Oxygen Evolution Electrocatalysts Research Articles

Stabilizing Lattice Oxygen through Mn Doping in NiCo2O4-δ Spinel Electrocatalysts for Efficient and Durable Acid Oxygen Evolution.

Design the electrocatalysts without noble metal is still a challenge for oxygen evolution reaction (OER) in acid media. Herein, we reported the manganese (Mn) doping method to decrease the concentration of oxygen vacancy (VO) and form the Mn-O structure adjacent octahedral sites in spinel NiCo2O4-δ (NiMn1.5Co3O4-δ), which highly enhanced the activity and stability of spinel NiCo2O4-δ with a low overpotential (η) of 280 mV at j=10 mA cm-2 and long-term stability of 80 h in acid media. The isotopic labelling experiment based on differential electrochemical mass spectrometry (DEMS) clearly demonstrated the lattice oxygen in NiMn1.5Co3O4-δ is more stable due to strong Mn-O bond and shows synergetic adsorbate evolution mechanism (SAEM) for acid OER. Density functional theory (DFT) calculations reveal highly increased oxygen vacancy formation energy (EVO) of NiCo2O4-δ after Mn doping. More importantly, the highly hydrogen bonding between Mn-O and *OOH adsorbed on adjacent Co octahedral sites promote the formation of *OO from *OOH due to the greatly enhanced charge density of O in Mn substituted sites.

Performance of Co9S8/Cu2S@LDH nanowires prepared by dual sulfur sources to promote oxygen evolution performance

Hydrogen production through water electrolysis is an effective method for obtaining clean energy. The oxygen evolution reaction plays a key role in the process of water electrolysis, and the development of efficient and stable oxygen evolution electrocatalysts is crucial to improve the efficiency of hydrogen production. This paper presents the preparation of Co9S8/Cu2S@LDH nanowire catalysts on copper foam using thiourea and sodium sulfide as dual sulfur sources by a hydrothermal method combined with electrochemical deposition. The composition, structure, and performance of the catalysts were investigated using characterization methods of XRD, SEM, Raman, and electrochemical analyses. Also, the mechanism of thiourea's role in the process of sulfurization of dual sulfur sources was also revealed. The results indicate that the introduction of thiourea facilitates the generation of the active center CoOOH and improves the catalytic efficiency of oxygen precipitation with a low overpotential of 199 mV at 10 mA/cm2 in 1.0 M KOH electrolyte. Additionally, the LDH deposited in the outer layer effectively prevents the oxidation of Co9S8/Cu2S and improves the catalyst's stability. Therefore, the prepared Co9S8/Cu2S@LDH nanowire catalyst exhibits high oxygen evolution reaction (OER) catalytic activity and stability in alkaline electrolyte, making it a promising OER catalyst.

Exploring Cu-Doped Co3O4 Bifunctional Oxygen Electrocatalysts for Aqueous Zn-Air Batteries.

The efficiency of oxygen electrocatalysis is a key factor in diverse energy domain applications, including the performance of metal-air batteries, such as aqueous Zinc (Zn)-air batteries. We demonstrate here that the doping of cobalt oxide with optimal amounts of copper (abbreviated as Cu-doped Co3O4) results in a stable and efficient bifunctional electrocatalyst for oxygen reduction (ORR) and evolution (OER) reactions in aqueous Zn-air batteries. At high Cu-doping concentrations (≥5%), phase segregation occurs with the simultaneous presence of Co3O4 and copper oxide (CuO). At Cu-doping concentrations ≤5%, the Cu ion resides in the octahedral (Oh) site of Co3O4, as revealed by X-ray diffraction (XRD)/Raman spectroscopy investigations and molecular dynamics (MD) calculations. The residence of Cu@Oh sites leads to an increased concentration of surface Co3+-ions (at catalytically active planes) and oxygen vacancies, which is beneficial for the OER. Temperature-dependent magnetization measurements reveal favorable d-orbital configuration (high eg occupancy ≈ 1) and a low → high spin-state transition of the Co3+-ions, which are beneficial for the ORR in the alkaline medium. The influence of Cu-doping on the ORR activity of Co3O4 is additionally accounted in DFT calculations via interactions between solvent water molecules and oxygen vacancies. The application of the bifunctional Cu-doped (≤5%) Co3O4 electrocatalyst resulted in an aqueous Zn-air battery with promising power density (=84 mW/cm2), stable cyclability (over 210 cycles), and low charge/discharge overpotential (=0.92 V).

