Active Oxygen Evolution Reaction Catalysts Research Articles

Coupling the vanadium-induced amorphous/crystalline NiFe2O4 with phosphide heterojunction toward active oxygen evolution reaction catalysts

Abstract Developing efficient and robust electrocatalysts is increasingly essential for water splitting, severely hindered by the sluggish four-electron transfer process of oxygen evolution reaction (OER). Amorphous/crystalline heterophase engineering has recently emerged as a promising electronic modulation approach for OER catalysts but suffers from poor conductivity of the amorphous structure. Here, we coupled the amorphous/crystalline NiFe2O4 induced by vanadium doping with NiP heterojunction, highlighting the synergistic effect in modulating the electronic structures and the complementary effect in promoting conductivity. As a superior electrocatalyst to commercial RuO2, the as-prepared NFO-V0.3-P showed a low overpotential of 277 mV at the current density of 20 mA cm−2, a Tafel slope of 45 mV dec−1, and long-term stability. The excellent OER catalytic activity is attributed to the synergistic effect at the heterophase interface with rich active sites, fine-tuning of electronic regulation, and enhanced conductivity.

Highly active and stable amorphous IrOx/CeO2 nanowires for acidic oxygen evolution

Development of highly active and durable electrocatalysts for acidic oxygen evolution reaction (OER) remains an unresolved grand challenge. Here, we reported the amorphous IrOx/CeO2 nanowires as highly active and acid-stable OER catalysts through a facile electro-spinning/calcination approach. The amorphous catalysts delivered a high mass activity of 167 A gIr−1 at 1.51 V, a low overpotential of 220 mV at 10 mA cm−2, and a stable performance for 300 h of continuous operation in acid. As revealed by complementary experimental and theoretical calculation results, the intimate nanoscale feature of IrOx/CeO2 creates abundant binary interfaces, at which CeO2 as an electron buffer regulates the adsorption of oxygen intermediates, lowers the activation barrier of OER, and suppresses the over-oxidation and dissolution of Ir, thereby significantly enhancing the OER activity and stability. This work provides a new strategy for designing highly active and acid-resistant OER catalysts.

Ir Nanoconfined and Doped MnO2 Nanosheets for an Enhanced Electrocatalytic Oxygen Evolution Reaction in Acidic and Basic Medium

One of the promising strategies to generate hydrogen for a future hydrogen economy is via electrocatalytic water splitting, where renewable electricity would be used to convert water into hydrogen and oxygen on an electrocatalyst within an electrolyzer. Oxygen evolution reaction (OER), the anodic half-reaction of water splitting is, however, kinetically sluggish and requires four electron-proton transfers making the overall reaction energy-intensive. For the economical production of hydrogen, improving the reaction kinetics in both low and high pH is of utmost necessity by designing an active and stable OER catalyst. Precious metals like Ir, Ru, and their oxides are known to be excellent OER catalysts but their large-scale application is hindered due to their low earth-abundancy. To address this issue, we have synthesized two types of OER catalysts with varying Ir wt.% to achieve a higher OER-activity per mass of Ir in the catalyst: Ir/MnO2, Ir nano-confined in the interlayer of layered MnO2 nanosheets and Ir-doped MnO2 nanosheets. In both acidic and basic environments, Ir/MnO2 exhibited better OER activity than Ir-doped MnO2. In 0.5 M H2SO4, 16.7 wt.% Ir/MnO2 and 17.5 wt.% Ir-doped MnO2 exhibited OER overpotentials (η) of 298 mV and 320 mV, respectively, at a current density of 10 mA cm-2. In 1M KOH electrolyte, 15.4 wt.% Ir/MnO2 and 17.5 wt.% of Ir-doped MnO2 exhibited OER η values of 213 mV and 245mV, respectively at 10 mA cm-2. These overpotentials are ~500 mV lower than the pristine MnO2 nanosheets and ~60-120 mV lower than commercial reference catalysts (20 wt.% Ir/C and IrO2) at both low and high pH conditions, respectively. These synthesized catalysts were able to outperform the commercial materials by exhibiting four times higher OER-activity per mass of Ir, in both acidic and basic media. Hence, this synthetic approach may be a method to achieve enhanced OER activity using a lower mass of Ir. The physical characterization of the materials was carried out with X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray absorption spectroscopy (XAS).

