Mesoporous nickel-sulfide/nickel/N-doped carbon as HER and OER bifunctional electrocatalyst for water electrolysis

The development of non-precious metal-based highly active bi-functional electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is critical factor for making water electrolysis a viable process for large-scale industrial applications. In this study, bi-functional water splitting electrocatalysts in the form of nickel-sulfide/nickel nanoparticles integrated into a three-dimensional N-doped porous carbon matrix, are prepared using NaCl as a porous structure-forming template. Microstructures of the catalytic materials are characterized by scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy and N₂ adsorption-desorption analysis. The most active catalyst synthesized in this study exhibits a low HER overpotential of 70 mV at 10 mA cm⁻² and a low Tafel slope of 45 mV dec⁻¹. In OER, the optimized sample performs better than a state-of-the-art RuO₂ catalyst and produces an overpotential of 337 mV at 10 mA cm⁻², lower than that of RuO₂. The newly obtained materials are also used as HER/OER electrocatalysts in a specially assembled two-electrode water splitting cell. The cell demonstrates high activity and good stability in overall water splitting.     

N,P-co-doped carbon coupled with CoP as superior electrocatalysts for hydrogen evolution reaction and overall water splitting

To achieve high activity and stability for both hydrogen and oxygen evolution reactions through the non-precious-metal based electrocatalysts is still facing the great challenge. Herein, we demonstrate a facile strategy to prepare CoP nanoparticles (NPs) loaded on N, P dual-doped carbon (NPC) electrocatalysts with high concentration N and P dopants through a pyrolysis-deposition-phosphidation process. The great bifunctional electrocatalytic activity for both HER (the overpotential of 98 mV and 86 mV at 10 mA cm⁻² in both 0.5 M H₂SO₄ and 1 M KOH electrolytes, respectively) and OER (the overpotential of 300 mV at 10  mA cm⁻² in 1 M KOH electrolyte) were achieved. When CoP@NPC hybrid was used as two electrodes in the 1 M KOH electrolyte system for overall water splitting, the needed cell potential for achieving the current density of 10 mA cm⁻² is 1.6 V, and it also showed superior stability for HER and OER after 10 h’ test with almost negligible decay. Experimental results revealed that the P atoms in CoP were the active sites for HER and the CoP@NPC hybrid showed excellent bifunctional electrocatalytic properties due to the synergistic effects between the high catalytic activity of CoP NPs and NPC, in which the doping of N and P in carbon led to a stronger polarization between Co and P in CoP, promoting the charge transfer from Co to P in CoP, enhancing the catalytic activity of P sites and Co sites in CoP for HER and OER, respectively. Specifically, the improvements could result from the changed charge state, the increased active specific surface area, and the facilitated reaction kinetics by N, P co-doping and admixture. This work provides a high-efficient, low-cost and stable electrocatalyst for overall water splitting, and throws light on rational designing high performance electrocatalysts.     

Uniform cobalt nanoparticles embedded in nitrogen-doped graphene with abundant defects as high-performance bifunctional electrocatalyst in overall water splitting

Co nanoparticles with uniform size (about 5 nm) embedded in N-doped graphene (Co-NG) were explored in this work. The introduction of a second carbon source of citric acid during synthesis prevented the Co atoms from growing up, thus regulating the size of the cobalt nanoparticles. N atoms in N-doped graphene had more lone-pair electrons, making it easier to capture electrons from hydrated ions, and facilitating the dynamics procedure of HER. Furthermore, N dopant rendered larger positive charge density on the adjacent carbon atoms, which was conducive to OER and HER. At 10 mA cm^?2 of the current density, the Co-NG/CC catalyst's overvoltage of HER was 78 mV, approaching that of 20% Pt/C (59 mV), an efficient precious metal electrocatalyst for HER, while its overvoltage of OER was about 225 mV, 12.5% lower than that of RuO₂ (257 mV, a common precious metal oxide OER electrocatalyst). In addition, this Co-NG/CC composite bifunctional catalyst displayed good electrochemical stability in alkali solution and might be designed as a dual-function catalyst in the application of overall water splitting. The cell voltage of Co-NG/CC//Co-NG/CC was only 1.66 V, approaching to that of full precious metal cell of Pt/C//RuO₂ (1.52 V), and revealing the good commercial application prospects of this composite bifunctional catalyst.     

