Electrocatalytic Oxygen Evolution Reaction Research Articles

Understanding the Anion Effect of Basic Cobalt Salts for the Electrocatalytic Oxygen Evolution Reaction

Basic cobalt salts with distinct acidic anion Co(OH)x(A)y are efficient electrocatalysts toward the oxygen evolution reaction (OER). However, plenty of present studies are still in the try-and-wrong stage, while the underlying anion effect remains unclear. Herein, a series of Co(OH)x(A)y (A = F–, Cl–, and CO32–) are synthesized via a hydrothermal strategy, and the order of the OER activity is determined to be Co(OH)(CO3)0.5 > Co2(OH)3Cl > Co(OH)F > Co(OH)2. X-ray photoelectron spectroscopy and soft X-ray absorption spectroscopy studies reveal that these Co(OH)x(A)y materials undergo surface oxidation during the OER process, and the highly active Co(IV) content is the dominant factor in deciding the OER performance. Furthermore, quantitative analysis of anion concentrations in the electrolyte solution reveals that the anion leaching ability in Co(OH)x(A)y directly relates to the Co(IV) content and finally the OER catalytic activity, which is the essence of the anion effect and can be summarized as an “anion leaching─metal oxidation” model. Our work not only provides deep understanding of the anion effect of metal basic salt-based OER catalysts but also profound insights for the activation process of the OER pre-catalysts.

Delocalization of d-electrons induced by cation coupling in ultrathin Chevrel-phase NiMo3S4 nanosheets for efficient electrochemical water splitting

Chevrel-phase metal sulfides are known to be promising materials for energy conversion and storage applications. However, a detailed understanding of the intrinsic kinetic mechanisms of electrocatalytic bifunctional hydrogen and oxygen evolution reactions (HER/OER) on NiMo3S4-based Chevrel-phases is lacking. Herein, novel ultrathin self-assembled nanosheets of NiMo3S4 are coupled with transition metal atoms (M/N-NiMo3S4; where M = Co, Fe, and Cu) were formed by a facile hydrothermal approach. Notably, the Co/N-NiMo3S4 electrocatalyst exhibits excellent performance in terms of ultralow overpotentials of 78, 208, 282, and 307 mV at 10, 100, 500, and 1000 mA cm−2 for the HER; and 186, 204, and 225 mV at 50, 100, and 300 mA cm−2 for the OER, respectively. Experimental and first principle calculations demonstrate that Co atoms coupling with edge Ni atoms results in d‐electron delocalization on Co/N-NiMo3S4, signifying the efficient charge transfer to improve overall water electrolysis. In addition, an upshift in the d‐band center of Co/N-NiMo3S4 can optimize the free energies of a variety of reaction intermediates for water adsorption and dissociation; thereby facilitating the robust alkaline overall water electrolysis at 1.47 V. This work therefore greatly deepens the understanding of the bifunctional hydrogen and oxygen evolution reaction of Chevrel-phase electrocatalysts.

Tracing the Effects of Nitrogen Doping and Sulfur Vacancy in Surging OER Electrocatalytic Activity of Bismuth Sulfide Nanorods

AbstractBismuth sulfide has become an attractive candidate in the electrocatalytic oxygen evolution reaction (OER), owing to its open coordination sites, unpaired electron orbitals, and mild electronegativity, but its OER performance is hindered by many deficiencies. The combination of heteroatom construction and vacancy engineering has been used to improve the OER performance of bismuth‐based electrocatalysts, but few studies can clearly describe the roles of dopants and vacancies in improving OER performance. Herein, N dopants and S vacancies in Bi2S3 nanorods via one‐step NH3/Ar plasma etching to investigate the enhanced OER performance are constructed. N dopants and S vacancies both regulated the intrinsic charge ordering of Bi2S3 nanorods, enhancing the p‐band center and Fermi level while also boosting the electroconductivity and wettability of the material. In addition, density functional theory calculations suggest that N doping promoted the adsorption of Bi sites, while S vacancies favored the desorption of S sites. Under the synergistic effects of N dopants and S vacancies, Bi2S3‐based electrocatalysts exhibited a low overpotential of 374 mV at 10 mA cm−2 and satisfactory durability, demonstrating a feasible strategy for exploiting main group element OER electrocatalysts.

