Draft:Contact-electro-catalysis (CEC)

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Contact-electro-catalysis (CEC)

Fig.1 Contact-electro-catalysis (CEC)

Contact-electro-catalysis (CEC), first proposed by Prof. Zhong Lin Wang's group in 2022, refers to a process that exploits the electron transfer during contact-electrification (CE) to promote chemical reactions.Despite being a relatively new field, CEC has quickly garnered significant attention and demonstrated its potential for groundbreaking advancements in catalysis and mechanochemistry. This paradigm-shift technology offers promising avenues for the development of innovative catalysts and catalytic processes.

1.    Fundamental principles of CEC

Contact-electrification (CE) manifests as 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 contact surfaces after CE become oppositely charged, and a series of recent investigations have ascribed it to CE-driven electron transfer at L-S interfaces. An “electron-cloud-potential-well” model has been proposed by Prof. Zhong Lin Wang to elucidate the mechanism of electron transfer during CE. In association with the electron exchange process in a typical catalytic process, the contribution from the CE effect to promote chemical reactions might be ignored. As a consequence, contact-electro-catalysis (CEC) has been proposed as a bridging concept that connects CE with mechanochemistry, offering a valuable supplement to existing catalytic strategies.

Fig.2 From CE-driven electron transfer to contact-electro-catalysis

The definition of CEC refers to a catalytic process that employs electrons exchanged during contact electrification to promote the rate of chemical reactions. Fig. 2 illustrates a typical CEC process for facilitating redox reactions: Reactant “A” first contacts with the CEC catalyst “C”, and the CE effect is supposed to drive electrons transferring from “A” to “C”. “A” will be oxidized to “A_oxd” by such electron exchange, while the designation “C^*” signifies the charged state of “C”. After the desorption of “A_oxd”, the residual electrons on the surface of “C^*” can be acquired by another reactant “B” upon their contact. Consequently, “B” is reduced to “B_red”, and “C^*” reverts to its original uncharged state “C”, completing a full catalytic cycle. External mechanical agitation serves as the energy source for CEC, and feasible catalysts encompass almost all organic (e.g., FEP, PTFE) and inorganic materials (e.g., SiO₂, Al₂O₃).

2.    The significance of CEC

Contact-electro-catalysis (CEC) represents an innovative catalytic approach with the potential to transform conventional perspectives on catalysis. By leveraging the ubiquitous nature of contact-electrification, CEC holds the promise of broadening the scope of materials suitable for catalytic purposes. For instance, CEC has enabled pristine polymers to catalyze the generation of reactive oxygen species (ROS), which is a direct contradiction to the well-accepted common sense that polymer is intrinsic catalytic inertness. CEC is expected to inspire the exploration of novel materials for catalytic applications, potentially leading to the discovery of previously unrecognized catalysts and catalytic reactions. Furthermore, CEC has the potential to significantly diversify the array of available catalytic strategies. Nowadays, the CEC has been successively applied to a variety of research frontiers, such as recycling cathode materials from spent lithium-ion batteries (LIBs), synthesizing ammonia from N₂ gas under ambient conditions, and producing hydrogen peroxide even under anaerobic environment. This paradigm shift opens up exciting possibilities for the development of novel catalysts and catalytic processes.

3.    Broad catalysts enabled by CEC

Contact-electrification is widely observed across various surfaces, such as solid-solid, solid-liquid, and even liquid-liquid or solid-gas interfaces. The prevalence of CE provides contact-electro-catalysis (CEC) with a significantly expanded range of catalyst options (Fig. 3). On the one hand, CE can occur at both micro- and macro-scales, thereby reducing limitations on the design of CEC catalysts. For example, powders at the micrometer scale, millimeter-sized spheres, and even centimeter-scale films have all been demonstrated as feasible options for CEC catalysts. On the other hand, the operating principle of CEC refers to promote chemical reactions through CE-driven interfacial electron transfer, which largely depends on the surface properties of the materials used. Consequently, the spectrum of CEC catalysts can be substantially diversified, typically falling into three categories: polymers, oxides, and matrix composites.

Fig.3 CEC-expanded selection range of materials that can be envisaged as catalysts

Pristine polymers, recognized for their exceptional contact-electrification (CE) capabilities and inherent catalytic inertness, are the first proposed CEC catalysts. Their successful utilization also serves as compelling evidence for the viability of CEC.^] However, the reduced CE ability of polymers at elevated temperatures, because of their glass transition, may hinder the application of CEC in catalyzing high-temperature chemical reactions. In response to this challenge, oxides have been proposed to withstand such high temperatures. The diversity of CEC catalysts also provides abundant opportunities for synergy with existing catalytic strategies, including metal-organic frameworks (MOFs) and piezocatalysis. By combining various configurations and categories of CEC catalysts, numerous opportunities are provided for designing innovative catalytic systems with enhanced sustainability and efficiency.

4.    Strategies for initiating CEC

The key for initiating CEC refers to implement efficient contact-separation cycles on specific interfaces. Ultrasonication is first proposed for inducing CEC by 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.

Fig.4 Strategies for initiating the contact-electro-catalysis

Ball milling represents another effective approach for initiating CEC, which could naturally bring about frequent contacts and separations. The utilization of triboelectric materials in the ball milling setup is anticipated to induce evident CE phenomena during these collisions. Distinguished from conventional ball milling methods, high-energy impacts are not essential for CEC-based grinding. Therefore, the recyclability and reusability of catalysts could be notably enhanced in CEC.

