Phase regulation of Ni(OH)2 nanosheets induced by W doping as self-supporting electrodes for boosted water electrolysis

Developing high-performance and low-cost electrodes for hydrogen and oxygen evolution reactions (HER and OER, respectively) represents a pivotal challenge in the field of water electrolysis. Herein, W doped NiFe LDH nanosheets (NiFe-Wx/NF) were immobilized on nickel foam (NF) through one-step corrosion engineering, which induced the coexistence of α-Ni(OH)2 and β-Ni(OH)2. The doping of large atomic radius W influenced the growth of crystal planes of Ni(OH)2, promoting the formation of α-Ni(OH)2, which results in large layer spaces and neatly arranged nanosheets structure. The optimized NiFe-W0.5/NF catalyst require potentials of only 69 to attain 10 mA/cm2 for HER, and require overpotentials of 269 mV to reach 100 mA/cm2 current density for OER, respectively. The W6+ with high oxidation state can withdraw neighboring electrons from Ni, altering the adsorption energy of hydrogen intermediates, which improves the Volmer step and electrical conductivity in HER. And the large layer space of α-Ni(OH)2 in NiFe-W0.5/NF can be contributed to accelerating the formation of high valence γ-NiOOH, which can accelerate OER kinetics. In addition, the NiFe-W0.5/NF catalyst also provides an overall water splitting activity of 780 mA/cm2 current density at a cell voltage of only 1.90 V, and remains highly stable for over 70 h at 100 mA/cm2, which makes it a bifunctional efficient catalyst for water electrolysis.

Phase-dependent electronic structure modulation of nickel selenides by Fe doping for enhanced bifunctional oxygen electrocatalysis

Bifunctional oxygen electrocatalysis is a pivotal process that underpins a diverse array of sustainable energy technologies, including electrolyzers and fuel cells. Metal selenides have been identified as highly promising candidates for oxygen electrocatalysts with electronic structure engineering that lies at the heart of catalyst design. Two-phase Fe-doped nitrogen carbon (NC)-supported nickel selenides were synthesized using a coordination polymer template. Fe doping offers significant advantages as it enhances electronic interactions, resulting in higher availability of active sites than nickel selenides and optimizing the adsorption energy for reaction intermediates. Owing to the intriguing compositional and structural features, the obtained NixFe1-xSe2-NC@400 electrocatalyst displays better catalytic activity with an overpotential (η10) of 253 mV and a lower Tafel slope of 57.1 mV dec-1 for the Oxygen Evolution Reaction (OER) in 1 M KOH. Likewise, the catalyst demonstrated remarkable efficiency in Oxygen Reduction Reaction (ORR) catalysis, achieving a limiting current density comparable to that of the standard Pt/C catalyst and exhibiting an improved Tafel slope of 35.4 mV dec-1 in 0.1 M KOH. This work reveals the influence of Fe dopants in oxygen electrocatalysis and presents an effective approach to tuning the electronic structure for the development of highly active electrocatalysts in alkaline media.

Low-spin state of Fe in Fe-doped NiOOH electrocatalysts

Doping with Fe boosts the electrocatalytic performance of NiOOH for the oxygen evolution reaction (OER). To understand this effect, we have employed state-of-the-art electronic structure calculations and thermodynamic modeling. Our study reveals that at low concentrations Fe exists in a low-spin state. Only this spin state explains the large solubility limit of Fe and similarity of Fe-O and Ni-O bond lengths measured in the Fe-doped NiOOH phase. The low-spin state renders the surface Fe sites highly active for the OER. The low-to-high spin transition at the Fe concentration of ~ 25% is consistent with the experimentally determined solubility limit of Fe in NiOOH. The thermodynamic overpotentials computed for doped and pure materials, η = 0.42 V and 0.77 V, agree well with the measured values. Our results indicate a key role of the low-spin state of Fe for the OER activity of Fe-doped NiOOH electrocatalysts.

A rare polyoxometalate cluster [NiW12O44]14- based solid as a pre-catalyst for efficient and long-term oxygen evolution

Polyoxometalates (POMs) are an eminent class of metal oxide anionic clusters of early transition metals with huge structural diversity. Herein, a [NiW12O44]14- cluster based solid, (C5H7N2)6[NiW12O44], has been reported (PS-78). The [NiW12O44]14- cluster bridges the missing gap of 1 : 12 hetero-POMs of Keggin and Silverton together with a coordination number of 8 of the central heteroatom (Ni). Furthermore PS-78 has been explored as an efficient and highly sustained oxygen evolution pre-catalyst in alkaline medium with an overpotential of 347 mV to attain a current density of 10 mA cm-2 and long-term stability up to 96 hours. Furthermore, mechanistic investigation showed that in situ generated NiO and WO x (x = 1, 2) species act as active species for the oxygen evolution reaction. This study will open up new avenues for exploring POMs' new topologies and the potential of POMs as effective pre-catalysts in electrocatalytic applications.

