First demonstration of photoelectrochemical water splitting by commercial W–Cu powder metallurgy parts converted to highly porous 3D WO3/W skeletons

Hydrogen evolution through photoelectrochemical (PEC) water splitting by tungsten oxide-based photoanodes, as a stable and environmental-friendly material with moderate band gap, has attracted significant interest in recent years. The performance of WO₃ photoanode could be hindered by its poor oxygen evolution reaction kinetics and high charge carrier recombination rate. Additionally, scalable and cost-effective commercial procedure to prepare nanostructured electrodes is still challenging. We present, for the first time, a novel and scalable method to fabricate highly efficient self-supported WO₃/W nanostructured photoanodes from commercial W–Cu powder metallurgy (P/M) parts for water splitting. The electrodes were prepared by electrochemical etching of Cu networks followed by hydrothermal growth of WO₃ nanoflakes. Interconnected channels of W skeleton provided high active surface area for the growth of WO₃ nanoflakes with a thickness of ~40 nm and lateral dimension of ~250 nm. The optimized photoelectrode having 35% interconnected porosity exhibited an impressive current density of 4.36 mA cm⁻² comprising a remarkable photocurrent of 1.71 mA cm⁻² at 1.23 V vs. RHE under 100 mW cm⁻² simulated sunlight. This achievement is amongst the highest reported photocurrents for WO₃ photoelectrodes with tungsten substrate reported so far. Impedance and Mott-Schottky analyses evidenced fast charge transfer, low recombination rate, and accelerated O₂ detachment provided by optimum 3D porous WO₃/W electrode. Due to the nature of the commercial P/M parts and low-temperature hydrothermal processing, the procedure is cost-effective and scalable which can pave a new route for the fabrication of highly porous and efficient water splitting electrodes.     

Sulfuration of hierarchical Mn-doped NiCo LDH heterostructures as efficient electrocatalyst for overall water splitting

Constructing efficient bifunctional electrocatalysts for both cathode and anode is of great importance for obtaining green hydrogen by water splitting. Herein, sulfuration of hierarchical Mn-doped NiCo LDH heterostructures (Mn–NiCoS₂/NF) is constructed as a bifunctional electrocatalyst for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) via a facile hydrothermal-annealing strategy. Mn–NiCoS₂/NF shows an overpotential of 310 mV at 50 mA cm⁻² for OER and 100 mV at 10 mA cm⁻² for HER in 1.0 M KOH. Moreover, only 1.496 V@10 mA cm⁻² is required for overall water splitting by using Mn–NiCoS₂/NF as catalyst dual electrodes in a two-electrode system. The excellent performance of Mn–NiCoS₂/NF should be attributed to the ameliorative energy barriers of adsorption/desorption for HO⁻/H₂O through the modification of electronic structure of NiCo basal plane by Mn-doping and the acceleration of water dissociation steps via rich delocalized electron inside sulfur vacancies. The construction of hierarchical Mn–NiCoS₂/NF heterostructures provides new prospects and visions into developing efficient-advanced electrocatalysts for overall water splitting.     

Strategies for improving Co/Ni-based bimetal-organic framework to water splitting

The preparation of high-efficiency, stable, and low-cost oxygen evolution reactions (OER) and hydrogen evolution reactions (HER) electrocatalysts remains a challenge for new energy systems. In this study, three-dimensional (3D) cobalt-nickel bimetal MOFs were used as precursors to synthesize catalysts through thermal decomposition, carbonization, nitriding, oxidation, phosphating, sulfurizing, and selenization, respectively. In 1.0 M KOH electrolyte, the overpotential of Co/Ni-MOFs@Se for OER was 238 mV and the that of Co/Ni-MOFs@P for HER was 194 mV at a current density of 10 mA cm⁻². Based on the excellent OER and HER performances of Co/Ni-MOFs@Se and Co/Ni-MOFs@P, these two materials were further assembled into electrodes for overall water splitting. Results showed that a potential of only 1.59 V was required to provide a current density of 10 mA cm⁻². The electrodes also exhibited long-term durability in a 2000 min stability test without significant changes in the catalytic performances. According to the difference in the doped non-metal elements, an electrode pair with a suitable matching degree was constructed, thereby improving the overall water splitting performance. Thus, the controllable modification of the metal-organic frameworks (MOFs)-derived carbon materials (CMs) effectively improved the materials’ catalytic water splitting performance. It was possible to further develop an efficient, inexpensive, and low-cost assembled electrode pair.     

