Two different types of H2O2 selectivity are reported for the electrochemical synthesis of H2O2: molar fraction selectivity and Faradaic selectivity. Here we revisit their definitions and discuss the best way to report H2O2 selectivity, which can help to avoid misunderstandings or unfair performance comparisons in this growing field.

Electrochemical two-electron water oxidation is a promising route for renewable and on-site H2O2 generation as an alternative to the anthraquinone process. However, it is currently restricted by low selectivity due to strong competition from the traditional four-electron oxygen evolution reaction, as well as large overpotential and low production rates. Here we report an interfacial engineering approach, where by coating the catalyst with hydrophobic polymers we confine in situ produced O2 gas to tune the water oxidation reaction pathway. Using carbon catalysts as a model system, we show a significant increase of the intrinsic H2O-to-H2O2 selectivity and activity compared to that of the pristine catalyst. The maximal H2O2 Faradaic efficiency was enhanced by sixfold to 66% with an overpotential of 640 mV, under which a H2O2 production rate of 23.4 µmol min−1 cm−2 (75.2 mA cm−2 partial current) was achieved. This approach was successfully extended to nickel metal, demonstrating the wide applicability of our local gas confinement concept.

In light of environmental concerns and energy transition, electrochemical CO2 reduction (ECR) to value-added multicarbon (C2+) fuels and chemicals, using renewable electricity, presents an elegant long-term solution to close the carbon cycle with added economic benefits as well. However, electrocatalytic C─C coupling in aqueous electrolytes is still an open challenge due to low selectivity, activity, and stability. Design of catalysts and reactors holds the key to addressing those challenges. We summarize recent progress in how to achieve efficient C─C coupling via ECR, with emphasis on strategies in electrocatalysts and electrocatalytic electrode/reactor design, and their corresponding mechanisms. In addition, current bottlenecks and future opportunities for C2+ product generation is discussed. We aim to provide a detailed review of the state-of-the-art C─C coupling strategies to the community for further development and inspiration in both fundamental understanding and technological applications.

Electrochemical CO2 reduction reaction (CO2RR) to liquid fuels is currently challenged by low product concentrations, as well as their mixture with traditional liquid electrolytes, such as KHCO3 solution. Here we report an all-solid-state electrochemical CO2RR system for continuous generation of high-purity and high-concentration formic acid vapors and solutions. The cathode and anode were separated by a porous solid electrolyte (PSE) layer, where electrochemically generated formate and proton were recombined to form molecular formic acid. The generated formic acid can be efficiently removed in the form of vapors via inert gas stream flowing through the PSE layer. Coupling with a high activity (formate partial current densities ~450 mA cm−2), selectivity (maximal Faradaic efficiency ~97%), and stability (100 hours) grain boundary-enriched bismuth catalyst, we demonstrated ultra-high concentrations of pure formic acid solutions (up to nearly 100 wt.%) condensed from generated vapors via flexible tuning of the carrier gas stream.

Hydrogen peroxide (H2O2) is a valuable chemical with a wide range of applications. A recent trend in H2O2 production focuses on electrochemical reduction of oxygen, which represents an environmentally friendly method for on-site H2O2 generation. To realize highly efficient practical-scale electrosynthesis of H2O2, catalytic materials design and electrochemical reactor engineering play equally important roles and must be carefully investigated. A highly active catalyst with superior selectivity and stability is the foundation for efficient H2O2 production, and well-configured reactors are important to realize practical deployment of on-site H2O2 production in large scales. In this review, an overview of the recent progress (over the past 3 years) in electrochemical production of H2O2 is presented, with an emphasis on the development of selective catalysts, especially the atomic tuning of carbon-based catalysts and single-atom catalysts, as well as electrode engineering and electrochemical cell design.

The rapid development of miniaturized and wearable electronics has stimulated growing needs for compatibleminiaturized energy storage components. Owing to their unlimited lifetime and high-power density, miniaturizedelectrochemical capacitors (microsupercapacitors) are considered to be an attractive solution for the developmentof these microelectronics, but they often depend on the choice of electrode materials and fabrication protocols forscalable production. Recently, a new family of two-dimensional transition metal carbides, carbonitride and nitrides (referred to as MXenes) has shown great promise in advanced microsupercapacitors with high energy andpower densities. This was achieved thanks to the high pseudocapacitance, metallic conductivity and ease of solution processing of MXene. In this review, recent progress on MXene synthesis, microstructure design, andfabrication strategies of MXene microsupercapacitors are discussed, and their electrochemical performance issummarized. Further, we briefly discuss the technical challenges and future directions

In contrast to alkaline water electrolysis, acidic water electrolysis remains an elusive goal due to the lack of earth-abundant, efficient, and acid-stable water oxidation electrocatalysts. Here, we show that materials with intrinsically poor electrocatalytic activity can be turned into active electrocatalysts that drive the acidic oxygen evolution reaction (OER) effectively. This development is achieved through ultrafast plasma sputtering, which introduces abundant oxygen vacancies that reconstruct the surface electronic structures, and thus, regulated the surface interactions of electrocatalysts and the OER intermediates. Using tungsten oxide (WO3) as an example, we present a broad spectrum of theoretical and experimental characterizations that show an improved energetics of OER originating from surface oxygen vacancies and resulting in a significantly boosted OER performance, compared with pristine WO3. Our result suggests the efficacy of using defect chemistry to modify electronic properties and hence to improve

As a thriving member of the 2D nanomaterials family, MXenes, i.e., transition metal carbides, nitrides, and carbonitrides, exhibit outstanding electrochemical, electronic, optical, and mechanical properties. They have been exploited in many applications including energy storage, electronics, optoelectronics, biomedicine, sensors, and catalysis. Moreover, printing can allow for complex 2D architectures and multifunctionality that are highly required in various applications. By means of printing and patterned coating, the performance and application range of MXenes can be dramatically increased through careful patterning in three dimensions; thus, printing/coating is not only a device fabrication tool but also an enabling tool for new applications as well as for industrialization

Hydrogels have recently garnered tremendous interest due to their potential application in soft electronics, human–machine interfaces, sensors, actuators, and flexible energy storage. Benefiting from their impressive combination of hydrophilicity, metallic conductivity, high aspect ratio morphology, and widely tuneable properties, when two-dimensional (2D) transition metal carbides/nitrides (MXenes) are incorporated into hydrogel systems, they offer exciting and versatile platforms for the design of MXene-based soft materials with tunable application-specific properties. We elucidate the existing structures of various MXene-containing hydrogel systems along with their gelation mechanisms and the interconnecting driving forces. We then discuss their distinctive properties stemming from the integration of MXenes into hydrogels, which have revealed an enhanced performance, compared to either MXenes or hydrogels alone, in many applications (energy storage/harvesting, biomedicine, catalysis, electromagnetic interference shielding, and sensing).

Ultrasound is a source of ambient energy that is rarely exploited. In this work, a tissue-mimicking MXene-hydrogel (M-gel) implantable generator has been designed to convert ultrasound power into electric energy. Unlike the present harvesting methods for implantable ultrasound energy harvesters, our M-gel generator is based on an electroacoustic phenomenon known as the streaming vibration potential. Moreover, the output power of the M-gel generator can be improved by coupling with triboelectrification. We demonstrate the potential of this generator for powering implantable devices through quick charging of electric gadgets, buried beneath a centimeter thick piece of beef. The performance is attractive, especially given the extremely simple structure of the generator, consisting of nothing more than encapsulated M-gel. The generator can harvest energy from various ultrasound sources, from ultrasound tips in the lab to the probes used in hospitals and households for imaging and physiotherapy.