Glycerol is an abundant, inexpensive fuel for miniaturized fuel cells with practical power output. Glycerol electrooxidation is coupled to a cathodic reaction, usually oxygen reduction on noble-metal-based catalysts. However, the slow kinetics of oxygen reduction on nonprecious metal catalysts has limited its applications. Here, we propose the use of hypochlorous acid (HClO from bleach) as a potential liquid oxidant and graphene oxide (GO)-modified carbon paper as a metal-free cathode. GO with high oxidation levels is synthesized and used for in situ flowing deposition on a carbon paper cathode. This unique GO deposition method leads to a homogeneous "blanket" coverage of the wetted fibers exposed to the flow, increasing the active surface area by 3.8 times. The prototype mixed-media microfluidic fuel cell with electrodes in a flowthrough configuration featuring glycerol electrooxidation coupled to HClO reduction on GO-modified carbon paper shows 1.72 V open-circuit voltage, 354 mA cm −2 maximum current density, and 110 mW cm −2 peak power density. This unprecedented performance proves that a well-designed in situ electrode modification can be applied to reach high power density without the use of noble metals.