Interface Engineering of Electrocatalysts for Efficient and Selective Oxygen Evolution in Alkaline/Seawater

Electrochemical water splitting is regarded as an effective technology for producing green hydrogen, which is crucial for addressing energy and environmental challenges. In particular, direct seawater splitting offers significant economic and environmental advantages. However, its efficiency is hindered by the high overpotential required for the oxygen evolution reaction (OER) and the competition from chloride oxidation. This review highlights the potential of interface engineering to overcome these limitations and develop efficient OER electrocatalysts. We comprehensively explore recent advancements in interface engineering for OER in both alkaline and seawater environments. We begin by introducing the mechanisms of freshwater and seawater electrolysis, emphasizing key considerations for OER catalyst design. Subsequently, we review the recent progress made in various interface engineering strategies, analyzing their impact on OER performance in both electrolytes. Finally, we outline promising future directions for developing efficient seawater oxidation catalysts through interface engineering.

Biochar from olive tree twigs and spent malt rootlets as electrodes in Zn-air batteries

Biochars, i.e. porous carbons obtained by pyrolysis of biomass, can act as electrocatalysts for oxygen evolution and oxygen reduction reaction. In the present work, two biochars have been prepared by using materials of completely different biomass origin: olive-tree twigs and spent malt rootlets (brewery wastes). Both biomass species were subjected to pyrolysis under limited oxygen supply and then they were activated by mixing with KOH and pyrolysis again. The obtained biochars were characterized by several techniques in order to determine their structural characteristics and the composition of their active components. Despite their different origin, the two biochars demonstrated similar structural and compositional characteristics thus highlighting the importance of the pyrolysis and activation procedure. Both biochars were used as electrocatalysts in the operation of rechargeable Zn-air batteries, where they also demonstrated similar electrocatalytic capacities with only a small advantage gained by olive-tree-twigs biochar. Compared to bare nanoparticulate carbon (carbon black), both biochars demonstrated a marked advantage towards oxygen evolution reaction.

Stabilizing Lattice Oxygen through Mn Doping in NiCo2O4−d Spinel Electrocatalysts for Efficient and Durable Acid Oxygen Evolution

Design the electrocatalysts without noble metal is still a challenge for oxygen evolution reaction (OER) in acid media. Herein, we reported the manganese doping method to decrease the concentration of oxygen vacancy (Vo) and form the Mn−O structure adjacent octahedral sites in spinel NiCo2O4−δ (NiMn1.5Co3O4−δ), which highly enhanced the activity and stability of spinel NiCo2O4−δ with a low overpotential (η) of 280 mV at j = 10 mA cm−2 and long‐term stability of 80 h in acid media. The isotopic labelling experiment based on differential electrochemical mass spectrometry (DEMS) clearly demonstrated the lattice oxygen in NiMn1.5Co3O4−δ is more stable due to strong Mn‐O bond and synergetic adsorbate evolution mechanism (SAEM) for acid OER. Density functional theory (DFT) calculations reveal highly increased oxygen vacancy formation energy (EVO) of NiCo2O4−δ after Mn doping. More importantly, the highly hydrogen bonding between Mn−O and *OOH adsorbed on adjacent Co octahedral sites promote the formation of *OO from *OOH due to the greatly enhanced charge density of O in Mn substituted sites.

