The types of carbonaceous material and electrocatalyst are the critical components in fabrication of gas diffusion electrodes for fuel cells and metal-air batteries. In this study, carbon nanocapsule (15-30 nm) and Ag-carbon nanocapsule powders were synthesized and characterized for their electrochemical performances as oxygen reduction electrocatalyst in alkaline electrolyte using commercial noncatalyzed gas diffusion electrode as substrate. The i-V polarization response of the carbon nanocapsule demonstrated enhanced electrocatalytic capability over those of XC-72R and vapor growth carbon fiber, delivering 0.80 V at 200 mA/cm(2). In addition, Ag-carbon nanocapsule powders exhibited a value of 0.99 V at 200 mA/cm(2), surpassing commercially available Mn-catalyzed, and MnCo-catalyzed gas diffusion electrodes. Galvanostatic discharge of these Ag-carbon nanocapsule powders from 10 to 200 mA/cm(2) confirmed their stability and sustainability. This report identified a class of carbon material that not only exhibits electrocatalytic capability itself but also provides opportunity as substrate for known electrocatalysts. (c) 2007 The Electrochemical Society.