Recent Development of CO2 Electrochemistry from Li–CO2 Batteries to Zn–CO2 Batteries

Metal-CO2 batteries with CO2 as cathode active species give rise to opportunities to deal with energy and environmental issues simultaneously. This technology is more appealing when CO2 is flexibly reduced to chemicals and fuels driven by surplus electricity because it represents a low-cost and controllable approach to maximized electricity utilization and value-added CO2 utilization. Nonaqueous metal-CO2 batteries exhibited high discharge voltage and capacity with carbon and oxalate as reduction products from CO2 electrochemistry that lacks proton. In contrast, aqueous Zn-CO2 batteries implemented flexible CO2 electrochemistry for more value-added products accompanied by energy storage based on a proton-coupled electron transfer mechanism. In this Account, we have exemplified our recent results in the development of CO2 electrochemistry from nonaqueous Li-CO2 batteries to aqueous Zn-CO2 batteries toward practical value-added CO2 conversion. Aimed at the challengingly limited CO2 electrochemistry and high cost of nonaqueous Li-CO2 batteries, we proposed aqueous Zn-CO2 batteries. Our previous works on nonaqueous Li-CO2 batteries, aqueous Zn-air batteries, and aqueous CO2 reduction electrocatalysts further shed light on battery mechanism, device construction, and electrocatalyst design. For example, bipolar membranes maintain the stability of the basic anolyte and neutral catholyte, as well as the kinetics of ion transport at the same time, forming the device base for aqueous Zn-CO2 batteries. Moreover, in terms of the electrocatalyst catalyzing both discharge and charge reactions on the cathode, the design of multifunctional electrocatalysts is of great importance for not only CO2 electrochemistry but also spontaneous discharge and energy efficiency of aqueous Zn-CO2 batteries. We have explored a series of multifunctional electrocatalyst cathodes, including noble metal, transition metal, and metal-free materials, all of which facilitated CO2 electrochemistry in aqueous Zn-CO2 batteries with value-added carbon-based products. Meanwhile, several operating models for practical complicated situations are presented, such as rechargeable, reversible, dual-model, and solid-state batteries. Zn-CO2 batteries with different models require different design mechanisms for electrocatalyst cathodes. Reversible aqueous Zn-CO2 batteries with HCOOH generation were enabled by electrocatalysts capable of catalyzing the interconversion of CO2 and HCOOH at low overpotentials, rechargeable aqueous Zn-CO2 batteries were allowed by electrocatalysts capable of catalyzing efficient CO2 reduction and O2 evolution, and dual-model aqueous Zn-CO2 batteries were realized by electrocatalysts capable of catalyzing CO2 reduction, water oxidation, and oxygen reduction. Concluding remarks include a summary of recent CO2 electrochemistry in metal-CO2 batteries and a brief discussion of future challenges and opportunities for practical aqueous Zn-CO2 batteries, such as highly reduced products and high production rate.