Synergistically Driven CoCr-LDH@VNiS2 as a Bifunctional Electrocatalyst for Overall Water Splitting and Flexible Supercapacitors

In Situ Fabrication of Hierarchical CuO@CoNi-LDH Composite Structures for High-Performance Supercapacitors

Layered double hydroxide (LDH) materials, despite their high theoretical capacity, exhibit significant performance degradation with increasing load due to their low conductivity. Simultaneously achieving both high capacity and high rate performance is challenging. Herein, we fabricated vertically aligned CuO nanowires in situ on the copper foam (CF) substrate by alkali-etching combined with the annealing process. Using this as a skeleton, electrochemical deposition technology was used to grow the amorphous α-phase CoNi-LDH nanosheets on its surface. Thanks to the high specific surface area of the CuO skeleton, ultrahigh loading (̃16.36 mg cm-2) was obtained in the fabricated CF/CuO@CoNi-LDH electrode with the cactus-like hierarchical structure, which enhanced the charge transfer and ion diffusion dynamics. The CF/CuO@CoNi-LDH electrode achieved a good combination of high areal capacitance (33.5 F cm-2) and high rate performance (61% capacitance retention as the current density increases 50 times). The assembled asymmetric supercapacitor device demonstrated a maximum potential window of 0-1.6 V and an energy density of 1.7 mWh cm-2 at a power density of 4 mW cm-2. This work provides a feasible strategy for the design and fabrication of high-mass-loading LDH composites for electrochemical energy storage applications.

Tuning the Activity and Stability of CoCr-LDH by Forming a Heterostructure on Surface-Oxidized Nickel Foam for Enhanced Water-Splitting Performance

The development of low-cost, efficient catalysts for electrocatalytic water splitting to generate green hydrogen is a hot topic among researchers. Herein, we have developed a highly efficient heterostructure of CoCr-LDH on NiO on nickel foam (NF) for the first time. The preparation strategy follows the simple annealing of a cleaned NF without using any Ni salt precursor, followed by the growth of CoCr-LDH nanosheets over the surface-oxidized NF. The CoCr-LDH/NiO/NF catalyst shows excellent electrocatalytic activity and stability toward oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in a 1 M KOH solution. For OER, only 253 mV and for HER, only 185 mV overpotentials are required to attain a 50 mA cm-2 current density. Also, the long-term stability of both the OER and HER for 60 h proves its robustness. The turnover frequency value for the OER increased 1.85 times after the heterostructure formation compared to bare CoCr-LDH. The calculated Faradaic efficiency values of 97.4 and 94.75% for the OER and HER revealed the high intrinsic activity of the heterostructure. Moreover, the heterostructure only needs 1.57 V of cell voltage when acting as both the anode and the cathode to achieve a 10 mA cm-2 current density. The long-term stability of 60 h for the total water-splitting process proves its excellent performance. Several systematic pre- and post-experiment characterizations prove its durable nature. These excellent OER and HER activities and stabilities are attributed to the surface-modified electronic structure and thin nanosheet-like surface morphology of the heterostructure. The thin, wide, and modified surface of the catalyst facilitates the diffusion of ions (reactants) and gas molecules (products) at the electrode/electrolyte interface. Furthermore, electron transfer from n-type CoCr-LDH to p-type NiO results in enhanced electronic conductivity. This study demonstates the effective design of a self-supported heterostructure with minimal synthetic steps to generate a bifunctional electrocatalyst for water splitting, contributing to the greater cause of green hydrogen economy.

Crystalline Ni5P4/amorphous CePO4 core/shell heterostructure arrays for highly-efficient electrocatalytic overall water splitting

Exploring low-cost and highly efficient bifunctional electrocatalysts for overall water splitting has become a research focus recently. Crystalline/amorphous core/shell heterostructures have great potential for applications in the field of electrocatalytic overall water splitting. However, related research is still challenging. Herein, crystalline Ni5P4 nanosheets/amorphous CePO4 nanocrystals core/shell heterostructure arrays were developed for electrocatalytic overall water splitting. It is shown that the heterostructure array required competitive HER and OER overpotentials of 94 and 191 mV in alkaline environment (10 mA/cm2), respectively. Encouragingly, the symmetrical two-electrode system constructed with the heterostructure array only required an ultra-low cell voltage of 1.535 V to achieve a current density of 10 mA/cm2. This indicates the system has huge potential in overall water splitting. The electrocatalytic mechanism was studied systematically by combining theoretical calculation and experimental characterization. It was found that the surface coating of amorphous CePO4 could not only significantly increase the electrochemical active surface area and improve the charge transfer of crystalline Ni5P4 nanosheets, but could also regulate d-band center of Ni5P4 and optimize the adsorption towards reaction intermediates in water splitting. The results not only provide an excellent crystalline/amorphous core/shell heterostructure bifunctional electrocatalyst for overall water splitting but also greatly expand the application of rare earth metal phosphate CePO4.