Interface Catalysts of Ni3Fe1 Layered Double Hydroxide and Titanium Carbide for High-Performance Water Oxidation in Alkaline and Natural Conditions.

The electrocatalytic oxygen evolution reaction (OER) is important for many renewable energy technologies. Developing cost-effective electrocatalysts with high performance remains a great challenge. Here, we successfully demonstrate our novel interface catalyst comprised of Ni3Fe1-based layered double hydroxides (Ni3Fe1-LDH) vertically immobilized on a two-dimensional MXene (Ti3C2Tx) surface. The Ni3Fe1-LDH/Ti3C2Tx yielded an anodic OER current of 100 mA cm-2 at 0.28 V versus reversible hydrogen electrode (RHE), nearly 74 times lower than that of the pristine Ni3Fe1-LDH. Furthermore, the Ni3Fe1-LDH/Ti3C2Tx catalyst requires an overpotential of only 0.31 V versus RHE to deliver an industrial-level current density as high as 1000 mA cm-2. Such excellent OER activity was attributed to the synergistic interface effect between Ni3Fe1-LDH and Ti3C2Tx. Density functional theory (DFT) results further reveal that the Ti3C2Tx support can efficiently accelerate the electron extraction from Ni3Fe1-LDH and tailor the electronic structure of catalytic sites, resulting in enhanced OER performance.

Phosphorus-Modulated Generation of Defective Molybdenum Sites as Synergistic Active Centers for Durable Oxygen Evolution.

It is well known that electrocatalytic oxygen evolution reaction (OER) activities primarily depend on the active centers of electrocatalysts. In some oxide electrocatalysts, high-valence metal sites (e.g., molybdenum oxide) are generally not the real active centers for electrocatalytic reactions, which is largely due to their undesired intermediate adsorption behaviors. As a proof-of-concept, molybdenum oxide catalysts are selected as a representative model, in which the intrinsic molybdenum sites are not the favorable active sites. Via phosphorus-modulated defective engineering, the inactive molybdenum sites can be regenerated as synergistic active centers for promoting OER. By virtue of comprehensive comparison , it is revealed that the OER performance of oxide catalysts is highly associated with the phosphorus sites and the molybdenum/oxygen defects. Specifically, the optimal catalyst delivers an overpotential of 287mV to achieve the current density of 10mAcm-2 , accompanied by only 2% performance decay for continuous operation up to 50h. It is expected that this work sheds light on the enrichment of metal active sites via activating inert metal sites on oxide catalysts for boosting electrocatalytic properties.

Covalent Linking of the Porphyrin Light Harvester to the Multiwalled-Carbon-Nanotube-Based Polypyridineruthenium(II) Complex for Boosting the Electrocatalytic Oxygen Evolution Reaction

The oxygen evolution reaction (OER) is a key step in artificial photosynthesis, and protons and electrons generated in the reaction are vital for subsequent cathodic reactions. In this paper, a light-assisted photoelectrocatalytic system constructed by the OER catalyst (tpyRubpy) and light-harvesting moiety (porphyrin) was further covalently grafted onto the multiwalled carbon nanotubes (MWCNTs). The formed nanocatalyst preserves both the high-efficiency catalytic OER performance of tpyRubpy and the light-harvesting performance of porphyrin, and the graft of MWCNT results in a decrease in the charge-transfer resistance of the composite electrode and achieved an excellent light-assisted electrocatalytic OER property with a lower onset potential of 1.57 V and a lower overpotential of 440 mV at a current density of 10 mA cm–2. This photoelectrocatalytic system successfully opens a new avenue for the rational design and fabrication of artificial photosynthesis systems through a covalent assembly strategy that mimics the key functions of photosystem II.

