Phosphorous Vacancy and Built-In Electric Field Effect of Co-Doped MoP@MXene Heterostructures to Tune Catalytic Activity for Efficient Overall Water Splitting.

Developing cost-effective, durable bifunctional electrocatalysts is crucial but remains challenging due to slow hydrogen/oxygen evolution reaction (HER/OER) kinetics in water electrolysis. Herein, a combined engineering strategy of phosphorous vacancy (Vp) and spontaneous built-in electric field (BIEF) is proposed to design novel highly-conductive Co-doped MoP@MXene heterostructures with phosphorous vacancy (Vp-Co-MoP@MXene). Wherein, Co doping regulates the surface electronic structure and charge re-distribution of MoP, Vp induces more defects and active sites, while BIEF accelerates the interfacial charge transfer rate between Vp-Co-MoP and MXene. Therefore, the synergistic integration of Vp-Co-MoP/MXene efficiently decreases activation energy and kinetic barrier, thus promoting its intrinsically catalytic activity and structural stability. Consequently, the Vp-Co-MoP@MXene catalyst displays low overpotentials of 102.3/196.5 and 265.0/320.0 mV at 10/50 mA cm-2 for HER and OER, respectively. Notably, two-electrode electrolyzers with the Vp-Co-MoP@MXene bifunctional catalysts to achieve 10/50 mA cm-2, only need low-cell voltages of 1.57/1.64 V in alkaline media. Besides, experimental and theoretical results confirm that the hetero-structure effectively reduces hydrogen adsorption free energy and rate-determining-step energy barrier of OER intermediates, thereby greatly boosting its intrinsically catalytic activity. This work verifies an effective strategy to fabricate efficient non-precious bifunctional electro-catalysts for water splitting via combination engineering of phosphorous vacancy, cation doping, and BIEF.

3D Dense Encapsulated Architecture of 2D Bi Nanosheets Enabling Potassium-Ion Storage with Superior Volumetric and Areal Capacities.

2D alloy-based anodes show promise in potassium-ion batteries (PIBs). Nevertheless, their low tap density and huge volume expansion cause insufficient volumetric capacity and cycling stability. Herein, a 3D highly dense encapsulated architecture of 2D-Bi nanosheets (HD-Bi@G) with conducive elastic networks and 3D compact encapsulation structure of 2D nano-sheets are developed. As expected, HD-Bi@G anode exhibits a considerable volumetric capacity of 1032.2 mAh cm-3, stable long-life span with 75% retention after 2000 cycles, superior rate capability of 271.0 mAh g-1 at 104 C, and high areal capacity of 7.94 mAh cm-2 (loading: 24.2 mg cm-2) in PIBs. The superior volumetric and areal performance mechanisms are revealed through systematic kinetic investigations, ex situ characterization techniques, and theorical calculation. The 3D high-conductivity elastic network with dense encapsulated 2D-Bi architecture effectively relieves the volume expansion and pulverization of Bi nanosheets, maintains internal 2D structure with fast kinetics, and overcome sluggish ionic/electronic diffusion obstacle of ultra-thick, dense electrodes. The uniquely encapsulated 2D-nanosheet structure greatly reduces K+ diffusion energy barrier and accelerates K+ diffusion kinetics. These findings validate a feasible approach to fabricate 3D dense encapsulated architectures of 2D-alloy nanosheets with conductive elastic networks, enabling the design of ultra-thick, dense electrodes for high-volumetric-energy-density energy storage.