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Low-cost and highly efficient bifunctional electrocatalysts for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) are intensively investigated for overall water splitting. Herein, we combined experimental research with first-principles calculations based on density functional theory (DFT) to engineer the NiCoS@NiFe LDH heterostructure interface for enhancing overall water-splitting activity. The DFT calculations exhibit strong interaction and charge transfer between NiCoS and NiFe LDH, which change the interfacial electronic structure and surface reactivity. The calculated chemisorption free energy of hydroxide (ΔE) is reduced from 1.56 eV for pure NiFe LDH to 1.03 eV for the heterostructures, indicating a dramatic improvement in OER performance, while the chemisorption free energy of hydrogen (ΔE) maintains almost invariable. By the use of the facile hydrothermal method, NiCoS nanotubes, NiFe LDH nanosheets, and NiCoS@NiFe LDH heterostructures are prepared on nickel foam, of which the corresponding experimental OER overpotentials are 306, 260, and 201 mV at 60 mA cm, respectively. These results are good agreement with the theoretical predictions. Meanwhile, the HER performance has little improvement, with an overpotential of about 200 mV at 10 mA cm. Due to the dramatic improvement in OER performance, there was an enhancement in the overall water-splitting activity of the NiCoS@NiFe LDH heterostructures, with a low voltage of 1.6 V.

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Two-dimensional Ti3C2T x MXenes have been extensively studied as pseudocapacitive electrode materials. This Letter aims at providing further insights into the charge storage mechanism of the Ti3C2T x MXene electrode in the acidic electrolyte by combining experimental and simulation approaches. Our results show that the presence of H2O molecules between the MXene layers plays a critical role in the pseudocapacitive behavior, providing a pathway for proton transport to activate the redox reaction of the Ti atoms. Also, thermal annealing of the samples at different temperatures suggests that the presence of the confined H2O molecules is mainly controlled by the surface termination groups. These findings pave the way for alternative strategies to enhance the high-rate performance of MXenes electrodes by optimizing their surface termination groups.