Homogeneous NiMoO4–Co(OH)2 bifunctional heterostructures for electrocatalytic oxygen evolution and urea oxidation reaction

Developing bifunctional electrocatalysts for catalyzing water oxidation and urea oxidation reactions is of vital importance for alleviating the energy crisis and environmental damage, but it is still formidably challenging. Here, the potential of a heterostructure of NiMoO4 nanorods conformally covered by metal-organic framework (MOF) derived Co(OH)2 nanosheets is demonstrated. Under the assistance of a series of advanced characterizations and electrochemical measurements, the rich heterointerfaces in the NiMoO4–Co(OH)2 are found to be beneficial for promoting the electrocatalytic performance. As a bifunctional catalyst for the electrooxidation of water and urea, the optimized NiMoO4–Co(OH)2-100 demonstrates higher catalytic activity, kinetics and durability than individual NiMoO4 and Co(OH)2. Specifically, an overpotential as low as 285 mV can drive 10 mA cm−2 for oxygen evolution reaction (OER), and a voltage of 1.32 V is demanded for achieving 10 mA cm−2 towards urea oxidation reaction (UOR), significantly lower than the benchmarked catalysts. Presented outcomes can greatly contribute to the industrial application of urea-containing wastewater purifying, by offering a deep understanding on the interface engineering of nanocatalysts.

The Microstructure–Activity Relationship of Metal–Organic Framework-Based Electrocatalysts for the Oxygen Evolution Reaction at the Single-Particle Level

Metal–organic framework (MOF)-based materials have garnered attention as promising candidates for electrocatalytic oxygen evolution reactions (OER). However, it remains a challenge to understanding the structure–activity relationship of these materials at the single-particle level due to the limited resolution of characterizations. Here, we present an efficient approach that combines a single-particle-at-the-tip technique with scanning electrochemical cell microscopy to reveal a relationship between the microstructure and electrochemical activity for sheet-like nickel-based MOF particles. We find a negative correlation between the sheet thickness and the OER activity based on statistical data at the single-particle level. The origin of the enhanced OER activity can be rationalized by the larger fraction of the (001) facet. Simultaneously, the derivation process of catalysts is documented to ensure the accuracy of structural information. This study enables a simple single-entity electrochemical strategy to understand the microstructure–activity relationship of MOF-based catalysts (MOF-C) and offers new insights into the design of high-performance MOF-C.

Single‐Source‐Precursor‐Derived Binary FeNi Phosphide Nanoparticles Encapsulated in N, P Co‐Doped Carbon as Electrocatalyst for Hydrogen Evolution Reaction and Oxygen Evolution Reaction

Water‐splitting processes require advanced catalysts for the electrocatalytic hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). A facile and cost‐effective approach using single‐source precursors (SSPs) to prepare active and durable bifunctional electrocatalysts consisting of core–shell structured transition metal phosphide (TMPs) nanoparticles dispersed and immobilized in a highly defective N‐, P‐codoped carbon matrix. The bimetallic FeNi phosphide supported on N‐, P‐codoped carbon is synthesized through pyrolysis at 900 °C under an argon atmosphere (FeNiP@NPC‐900), displaying promising electrocatalytic performance for both HER and OER, with low overpotentials of 191 and 278 mV, respectively. Furthermore, FeNiP@NPC‐900 exhibits remarkable durability in both acidic and alkaline electrolytes. The excellent catalytic and long‐term performance is attributed to several factors, including the novel SSP approach, which prevents the agglomeration of active TMPs particles, the in situ formation of exposed nanopores during the carbonization of the precursor without using surfactants, and the heterostructure between highly defective carbon and TMPs, which positively influences the electronic structure at the interface and facilitates charge transfer. The unique core–shell nanostructure protects the catalytically active phase from corrosion and synergistically promotes the activity performance. Therefore, a new strategy for designing active and stable heterostructured TMPs@carbon‐based electrocatalysts is provided.

Side-Chain Modification in Conjugated Polymer Frameworks for the Electrocatalytic Oxygen Evolution Reaction.