Ruthenium-Zirconium Oxides As Highly Stable Oxygen Evolution Electrocatalysts

The further development and utilization of proton exchange membrane water electrolyzers (PEMWEs) is hindered by the cost, activity, and stability of the oxygen evolution reaction (OER) electrocatalyst. Iridium oxide (IrOx) is currently the go-to OER electrocatalyst, as it has been shown to have relative high activity and stability when compared to other OER active catalysts. However, iridium is one of the rarest elements in the Earth’s crust and therefore cost is a major limitation of iridium-based electrocatalysts. Ruthenium oxide (RuO2) is a highly active OER catalyst; although, it is highly unstable in acidic media and undergoes catalyst degradation over time. We investigated modifying RuO2 by substituting zirconium, which is highly stable in acidic conditions, to provide an electrocatalyst with increased stability. Our study explored the effect of low and moderate zirconium concentrations within RuO2 (Ru1-xZrxO2) on the structure, morphology, OER activity, and stability. The structure and morphology were characterized by X-ray diffraction and scanning electron microscopy. Preliminary results from XRD showed no observable phase separation at low Zr concentrations, and peak shifts were indicative of the incorporation of the larger Zr ion into the crystal structure of rutile RuO2. The OER activities and stabilities of Ru1-xZrxO2 were measured using a rotating disk electrode configuration and compared with RuO2. Our preliminary results show that the OER activity and stability are strongly affected by the addition of Zr and there may be an optimal concentration range for obtaining a balance between OER activity and stability. Our work furthers the understanding of how to develop OER electrocatalysts with increased stability while maintaining a high OER activity, which is crucial to the large-scale adoption of PEMWE’s.

Stability of Metal Hydroxide Organic Frameworks for Oxygen Evolution

The oxygen evolution reaction (OER) is central to storing electrical energy via chemical bonds in energy carriers and fuels through reactions such as electrochemical water splitting to produce hydrogen, CO2 reduction for CO and liquid hydrocarbons, and nitrogen to ammonia. While there has been considerable work towards the engineering of inexpensive yet highly active OER catalysts, current state-of-the-art materials are still at least an order of magnitude less active than oxygen evolving complexes found in biological systems that intricately combine inorganic metal-oxo clusters with organic ligands.1 As a result, metal organic frameworks (MOFs) have drawn considerable attention as hybrid organic-inorganic systems that can potentially mimic the unique structure of biological oxygen evolving complexes. Recently, metal hydroxide organic frameworks (MHOFs),2 a new class of MOFs that combine layered hydroxides with aromatic carboxylate linkers that stabilize the structure via π-π interactions, have been shown to display three times the tunability of layered hydroxides, offering extensive opportunities for further engineering of its electrochemical properties for numerous applications. However, the long-term stability of these material during OER is still unclear, which would be paramount to understand for further rational design of this class of material.In this study, we investigated Ni-based MHOFs with carboxylate linkers of varying π-π interaction strengths to understand how these differences affect the electrochemical stability of these materials during OER. We observed that the MHOFs all undergo activation during OER leading to two orders of magnitude increase in OER activity, where the MHOFs with weaker π-π interaction strengths tend to transform at a faster rate than the MHOFs with stronger π-π interaction strengths. We further characterized the MHOFs using a wide range of analytical techniques, including scanning transmission electron microscopy, Raman spectroscopy, x-ray photoelectron spectroscopy, and hard x-ray absorption spectroscopy, before and after extended OER cycling and galvanostatic tests to understand the transformed phase, which suggested that while the bulk structure largely remains unchanged, the surface undergoes significant restructuring into a Ni(OH)2-like phase during OER. Using operando UV-vis and Raman spectroscopy measurements on the MHOFs during OER to understand the factors that induce the transformation process, we found that there was a clear link between the Ni2+/3+, 4+ redox couple observed around 1.4 VRHE and a loss in the carboxylate organic linkers for the linkers with weak π-π interactions. However, for the MHOFs synthesized with linkers exhibiting strong π-π interaction strengths, there were smaller changes to the overall material during OER, suggesting that the bulk stability of these materials is largely dictated by the linker interaction strength and activation are primarily only surface transformations. These results directly demonstrate that linker selection also plays a key role in the stability MOFs under electrochemical conditions and are pertinent for rational design and understanding of the stability and activity of hybrid organic-inorganic materials as electrocatalysts.

