Recent Development of CO Electrochemistry from Li-CO Batteries to Zn-CO Batteries.

Metal-CO batteries with CO as cathode active species give rise to opportunities to deal with energy and environmental issues simultaneously. This technology is more appealing when CO 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 CO utilization. Nonaqueous metal-CO batteries exhibited high discharge voltage and capacity with carbon and oxalate as reduction products from CO electrochemistry that lacks proton. In contrast, aqueous Zn-CO batteries implemented flexible CO 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 CO electrochemistry from nonaqueous Li-CO batteries to aqueous Zn-CO batteries toward practical value-added CO conversion. Aimed at the challengingly limited CO electrochemistry and high cost of nonaqueous Li-CO batteries, we proposed aqueous Zn-CO batteries. Our previous works on nonaqueous Li-CO batteries, aqueous Zn-air batteries, and aqueous CO 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-CO 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 CO electrochemistry but also spontaneous discharge and energy efficiency of aqueous Zn-CO batteries. We have explored a series of multifunctional electrocatalyst cathodes, including noble metal, transition metal, and metal-free materials, all of which facilitated CO electrochemistry in aqueous Zn-CO 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-CO batteries with different models require different design mechanisms for electrocatalyst cathodes. Reversible aqueous Zn-CO batteries with HCOOH generation were enabled by electrocatalysts capable of catalyzing the interconversion of CO and HCOOH at low overpotentials, rechargeable aqueous Zn-CO batteries were allowed by electrocatalysts capable of catalyzing efficient CO reduction and O evolution, and dual-model aqueous Zn-CO batteries were realized by electrocatalysts capable of catalyzing CO reduction, water oxidation, and oxygen reduction. Concluding remarks include a summary of recent CO electrochemistry in metal-CO batteries and a brief discussion of future challenges and opportunities for practical aqueous Zn-CO batteries, such as highly reduced products and high production rate.