Ti3C2Tx MXene has been considered a potential supercapacitor electrode material on account of its fast electron transfer performance, large volume pseudocapacitance, excellent hydrophilicity and mechanical properties. However, the serious stacking of MXene nanosheets reduced the exposed active sites and hindered diffusion of ions, leading to bad electrochemical property. Here, we report a strategy to transform MXene nanosheets into 3D MXene microgels induced by 2-methylimidazole. Benefitting from the existence of microgels, it facilitates the rapid vacuum filtration to prepare the porous MXene film, thus enhancing the ion transfer efficiency. Furthermore, 2-methylimidazole can also act as an intercalated agent to increase the layer spacing of MXene nanosheets. Therefore, the as-prepared porous MXene film demonstrates a superior rate capacity, and its capacitance reaches 223.4 F g−1 at the scan rates of 1000 mV s−1. Moreover, the assembled MXene//AC asymmetrical supercapacitor displays a 16.88 Wh kg−1 at a power density of 799 W kg−1, and maintains 13.11 Wh kg−1 at a power density of 8 kW kg−1. This work develops a simple way to prepare the porous MXene film for supercapacitors with excellent electrochemical performance. 

Self-standing porous MXene film via in situ formed microgel induced by 2- methylimidazole for high rate supercapacitor

This review systematically provides various insights into the pH effect on hydrogen electrocatalysis, and thus providing a reference for future development of hydrogen electrocatalysis based on these insights. 

Insights into the pH effect on hydrogen electrocatalysis

Developing abundant Earth-element and high-efficient electrocatalysts for hydrogen production is crucial in effectively reducing the cost of green hydrogen production. Herein, a strategy by comprehensively considering the computational chemical indicators for H* adsorption/desorption and dehydrogenation kinetics to evaluate the hydrogen evolution performance of electrocatalysts is proposed. Guided by the proposed strategy, a series of catalysts are constructed through a dual transition metal doping strategy. Density Functional Theory (DFT) calculations and experimental chemistry demonstrate that cobalt-vanadium co-doped Ni3 N is an exceptionally ideal catalyst for hydrogen production from electrolyzed alkaline water. Specifically, Co,V-Ni3 N requires only 10 and 41 mV in alkaline electrolytes and alkaline seawater, respectively, to achieve a hydrogen evolution current density of 10 mA cm-2 . Moreover, it can operate steadily at a large industrial current density of 500 mA cm-2 for extended periods. Importantly, this evaluation strategy is extended to single-metal-doped Ni3 N and found that it still exhibits significant universality. This study not only presents an efficient non-precious metal-based electrocatalyst for water/seawater electrolysis but also provides a significant strategy for the design of high-performance catalysts of electrolyzed water.

Designing Efficient Non-Precious Metal Electrocatalysts for High-Performance Hydrogen Production: A Comprehensive Evaluation Strategy.

MXenes have unique properties such as high electrical conductivity, excellent mechanical properties, rich surface chemistry, and convenient processability. These characteristics make them ideal for producing flexible materials with tunable microstructures. This paper reviews the laboratory research progress of flexible MXene and its composite materials for supercapacitors. And introduces the general synthesis method of MXene, as well as the preparation and properties of flexible MXene. By analyzing the current research status, the electrochemical reaction mechanism of MXene was explained from the perspectives of electrolyte and surface terminating groups. This review particularly emphasizes the composite methods of freestanding flexible MXene composite materials. The review points out that the biggest problem with flexible MXene electrodes is severe self-stacking, which reduces the number of chemically active sites, weakens ion accessibility, and ultimately lowers electrochemical performance. Therefore, it is necessary to composite MXene with other electrode materials and design a good microstructure. This review affirms the enormous potential of flexible MXene and its composite materials in the field of supercapacitors. In addition, the challenges and possible improvements faced by MXene based materials in practical applications were also discussed. 

Recent Advances of Flexible MXene and its Composites for Supercapacitors

Cost-effective transition metal chalcogenides (TMCs) are highly promising electrocatalysts for both alkaline and acidic hydrogen evolution reactions (HER). However, unsatisfactory HER kinetics and stability have severely hindered their applications in industrial water electrolysis. Herein, a nanoflowers-shaped W-doped cubic/orthorhombic phase-mixed CoSe2 catalyst ((c/o)-CoSe2-W) is reported. The W doping induces spontaneous phase transition from stable phase cubic CoSe2 to metastable phase orthorhombic CoSe2, which not only enables precise regulation of the ratio of two phases, but also realizes W doping at the interfaces of two phases. The (c/o)-CoSe2-W catalyst exhibits a Pt-like HER activity in both alkaline and acidic media, with record-low HER overpotentials of 29.8 mV (alkaline) and 35.9 mV (acidic) at 10 mA cm-2, respectively, surpassing the vast majority of previously reported non-precious metal electrocatalysts for both alkaline and acidic HER. The Pt-like HER activities originate from the formation of Co-Se-W active species on the c-CoSe2 side at the phase interface, which effectively modulates electron structures of active sites, not only enhancing H2O adsorption and dissociation at Co sites, but also optimizing H* adsorption to ΔGH* ≈ 0 at W sites. Benefiting from the abundant phase interfaces, the catalyst also displays outstanding long-term durability in both acidic and alkaline media. This article is protected by copyright. All rights reserved.

