Trapping [PMo₁₂O₄₀]³⁻ clusters into pre-synthesized ZIF-67 toward Mo_xCo_xC particles confined in uniform carbon polyhedrons for efficient overall water splitting

Abstract

Bi-transition metal carbides (BTMCs) are promising in catalytic fields, but the synthesis of small-sized BTMCs remains a challenge. Here, Mo _<i>x</i> Co _<i>x</i> C (mainly below 20 nm in size) confined in uniform carbon polyhedrons (Mo _<i>x</i> Co _<i>x</i> C@C) was synthesized based on trapping [PMo₁₂O₄₀]^3- (PMo₁₂) clusters into pre-synthesized, uniform ZIF-67 (PMo/ZIF-67). The opening of the windows (0.34 nm) of ZIF-67 cages through heating is essential to allow the trapping of PMo₁₂ into the cages. This trapping route provides a new method to successfully combine POMs and MOFs that can not be effectively combined <i>via</i> traditional POMOF-based (simultaneous assembly) routes. It also has advantages in controlling the uniformity and components of the materials. The size matching of PMo₁₂ (1 nm) and the cages (1.16 nm) of ZIF-67 enables effective contact of the Co and Mo sources, thus giving small-sized Mo _<i>x</i> Co _<i>x</i> C protected by carbon <i>via</i> calcination. The optimized catalysts showed good performance for water splitting with a low <i>η</i>10 of 83 mV (295 mV) for the hydrogen (oxygen) evolution reaction, which is superior to those derived from ZIF-67 and precursors from POMOF-based routes. Our results also indicated that the HER activity is determined by the kind of BTMC, and the activity for the OER is relative to the oxygen-containing species formed during the initial OER test.