fig3

Figure 3. (A) Diagrams of the structure and working principle of proton-exchange membrane fuel cells (reproduced with permission from Springer Nature, copyright 2021)^[73]; Schematics of (B) an anion-exchange membrane fuel cell (reproduced with permission from Elsevier, copyright 2021)^[78], (C) a liquid Zn-air battery (reproduced with permission from Wiley-VCH, copyright 2023)^[88], and (D) a solid flexible Zn-air battery (reproduced with permission from Wiley-VCH, copyright 2023)^[90]. ORR: Oxygen reduction reaction; OER: oxygen evolution reaction; AEM: anion-exchange membrane.