The unusual ability of O2-tolerant hydrogenases (H2ase) to produce electricity from a H2–air mixture (when used as the anodic electrocatalyst in a simple, membrane-less fuel cell) is investigated with the aim of establishing a strategy for raising volume power density, the measure of importance for miniature devices. Compacted mesoporous carbon electrodes provide a simple and inexpensive method for obtaining a large increase in productive enzyme loading, greatly increasing current densities and stability. Operated under a 78% H2–22% air mixture at 25 °C, typical current densities at a stationary H2ase anode and bilirubin oxidase cathode are 4.60 ± 0.32 mA cm−2 and 1.23 ± 0.12 mA cm−2, respectively. The power limitation due to low O2 concentration is addressed by re-proportioning the cathode/anode area ratio to balance the cathodic and anodic currents. At room temperature, the maximum power density of the fuel cell with an anode/cathode (A/C) ratio of 1:3 (1A/3C) is 1.67 ± 0.24 mW cm−2 (per anode area) or 0.42 ± 0.06 mW cm−2 (per total area). Good prospects for stability are demonstrated by the fact that 90% of the power is retained after continuously working for 24 h, and more than half of the power is retained after one week of non-stop operation. Using an even weaker O2 mixture (89% H2, 11% air) the 1A/3C cell gives over 0.8 mW cm−2 (anode) or 0.2 mW cm−2 (total electrode area). The results demonstrate the feasibility of membrane-less hydrogen–air fuel cells delivering volume power densities well in excess of 1 mW cm−3.