistries Leading to Similar Overpotentials.

β-Nickel oxyhydroxide (β-NiOOH) is a promising electrocatalyst for the oxygen evolution reaction (OER), which is the more difficult half-reaction involved in water splitting. In this study, we revisit the OER activities of the two most abundant crystallographic facets of pristine β-NiOOH, the (0001) and (1010) facets, which expose 6-fold-lattice-oxygen-coordinated and 5-fold-lattice-oxygen-coordinated Ni sites, respectively. To this end, we model various active sites on these two facets using hybrid density functional theory, which includes a fraction of the exact nonlocal Fock exchange in the electronic description of the system. By evaluating thermodynamic OER overpotentials, we show that the two active sites considered on each crystallographic facet demonstrate OER activities remarkably different from each other. However, the lowest OER overpotentials calculated for the two facets were found to be similar to each other and comparable to the overpotential for the 4-fold-lattice-oxygen-coordinated Ni site on the (1211) facet of β-NiOOH previously examined in J. Am. Chem. Soc. 2019 , 141 , 1 , 693 - 705 . This finding shows that all of the low-index facets investigated so far could be responsible for the experimentally observed OER activity of pristine β-NiOOH. However, the lowest overpotential active sites on these three crystallographic facets operate via different mechanisms, underscoring the importance of considering multiple OER pathways and intermediates on each crystallographic facet of a potential electrocatalyst. Specifically, our work demonstrates that consideration of previously overlooked active sites, transition-metal-ion oxidation states, reaction intermediates, and lattice-oxygen-stabilization are critical to reveal the lowest overpotential OER pathways on pristine β-NiOOH.

Structure and reactivity of a water-covered anatase TiO2(001) surface.

We systematically studied water adsorption and oxidation on the unreconstructed TiO2(001) surface using first-principles calculations. Water first adsorbs on the surface in a dissociative state and then in a molecular state, as water coverage increases. The geometric properties of all adsorption structures suggest that the dissociative water molecules can induce stress release of the (001) surface at low coverage, reducing reactivity of the surface and thus leading to molecular adsorption of water on the surface at high coverage. The adsorption energy (or the surface energy) monotonously increases (or decreases) with the increase of the coverage, which further confirms that water, irrespective of its dissociative or molecular state, can improve the stability of the (001) surface and reduce its activity. We deeply investigated the mechanism of the oxygen evolution reaction (OER) on the water-covered (001) surface. A new water-assisted OER pathway is identified on the (001) surface, which includes the sequential transfer of protons from molecular water and surface hydroxyls, and O-O coupling processes. During the OER pathway, the O-O coupling step exhibits the largest thermodynamic energy and highest energy barrier, clarifying that it is the rate-determining step in the whole pathway. Our findings provide new insights into the strong dependence of water adsorption modes on coverage for the anatase TiO2(001) surface and may explain the high oxidation activity of the TiO2(001) surface in aqueous environments typical of TiO2 photocatalysis.

Charge transfer mechanisms underlying Contact Glow Discharge Electrolysis

Understanding charge transfer at strongly polarised gas-evolving electrodes is crucial for the development and control of Contact Glow Discharge Electrolysis (CGDE), a method of interest in a wide range of applications spanning from biochemistry to materials engineering. Since the pioneering work of Kellogg in 1950, the transition from conventional electrolysis to the CGDE regime has been generally associated with excessive ohmic heating at the working electrode, disregarding basic principles of electrochemical kinetics. Here we attempt to address this controversy by investigating of a CGDE process at a Fe-13%Cr anode in aqueous solutions of (NH4)2SO4 using an original method of in-situ impedance spectroscopy combined with voltammetric, gravimetric, chemical analysis and optical emission spectroscopy techniques. The study reveals three characteristic relaxation processes representing charge transfer through the surface oxide-hydroxide layer, the presence of which is confirmed by Glow Discharge Optical Emission and Raman Spectroscopies, and adsorption-desorption behaviour which we identify with oxo- and peroxo-intermediates of the oxygen evolution reaction (OER) coupled with anodic metal dissolution. We explain the transition to CGDE by the OER pathway change from direct recombination of two terminally coordinated oxides to a single-site route involving early peroxide formation. This change triggers electrochemical luminescence which proceeds via a co-reactant mechanism involving fragments of metastable peroxide structures and metal cations at the active sites on the anode surface that produce electrically excited intermediates prone to radiative relaxation. Our results undermine existing CGDE theories whilst emphasising common bases in behaviour of conventional and plasma-assisted electrochemical systems, thus providing far-reaching implications for understanding, development and practical use of electrolytic plasma processes.