Direct Synthesis of CuP2 and Cu3P and Their Performance as Electrocatalysts for Hydrogen Evolution, Oxygen Evolution, and Oxygen Reduction Reactions

Copper phosphides are promising materials for energy conversion applications because of their unique electronic structure and controllable composition. Two stoichiometric copper phosphides, CuP2 and Cu3P, were prepared by direct wet-chemical synthesis using red phosphorus. They were characterized by powder X-ray diffraction, scanning and transmission electron microscopy, and X-ray photoelectron spectroscopy. The precursor selection, reaction temperature, time and solvent composition were also studied. CuP2 is the thermodynamically more stable product, but Cu3P is more commonly obtained. This work demonstrated that higher temperature helps in CuP2 formation. More importantly, using more trioctylphosphine oxide helps control the morphology leading to crystal growth along the crystallographic a-axis. CuP2 and Cu3P were tested for hydrogen evolution, oxygen evolution, and oxygen reduction reactions. CuP2 works better for HER in acidic conditions and OER in general, and Cu3P showed better activity than CuP2 for HER and ORR in an alkaline medium. This study has led to a simple approach to the synthesis of CuP2 nanowires.

Cobalt molybdenum di-selenide surface modification: A path to improved trifunctional catalysis via partial oxygenation

Efficient, stable, and cost-effective electrocatalysts for hydrogen evolution, oxygen evolution, and oxygen reduction reactions have been in demand for decades. Recently, layered transition metals (Co, Mo, Ni, and Fe) and chalcogen compounds (S/Se/Te) gained prominence in catalysis and device studies. Density functional theory identified the covalency of Co and Mo, oxygenated bimetallic selenium bond, crystal structure distortions, and adsorption properties as crucial for enhanced electrocatalytic activity. Hybridizing the Co and Mo d-orbitals around the Fermi level of 0.463 eV improves bonding with metal surface-adsorbate intermediates validated by partial density of states. Oxygenated cobalt molybdenum selenide (O-CoMoSe2) (210) exhibited remarkable electrocatalytic properties at various active sites, with Gibbs free energy and theoretical overpotential at an active site of Co = 0.086 eV, 0.41 V, and Se = 0.50 V.

Nickel molybdenum selenide on carbon cloth as an efficient bifunctional electrocatalyst for alkaline seawater splitting

Exploring cost-effective and highly efficient electrocatalysts is desirable for the overall water-splitting challenge. In this work, a promising bifunctional electrocatalyst for oxygen evolution and hydrogen evolution reactions (OER/HER) in an alkaline medium (1 M KOH) and simulated alkaline seawater (1 M KOH + 0.5 M NaCl) was rationally developed by a simple, one-step, inexpensive, and eco-friendly method. Mo-doped nickel selenide supported on activated carbon cloth (NiMoSe@CC) was successfully synthesized via a hydrothermal method. NiMoSe nanoparticles were observed to be densely grown with better uniformity on the carbon fibers, as confirmed by different characterization techniques. The NiMoSe@CC showed an enhanced OER activity with a low overpotential of 320 mV (alkaline medium) and 360 mV (alkaline seawater) at 10 mA cm−2. The HER performance of the NiMoSe@CC was also the highest amongst NiMoSe, NiSe@CC, and MoSe@CC in both the alkaline and seawater environments. In addition, NiMoSe@CC bears a high electrochemically active surface area, low charge transfer resistance of 1.77 Ω, and high stability for both OER and HER. The NiMoSe@CC//NiMoSe@CC system was stable for 5000 LSV cycles with η10 values of 1.63 V (1st LSV cycle) and 1.71 V (5000th LSV cycle).

Self-etching assembly of designed NiFeMOF nanosheet arrays as high-efficient oxygen evolution electrocatalyst for water splitting.

2D metal-organic frameworks (MOFs) have emerged as potential candidates for electrocatalytic oxygen evolution reactions (OER) due to their inherent properties like abundant coordination unsaturated active sites and efficient charge transfer. Herein, a versatile and massively synthesizable self-etching assembly strategy wherein nickel-iron foam (NFF) acts as a substrate and a metal ion source. Specifically, by etching the nickel-iron foam (NFF) surface using ligands and solvents, Ni/Fe metal ions are activated and subsequently reacted under hydrothermal conditions, resulting in the formation of self-supporting nanosheet arrays, eliminating the need for external metal salts. The obtained 33 % NiFeMOF/NFF exhibits remarkable OER performance with ultra-low overpotentials of 188/231 mV at 10/100 mA cm-2 , respectively, outperforming most recently reported catalysts. Besides, the built 33 % NiFeMOF/NFF(+) ||Pt/C(-) electrolyzer presents low cell voltages of 1.55/1.83 V at 10/100 mA cm-2 , superior to the benchmark RuO2 (+) ||Pt/C(-) , implying good industrialization prospects. The excellent catalytic activity stems from the modulation of the electronic spin state of the Ni active site by the introduction of Fe, which facilitates the adsorption process of oxygen-containing intermediates and thus enhances the OER activity. This innovative approach offers a promising pathway for commercial-scale sustainable energy solutions.