In Situ Grown CoMn-Based Metal–Organic Framework on Nickel Foam as Efficient and Robust Electrodes for Electrochemical Oxygen Evolution Reaction

Abstract It is of great significance to develop efficient and robust oxygen evolution reaction (OER) electrocatalysts based on inexpensive and earth-abundant materials to enable water splitting as a future renewable energy source. Herein, the in situ grown CoMn-MOF-74 on nickel foam and their use as active electrodes for high-performance water-oxidation catalysis are reported. In alkaline media, the binder-free three-dimensional (3D) electrode shows superior OER activity with a current density of 10 mA cm−2 at a small overpotential of 260 mV, a Tafel slope of 58.2 mV dec−1, as well as excellent stability, making it one of the most active OER catalyst. Such high performance is attributed to increased electrochemically active areas, accelerated electron transport capability and the synergy between metal–organic framework (MOFs) and Ni substrate. This work elucidates a promising electrode for electrochemical water oxidation and enriches the direct application of MOF materials for future clean energy conversion and storage systems.

Construction dual-regulated NiCo2S4 @Mo-doped CoFe-LDH for oxygen evolution reaction at large current density

Developing efficient and nonprecious large-current-density based oxygen evolution reaction (OER) electrocatalysts is strongly required for sustainable industrial water splitting. Hence, a unique heterostructure erecting by Mo-doped CoFe layered double hydroxides coating NiCo2S4 nanotube arrays grown on nickel foam (NCS@CFM-LDH/NF) is elaborately demonstrated. It only needs an overpotential of 295/332 mV to achieve current density of 500/1000 mA cm−2, respectively, with a low Tafel slope of 83.0 mV dec−1 in alkaline media. NCS@CFM-LDH/NF also shows an ultra-long-term stability at 1000 mA cm−2 over 100 h. Its remarkable performance is ascribed to the synergic effect of multi-component and hierarchical structure. Additionally, Theoretical calculations disclose that the doping of molybdenum is beneficial to the adsorption of the *O intermediate, thus promotes OER activity. This study provides an attractive approach to design highly active and durable OER catalysts for industrial application in electrolysis of water.

Introducing Brønsted acid sites to accelerate the bridging-oxygen-assisted deprotonation in acidic water oxidation

Oxygen evolution reaction (OER) consists of four sequential proton-coupled electron transfer steps, which suffer from sluggish kinetics even on state-of-the-art ruthenium dioxide (RuO2) catalysts. Understanding and controlling the proton transfer process could be an effective strategy to improve OER performances. Herein, we present a strategy to accelerate the deprotonation of OER intermediates by introducing strong Brønsted acid sites (e.g. tungsten oxides, WOx) into the RuO2. The Ru-W binary oxide is reported as a stable and active iridium-free acidic OER catalyst that exhibits a low overpotential (235 mV at 10 mA cm−2) and low degradation rate (0.014 mV h−1) over a 550-hour stability test. Electrochemical studies, in-situ near-ambient pressure X-ray photoelectron spectroscopy and density functional theory show that the W-O-Ru Brønsted acid sites are instrumental to facilitate proton transfer from the oxo-intermediate to the neighboring bridging oxygen sites, thus accelerating bridging-oxygen-assisted deprotonation OER steps in acidic electrolytes. The universality of the strategy is demonstrated for other Ru-M binary metal oxides (M = Cr, Mo, Nb, Ta, and Ti).

