Three-Dimensional Mesoporous Phosphide–Spinel Oxide Heterojunctions with Dual Function as Catalysts for Overall Water Splitting

Abstract

Highly efficient electrocatalysts for oxygen evolution (OER) and hydrogen evolution reactions (HER) are critical in the development of efficient and sustainable alternative energy. Toward this goal, we report on the nanocasting synthesis of a three-dimensionally (3D) mesoporous CoP/CoCr2O4 heterojunction as efficient bifunctional electrodes for use in overall water splitting in alkaline media. The phosphorization of mesoporous Co2CrO4 nanocast from a KIT-6 hard silica template leads to phase transformation and separation, spontaneously forming a heterojunction between CoP and spinel CoCr2O4 with attractive nanostructured properties (large surface area of 83 m2/g with 4.8 nm diameter accessible mesopores). This phosphide–spinel oxide heterojunction catalyst yields excellent catalytic activity for both OER and HER with overpotentials of only 290 and 212 mV, respectively, to achieve the benchmark current density of 10 mA cm–2 in a 1.0 M KOH electrolyte. Moreover, CoP/CoCr2O4 shows small Tafel slopes with outstanding durability and long-life stability in alkaline media. Finally, the bifunctionality of CoP/CoCr2O4 for both OER and HER was shown by producing symmetric electrodes in an alkaline water electrolyzer, resulting in a low cell voltage of 1.68 V with 24 h durability, making it among the best Co-based electrocatalysts for overall water splitting. Such bifunctionality suggests a cost-effective heterojunction-based system for water electrolysis using inexpensive earth-abundant metals. The simple route to achieving an effective bifunctional material, which can be applied to various transition-metal-based homo- or heterojunctions, offers a paradigm for templated bifunctional electrode materials.

Comparison of the effects of cation and phosphorus doping in cobalt-based spinel oxides towards the oxygen evolution reaction

“…3b indicated that the main form of surface P was phosphate or phosphite (denoted as P-O at 133.65-134 eV) with a trivial amount of phosphide (denoted as P-metal at 129.4-129.8 eV). [34][35][36] Therefore, we concluded that most P incorporated into the spinel structure simply as heteroatoms without generating a relevant crystalline phase, in accordance with the XRD patterns. Fig.…”

“…6c, S5d and e, † the O 1s energy level was deconvoluted into 3 peaks associated with oxygen in the crystalline lattice (O1), oxygen vacancies (oxygen species adsorbed on these oxygen-deficient sites, O2) and surface adsorbed water from the thermal decomposition of NaH 2 -PO 2 •H 2 O 28,45,46 and/or carbonates (O3). 5,12,34,47 Evidently, the percentage of O1 at 529.4-530.2 eV sharply decreased, indicating the weakened M-O bonds as well as destruction of the pristine crystalline structure on the surface after incorporation of P. 15 Taking P-ZNMC as an example, the O1 even disappeared after deconvolution (Fig. 6c), although obvious crystalline phases of spinel and CoO were observed from the XRD pattern (Fig.…”

“…6a and S5a and b † and Table 2), oxygen vacancies were inclined to appear conforming to the charge balance. All of the above structural changes accounted for the generation of ample active sites to optimize the charge transfer, adsorption and activation energy of intermediates 3,5,34 for higher electrochemical performance.…”

Mesoporous Ternary Nitrides of Earth-Abundant Metals as Oxygen Evolution Electrocatalyst

“…4d), the smallest semicircle for Ni-Fe nitride suggests the lowest charge-transfer resistance for OER at an overpotential of 1.5 V compared with the other mesoporous catalysts, which is consistent with the superior OER activity and smaller Tafel slope. Double-layer capacitance (C dl ) measurements were performed to evaluate the electrochemically active surface area (ECSA) of the mesoporous Ni 3 FeN catalyst through electrical double-layer capacitance (EDLC) measurements in 1 M KOH electrolyte [56,58,59]. The charging currents were collected at different scan rates (20,40,60,80, and 100 mV s -1 ) as shown in Fig.…”

Sc Modification Induced Short-range Ordering in ZnGa2O4 Spinel Leading to High Microwave Dielectric Performance

“…Spinel-type compounds AB2O4 have been extremely investigated in science and technical fields such as phosphor [1][2], catalysts [3][4], semiconductors [5][6], superconductors [7][8] and microwave dielectric materials [9][10][11][12]. Among these applications, microwave dielectric materials in high frequency (5G) is one of the most important applications with wide investigations.…”