Remarkable improvement of photoelectrochemical water splitting in pristine and black anodic TiO2 nanotubes by enhancing microstructural ordering and uniformity

Efficient photoelectrochemical (PEC) water splitting is crucial for future energy and sustainable world. We here report on the improvement of PEC activity of anodic TiO₂ nanotubes (TNTs) by enhancing tube ordering and subsequent electrochemical reduction. TNTs were prepared by two-step anodic oxidisation from an organic electrolyte containing fluoride ions. The effects of first-step anodisation time on the ordering of TNTs and subsequent electrolytic reduction were investigated on the PEC performance under simulated solar light spectrum. The photocurrent densities of TNTs anodised for 1 h, 4 h and subsequently reduced are about 25.12 μA cm⁻², 51.76 μA cm⁻² and 126.89 μA cm⁻², respectively, at 1.23 V vs RHE and their conversion efficiency of light to electrical energy achieved are about 0.016%, 0.04% and 0.08% respectively. Electrochemical impedance spectroscopy (ESI) curves revealed the improved PEC water splitting confirmed by sharper charge carrier separation and enhanced charge transfer in highly ordered pristine and black TNTs. This improvement of PEC in dopant-free TNT is at the first instance interpreted by enhancing TNT ordering and uniformity achieved by prolonging of the first-step anodisation time and its effect on the electronic band structure of TNTs. This significant effect on PEC performance of pristine TNT under visible light absorption takes place due to the induced surface defects and slower recombination rates of hole and electron. This demonstrates an efficient economic materials production appraoch for PEC hydrogen production.     

CoSe2@NiSe2 nanoarray as better and efficient electrocatalyst for overall water splitting

Electrocatalytic water splitting is identified as one of the most promising solutions to energy crisis. The CoSe₂@NiSe₂ materials were first prepared and in situ grown on nickel foam by typical hydrothermal and selenification process at 120 °C. The results show that the CoSe₂@NiSe₂ material used as the 3D substrates electrode can maximize the synergy between the CoSe₂ and NiSe₂, and also exhibits high efficiency of water splitting reaction. The lower overpotential of only 235 mV is presented to attain 20 mA cm⁻² compared to the benchmark of RuO₂ electrodes (270 mV @ 20 mA cm⁻²). Besides, the CoSe₂@NiSe₂ material also shows a remarkable improved hydrogen evolution reaction activity compared to NiSe₂ (192 mV@10 mA cm⁻²) and Co precursor catalysts (208 mV@10 mA cm⁻²) individually, which a low overpotential of only 162 mV is achieved at 10 mA cm⁻². The CoSe₂@NiSe₂ catalysts exhibit excellent water splitting performance (cell voltage of 1.50 V@ 10 mA cm⁻²) under alkaline conditions. It was proved that the high water splitting performance of the catalyst is attributed to high electrochemical activity area and synergistic effect. The work offers new ideas for the exploitation of synergistic catalysis of composite catalysts and adds new examples for the exploitation of efficient, better and relatively non-toxic electrocatalysts.     

Phase transition in V-doped cobalt hydroxide for efficient oxygen evolution and urea oxidation reaction

In this work, many kinds of V doped Co(OH)₂ electrodes were in situ synthesized on Ni foam by a one-step typical hydrothermal process. It is worth noting that the phase transition composition of the V doped Co(OH)₂ material can be modulated by the difference of the amount of the V introduced. Different crystal phase compositions show different water oxidation activities. It is worth noting that the V2–Co(OH)₂/NF electrode shows better oxygen evolution performance (Overpotential of 320 mV@50 mA cm⁻²) compared with Co(OH)₂/NF (450 mV@50 mA cm⁻²), V1–Co(OH)₂/NF (340 mV@50 mA cm⁻²) and V3–Co(OH)₂/NF (350 mV@50 mA cm⁻²) electrodes. The experimental results show that not all doping can improve the electrochemistry performance of electrodes, such as the oxidation of urea. Density functional theory calculation further proves that the doping of the V is favorable to the adsorption of water and inhibits the adsorption of urea. This study provides a new idea for the development of efficient overall water splitting catalysts.     

