Electrochemical Oxygen Evolution Reaction Activity Research Articles

Polyaniline Induced Trivalent Ni in Laser-Fabricated Nickel Oxides for Efficient Oxygen Evolution Reaction.

Although it is generally acknowledged that transition metals at high oxidation states represent superior oxygen evolution reaction (OER) activity, the preparation and stability of such a high-valence state are still a challenge, which requires relatively harsh reaction conditions and is unstable under ambient conditions. Herein, we report the formation of trivalent nickel (Ni3+) in laser-fabricated nickel oxides induced by polyaniline (PANI) under electrochemical activation via a significant charge transfer between Ni and N, as confirmed by X-ray photoelectron spectroscopy and density functional theory calculations. Thereafter, the presence of Ni3+ and the improved conductivity by PANI effectively increase the electrochemical OER activity of the samples together with excellent long-term stability. This work provides new insights for the rational manufacture of high-valence metal for electrochemical reactions.

Bimetallic-based CuxCo3−xO4 nanoparticle-embedded N-doped reduced graphene oxide toward efficient oxygen evolution reaction and hydrogen evolution reaction for bifunctional catalysis

The fabrication of efficient electrocatalysts for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is essential for hydrogen production to obtain sustainable renewable energy. Owing to the increase in chemical complexity, multimetallic catalysts provide flexibility to alter their electronic structure to attain high intrinsic catalytic activity via synergistic effects. In this study, bimetallic copper-cobalt oxide nanoparticles (CuxCo3−xO4 NPs) embedded on nitrogen-doped reduced graphene oxide (N-RGO) substrates are fabricated as catalysts for the OER and HER via a facile solution-hydrothermal synthesis. In the fabricated N-RGO-supported Cu-doped Co3-xO4 nanoparticle (CuxCo3−xO4 NPs@N-RGO) hybrid catalyst, the RGO sheet acts as a conductive network to promote electron transfer, thereby increasing the electrical conductivity of the catalyst. Additional active sites acting as bridged sites from the Cu and N dopants improve the catalytic activity, resulting in outstanding electrochemical activity for OER and HER. The optimal Cu0.35Co2.65O4 NPs@N-RGO catalyst exhibits an overpotential of 170 mV at 10 mA cm−2 for HER and a low overpotential of 270 mV for the OER in basic media, which are comparable to those of Pt/C and IrO2. The synthesized catalyst is a promising bifunctional catalyst for overall water splitting.

Binder-Free Fabrication of Prussian Blue Analogues Based Electrocatalyst for Enhanced Electrocatalytic Water Oxidation.

Developing a cost-effective, efficient, and stable oxygen evolution reaction (OER) catalyst is of great importance for sustainable energy conversion and storage. In this study, we report a facile one-step fabrication of cationic surfactant-assisted Prussian blue analogues (PBAs) Mx[Fe(CN)5CH3C6H4NH2]∙yC19H34NBr abbreviated as SF[Fe-Tol-M] (where SF = N-tridecyl-3-methylpyridinium bromide and M = Mn, Co and Ni) as efficient heterogeneous OER electrocatalysts. The electrocatalysts have been characterized by Fourier transform infrared (FT-IR) spectroscopy, powder X-ray diffraction (PXRD), scanning electron microscopy (SEM) coupled with energy dispersive X-ray (EDX) analysis, and X-ray photoelectron spectroscopy (XPS). In the presence of cationic surfactant (SF), PBAs-based electrodes showed enhanced redox current, high surface area and robust stability compared to the recently reported PBAs. SF[Fe-Tol-Co] hybrid catalyst shows superior electrochemical OER activity with a much lower over-potential (610 mV) to attain the current density of 10 mA cm−2 with the Tafel slope value of 103 mV·dec−1 than that for SF[Fe-Tol-Ni] and SF[Fe-Tol-Mn]. Moreover, the electrochemical impedance spectroscopy (EIS) unveiled that SF[Fe-Tol-Co] exhibits smaller charge transfer resistance, which results in a faster kinetics towards OER. Furthermore, SF[Fe-Tol-Co] offered excellent stability for continues oxygen production over extended reaction time. This work provides a surface assisted facile electrode fabrication approach for developing binder-free OER electrocatalysts for efficient water oxidation.