(Digital Presentation) Improved Nanocatalysts, Supported IrOx and IrRuOx, for Enhanced Oxygen Evolution Reaction

The production of hydrogen via acidic electrochemical water splitting is an important pillar towards more sustainability. The overall water electrolysis is hindered by the slow kinetics of the oxygen evolution reaction (OER). Therefore, the catalyst material for the anode still requires optimization. Hereby, we present two highly active OER catalysts, IrOx and Ir0.4Ru0.6Ox nanoparticles (NPs), prepared by a straightforward, surfactant-free, colloidal synthetic route.1 The metallic NPs are ~1.5 nm in diameter and were deposited on carbon Ketjen Black (CKB) as well as on antimony doped tin oxide (ATO) to investigate the active phase as well as potential influences of the support material. After the colloidal synthesis and supporting, the catalysts were activated, thus undergoing a particle growth of ~1 nm. Their performance was benchmarked in terms of activity and stability using our recently developed electrochemical flow cell called gas diffusion electrode (GDE) setup.2 This novel approach allows closing the experimental gap between the two most common methods, namely the rotating disk electrode (RDE) and the membrane electrode assembly (MEA). Independently from the support, Ir0.4Ru0.6Ox shows a higher activity towards OER than IrOx at three tested temperatures, i.e. 30, 40, 60 °C. The two different support materials were also compared, and it was found that independently from the material, NPs deposited on CKB showed slightly higher activity at low current densities (up to 40 mA mgIr -1). As expected, accelerated potentiostatic stability test reveals a rapid degradation of the IrOx/CKB, while IrOx/ATO and Ir0.4Ru0.6Ox/ATO exhibit similar stability than commercially available Ir black.[1] Francesco Bizzotto, Jonathan Quinson, Alessandro Zana, Jacob J. K. Kirkensgaard, Alexandre Dworzak, Mehtap Oezaslan, Matthias Arenz, Catal. Sci. Technol., 2019, 9, 6345-6356.[2] Johanna Schröder, Vladislav A. Mints, Aline Bornet, Etienne Berner, Mohammad Fathi Tovini, Jonathan Quinson, Gustav K. H. Wiberg, Francesco Bizzotto, Hany A. El-Sayed, Matthias Arenz, JACS Au, 2021, 1, 247-251. Figure 1

Interfacial microenvironment modulation enhancing catalytic kinetics of bimetallic (oxy)hydroxide heterostructures for highly efficient oxygen evolution reaction

Water splitting is a promising hydrogen production technology limited by the slow kinetic oxygen evolution reaction (OER). The development of highly active and robust OER catalysts is in high demand. A series of three-dimensional (3D) self-supported [email protected](OH)2 heterostructures supported on nickel foam are synthesized and used as efficient OER catalysts using a simple hydrothermal method. The benefits of the appropriate 3D nanoflower morphology, the large number of FeOOH nanosheets adhered on the nanoflower surface, the formation of heterogeneous interfaces, and the synergistic effect of Ni(OH)2 and FeOOH significantly improve the OER catalytic performance of the material. The optimal catalyst in 1 M KOH, alkaline artificial seawater (1 M KOH + 1 M NaCl), and alkaline nature seawater (1 M KOH + seawater) require only 285, 286, and 325 mV overpotential, respectively, to achieve 100 mA cm−2 current density and has robust stability. This study contributes to the rational design of interface-engineered heterostructures for seawater splitting by synthesizing high catalytic activity catalysts.

Activity-Stability Relationships in Oxide Electrocatalysts for Water Electrolysis.

The oxygen evolution reaction (OER) is one of the key kinetically limiting half reactions in electrochemical energy conversion. Model epitaxial catalysts have emerged as a platform to identify structure-function-relationships at the atomic level, a prerequisite to establish advanced catalyst design rules. Previous work identified an inverse relationship between activity and the stability of noble metal and oxide OER catalysts in both acidic and alkaline environments: The most active catalysts for the anodic OER are chemically unstable under reaction conditions leading to fast catalyst dissolution or amorphization, while the most stable catalysts lack sufficient activity. In this perspective, we discuss the role that epitaxial catalysts play in identifying this activity-stability-dilemma and introduce examples of how they can help overcome it. After a brief review of previously observed activity-stability-relationships, we will investigate the dependence of both activity and stability as a function of crystal facet. Our experiments reveal that the inverse relationship is not universal and does not hold for all perovskite oxides in the same manner. In fact, we find that facet-controlled epitaxial La0.6Sr0.4CoO3-δ catalysts follow the inverse relationship, while for LaNiO3-δ, the (111) facet is both the most active and the most stable. In addition, we show that both activity and stability can be enhanced simultaneously by moving from La-rich to Ni-rich termination layers. These examples show that the previously observed inverse activity-stability-relationship can be overcome for select materials and through careful control of the atomic arrangement at the solid-liquid interface. This realization re-opens the search for active and stable catalysts for water electrolysis that are made from earth-abundant elements. At the same time, these results showcase that additional stabilization via material design strategies will be required to induce a general departure from inverse stability-activity relationships among the transition metal oxide catalysts to ultimately grant access to the full range of available oxides for OER catalysis.