New method for improving the bulk charge separation of hematite with enhanced water splitting

Due to its poor bulk charge separation efficiency, the photoelectrochemical (PEC) performance of pristine hematite prepared directly from an electrodeposited Fe film is limited. Au-modification of hematite via a simple immersion method improves the PEC performance two-fold to 0.31 mA cm⁻². The Au nanoparticles deposited from HAuCl₄ act as plasmonic photosensitizers and electron collectors to improve the light absorption and bulk charge separation efficiency of the photoanode. In addition, the increase in the (110) plane and specific surface area induced by HAuCl₄ enhances the bulk charge separation efficiency. After further modification with Ti, the photocurrent response of the resulting Ti/Au/α-Fe₂O₃ photoanode improves to 0.51 mA cm⁻²; this increase is attributed to its increased light absorption, bulk charge separation efficiency (η_bulk), and surface charge injection efficiency (η_surface). In this work, the effect of Au and Ti on the crystalline structure, morphology and PEC performance of the novel electrodeposited hematite photoanode are investigated by systematical characterization.     

The P/NiFe doped NiMoO4 micro-pillars arrays for highly active and durable hydrogen/oxygen evolution reaction towards overall water splitting

Water splitting is an efficient strategy to produce purity hydrogen and convert intermittent electricity from renewable wind and solar sources. In this work, dense NiMoO₄ micro-pillars arrays (MPAs) were in-situ grown on nickel foam (NF) through facile hydrothermal method, then the NiMoO₄/NF were converted into NiMoO₄–P/NF and NiFe/NiMoO₄/NF via phosphating and electrodeposition method, respectively. The NiMoO₄–P/NF electrode required small overpotentials of 34 mV@10 mA cm⁻² and 130 mV@100 mA cm⁻² for hydrogen evolution reaction (HER). The NiFe/NiMoO₄/NF electrode exhibited excellent oxygen evolution reaction (OER) activity with overpotentials of 210 mV@10 mA cm⁻² and 300 mV@100 mA cm⁻². The overall water splitting using the anode-cathode couple of NiFe/NiMoO₄/NF||NiMoO₄–P/NF only consumes low voltages of 1.47 V@10 mA cm⁻² for 100 h and 1.66 V@100 mA cm⁻² for 50 h in 1 M KOH. The electronic modification and the well-designed hierarchical structure contribute the high energy-efficient and stabile overall water splitting.     

Autologous growth of Fe-doped Ni(OH)2 nanosheets with low overpotential for oxygen evolution reaction

Highly active and durable electrocatalysts for oxygen evolution reaction (OER) play a vital role in water splitting. Despite numerous efforts, the strategies to prepare durable and effective electrocatalysts via scalable methods still remain a great challenge. In this work, we fabricated Fe-doped Ni(OH)₂ ultrathin nanosheets (Fe–Ni–OH/Ni) via autologous growing of Ni(OH)₂ from Ni foam, and in situ electrochemical-assisted doping Fe into Ni(OH)₂. Benefiting from the unique structure with large surface areas and strong coupling effects between Fe and Ni, the optimal Fe–Ni–OH electrodes exhibit remarkable catalytic performance toward OER, which requires an overpotential of 220 mV to achieve a current density of 10 mA cm⁻² with a Tafel slope of 48.3 mV dec⁻¹. The Fe–Ni–OH electrodes also possess high stability even under a high current density of 500 mA cm⁻² for 600 h with an ultralow overpotential of 290 mV. Using Ni–Fe–OH electrodes as both anode and cathode for overall water splitting, only a small overpotential of 1.57 V is required to reach a current density of 10 mA cm⁻². Moreover, the high catalytic performance and scalable preparation method can meet the emergency needs for the practical application.     

Design of advanced self-supported electrode by surface modification of copper foam with transition metals for efficient hydrogen evolution reaction

Electrocatalytic water splitting is one of the most favorable methods for industrial-scale hydrogen production, but high cost and scarcity of commercially available noble metals restrict its application for hydrogen evolution reaction (HER). It is challenging to develop efficient non-noble metal-based electrocatalysts for HER. Herein, a Ni–Cr was doped on Copper foam (CF) substrate by adopting a simple annealing process. The high electrocatalytic efficiency for HER was achieved with Ni–Cr@CF electrode in strong basic medium with a lower overpotential of 144 mV to gain a current density of 10 mA cm⁻² with a small Tafel slope of 88 mV dec⁻¹. After surface modification, the CF substrate exhibits that the entire surface was uniformly covered with Ni–Cr species ensuring the fast reaction kinetics due to the efficient electron transfer process between the substrate and active catalyst. Moreover, the Ni–Cr@CF electrode exhibits excellent stability up to 2000 cycles under the strong basic medium.     

