Draft:Contact-electro-catalysis

Contact-electro-catalysis (CEC), is a bridging concept between contact-electrification effect (also know as triboelectricity) and mechanochemistry. It was first proposed in 2022 by using chemically inert triboelectric materials (FEP) to catalyze the degradation of methyl orange (MO) aqueous solution. , The definition of CEC refers to a process that exploits the electron transfer during contact-electrification (CE) to promote chemical reactions. The solid to be used in CEC involves pristine polymers (FEP, PTFE),  inorganics (SiO2),  and matrix composites. The energy source of CEC is mechanical stimuli such as ultrasonication and ball milling. CEC has appeared as a significant branch of mechanochemistry due to its broad materials selection range and application fields.

The origin of CEC
Force-induced increase of defects, extreme conditions,  or other effects (such as piezoelectric effect)  are three governing operating mechanisms for mechanochemical processes. As a matter of fact, mechanical stimuli would inevitably result in frequent contact-separations and contact-electrification effect between friction pairs, but the contribution of contact-electrification effect to chemical reactions has long been ignored. Contact-electrification (CE), also known as triboelectrification, is a ubiquitous phenomenon across various interfaces. In addition to the well-known CE phenomenon at solid-solid interfaces, CE can also take place when a liquid contacts with a solid. The two surfaces after CE become oppositely charged, and a series of recent investigations have ascribed it to the CE-driven electron transfer. In association with the electron exchange process in a typical catalytic process, the concept of CEC has been proposed by using the CE-driven electron transfer for promote chemical reactions.

The catalysts of CEC
Pristine polymers. Pristine polymers is the first proposed CEC catalysts. In virtue of the high CE ability of polymers, the polymer-based CEC has been proposed for organic pollutants,  synthesis of H2O2 under ambient and anaerobic conditions, direct oxidation of methane, and continuous synthesis of ammonia. Owing to the inherent catalytic inertness, the successful utilization of pristine polymers also serves as compelling evidence for the viability of CEC.

Oxides. The reduced CE ability of polymers at elevated temperatures may hinder the application of CEC in catalyzing high-temperature chemical reactions. In response to this challenge, oxide has been proposed for CEC at enhanced temperatures, and the SiO2-based CEC have been proposed for promoting the leach process of cathode materials in lithium ion batteries (LIBs) with a temperature at 90 °C. The CEC at TiO2 surface has also been reported for atom transfer radical polymerization and pollutants degradation.

Matrix composites. The ubiquity of CE also provides abundant opportunities for synergy with existing catalytic strategies. For example, the pristine MIL-101 (Cr) metal-organic frameworks (MOFs) can be employed for CEC after grafting pyridine molecular groups. A ZnO@PTFE composites that combines CEC with piezocatalysis in one system has been devised with an overall enhancement of degradation rate by 444.23 %. A RGO/ZnO nanohybrid has also been developed for degrading malachite green dye via CEC.

Strategies for initiating CEC
Ultrasonication. Ultrasonication is the first proposed strategy for inducing CEC, which mainly uses the variation of cavitation bubbles during the propagation of ultrasonic waves. In particular, cavitation bubble nuclei tend to develop near dissolved gases (such as O2), and their growth will encapsulate these neighboring gas molecules. Upon reaching a critical size, the collapse of a cavitation bubble releases the trapped gas molecules, generating a high-pressure microjet capable of inducing contact-separation cycles and subsequent electron exchange.

Ball milling. Ball milling is also effective for initiating CEC. The utilization of triboelectric materials in a ball milling setup is anticipated to induce evident CE phenomena during collisions. In virtue of the grinding-based CEC, 50 mL 5-ppm MO aqueous solution can be degraded in 2 hours.

Significant applications of CEC
Organic pollutants degradation. The methyl orange (MO) aqueous solution can be degraded by FEP powder or other dielectrics through CEC despite they are highly chemically inert and has never been reported with any catalytic activity. Other organic pollutants, such as acid orange 17 (AO-17) and rhodamine B (RhB), can also be degraded through a similar process.

Direct synthesis of H2O2. The fabrication of H2O2 via CEC can be achieved under both ambient and anaerobic conditions by ultrasonicating PTFE powder in DI water. The yield can reach as high as 313 μmol L-1 h-1, and this strategy is feasible even under anerobic conditions. The formation mechanism of H2O2 during CEC is further illustrated by a subsequent study.

Recycle of spent lithium-ion batteries (LIBs). By using the CE-driven electron transfer on SiO2 particle surfaces, a high leaching efficiency of 100 % for Li and 92.19 % for Co for lithium cobalt (Ⅲ) oxide (LCO) batteries, and the used SiO2 could be easily recycled with nearly no diminution in catalytic efficiency.

Contiuous synthesis of ammonia. CEC is also feasible for synthesizing ammonia from water and dissolved nitrogen. By ultrasonicating PTFE powder in DI water with N2 gas, the yield of ammonia is as high as 420 μmol L−1 h−1 per gram of PTFE under room termperature.