Conjugated polymer frameworks (CPFs) have recently sparked tremendous research interest due to their broad potentials in various frontline application areas such as photocatalysis, sensing, gas storage, energy storage, etc. These framework materials, without sidechains or functional groups on their backbone, are generally insoluble in common organic solvents and less solution processable for further device applications. There are few reports on metal-free electrocatalysis, especially oxygen evolution reaction (OER) using CPF. Herein, we have developed two triazine-based donor-acceptor conjugated polymer frameworks by coupling a 3-substituted thiophene (donor) unit with a triazine ring (acceptor) through a phenyl ring spacer. Two different sidechains, alkyl and oligoethylene glycol, were rationally introduced into the 3-position of thiophene in the polymer framework to investigate the effect of side-chain functionality on the electrocatalytic property. Both the CPFs demonstrated superior electrocatalytic OER activity and long-term durability. The electrocatalytic performance of CPF2, which achieved a current density of 10 mA/cm2 at an overpotential (η) of 328 mV, is much superior to CPF1, which reached the same current density at an overpotential of 488 mV. The porous and interconnected nanostructure of the conjugated organic building blocks, which allowed for fast charge and mass transport processes, could be attributed to the higher electrocatalytic activity of both CPFs. However, the superior activity of CPF2 compared to CPF1 may be due to the presence of a more polar oxygen-containing ethylene glycol side chain, which enhances the surface hydrophilicity, promotes better ion/charge and mass transfer, and increases the accessibility of the active sites toward adsorption through lower π-π stacking compared to hexyl side chain present in CPF1. The DFT study also supports the plausible better performance toward OER for CPF2. This study confirms the promising potentiality of metal-free CPF electrocatalysts for OER and further sidechain modification to improve their electrocatalytic property.

Atomic Ru-modulated Ru-CoO heterostructures as efficient bifunctional electrocatalyst for oxygen and hydrogen evolution reactions

Regulating morphology and composition of catalysts is of grand importance to enhance their electrocatalytic activity in energy-related fields. Herein, an electrocatalyst of ruthenium-cobalt oxide (Ru-CoO) heterojunctions decorated with atomically dispersed Ru atoms is constructed with a simple and effective salt fractionation approach. The morphology and structure of the resulting Ru-CoO were analyzed using scanning electron microscopy, transmission electron microscopy, high-angle annular dark-field scanning transmission electron microscopy, powder X-ray diffraction, N2 sorption, and X-ray photoelectron spectroscopy, respectively. Owing to the synergistic contributions of heterostructured Ru-CoO with abundant interfaces and single atomic Ru at doping level, the resulted Ru-CoO exhibits excellent bifunctional electrocatalytic oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) performances along with long-term durability under alkaline electrolytes. To attain a current density of 10 mA cm−2, the hybrid needs overpotentials of 340 and 359 mV for the OER and HER, respectively, excelling many reported catalysts to date. Importantly, this work paves a new route toward the development of efficient catalysts for the future of hydrogen economy.

Manipulating electron redistribution induced by asymmetric coordination for electrocatalytic water oxidation at a high current density

Electronic structure manipulation with regard to active site coordination is an effective strategy to improve the electrocatalytic oxygen evolution reaction (OER) activity. Herein, we present the structure–activity relationship between oxygen-atom-mediated electron rearrangement and active site coordination asymmetry. Ni2+ ions are introduced to FeWO4 on Ni foam (NF) via self-substitution to break the symmetry of the FeO6 octahedron and regulate d-electron structure of Fe sites. Structural regulation optimizes the adsorption energy of hydroxyl on the Fe sites and promotes the partial formation of hydroxyl oxide with high OER activity on the tungstate surface. Fe0.53Ni0.47WO4/NF with the asymmetric FeO6 octahedron of Fe sites can achieve an ultralow overpotential of 170 mV at 10 mA cm−2 and 240 mV at 1000 mA cm−2 with robust stability for 500 h at high current density under alkaline conditions. This research develops novel electrocatalysts with impressive OER performance and provides new insights into the design of highly active catalytic systems.