Inverse Spinel Cobalt–Iron Oxide and N-Doped Graphene Composite as an Efficient and Durable Bifuctional Catalyst for Li–O2 Batteries

Rational design of efficient bifunctional oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) electrocatalysts are critical for rechargeable Li–O2 batteries. Here, we report inverse spinel Co[Co,Fe]O4/nitrogen-doped graphene (NG) composite used as a promising catalyst for rechargeable Li–O2 batteries. The cells with Co[Co,Fe]O4/NG catalyst exhibit high initial capacity, remarkable cyclability, and good rate capability. Moreover, the overpotential of the Li–O2 batteries is reduced significantly. The improved ORR/OER performances are attributed to the good property of Co[Co,Fe]O4 with an inverse spinel structure toward ORR and the improved electronic conductivity of N-doped graphene. The density functional theory (DFT) calculation shows the rate limitation step for ORR on the inverse spinel surface is the growth of the Li2O2 cluster while the rate limitation step for the OER pathway is the oxidation of Li2O2. The inverse spinel surface in Co[Co,Fe]O4/NG is more active than that of the normal...

Mechanistic understanding on oxygen evolution reaction on γ-FeOOH (010) under alkaline condition based on DFT computational study

An efficient and economical oxygen evolution reaction (OER) catalyst is critical to the widespread application of solar energy to fuel conversion. Among many potential OER catalysts, the metal oxyhydroxides, especially FeOOH, show promising OER reactivity. In the present work, we performed a DFT + U study of the OER mechanism on the γ-FeOOH (010) surface. In particular, we established the chemical potential of the OH− and hole pair and included the OH− anion in the reaction pathway, accounting to the alkaline conditions of anodic OER process. We then analyzed the OER pathways on the surface with OH-, O- and Fe-terminations. On the surface with OH- and O- terminations, the O2 molecule could form from either OH reacting with the surface oxygen species (–OH* and –O*) or the combination of two surface oxygen species. On the Fe-terminated surface, O2 can only form by adsorbing OH on the Fe sites first. The potential-limiting step of the oxygen evolution with different surface terminations was determined by following the free-energy change of the elementary steps along each pathway. Our results show that oxygen formation requires recreating the surface Fe sites, and consequently, the condition that favors the partially exposed Fe sites will promote oxygen formation.

Developing Cobalt Doped Pr0.5Ba0.5MnO3-δ Electrospun Nanofiber Bifunctional Catalyst for Oxygen Reduction Reaction and Oxygen Evolution Reaction

