Water-Activated Metal–Air Paper Batteries: Toward Realizing Safe, Sustainable, and High-Performance Wearable Sensing and Emergency Power Sources

The rapid development of wearable electronics has increased demand for power sources that are lightweight, safe, sustainable, and capable of delivering sufficient power and capacity. Water-activated metal–air paper batteries have recently emerged as a promising alternative, combining high energy density with intrinsic safety and environmental compatibility. In this spotlight, we summarize the structure, operating principles, and performance characteristics of water-activated metal–air paper batteries. These batteries employ dry paper as both a separator and electrolyte reservoir, enabling electricity generation upon water absorption via capillary action. The output performance is governed by the choice of anode metals (Al, Zn, and Mg), cathode catalysts for the oxygen reduction reaction, paper properties, and electrolyte conditions. In particular, Mg-based paper batteries operating with neutral electrolytes exhibit a favorable balance of high output voltage, power density, and capacity while avoiding hazardous materials. We further review reported performance metrics of Al-, Zn-, and Mg-anode paper batteries and highlight recent advances using rare-metal-free cathode catalysts, including macrocyclic metal complexes and biomass-derived carbon catalysts. Owing to their thin form factor, water-triggered activation, and high safety, metal–air paper batteries represent a promising energy solution for both wearable technologies and emergency-use scenarios.