Heterostructural Co/CeO2/Co2P/CoP@NC dodecahedrons derived from CeO2-inserted zeolitic imidazolate framework-67 as efficient bifunctional electrocatalysts for overall water splitting

The design and fabrication of highly active, robust and cost-efficient electrocatalysts for both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is of great significance towards overall water splitting, but remains challenging as well. Herein, we report, for the first time, heterostructural Co/CeO₂/Co₂P/CoP@NC dodecahedrons as bifunctional electrocatalyst, in which abundant interfaces are formed between different components. Typical ZIF-67 (ZIF = zeolitic imidazolate framework) dodecahedrons with pre-inserted CeO₂ nanowires were selected as precursors to synthesize Co/CeO₂/Co₂P/CoP@NC via a direct carbonization process followed by phosphidation, simultaneously generating the strong coupled heterojunction interfaces through interactions between CeO₂ and Co_xP species. Abundant porous structure leads to the exposure of more active sites and the carbon encapsulation of nanodomains sustains the high robustness and conductivity and the synergistic effect between the multi-components heterostructure. Benefiting from the above collective advantages, the Co/CeO₂/Co₂P/CoP@NC electrocatalysts exhibit small overpotentials of 307 and 195 mV to derive 10 mA cm⁻² for OER and HER, respectively. Furthermore, an alkaline electrolyzer assembled by using Co/CeO₂/Co₂P/CoP@NC as both cathode and anode can achieve a current density of 10 mA cm⁻² at a low voltage of 1.76 V and work continuously for over 15 h. This work would provide a rational protocol for fabrication multi-phase interface enriched electrocatalysts toward highly efficient energy conversion.     

One-pot solvothermal synthesis of Co2P nanoparticles: An efficient HER and OER electrocatalysts

Development of an inexpensive electrocatalyst for hydrogen evolution (HER) and oxygen evolution reactions (OER) receives much traction recently. Herein, we report a facile one-pot ethyleneglycol (EG) mediated solvothermal synthesis of orthorhombic Co₂P with particle size ~20–30 nm as an efficient HER and OER catalysts. Synthesis parameters like various solvents, temperatures, precursors ratios, and reaction time influences the formation of phase pure Co₂P. Investigation of Co₂P as an electrocatalyst for HER in acidic (0.5 M H₂SO₄) and alkaline medium (1.0 M KOH), furnishes low overpotential of 178 mV and 190 mV, respectively to achieve a 10 mA cm^?2 current density with a long term stability and durability. As an OER catalyst in 1.0 M KOH, Co₂P shows an overpotential of 364 mV at 10 mA cm^?2 current density. Investigation of Co₂P NP by XPS analysis after OER stability test under alkaline medium confirms the formation of amorphous cobalt oxyhydroxide (CoOOH) as an intermediate during OER process.     

Spherical Co/Co9S8 as electrocatalyst for hydrogen production from alkaline solution and alkaline seawater

Cobalt-based sulfide catalysts are considered as potential materials for electrocatalytic hydrogen production from seawater. Here, we have successfully prepared a Co/Co₉S₈ electrocatalyst with hollow spherical structure. As-prepared material exhibited excellent electrocatalytic activity in hydrogen evolution reaction (HER) in alkaline seawater. The overpotentials for Co/Co₉S₈ in alkaline seawater were measured as low as 136.2 mV, when reached a current density of 10 mA cm^{− 2}. It also had good stability and could be maintained for 24 h in 1.0 M KOH and alkaline seawater. The results of SEM and TEM confirmed that the catalyst had excellent reaction structure. Due to the hollow structure, Co/Co₉S₈ showed remarkable catalytic performance for HER. The construction method of Co/Co₉S₈ hollow structure is an effective strategy to improve the performance of HER for seawater splitting.     

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.     

The Mo as electron trapper and donor to modulate the electron of CoP2 in phosphating process

To prepare bifunctional electrocatalyst towards HER and OER is extremely important for promoting the development of electrochemical water splitting technology. Herein, the element doping method is employed to tune the electron environment of cobalt phosphide (CoP). The Mo-doped CoP supported on carbon cloth (CC) is constructed by solvothermal and annealing method. The effect of Mo on the electron modulation of CoP during different phosphating time was studied carefully. It is noted that the Mo play an important role in tuning the electron state of Co and P elements which can trap the electron and was reduced to low valence, then transfer the electron to Co and P. With increasing the phosphating time, the electron transfer phenomenon between Mo and CoP is obvious. Benefiting from the electron engineering of Mo, Co and P as well as thin and wrinkle sheets structure, the optimal electrocatalyst only requires 39 mV and 251 mV to deliver 10 mA cm⁻² for HER and OER, respectively. Also, as for the whole water splitting performance, it delivers 10 mA cm⁻² at cell voltage of 1.56 V. Importantly, Faraday efficiency of the optimal catalyst achieves 99.9% for HER due to the tuned electron state of Co and P, high ECSA and low R_ct.     