P-doped RuPd nanoparticles anchored on Y2Ru2-xPdxO7 pyrochlore oxide surface as oxygen evolution and reduction electrocatalysts for Zn-air battery

Metal-air battery technology is the most promising green technology. However, the sluggish kinetics of the oxygen evolution/reduction reaction (OER and ORR), which are key reactions in air cathode, must be improved. In this study, Pd substitution was introduced into Y2Ru2O7 pyrochlore oxides and the ratio of Pd was varied (YRPO-x). Then, P-doped RuPd nanoparticles were synthesized on Pd-substituted YRPO pyrochlore (YRPO/RuPd-P) by an in situ exsolution process to create bifunctional electrocatalysts facilitating both reactions. The uniquely designed YRPO/RuPd-P catalyst exhibited the OER overpotential and Tafel slope (Ej10 = 232 mV; Tafel slope = 37.1 mV/dec). Furthermore, YRPO/RuPd-P shows an E1/2 of 0.82 V, indicating superior ORR activity. This was further investigated by applying in a unit-cell battery system, which showed an outstanding power density (163 mW/cm2) and robust charge–discharge stability. This study proposes a novel design strategy for bifunctional electrocatalysts.

Construction of nickel phosphide/iron oxyhydroxide heterostructure nanoparticles for oxygen evolution

Active and stable oxygen evolution electrocatalysts are essential in increasing the efficiency of water electrolyzers. The Ni2P/Fe(O)OH heterostructure nanoparticles are prepared via solvothermal phosphidization of Ni metal-organic frameworks (MOF) followed by immersing in Fe3+ aqueous solution. Characterizations reveal that the Ni2P/Fe(O)OH heterostructure nanoparticles are 12.83 nm in size averagely, and the heterointerface induces electron interactions between the Ni2P and Fe(O)OH phases. When used to catalyze OER in alkaline solutions, the Ni2P/Fe(O)OH-40/nickel foam (NF) is the most active and exhibits 240 mV overpotential to reach 10 mA cm−2 oxygen evolution (OER) current densities, which is significantly better than the Ni2P/NF. Lower apparent activation energy, charge transfer resistance, and Tafel slope, along with higher electron rate constant are observed at Ni2P/Fe(O)OH-40/NF, which suggests that the OER kinetics is more facile at the surface of heterostructure nanoparticles. The OER mechanistic pathways of both Ni2P/Fe(O)OH-40/NF and Ni2P/NF involve decoupled electron and proton transfer processes, and higher degree of lattice oxygen oxidation mechanism (LOM) participation is observed at Ni2P/Fe(O)OH-40/NF, which results from the increased acidity of the Ni sites. Density functional theory calculations prove that the formation of heterostructure with Fe(O)OH alters the band structure and the adsorption energies of OER intermediates, which leads to lower energy barrier in the rate-determining step. The Ni2P/Fe(O)OH-40/NF is also stable towards OER in alkaline solutions.

NiCo2O4/MXene Hybrid as an Efficient Bifunctional Electrocatalyst for Oxygen Evolution and Reduction Reaction

