Facile Synthesis of Vanadium-Doped Ni3S2 Nanowire Arrays as Active Electrocatalyst for Hydrogen Evolution Reaction

Promoted surface reconstruction of pentlandite via phosphorus-doping for enhanced oxygen evolution reaction

The pentlandite Fe5Ni4S8(abbreviated as FNS) is not efficient for water splitting because of its inferior performance for the oxygen evolution reaction (OER). This issue originates from the low activity and instability of FNS during the OER process but can be solved through appropriate doping. Herein, a P-doping strategy based on annealing in the presence of NaH2PO2as a phosphorus source upstream was employed on FNS to enhance its activity and stability toward OER. The results demonstrated fine-tuned electronic structures of Fe and Ni in FNS through P-doping, resulting in suppressed Fe leaching,improved electrical conductivity of FNS, and easier formation of NiOOH on the surface of the catalyst. In turn, these features enhanced the OER activity and stability. The optimal P-doped FNS catalyst FNSP-40 exhibited a 4-fold greater electrochemical surface area compared to that of FNS, accompanied by an overpotential of 235 mV at 10 mA cm-2. The optimized FNSP-40 catalyst was used as an anode, and platinum-decorated FNS was used as a cathode. This combination demonstrated an electrolysis performance with a cell voltage of 1.57 V, reaching a current density of 100 mA cm-2,which indicates efficient operation. The advantages of P-doping engineering were also verified in simulated seawater with enhanced OER performance. Overall, the proposed strategy looks promising for the fabrication of pentlandite-structured catalysts for efficient alkaline water and seawater oxidation.

Synergistic dual built-in electric fields in 1T-MoS2/Ni3S2/LDH for efficient electrocatalytic overall water splitting reactions

Designing cost-effective electrocatalysts for water decomposition is crucial for achieving environmental-friendly hydrogen production. A transition metal sulfide/hydroxide electrocatalyst (1T-MoS2/Ni3S2/LDH) with double heterogeneous interfaces was developed through a two-step hydrothermal assisted electrodeposition method. The presence of the two built-in electric fields not only accelerated the charge transfer at the interface, but also enhanced the adsorption of the reactants and intermediate groups, and therefore improved the reaction rate and overall catalytic performance. The results suggest that the 1T-MoS2/Ni3S2/LDH catalysts display exceptional electrocatalytic reactivity. Under alkaline conditions, the overpotential of the electrocatalyst was 187 (η50) mV for OER and 104 (η10) mV for HER. Furthermore, the two-electrode system assembled by the electrocatalyst needs only a voltage of 1.55 V to deliver a current density of 10 mA cm-2. Our result provides a simple and effective methodical approach to the design of dual heterogeneous interfacial electrocatalysts.

Influence of deposition conditions on performance of Ni3S2 as the bifunctional electrocatalyst in alkaline solutions by galvanostatic deposition

The electrodeposition method is a popular synthesis method due to its low cost, simplicity, and short synthesis time. In addition, this synthesis route results in the preparation of a self-supporting electrocatalyst, which eliminates the use of binders and ultimately facilitates the durability as well as the activity of the catalyst. In this work, a series of Ni3S2/Ni mesh electrodes are prepared by galvanostatic deposition at different deposition current densities and times. The morphology, microstructure, and elemental composition distribution of these obtained electrodes are characterized, and the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) performance of the series of Ni3S2/Ni meshes are tested. The results show that the Ni3S2/Ni mesh electrodes electrodeposited at 30 mA cm-2 for 1200 s have superior electrochemical performance for HER and OER. The overpotentials of Ni3S2/Ni mesh 30 mA cm-2-1200 s are 236 and 244 mV for HER and OER, respectively, at a current density of 10 mA cm-2. In addition, the Tafel slopes for HER and OER are 113 mV dec-1 and 176 mV dec-1, respectively. This research provides some valuable insights into the use of the electrodeposition method for the fabrication of electrocatalysts.

Triggering efficient reconstructions of Co/Fe dual-metal incorporated Ni hydroxide by phosphate additives for electrochemical hydrogen and oxygen evolutions

Alkaline electrochemical water splitting has been considered as an efficient way for the green hydrogen production in industry, where the electrocatalysts play the critical role for the electricity-to-fuel conversion efficiency. Phosphate salts are widely used as additives in the fabrication of electrocatalysts with improved activity, but their roles on the electrocatalytic performance have not been fully understood. Herein, we fabricate Co, Fe dual-metal incorporated Ni hydroxide on Ni foam using NaH2PO4 ((Co, Fe)NiOxHy-pi) and NaH2PO2 ((Co, Fe)NiOxHy-hp) as additive, respectively. We find that (Co, Fe)NiOxHy-hp with NaH2PO2 in the fabrication shows high activity and stability for both HER and OER (a overpotential of -0.629 V and 0.65 V at 400 mA cm-2 for HER and OER, respectively). Further experiment reveals that the reconstructed structures of electrocatalyst by using NaH2PO2 (hp) endow high electrocatalytic performances: (1) in-situ generated active metal improves the accumulation, transportation and activity of hydrogen species in the HER process; and (2) in-situ generated poor-crystalline hydroxide endows superior charge/mass transportation and kinetics improvements in the OER process. Our study may provide an insightful understanding on the catalytic performance of non-precious metal electrocatalysts by controlling additives and guidance for the design and synthesis of novel electrocatalysts.

