Polarization data of BH-4 oxidation on Cu anode in alkaline solution were measured at steady state by a self-made experimental cell.The experimental polarization curve showed three regions:the region at lower over-potential η(below 0.4 V approximately),the region at higher η(above 0.6 V approximately)and the transitional region at medium η.It was found that the limiting current density is caused by the limiting elemental step rather than the external mass transport.The apparent reaction order with respect to BH-4 changes from 0 to 1.The active energy is about 40 kJ·mol-1 in both regions with lower and higher η,and is higher than 40 kJ·mol-1 in the transitional region.With the experimental temperatures and NaOH concentration,the hydrogen production rates released by the electrode reaction were measured at [BH-4]/[OH-]ratios of 1:40,1:20,1:6.7 and 1:4,which is strongly dependent on the over-potential.The apparent number of released electron n by the reaction was calculated by using the measured amounts of hydrogen and coulomb.Higher value of n was obtained at lower [BH-4]/[OH-]ratio,higher temperature as well as higher over-potential.Under the experimental conditions,the value of n varies from 0 to 7,while in the transitional region it is just in the range of 3 to 5.The experimental results indicate that the limiting current density or n value will increase by virtue of proper experimental conditions.However,to considerably increase the current density at lower over-potential requires a modified anode with higher activity.
A mediator microbial fuel cell (MFC) was constructed by using E. coli as biocatalyst and new methylene blue as electron mediator. E. coli cells were carried out in anaerobic growth prior to inoculating them into the MFC in order to pre-adapt bacterial metabolism in an anaerobic environment, the electricity generation of MFC was tested, its maximum power density reached 263.94 mW/m2 with the corresponding current density 1287.50 mA/m2, the internal resistance of MFC was 200Ω, and capability of the MFC was even better than those reported so far. Moreover, on-electrode taming method was adopted to improve electrochemical activity of E. coli, namely a combination of E. coli taming and electricity generation simultaneously in the same MFC without scraping off the biofilm of MFC, after the 4th on-electrode taming, the tamed E. coli MFC showed a 54% improvement in peak current density, being 612.50 mA/m2, and a 64% improvement in the maximum power output, being 166.67 mW/m2, compared with that of parental E. coli MFC. And the maturation time of tamed biofilm was obviously reduced to 240 min, quickening up 1 times compared with that of parental E. coli biofilm.