Nitrogen-doped carbon dot anchored 1-D WO3 for enhanced solar water splitting: A nano surface imaging evidence of charge separation and accumulation

Combining WO₃ with suitable materials to form heterojunction is essential to overcome the limitations of WO₃ to enhance its photoelectrochemical (PEC) water splitting activity. Moreover, a clear understanding of photo-response and charge behavior of materials could lead to the rational design of efficient photoelectrodes. Given this, an efficient strategy is applied to fabricate WO₃ heterojunction with nitrogen-doped carbon dots (NCDs) and in-depth characterization to investigate the surface charge dynamics using nano imaging in a relation to the enhanced PEC water splitting activity. The optimized NCDs loading to the WO₃ NRs exhibited the enhanced photocurrent density of 1.54 mA cm⁻² at 1.23V vs RHE under AM 1.5 G illumination, highest IPCE of ~82 % (at 308.32 nm). The Kelvin probe force microscopy and electrostatic force microscopy reveal that after loading NCDs to the WO₃, a relatively smooth charge transport has been observed, which improves the PEC. Furthermore, this work demonstrates the effect of photogenerated charges caused by the NCDs that assist in enhancing the increased photocurrent, hydrogen production efficiency, and stability of the PEC water splitting system. Significantly, the nano imaging characterization utilized in this work could be extended to various photoanodes to study the surface charge dynamics.     

Photoelectrochemical performance of bismuth vanadate photoanode for water splitting under concentrated light irradiation

Photoelectrochemical (PEC) water splitting is a promising way to convert solar energy into hydrogen energy. It is typically carried out at room temperature (RT) and 1 sun illumination. The PEC water splitting under concentrated light is expected to be an effective route to improve PEC performance, but there are few studies on it. Herein, CoPi/Mo:BiVO₄ photoanode was selected to investigate the effect of concentrated light and the reaction temperature on its PEC performance. It was revealed that CoPi/Mo:BiVO₄ showed enhanced PEC performance under concentrated light. The photocurrent density was enhanced with increased light intensity and increased reaction temperature. At a high temperature (60 °C), the normalized photocurrent density (3.31 mA cm⁻² at 1.23 V vs. RHE) was found to be optimal at 4 suns, which was attributed to the synergistic effect of concentrating light and heating. It is proved that concentrated light can effectively improve the PEC performance, which has important guiding significance to realize the low-cost and efficient PEC water splitting.     

Porous NiFe alloys synthesized via freeze casting as bifunctional electrocatalysts for oxygen and hydrogen evolution reaction

Binder-free NiFe-based electrocatalyst with aligned pore channels has been prepared by freeze casting and served as a bifunctional catalytic electrode for oxygen and hydrogen evolution reaction (OER and HER). The synergistic effects between Ni and Fe result in the high electrocatalytic performance of porous NiFe electrodes. In 1.0 M KOH, porous Ni₇Fe₃ attains 100 mA cm⁻² at an overpotential of 388 mV with a Tafel slope of 35.8 mV dec⁻¹ for OER, and porous Ni₉Fe₁ exhibits a low overpotential of 347 mV at 100 mA cm⁻² with a Tafel slope of 121.0 mV dec⁻¹ for HER. The Ni₉Fe₁//Ni₉Fe₁ requires a low cell voltage of 1.69 V to deliver 10 mA cm⁻² current density for overall water splitting. The excellent durability at a high current density of porous NiFe electrodes has been confirmed during OER, HER and overall water splitting. The fine electrocatalytic performances of the porous NiFe-based electrodes owing to the three-dimensionally well-connected scaffolds, aligned pore channels, and bimetallic synergy, offering excellent charge/ion transfer efficiency and sizeable active surface area. Freeze casting can be applied to design and synthesize various three-dimensionally porous non-precious metal-based electrocatalysts with controllable multiphase for energy conversion and storage.     

