The liquid level difference between the cathode and the anode increased obviously with increase of operation cycles in the dual chamber microbial fuel cell (DCMFC). To analyze this phenomenon

the transport behavior of proton and water was investigated from evaporation

osmotic pressure

metabolism and electric field. The relationship between water production and the fuel's performance was studied. The results showed that within 360 h

the liquid change due to evaporation and osmotic pressure was less than 0.50 ml (liquid level declined near 0.5 mm). Within 312 h of circuit breakage

the anodic metabolism gas led to the proton exchange membrane (PEM) deformation convex to the cathode. The anodic liquid decreased 6.20 ml (liquid level reduced near 6.5 mm)

the cathodic liquid increased 10.75 ml (liquid level rose near 11.2 mm) and the liquid level difference reached 17.7 mm. Under the circuit connection

except the PEM deformation

the protons were dragged by electro-osmotic to the cathode and reduced to water. Within 312 h

the anodic liquid decreased 10.70 ml (liquid level reduced about 11.1 mm)

the cathodic liquid increased 17.00 ml (liquid level rose about 17.7 mm)

and then formed a 28.8 mm liquid level difference. Moreover

the water transmission increased with the increase of output voltage. The results implied that the biological metabolism and electro-osmotic had an important influence on DCMFC liquid difference. It was possible to calculate proton transfer rate based on its water production. The proton transfer rate in the system was over 54%. This study provided a simple and intuitive basis for judging the electricity production efficiency.