A review of modulation strategies for improving the catalytic performance of transition metal sulfide self-supported electrodes for the hydrogen evolution reaction

Electrocatalytic water splitting is considered to be one of the most promising technologies for large-scale sustained production of H2. Developing non-noble metal-based electrocatalytic materials with low cost, high activity and long life is the key to electrolysis of water. Transition metal sulfides (TMSs) with good electrical conductivity and a tunable electronic structure are potential candidates that are expected to replace noble metal electrocatalysts. In addition, self-supported electrodes have fast electron transfer and mass transport, resulting in enhanced kinetics and stability. In this paper, TMS self-supported electrocatalysts are taken as examples and their recent progress as hydrogen evolution reaction (HER) electrocatalysts is reviewed. The HER mechanism is first introduced. Then, based on optimizing the active sites, electrical conductivity, electronic structure and adsorption/dissociation energies of water and intermediates of the electrocatalysts, the article focuses on summarizing five modulation strategies to improve the activity and stability of TMS self-supported electrode electrocatalysts in recent years. Finally, the challenges and opportunities for the future development of TMS self-supported electrodes in the field of electrocatalytic water splitting are presented.

Electrochemical Investigations of Double Perovskite M2NiMnO6 (Where M = Eu, Gd, Tb) for High-Performance Oxygen Evolution Reaction

Double perovskites are known for their special structures which can be utilized as catalyst electrode materials for electrochemical water splitting to generate carbon-neutral hydrogen energy. In this work, we prepared lanthanide series metal-doped double perovskites at the M site such as M2NiMnO6 (where M = Eu, Gd, Tb) using the solid-state reaction method, and they were investigated for an oxygen evolution reaction (OER) study in an alkaline medium. It is revealed that the catalyst with a configuration of Tb2NiMnO6 has outstanding OER properties such as a low overpotential of 288 mV to achieve a current density of 10 mAcm-2, a lower Tafel slope of 38.76 mVdec-1, and a long cycling stability over 100 h of continuous operation. A-site doping causes an alteration in the oxidation or valence states of the NiMn cations, their porosity, and the oxygen vacancies. This is evidenced in terms of the Mn4+/Mn3+ ratio modifying electronic properties and the surface which facilitates the OER properties of the catalyst. This is discussed using electrochemical impedance spectroscopy (EIS) and electrochemical surface area (ECSA) of the catalysts. The proposed work is promising for the synthesis and utilization of future catalyst electrodes for high-performance electrochemical water splitting.

Synergistic modulation in a triphasic Ni5P4-Ni2P@Ni3S2 system manifests remarkable overall water splitting

The potential for water splitting electrocatalysts with high efficiency paves the way for a sustainable future in hydrogen energy. However, this task is challenging due to the sluggish kinetics of the oxygen evolution reaction (OER), which has a significant impact on the hydrogen evolution reaction (HER). Herein multi-heterointerface of Ni5P4-Ni2P@Ni3S2 was fabricated by a two-step synthesis procedure that consist the development of Ni5P4-Ni2P nanosheets over nickel foam followed by the electrodeposition of Ni3S2. The HR-TEM analysis shows that the Ni5P4-Ni2P@Ni3S2 nanosheets array provide numerous well-exposed diverse heterointerfaces. The electrochemical investigations conducted on the Ni5P4-Ni2P@Ni3S2 nanosheets for complete water splitting indicate that they possess an overpotential of 73 mV and 230 mV in HER and OER respectively, enabling them to generate a current density of 10 and 50 mA cm-2. The nanosheets also demonstrate Tafel slope values of 95 mV dec-1 and 83 mV dec-1 for HER and OER, respectively. The HER stability of the catalyst was conducted for 45 h using chronoamperometric technique under a current density of 20 mA cm-1, while the stability test for OER was carried out at current densities of 100 and 200 mA cm-1 for 100 h each. Furthermore, in the overall water splitting, the catalyst exhibits a cell voltage of 1.47 V@10 mA cm-2 and displayed a stability operation for 100 h at a current density of 150 mA cm-1.