Ni3S2-MoSx nanorods grown on Ni foam as high-efficient electrocatalysts for overall water splitting

In order to solve the problem of large overpotential in water electrolysis for hydrogen production, transition metal sulfides are promising bifunctional electrocatalysts for hydrogen evolution reaction/oxygen evolution reaction that can significantly reduce overpotential. In this work, Ni₃S₂ and amorphous MoS_x nanorods directly grown on Ni foam (Ni₃S₂-MoS_x/NF) were prepared via one-step solvothermal process, which were used as a high-efficient electrocatalyst for overall water splitting. The Ni₃S₂-MoS_x/NF composite exhibits very low overpotentials of 65 and 312 mV to reach 10 mA cm⁻² and 50 mA cm⁻² in 1.0 M KOH for HER and OER, respectively. Besides, it exhibits a low Tafel slope (81 mV dec⁻¹ for HER, 103 mV dec⁻¹ for OER), high exchange current density (1.51 mA cm⁻² for HER, 0.26 mA cm⁻² for OER), and remarkable long-term cycle stability. This work provides new perspective for further the development of highly effective non-noble-metal materials in the energy field.     

Co-based MOF-derived Co/CoN/Co2P ternary composite embedded in N- and P-doped carbon as bifunctional nanocatalysts for efficient overall water splitting

In this work, we developed ternary metallic cobalt-cobalt nitride-dicobalt phosphide composite embedded in nitrogen and phosphorus co-doped carbon (Co/CoN/Co₂P-NPC) as bifunctional catalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The as-prepared Co/CoN/Co₂P-NPC is achieved by simultaneous annealing and phosphating of a Co–N rich metal-organic frameworks (MOFs) precursor. Compare with the phosphorus-free Co/CoN embedded nitrogen-doped carbon electrocatalyst (Co/CoN-NC), the as-prepared Co/CoN/Co₂P-NPC display superior HER and OER low overpotential of 99 mV and 272 mV at current density of 10 mA cm⁻². When Co/CoN/Co₂P-NPC electrocatalyst is use as bifunctional catalysts in overall alkaline water splitting, it exhibit excellent behaviour with 10 mA cm⁻² current at overall cell potential of 1.60 V. The excellent performance of Co/CoN/Co₂P-NPC electrocatalyst is attributed to the phosphating process that could further enhance synergistic effect, create stronger electronic interactions, and form efficient dual heteroatom doping to optimize the interfacial adhesion within the electrocatalyst. This present work will create more opportunities for the development of new, promising and more active sites electrocatalysts for alkaline electrolysis.     

One-pot preparation of Ni3S2@3-D graphene free-standing electrode by simple Q-CVD method for efficient oxygen evolution reaction

Efficient non-noble metal catalysts for the oxygen evolution reaction (OER) are particularly important in the practical applications of electrocatalytic water splitting (ECWS). Herein, based on a simple quasi chemical vapor deposition (Q-CVD) method, we fabricate a newly Ni₃S₂@3-D graphene free-standing electrode for efficient OER applications. The Ni₃S₂@3-D graphene integrates the advantageous features of 3-D graphene and Ni₃S₂ towards OER, such as more interfacial catalytic sites, pore-rich structure, N-doped structure and good electrical conductivity. Benefiting from the favorable features, the Ni₃S₂@3-D graphene (especially 900 °C sample) exhibits excellent OER performances in alkaline medium, which includes a low on-set potential (1.53 V), low overpotential of 305 mV at a current density of 10 mA cm⁻², and a smaller Tafel slope (50 mV dec⁻¹). This catalyst also shows ultrahigh stability after chronoamperometry response at 10 mA cm⁻² for 48 h with 30% increase in the current density. The present work opens a new approach for the one-pot construction of hybrid materials between metal sulfide and graphene to increase the electrocatalytic activity of non-noble metal OER catalysts.     

Preparation of ultrathin molybdenum disulfide dispersed on graphene via cobalt doping: A bifunctional catalyst for hydrogen and oxygen evolution reaction

The development of highly active and low-cost catalysts for hydrogen evolution reaction (HER) is significant for the development of clean and renewable energy research. Owing to the low H adsorption free energy, molybdenum disulfide (MoS₂) is regarded as a promising candidate for HER, but it shows low activity for oxygen evolution reaction (OER). Herein, graphene-supported cobalt-doped ultrathin molybdenum disulfide (Co–MoS₂/rGO) was synthesized via a one-pot hydrothermal method. The obtained hybrids modified electrode exhibits a high HER catalytic activity with a low overpotential of 147 mV at the current density of 10 mA cm⁻², a small Tafel slope of 49.5 mV dec⁻¹, as well as good electrochemical stability in acidic electrolyte. Meanwhile, the catalyst shows remarkable OER activity with a low overpotential of 347 mV at 10 mA cm⁻². The superior activity is ascribed not only to the high conductivity originated from the reduced graphene, but also to the synergistic effect between MoS₂ and cobalt.     