CoO/NF nanowires promote hydrogen and oxygen production for overall water splitting in alkaline media

The exploration of catalysts with high activity and low cost for water splitting is still necessary. Herein, a nanowire-like morphology CoO/NF electrode is synthesized using facile hydrothermal reaction and calcination treatment. The urea can regulate its morphology during the synthetic process of CoO/NF. Electrochemical studies reveal that the as-obtained CoO/NF exhibits excellent electrocatalytic performance with overpotential of 307 mV at current density of 10 mA cm⁻² and Tafel slope of 72 mV dec⁻¹ for oxygen evolution reaction, and CoO/NF delivers current density of 10 mA cm⁻² at overpotential of 224 mV for hydrogen evolution reaction. The results of the oxygen evolution reaction stability show that the overpotential of CoO/NF electrode is only increased by 4 mV at current density of 10 mA cm⁻². The two-electrode water splitting with CoO/NF electrodes as both anode and cathode needs a cell potential of 1.76 V to reach 10 mA cm⁻². Therefore, this simple method to prepare CoO/NF electrode can enhance the properties of electrocatalysts, which makes CoO/NF a promising material to replace noble metal-based catalysts.     

Construction of hierarchical Prussian Blue Analogue phosphide anchored on Ni2P@MoOx nanosheet spheres for efficient overall water splitting

In response to the energy crisis, molybdenum-based catalyst has been proposed as a high-performance electrocatalytic material due to its low price and excellent HER performance. However, in contrast with its excellent HER performance, its poor OER performance often limits practical application as a high-performance overall water splitting catalyst. In this study, Prussian blue analogue (PBA) is grown in-situ on molybdenum-based nanosheet spheres by a simple and ingenious method and then subjected to phosphorization. The resulting composite catalyst exhibits highly efficient overall water splitting performance, overpotentials at current densities of 10 mA cm⁻² and 100 mA cm⁻² for the HER and OER are −61 mV and 268 mV, respectively. Moreover, an alkaline electrolyzer makes up by the catalyst both as positive and negative can reach a cell voltage 1.494 V at 10 mA cm⁻² for the overall water splitting. This method has provided a new strategy to effective combine PBA and molybdenum-based catalyst.     

Heterogeneous layered multiple transition metal composite electrocatalysts with controlled composition for hydrogen production

The exploration of highly efficient and low-cost bifunctional electrocatalyst is essential for overall water splitting, especially for industrial application under alkaline conditions. Herein, we propose a controllable structural engineering strategy of constructing heterogeneous layered electrocatalyst with wetting surface for hydrogen evolution reaction and oxygen evolution reaction. Heterogeneous layered NiFe LDH (layered double hydroxide)/CoFeP/NF (Ni foam) with superhydrophilic surfaces is successfully fabricated by successive electrodeposition, phosphorization and solvothermal method. The NiFe LDH/CoFeP/NF for hydrogen evolution achieves a low overpotential of 198 mV at 50 mA cm^?2 in 1.0 M KOH. An overpotential of 269 mV is required at 50 mA cm^?2 for oxygen evolution. Meanwhile, the practical utilization of NiFe LDH/CoFeP/NF as bifunctional electrocatalysts for overall water splitting yields 1.73 V at 50 mA cm^?2 in the two-electrode cell. Moreover, NiFe LDH/CoFeP/NF can retain over 50 h without an obvious degradation at 10 mA cm^?2. The satisfactory operating stability and high activity of NiFe LDH/CoFeP/NF in alkaline solution can be attributed to the heterogeneous layered structure and excellent hydrophilic surface. The study provides a strategy to engineering heterogeneous layered structures with wetting surface for excellent electrocatalytic activities toward overall water splitting.     

Electrodeposition of NiS/Ni2P nanoparticles embedded in amorphous Ni(OH)2 nanosheets as an efficient and durable dual-functional electrocatalyst for overall water splitting

To develop earth-abundant and cost-effective catalysts for overall water splitting is still a major challenge. Herein, a unique “raisins-on-bread” Ni–S–P electrocatalyst with NiS and Ni₂P nanoparticles embedded in amorphous Ni(OH)₂ nanosheets is fabricated on Ni foam by a facile and controllable electrodeposition approach. It only requires an overpotential of 120 mV for HER and 219 mV for OER to reach the current density of 10 mA cm⁻² in 1 M KOH solution. Employed as the anode and cathode, it demonstrates extraordinary electrocatalytic overall water splitting activity (cell voltage of only 1.58 V @ 10 mA cm⁻²) and ultra-stability (160 h @ 10 mA cm⁻² or 120 h @50 mA cm⁻²) in alkaline media. The synergetic electronic interactions, enhanced mass and charge transfers at the heterointerfaces facilitate HER and OER processes. Combined with a silicon PV cell, this Ni–S–P bifunctional catalyst also exhibits highly efficient solar-driven water splitting with a solar-to-hydrogen conversion efficiency of 12.5%.     