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%.     

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.     

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.     

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.     

Structural engineering of hierarchically hetestructured Mo2C/Co conformally embedded in carbon for efficient water splitting

Developing low-cost and high efficient electrocatalysts for both oxygen and hydrogen evolution reaction in an alkaline electrolyte toward overall water splitting is still a significant challenge. Here, a novel hierarchically heterostructured catalyst composed of ultrasmall Mo₂C and metallic Co nanoparticles confined within a carbon layer is produced by a facile phase separation strategy. During thermal reduction of CoMoO₄ nanosheets in CO ambient, in-situ generated nanoscale Co and ultrafine Mo₂C conformally encapsulated in a conductive carbon layer. In addition, some carbon nanotubes catalyzed by Co nanoparticles vertically grew on its surface, creating 3D interconnected electron channels. More importantly, the integrated C@Mo₂C/Co nanosheets assembled into the hierarchical architecture, providing abundant active surface and retaining the structural integrity. Benefiting from such unique structure, the constructed hierarchical heterostructure shows low overpotentials of 280 mV and 145 mV to reach a current density of 10 mA cm⁻² for OER and HER in an alkaline electrolyte. Furthermore, the symmetrical electrolyzer assembled with catalyst exhibits a small cell voltage of 1.67 V at 10 mA cm⁻² in addition to outstanding durability, demonstrating the great potential as a high efficient bifunctional electrocatalyst for overall water splitting.     

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.     

Quick-to-synthesize hybrid electrodes composed of activated carbon cloth with Co/Fe-based films as bifunctional electrocatalysts for water splitting

Hybrid electrodes have recently been investigated as attractive alternatives to noble-metal-based electrocatalysts for hydrogen production by water splitting. Herein, we propose an electrode composed of an oxidized carbon cloth with an electrodeposited bimetallic Co/Fe-based film. By optimizing the electrodeposition conditions and applying electrochemically activated carbon cloth as a substrate, one can prepare a free-standing noble-metal-free electrocatalytic electrode with high bifunctional electrocatalytic activity in hydrogen and oxygen evolution from alkaline solution. The developed Fe_0.25Co_0.75 electrode requires overpotentials of 245 mV for HER and 360 mV for OER at high current densities of −100 and 100 mA cm⁻², respectively. Furthermore, its overall synthesis time from commercially available raw materials is only approximately 20 min. The electrode material was used as both a cathode and an anode in the model electrolyzer, which can deliver 10 mA cm⁻² of current density at 1.66 V without loss of activity during 100 h of performance.     

Constructing microstructures in nickel-iron layered double hydroxide electrocatalysts by cobalt doping for efficient overall water splitting

The development of Ni–Fe layered double hydroxide (NiFe LDH) catalysts for overall water splitting (OWS) is urgently required. NiFe LDHs are promising catalysts for the oxygen evolution reaction (OER). However, their hydrogen evolution reaction (HER) performance is restricted by slow kinetics. The construction of multiple types of active sites to simultaneously optimise the OER and HER performance is significant for OWS using NiFe LDHs. Hence, a Co-doped NiFe LDH electrocatalyst with dislocations and stacking faults was designed to modulate the electronic structure and generate multiple types of activity sites. The Co_0.03-NiFe_0.97 LDH catalyst only required overpotentials of 280 (50 mA cm⁻², OER) and 170 mV (10 mA cm⁻², HER). However, it reached a current density of 50 mA cm⁻² at 1.53 V during OWS. Co_0.03-NiFe_0.97 LDHs could be stabilised for 140 h at 1.52 V. Furthermore, Co_0.03-NiFe_0.97 LDHs exhibited a higher electrocatalytic activity than commercial Raney nickel and Pt/C||IrO₂ under industrial conditions. The significant specific surface area, high conductivity, and unique microstructures are the major factors contributing to the excellent OWS performance. This study suggests an efficient strategy for introducing microstructures to fabricate catalysts with high activity for application in OWS.     

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.     