Metal organic framework-derived transition metal-doped CoSx nanocage for enhanced visible light-assisted methanol electrocatalytic oxidation

Designing noble metal-free anode catalysts for visible light-assisted direct methanol fuel cells still remains a significant challenge. In this study, combining the photocatalytic and electrocatalytic properties of CoSx, a visible light-assisted methanol electrocatalytic oxidation strategy was provided. Doping engineering was employed to adjust the electronic structure of CoSx and improve their photoassisted methanol electrocatalytic oxidation activity. Using ZIF-67 as precursor, transition metal-doped CoSx (M-CoSx, M = Zn, Cu, Ni, and Cd) nanocage was synthesized by cation exchange and L-cysteine-controlled etching. Cd doping not only widens the light adsorption to the visible region but also enhances the separation efficiency of photogenerated electron-hole pairs. The electrochemical and photochemical results indicated that the strong oxidative photogenerated hole, OH˙, and O2˙- are beneficial for methanol electrocatalytic oxidation. The synergistic electrocatalytic and photocatalytic effect will be a practical strategy for improving the methanol electrocatalytic oxidation activity of noble metal-free semiconductor catalysts.

Fe and Mo Co-Modulated Coral-like Nickel Pyrophosphate in situ Derived from Nickel-Foam for Oxygen Evolution

A highly active catalyst for the oxygen evolution reaction (OER) is critical to achieve high efficiency in hydrogen generation from water splitting. Direct conversion of nickel foam (NF) into nickel-based catalysts has attracted intensive interest due to the tight interaction of the catalysts to the substrate surface. However, the catalytic performances are still far below expectation because of the problems of low catalyst amount, thin catalyst layer, and small active area caused by the limitations of the synthesis method. Herein, we develop a Fe3+ -induced synthesis strategy to transform the NF surface into a thicker catalyst layer. In addition to the excellent conductivity and high stability, the as-prepared FeMo-Ni2 P2 O7 /NF catalysts expose more active sites and facilitate mass transfer due to their thicker catalyst layer and highly dense coral-like micro-nano structure. Furthermore, the Mo, Fe co-modulation optimizes the adsorption free energies of the OER intermediates, boosting catalytic activities. Its catalytic activity is among the highest, and it exhibits a small Tafel slope of 34.71 mV dec-1 and a low overpotential of 161 mV for delivering a current density of 100 mA cm-2 compared to reported Ni-based catalysts. The present strategy can be further used in the design of other catalysts for energy storage and conversion.

3D nanostructured Ce-doped CoFe-LDH/NF self-supported catalyst for high-performance OER

Powder electrocatalysts for oxygen evolution reactions usually need adhesives for electrocatalytic performance tests, leading to the increase of resistance, reduction of catalyst loading, and easy stripping of the catalyst under long-time or high current operation. In this study, Ce-doped CoFe layered double hydroxides were uniformly grown on nickel foam by a one-step hydrothermal route. A nanostructured self-supported electrode Ce-CoFe-LDH/NF without adhesive was obtained directly, which has a regular nanoneedle morphology with a length of ∼1.2 μm and tip width of ∼20 nm. Adopting Ce3+ ions with a large radius to partially displace Fe3+ ions with a small radius produced lattice distortion and more defects in the host layer of CoFe-LDH, whereby possessing the great potential to enhance catalytic behaviors. Once used as an electrocatalyst for the oxygen evolution reaction, Ce-CoFe-LDH/NF shows an outstanding electrocatalytic performance, including an optimized overpotential of 225 mV at 10 mA cm-2, a decreased Tafel slope of 34.34 mV dec-1, and a low charge transfer impedance of 2.4 Ω in 1 M KOH electrolyte. Moreover, the overpotential of the working electrode increased by only 0.04 V after 24 hours and was maintained at a current density of 50 mA cm-2. These results demonstrate a low-cost strategy compared to using noble metal OER electrocatalysts. Thus, this study highlights a ready universal approach to fabricate high-performance supported catalysts for energy-related applications.

Synergistic Effect of P Doping and Mo-Ni-Based Heterostructure Electrocatalyst for Overall Water Splitting

Heterostructure construction and heteroatom doping are powerful strategies for enhancing the electrolytic efficiency of electrocatalysts for overall water splitting. Herein, we present a P-doped MoS₂/Ni₃S₂ electrocatalyst on nickel foam (NF) prepared using a one-step hydrothermal method. The optimized P_[0.9mM]-MoS₂/Ni₃S₂@NF exhibits a cluster nanoflower-like morphology, which promotes the synergistic electrocatalytic effect of the heterostructures with abundant active centers, resulting in high catalytic activity for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline electrolyte. The electrode exhibits low overpotentials and Tafel slopes for the HER and OER. In addition, the catalyst electrode used in a two-electrode system for overall water splitting requires an ultralow voltage of 1.42 V at 10 mA·cm^-2 and shows no obvious increase in current within 35 h, indicating excellent stability. Therefore, the combination of P doping and the heterostructure suggests a novel path to formulate high-performance electrocatalysts for overall water splitting.