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%.     

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.     

MoS2/NiFeS2 heterostructure as a highly efficient electrocatalyst for overall water splitting at high current densities

Searching for efficient, stable and low-cost nonprecious catalysts for oxygen and hydrogen evolution reactions (OER and HER) is highly desired in overall water splitting (OWS). Herein, presented is a nickel foam (NF)-supported MoS₂/NiFeS₂ heterostructure, as an efficient electrocatalyst for OER, HER and OWS. The MoS₂/NiFeS₂/NF catalyst achieves a 500 mA cm⁻² current density at a small overpotential of 303 mV for OER, and 228 mV for HER. Assembled as an electrolyzer for OWS, such a MoS₂/NiFeS₂/NF heterostructure catalyst shows a quite low cell voltage (≈1.79 V) at 500 mA cm⁻², which is among the best values of current non-noble metal electrocatalysts. Even at the extremely large current density of 1000 mA cm⁻², the MoS₂/NiFeS₂/NF catalyst presents low overpotentials of 314 and 253 mV for OER and HER, respectively. Furthermore, MoS₂/NiFeS₂/NF shows a ceaseless durability over 25 h with almost no change in the cell voltage. The superior catalytic activity and stability at large current densities (>500 mA cm⁻²) far exceed the benchmark RuO₂ and Pt/C catalysts. This work sheds a new light on the development of highly active and stable nonprecious electrocatalysts for industrial water electrolysis.     

Fe doped metal organic framework (Ni)/carbon black nanosheet as highly active electrocatalyst for oxygen evolution reaction

To alleviate the sluggish oxygen evolution reaction (OER) kinetics, it's urgent to develop electrocatalysts with high activity and low cost. In this work, Fe doped metal organic frameworks (Ni)/carbon black composites were synthesized via a facile hydrothermal method. Benefiting from the direct use of metal organic frameworks (MOFs) for OER, numerous and highly dispersed active sites are exposed to the electrolyte and reactants. By regulating Ni/Fe ratios, a high electrochemical active surface area (ECSA) and high relative surface content of active Ni³⁺ species are obtained, which mainly contribute to the high OER activity. Besides, the introduced carbon black (CB) was found to enhance the charge-transfer efficiency of the electrocatalysts, which is also favorable for OER. The optimal Ni9Fe1-BDC-0.15CB electrocatalyst shows excellent OER activity with the low overpotential of ~290 mV at 10 mA cm⁻² and the Tafel slope of ~76.1 mV dec⁻¹, which is comparable to RuO₂ and other MOFs-based OER electrocatalysts reported in recent years.     

Highly efficient ternary CuP2/Ni1−xCux‒P@g-C3N4 nanostructure for improved hydrogen and oxygen evolution reactions

Herein, a highly active, and stable copper phosphide/nickel-copper phosphide (CuP₂/NiCuP) nanosheet supported by a g-C₃N₄ is constructed via phosphorization for hydrogen and oxygen evolution reactions (HER and OER, respectively). Strong coupling between CuP₂/NiCuP heteronanosheets and the structured g-C₃N₄ provides CuP₂/NiCuP/g-C₃N₄ with a regulated electronic state, sufficient anchored active sites, reduced mass transport distance, and improved structural stability. Consequently, the obtained CuP₂/(NiCu)_1:1P/g-C₃N₄ electrocatalyst manifests excellent performance in HER and OER, with overpotentials of 96 and 280 mV at 10 mA cm⁻², respectively, as well as stability. The excellent HER and OER activities are mainly ascribed to the collective effects of electronic structure engineering, strong interaction between CuP₂ and NiCuP in the heteronanojunction, abundant catalytic active sites, and highly conductive g-C₃N₄ support. This study provides novel insights for designing cost-effective bimetallic alloy-phosphide HER/OER electrocatalysts for various sustainable energy-conversion applications.     