Porous Co2P film coated on carbon fiber as highly performance electrocatalyst toward overall water splitting

Designing porous active materials and enhancing their contact with conductive substrates is an effective strategy to improve electrolytic water splitting performance of noble metal-free catalysts. Herein, a facile nanostructured electrode, composed of porous Co₂P films coated on carbon fiber (CF@P–Co₂P), is designed and prepared. The unique three-dimensional interconnected pore structure of Co₂P and the close contact between porous Co₂P and CF not only increase specific surface areas to expose abundant catalytic sites but also stimulate the transport of electron, mass and gaseous products in catalytic process. Benefit from the reasonable electrode structure, the self-supported CF@P–Co₂P electrodes present perfect performance with only needing overpotentials of 107.7/175.5/141.8 mV for hydrogen evolution reaction (HER) in acidic/neutral/alkaline solution and 269.4 mV for oxygen evolution reaction (OER) in alkaline solution to get current density of 20 mA cm^?2. In addition, alkaline electrolyzer equipped with CF@P–Co₂P bifunctional electrodes only needs a cell voltage of 1.657 V to get water-splitting current density of 20 mA cm^?2. Even better, the electrolyzer can continuously electrolyze over 50 h with negligibly decreasing current density and the Faraday efficiency is close to 100% toward both HER and OER.     

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.     

Co0.85Se on three-dimensional hierarchical porous graphene-like carbon for highly effective oxygen evolution reaction

Designing an efficient, cheap and abundant catalyst for oxygen evolution reaction (OER) is crucial for the development of sustainable energy sources. A novel catalyst which could be a promising candidate for such electrocatalysts is described. Co_0.85Se supported on three-dimensional hierarchical porous graphene-like carbon (HPG) exhibits outstanding catalytic performances for OER in alkaline medium. It is found that the onset overpotential is 311 mV on the Co_0.85Se/HPG electrode, which is more 28 and 41 mV negative than that on the Co/HPG and Co₃O₄/HPG electrodes. What's more, the value of Tafel slope is 61.7 mV dec⁻¹ and the overpotential at the current density of 10 mA cm⁻² is 385 mV on this electrode. The Co_0.85Se/HPG of this work is an appealing electrocatalyst for OER in basic electrolyte.     

Efficient overall water splitting over a Mo(IV)-doped Co3O4/NC electrocatalyst

To meet the demand of producing hydrogen at low cost, a molybdenum (Mo)-doped cobalt oxide (Co₃O₄) supported on nitrogen (N)-doped carbon (x%Mo–Co₃O₄/NC, where x% represents Mo/Co molar ratio) is developed as an efficient bifunctional electrocatalyst for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). This defect engineering strategy is realized by a facile urea oxidation method in nitrogen atmosphere. Through X-ray diffraction (XRD) refinement and other detailed characterizations, molybdenum ion (Mo⁴⁺) is found to be doped into Co₃O₄ by substituting cobalt ion (Co²⁺) at tetrahedron site, while N is doped into carbon matrix simultaneously. 4%Mo–Co₃O₄/NC is the optimized sample to show the lowest overpotentials of 91 and 276 mV to deliver 10 mA cm^?2 for HER and OER in 1 M potassium hydroxide solution (KOH), respectively. The overall water splitting cell 4%Mo–Co₃O₄/NC||4%Mo–Co₃O₄/NC displays a voltage of 1.62 V to deliver 10 mA cm^?2 in 1 M KOH. The Mo⁴⁺ dopant modulates the electronic structure of active cobalt ion (Co³⁺) and boosts the water dissociation process during HER, while the increased amount of lattice oxygen and formation of pyridinic nitrogen due to Mo doping benefits the OER activity. Besides, the smaller grain size owing to Mo doping leads to higher electrochemically active surface area (ECSA) on 4%Mo–Co₃O₄/NC, resulting in its superior bifunctional catalytic activity.     

Urchin-like Co0.8-Mn0.2-P/CC nanowires array: a high-performance and cost-effective hydrogen evolution electrocatalyst

Using cost-effective materials to replace precious Pt-based hydrogen evolution reaction (HER) catalysts holds great foreground for energy saving and environmental protection. In this work, we successfully prepared an urchin-like Co_0.8-Mn_0.2-P nanowires array supported on carbon cloth (CC) through a hydrothermal-phosphatization strategy and we also systematically studied its electrocatalytic HER performance. Electrochemical tests demonstrate that our urchin-like Co_0.8-Mn_0.2-P/CC possesses outstanding HER activity in acidic and alkaline media. In 0.5 M H₂SO₄, this urchin-like Co_0.8-Mn_0.2-P/CC only requires an overpotential of 55 mV to drive a current density of 10 mA cm⁻², with the Tafel slope of 55.9 mV dec⁻¹. Similarly, when reaching the same current density, just a particularly low overpotential of 61 mV is required with a corresponding Tafel slope of 41.7 mV dec⁻¹ in 1 M KOH. Furthermore, this electrocatalyst exhibits superior stability with 1000 cycles of cyclic voltammetry and 24 h in the I-T test. Such excellent HER catalytic performance can be attributed to the synergistic effect between Co and Mn atoms and high electrochemical active surface area (ECSA). Our work provides a valuable synthesis strategy of non-precious and high HER performance catalytic material.     

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.