A highly active composite electrocatalyst Ni–Fe–P–Nb2O5/NF for overall water splitting

This work demonstrates a facile Nb₂O₅-decorated electrocatalyst to prepare cost-effective Ni–Fe–P–Nb₂O₅/NF and compared HER & OER performance in alkaline media. The prepared electrocatalyst presented an outstanding electrocatalytic performance towards hydrogen evolution reaction, which required a quite low overpotential of 39.05 mV at the current density of ?10 mA cm^?2 in 1 M KOH electrolyte. Moreover, the Ni–Fe–P–Nb₂O₅/NF catalyst also has excellent oxygen evolution efficiency, which needs only 322 mV to reach the current density of 50 mA cm^?2. Furthermore, its electrocatalytic performance towards overall water splitting worked as both cathode and anode achieved a quite low potential of 1.56 V (10 mA cm^?2).     

Bifunctional nanoporous ruthenium-nickel alloy nanowire electrocatalysts towards oxygen/hydrogen evolution reaction

A class of ruthenium-nickel alloy catalysts featured with nanoporous nanowires (NPNWs) were synthesized by a strategy combining rapid solidification with two-step dealloying. RuNi NPNWs exhibit excellent electrocatalytic activity and stability for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in which the RuNi-2500 NPNWs catalyst shows an OER overpotential of 327 mV to deliver a current density of 10 mA cm^?2 and the RuNi-0 NPNWs catalyst requires the overpotential of 69 mV at 10 mA cm^?2 showing the best HER activity in alkaline media. Moreover, the RuNi-1500 NPNWs catalyst was used as the bifunctional electrocatalyst in a two-electrode alkaline electrolyzer for water splitting, which exhibits a low cell voltage of 1.553 V and a long-term stability of 24 h at 10 mA cm^?2, demonstrating that the RuNi NPNWs catalysts can be considered as promising bifunctional alkaline electrocatalysts.     

Multi-active site CoNi-MOFs as non-noble bifunctional electrocatalysts for highly efficient overall water splitting

The high-efficiency non-precious metal catalysts for oxygen evolution (OER) and hydrogen evolution (HER) are of great significance to the development of renewable energy technologies. Herein, a multiple active sites CoNi-MOFs-DBD electrocatalyst modified by low temperature plasma (DBD) was successfully synthesized by converting metal hydroxyfluoride on nickel foam into a well-arranged MOFs array using vapor deposition. The as-prepared CoNi-MOFs-DBD electrode showed better HER and OER catalytic activity, super hydrophilicity, and excellent stability. In an alkaline medium, the overpotential of HER is 203 mV at 10 mA cm^?2 and that of OER is 168 mV at 40 mA cm^?2. When CoNi-MOFs-DBD was used as a bifunctional electrocatalyst for overall water splitting in a two-electrode system, a current density of 10 mA cm^?2 can be achieved at a low voltage of 1.42 V, which shows great potential in electrocatalytic water splitting.     

Hierarchically hollow interconnected rings of nickel substituted cobalt carbonate hydroxide hydrate as promising oxygen evolution electrocatalyst

The present work highlights fabrication of nanostructured nickel-substituted cobalt carbonate hydroxide hydrates (NCCHH) through one-step reflux method. It is noted that optimized 30 mol% nickel-substituted cobalt carbonate hydroxide hydrate (NCCHH-30) nanostructures show quite high specific surface area (～229.55 m² g^?1) owing to the formation of hierarchically hollow interconnected ring-type morphology facilitating the electrode-electrolyte interfacial interaction. As a result, NCCHH-30 showed significant amplification in electrocatalytic oxygen evolution reaction (OER) activity with ultralow overpotential (～141 mV @ 10 mA cm^?2), Tafel slope (～49 mV dec^?1), and excellent durability (12 h and 2000 cycles) in 1.0 M KOH. Notably, to the best of our knowledge, interconnected NCCHH-30 hierarchical hollow rings exhibited the best overpotential (η₁₀₀ ～198 mV) value reported for cobalt-based electrocatalysts in alkaline OER. In addition, this material exhibited exceptionally high oxygen evolution performance in comparison to the state-of-the-art commercial RuO₂ electrocatalyst in 1.0 M KOH. Such interconnected hierarchically hollow nickel (30 mol%)-substituted cobalt carbonate hydroxide hydrate nanostructured rings could act as an ultraefficient, cost-effective, and stable electrocatalyst for OER in alkaline medium.     