Bimetallic metal-organic framework derived electrocatalyst for efficient overall water splitting

The development of bifunctional catalysts that can be applied to both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is widely regarded as a key factor in the production of sustainable hydrogen fuel by electrochemical water splitting. In this work, we present a high-performance electrocatalyst based on nickel-cobalt metal-organic frameworks for overall water splitting. The as-obtained catalyst shows low overpotential to reaches the current density of 10 mA cm⁻² with 249 mV for OER and 143 mV for HER in alkaline media, respectively. More importantly, when the electrolyzer was assembled with the as-prepared catalyst as anode and cathode simultaneously, it demonstrates excellent activity just applies a potential of 1.68 V to achieve 10 mA cm⁻² current density for overall water splitting.     

Novel 3-D urchin-like Ni– Co–W porous nanostructure as efficient bifunctional superhydrophilic electrocatalyst for both hydrogen and oxygen evolution reactions

Fabrication of an electrocatalyst with remarkable electrocatalytic activity for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is important for the production of hydrogen energy. In this study, Ni–Co–W alloy urchin-like nanostructures were fabricated by binder-free and cost-effective electrochemical deposition method at different applied current densities and HER and OER electrocatalytic activity was studied. The results of this study showed that the microstructure and morphology are strongly influenced by the electrochemical deposition parameters and the best electrocatalytic properties are obtained at the electrode created at the 20 mA.cm⁻²applied current density. The optimum electrode requires −66 mV and 264 mV, respectively, for OER and HER reactions for delivering the 10 mA cm⁻² current density. The optimum electrode also showed negligible potential change after 10 h electrolysis at 100 mA cm⁻², which means remarkable electrocatalytic stability. In addition, when this electrode used as a for full water splitting, it required only 1.58 V to create a current density of 10 mA cm⁻². Such excellent electrocatalytic activity and stability can be related to the high electrochemical active surface area, being binder-free, high intrinsic electrocatalytic activity and hydrophilicity. This study introduces a simple and cost-effective method for fabricating of effective electrodes with high electrocatalytic activity.     

Co3O4 nanoparticles decorated Polypyrrole/carbon nanocomposite as efficient bi-functional electrocatalyst for electrochemical water splitting

An effective bi-functional electrocatalyst of Co₃O₄/Polypyrrole/Carbon (Co₃O₄/Ppy/C) nanocomposite was prepared through a simple dry chemical method and used to catalyze the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Three types of carbon support as Vulcan carbon, reduced graphite oxide (RGO) and multi-walled carbon nanotubes (MCNTs) were used to study the influence on electrochemical reactions. Spherical shaped Co₃O₄ nanoparticles with 8–10 nm was found uniformly distributed on Ppy/C composite, which were analyzed by X-ray diffraction and transmission electron microscopy techniques. Amongst, Co₃O₄/Ppy/MWCNT shows improved bifunctional electrocatalytic activity towards both OER and HER with relatively low over potential (340 mV vs. 490 mV at 10 mA cm⁻²) and Tafel slope (87 vs. 110 mV dec⁻¹). In addition to that, MWCNT supported Co₃O₄/Ppy nanocomposite exhibits good electronic conductivity and electrochemical stability up to 2000 potential cycles. The results clearly indicate that the Co₃O₄/Ppy/MWCNT nanocomposite could be the promising bi-functional electrocatalyst for efficient water electrolysis.     

MOF-derived Zn,S, and P co-doped nitrogen-enriched carbon as an efficient electrocatalyst for hydrogen evolution reaction

In this study, Zn, S, and P co-doped nitrogen N-enriched carbon (ZnSP/NC) was successfully fabricated as an efficient electrocatalyst for the hydrogen evolution reaction (HER) process via pyrolysis, sulfurization, and phosphorization. The metal Zn derived from zeolitic imidazolate framework-8 (ZIF-8) was combined with the S element to form ZnS nanoparticles, and then embedded in N-enriched carbon during the sulfurization process. Following this, the P element was well-dispersed in the catalyst via phosphorization. It was found that ZnSP/NC exhibits excellent electrocatalytic activity when used as a catalyst for the HER process. ZnSP/NC, with an overpotential of 171 mV and a Tafel slope of 54.78 mV dec⁻¹, demonstrates superior electrocatalytic activity as compared to Zn/NC (277 mV, 92.34 mV dec⁻¹) and ZnS/NC (241 mV, 76.41 mV dec⁻¹). During the HER process, ZnS and P serve as active sites, while the N-enriched carbon provides reliable electronic transmission. The synergistic effects among the ZnS, P, and N-enriched carbon result in an excellent electrocatalytic activity of ZnSP/NC for the HER process.     