AbstractFor the advancement of electrochemical energy conversion and storage technologies, bifunctional electrocatalysts are crucial for efficiently driving both the oxygen evolution (OER) and reduction reactions (ORR). Cobalt‐based spinel oxides are a class of promising bifunctional electrocatalysts. However their low electrical conductivity and stability may hinder further improvement. A novel composite material composed of NiCo2O4 nanoparticles integrated with emerging two dimensional MXene nanosheets (NiCo2O4/MXene) was developed. The successful integration of NiCo2O4 with MXene brings about a number of attractive structural features. This includes synergistic effects between NiCo2O4 and MXene, highly accessible surface areas, complete exposure of numerous active sites, and excellent electronic conductivity, all of which collectively contribute to the desirability of composite material for OER and ORR. The synthesized NiCo2O4/MXene composite showed extraordinary OER electrocatalytic activity with a lower overpotential of 360 mV at a current density of 10 mA/cm2, and a small Tafel slope of 64 mV/dec compared to NiCo2O4, MXene and NiCo2O4+MXene (physically mixed). Additionally, NiCo2O4/MXene displays an ORR limiting current density of −4 mA/cm2 and exhibited highest onset potential and half wave potential of 0.92 V and 0.72 V vs. RHE, respectively, for the ORR in alkaline media compared to NiCo2O4, MXene and NiCo2O4+MXene (physically mixed).

Ionic liquid in-situ functionalized carbon nanotube film as self-supported metal-free electrocatalysts for oxygen evolution

The oxygen evolution reaction (OER) is a critical cathode reaction for hydrogen production from water splitting. However, due to the sluggish kinetics of OER, the water-splitting process is impeded. Thus, it is imperative to develop efficient and low-cost electrocatalysts that reduce the overpotential and improve the kinetics of OER. Herein, an in-situ synthesis strategy was applied to prepare the ionic liquid functionalized carbon nanotube (IL-CNT) that served as a metal-free electrocatalyst for OER. This powder material exhibited exceptional electrocatalytic activity for OER, with an overpotential of 335 mV at 10 mA cm−2 and a Tafel slope of 71 mV dec-1. Then, it was found that catalytic activity was influenced by the IL anion. Moreover, this IL-CNT powder could self-assemble to form carbon film during preparation, exhibiting outstanding catalytic activity for OER (a low overpotential of 153 mV at 10 mA cm−2), which was highly beneficial for practical application. Based on the theoretical calculations and experimental results, these in-situ functionalization sites on CNT surface played a significant role in regulating their electron structure. This, in turn, promoted electron transfer and hydroxyl adsorption, resulting in a substantial increase in catalytic activity for the OER.

Hierarchically Designed Co4Fe3@N-Doped Graphitic Carbon as an Electrocatalyst for Oxygen Evolution in Anion-Exchange-Membrane Water Electrolysis

Hierarchically Designed Co₄Fe₃@N-Doped Graphitic Carbon as an Electrocatalyst for Oxygen Evolution in Anion-Exchange-Membrane Water Electrolysis

Three-dimensional ordered macroporous design of heterogeneous cobalt–iron phosphides as oxygen evolution electrocatalyst

Oxygen evolution reaction (OER) plays a key role in electrochemical conversion, which needs efficient and economical electrocatalyst to boost its kinetics for large-scale application. Herein, a bimetallic CoP/FeP2 heterostructure with a three-dimensional ordered macroporous structure (3DOM-CoP/FeP2) was synthesized as an OER catalyst to demonstrate a heterogeneous engineering induction strategy. By adjusting the electron distribution and producing a lot of active sites, the heterogeneous interface enhances catalytic performance. High specific surface area is provided by the 3DOM structure. Additionally, at the solid–gas-electrolyte threephase interface, the electrocatalytic reaction exhibits good mass transfer. In sit u Raman spectroscopy characterization revealed that FeOOH and CoOOH reconstructed from CoP/FeP2 were the true OER active sites. Consequently, the 3DOM-CoP/FeP2 demonstrates superior OER activity with a low overpotentials of 300/420 mV at 10/100 mA cm–2 and meritorious OER durability. It also reveals promising performance as the overall water splitting anode.