Defect Engineering in Bimetallic NiFe‐BTC for Boosting Electrocatalytic Oxygen Evolution Reaction through Coordinated Ionic Liquids

AbstractCoordinatively unsaturated metal (CUM) sites in metal organic frameworks (MOFs) have demonstrated distinct catalytic activities, yet remain great challenges in their designing and engineering. Herein, a novel “Coordination Geometric Hindrance” approach based on ionic liquids (ILs) is proposed for defects construction in bimetallic MOF, of which the steric effect of competitively coordinated anions of imidazole ILs ([EMIM]X) enables the interference with the ordered growth of bimetallic NiFe‐MOFs. Specifically, the PF6− anion displays the strongest binding energy with metals Ni and Fe, accordingly, facilitates the formation of CUM sites in NiFe‐BTC/IL‐PF6. Taking both advantages of CUM defects and good intrinsic conductivity of IL, NiFe‐BTC/IL‐PF6 achieves a superior electrocatalytic performance in the oxygen evolution reaction (OER), with the overpotential as low as 210 mV at 10 mA cm−2, and the Tafel slope of 103 mV dec−1, thus, giving an 18‐fold larger TOF of the commercial OER catalyst of RuO2.

Insight into the surface-reconstruction of metal–organic framework-based nanomaterials for the electrocatalytic oxygen evolution reaction

Insight into the surface-reconstruction of metal–organic framework-based nanomaterials for the electrocatalytic oxygen evolution reaction

Tuned d-band states over lanthanum doped nickel oxide for efficient oxygen evolution reaction

The d-band state of materials is an important descriptor for activity of oxygen evolution reaction (OER). For NiO materials, there is rarely concern about tuning their d-band states to tailor the OER behaviors. Herein, NiO nanocrystals with doping small amount of La3+ were used to regulate d-band states for promoting OER activity. Density of states calculations based on density functional theory revealed that La3+ doping produced upper shift of d-band center, which would induce stronger electronic interaction between surface Ni atoms and species of oxygen evolution reaction intermediates. Further density functional theory calculation illustrated that La3+ doped NiO possessed reduced Gibbs free energy in adsorbing species of OER intermediate. Predicted by theoretical calculations, trace La3+ was introduced into crystal lattice of NiO nanoparticles. The La3+ doped NiO nanocrystal showed much promoted OER activity than corresponding pristine NiO product. Further electrochemical analysis revealed that La3+ doping into NiO increased the intrinsic activity such as improved active sites and reduced charge transfer resistance. The in-situ Raman spectra suggested that NiO phase in La3+ doped NiO could be better maintained than pristine NiO during the OER. This work provides an effective strategy to tune the d-band center of NiO for efficient electrocatalytic OER.

Doping of high‐valence metals and their electrocatalytic oxygen evolution reaction in metal/porous carbon composites

AbstractPreparing high‐efficiency, durable and low overpotential electrocatalytic materials for oxygen evolution reaction (OER) plays a key role in water electrolysis. At present, electrochemical water separation technology has become the most promising sustainable hydrogen production process. The inherent hysteresis kinetics of oxygen evolution reaction (OER) is the key bottleneck of water electrolysis, which greatly restricts the energy conversion efficiency and practical large‐scale application. Herein, a facile method is developed to synthesize a series of trimetallic (Mo/Ni/Fe) metal‐organic frameworks (MOFs)‐derived carbon nanoparticles (CNPs) with various Fe content, and a Fe‐dependent volcano‐type plot can be drawn out for MoNiFex‐CNPs. The optimized MoNiFe0.4‐CNPs demonstrate superior electrocatalytic performance with a low overpotential of only 227 mV@10 mA cm−2 and excellent durability of 100 h. Its excellent OER performance is attributed to the synergistic effect of Fe/Ni bimetallic regulated by Mo. This work improved the catalytic performance of MOFs in the OER and proposed a new strategy for the structural design of MOFs.

