Dynamic Electrodeposition on Bubbles: An Effective Strategy toward Porous Electrocatalysts for Green Hydrogen Cycling

Conspectus Closed-loop cycling of greenhydrogen is a promising alternativeto the current hydrocarbon economy for mitigating the energy crisisand environmental pollution. It stores energy from renewable energysources like solar, wind, and hydropower into the chemical bond ofdihydrogen (H-2) via (photo)electrochemical water splitting,and then the stored energy can be released on demand through the reversereactions in H-2-O-2 fuel cells. The sluggishkinetics of the involved half-reactions like hydrogen evolution reaction(HER), oxygen evolution reaction (OER), hydrogen oxidation reaction(HOR), and oxygen reduction reaction (ORR) limit its realization.Moreover, considering the local gas-liquid-solid triphasemicroenvironments during H-2 generation and utilization,rapid mass transport and gas diffusion are critical as well. Accordingly,developing cost-effective and active electrocatalysts featuring three-dimensionalhierarchically porous structures are highly desirable to promote theenergy conversion efficiency. Traditionally, the synthetic approachesof porous materials include soft/hard templating, sol-gel,3D printing, dealloying, and freeze-drying, which often need tediousprocedures, high temperature, expensive equipment, and/or harsh physiochemicalconditions. In contrast, dynamic electrodeposition on bubbles usingthe in situ formed bubbles as templates can be conductedat ambient conditions with an electrochemical workstation. Moreover,the whole preparation process can be finished within minutes/hours,and the resulting porous materials can be employed as catalytic electrodesdirectly, avoiding the use of polymeric binders like Nafion and theconsequent issues like limited catalyst loading, reduced conductivity,and inhibited mass transport. In this Account, we summarizeour contributions to the dynamicelectrodeposition on bubbles toward advanced porous electrocatalystsfor green hydrogen cycling. These dynamic electrosynthesis strategiesinclude potentiodynamic electrodeposition that linearly scans theapplied potentials, galvanostatic electrodeposition that fixes theapplied currents, and electroshock which quickly switches the appliedpotentials. The resulting porous electrocatalysts range from transitionmetals to alloys, nitrides, sulfides, phosphides, and their hybrids.We mainly focus on the 3D porosity design of the electrocatalystsby tuning the electrosynthesis parameters to tailor the behaviorsof bubble co-generation and thus the reaction interface. Then, theirelectrocatalytic applications for HER, OER, overall water splitting(OWS), biomass oxidation (to replace OER), and HOR are introduced,with a special emphasis on the porosity-promoted activity. Finally,the remaining challenges and future perspective are also discussed.We hope this Account will encourage more efforts into this attractiveresearch field of dynamic electrodeposition on bubbles for variousenergy catalytic reactions like carbon dioxide/monoxide reduction,nitrate reduction, methane oxidation, chlorine evolution, and others.

Automated summary

Closed-loop cycling of green hydrogen, stored from renewable energy sources, is a promising alternative for mitigating the energy crisis and environmental pollution. However, the sluggish kinetics of involved reactions and the need for rapid mass transport and gas diffusion pose challenges. This Account summarizes the contributions of dynamic electrodeposition on bubbles to the development of advanced porous electrocatalysts for green hydrogen cycling.