Tan Dang,Lei Wang,Donghai Wei,Guiqing Zhang,Qinggang Li,Xiaojia Zhang,Zuoying Cao,Guanhua Zhang,Huigao Duan
Low-cost and resourceful transition metal phosphides (TMPs), as a new kind of pseudocapacitive materials, have received considerable interest for electrochemical energy storage/conversion. Combination of TMPs and carbon nanostructures is supposed to greatly improve the charge storage capability due to the synergistic effect. In this work, nickel cobalt phosphides nanosheets supported by P doped C spheres (P-CSs@Ni-Co-P NSs) composites with different Ni/Co molar ratios (1:0, 1:1, 1:2, 1:4 and 0:1) were prepared by in-situ bifunctional phosphorization of the as-synthesized CSs@Ni-Co-OH NSs precursor. The optimized 3D networked mesoporous P-CSs@Ni 1 -Co 2 -P NSs electrode exhibits a remarkable specific capacitance of 1040.3 F g −1 at 1 A g −1 and excellent rate capability with a capacitance retention of 49.2% even at 30 A g −1 , which are much higher than those of A-CSs@Ni 1 -Co 2 -O NSs (870.5 F g −1 at 1 A g −1 , 37.4% at 30 A g −1 ) and Ni 1 -Co 2 -P NSs (500.5 F g −1 at 1 A g −1 , 40.0% at 30 A g −1 ). The improved supercapacitive performances are attributed to the 3D mesoporous nanosheets network and bifunctional phosphorization treatment which lead to enhancement of conductivity. Meanwhile, Ni-Co bimetallic phosphide with abundant oxidation states facilitates redox charge transfer efficiently. Furthermore, P-CSs@Ni 1 -Co 2 -P NSs assembled asymmetric supercapacitors (ASC) achieve outstanding cycling stability with no decay of capacitance after 20000 cycles. Additionally, the charge storage kinetic of the device is further investigated, indicating that fast surface-control capacitive behavior is dominant at high current densities.
As one class of important functional materials, transition metal phosphides (TMPs) nanostructures show promising applications in catalysis and energy storage fields. Although great progress has been achieved, phase-controlled synthesis of cobalt phosphides nanocrystals or related nanohybrids remains a challenge, and their use in overall water splitting (OWS) is not systematically studied. Herein, three kinds of cobalt phosphides nanocrystals encapsulated by P-doped carbon (PC) and married with P-doped graphene (PG) nanohybrids, including CoP@PC/PG, CoP-Co 2 P@PC/PG, and Co 2 P@PC/PG, are obtained through controllable thermal conversion of presynthesized supramolecular gels that contain cobalt salt, phytic acid, and graphene oxides at proper temperature under Ar/H 2 atmosphere. Among them, the mixed-phase CoP-Co 2 P@PC/PG nanohybrids manifest high electrocatalytic activity toward both hydrogen and oxygen evolution in alkaline media. Remarkably, using them as bifunctional catalysts, the fabricated CoP-Co 2 P@PC/PG || CoP-Co 2 P@PC/PG electrolyzer only needs a cell voltage of 1.567 V for driving OWS to reach the current density at 10 mA cm −2 , superior to their pure-phase counterparts and recently reported bifunctional catalysts based devices. Also, such a CoP-Co 2 P@PC/PG || CoP-Co 2 P@PC/PG device exhibits outstanding stability for OWS. This work may shed some light on optimizing TMPs nanostructures based on phase engineering, and promote their applications in OWS or other renewable energy options.
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