Facile synthesis of molybdenum-based layered double hydroxide nanorods for boosting water oxidation reaction

Exploiting environmentally friendly and robust oxygen evolution reaction (OER) electrodes is still a great difficulty to promote the water oxidation for electrocatalytic water splitting. In the work, molybdenum-doped nickel copper hydrotalcite (NiCuMo_x LDH) nanoarrays have been firstly in situ grown on nickel foam (NF) via a typical hydrothermal process. When NiCuMo_0.2 LDH/NF was used as a OER electrocatalyst, it displays superior electrocatalytic performance with the need of small overpotentials of only 290 mV to drive 40 mA cm⁻² and a low Tafel slope of 39.8 mV dec⁻¹, which are almost one of the best water oxidation activities reported so far. The enhanced electrocatalytic performance is attributed to unique urchin-like structure, more exposure to active sites and enhanced charge transfer rate owing to the synergistic effect of Mo doping and NiCu LDH. The work put forward a new method for the development of efficient water oxidation electrocatalysts, which will fill the gap for the exploitation of trimetal LDH-based electrodes in large-scale water splitting applications in the future.     

Mesoporous spinel NiFe oxide cubes as advanced electrocatalysts for oxygen evolution

Oxygen evolution reaction (OER) is the rate-controlling step of the electrochemical water splitting. The slow kinetics hinders large-scale H₂ production. Herein, the spinel NiFe oxides were prepared by directly pyrolyzing nickel hexacynoferrate precursors in air. The NiFe oxides were presented as mesoporous nanocubes with a specific surface area of 125 m² g⁻¹. The mesoporous spinel NiFe oxide nanocubes can afford a geometric current of 10 mA cm⁻² at a low overpotential of a 0.24 V and a small Tafel slope of 41 mV dec⁻¹ in alkaline solution. The specific activity can reach up to 0.37 mA cm⁻² with a turnover frequency of 0.93 s⁻¹. The superior OER activity of the NiFe oxide nanocubes (NiFeO NCs) can outperform those of the state-of-the-art IrO₂ catalysts, and compare favorably with other spinel transition metal oxides reported recently under identical condition. NiFeO NCs also show a long-term durability without significant loss of the OER activity. Our works provide a new strategy to develop efficient, robust and earth-abundant spinel NiFe oxides as advanced OER electrocatalysts to replace the expensive commercial IrO₂ catalysts for water splitting in the industrial scale.     

In-Situ solid-state synthesis of 2D/2D interface between Ni/NiO hexagonal nanosheets supported on g-C3N4 for enhanced photo-electrochemical water splitting

Herein, we report a synthesis of 2D/2D interfaces between nickel/nickel oxide (Ni/NiO) hexagonal nanosheets with graphitic-carbon nitride (g-C₃N₄) using an in-situ solid-state heat treatment that shows enhanced activity for electrochemical as well as photo-electrochemical (PEC) water splitting. The transmission electron microscopy characterization confirms the homogenous dispersion of 2D hexagonal nanosheets of Ni/NiO on the surface of g-C₃N₄. The higher electrochemical and PEC water splitting activity of 2D/2D interface may be due to the more intimate contact between 2D sheets of NiO with g-C₃N₄. Moreover, the effect of NiO loading in nanoheterostructures have been studied towards overall water splitting by varying the ratio of precursors for NiO to that of g-C₃N₄ viz. 1:1, 1:8, and 1:16. A compositional ratio of 1:8 have been found to show the best PEC activity towards OER depicting a maximum photocurrent density of 20 mA cm⁻² at an over potential of 190 mV. Whereas, the highest ratio of NiO to g-C₃N₄ nanosheets (i.e. 1:1) was noted to demonstrate the best performance towards electrochemical hydrogen evolution reaction showing dramatically reduced over potential of 26 mV to drive a current density of 10 mA cm⁻².