Microbial electrolysis cell (MEC): Strengths, weaknesses and research needs from electrochemical engineering standpoint

Abstract

Microbial electrolysis cells (MECs) produce hydrogen at the cathode associated with the oxidation of organic matter at the anode. This technology can produce hydrogen by consuming less electrical energy than water electrolysis does. However, it has been very difficult so far to scale up efficient MECs beyond the size of small laboratory cells. This article firstly revisits the fundamentals of MECs to assert their theoretical advantages. The low formal equilibrium cell voltage of 0.123 V and electrical and thermal energy yields as high as 10 and 12, respectively, are major assets. Other theoretical strengths are discussed, including the possibility to produce methane, and some safety advantages. The experimental achievements at pilot scale (several litres volume) are analysed through the prism of electrochemical engineering. This analysis leads to recommendations to modify some research efforts, notably by giving priority to increasing current density rather than working with volumetric parameters, using Faradaic yields to detect dysfunctions, and systematizing control experiments at open circuit. The critical analysis successively addresses electrolytes, electrode kinetics, temperature, substrate concentration, reactor architecture, and control procedures. It brings to light intrinsic weaknesses of the MEC concept and identifies improvements that can be made using current technology, for instance, by the catalysis of hydrogen evolution at neutral pH. The problem of the low electrolyte conductivity is pointed out and, in return, how increasing it can be detrimental to the key issue of anode acidification. Finally, research lines are proposed with the objective of moving ahead towards MEC development.

Optimising the Hydraulic Retention Time in a Pilot-Scale Microbial Electrolysis Cell to Achieve High Volumetric Treatment Rates Using Concentrated Domestic Wastewater

“…The fastest HRT of 0.015-days achieved no measureable COD removal with a VTR of 0-kgCOD/m 3 •day. Increasing the HRT increased VTRs until a 0.5-day HRT, where it peaked at 3.82 kgCOD/m 3 •day. The rise shows a logarithmic increase (r 2 = 0.99) and, following this peak, VTRs exponentially decrease (r 2 = −0.862).…”

“…These can be compared to the energetic treatment costs for AS (which is currently the method used to treat this waste), modelling the difference in energetic costs of a full scale BES reactor. From Figure 4, it shows there is a high energetic cost saving of £2967 per day in the first 18 m. Following this, savings decline and then turn negative, with the effluent only reaching 569 mg/L (higher than discharge standards) after a tank length of 268 m and total size of 28,944 m 3 . Between an influent of 668 mg/L and 569 mg/L, a higher COD concentration than would be found in typical raw domestic wastewater, the MEC was energetically more expensive than AS (>2 kJ/gCOD) ( Table 3).…”

“…The reactor was situated at Northumbrian Water Ltd.'s wastewater treatment site at Howdon, South Banks, in their pump control building. Wastewater was piped up from the RSL pipe into a 1m 3 intermediate bulk container (IBC) which was filled and replenished every 2 days with fresh wastewater. The room was not heated, so the RSL remained at ambient temperature.…”

“…This is due to a number of reasons: pilot reactors are expensive; they are difficult to engineer; scale-up shows decreased performance; and data from a pilot-scale reactor is often limited and rarely in duplicate. Therefore, only a small number of pilot-scale BESs have been built, as reviewed by Rousseau et al [3] and Gajda et al [4]. All use a range of configurations, operate under different conditions and use different substrates.…”

Material–Microbe Interfaces for Solar-Driven CO2 Bioelectrosynthesis

“…This is a relatively underexplored area for biohybrids. Several factors such as type of biocatalyst, nanomaterials, concentration of NPs, hydraulic retention time, temperature, electron transfer mechanism ( Table 2 gives the equations and Gibbs free energy for CO 2 reduction into various products), CO 2 solubility, role of electrolyte, and design of reactor need to be optimized to ensure the maximum efficiency and cost-effectiveness [75].…”