The development of energy storage and conversion devices provides a beneficial approach for the renewable energy application, which can help relieve the severe reliance on fossil fuels and also address problems related to global climate change. Currently, the efficiencies of energy conversion devices such as the metal–air batteries and fuel cells are mainly limited by the sluggish kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) processes. Developing catalytically active and cost effective catalyst for the ORR and OER is of prime importance. So far, noble metals and their alloys, such as Pt and Pt-Pd, have been exclusively used as electrochemical bifunctional catalyst in ORR and OER due to their superior catalytic activity. However, the high cost, limited availability, poor durability and sluggish electron transfer kinetics of noble metal based bifunctional catalysts have impeded the practical application of metal-air batteries. Therefore, the discovery of cost effective, catalytically active alternative bifunctional catalyst such as non-precious metals, carbonaceous materials, and transition metal oxides is highly desirable. Perovskite oxide (ABO3) possesses a unique electronic structure and chemical defect properties, and has been demonstrated to be a promising non-precious metal catalyst among the transition metal oxides in the application of metal air batteries and alkaline fuel cells. It is known that the intrinsic electrochemical catalytic activity is mainly determined by the B site cation in perovskite oxide. Extensive research conducted by Yang et al. demonstrated that the ORR and OER catalytic activities of perovskite oxides follow a volcanic relationship with the filling of electrons in antibonding orbitals [1]. Among the typical LaMO3 (M= Mn, Co, Fe, Ni, Cr), LaMnO3 and LaCoO3 have exhibited highest ORR and OER catalytic activities, respectively. In addition, due to the desirable electronic properties of the perovskite oxides, strategies such as cation partial substitution and oxygen non-stoichiometry formation could, therefore, be utilized as the effective approaches to fine-tune the catalytic properties and to achieve a better bifunctional activity [2,3]. To enhance the bifunctional catalytic performance, improved specific surface area as well as enhanced oxygen channel are desired. Researchers have found that electrochemical catalysts with porous nanofiber structures could favor the ORR and OER pathways by providing uniform O2 electrolyte distribution, and beneficial oxygen diffusion channels. Zhao et al. have synthesized mesoporous La0.5Sr0.5CoO2.91 nanowires through the multistep microemulsion method, showing significant enhanced catalytic performance [4]. Xu et al. have demonstrated that the porous La0.75Sr0.25MnO3 nanotube catalyst fabricated through facile electrospinning technologies provides favorable advantages to the availability of the catalytic active sites in the organic solvent electrolyte in Li-O2 batteries [5].Herein, we developed a bifunctional perovskite catalyst towards both ORR and OER in alkaline solution. Cobalt cations were doped into Pr0.5Ba0.5MnO3-δ perovskite to achieve the higher intrinsic ORR and OER activities by engineering the structure symmetry, electronic and the oxygen vacancy defects of the material. The bifunctional catalyst with a unique porous nanofiber structure was fabricated by electrospinning technology (Figure.1). A single phase perovskite oxide Co doped Pr0.5Ba0.5MnO3-δ was obtained after sintering the electrospun precursor at high temperature. The ORR and OER activities of the composite catalysts consisting 50 wt% of the as-synthesized perovskite oxides and 50 wt% carbon black were investigated in 0.1 KOH with rotating disk electrode (RDE). Significant enhancement in ORR and OER performance was achieved via using the composite catalyst with Co doped Pr0.5Ba0.5MnO3-δ nanofiber/carbon black with respect to the reduced overpotential and improved current density. In particular, the Co doped Pr0.5Ba0.5MnO3-δ nanofiber/carbon black composite demonstrated enhanced electron transfer number in the ORR process, indicating preferable dominance of four electron transfer pathway. Moreover, the Co doped Pr0.5Ba0.5MnO3-δ composite exhibited high stability during the cycling tests, indicating its promising applications in metal-air batteries.

A Functionally Stable Manganese Oxide Oxygen Evolution Catalyst in Acid

First-row metals have been a target for the development of oxygen evolution reaction (OER) catalysts because they comprise noncritical elements. We now report a comprehensive electrochemical characterization of manganese oxide (MnOx) over a wide pH range, and establish MnOx as a functionally stable OER catalyst owing to self-healing, is derived from MnOx redeposition that offsets catalyst dissolution during turnover. To study this process in detail, the oxygen evolution mechanism of MnOx was investigated electrokinetically over a pH range spanning acidic, neutral, and alkaline conditions. In the alkaline pH regime, a ∼60 mV/decade Tafel slope and inverse first-order dependence on proton concentration were observed, whereas the OER acidic pH regime exhibited a quasi-infinite Tafel slope and zeroth-order dependence on proton concentration. The results reflect two competing mechanisms: a one-electron one-proton PCET pathway that is dominant under alkaline conditions and a Mn(3+) disproportionation process, which predominates under acidic conditions. Reconciling the rate laws of these two OER pathways with that of MnOx electrodeposition elucidates the self-healing characteristics of these catalyst films. The intersection of the kinetic profile of deposition and that of water oxidation as a function of pH defines the region of kinetic stability for MnOx and importantly establishes that a non-noble metal oxide OER catalyst may be operated in acid by exploiting a self-healing process.