Study Concludes That Microbial Electrolysis Cells Are a Promising Approach to Renewable and Sustainable Hydrogen Production
A review of the materials, architectures, performance, and energy efficiencies of emerging microbial electrolysis cell systems (MECs) finds that MECs can efficiently convert a wide range of organic matter into hydrogen and are therefore a promising technology for renewable and sustainable hydrogen gas production from organic feedstocks.
However, the researchers conclude, there are a number of outstanding research questions that must be resolved for MECs to develop into a mature, commercial hydrogen production technology. The paper was published online 1 November in the ACS journal Environmental Science & Technology.
The review team included the two research groups who independently discovered several years ago that bacteria could be used to make hydrogen gas in an electrolysis-type process based on microbial fuel cells (MFCs). One group was led by Dr. Bruce Logan at Penn State, the other by Dr. René A. Rozendal at the University of Queensland (Australia).
MECs show high hydrogen yields and they need only a relatively small electrical energy input. Given these interesting properties, MECs could become viable technology to produce renewable hydrogen, provided a clean and renewable electricity input is used. Renewable hydrogen has many applications, the most prominent ones being for transportation and industry.
Transportation fuels are currently responsible for about 20 to 25% of the global fossil fuel consumption. Because of climate change, and instabilities in the fossil fuel market, there is great interest in hydrogen as a transportation fuel (i.e., the hydrogen economy). Moreover, even without a hydrogen economy, there exists a large hydrogen demand.
In 2000, the global hydrogen consumption was already estimated to be 50 million tons per year, with about two-thirds used by the petrochemical industry. This hydrogen is used for upgrading fossil fuels and synthesis of industrial chemicals such as ammonia and methanol. Other industries that consume significant amounts of hydrogen include the food industry (saturation of fats and oils) and the metal industry (as a reducing agent for metallic ores).
MECs can contribute significantly to these hydrogen demands by producing large quantities of hydrogen from renewable resources such as biomass and wastewaters. The MEC concept is now well proven, and significant advancements have been made with respect to the performance in only a few years since its discovery.—Logan et al. (2008)
In a microbial fuel cell, bacteria oxidize organic matter and release carbon dioxide and protons into solution and electrons to an electrode (anode). (Earlier post.) The electrons flow from the anode through an electrical circuit to the cathode where they are consumed in the reduction of oxygen. Without oxygen, current generation is not spontaneous. However, if a small voltage (>0.2 V in practice) is applied between the anode and the cathode, hydrogen gas is produced at the cathode through the reduction of protons. The system based on this latter process is termed a microbial electrolysis cell.
MEC systems are based on a number of components, each of which will require much additional investigation.
Microorganisms. Little is known about the composition of the microbial communities in MECs, the authors note. The only study of a community analysis of an MEC found that Pseudomonas spp. and Shewanella spp. were present on the anode. Microorganisms are observed to be attached to the cathode, but to what extent they affect the function of the MEC is not clear.
Nor is it clear to what extent the operation of an MEC is affected by the inoculum source.
Another production issue to be resolved is that high concentrations of hydrogen gas also favors the growth of methanogens, reducing hydrogen gas production and contaminating the product gas with methane.
Materials. The anode material in a MEC can be the same as the material in a MFC—e.g., carbon cloth carbon paper, graphite felt, graphite granules or graphite brushes. Hydrogen production in an MEC occurs at the cathode. Because the hydrogen evolution reaction (HER) on plain carbon electrodes is very slow, a high overpotential is required to drive hydrogen production. To reduce this overpotential, platinum is usually used as a catalyst.
There are many disadvantages to using platinum, the authors note, including the high cost and the negative environmental impacts incurred during mining/extraction. Exploration of biocathodes are underway.
Other materials issues include membranes (although some MECs are membraneless), and tubing and gas collection systems.
Potential feedstock sources for MECs include wastewater (and wastewater treatment is a major potential application for MECs) and cellulosic biomass.
So far, MECs have achieved hydrogen production of up to 3.12 m3H2/m3 d with energy input of 0.8 V, values which are in the same order as those of fermentation systems, according to the reviewers. MEC systems have reached a maximum current density of 186 A/m3. This, the authors note, is much lower than those in the more-extensively studied MFCs (5,600 A/m3, 10 A/m2), and thus it is likely that with additional research, higher current densities will be achieved in MECs in the future.
The reviewers suggest that for MECs to become a mature hydrogen production technology, several research questions still need to be addressed:
More experience is required with real organic feedstocks containing complex organic substrates such as polymeric and particulate substances;
Novel, more cost-effective chemical and/or biological cathodes need to be developed that show low potential losses and are not platinum-based;
Membrane pH gradients need to be eliminated, or membranes should not be used in the reactor;
Methanogenic consumption of the hydrogen product needs to be prevented (in case of membrane-less MECs and/or MECs with a biocathode); and, most critically,
A cost-effective, scalable MEC design needs to be developed.
Bruce E. Logan, Douglas Call, Shaoan Cheng, Hubertus V. M. Hamelers, Tom H. J. A. Sleutels, Adriaan W. Jeremiasse, and René A. Rozendal (2008) Microbial Electrolysis Cells for High Yield Hydrogen Gas Production from Organic Matter. ASAP Environ. Sci. Technol., doi: 10.1021/es801553z