State of the Active Site in La1–xSrxCoO3−δ Under Oxygen Evolution Reaction Investigated by Total-Reflection Fluorescence X-Ray Absorption Spectroscopy

Recent developments in hydrogen energy devices have furthered the research on sustainable hydrogen production methods. Among these, the water splitting process has been considered a promising hydrogen production method, particularly, in alkaline media. The lack of information on the reaction active sites under the conditions of the oxygen evolution reaction (OER) hinders establishing guidelines for catalyst development. In the case of powder catalysts, many operando techniques also measure bulk information, and therefore, extracting information on the reaction active sites is challenging. Accordingly, film electrodes were used in this study, and the electrochemical performance and reaction kinetics of perovskite-type La1–xSrxCoO3−δ films as OER catalysts synthesized by pulsed laser deposition were investigated. By combining ex situ X-ray absorption spectroscopy (XAS) and operando total-reflection fluorescence X-ray absorption spectroscopy (TRF-XAS), we succeeded in observing a significant oxidation state change on the surface of La0.6Sr0.4CoO3−δ, which indicated that the active surface sites were formed upon applying the OER potential. This surface reconstruction resulted in numerous active sites at the reaction interface, thereby enhancing the OER activity. This study provides definitive evidence for the surface reconstruction of OER catalysts, which enhances the fundamental understanding of OER catalyst behaviors, and can inspire the design of active OER catalysts by suitable surface modulation.

Silver decorated hydroxides electrocatalysts for efficient oxygen evolution reaction

Electrocatalysts play a pivotal role in electrochemical water splitting, where the development of efficient oxygen evolution reaction (OER) electrocatalysts remains challenging. Here, we report a facile one-step blending approach to synthesize efficient and durable OER electrocatalysts of silver decorated hydroxides (Ag@hydroxides). The general efficacy of this methodology is demonstrated in five hydroxides, in form of powders or self-supported electrodes. The decoration of Ag lowers the overpotentials by 41–56 mV, rendering Ag@NiFe LDH among the most active OER catalysts. It requires an overpotential of 246 mV only to deliver a current density of 10 mA cm−2. Under industrial electrolysis conditions (30 wt% KOH and 80 °C), the cell voltage is 1.76 V for 500 mA cm−2. It is 270 mV lower than the intact nickel mesh, corresponding to a 9% improvement in the energy conversion efficiency. Furthermore, by combining spectroscopic analysis and theoretical calculation we unravel that the strong interfacial charge transfer is the root cause for the promoting catalytic activity, where the binding energies of catalytic intermediates are optimized. This work provides a facile and ease-to-scale-up approach for the synthesis of efficient and durable electrocatalysts and demonstrates the power of regulating the interface in improving electrocatalytic kinetics.

Surface carbon layer controllable Ni3Fe particles confined in hierarchical N-doped carbon framework boosting oxygen evolution reaction