Vanadium Oxide-Doped Laser-Induced Graphene Multi-Parameter Sensor to Decouple Soil Nitrogen Loss and Temperature

Monitoring nitrogen utilization efficiency and soil temperature in agricultural systems for timely intervention is essential for crop health with reduced environmental pollution. Herein, this work presents a high-performance multi-parameter sensor based on vanadium oxide (VOX )-doped laser-induced graphene (LIG) foam to completely decouple nitrogen oxides (NOX ) and temperature. The highly porous 3D VOX -doped LIG foam composite is readily obtained by laser scribing vanadium sulfide (V5 S8 )-doped block copolymer and phenolic resin self-assembled films. The heterojunction formed at the LIG/VOX interface provides the sensor with enhanced response to NOX and an ultralow limit of detection of 3 ppb (theoretical estimate of 451 ppt) at room temperature. The sensor also exhibits a wide detection range, fast response/recovery, good selectivity, and stability over 16 days. Meanwhile, the sensor can accurately detect temperature over a wide linear range of 10-110 °C. The encapsulation of the sensor with a soft membrane further allows for temperature sensing without being affected by NOX . The unencapsulated sensor operated at elevated temperature removes the influences of relative humidity and temperature variations for accurate NOX measurements. The capability to decouple nitrogen loss and soil temperature paves the way for the development of future multimodal decoupled electronics for precision agriculture and health monitoring.

Nickel sulfide and phosphide electrocatalysts for hydrogen evolution reaction: challenges and future perspectives

The search for environmentally friendly and sustainable energy sources has become necessary to alleviate the issues associated with the consumption of fossil fuel such as air pollution and global warming. Furthermore, this is significant considering the exhaustible resources and burgeoning energy demand globally. In this regard, hydrogen, a clean fuel with high energy density, is considered a reliable alternative energy source. The hydrogen evolution reaction (HER) is one of the most promising methods to produce green hydrogen from water on a large scale. However, the HER needs effective electrocatalysts to address the concerns of energy consumption; thus, finding active materials has recently been the main focus of researchers. Among the various electrocatalysts, nickel sulfides and phosphides and their derivatives with low cost, high abundance, and relatively straightforward preparation have shown high HER activity. In this review, we compare the diverse methods in the synthesis of nickel sulfides and phosphides together with effective synthesis parameters. Also, the optimum conditions for the preparation of the desired active materials and their properties are provided. Then, the performance of nickel sulfide and phosphide electrocatalysts in the HER is addressed. The HER activity of the various crystalline phases is compared, and their most active crystalline phases are introduced. Finally, the present challenges and perspectives for future HER electrocatalysts are presented.

In Situ Synthesis and Microfabrication of High Entropy Alloy and Oxide Compounds by Femtosecond Laser Direct Writing under Ambient Conditions

Synthesis and coating of multi-metal oxides (MMOs) and alloys on conductive substrates are indispensable to electrochemical applications, yet demand multiple, resource-intensive, and time-consuming processes. Herein, an alternative approach to the synthesis and coating of alloys and MMOs by femtosecond laser direct writing (FsLDW) is reported. A solution-based precursor ink is deposited and dried on the substrate and illuminated by a femtosecond laser. During the illumination, dried precursor ink is transformed to MMO/alloys and is simultaneously bonded to the substrate. The formulation of the alloy and MMO precursor ink for laser processing is universally applicable to a large family of oxides and alloys. The process is conducted at room temperature and in an open atmosphere. To demonstrate, a large family of 57 MMOs and alloys are synthesized from a group of 13 elements. As a proof of concept, Ni_0.24 Co_0.23 Cu_0.24 Fe_0.15 Cr_0.14 high entropy alloy synthesized on stainless-steel foil by FsLDW is used for the oxygen evolution reaction, which achieves a current density of 10 mA cm^-2 at a significantly low overpotential of 213 mV. Further, FsLDW can also achieve microfabrication of alloys/MMO with feature sizes down to 20 µm.

V5+-Doped Potassium Ferrite as an Efficient Trifunctional Catalyst for Large-Current-Density Water Splitting and Long-Life Rechargeable Zn-Air Battery

Developing non-noble metal catalyst with super trifunctional activities for efficient overall water splitting (OWS) and rechargeable Zn-air battery (ZAB) is urgently needed. However, catalysts with excellent oxygen evolution reaction (OER), oxygen reduction reaction (ORR), and hydrogen evolution reaction (HER) performances are relatively few. Although metal-ionic-conductor K₂Fe₄O₇ (KFO) can output large current densities for OER/HER even in 10.0 M KOH electrolyte, its water-splitting property still needs to be further improved. Herein, we introduced V⁵⁺ directly into KFO and synthesized the binder-free nickel foam (NF) basal V-KFO nanoparticles (labeled as V-KFO/NF). Both the theoretical analysis and actual experimental data certify that V⁵⁺ doping enhances the instinct water-splitting property of V-KFO/NF. Additionally, V-KFO/NF can directly serve as the air cathode of liquid/flexible ZABs. The assembled liquid ZAB can continue the charge-discharge cycling testing with a lower voltage gap (0.834 V) and a longer operation life (>550 h) at 10 mA cm^-2. Meanwhile, the assembled flexible ZAB can drive the two-electrode water-splitting unit of V-KFO/NF and needs only 1.54 V to achieve the current density of 10 mA cm^-2, which is much lower than that of KFO/NF (1.59 V). This work not only provides a novel and efficient trifunctional catalyst for a self-powered water-splitting device but also is the foundation support for other heteroatom-doped low-cost materials.

