by Kudzai Mtambanengwe and Ronen Fogel
Fossil fuels have been providing energy world-wide for the last few centuries. The discovery of fossil fuels was the major driving force behind modern industry. During the 19th century, when the use of fossil fuel was still being uncovered, many deposits were being discovered, the human population was small with limited energy demands and fossil fuels therefore seemed at the time to be abundant. As our understanding of how to refine and better release the chemical energy found in them grew, so too did our reliance on them to power the newly-invented equipment that drove industry.
Skip ahead to the present day and humankind’s dependence on fossil fuels has increased exponentially since the 1800’s – during 2008, fossil fuels accounted for more than 86% of the global demand energy. The increase is a direct result of the global population boom and technological advances that lead the development of coal- or oil- dependent technologies, such as cars. The demand for fossil fuels has been projected to increase continuously and it has become abundantly clear that fossil fuels are a limited, non-renewable natural resource. The combustion of fossil fuels has also led to the release of greenhouse gases that have contributed significantly to air pollution and climate change. This has led to the investigation into a range of alternative energy sources that are renewable and do not contribute to greenhouse gas emissions.
Many forms of renewable energy are currently being researched around the world, including solar-, wind-, hydro- and biomass-based energy. Biomass, a term used for organic material produced by living organisms, is abundant in various forms. Burning wood is a form of biomass energy production, as you’re converting the chemical energy found in the bonds between atoms in the organic matter to heat and light. So, from crop and plant biomass even to domestic and certain industrial waste products, there’s plenty of biomass as possible fuel sources.
A useful advantage to using waste products is that it is abundant and largely quite cheap. Waste generally also tends to be quite a problem for industry, the environment and usually is an additional cost to treat. The trick is finding ways to utilise waste in a way that it can either be recycled or utilised in a beneficial way.
Fuel cells represent a promising alternative energy source that does just that. Traditional chemical fuel cells break down simple chemicals, converting the chemical energy of the chemical bonds into electrical energy. Biological fuel cells utilise the energy in biological materials, which can be more complex chemicals, or a mixture of chemicals, such as those found in biomass. A subcategory of the biological fuel cells is the microbial fuel cell or MFC for short.
MFCs make use of microorganisms, such as bacteria, as catalysts to convert chemical energy through enzymatic reactions to generate electricity. The real beauty of using living microorganisms is that they replicate, thus creating a continuous supply of catalysts for generating power. MFCs can generate bioelectricity while “cleaning up” waste – effectively using the organic waste they break down as a source of electrons. This handily solves the two problems of removing pollution and generating energy at the same time. Bacteria are commonly used in MFCs because they can use a wide range of substrates as their food sources (again, useful for breaking down biomass), grow quite rapidly, they are easy to handle and adaptable.
In two chambered fuel cells, the design is fairly straightforward comprising an anode and a cathode. The removal of electrons from atoms and molecules happens at the anode and the cathode donates electrons to other compounds. The bacteria are added to an anode chamber, which is designed to be anaerobic. Once there, the bacteria metabolise the organic substrate as a food source, which releases a mixture of protons and electrons. The electrons travel from the anode chamber through an external circuit (where the electrical energy can be put to use) to a cathode chamber, while the protons travel from the anode chamber to the cathode chamber through a selective membrane that is proton-restrictive. In the cathode chamber, the electrons react with a final electron acceptor, such as oxygen, which they reduce and the reduced oxygen reacts with the protons to form harmless water.
In essence, MFCs harness cellular mechanisms that are already present to produce energy. Cellular respiration relies on the breakdown of organic molecules (like glucose) to simpler molecules (Carbon dioxide). In so doing, it carries the energy from the breaking of chemical bonds in electrons and protons and uses that energy to synthesise energy-containing compounds, like ATP in several stages. Once as much energy as possible is extracted from the electrons, they are transferred to oxygen, to produce water. MFCs just add an additional step to this, by making the electrons and protons travel to the cathode before being united with oxygen. This is also why it’s important to have no oxygen present where the bacteria are – if there was, they’d take the simpler and more energy-efficient step and just transfer their electrons to the oxygen close to them.
Curently, research is divided pretty evenly between improving the electrodes used in MFCs, the membrane separating the two electrode chambers and the bacteria’s interaction with the fuel of choice. Improving the bacteria’s ability to break down biomass is an obvious strategy to improving the amount of electrical energy we can get from MFCs – the faster and more efficiently they can use the biomass as a food source, the more energy we can siphon off in the form of electrical energy. Finding better-suited bacteria for biomass breakdown and coaxing them into more efficient metabolic pathways is a major research area for this.
As we’ve said previously, the ability of MFCs to use complex organics allows the use of waste products as fuel. This will allow MFCs to remove pollutants while simultaneously producing electrical energy. Various waste streams have been studied in MFCs with some degree of success. The robustness of MFCs makes the technology very useful in bioremediation. The use of waste streams would make MFCs a viable option for energy harvesting because waste streams are readily available and are a rich source of energy that is usually disposed of.
The proton-selective membrane used is also widely researched. Ideally, it should also prevent oxygen from travelling from the cathode to the anode, while also letting the protons travel rapidly in the other direction (to faster get protons to the cathode). Research in nanotechnology is being rapidly integrated in the last area of improvement – that of improving the electrodes for more rapid and efficient electron transfer while still making their production cost-effective. As we’ve discussed, when bacteria start to break down the biomass, electrons travel from the bacterial cells to the anode electrode. Most research in this field shows that modifying the electrode generally improves electron transfer, in turn improving the current generated from MFCs. Size is important here and the greater the surface area the better. The use of nanomaterials (such as carbon nanotubes) to improve this aspect of fuel cells are currently showing great promise here.
The cathode electrode is responsible for the oxygen reduction reaction (ORR), where oxygen is reduced by the electrons harvested by the anode and reacts with the protons to produce water. This reaction generates the electrical current of an MFC. Catalysts to help the rate and efficiency of this reaction are highly sought after. The less energy that the system spends in pushing electrons into oxygen, the more energy the system has to power whatever’s in the electrical circuit. Currently, the best known catalyst for the ORR is platinum, which is very expensive and not quite abundant enough for widespread use. Cheap alternative catalysts to this have been studied and a group of organo-metallic compounds known as phthalocyanines have shown particular promise as catalysts for oxygen reduction, especially when combined with nanomaterials to give a better surface area to the electrode.
MFCs are definitely a technology to watch for in the foreseeable future and with a little more research, they are bound to be used in many waste treatment facilities in the downstream process for further energy harvesting and bioremediation.