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.