In the past 10–15 years, the microbial fuel cell (MFC) technology has captured the attention of the scientific community for the possibility of transforming organic waste directly into electricity through microbially catalyzed anodic, and microbial/enzymatic/abiotic cathodic electrochemical reactions. In this review, several aspects of the technology are considered. Firstly, a brief history of abiotic to biological fuel cells and subsequently, microbial fuel cells is presented. Secondly, the development of the concept of microbial fuel cell into a wider range of derivative technologies, called bioelectrochemical systems, is described introducing briefly microbial electrolysis cells, microbial desalination cells and microbial electrosynthesis cells. The focus is then shifted to electroactive biofilms and electron transfer mechanisms involved with solid electrodes. Carbonaceous and metallic anode materials are then introduced, followed by an explanation of the electro catalysis of the oxygen reduction reaction and its behavior in neutral media, from recent studies. Cathode catalysts based on carbonaceous, platinum-group metal and platinum-group-metal-free materials are presented, along with membrane materials with a view to future directions. Finally, microbial fuel cell practical implementation, through the utilization of energy output for practical applications, is described.

Microbial electrochemical reduction of CO₂was carried out under two different applied potentials, −0.36 V and −0.66 Vvs.SHE, using a biological sludge as the inoculum.

The abilities of carbon cloth, graphite plate and stainless steel to form microbial anodes were compared under identical conditions. Each electrode was polarised at −0.2 V vs. SCE in soil leachate and fed by successive additions of 20 mM acetate. Under these conditions, the maximum current densities provided were on average 33.7 A m⁻² for carbon cloth, 20.6 A m⁻² for stainless steel, and 9.5 A m⁻² for flat graphite. The high current density obtained with carbon cloth was obviously influenced by the three-dimensional electrode structure. Nevertheless, a fair comparison between flat electrodes demonstrated the great interest of stainless steel. The comparison was even more in favour of stainless steel at higher potential values. At +0.1 V vs. SCE stainless steel provided up to 35 A m⁻², while graphite did not exceed 11 A m⁻². This was the first demonstration that stainless steel offers a very promising ability to form microbial anodes. The surface topography of the stainless steel did not significantly affect the current provided. Analysis of the voltammetry curves allowed two groups of electrode materials to be distinguished by their kinetics. The division into two well-defined kinetics groups proved to be appropriate for a wide range of microbial anodes described in the literature.

Bacterial respiration of nitrate is a natural process of nitrate reduction, which has been industrialized to treat anthropic nitrate pollution. This process, also known as “microbial denitrification”, is widely documented from the fundamental and engineering points of view for the enhancement of the removal of nitrate in wastewater. For this purpose, experiments are generally conducted with heterotrophic microbial metabolism, neutral pH and moderate nitrate concentrations (<50 mM). The present review focuses on a different approach as it aims to understand the effects of hydrogenotrophy, alkaline pH and high nitrate concentration on microbial denitrification. Hydrogen has a high energy content but its low solubility, 0.74 mM (1 atm, 30 °C), in aqueous medium limits its bioavailability, putting it at a kinetic disadvantage compared to more soluble organic compounds. For most bacteria, the optimal pH varies between 7.5 and 9.5. Outside this range, denitrification is slowed down and nitrite (NO₂⁻) accumulates. Some alkaliphilic bacteria are able to express denitrifying activity at pH levels close to 12 thanks to specific adaptation and resistance mechanisms detailed in this manuscript, and some bacterial populations support nitrate concentrations in the range of several hundred mM to 1 M. A high concentration of nitrate generally leads to an accumulation of nitrite. Nitrite accumulation can inhibit bacterial activity and may be a cause of cell death.