Electron transfer is not quite trivial in an MFC. Some bacteria have the ability to transfer electrons directly to the anode material, typically some form of carbon, while others require a shuttle or mediator. This adds another layer of complexity to the electrode design for the MFC. As the anode is the electrode upon which the biological community exists, preparation of anodes for MFC is radically different from that for other fuel cells. The anode material is inoculated with a source of bacteria, often a colony from an already active MFC. All manner of different forms of carbon (brush, felt, fiber, mesh, granular graphite, and more) have been used as the scaffold for the anode. As expected, materials that provide a larger surface area have demonstrated better performance, with respect to both electron transfer and bacterial growth. Because of the variety of inoculation sources and reactor types, it is otherwise difficult to draw conclusions about the performance of these various types of carbon materials.
Metals such as stainless steel, titanium, and gold have also been used for MFC anode fabrication, with the requirement being that the metal must not corrode under the operating conditions of the cell. Because bacteria do not adhere well to a smooth metal surface, metal-based anodes have generally shown lower power densities than comparable graphite anodes (Wei et al. 2011). It is interesting to note that some sort of surface treatment is helpful even for carbonaceous materials, and several approaches (primarily chemical modifications such as coatings or surface changes) have been tried with positive results. In addition, composite materials (e. g., a metal – graphite pair) have been investigated. However, all of these alterations must take into account the impact on the growth and health of the bacterial electrocatalysts.