Dear Roohangiz,

Are you using a three cell electrode?

Then, you have to think that you actually have a working electrode and a counter electrode and both can act as anode or cathode depending on your cell conditions.

It seems that your working electrode began to work as anode but it finally acted as cathode.

I am sorry about my lack of knowledge in MFC to know if this is normal or not.

I work on ethanol oxidation (not methanol) and it is unusual to detect negative CA current for the oxidation reaction.

In this instance, it is probably that you have got a reduction reaction (of oxygen for example) in stead of the oxidation of methanol.

Do you degas your electrolytes before running your CA and CV?

20-40 min of N2 or Ar bubbling might help take the dissolved gases in your solution.

Also, the working electrode preparation is most sensitive.

Hope this help you.

The air-cathode phototrophic MFC, however, had no Biofilm formed on the cathode and did not show a negative current under the light (Figure S1), suggesting that the cathodic Biofilm of the sediment phototrophic MFC played a role in the reversed current generation. In addition to oxygen effect, substrate changes may also contribute to current reversal (30), though the exact reason remains unclear.

The negative effect of oxygen can be alleviated by using color filters to filtrate the full-spectrum light or increasing the thickness of the sediment layer on the anode. Color filters would not completely inhibit the activities of photosynthetic microorganisms because most of them have a wide adsorption spectrum (31). Instead, color filters may lessen oxygen evolvement by reducing light strength. However, it requires further investigation on how the photosynthetic microorganisms in this study were affected by red or blue light.

Zhen He et al., 2009 : Self-Sustained Phototrophic Microbial Fuel Cells Based on the Synergistic Cooperation between Photosynthetic Microorganisms and Heterotrophic Bacteria. Environ. Sci. Technol., 2009, 43 (5), 1648-1654• DOI: 10.1021/es803084a • Publication Date (Web): 02 February 2009

Downloaded from http://pubs.acs.org on April 12, 2009

@Ahmed M Elsheikh Actually I purge N2 before CV test and I repeated this test more than 50 time and almostly I get similar results. Anyway thanks for your help...

@Jordi Carrillo Abad thanks for your help

@Abdullah Ahmad Thank you sincerely for your descriptions,

It seems your writings are related to the topic of Photocatalyst, but my project is related to electrocatalysts, and unfortunately I do not have any information about photocatalysts. Anyway thanks for your time...