Cellulose is one of the most abundant renewable resources in the world. Cellulose has previously been investigated to produce H2, ethanol and biogas mainly through biological fermentation processes. It was recently shown that cellulose could also be used to directly generate electricity in a two-chamber Microbial fuel cell (MFC) or to generate hydrogen in a two-chamber microbial electrolysis cell (MEC). However, the power density or H2 rate was low. We report here that a maximum power density of 1080 mW/m2 can be obtained using cellulose in a single-chamber MFC with a Pt-catalyzed air-cathode and carbon fiber brush anode. This power density is around 20 times higher than that previously obtained using a two-chamber bottle MFC. By using single-chamber MEC, we achieved hydrogen recoveries of up to 75 % and a hydrogen production rate of 0.5 m3-H2 per m3 reactor volume per day at an applied voltage of 0.8 V.