Effect of cobalt doping on the enhanced energy storage performance of 2D vanadium diselenide: experimental and theoretical investigations

Vanadium Diselenide (VSe₂) is a prominent candidate in the 2D transition metal dichalcogenides family for energy storage applications. Herein, we report the experimental and theoretical investigations on the effect of cobalt doping in 1T-VSe₂. The energy storage performance in terms of specific capacitance, stability and energy and power density is studied. It is observed that 3% Co doped VSe₂exhibits better energy storage performance as compared to other concentrations, with a specific capacitance of ∼193 F g^-1in a two-electrode symmetric configuration. First-principles Density Functional Theory based simulations support the experimental findings by suggesting an enhanced quantum capacitance value after the Co doping in the 1T-VSe₂. By making use of the advantages of the specific electrode materials, a solid state asymmetric supercapacitor (SASC) is also assembled with MoS₂as the negative electrode. The assembled Co-VSe₂//MoS₂SASC device shows excellent energy storage performance with a maximum energy density of 33.36 Wh kg^-1and a maximum power density of 5148 W kg^-1with a cyclic stability of 90% after 5000 galvano static charge discharge cycles.

Key Role of Lorentz Excitation in the Electromagnetic-Enhanced Hydrogen Evolution Reaction

Alternating magnetic fields (AMFs) are recently demonstrated as a promising strategy to promote the electrochemical catalytic reactions. However, the underlying mechanisms are still an open question. In this work, we systematically investigated the influence of AMFs on the hydrogen evolution reaction (HER) by using a Fe-Co-Ni-P-B magnetic catalyst. The HER catalytic efficiency is boosted significantly by AMFs, with 27% increase in current density at 20 mT. This is attributed to the enhancement of charge-transfer efficiency by Lorentz interaction with a minor contribution from the heating effect. The high magnetic permeability and skin effect of electromagnetic eddy current for the Fe-Co-Ni-P-B electrode can magnify the Lorentz effect. These findings clarify the mechanism of AMF-enhanced HER catalytic activities and open a door for designing a high-efficiency electrocatalysis system.

Regulation of morphology and electronic configuration of NiCo2O4 by aluminum doping for high performance supercapacitors

Morphology engineering and element doping are two effective strategies to boost the capacitive performance of electroactive materials. The morphology control through doping process is conducive to simplifying the preparation process. Herein, an aluminum-doped (Al-doped) strategy was used to prepare Al-doped NiCo₂O₄ nanosheet-wire structure (Al-NiCo₂O₄ NSW) by hydrothermal method and subsequent calcination. The nanosheet-wire structure was composed of one-dimensional (1D) nanowires and two-dimensional (2D) ultrathin nanosheets. 1D nanowires can provide efficient pathways for the electrons/ions transport. 2D nanosheets can enlarge the specific surface area and expose more active sites. The Al doping can change the electronic structure of NiCo₂O₄ with enhanced electrical conductivity as revealed by density functional theory (DFT) calculations. Meanwhile, a strong adsorption capacity of OH^- was obtained on Al-NiCo₂O₄ NSW for redox reactions. The Al-NiCo₂O₄ NSW electrode demonstrated a high specific capacity of 1441C g^-1 (2446F g^-1) at 1 A g^-1 and excellent cycling stability (87.6% capacity retention at 10 A g^-1 for 5000 charge-discharge cycles). The assembled asymmetric supercapacitor manifested a superior energy density of 46.2 Wh Kg^-1 at a power density of 800 W kg^-1.