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.     

Autogenous growth of highly active bifunctional Ni–Fe2B nanosheet arrays toward efficient overall water splitting

Developing an effective and low-cost bifunctional electrocatalyst for both OER and HER to achieve overall water splitting is remaining a challenge to meet the needs of sustainable development. Herein, an electroless plating method was employed to autogenous growth of ultrathin Ni–Fe₂B nanosheet arrays on nickel foam (NF), in which the whole liquid phase reduction reaction took no more than 20 min and did not require any other treatments such as calcination. In 1.0 M KOH electrolyte, the resulted Ni–Fe₂B ultrathin nanosheet displayed a low overpotential of 250 mV for OER and 115 mV for HER to deliver a current density of 10 mA cm^?2, and both OER and HER activities remained stable after 26 h stability testing. Further, the couple electrodes composed of Ni–Fe₂B could afford a current density of 10 mA cm^?2 towards overall water splitting at a cell voltage of 1.64 V in 1.0 M KOH and along with excellent stability for 26 h. The outstanding electrocatalytic activities can be attributed to the synergistic effect of electron-coupling across Ni and Fe atoms and active sites exposed by large surface area. The effective combination of low cost and high electrocatalytic activity brings about a promising prospect for Ni–Fe₂B nanosheet arrays in the field of overall water splitting.     

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.     

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%.     

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.     

Bifunctional electrocatalysts for water splitting from a bimetallic (V doped-NixFey) Metal–Organic framework MOF@Graphene oxide composite

An ongoing challenge still lies in the exploration of proficient electrocatalysts from earth-abundant non-precious metals instead of noble metal-based catalysts for clean hydrogen energy through large-Scale electrochemical water splitting. However, developing a non-precious transition metals based, stable electrocatalyst for cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER) is important challenge for modern energy conversion technology. In this report Vanadium doped bimetallic nickel-iron nanoarray, fabricated by carbon supported architecture through carbonization process for electrochemical water splitting. Three types of catalysts were prepared in different molar ratio of Ni/Fe. The electrocatalytic performance demonstrated that the catalyst with equal mole ratio (0.06:0.06) of Ni/Fe possess high catalytic activity for both OER and HER in alkaline and acidic medium. Besides, our findings revealed that the doping of vanadium could play a strong synergetic effect with Ni/Fe, which provide a small overpotential of 90 mV and 210 mV at 10 mA cm^?2 for HER and OER respectively compared to the other two catalyst counterparts. Also, the catalyst with 1:1 (Ni/Fe) molar ratio showed a high current density of 208 mA cm^?2 for HER at 0.5 M H₂SO₄ and 579 mA cm^?2 for OER at 1 M KOH solution, the both current densities are much higher than the other two catalysts (different Ni/Fe ratio). In addition, the presented catalysts showed extremely good durability, reflecting in more than 20 h of consistent Chronoamprometry study at fixed overpotential η = 250 mV without any visible voltage elevation. Similarly, the (Ni/Fe) equal ratio catalyst showed better corrosion potential 0.209 V vs Ag/AgCl and lower current density 0.594 × 10^?12 A cm^?2 in high alkaline medium. The V-doping, MOF/GO surface defects are significantly increased the corrosion potential of the V-Ni_xFe_y-MOF/GO electrocatalyst. Besides, the water electrolyzed products were analysed by gas chromatography to get clear insights on the formed H₂ and O₂ products.     

Petal-like NiCoP sheets on 3D nitrogen-doped carbon nanofiber network as a robust bifunctional electrocatalyst for water splitting

Searching high-active, stable and abundant bifunctional catalysts to replace noble metals for hydrogen and oxygen evolution reactions (HER and OER) is desired. Herein, petal-like NiCoP sheets were synthesized on carbon paper covered with a 3D nitrogen-doped carbon nanofiber network (NiCoP/CNNCP) by a simple hydrothermal process followed by phosphorization. The HER overpotential in 0.5 M H₂SO₄ and OER overpotential in 1 M KOH of the NiCoP/CNNCP electrode only required 55 mV and 260 mV to drive a current density of 10 mA cm^?2, respectively, which was comparable or even better than most nickel-and cobalt-based phosphide catalysts. The overall water-splitting electrolyzer with an asymmetric electrolyte system assembled using NiCoP/CNNCP as bifunctional electrodes required an extremely low cell voltage of 1.04 V to achieve a current density of 10 mA cm^?2, which was much lower than almost all alkaline electrolysis systems.     