Integrated core-shell assembly of Ni3S2 nanowires and CoMoP nanosheets as highly efficient bifunctional electrocatalysts for overall water splitting

A paradigm for the rational design of active, abundant, and inexpensive bifunctional electrocatalysts with acceptable electrochemical energy conversion rates has yet to be achieved. Here, we describe the assembly of a superior bifunctional electrocatalyst for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) activities based on a CoMoP/Ni3S2 multicomponent, heterointerface consisting of a freestanding electrocatalyst prepared by hydrothermal-phosphidation, consisting of one-dimensional Ni3S2 covered with Cereus cactus–like hierarchical CoMoP nanosheets. The resulting three dimensional CoMoP/Ni3S2 core-shell heterostructure exhibited remarkable HER and OER electrocatalysis, benefiting from modulated electronic structures, rapid mass diffusion, reduced charge-transfer resistance, and a larger electrochemically active surface area. Due to these superior functionalities, the CoMoP/Ni3S2 requires only 96.8 mV (at η10) for HER catalysis and 270 mV (at η50) for an OER process. Furthermore, a stable water-splitting device using CoMoP/Ni3S2 for both the anode and cathode required a low cell voltage of 1.54 V at 10 mA cm−2. This work represents a significant advance in interface construction of transition-metal phosphide/sulfide for long-term water splitting.

Fe-Incorporated Nickel-Based Bimetallic Metal-Organic Frameworks for Enhanced Electrochemical Oxygen Evolution.

Developing cost-effective and high-efficiency catalysts for electrocatalytic oxygen evolution reaction (OER) is crucial for energy conversions. Herein, a series of bimetallic NiFe metal-organic frameworks (NiFe-BDC) were prepared by a simple solvothermal method for alkaline OER. The synergistic effect between Ni and Fe as well as the large specific surface area lead to a high exposure of Ni active sites during the OER. The optimized NiFe-BDC-0.5 exhibits superior OER performances with a small overpotential of 256 mV at a current density of 10 mA cm-2 and a low Tafel slope of 45.4 mV dec-1, which outperforms commercial RuO2 and most of the reported MOF-based catalysts reported in the literature. This work provides a new insight into the design of bimetallic MOFs in the applications of electrolysis.

Mechanochemical Post‐Synthesis of Metal–Organic Framework‐Based Pre‐Electrocatalysts with Surface FeONi/Co Bonding for Highly Efficient Oxygen Evolution

AbstractRational surface engineering of metal–organic frameworks (MOFs) provide potential opportunities to address the sluggish kinetics of oxygen evolution reaction (OER). However, the development of MOF‐based materials with low overpotentials remains a great challenge. Herein, a post‐synthesis strategy to prepare highly efficient MOF‐based pre‐electrocatalysts via all‐solid‐phase mechanochemistry is demonstrated. The surface of a Fe‐based MOF (MIL‐53) can be reconstructed and anchored with atomically dispersed Ni/Co sites. As expected, the optimized M‐NiA‐CoN exhibits a very low overpotential of 180 mV at 10 mA cm−2 and a small Tafel slope of 41 mV dec−1 in 1 m KOH electrolyte. The superior electrocatalytic OER activity is mainly due to the formation of surface FeONi/Co bonding. Furthermore, density functional theory calculations reveal that the transformation from *OH to *O is the rate‐determining step and the electrocatalytic OER activity trend at different metal sites is Co > Ni≈Fe.

Inert Mg Incorporation to Break the Activity/Stability Relationship in High-Entropy Layered Hydroxides for the Electrocatalytic Oxygen Evolution Reaction

Ni-based layered double hydroxides (LDHs) have been demonstrated as the most active catalyst for the oxygen evolution reaction in alkaline media but suffer from poor stability. Herein, we report the rational breaking of the activity/stability trade-off by a strategy of inert Mg-involved high-entropy coordination. The as-developed high-entropy FeCoNiMg-LDH nanosheets exhibit not only high catalytic activity (302 mV at 100 mA cm–2) but also an unprecedented stability with almost no performance decline at a large current density of 300 mA cm–2. The combined structural investigations and theoretical calculations reveal that the synergistic electronic coupling provides active Ni in desired electronic states, leading to an optimized adsorption energy of intermediates, while the construction of high configuration entropy as well as the formation of the Mg–O interfacial bond can effectively mitigate phase segregation and transformation and thus improve the stability. The current insights into unveiling the composition–activity–stability of high-entropy coordination may provide valuable guidance for the design of catalysts for various electrochemical reactions.