Other strategies, such as sliding droplets across polymers or submerging a triboelectric nanogenerator (TENG) in aqueous solutions, have also been reported. However, physical adsorption due to electrostatic attraction may play a dominant role in these methods. To improve the contribution from chemical degradation, endeavors should concentrate on enhancing the frequency of CE or providing greater energy for the CE process.

5.    “Two-step” mechanism of CEC

Wang et al. were the first to propose the two-step mechanism for generating reactive oxygen species (ROS) through CEC, which encompasses the oxidation of water and the reduction of oxygen molecules, as detailed in equations 1 and 2.

(1).

(1)

(2)

Fig.5 Two-step mechanism of producing reactive oxygen species by CEC

A more detailed study indicates that the generation of superoxide radicals should be the rate determining step since electrons need to be removed from the electron-affinitive surface of PTFE, resulting in a much higher energy for accomplishing this process. A more completed two-step mechanism for generating ROS through CEC was described in Fig. 5. At the first step, electrons will be transferred from H₂O to PTFE upon their contact, producing water radical cations that would be converted to hydronium cations and hydroxyl radicals through a rapid proton transfer from water. In the second step, electrons accumulated on the charged surface of PTFE^* are transferred to dissolved O₂ molecules upon collision with sufficient energy. Consequently, PTFE^* reverts to its initial uncharged state, and O₂ is converted to superoxide radicals after acquiring these electrons. This cycle will repeat itself as mechanical stimuli persist.

6.    Significant applications of CEC

6.1 Organic pollutants degradation

The first representative application of CEC involves the degradation of organic pollutants in wastewater. Utilizing CEC for this purpose offers several distinct advantages. Firstly, CEC imposes minimal constraints on catalysts, with even natural substances like rocks potentially serving as CEC catalysts. Secondly, the energy source for CEC derives from mechanical energy such as collision, stirring, and ultrasound, enabling organic pollutant degradation via CEC even in remote wilderness areas where are far away from electricity, light, and synthetic materials. This suggests that CEC may be the potential mechanism of “running water does not decay.” The methyl orange (MO) aqueous solution can be degraded by FEP powder through CEC despite FEP is 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.

6.2 Direct synthesis of H₂O₂

Hydrogen peroxide serves as a crucial chemical precursor with diverse applications, yet achieving controlled preparation of H₂O₂ at room temperature poses considerable challenges. Traditional synthesis routes often entail multiple reaction steps, rendering the process thermodynamically unfavorable. However, by dispersing commercially available PTFE particles as catalysts in deionized water and employing ultrasonic waves to initiate CEC, H₂O₂ synthesis becomes feasible under ambient and even anaerobic conditions. More importantly, the production of H₂O₂ via CEC is safe and scalable since no risk of H₂ explosion is existed and each dispersed PTFE particle can serve as a reaction site. Additionally, this method yields no environmentally harmful intermediates or by-products, further highlighting the practicability of using CEC to prepare H₂O₂.

6.3 Recycle of spent lithium-ion batteries (LIBs)

As the charge-discharge cycle of numerous LIBs comes to an end, the importance and urgency of effectively recycling these depleted LIBs have become increasingly evident. CEC has presented an efficient and sustainable method for extracting valuable Li and Co metals from cathode materials in spent LIBs under mild conditions. By using the CE-driven electron transfer on SiO₂ particle surfaces, a high leaching efficiency of 100 % for Li and 92.19 % for Co was achieved for lithium cobalt (Ⅲ) oxide (LCO) batteries, and the used SiO₂ could be easily recycled with nearly no diminution in catalytic efficiency. More importantly, the utilization of citric acid instead of hazardous and expensive strong inorganic acids substantially mitigates environmental impact while offering considerable economic benefits.

7.    Connection and difference between CEC and tribocatalysis

Fig.6 The similarity and disparity between tribocatalysis and contact-electro-catalysis

Both tribocatalysis and contact-electro-catalysis (CEC) are originated from interactions between two surfaces under external mechanical forces. Tribocatalysis primarily leverages temperature elevation or bond breakage induced by intense friction to expedite reaction rates. Although the CE effect is supposed to occur during typical tribocatalytic processes, its role may not be predominant, partly because electrons tend to be thermionically emitted rather than exchanged at elevated temperatures. In virtue of the CE-driven interfacial electron transfer, the concept of contact-electro-catalysis (CEC) has recently been introduced as a promising avenue for mechanochemical exploration. Given that the CE effect can occur even without significant friction during contact, CEC might offer a milder condition for enhancing reaction rates. Furthermore, in tribocatalysis, catalysts typically comprise metals or their oxides, necessitating resilience to high-temperature environments and severe surface abrasions, while also providing active catalytic sites. Benefitted from the ubiquitous nature of CE effect, CEC enriches the spectrum of available catalysts, with expectations for enhanced recyclability and reusability due to minimal damage during CEC processes. Moreover, while tribocatalysis typically relies on solid-solid friction pairs to induce intense frictions for creating suitable catalytic conditions, CEC could circumvent such constraints, as CE is able to facilitate direct electron exchange between substrates even when in gaseous or liquid phases.