Nickel foam-supported Fe,Ni-Polyporphyrin microparticles: Efficient bifunctional catalysts for overall water splitting in alkaline media

Developing highly active, stable and sustainable electrocatalysts for overall water splitting is of great importance to generate renewable H₂ for fuel cells. Herein, we report the synthesis of electrocatalytically active, nickel foam-supported, spherical core-shell Fe-poly(tetraphenylporphyrin)/Ni-poly(tetraphenylporphyrin) microparticles (FeTPP@NiTPP/NF). We also show that FeTPP@NiTPP/NF exhibits efficient bifunctional electrocatalytic properties toward both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). Electrochemical tests in KOH solution (1 M) reveal that FeTPP@NiTPP/NF electrocatalyzes the OER with 100 mA cm⁻² at an overpotential of 302 mV and the HER with 10 mA cm⁻² at an overpotential of 170 mV. Notably also, its catalytic performance for OER is better than that of RuO₂, the benchmark OER catalyst. Although its catalytic activity for HER is slightly lower than that of Pt/C (the benchmark HER electrocatalyst), it shows greater stability than the latter during the reaction. The material also exhibits electrocatalytic activity for overall water splitting reaction at a current density of 10 mA cm⁻² with a cell voltage of 1.58 V, along with a good recovery property. Additionally, the work demonstrates a new synthetic strategy to an efficient, noble metal-free-coordinated covalent organic framework (COF)-based, bifunctional electrocatalyst for water splitting.     

Flower-shaped and sea urchin-shaped structures coexisting in cobalt-based phosphate and oxides achieve efficient electrochemical overall water electrolysis

Active site engineering for electrocatalysts is an essential strategy to improve their intrinsic electrocatalytic capability for practical applications and it is of great significance to develop a new excellent electrocatalyst for overall water splitting. Here, Co₃O₄/nickel foam (NF) and Co₂(P₄O₁₂)/NF electrocatalysts with flower-shaped and sea urchin-shaped structures are synthesized by a simple hydrothermal process and followed by a post-treatment method. Among them, Co₂(P₄O₁₂)/NF shows good catalytic activity for hydrogen evolution reaction (HER), and at the current density of 10 mA cm^?2, the overpotential is only 113 mV Co₃O₄/NF exhibits good catalytic activity for oxygen evolution reaction (OER), and the overpotential is 327 mV at 20 mA cm^?2. An alkaline electrolyzer with Co₃O₄/NF and Co₂(P₄O₁₂)/NF catalysts respectively as anode and cathode displays a current density of 10 mA cm^?2 at a cell voltage of 1.59 V. This work provides a simple way to prepare high efficient, low cost and rich in content promising electrocatalysts for overall water splitting.     

Cobalt phosphide nanoparticles embedded in 3D N-doped porous carbon for efficient hydrogen and oxygen evolution reactions

An electrocatalyst based on a unique three-dimensional (3D) N-doped porous carbon sheet networks embedded with CoP₂ nanoparticles (CoP₂@3D-NPC) was synthesized by a facile pyrolysis process as well as an in-situ phosphatization method. The improved CoP₂@3D-NPC hybrid materials show excellent electrocatalytic activity toward HER and OER. This material provides a low overpotential of 126 mV at 10 mA cm⁻² in 0.5 M H₂SO₄ and 167 mV at 20 mA cm⁻² in 1.0 M KOH for HER with a small Tafel slope value of 59 mV dec⁻¹, respectively. Besides, it is also active for the OER under alkaline conditions. Such a prominent property of the CoP₂@3D-NPC electrocatalyst could be attributed to its excellent electrical conductivity of 3D carbon substrate, strong synergistic effect between CoP₂ nanoparticles and carbon nanosheet as well as extra active sites created by the N-doped structure.     

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.     

