Chemically modified carbon nanotube fibers enable unique power sources driven entirely by a chemical potential gradient. Electrical current (11.9 mu A mg(-1)) and potential (525 mV) are reversibly produced by localized acetonitrile doping under ambient conditions. An inverse length-scaling of the maximum power as L-1.03 that creates specific powers as large as 30.0 kW kg(-1) highlights the potential for microscale energy generation.