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.     

Controlled synthesis of three-dimensional branched Mo–NiCoP@NiCoP/NiXCoYH2PO2 core/shell nanorod heterostructures for high-performance water and urea electrolysis

It is necessary to design reasonably efficient bifunctional electrocatalyst, but it is still a difficult problem for the water and urea electrolysis. Therefore, we firstly constructed a novel Mo–NiCoP@NiCoP/Ni_XCo_YH₂PO₂ (MNCP@NCP/Ni_XCo_YH₂PO₂) core/shell nanorod heterostructure by hydrothermal and two-step phosphating on nickel foam (NF). It is worth noting that Mo-doping could availably regulate the electronic structure of NiCoP(NCP), resulting in the increased exposure of the active center and the increased inherent activity of each site. Furthermore, a strategy of improving catalyst activity was proposed, that is, the NiCoP nanorod core and Mo–NiCoP/Ni_XCo_YH₂PO₂ nanorod shell was constructed by the two phosphating reactions to come into being mixed transition-metal phosphides (TMPs), thus improving the synergistic catalytic effect of the material. In addition, the water and urea electrolysis apparatus was installed from two MNCP@NCP/Ni_XCo_YH₂PO₂ electrodes to actuate a current density of 10 mA cm^?2, the necessary cell voltage was merely 1.348 V in 1.0 M KOH with 0.5 M urea for urea electrolysis, while the higher 1.522 V of cell voltage was required in 1.0 M KOH for water electrolysis, which is one of the best catalytic activities reported so far. Experimental results show that the oxyhydroxide is the real active site during urea electrolysis process. Density functional theory calculation shows that the doping of Mo and Co increase the water adsorption energy and conductivity of the oxyhydroxide material, so the water splitting performance of the catalyst is improved. Therefore, this work provided a new way to design bifunctional electrocatalysts by Mo-doping and two-step phosphating process.     

P doped NiCoZn LDH growth on nickel foam as an highly efficient bifunctional electrocatalyst for Overall Urea-Water Electrolysis

Water electrolysis is one of the important methods for hydrogen production, but the oxygen evolution reaction (OER) on the anode has a higher theoretical potential. Using urea oxidation reaction (UOR) instead of OER has been an energy-saving method because it has a lower theoretical potential and also can reduce pollution. In this paper, NiCoZn LDH/NF, P–NiCoZn LDH/NF-X (X is the atom ratio of P) = 10%, 15%, 20%) were synthesized through typically hydrothermal and calcination methods. P–NiCoZn LDH/NF-15% was demonstrated to be abifunctional electrocatalyst towards HER and UOR. When P–NiCoZn LDH/NF-15% is used as the anode and cathode for urea-water electrolysis,it shows that when the current density is 100 mA cm^?2, the voltage is 1.479 V for urea-water electrolysis, which is much better than that of IrO₂/NF||Pt/C/NF (1.698 V). Thus, P–NiCoZn LDH/NF-15% is expected to replace precious metals for practical applications.     

An alkaline water electrolyzer with nickel electrodes enables efficient high current density operation

Low-temperature industrial water electrolysis is typically conducted using either liquid alkaline electrolytes or acidic polymer electrolyte membranes (PEMs). The latter approach is considered to be more efficient but also more expensive as it requires Pt and Ir based catalysts. This study reports on an alkaline water electrolyzer with Ni electrodes that operates at a current density of 2 A/cm² with a cell voltage of 1.85 V, which provides a comparable voltage-current characteristic to the state-of-the-art PEM water electrolyzers. Thin Ni mesh electrodes with surface areas that are thousand times higher than the geometric area were manufactured by an easily scalable and cheap process, i.e. metallurgical hot dip galvanization with subsequent de-alloying. With a thin porous polymer of approximately $140\text{μm}$ as the diaphragm a low cell resistance of 0.11 Ω cm^?2 was obtained.     