Nanostructured cobalt-based metal-organic framework/cadmium sulfide electrocatalyst for enhanced oxygen evolution reaction and anion exchange membrane-based water electrolysis: Synergistic effect

Precious metal-free electrocatalysts are essential for green hydrogen using low-cost electrolysis technology for future power supply systems. In this regard, anion-exchange membrane (AEM) electrolysis is the technology of best choice due to its compactness and high efficiency. Here, we proposed a low cost electrocatalyst composed of cadmium sulfide (CdS) nanoparticles decorated in a Cobalt-based metal-organic framework (CdS/ZIF-67), synthesized using a simple coprecipitation method. Compared with pristine Cobalt-based metal-organic framework (ZIF-67), 5 wt%CdS/ZIF-67 has excellent electrochemical oxygen evolution reaction activity, low overpotential (294 mV), low Tafel-slope (100 mV dec−1), higher mass activity (25.8 mA mg−1), and better stability. The AEM electrolyzer with the 5 wt%CdS/ZIF-67 anode can attain a potential of 1.85 V at a current-density of 1 A cm−2, which is lower than that of bare Cobalt-based metal-organic framework electrolyzers. Thus, these catalysts are not only very active but also scalable, making them a promising electrolyzer for application in commercial power market.

Sacrificial Cu Layer Mediated the Formation of an Active and Stable Supported Iridium Oxygen Evolution Reaction Electrocatalyst.

The production of hydrogen via a proton-exchange membrane water electrolyzer (PEM-WE) is directly dependent on the rational design of electrocatalysts for the anodic oxygen evolution reaction (OER), which is the bottleneck of the process. Here, we present a smart design strategy for enhancing Ir utilization and stabilization. We showcase it on a catalyst, where Ir nanoparticles are efficiently anchored on a conductive support titanium oxynitride (TiONx) dispersed over carbon-based Ketjen Black and covered by a thin layer of copper (Ir/CuTiONx/C), which gets removed in the preconditioning step. Electrochemical OER activity, stability, and structural changes were compared to the Ir-based catalyst, where Ir nanoparticles without Cu are deposited on the same support (Ir/TiONx/C). To study the effect of the sacrificial less-noble metal layer on the catalytic performance of the synthesized material, characterization methods, namely X-ray powder diffraction, X-ray photoemission spectroscopy, and identical location transmission electron microscopy were employed and complemented with scanning flow cell coupled to an inductively coupled plasma mass spectrometer, which allowed studying the online dissolution during the catalytic reaction. Utilization of these advanced methods revealed that the sacrificial Cu layer positively affects both Ir OER mass activity and its durability, which was assessed via S-number, a recently reported stability metric. Improved activity of Cu analogue was ascribed to the higher surface area of smaller Ir nanoparticles, which are better stabilized through a strong metal–support interaction (SMSI) effect.

Growth of CeO2 nanocubes showing size-dependent optical and oxygen evolution reaction behaviors.

CeO2 nanocubes with average sizes of 9, 13, and 18 nm have been synthesized by preparing a slightly basic aqueous mixture of Ce(NO3)3, Na2SO4, and NH4OH and heating the solution to 100 to 150 °C in 4 or 9 h. The nanocubes possess high crystalline quality. Their band gaps decrease gradually beyond the quantum confinement regime from 3.57 eV to 3.45 eV with increasing particle sizes. The 9 nm CeO2 nanocubes have the most positive valence band energy and correspondingly they exhibit the best electrochemical oxygen evolution reaction activity. Since band gaps of semiconductor nanocrystals can be tuned substantially through particle size control to yield different band energies, this fact can be utilized to enhance the electrochemical and photocatalytic properties.

S-doped Co-Fe-Pi nanosheets as highly efficient oxygen evolution electrocatalysts in alkaline media

The cobalt-based phosphates are important and efficient catalysts for improving the activity of oxygen evolution reaction (OER), which is an essential reaction in water splitting. Here, a series of S-doped cobalt-based phosphate nanomaterials are synthesized through a simple co-precipitation process. It is found that the amorphous S-doped cobalt-iron phosphate (S-doped Co-Fe-Pi) nanosheets with an Co/Fe ratio of 6.9:1 can display the best electrochemical activity for OER, requiring an overpotential of only 273 mV to drive the current density of 10 mA cm−2 in 1M KOH. As a highly efficient catalyst, both the structure features and the electrocatalytic performance of S-doped Co-Fe-Pi catalysts are discussed in this work. More importantly, the OER performance of the catalysts affected by thiophosphate anions and Fe incorporated cations is investigated. Finally, it is suggested that the outstanding activity can be attributed to not only the porous nanostructure existing in amorphous morphology, but also the proper electronic state of Co atoms induced by the decoration of Fe and S atoms.

