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The advancement of technology has led to the development of new forms of energy storage options. Energy storage devices such as rechargeable batteries continue to dominate the market, but fuel cells are on the rise. It has been almost a century since the first commercially hydrogen-oxygen fuel cell was introduced. Recent advances in fuel cell energy efficiency1, and power densities, coupled with efforts to combat climate change and the push for green energy generation, have made them more attractive, especially those using green hydrogen. This post will look at the chemistry behind fuel cells and the different fuel cell variations that have been developed to support a wide range of applications.
Fuel cells are an energy-generation technology similar to batteries but with some fundamental differences. One of the key differences between fuel cells and batteries is that fuel cells require a continuous fuel source to function. In contrast, the shuttling of already present ions between electrodes produces energy in batteries. If a continuous fuel source is supplied—the most common choices being hydrogen and oxygen—the fuel cell can continue to produce energy. The fuel cell’s ability to provide continuous power can be advantageous in certain applications—for example, as backup power2 for data centers to move away from fossil-fuel-burning standby diesel generators and as an alternative source of electricity (microgrids) for rural areas that will not only have power but also be able to save significantly on infrastructure costs.
Like the standard battery setup, fuel cells possess an anode, cathode, and electrolyte (between the electrodes) and rely on various electrochemical reactions to produce energy. For fuel cells, it is the reactions that occur at both electrodes that generate electricity, and although there are many different types of fuel cells, the most common working mechanism involves the movement of protons—i.e., positively charged hydrogen ions—between the electrodes. Both electrodes contain a catalyst that helps to facilitate the electrochemical reactions and break down the feedstock ‘fuel’ into their corresponding ionized species. The catalyst does vary from fuel cell to fuel cell, but it needs to be a material that can facilitate oxygen and hydrogen reactions, so common choices include platinum and nickel.
While hydrogen is the basic fuel for a cell, they also require oxygen, and these are the two basic feedstocks that need to be continually supplied for most fuel cells to work. Because the electrolyte separates the two electrodes, there are two distinct electrode-electrolyte interfaces, and these are the areas where the electrochemical reactions take place.
When the hydrogen enters via the anode, the electrochemical reactions at the anode remove their electrons, creating positively charged hydrogen ions— i.e., protons. The released electrons then enter the external circuit, generating a current, while the hydrogen ions pass through the electrolyte and to the cathode. The electrolyte acts as a proton exchange membrane (PEM) that only allows positively charged ions through, excluding any removed electrons so that they only enter the external circuit and don’t try to diffuse to the other electrode, as this would stop the relevant chemical reactions from occurring. Oxygen is also fed into the cathode, and when the catalyst breaks the molecular oxygen down into negatively charged oxygen ions, the electron ejected to the external circuit, recombines with the positively charged hydrogen ions as well as the oxygen ions at the cathodic interface. The result is that water is produced as the end product, which is removed via an exhaust from the fuel cell.
This basic fuel cell mechanism is known as the ‘hydrogen fuel cell’ and is the most common type of fuel cell in existence. Though this is the default fuel cell mechanism, there are variations. Most fuel cells work using similar principles, and all require hydrogen and oxygen as the fuel, but some require others in addition. The main differentiator of all the fuel cells is the type of electrolyte used to transport hydrogen ions to the cathode.
The main types of fuel cells are:
As with anything, the different variations all have their place, and some are only suitable in certain situations.
The ‘alkali’ in alkali fuel cells comes from the electrolyte used, which is potassium hydroxide (an alkaline substance) and is the only differentiator from other types of fuel cells, other than being a fuel cell that operates at low temperatures. MCFCs, on the other hand, operate at higher temperatures and have an inlet of carbon dioxide as well as oxygen and hydrogen because the carbonate ions in the electrolyte get used up and needs to be replenished by injecting carbon dioxide). MCFCs use salt carbonates as the electrolyte. Because they operate at higher temperatures, they are not suitable for all applications, particularly home use, as the high temperatures can cause leaks.
PAFCs, another low-temperature fuel cell, use phosphoric acid as the electrolyte. However, PAFCs are an interesting variation as the internal workings can tolerate the formation of carbon monoxide, meaning that gasoline can be used as a fuel, although this is not a green option. Unlike the others, SOFCs and PEM fuel cells use a non-liquid electrolyte, with SOFCs using a metal oxide ceramic compound (such as zirconia) and PEM fuel cells using a thin and permeable polymer sheet. SOFCs operate at very high temperatures and are limited in use again as the electrolyte can crack instead of leak. PEM fuel cells, on the other hand, are a very low-temperature fuel cell but their efficiencies are lower, and the fuel must be purified before use.
In addition to producing energy through the electrochemical reactions, the heat produced can also be harnessed to produce extra electricity, so while some of the higher temperature fuel cells are less stable, more energy can be generated if both the fuel and the heat by-product are harnessed simultaneously.
While there are many different types of fuel cells, they all work via a similar mechanism. All fuel cells require hydrogen and oxygen to function and electrochemical reactions at the electrodes break down these gases to produce water, while subsequently producing electricity in the process. Therefore, the overall electrochemical reaction for fuel cells is: Hydrogen + Oxygen = Electricity + Water Vapor.
Fuel cells are an alternative technology to batteries and are seen as a much greener option because they only produce water, which is not harmful to the environment. They are often trickier to implement than many of the commercially available batteries, but aside from being green, one of the key advantages of fuel cells is that will always produce electricity so long as they are being fed with both hydrogen and oxygen gas.
Liam Critchley is a writer, journalist and communicator who specializes in chemistry and nanotechnology and how fundamental principles at the molecular level can be applied to many different application areas. Liam is perhaps best known for his informative approach and explaining complex scientific topics to both scientists and non-scientists. Liam has over 350 articles published across various scientific areas and industries that crossover with both chemistry and nanotechnology.
Liam is Senior Science Communications Officer at the Nanotechnology Industries Association (NIA) in Europe and has spent the past few years writing for companies, associations and media websites around the globe. Before becoming a writer, Liam completed master’s degrees in chemistry with nanotechnology and chemical engineering.
Aside from writing, Liam is also an advisory board member for the National Graphene Association (NGA) in the U.S., the global organization Nanotechnology World Network (NWN), and a Board of Trustees member for GlamSci–A UK-based science Charity. Liam is also a member of the British Society for Nanomedicine (BSNM) and the International Association of Advanced Materials (IAAM), as well as a peer-reviewer for multiple academic journals.
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