One-step synthesis of amorphous Ni–Fe–P alloy as bifunctional electrocatalyst for overall water splitting in alkaline medium

Design of inexpensive and highly efficient bifunctional electrocatalyst is paramount for overall water splitting. In this study, amorphous Ni–Fe–P alloy was successfully synthesized by one-step direct-current electrodeposition method. The performance of Ni–Fe–P alloy as a bifunctional electrocatalyst toward both hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) was evaluated in 30 wt% KOH solution. It was found that Ni–Fe–P alloy exhibits excellent HER and OER performances, which delivers a current density of 10 mA cm^?2 at overpotential of ～335 mV for HER and ～309 mV for OER with Tafel slopes of 63.7 and 79.4 mV dec^?1, respectively. Moreover, the electrolyzer only needs a cell voltage of ～1.62 V to achieve 10 mA cm^?2 for overall water splitting. The excellent electrocatalytic performance of Ni–Fe–P alloy is attributed to its electrochemically active constituents, amorphous structure, and the conductive Cu Foil.     

Crossed FeCo2S4 nanosheet arrays grown on 3D nickel foam as high-efficient electrocatalyst for overall water splitting

Great efforts in developing low-cost, highly efficient and stable electrocatalysts are to tune the chemical compositions and morphological characteristics for enhancing efficiency of water splitting. In this communication, FeCo₂S₄ nanosheet was grown in situ on nickel foam (FeCo₂S₄/NF) via a facile hydrothermal sulfidization method and served as a high-efficient bifunctional electrocatalyst for overall water splitting. As-synthesized FeCo₂S₄/NF self-supported electrode delivers 20 mA cm^?2 at an overpotential of 259 mV toward OER and 10 mA cm^?2 at an overpotential of 131 mV toward HER in alkaline media. Moreover, when used as both anode and cathode in a two-electrode electrolyzer, only a small cell voltage of 1.541 V is needed to afford a current density of 10 mA cm^?2 for overall water splitting. Bifunctional electrode FeCo₂S₄/NF also revealed a distinguished electrochemical durability during a 12 h stability test at 1.63 V, which would provide a promising water splitting installation for commercial hydrogen production.     

Evidencing enhanced oxygen and hydrogen evolution reactions using In–Zn–Co ternary transition metal oxide nanostructures: A novel bifunctional electrocatalyst

Ternary transition metal oxides are gaining popularity for cost effective bifunctional electrocatalytic activities and to realization of novel water splitting devices. In this regard, In₂O₃/ZnO/Co₃O₄ based ternary oxide nanostructures were investigated in detail for their oxygen/hydrogen evolution reaction (OER/HER) in alkaline environment. The ternary oxides were at first processed through a simple chemical route involving hydrothermal treatment. The prepared nanostructures were then investigated by using high-resolution transmission electron microscopy (TEM/HRTEM) to ascertain their morphological traits. X-ray diffraction, Raman signals and photoluminescence data demonstrated the In₂O₃ phase to be prevalent in the ternary mixture on par with that of ZnO and Co₃O₄. The valence state of various metal ions and the In–O, Zn–O and Co–O bonding was verified using XPS. The ternary oxide coated electrodes exhibited excellent overall water splitting activity. Overpotential values of 398 and 510 mV were registered for OER and HER experiments under a current density of ±10 mA cm⁻², demonstrating the material to be an ideal OER/HER electrocatalyst at room temperature. The exceptional long-term stability in ternary oxides and their Tafel slope (88 mV/dec for OER and 60 mV/dec for HER) further affirmed their unique anodic/cathodic characteristics for water splitting applications.     

Deposited RuO2–IrO2/Pt electrocatalyst for the regenerative fuel cell

A bifunctional RuO₂–IrO₂/Pt electrocatalyst for the unitized regenerative fuel cell (URFC) was synthesized by colloid deposition and characterized by analytical methods like TEM, XRD, etc. The result reveals that RuO₂–IrO₂ was well dispersed and deposited on the surface of Pt black. With deposited RuO₂–IrO₂/Pt as the catalyst of oxygen electrode, the performance of fuel cell/water electrolysis of unitized regenerative fuel cell (URFC) was studied in detail. URFC with deposited RuO₂–IrO₂/Pt shows better performance than that of URFC with mixed RuO₂–IrO₂/Pt catalyst. Cyclic performance of URFC with deposited RuO₂–IrO₂/Pt is very stable during 10 cyclic tests.