Developing high-efficiency and low-cost catalysts towards oxygen evolution reaction (OER) is extremely important for overall water splitting and rechargeable metal−air batteries. Herein we propose a promising organometallic coordination polymer (OCP) induced strategy to construct hierarchical N-doped carbon framework with NiFe nanoparticles encapsulated inside (N x Fe@N–C) as a highly active and stable OER catalyst. The synthesis of OCP precursor depends on the unique molecular structure of iminodiacetonitrile (IDAN), which can coordinate with metal ions to form Ni 2 Fe(CN) 6 with prussian blue analogs (PBA) structure. Unlike previous PBA-induced methods, the thickness of the carbon layer covering the surface of the metal core can be well controlled during the pyrolysis through adjusting the amount of IDAN, which builds a wonderful bridge for investigating the relationship between carbon layer thickness and catalytic performance. Both the experimental characterizations and theoretical studies validate that a suitable carbon layers thickness leads to optimal OER activity and stability. By optimizing the structure and composition, the optimized Ni 3 Fe@N–C with hierarchical framework exhibits the low overpotentials (260 ​mV at 10 ​mA ​cm −2 ; 320 ​mV at 50 ​mA ​cm −2 ), improved kinetics (79 ​mV dec −1 ), and robust long-term stability, which exceeds those of benchmark RuO 2 . • Iminodiacetonitrile and metal ions are coordinated to form Ni 2 Fe(CN) 6 ) prussian blue analogues. • Hierarchical Ni 3 Fe@N–C framework can be formed through the direct pyrolysis of Ni 2 Fe(CN) 6 ). • Thickness of carbon layer covered on Ni 3 Fe can be well controlled by adjusting IDAN amount. • Hierarchical Ni 3 Fe@N–C framework presents excellent electrocatalytic performance for the OER. • OER improvement mechanism is clarified through the experiment and theoretical combination.

The modulated oxygen evolution reaction performance in La2/3Sr1/3CoO3 by a design of stoichiometry offset

The accurate stoichiometry control of single-crystalline transition metal oxide thin film and the electrochemical performance such as in oxygen evolution reaction (OER) offer a useful pathway towards the mechanism understanding and the strategy of improving electrochemical activity at the atomic level. As a stable, low cost, and high active OER catalyst, La2/3Sr1/3CoO3 (LSCO) has drawn substantial attention recently. Here we report a modulated OER performance of LSCO by different stoichiometry offsets, an overall ∼2074.63% tunability can be seen. Meanwhile, the overpotential of LSCO exhibits a volcano trend as a function of oxygen concentration. These phenomena can be understood on the competition of double- and super-exchanges in Co3+/Co4+ with the help of the overlap of O 2p and Co 3d orbits. Our investigation provides an effective platform for obtaining a narrow window of improved electrochemical activity in correlated transition metal oxides.

Statistical analysis of breaking scaling relation in the oxygen evolution reaction

The application of proton exchange membrane electrolyzers in practice to produce gaseous hydrogen as energy vector is majorly hampered by the sluggish oxygen evolution reaction (OER) at the anode. On the atomic scale, a scaling relation between the OH and OOH intermediates has been recognized as main limitation for the development of highly active OER catalysts. Breaking scaling relation is considered as a universal remedy to obtain OER materials with enhanced electrocatalytic activity. While it is a well-accepted paradigm that the optimum OER catalyst reveals a symmetric thermodynamic free-energy landscape, recently, it was suggested that the thermodynamic ideal may correspond to a free-energy landscape with asymmetric shape, allowing thermoneutral stabilization of the key intermediate at the target overpotential. In the present manuscript, we analyze breaking scaling relation in the OER to the symmetric and asymmetric thermodynamic free-energy landscapes by statistical methods at different applied overpotentials. Our analysis reveals that breaking scaling relation to the asymmetric rather than to the symmetric picture is statistically more significant as soon as an overpotential is applied, calling for a change in mindset when thermodynamic considerations are used for catalyst optimization.

The low overpotential regime of acidic water oxidation part I: the importance of O2 detection.

The high overpotential required for the oxygen evolution reaction (OER) represents a significant barrier for the production of closed-cycle renewable fuels and chemicals. Ruthenium dioxide is among the most active catalysts for OER in acid, but the activity at low overpotentials can be difficult to measure due to high capacitance. In this work, we use electrochemistry – mass spectrometry to obtain accurate OER activity measurements spanning six orders of magnitude on a model series of ruthenium-based catalysts in acidic electrolyte, quantifying electrocatalytic O2 production at potential as low as 1.30 VRHE. We show that the potential-dependent O2 production rate, i.e., the Tafel slope, exhibits three regimes, revealing a previously unobserved Tafel slope of 25 mV decade−1 below 1.4 VRHE. We fit the expanded activity data to a microkinetic model based on potential-dependent coverage of the surface intermediates from which the rate-determining step takes place. Our results demonstrate how the familiar quantities “onset potential” and “exchange current density” are influenced by the sensitivity of the detection method.