Ta-doping triggered electronic structural engineering and strain effect in NiFe LDH for enhanced water oxidation

Structural manipulation of electrocatalyst via doping strategy has always held great interests for developing advanced and stable non-noble metal electrocatalyst for oxygen evolution reaction (OER). Herein, a high-valence state tantalum (Ta) was incorporated into the pristine NiFe layered double hydroxide (LDH) by hydrothermal method. Interestingly, as revealed by structural characterizations, Ta doping causes the lattice expansion of LDH and electronic structure modification through electron transfer from Fe to Ta. DFT calculations further verified that the modulated electronic structure among Ni, Fe and Ta and the modified eg orbital of Ta induced by charge transfer are beneficial for the adsorption of OH species on Ta site in Ta-doped NiFe LDH and increasing the intrinsic metallic property of NiFe LDH. Consequently, the Ta site has lower overpotential compared with other sites on NiFe LDH including the pristine oxygen vacancies, which can improve the electrocatalytic activity for OER. Furthermore, the optimized Ta-NiFe LDH (0.5:6:1.5) exhibited a superior OER activity in contrast to bimetallic LDH, with a low overpotential of 260 mV to drive the current density of 50 mA·cm−2 and a small Tafel slope of 58.95 mV·dec-1. This work provides the theoretical basis for the enhancement of electrochemical OER activity by doping LDH with high-valence state foreign metal.

Enhanced electrochemical oxygen evolution reaction activity on natural single-atom catalysts transition metal phthalocyanines: the substrate effect

Some TMPc are excellent OER electrocatalysts and photocatalysts with a single-wall nanotube (SWNT) substrate which can enhance their catalytic activity.

Self-supported Ni2P nanotubes coated with FeP nanoparticles electrocatalyst (FeP@Ni2P/NF) for oxygen evolution reaction

Bimetallic phosphides have been widely investigated as electrocatalysts for oxygen evolution reaction (OER) due to their efficient activity and environmental friendliness. While the reasonable design and controllable synthesis of bimetallic phosphide with typical nanostructure is still a great challenge. Hence, we put forward a novel and straightforward way for constructing FeP nanoparticles coated Ni2P ultrathin nanotube arrays on the surface of Ni foil (FeP@Ni2P/NF), which is synthesized through two steps of electrodeposition and subsequent in-situ phosphorization process. The obtained FeP@Ni2P/NF shows excellent electrochemical activity for OER, and it only needs potential of 1.52 V vs. RHE to reach the current density of 50 mA cm−2 in an alkaline media. The excellent electrocatalytic activity of FeP@Ni2P/NF mainly benefits from: (i) the synergistic effect between FeP and Ni2P promoting electron transfer; (ii) the formation of the unique 3D ultrathin nanotube arrays increasing the quantity of active sites and avoiding the agglomeration of catalysts during testing. In addition, the influence of reaction condition on the electrochemical activity for OER has also been investigated through altering the phosphorization temperature of precursor.

Solution blow spun nickel oxide/carbon nanocomposite hollow fibres as an efficient oxygen evolution reaction electrocatalyst

The development of efficient electrocatalysts for slow reaction of the oxygen evolution reaction (OER) is fundamental for viability of the electrochemical water splitting technologies. Here we report for the first time the synthesis of NiO/carbon hollow fibres (NiO-HF) by the Solution Blow Spinning (SBS) technique, and a study of their catalytic activity towards the OER in alkaline medium. The hollow fibres were obtained with ca. 300 nm in diameter consisting of agglomerated NiO nanoparticles with an average size of 50 nm which is close to the tubular wall thickness. The formation mechanism of the hollow structure was discussed. It was revealed that the carbon from polyenic branch of polyvinylpyrrolidone (PVP) resists the firing treatment and acts as an agglomerating agent, thus ensuring a conductive and percolating path between NiO nanoparticles along the fibres. A battery of electrochemical tests of NiO-HF supported by commercial Ni foam reveals excellent electrochemical activity for OER in 1 M KOH, in comparison with reference NiO nanoparticles (NiO-NP, diameter ca. 23 nm). NiO-HF attains an overpotential of 340 mV vs. RHE at a current density of 10 mA cm−2, which is amongst the lowest values reported in the literature for undoped NiO. Chronopotentiometry reveals stable NiO-HF electrodes over 15 h under an electrolysis current of 25 mA cm−2. Microscopic analysis shows that the fibrillar morphology is completely preserved after the electrolysis test. The remarkable performance of the NiO-HF catalyst is ascribed to the enhanced electronic conductivity resulting from the interpenetrating NiO-HF/carbon microstructure.

