Enhanced electrocatalytic performance of NiOx@MnOx@graphene for oxygen reduction and evolution reactions

Oxygen electrodes for oxygen evolution reaction and oxygen reduction reaction were intensively investigated due to high overpotential required to drive the four-electron process. A NiOx@MnOx@G nanostructure supported homogenously on graphene nanosheets through an easy and scalable self-assembly method was studied. The NiOx@MnOx@G exhibited a nanostructured NiO_x nanocrystalline with size around 2.3 nm and an amorphous MnO_x with controllable thickness. The 25% NiOx@MnOx@G showed remarkable activity for oxygen reduction reaction with a 4-electron process, the half-wave potential was 50 mV, but the stability of 25% NiOx@MnOx@G was better than Pt/C-JM. NiOx@MnOx@G nanostructure exhibited significantly better activity for oxygen evolution reaction compared with MnO_x, which can demonstrate that NiO_x could tune the activity of surface amorphous MnO_x and dramatically increased oxygen evolution reaction activity. NiOx@MnOx@G is demonstrated with superior oxygen catalysts performance for reversible oxygen evolution and oxygen reduction reaction, due to the synergistic effect of NiO_x and amorphous MnO_x.     

Synthesis and characterization of electrocatalysts for the oxygen evolution in PEM water electrolysis

IrO₂, Ir_xSn_(1−x)O₂ (x = 0.7, 0.5) and Ir_xRu_(1−x)O₂ (x = 0.7, 0.5) electrocatalysts for the oxygen evolution reaction (OER) have been synthesized using the Adams fusion method. The metal oxides were characterized via X-ray diffraction, scanning electron microscopy, inductively coupled plasma-atomic emission spectrometry and nitrogen adsorption-desorption measurements to have information about their crystallographic structure, chemical composition and morphology, respectively. A controlled bulk molar fraction of Ru or Sn was introduced in the IrO₂ lattice during the synthesis with no phase separation. The electrocatalytic activity of the synthesized oxides in the OER was studied in liquid electrolyte using porous rotating-disk electrodes, in “half-cell” configuration and in a 5 cm² proton-exchange membrane water electrolysis cell. An increase of the electrical performance was observed upon Ru insertion and a severe depreciation upon Sn insertion.     

Electrodeposited Ni–Co-oxide electrodes:characterization and kinetics of the oxygen evolution reaction

In this paper we present results on the characterization of Ni–Co-oxide electrodes, prepared by anodic deposition from Co(NO₃)₂ aqueous solutions on Ni substrates. The kinetics and mechanism of oxygen evolution was analysed. Tafel slopes close to 40 mV per decade were measured. The reaction order with respect to [OH⁻] was found to be approximately 2 at 25°C. A possible mechanism for oxygen evolution on these electrodes is presented, which accounts for the values of the kinetic parameters experimentally obtained.     

Vertical Nickel–Iron layered double hydroxide nanosheets grown on hills-like nickel framework for efficient water oxidation and splitting

Herein, the vertical thin nickel–iron layered double hydroxide nanosheets grown on the hills-like nickel framework (NiFe LDH/Ni@NF) are employed for the oxygen evolution reaction (OER), securing at the low overpotentials of 197 and 270 mV to obtain the current densities of 20 and 100 mA cm⁻², respectively, with a Tafel slope of 73.34 mV dec⁻¹. The electrodeposited nickel film induces the NiFe LDH nanosheets grow vertically and thinly. As well, the nickel abundant interfaces and inner space makes this catalyst effective for OER. It was further served as the OER electrode in a water splitting system coupled the Pt/C cathode, and a cell voltage was at 1.52 and 1.67 V to achieve the current density of 10 mA cm⁻² and 50 mA cm⁻². In addition, the water electrolyzer can suffer a long time of 24 h at 50 mA cm⁻², showing the feasibility in a practical unbiased alkaline water splitting system.     

A facile method for reduced CoFe2O4 nanosheets with rich oxygen vacancies for efficient oxygen evolution reaction