Cadmium sulfide embedded Prussian Blue as highly active bifunctional electrocatalyst for water-splitting process

Hydrogen production by water-splitting has limited commercial application as substantial amount of energy is required for the favorable kinetics of the process. We present an interface engineering strategy for constructing a bifunctional electrode material for an efficient water splitting process. Designed cadmium sulphide and Prussian blue nanorods (CdS-NRs@PBNPs) heterostructures acts as bifunctional electrocatalyst improved water splitting performance, for both HER and OER. For HER, the optimized hybrid CdS-NRs@PBNPs (1:1) showed significantly a low overpotentials of 126 mV and 181 mV at current densities of 10 mA cm^?2 and 20 mA cm^?2 respectively. For OER it displays an overpotential of 250 mV and 316 mV at current densities of 10 mA cm^?2 and 20 mA cm^?2. Additionally, the CdS-NRs@PBNPs (1:1) has demonstrated long-term stability. The hybrid's enhanced OER and HER activity is attributable to a synergetic impact between CdS-NRs and PBNPs, as well as the active site modification due to the presence of Cadmium and iron in the hybrid.     

Self-standing and efficient bifunctional electrocatalyst for overall water splitting under alkaline media enabled by Mo1-xCoxS2 nanosheets anchored on carbon fiber paper

The layered MoS₂ nanostructures have been widely used in the electrochemical hydrogen evolution reaction (HER), but rarely applied in overall water splitting application for their ignorable oxygen evolution reaction (OER) activity. To address this issue, a novel self-standing and bifunctional electrocatalyst, consisting of Co-doped MoS₂ nanosheets anchored on carbon fiber paper, has been prepared via hydrothermal method. Taking advantage of conductive substrate of carbon fiber paper, sufficient-exposed active edges of MoS₂ sheets, and metallic character caused by Co-doping, our electrode exhibits high-efficient bifunctional activities for the overall water splitting in alkaline electrolyte (1 M KOH), which can produce a current density of 20 mA cm⁻² at an overpotential of 197 mV for HER and 235 mV for OER.     

Facile fabrication of flower-like CuS/MnCO3 microspheres clusters on nickel foam as an efficient bifunctional catalyst for overall water splitting

Utilizing the abundant elements on earth to product inexpensive, high-active and stable catalysts for water splitting is very significant but still remains serious challenge to produce hydrogen. Herein, heterostructures of CuS/MnCO₃ on nickel foam substrate are firstly successfully synthesized via a facile one-step hydrothermal strategy. The as-prepared electrocatalyst displays an enhanced oxygen evolution reaction (OER) performance in alkaline conditions with a minimum overpotential of 70 mV and a small Tafel slope of 42.5 mV/dec to achieve 10 mA cm^?2. The catalyst also exhibits an excellent HER activity with a low overpotential of 143 mV and the Tafel slope of 51.4 mV/dec to acquire 10 mA cm^?2 in 1.0 M KOH. Moreover, when the CuS/MnCO₃//CuS/MnCO₃ electrode is applied for the overall water splitting, the electrolyzer cell device affords 10 mA cm^?2 at a relative low voltage of 1.43 V, which is one of the best catalysts ever reported. In stability test, its activity first decreases and then remains stable in 1 M KOH solution for about 10 h, indicating that the electrode has good electrochemical stability. Density functional theory calculations (DFT) show that MnCO₃ has a stronger adsorption energy for water than CuS does, indicating that MnCO₃ is a real active center and CuS plays a certain synergistic effect. This work not only provides a low-cost and efficient bifunctional catalyst for water splitting technology, but also extends the application of bifunctional catalyst based on transition metal sulfide and carbonate compound.     

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.     

MoS2/NiS heterostructure grown on Nickel Foam as highly efficient bifunctional electrocatalyst for overall water splitting

It is great important to develop and explore a non-precious bifunctional electrocatalyst with high efficiency and good stability for Hydrogen Evolution Reaction (HER) and Oxygen Evolution Reaction (OER) in alkaline electrolyte. Herein, a three-dimensional (3D) needle-like MoS₂/NiS heterostructure supported on Nickel Foam (NF) (MoS₂/NiS/NF) is synthesized by a simple hydrothermal method for the first time, which can act as a good bifunctional electrocatalyst for overall water splitting. As expected, the optimal MoS₂/NiS/NF exhibits excellent catalytic performance with a low overpotential of 87 and 216 mV at 10 mA cm⁻² for HER and OER in 1 M KOH electrolyte, respectively, accompanied by good cycle stability. Furthermore, the MoS₂/NiS/NF as bifunctional electrocatalyst in an electrolyzer shows high efficiency with a cell voltage of 1.5 V at 10 mA cm⁻², as well as superior durability. The present work may open a new direction to design and develop a non-precious bifunctional electrocatalyst with excellent catalytic activity for water splitting in the future.