Laser In-Situ synthesis of metallic cobalt decorated porous graphene for flexible In-Plane microsupercapacitors

Transition metal nanoparticles-graphene nanocomposites incorporate the advantages of graphene and metal nanoparticles, which arouse extensive attention. Here, we design a novel, facile and versatile method for in-situ synthesis of laser-induced porous graphene (LIG) decorated with cobalt particles (Co). The LIG/Co nanocomposites are fabricated through one-step laser direct scribing on a customized film composed of polyimide (PI) powder, polyvinyl alcohol (PVA), and cobalt chloride (CoCl₂·6H₂O) precursors. Benefiting from the unique properties of Co nanoparticles embedded LIG, the obtained optimal in-plane micro-supercapacitors (IMSC) based on LIG/Co-1.5 possesses an excellent areal capacitance of 110.11 mF cm^-2 and a superior energy density of 9.79 μWh cm^-2, which are about 79 times that of pure LIG-based IMSCs. Simultaneously, the LIG/Co-1.5 MSCs also present good cycling stability, remarkable modular integration capability, and outstanding mechanical flexibility, showing potential for practical applications. Additionally, the density functional theory (DFT) calculations indicate that the decorating of cobalt particles elevates electron transfer. Moreover, the interaction between electrolyte and electrodes is also improved with the introduction of cobalt particles. Therefore, this strategy offers a new avenue for facile and large-scale manufacturing of various metallic atoms in-situ decorating in porous graphene.

Facile Synthesis of Copper Oxide-Cobalt Oxide/Nitrogen-Doped Carbon (Cu2O-Co3O4/CN) Composite for Efficient Water Splitting

Herein, we report a copper oxide-cobalt oxide/nitrogen-doped carbon hybrid (Cu2O-Co3O4/CN) composite for electrochemical water splitting. Cu2O-Co3O4/CN is synthesized by an easy two-step reaction of melamine with Cu2O-Co3O4/CN composite. The designed composite is aimed to solve energy challenges by producing hydrogen and oxygen via electrochemical catalysis. The proposed composite offers some unique advantages in water splitting. Carbon imparts superior conductivity, while the water oxidation abilities of Cu2O and Co3O4 are considered to constitute a catalyst. The synthesized composite (Cu2O-Co3O4/CN) is characterized by SEM, EDS, FTIR, TEM, and AFM in terms of the size, morphology, shape, and elemental composition of the catalyst. The designed catalyst’s electrochemical performance is evaluated via linear sweep voltammetry (LSV) and cyclic voltammetry (CV). The Cu2O-Co3O4/CN composite shows significant electrocatalytic activity, which is further improved by introducing nitrogen doped carbon (current density 10 mA cm−2, onset potential 91 mV, and overpotential 396 mV).

Ar plasma-assisted P-doped Ni3S2 with S vacancies for efficient electrocatalytic water splitting

Doping engineering is considered an effective way to improve the electrocatalytic water splitting performance of catalysts. In this paper, P-doped Ni₃S₂/NF was prepared by Ar plasma-assisted chemical vapor deposition, where the P dopant was efficiently introduced into Ni₃S₂/NF under the assistance of Ar plasma. Meanwhile, numerous vacancies were generated due to plasma bombardment. In the doping process, the P dopants replace the S vacancies, which contributes to the strong bonding between the P dopants and Ni₃S₂. Due to the synergistic effect of the P dopants and S vacancies, the S_v-Ni₃S_2-xP_x-4 catalyst has low HER and OER overpotentials of 89 mV and 216 mV at 10 mA cm^-2, with a lower impedance value and good stability. The present work shows a facile route to introduce dopants and vacancies into catalyst materials for adding active sites, eventually improving their electrocatalytic performance.

Formation of porous NiCoV-LTH nanosheet arrays by in situ etching of nickel foam for the hydrogen evolution reaction at large current density

We report the in situ generation of NiCoV-LTH on nickel foam for the HER. Interestingly, the introduction of Co into NiV-LDH can induce the formation of porous nanosheets to expose a large number of active sites and change the electron density around Ni and V to promote the absorption of hydrogen species and thus accelerate the HER kinetics.

Surface covalent sulfur enriching Ni active sites of Ni3S2 nanoparticles for efficient oxygen evolution

Surface covalent sulfur enhanced Ni₃S₂ nanoparticles are demonstrated to maximize the catalytically active Ni³⁺ density and further boost OER performance.

Al-doped nickel sulfide nanosheet arrays as highly efficient bifunctional electrocatalysts for overall water splitting

The development of low-cost, high-activity, durable non-precious metal bifunctional electrocatalysts is of great importance in the production of hydrogen by water electrolysis. In this work, we have prepared new Al-doped Ni₃S₂ nanosheet arrays grown on Ni foam (Al-Ni₃S₂/NF) as an excellent bifunctional electrocatalyst in the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The Al-Ni₃S₂/NF electrode obtained only requires extremely low overpotentials of 86 and 223 mV for the HER and OER to achieve a current density of 10 mA cm^-2 in 1 M KOH, respectively. Moreover, the electrolytic cell assembled using this electrode as both cathode and anode provides a current density of 10 mA cm^-2 at an extremely low battery voltage of 1.58 V relative to that with Ni₃S₂/NF (1.71 V). Additionally, both experimental results and theoretical calculations reveal that the increased electrochemical active surface area and optimized intermediate adsorption free energies are responsible for the enhanced electrocatalytic performance. This work provides a promising bifunctional electrocatalyst for water electrolysis in alkaline media with broad application prospects.