Quasi-Parallel NiFe Layered Double Hydroxide Nanosheet Arrays for Large-Current-Density Oxygen Evolution Electrocatalysis

Designing advanced electrocatalysts for oxygen evolution at large current density (>500 mA cm^-2 ) is critical to practical water splitting applications. Herein, a novel quasi-parallel NiFe layered double hydroxide (NiFe LDH) nanosheet arrays with pattern alignment on Ni foam was developed. The initial α-Ni(OH)₂ layer induced effective coprecipitation between Ni²⁺ and Fe³⁺ for the formation of LDH phase, guaranteeing the electronic pulling effect among metal cations and enhancing the interaction between active materials and substrate for excellent adhesion and electrical conductivity. Quasi-parallel NiFe LDH nanoarrays exhibited outstanding oxygen evolution activity with a small Tafel slope of 30.1 mV dec^-1 and overpotentials of 196, 255, and 284 mV at a current density of 10, 500, and 1000 mA cm^-2 in 1.0 m KOH solution, respectively, and high stability over 40 h at 750 mA cm^-2 . This work presents a new strategy towards fabricating electrode materials with exceptional performance.

Doping of MoS2 by "Cu" and "V": An Efficient Strategy for the Enhancement of Hydrogen Evolution Activity

To replace Pt-based compounds in the electrocatalytic hydrogen evolution reaction (HER), MoS₂ has already been established as an efficient catalyst. The electrocatalytic activity of MoS₂ is further improved by tuning the morphology and the electronic structure through doping, which helps the band energy position to be modified. Presently, thin sheets of MoS₂ (MoS₂-TSs) are synthesized via a microwave technique. Thin sheets of MoS₂ can outperform nanosheets of MoS₂ in the HER. Further, the efficiency of the thin sheets is improved by doping with different metals like Cu, V, Zn, Mn, Fe, Sn, etc. "Cu"- and "V"-doped MoS₂-TSs are highly efficient for the HER. At a fixed potential of -0.588 V vs RHE, Cu-doped MoS₂ (Cu-MoS₂-TS), V-doped MoS₂ (V-MoS₂-TS), and MoS₂-TS can generate current densities of 327.46, 308.45, and 127.82 mA/cm², respectively. The electrochemically active surface area increases nearly 7.7-fold and 2.5-fold for Cu-MoS₂-TS and V-MoS₂-TS than for MoS₂-TS, respectively. Cu-MoS₂-TS shows exceptionally high electrocatalytic stability up to 140 h in an acidic medium (0.5 M H₂SO₄). First-principles calculations using density functional theory (DFT) are performed, which are well matched with the experimental observations. DFT calculations dictate that after doping with "V" and "Cu" both valance band maxima and conduction band minima are uplifted, which indicates the higher hydrogen-ion-reducing ability of M-MoS₂-TS (M = Cu, V) compared to bare MoS₂-TS.

Ethylene glycol-mediated one-pot synthesis of Fe incorporated α-Ni(OH)2 nanosheets with enhanced intrinsic electrocatalytic activity and long-term stability for alkaline water oxidation

Sustainable electrocatalytic water splitting stipulates the development of cheap, efficient and stable electrocatalysts to promote comparatively sluggish oxygen evolution reaction. We have synthesized iron-incorporated pure phase α-nickel hydroxide, Ni0.8Fe0.2(OH)2 electrocatalyst utilizing N,N,N',N'-Tetramethylethane-1,2-diamine (TMEDA) and ethylene glycol (EG) following a simple one-pot synthesis process. PXRD and FTIR data suggest that the intercalation of EG in the interlayer spacing promotes amorphousness of the material. FESEM and TEM analyses suggest that the catalyst possesses hierarchical sheet-like morphology and BET measurements indicated the surface area of 50 m2 g-1 with high mesoporosity. Electrochemical studies suggest that Ni0.8Fe0.2(OH)2 prepared using water-EG mixture is the most efficient electrocatalyst for OER activity as it requires only 258 mV overpotential (considering backward LSV) on a glassy carbon electrode to achieve the benchmark current density of 10 mA cm-2geo. Additionally, the catalyst shows remarkable long-term stability for up to 7 days. The efficiency of Ni0.8Fe0.2(OH)2 electrocatalyst is reflected in its low Tafel slope (43 mV dec-1) and high OER faradaic efficiency (93%). The enhanced activity is attributed to the increase in the interlayer spacing due to the intercalation of EG into the material, which facilitates the transport of ions during the OER process. The overall improved catalytic property is due to the enhanced ionic mobility, controllable textural property, higher per-site activity and increased conductivity for the Ni0.8Fe0.2(OH)2 catalytic network.

