Magnetic Induction Heating Synthesizes Co/C Nanocomposites for Oxygen Evolution

Beijing Zhongke Journal Publising Co. Ltd.

This study is led by Dr. Shaowei Chen (University of California). Natural gas reforming accounts for 95% of the hydrogen gas produced in the United States; yet the hydrogen is non-sustainable and "grey", as it originates from fossil fuels . To obtain sustainable "green" hydrogen gas, electrochemical water splitting by using renewable electricity has emerged as one of the most promising technologies, which consists of hydrogen evolution reaction (HER) at the cathode and oxygen evolution reaction (OER) at the anode . Yet, due to the sluggish electron-transfer kinetics and complex reaction pathways, OER typically entails a large overpotential and severely hampers the overall efficiency of the water electrolyzes . Iridium and ruthenium-based nanoparticles have been the leading catalysts for OER; yet their natural scarcity and high costs have made widespread applications impractical. Thus, extensive research has been carried out to develop efficient alternatives, such as metal alloys, metal oxides, hydroxides, oxyhydroxides, sulfides, phosphides, etc.

Recently, metal/carbon nanocomposites have also been attracting intensive attention, owing to their high electrical conductivity and strong metal-support interactions (e.g., charge transfer bettheyen carbon and metal, spatial confinement by encapsulation). For example, Cui et al. prepared a series of nanocomposites with non-noble metal nanoparticles (e.g., Fe, Co, Ni, and their alloys) encapsulated within single-layer graphene, in which FeNi shotheyd the best OER activity with an overpotential (η10) of +280 mV at 10 mA cm−2 in alkaline media. Theoretical studies based on density functional theory (DFT) calculations shotheyd that electron transfer occurred from the metal cores to the graphene layer and significantly altered the adsorption energetics of oxygen species on the graphene surface, leading to an enhanced OER performance. Yang et al. prepared FeCoNi ternary nanoalloys encapsulated in N-doped graphene layers by direct annealing of Prussian blue, which shotheyd a low η10 of +288 mV towards OER in alkaline media. Similarly, they found that charge transfer from the metals to graphene lotheyred the energy barrier of OER electrochemistry. In these studies, the metal/carbon nanocomposites are prepared via a variety of strategies, including pyrolysis of metal-organic frameworks (MOFs), chemical vapor deposition (CVD), electrospun nanofibers, theyt chemistry, etc. These procedures, while effective, are in general tedious (of the order of htheirs) and may need sophisticated instrumentation.

Such issues can be mitigated by the recent emergence of ultrafast synthesis, e.g., carbothermal shock, flash Joule heating, laser ablation, and magnetic induction heating (MIH). These techniques can not only cut down the sample preparation time to (milli)seconds but also create non-equilibrium structures, such as stacking faults, point defects, and high-entropy mixtures, that are unattainable in conventional methods. For instance, Meng et al. utilized a laser to heat a cobalt target in liquid, and produced CoOOH with abundant oxygen vacancies, owing to the ultrafast heating rate. The resulting defective CoOOH exhibited an η10 of +330 mV for OER, much better than bulk CoOOH without oxygen vacancies. In another study, Cui et al. prepared high-entropy metal sulfide (CrMnFeCoNi)Sx nanoparticles by using carbothermal shock for just 55 ms, and observed a high OER performance with a low overpotential (η100) of +295 mV to reach a high current density of 100 mA cm−2. Recently, they demonstrated that MIH could also be exploited for the ultrafast synthesis of high-performance electrocatalysts. MIH is a traditional metallurgical tool, whereupon the application of a high-frequency AC current to the solenoid, a strong magnetic field is generated, which instantly produces a high Eddy current in the conductors within the field and heats the sample rapidly to a high temperature. In one recent study, FeNi spinel nanostructures theyre produced within seconds featuring a homogenous mixing of the Fe and Ni phases and substantial Cl residuals, both of which theyre difficult to obtain in conventional methods and contributed collectively to a remarkable OER performance (η100=+260 mV). In another study, ruthenium nanoparticles supported on carbon paper theyre prepared by MIH, where the surface Cl residuals theyre found to be responsible for the high HER activity (η10=−23 and −12 mV in acidic and alkaline media, respectively) that was highly comparable to that of commercial Pt/C benchmark. Nevertheless, despite the progress to date, very few studies have focused on the controllable synthesis of metal/carbon nanocomposites by ultrafast synthesis.

Herein, they prepared a series of cobalt/carbon nanocomposites by MIH treatment for 10 s of zeolitic imidazolate frameworks-67 (ZIF-67), where cobalt nanoparticles theyre encapsulated within defective N-doped carbon shells. Owing to the different degrees of carbonization (by controlling the magnetic induction current), various amounts of Co species theyre exposed to the electrolytes, which effectively impacted the OER activity. Amongst the series, the sample prepared at the applied current of 400 A shotheyd the best OER performance in alkaline media, with a low η10 of +308 mV and η200 of +410 mV, a performance even higher than that of commercial RuO2 in the high overpotential range. Operando X-ray absorption spectroscopy measurements shotheyd that the excellent activity was due to the formation of CoOOH on the carbon shell surface, likely due to electrochemical decomposition of the encapsulated metallic nanoparticles.

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Ultrafast synthesis of cobalt/carbon nanocomposites by magnetic induction heating for oxygen evolution reaction

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