Enhancement of oxygen evolution reaction by X-doped (X = Se, S, P) holey graphitic carbon shell encapsulating NiCoFe nanoparticles: a combined experimental and theoretical study

Among the two half-reactions of the electrochemical water splitting, the anodic oxygen evolution reaction (OER) is a kinetically sluggish process and usually requires noble metal oxide catalysts. Therefore, the development of highly active, noble metal-free, and durable catalysts for OER is an urgent need for sustainable applications. The encapsulation of transition-metal alloyed nanoparticles in graphitic carbon layers, with core-shell features, is a viable approach for establishing an undegradable and highly efficient catalyst toward OER. Furthermore, the reactivity of the carbon shell surface can be further improved by the electron transfers between the core alloy and carbon shell, through a control of the alloyed nanoparticle compositions, or through a chemical doping. Nevertheless, it is still not clear whether the incorporation of a chemical dopant directly into the carbon shells systematically promotes the catalytic activities toward OER. To clarify this point, we synthetize trimetallic (CoNiFe) nanoparticles encapsulated in graphitic carbon shells by pyrolysis of metal−organic frameworks. We then investigate the effect of a doped graphitic carbon shell by incorporating non-metallic elements such as sulfur, phosphorus, and selenium. The main finding is that all doped CoNiFe@C core-shell catalysts exhibit an enhanced catalytic activity toward OER in the alkaline electrolyte with a low overpotential, a small Tafel slope, and long stability, which are all being comparable to those of RuO2 benchmark. Electrochemical impedance spectroscopy, X-ray photoelectron spectroscopic, and Raman measurements collected at different applied potentials during the OER process indicate that the doping of graphitic carbon shell significantly improves the interfacial electron-transfer kinetics and facilitates the adsorption of OH− ion as well as promotes the formation of metal oxyhydroxide, which positively affect the OER performances. Moreover, this improvement in OER efficiency upon incorporating of a chemical doping is rationalized by the optimization of the Gibbs free energy of OER intermediates, thereby remarkably reducing the required energy input of rate-determining step, as elucidated by the density functional theory calculations.

The role of proton dynamics on the catalyst-electrolyte interface in the oxygen evolution reaction

The development of non-precious metal catalysts that facilitate the oxygen evolution reaction (OER) is important for the widespread application of hydrogen production by water splitting. Various perovskite oxides have been employed as active OER catalysts, however, the underlying mechanism that occurs at the catalyst-electrolyte interface is still not well understood, prohibiting the design and preparation of advanced OER catalysts. Here, we report a systematic investigation into the effect of proton dynamics on the catalyst-electrolyte interfaces of four perovskite catalysts: La 0.5 Sr 0.5 CoO 3-δ (LSCO), LaCoO 3 , LaFeO 3 , and LaNiO 3 . The pH-dependent OER activities, H/D kinetic isotope effect, and surface functionalization with phosphate anion groups were investigated to elucidate the role of proton dynamics in the rate-limiting steps of the OER. For oxides with small charge-transfer energies, such as LSCO and LaNiO 3 , non-concerted proton-coupled electron transfer steps are involved in the OER, and the activity is strongly controlled by the proton dynamics on the catalyst surface. The results demonstrate the important role of interfacial proton transfer in the OER mechanism, and suggest that proton dynamics at the interface should carefully be considered in the design of future high-performance catalysts. A systematical study was conducted to probe the role of proton dynamics in the multiple PCET steps during OER on the catalyst-electrolyte interfaces using perovskite oxides as model catalysts.

A strategy to improve electrochemical water oxidation of FeOOH by modulating the electronic structure

The good performance of base metal oxides as alternative electrocatalysts for water oxidation has attracted great attention. However, the formed Fe-O bonds are not conducive to the dissociation of water molecule, which thus inhibit oxygen evolution reaction (OER). Herein, a facial on-site activation strategy is proposed to improve the OER activity of FeOOH by P doping. By optimizing the electronic structure, the obtained electrode exhibits an overpotential of 203 and 263 mV at 10 and 100 mA cm−2, respectively. Systematic experiments and theoretical analysis suggest that P doping can optimize the adsorption energy of OER intermediates by forming P-O bonds. Therefore, it enables P-FeOOH as a low-cost and efficient electrocatalyst and provides a strategy for designing new highly active and cost-effective catalysts for OER.