Heterogeneous cobalt phosphides nanoparticles anchored on carbon cloth realizing the efficient hydrogen generation reaction

Intrinsic activity modifying of electrocatalysts is crucial to realizing the excellent catalytic performance towards hydrogen evolution reaction. Herein, we demonstrate a highly efficient electrocatalyst based on heterogeneous cobalt phosphides nanoparticles. The ultrafine size of the as-prepared catalyst (∼5 nm) ensures the efficient extension of active sites. Furthermore, the incorporation of orthorhombic CoP and Co₂P contributes to the improvement of the inherent catalytic property. As a consequence, the as-prepared heterogeneous cobalt phosphides nanoparticles supported on carbon cloth exhibit impressive electrocatalytic activity, which only acquire a small overpotential of 90 mV at a current density of 10 mA cm⁻², and present a low Tafel slope (67.9 mV dec⁻¹), a large exchange current density (0.58 mA cm⁻²) as well as good durability. Therefore, this work provides a favorable guidance for exploring executable strategies to improve catalyst activity.     

Facile galvanic replacement method for porous Pd@Pt nanoparticles as an efficient HER electrocatalyst

In realm of renewable energy, development of an efficient and durable electrocatalyst for H₂ production through electrochemical hydrogen evolution reaction (HER) is indispensable. Herein, we demonstrate a simple preparation of carbon-supported nanoporous Pd with surface coated Pt (CS–PdPt) by a simple galvanic replacement reaction (GRR). The phase purity and porosity have been confirmed by XRD, HRTEM, and N₂ sorption techniques. As HER electrocatalyst, CS-PdPt showed a low overpotential of 26 mV in 0.5 M H₂SO₄ at current density of 10 mA cm⁻², which is lower than the commercial Pt/C electrode. The CS-PdPt catalyst exhibits an overpotential of 46 mV in 1 M KOH, and 50 mV in neutral buffer (1 M PBS) at 10 mA cm⁻². The CS-PdPt furnished with small Tafel values of 33, 88, and 107 mV dec⁻¹ in acidic, alkaline, and neutral medium, respectively. Accelerated durability test at 100 mV s⁻¹ for 1000 cycles demonstrated a negligible change in HER activity.     

High throughput preparation of Ni–Mo alloy thin films as efficient bifunctional electrocatalysts for water splitting

The synthesis of cost-effective and high-performance electrocatalysts for water splitting is the main challenge in electrochemical hydrogen production. In this study, we adopted a high throughput method to prepare bi-metallic catalysts for oxygen/hydrogen evolution reactions (OER/HER). A series of Ni–Mo alloy electrocatalysts with tunable compositions were prepared by a simple co-sputtering method. Due to the synergistic effect between Ni and Mo, the intrinsic electrocatalytic activity of the Ni–Mo alloy electrocatalysts is improved, resulting in excellent HER and OER performances. The Ni₉₀Mo₁₀ electrocatalyst shows the best HER performance, with an extremely low overpotential of 58 mV at 10 mA cm^?2, while the Ni₄₀Mo₆₀ electrocatalyst shows an overpotential of 258 mV at 10 mA cm^?2 in OER. More significantly, the assembled Ni₄₀Mo₆₀//Ni₉₀Mo₁₀ electrolyzer only needs a cell voltage of 1.57 V to reach 10 mA cm^?2 for overall water splitting.     

Facile one-step in-situ encapsulation of non-noble metal Co2P nanoparticles embedded into B,N, P tri-doped carbon nanotubes for efficient hydrogen evolution reaction

Developing high performance, good stability and noble-metal-free electrocatalysts for renewable hydrogen evolution reaction (HER) remain a substantial challenge. Herein, we introduce a novel facile one-step in-situ strategy through pyrolysis for the synthesis of Co₂P nanoparticles encapsulated Boron, Nitrogen, and Phosphorous tri-doped carbon nanotubes (Co₂P/BNP-CNTs). The synergetic effect between Co₂P nanoparticles and heteroatom doped CNTs contributes to the remarkable HER performance. The Co₂P/BNP-CNT-900 electrocatalyst shows a low overpotential of 133 mV at a current density of 10 mA cm⁻² and a small Tafel slope of 90 mV dec⁻¹ in 0.1 M KOH media. More importantly, the Co₂P/BNP-CNT-900 electrocatalyst exhibits superior long-term stability in alkaline solution at −0.25 V versus Reversible Hydrogen Electrode (RHE) for 15 h and up to 1000 cycles with negligible performance loss. Overall, our works suggest a one-pot facile synthesis strategy for rational designing high-performance electrocatalysts with enhanced HER performance.