Construction of Co-Se-W at Interfaces of Phase-Mixed Cobalt Selenide via Spontaneous Phase Transition for Platinum-Like Hydrogen Evolution Activity and Long-Term Durability in Alkaline and Acidic Media.

Water splitting is a promising technique in the sustainable "green hydrogen" generation to meet energy demands of modern society. Its industrial application is heavily dependent on the development of novel catalysts with high performance and low cost for hydrogen evolution reaction (HER). As a typical non-precious metal, cobalt-based catalysts have gained tremendous attention in recent years and shown a great prospect of commercialization. However, the complexity of the composition and structure of newly-developed Co-based catalysts make it urgent to comprehensively retrospect and summarize their advance and design strategies. Hence, in this review, the reaction mechanism of HER is first introduced and the possible role of the Co component during electrocatalysis is discussed. Then, various design strategies that could effectively enhance the intrinsic activity are summarized, including surface vacancy engineering, heteroatom doping, phase engineering, facet regulation, heterostructure construction, and the support effect. The recent progress of the advanced Co-based HER electrocatalysts is discussed, emphasizing that the application of the above design strategies can significantly improve performance by regulating the electronic structure and optimizing the binding energy to the crucial intermediates. At last, the prospects and challenges of Co-based catalysts are shown according to the viewpoint from fundamental explorations to industrial applications.

Recent Advances in Co-Based Electrocatalysts for Hydrogen Evolution Reaction.

MXenes have been extensively explored as supercapacitor electrodes, especially in acidic aqueous electrolytes, where ultrahigh specific capacitance can be achieved; however, their narrow working potential window (≤ 1.0 V) limits the acquisition of high energy. Neutral and alkaline electrolytes can be used to extend the working potential window but MXenes in these electrolytes are less pseudocapacitive active, which leads to reduced charge storage. In this study, it is shown that Ti3 C2 Tx MXene in a mild AlCl3 aqueous electrolyte can operate at a wide potential range from 0 to -1.3 V versus Hg/Hg2 SO4 and retain high pseudocapacitive activity. Thus, a high capacity of up to 85 mAh g-1 is achieved, surpassing its performance in H2 SO4 electrolyte of 78 mAh g-1 . More interestingly, most of the capacity is released at a more negative potential range than that in acidic electrolytes, making it more suitable as a negative electrode material. In situ electrochemical quartz crystal microbalance results suggest that the high capacity originates from the pseudocapacitive intercalation/deintercalation of H+ instead of Al3+ , providing the possibility of coupling MXene anodes with proton redox active cathodes to achieve high-energy and high-power devices.

Ti3 C2 Tx MXene with High Pseudocapacitive Activity and Large Potential Window in a Mild AlCl3 Aqueous Electrolyte.

Nanoflakes Co(OH)2 porous films were successfully synthesized by a facile electrochemical technique. The morphology was characterized by field emission scanning electron microscopy (FESEM). Electrochemical techniques such as cyclic voltammetry (CV), galvanostaitc charge/discharge and electrochemical impedance spectroscopy were used to study the effects of deposition temperatures on the capacitance of the films. The results exhibited that the Co(OH)2 films single electrode had high specific capacitance in KOH electrolyte. A maximum specific capacitance of 2780 F/g could be achieved for the Co(OH)2 film deposited at 50°C in 2 M aqueous KOH with 0 to 0.4V potential at a charge-discharge current density of 4 mA/cm2. Therefore, the obtained nanoflakes Co(OH)2 porous films can be a potential application electrode material for supercapacitors.

Effects of Deposition Temperatures on the Nanoflakes Co(OH)2 Porous Films for Supercapacitors

Background The high cost and scarcity of Pt metal limit the large-scale applicability of the excellent hydrogen evolution reaction (HER) catalyst. Objective The current study aims to generate high-performance HER electrodes with low Pt content. Methods Nitrogen-doped carbon support has been produced via hydrothermal and calcination treatments of ZIF-8 precursor. By applying drop coating, a CN/CC electrode support is fabricated before it is further decorated with Pt nanoparticles to afford a [email protected]/CC composite electrode through a cyclic voltammetry (CV) method. Results In 0.5 M H2SO4 at room temperature, the electrode achieves an HER overpotential of 74 mV at a current density of 10 mA cm–2. When the electrolyte is 1 M KOH, the electrode functions even more favorably with an overpotential of 55 mV under the same experimental conditions. The HER performance increases with a gain in temperature. At 80°C, alkaline overpotential of the electrode becomes 29 mV, much superior to the 20% Pt/C/CC electrode. Conclusion The study paves the method of feasibly constructing a high-performance HER electrode with low Pt content. This composite electrode imparts synergistic effects between PtZn nanoparticles and CN support that enhance the electrochemically active surface area while reducing the reaction impedance to provide outstanding hydrogen evolution performance. 

PtZn
 @
 CN
 /
 CC
 derived from
 ZIF
 –86 for high‐performance hydrogen evolution in acid and alkaline electrolyte

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Recent Advances in Co-Based Electrocatalysts for Hydrogen Evolution Reaction.

Ti3 C2 Tx MXene with High Pseudocapacitive Activity and Large Potential Window in a Mild AlCl3 Aqueous Electrolyte.

Effects of Deposition Temperatures on the Nanoflakes Co(OH)2 Porous Films for Supercapacitors

PtZn @ CN / CC derived from ZIF –86 for high‐performance hydrogen evolution in acid and alkaline electrolyte