(Invited) Operando X-Ray Absorption Investigations into the Oxygen Evolution Activity, Stability, and pH Dependency of NixFe1-XOy Nanoparticles

Mixed Ni-Fe metal oxides currently represent some of the most attractive anode catalysts for the electrochemical splitting of water due to their low overpotentials for the oxygen evolution reaction (OER) in alkaline electrolytes. The concentrated potassium hydroxide solutions and anion exchange membranes that are typically employed in alkaline water electrolyzers, however, are prone to carbonation which can lead to significant shifts in the electrolyte pH from alkaline to quasi-neutral conditions (pH = 7-10). So far a fundamental understanding of how changes in the local pH environment induced via carbonation affect the catalyst electronic and surface structure, and ultimately the catalyst stability and electrochemical OER activity, is lacking. The importance of understanding this behavior is further highlighted by the potential application of Ni-Fe oxides as anode catalysts in co-electrolysis, where the electrochemical reduction of CO2 and the anodic evolution of oxygen occur in a carbonated, quasi-neutral pH environment. Here we demonstrate a practical and scalable flame-spray pyrolysis synthesis capable of producing highly crystalline Ni-Fe oxide (Ni1-xFexOy) nanoparticles with high surface areas (SABET ≈ 20 - 75 m2/g). The research presented herein focuses on expanding the current understanding of the influence of the local electronic and surface structures on the OER activity and electrochemical stability in both alkaline (pH = 13) and carbonated, quasi-neutral (pH = 9) electrolyte. The resulting operando XANES and EXAFS analyses of the Ni and Fe K-edges permit useful insight into the nature of the valence states and rearrangements in local structure that occur under operating conditions representative of alkaline water electrolysis and co-electrolysis. Combined with a broad range of ex situ physical characterization techniques, we then relate the structural, electronic, and morphological changes to the observed electrochemical OER activity and stability.

Influence of Crystallinity, Particle Size, and Microstructure on the Oxygen Evolution Activity of IrO2

The development of cost-effective polymer electrolyte membrane (PEM) electrolyzers is key to the implementation of a hydrogen-based infrastructure. It cannot be denied that there is an ever-increasing demand to shift away from the use of nuclear power and the traditional burning of fossil fuels as primary energy systems. The only foreseeable long-term solution has been increasingly focused on the implementation of clean, renewable power supplies, such as wind farms and solar power stations. In this regard, the electrochemical conversion of water to hydrogen is expected to play a key role in the development of scalable energy storage that is required for such intermittent power supplies(1). The cathodic generation of hydrogen from water splitting is known to be extremely facile on Pt-based catalysts(2). The simultaneous oxygen evolution reaction (OER) occurring at the anode, however, is limited by sluggish kinetics and requires a considerable overpotential to achieve modest current densities. Moreover, the harsh acidic environment and high anodic operating potentials limits the choice of stable electrocatalyst materials to those of the noble metal oxides. Reduction of the noble metal loading at the anode and enhancing catalyst stability for OER in PEM electrolyzers remains a challenge. Perhaps the most widely implemented approach for reducing the noble metal content is that which considers reducing the catalyst particle size(3, 4). A major issue that arises from this approach, however, is that it becomes increasingly difficult to establish structure-activity relationships for the OER due to the transformation processes, e.g. changes in microstructure and crystallinity, that commonly occur during the preparation of the materials. The research reported herein is focused on expanding the fundamental understanding of the influence of crystallinity, particle size, and microstructure on the electrochemical OER activity of nanocrystalline IrO2 with particular emphasis pointed towards formation of a hydrous surface layer. Chlorine−free iridium oxide nanoparticles are synthesized using the modified Adams fusion method(5), which is capable of producing spherical 1.7 ± 0.4 nm particles with a specific surface area of 150 m2/g using a low temperature synthesis (350 °C). Increasing the synthesis temperature to 600 °C results in the formation of larger, rod−shaped particles mostly terminated by highly ordered non−defective (110) surfaces. X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), and electrochemical studies indicate the presence of a hydrous surface layer, i.e. Ir(O)OH, that leads to an enhanced OER activity. We report that it is possible to create a larger hydrous layer on smaller nanoparticles and thus increase the specific OER activity.

Active copper delafossite anode for oxygen evolution reaction

The design of highly active anode catalysts for water electrolysis is a critical challenge to achieve a sustainable energy-powered economy. Here we report on copper delafossite oxide (ABO2; A=Cu) as a novel anode for the oxygen evolution reaction (OER). Among the copper delafossites we synthesized, copper rhodium delafossite (CuRhO2) showed excellent cyclic voltammogram (CV) characteristics for OER in 1M KOH (aq), comparable to that of Co3O4. We also confirmed that the eg orbital occupancy at the B site of delafossites, calculated from the bulk structure, had a strong correlation with the electrochemical OER activity of those materials. The CV characteristics of Cu delafossites were more accurately explained by considering the electronic conductivity.