The efficiency of electrochemical water splitting is greatly hindered by the thermodynamic uphill reaction of oxygen evolution reaction (OER). Thus, it is important to synthesize an active OER electrocatalysts with abundant active sites, favorable conductivity and good durability. Herein, a facile reduction method using NaBH₄ as readily available reductant has been developed to fabricate the reduced CoFe₂O₄ nanosheets (NS). The obtained reduced CoFe₂O₄ NS are rich in oxygen deficient sites, leading to more active sites as well as the enhanced conductivity than the pristine CoFe₂O₄ hollow nanosphere, which reaches the current density of 10 mA cm^?2 at the overpotential of 320 mV in 1 M KOH. Meanwhile, CoFe₂O₄ samples with three different morphology nanostructures including hollow nanospheres, bulk and nanoparticles have been provided to study the effect of different morphology on NaBH₄ reduction efficiency. As expected, after NaBH₄ reduction, CoFe₂O₄ hollow nanosphere with relatively higher surface area exhibits most obvious improvement for OER activity and also its corresponding reduced CoFe₂O₄ NS showed best OER performance than the reduced CoFe₂O₄ bulk as well as the reduced CoFe₂O₄ nanoparticles, implying the hollow nanospheres feature more accessible surface area than bulk and nanoparticles samples, thus greatly facilitate efficiency of NaBH₄ reduction treatment.     

Facile synthesis of bimetallic Ni–Fe phosphide as robust electrocatalyst for oxygen evolution reaction in alkaline media

Bimetallic Ni–Fe phosphide electrocatalysts were in-situ synthesized through direct phosphorization of metal salts on carbon cloth (CC). The Fe dopant remarkably enhances the OER performance of Ni₂P in alkaline medium through the electronic structure modulation of Ni. The (Fe_0.5Ni_0.5)₂P/CC electrode, composed of uniform films coated on carbon fibers, delivers a low overpotential of 260 mV with a small Tafel slope of 45 mV·dec⁻¹ at the current density of 100 mA cm⁻², outperforming most reported non-noble electrocatalysts and commercial RuO₂ electrocatalyst. The (Fe_0.5Ni_0.5)₂P/CC also displays superior electrochemical stability at high current density. An appropriate Fe dopant level facilitates the in-situ transformation of Ni–Fe phosphides into active NiFeOOH during alkaline OER. This work simplifies the synthesis procedure of metal phosphides.     

Bifunctional doped transition metal CoSSeNi–Pt/C for efficient electrochemical water splitting

Reasonable design and synthesis of hetero atom metal coordination compounds on the atomic scale can significantly improve the performance of the catalysts. Herein, Pt/C (20%) is doped and inserted into the CoSSeNi catalyst through two steps of hydrothermal reaction and high temperature calcination process. Compared with the single metal Pt/C doped CoSSe–Pt/C, the bimetal doped CoSSeNi–Pt/C can greatly enhance the hydrogen evolution reaction (HER) activity. The optimized Pt content in CoSSeNi–Pt/C is 2.25 wt%, which achieves the optimal HER and OER activity. The OER and HER overpotentials of CoSSeNi–Pt/C-0.3 nanosheets at 10 mA cm^?2 only required 295 mV and 180 mV, respectively. During the accelerated durability test, in the presence of Pt/C dopants, CoSSeNi–Pt/C catalyst exhibited excellent long-time durability in alkaline media. Meanwhile, CoSSeNi–Pt/C showed much higher DOS near the Fermi level and the higher electron density near the Fermi level would facilitate the adsorption of adsorbates.     

IrO2-graphene hybrid as an active oxygen evolution catalyst for water electrolysis

In the present work, graphene supported IrO₂ catalyst (IrO₂/RGO) has been synthesized by hydrothermal method in ethanol/water mixture solvent. X-ray diffraction (XRD) and transmission electron microscopy (TEM) tests reveal that IrO₂ is uniformly supported on RGO surface with ultrafine IrO₂ nanoparticles (ca. 1.7 nm). Linear sweep voltammetry (LSV) tests indicate that the catalytic activity of IrO₂/RGO hybrid towards oxygen evolution reaction (OER) is 2.3 times that of commercial IrO₂. The superior OER activity of IrO₂/RGO hybrid is attributed to the enhanced surface area and the improved electrical conductivity of IrO₂ due to the introduction of graphene support. Lifetime tests demonstrate that IrO₂/RGO hybrid has unexpectedly high OER durability. It also displays an excellent performance in long-time water electrolysis. This may be interpreted in terms of the dispersion retention of IrO₂ nanoparticles on RGO surface, which is caused by the interaction between IrO₂ and Π-electrons of RGO.     

Rod-like nonstoichiometric Ni0.85Se as efficient electrocatalysts for hydrogen evolution reaction

Nickel selenides with different Ni/Se atomic ratio have been successfully synthesized by a simple solvothermal method. Among them, the nonstoichiometric compound (Ni_0.85Se) (A-10) behaved excellent electrochemical catalytic activity towards hydrogen evolution reaction. Furthermore, it presents an ultra-low overpotential (η) of 190 mV and extremely small Tafel slope of 57 mV/dec when the current density j_A reaches ～10 mA/cm². At the same time, it has large effective electrochemical active surface area and shows outstanding stability after a long time chronoamperometry testing.