1T/2H Mixed Phase MoS2 Nanosheets Integrated by a 3D Nitrogen-Doped Graphene Derivative for Enhanced Electrocatalytic Hydrogen Evolution

Molybdenum disulfide (MoS₂) has become one of the most promising non-platinum-based electrocatalysts for the hydrogen evolution reaction (HER) because of its unique layered structure. However, the catalytic performance of the thermodynamically stable MoS₂ is hindered by its poor conductivity and scarce active sites. We developed a 3D porous N-doped graphene derivative-integrated metal-semiconductor (1T-2H) mixed phase MoS₂ (MNG) using urea as a doping reagent. The highly exposed active sites were achieved by inducing the phase transition of MoS₂ from 2H phase to 1T phase and the inclusion of highly N-incorporated reduced graphene oxide, both of which were simultaneously realized by optimizing the concentration of the doping reagent. Moreover, the charge/proton transfer was enhanced by the well-designed porous architecture and hydrophilic 1T-MoS₂. With these advantages, the optimized MNG-40 catalyst has a small overpotential of 157 mV at a cathodic current density of 10 mA cm^-2, a relatively low Tafel slope of 45.8 mV dec^-1, and an excellent stability. This work represents a new strategy to design higher-performance HER catalysts and provides new insights into the structural regulation of metal composite transitions.

Superb Hydrogen Evolution by a Pt Nanoparticle-Decorated Ni3S2 Microrod Array

Ni₃S₂ has attracted great interest as a potential alternative catalyst for the oxygen evolution reaction; however, the formation of sulfur-hydrogen bonds on Ni₃S₂ suppressed the hydrogen evolution reaction (HER), which remains a significant challenge in interface engineering of Ni₃S₂ structures for enhancing its HER performance. Herein, we demonstrate an efficient strategy for constructing a Pt nanoparticle-decorated Ni₃S₂ microrod array supported on Ni foam (Pt/Ni₃S₂/NF) by electrodeposition of Pt nanoparticles on hydrothermally synthesized Ni₃S₂/NF. The Pt/Ni₃S₂/NF heterostructure array exhibits an extremely low overpotential of 10 mV at 10 mA cm^-2, surpassing that of commercial Pt/C and representing the best alkaline HER catalysts to date. Impressively, at an overpotential of 0.15 V, Pt/Ni₃S₂/NF displays a Pt mass activity and a normalized current density (against the electrochemical surface area) of 5.52 A mg^-1 and 1.84 mA cm^-2, respectively, which are 8.8 and 15.3 times higher compared to those of Pt/C, respectively. In addition, this electrode also shows much enhanced catalytic performance and stability in neutral media. Such enhanced HER activities are attributed to the constructed interface in the Pt/Ni₃S₂ heterostructure array, which synergistically favor water dissociation and subsequent hydrogen evolution, which is supported by density functional theory calculations.

Nanoporous V-Doped Ni5P4 Microsphere: A Highly Efficient Electrocatalyst for Hydrogen Evolution Reaction at All pH

Hydrogen economy is one of the most promising candidates to replace the current energy system on depleting fossil fuels. As a clean and sustainable way to produce hydrogen, electrocatalytic water splitting attracts ever-increasing interest from the research community. Although the wide application of platinum group metal (PGM) catalysts is limited because of the scarcity and high cost toward hydrogen evolution reaction (HER), the non-PGM electrocatalysts usually suffer from unsatisfactory activity and poor durability. In this work, we report an active and durable V-doped Ni₅P₄ electrocatalyst that can be used for all-pH HER. Particularly, V-Ni₅P₄ has an HER activity that is comparable to that of Pt in preferred alkaline media, with overpotentials as low as 13 mV and 295 mV at current densities of 10 and 1000 mA cm^-2, respectively. The low-cost V-Ni₅P₄ that enables ultrahigh current density (i.e., at the level of A cm^-2) would be of great interest to the hydrogen production industry.

Facile electrodeposition of V-doped CoP on vertical graphene for efficient alkaline water electrolysis

In this work, we demonstrate a highly enhanced electrocatalytic activity of vanadium-doped CoP (V-CoP), directly grafted on a vertical graphene/carbon cloth electrode (VG/CC) by a facile electrochemical deposition method. Impressively, V-CoP/VG/CC exhibited a superior catalytic activity toward the hydrogen evolution reaction (HER) in alkaline solution. Compared to CoP/VG/CC, V-doping decreased the overpotential for HER at 10 mA cm-2 by more than half to 40 mV. The new catalyst even outperformed Pt/C beyond 150 mA cm-2. The overpotential for OER at 50 mA cm-2 was merely 314 mV, more than 100 mV lower than that of IrO2. Moreover, our novel catalyst worked as an excellent bifunctional catalyst with a low cell voltage of 1.69 V to achieve a current density of 50 mA cm-2. Detailed characterizations revealed that the V-doping in CoP resulted in improved electrical conductivity and increased active sites. Our findings highlight the significant advantage of V doping on the catalytic activities of CoP, already boosted by VG. Furthermore, concurrent doping with the electrodeposition of catalyst offers a new approach for practical water electrolysis.

