Microbial fuel cells for hydrogen generation

Microbial fuel cells for hydrogen generation



INTRODUCTION


Microbial fuel cells (MFC) from biodegradable compounds offer us new proposals to produce energy sustainably. MFCs can work from different carbohydrates. Although it is the most effective, they can also work from complex substrates that are found in some residual waters. The bacteria found in the MFCs will use different methods based on the operational parameters of our MFC. As main competing or alternative with the current alternatives for the generation of hydrogen we evaluate the relation with the MFC technology.


Uses of hydrogen:

• Energy storage: Economic storage of renewable energies. 
• Automotive and transport industries: emissions-free fuel solution for the automotive and transport industries. 
• Refineries: hydrocracking – Green hydrogen for petroleum refineries. 
• Fertiliser production: climate-neutral hydrogen for ammonia production. 
• Hydrogen production from wastewater: cost-effective water purification with the help of plasma electrolyzers. 
• Gas separation: gas separation of oxygen/natural gas/hydrogen –innovatie membrane technology. 

PROCESSES


• Methanogenesis:


Is the formation of methane by microbes known as methanogens. Methanogens are organisms capable of producing methane a group phylogenetically distinct from both eukaryotes and bacteria, although many live in close association with anaerobic bacteria. The production of methane by anaerobic digesters is an important form of microbial metabolism.

Reaction:

CO2 + 4 H2 → CH4 + 2 H2O

CH3COOH → CH4 + CO2

Examples of methanogens:

• Methanobacterium bryantii 
• Methanobacterium formicum 
• Methanobrevibacter arboriphilicus 

• Electrogenesis

Electrogenesis is the generation of electricity by living organisms such us microbial fuel cells or even some aquatic creatures like electric rays, or electric catfish who use it for electrolocation, self-defense, electrocommunication and sometimes the stunning of prey.

There are several mechanisms for extracellular electron transport, for example some bacteria use nanowires in biofilm to transfer electrons towards the anode.

Furthermore, electron hopping from one bacteria to another in biofilm towards an anode through their outer membrane cytochromes is also another electron transport mechanism (Bonanni et al 2013).

Exoelectrogens are the main microorganisms that are aviable to generate electricity, usually related with Microbial fuel cells (MFCs)

Examples of elelectrogens:

Figure 2. Different examples of electrogens organisms. (Source: Huarachi-Olivera et al 2017)

• Electrohydrogenesis


Electrohydrogenesis or biocatalyzed electrolysis it’s a process that generates hydrogen gas from organic matter disintegrated by bacteria. This process uses a modified fuel cells to retain organic matter and water. Therefore, a study reported by Cheng and Logan shows that only a small of electricity is used. This is calculated relative to the amount of electricity used, wateheat lowers the overall efficiency.

Figure 3. Microbial electrolysis cell (Source: Zina Deretsky, National Science Foundation)

CURRENT SOURCES OF H2 PRODUCTION

Figure 4. Global production of hydrogen (Source: Air Products and Chemicals, 2003) 

ALTERNATIVE SOURCES OF H2 PRODUCTION


·Sediment microbial fuel cell


Sediment microbial fuel cells (SMFC’s) are a type of MFC’s systems. Anode and cathode are contained in two compartments, the cathode is in the water column (top of the reactor) and the anode is inside the sediment (bottom of the reactor), the device operates on the potential gradient at a sediment-water interface.

It’s possible to produce hydrogen gas (H2) removing the oxygen at the cathode and adding in a small voltage with a bioelectrochemically assisted microbial reactor process or the biocatalyzed electrolysis process.

This process have an important limitation, a low and variable power generation. To solve this problem we can use a power management system for store the energy produced by SMFC in rechargeable batteries and then use this energy to isolate H2.

Figure 5. Schematic diagram of two-chamber sediment microbial fuel cells (Source:US DOE. 2005. Genomics:GTL Roadmap, DOE/SC-0090, U.S. Department of Energy Office of Science. (p. 76) )

·Wastewater microbial fuel cell


Microbial fuel cells is a device where bacteria is used to produce an electrical energy current from, in these case, wastewater.

At the moment, we have lots of wastewater treatment plants that consume electrical power but, it could be possible to transform these treatment plants to power plants by take advantage of the bacteria or microbes in the wastewater?

In that conditions of wastewater we have billions of bacteria that break down organic matter to produce hydrogen electrons. They liberate their electrons to oxygen molecules in exchange for energy.

What happens in a microbial fuel cell is that the electrons take a deviation. In the device, we have a tube that connect electrons from an oxygen free conditions, which is isolated so the bacteria can’t get at the oxygen. The other side is directly exposed to oxygen.

In that device microbes are the most important part and they grow on the oxygen free side and send off the electrons to the electrode of their side. This electrons flow through the circuit and we extract the energy resulting from that.

The hydrogen electrons that arrive at the other side of the tube can combine with oxygen.

It’s actually difficult to move from a little device to a useful mechanism for the wastewater in a treatment plant, but if that it’s achieved wastewater will be no longer waste.

Figure 6. Device that produces energy and hydrogen gas from wastewater, (source:https://www.youtube.com/watch?v=ZotwUJAb8R4)

CONCLUSIONS



Hydrogen production can be made from different methods. The conventional methods are: methanogennesis, electrogenesis and electrohydrogenesis. Despite the different processes of hydrogen generation, the main source it is still the natural gas. 

Although, alternative source have been found that are more sustainable with the environment. One of them is through wastewaters.

The bacteria found in the wastewater use organic matter to produce energy. During this process they release hydrogen electrons. These electrons travel from the site without oxygen through the circuit producing energy. In the electrode on the other side The hydrogen electrons are reconciled with oxygen in the electrode of the side with oxygen.