Aggregates of Ni/Ni(OH)2/NiOOH Nanoworms on Carbon Cloth for Electrocatalytic Hydrogen Evolution

The development of an efficient electrocatalyst for hydrogen evolution reaction (HER) is essential to facilitate the practical application of water splitting. Here, we aim to develop an electrocatalyst, Ni/Ni(OH)₂/NiOOH, via electrodeposition technique on carbon cloth, which shows efficient activity and durability for HER in an alkaline medium. Phase purity and morphology of the electrodeposited catalyst are determined using powder X-ray diffraction and electron microscopic techniques. The compositional and thermal stability of the catalyst is checked using X-ray photoelectron spectroscopy and thermogravimetry analysis. Electrodeposited Ni/Ni(OH)₂/NiOOH material is an efficient, stable, and low-cost electrocatalyst for hydrogen evolution reaction in a 1.0 M KOH medium. The catalyst exhibits remarkable performance, achieving a current density of 10 mA/cm² at a potential of -0.045 V vs reversible hydrogen electrode (RHE), and the Tafel slope value is 99.6 mV/dec. The overall electrocatalytic water splitting mechanism using Ni/Ni(OH)₂/NiOOH catalyst is well explained, where formation and desorption of OH^- ion on the catalyst surface are significant at alkaline pH. The developed electrocatalyst shows significant durability up to 200 h in a negative potential window in a highly corrosive alkaline environment along with efficient activity. The electrocatalyst can generate 165.6 μmol of H₂ in ∼145 min of reaction time with 81.5% faradic efficiency.

NaBH4 induces a high ratio of Ni3+/Ni2+ boosting OER activity of the NiFe LDH electrocatalyst

Electrochemical water splitting is a promising way to produce hydrogen gas, but the sluggish kinetics of the oxygen evolution reaction (OER) extremely restrict the overall conversion efficiency of water splitting. Transition metal based LDHs (TM LDHs) are one of the most effective non-noble metal OER catalysts and have attracted wide interest, especially the nickel–iron LDH (NiFe LDH). The high valence Ni³⁺ species with a large coordination number play a vital role in OER catalysis. Herein, we report on a surprising discovery that reaction between NiFe LDH and NaBH₄ with multi-hydrides induces vacancy formation around Fe³⁺ and enrichment in Ni³⁺, crucially activating the OER performance. The ratio of Ni³⁺/Ni²⁺ is found to be closely tied to the OER performance, nicely accounting for the leading role of Ni³⁺ ions in octahedral sites in electrocatalysis. Significantly, the NaBH₄ treated NiFe LDH directly on nickel foam (NF), denoted as NaBH₄–NiFe LDH@NF exhibited an outstanding OER performance with an overpotential of only 310 mV at 100 mA cm⁻², and a Tafel slope of 47 mV dec⁻¹. For the series of TM LDHs we studied with different metal combinations, the high valence metal ion is found to be positively related to OER performance.

Reaction between NiFe LDH and NaBH₄ induces vacancies around Fe³⁺ and enrichment in Ni³⁺, crucially activating the OER catalyst leading to high performance.

Preparation of Hierarchical Cube-on-plate Metal Phosphides as Bifunctional Electrocatalysts for Overall Water Splitting

To rationally design efficient and cost-effective electrocatalysts, a simple but efficient strategy has been developed to directly anchor prussian blue analogue (PBA) nanocubes on cobalt hydroxide nanoplates (PBA@Co(OH)₂ ) via the in-situ interfacial precipitation process. Subsequently, the thermal treatment in the presence of sodium hydrogen phosphite enabled the successful transition into metal phosphides with the hierarchical cube-on-plate structure. When used as electrocatalytsts, the obtained bimetal phosphides exhibited good bifunctional electrocatalytic activities for hydrogen and oxygen evolution reactions with good long-term stability. Thus, an enhanced performance for overall water splitting can be achieved, which could be ascribed to the hierarchical structure and favorable composition of as-prepared bimetal phosphide for rapid electron and mass transfer. The present study demonstrates a favorable approach to modulate the composition and structure of metal phosphide for enhancing the electrocatalytic ability toward water splitting.

NiFe2O4 hollow nanoparticles of small sizes on carbon nanotubes for oxygen evolution

CNT-supported Ni–Fe bimetallic oxide hollow nanoparticles with an ultra-small size based on Kirkendall effect are fabricated and this catalyst exhibits excellent OER performances and robust stability, superior to the benchmark IrO₂ catalyst.

Study of “Ni-doping” and “open-pore microstructure” as physico-electrochemical stimuli towards the electrocatalytic efficiency of Ni/NiO for the oxygen evolution reaction

Added “Ni-doping” and “open-pore microstructure” act as physico-electrochemical stimuli towards enhanced electrocatalytic efficiency and electromechanical stability of Ni/NiO for the low-overpotential oxygen evolution reaction in alkaline medium.