Constructing NiS–VS heterostructured nanosheets for efficient overall water splitting

Self-supported porous NiS/VS heterostructured nanosheets were designed towards efficient electrocatalytic water splitting.

Electronic modulation of composite electrocatalysts derived from layered NiFeMn triple hydroxide nanosheets for boosted overall water splitting

Herein, we report the synthesis of Fe and Mn co-doped Ni₃S₂ nanosheet arrays (FM-NS NSAs) as well as layered NiFeMn triple hydroxide (NiFeMn-LTH)/FM-NS hybrid NSAs (HNSAs) via an easily controllable sulfidation method with NiFeMn-LTH NSAs as the precursor, which can be employed as coupled oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) electrodes to achieve boosted overall water splitting performance. It is discovered that the incorporation of the dual Fe and Mn cations can simultaneously modulate the morphology and optimize the electron density of the obtained catalysts. As a result, the full sulfidation product of FM-NS NSAs display a much-enhanced activity for the OER at a current density of 10 mA cm^-2 with an overpotential of 188 mV. The NiFeMn-LTH/FM-NS HNSAs obtained from partial sulfidation demonstrate an enhanced HER activity with a lower overpotential of 110 mV at 10 mA cm^-2. The FM-NS NSA anode and the NiFeMn-LTH/FM-NS HNSA cathode were coupled in an alkaline medium for overall water splitting and exhibited a much lower cell voltage of 1.48 V at 10 mA cm^-2, superior to most of the reported noble-metal-free electrocatalysts. Additionally, a battery-assisted electrolyzer (1.5 V) was assembled to explore the feasibility for practical energy-efficient water splitting.

New Way to Synthesize Robust and Porous Ni1-xFex Layered Double Hydroxide for Efficient Electrocatalytic Oxygen Evolution

Traditional catalysts are usually synthesized by sputtering, electrochemical deposition, or hydrothermal methods, and they also need to be combined with substrates to obtain the working electrodes. Here we introduce a new route to produce an efficient catalyst for oxygen evolution reaction (OER) that is made by ball milling and sintering. By using Se as a grinding aid, the bulk electrode Ni_1-xFe_xSe_1.15 is obtained with high porosity and robust mechanical strength after sintering. Active Ni_1-xFe_x layered double hydroxide (LDH) nanosheets are subsequently produced on the surface of the Ni_1-xFe_xSe_1.15 by in situ electrochemical oxidation. Compared with traditional synthesis methods, the new process displays superior advantages, such as producing an electrode that is substrate-free and exhibits robust mechanical strength as well as being cost-effective for mass production. Additionally, V- and Mn-doped Ni_0.75Fe_0.25-LDH exhibit comparable and competent OER performance in 1 M KOH solution. Ni_0.71V_0.04Fe_0.25-LDH achieves current densities of 100 and 1000 mA cm^-2 at overpotentials of 244 and 300 mV, respectively. This work demonstrates a promising way to synthesize highly efficient and robust electrocatalysts for water oxidation.

CoO Promoted the Catalytic Activity of Nitrogen-Doped MoS2 Supported on Carbon Fibers for Overall Water Splitting

Non-noble metal electrocatalysts have recently witnessed increasing attention for the hydrogen evolution reaction (HER) in acidic electrolytes. However, in alkaline electrolytes, the slow kinetics of water splitting leads to poor HER activities. In this study, we describe the preparation of a hybrid material consisting of cobalt oxide (CoO) decorated on nitrogen-doped MoS₂ supported on carbon fibers (CoO/N-MoS₂/CF) through a two-step process combining hydrothermal technique and electrochemical deposition. The electrochemical properties of the CoO/N-MoS₂/CF electrocatalyst were assessed in alkaline medium. The results revealed that CoO/N-MoS₂/CF exhibits excellent bifunctional electrocatalytic activity for the HER and oxygen evolution reaction (OER). The CoO/N-MoS₂/CF delivered a current density of 10 mA/cm² at an overpotential of only 78 mV for the HER and a current density of 50 mA/cm² at 458 mV for the OER in 1.0 M KOH, performing better than many noble metal-free electrocatalysts. The enhanced catalytic properties of the hybrid nanomaterial could be ascribed to its hierarchical structure, and increased number of active sites, as well as the synergetic cooperation between its different components. Additionally, the CoO/N-MoS₂/CF nanomaterial was investigated as both cathode and anode for full water splitting in 1.0 M KOH. The water electrolyzer delivered a maximum current density of 53 mA cm^-2 at an applied cell voltage of 1.5 V, which is very favorable for overall water-splitting applications.