Medium-Entropy Co–Fe–Cr–Mo Spinel Nanoflowers as Electrocatalysts for Oxygen Evolution

Medium-Entropy Co–Fe–Cr–Mo Spinel Nanoflowers as Electrocatalysts for Oxygen Evolution

Metal‐Organic Frameworks: Direct Synthesis by Organic Acid‐Etching and Reconstruction Disclosure as Oxygen Evolution Electrocatalysts

AbstractMetal‐organic frameworks (MOFs) have emerged as promising oxygen evolution reaction (OER) electrocatalysts. Chemically bonded MOFs on supports are desirable yet lacking in routine synthesis, as they may allow variable structural evolution and the underlying structure‐activity relationship to be disclosed. Herein, direct MOF synthesis is achieved by an organic acid‐etching strategy (AES). Using π‐conjugated ferrocene (Fc) dicarboxylic acid as the etching agent and organic ligand, a series of MFc‐MOF (M=Ni, Co, Fe, Zn) nanosheets are synthesized on the metal supports. The crystal structure is studied using X‐ray diffraction and low‐dose transmission electron microscopy, which is quasi‐lattice‐matched with that of the metal, enabling in situ MOF growth. Operando Raman and attenuated total reflectance Fourier transform infrared spectroscopy disclose that the NiFc‐MOF features dynamic structural rebuilding during OER. The reconstructed one showing optimized electronic structures with an upshifted total d‐band center, high M−O bonding state occupancy, and localized electrons on adsorbates indicated by density functional theory calculations, exhibits outstanding OER performance with a fairly low overpotential (130 mV at 10 mA cm−2) and good stability (144 h). The newly established approach for direct MOF synthesis and structural reconstruction disclosure stimulate the development of more prudent catalysts for advancing OER.

Metal-Organic Frameworks: Direct Synthesis by Organic Acid-Etching and Reconstruction Disclosure as Oxygen Evolution Electrocatalysts.

Metal-organic frameworks (MOFs) have emerged as promising oxygen evolution reaction (OER) electrocatalysts. Chemically bonded MOFs on supports are desirable yet lacking in routine synthesis, as they may allow variable structural evolution and the underlying structure-activity relationship to be disclosed. Herein, direct MOF synthesis is achieved by an organic acid-etching strategy (AES). Using π-conjugated ferrocene (Fc) dicarboxylic acid as the etching agent and organic ligand, a series of MFc-MOF (M=Ni, Co, Fe, Zn) nanosheets are synthesized on the metal supports. The crystal structure is studied using X-ray diffraction and low-dose transmission electron microscopy, which is quasi-lattice-matched with that of the metal, enabling in situ MOF growth. Operando Raman and attenuated total reflectance Fourier transform infrared spectroscopy disclose that the NiFc-MOF features dynamic structural rebuilding during OER. The reconstructed one showing optimized electronic structures with an upshifted total d-band center, high M-O bonding state occupancy, and localized electrons on adsorbates indicated by density functional theory calculations, exhibits outstanding OER performance with a fairly low overpotential (130 mV at 10 mA cm-2 ) and good stability (144 h). The newly established approach for direct MOF synthesis and structural reconstruction disclosure stimulate the development of more prudent catalysts for advancing OER.

A high-efficient electrocatalyst for oxygen evolution and methanol oxidation reactions prepared by a facile and scale-up method

Developing the facile, universal and scale-up method to prepare electrocatalyst for oxygen evolution reaction (OER) and methanol oxidation reaction (MOR) is crucial for the high-efficient hydrogen production through water splitting. Herein, we load the CeO2 nanoparticles on the various metal foams by a very simple and scalable impregnation method. Taking iron-nickel foam (INF) as an example, we can obtain a high-efficient OER and MOR electrocatalyst of CeO2 @INF-0.6. CeO2 @INF-0.6 shows the superior electrocatalytic activities toward both OER and MOR with the oxidation potentials of 1.52 V and 1.47 V at 100 mA cm−2, respectively. This can be ascribed to the CeO2 @INF-0.6 with strong adsorption and fast conversion ability of OH- in the alkaline electrolyte. CeO2 @INF-0.6 also has good stability and selectivity in MOR-assisted alkaline water splitting, and the Faraday efficiency of formate is up to 99.2% at 1.42 V (vs. RHE). In addition, the Na+ and Cl- ions with high concentrations almost have the negligible effects on the OER and MOR activities of our CeO2 @INF-0.6 in the alkaline solution. This indicates that using CeO2 @INF-0.6 as anode for seawater electrolysis not only can greatly reduce the energy consumption, but produce the value-added anode product of formate.