History

The history of fuel cells dates to the 18th century, with Alessandro Volta and J.W. Ritter making significant contributions to the understanding of electricity. Sir Humphrey Davy created a remarkably simple fuel cell in 1802, which produced a small electrical shock. Christian Friedrich Schonbein is credited with discovering the principle of the fuel cell in 1839, and Sir William Grove developed the first cell type in 1839. Ceramic fuel cells were developed later, with Nernst's discovery of solid oxide electrolytes in 1899, and the first ceramic fuel cell operating at 1000 °C by Baur and Preis in 1937. Research groups in the USA, Germany, and the former USSR improved fuel cell technologies for industrial development purposes after World War II.^[1] 

During the 1960 Apollo mission to the moon, NASA invested tens of millions of dollars in a program that utilized fuel cells based on hydrogen to power the electrical systems onboard the spacecraft, achieving successful results. Throughout the 1980s, there was an upswing in government investment in fuel cell research and development across the USA, Canada, and Japan. In response to this trend, Ballard, a prominent manufacturer of fuel cells, commenced the creation of increasingly innovative prototype buses that could run on compressed hydrogen. Fuel cells are now commonly used in spaceflight, transportation, and for portable power, home power generation, and large power generation.^[1]

Fuel Cells 

It is defined as the cell in which chemical energy is converted into the electrical energy with high efficiency. The core of the fuel cell is a unit cell. It is the main part where actual process happens. It consists of the cathode positive electrode and anode negative electrode both of which are in contact with the electrolyte solution (Figure 1). The anode is supplied with the fuel and the cathode is supplied with the oxidant. The electrolyte present allows the flow of ions. The operation of fuel cells is characterized by electrochemical processes that differ from those of internal combustion (IC) engines and are not constrained by Carnot's cycle. This aspect gives fuel cells a less complex and more efficient mode of operation.^[2]

For a fuel cell the anodic reactions are direct oxidation of hydrogen or oxidation of methanol. Apart from this an indirect oxidation through reforming step also can occur. Cathodic reactions are oxygen reduction and the source for oxygen is directly from the air. 

Fig 1: Fuel cell basic working. Figure by user Paulsmith99 at Wikipedia. License: CC BY-SA.

Oxygen Reduction:

Oxygen reduction happens by two pathways Four electron pathway and peroxide pathway. The four electron pathway is preferred. ^{[3, p.164-166]}

Four electron pathway:
a)Alkaline electrolyte
O₂ + 2H₂O+4e^-→4OH^-
b)Acidic electrolyte
O₂ + 4H⁺+4e^-→2H₂O

Hydrogen Oxidation:

It is performed via platinum catalyst it is kinetically very fast and is controlled by mass transfer. ^{[3, p.166]}

Overall reaction:
H₂→2H⁺+2e^-

Efficiencies of fuel cell

Overall the efficiency of a fuel cell is more than the other systems which are present. If we compare fuel cell with IC engine we see that the chemical energy is first transferred to the mechanical and then finally to the electrical energy with the help of AC generator in ICE.

Types of Fuel cells

Fuel cells are classified on the basis of the type of electrolyte and fuel these cells use. The other criteria of classification is the temperature low temperature and high temperature fuel cell.^{[3, p.168-180][4]}

• PEMFC- The PEMFC, which stands for proton exchange membrane or polymer electrolyte membrane fuel cell, utilizes a polymer membrane that is both water-based and acidic as its electrolyte. Platinum catalysed electrodes. This type of fuel cell operates using either pure hydrogen or reformed natural gas, with the ability to remove carbon monoxide. Additionally, it operates at a temperature below 100 degrees Celsius. It was used first time in the space and was also employed for providing the astronauts with clean drinking water. The membrane used in PFMFC is PSS(polystyrene sulfonate) and was very instable. This was later replaced in the space programs.
• HT-PEMFC (high temperature PEMFC). This type of fuel cell uses the mineral based acid and operates at high temperature 200°C.
• DMFC (direct methanol fuel cell). It is an exception to the classification and uses polymer membrane as an electrode. On its anode it uses platinum-ruthenium based catalyst. It is worth to note that the hydrogen is utilised from the liquid methanol which is directly oxidised in the fuel cell. It is a low temperature fuel cell but can operates slightly at higher temperature than the PEMFC.
• MCFC (molten carbonate fuel cell) molten carbonate salt suspended in a porous ceramic matrix is used as the electrolyte. It uses coal derived fuel gas methane or natural gas and operates at higher temperatures 650°C.
• PAFC (phosphoric acid fuel cell) It is one of the advanced fuel cells used in the power plant generations. It has uses cathode and anode as finely dispersed platinum. The operating temperature is 150-200°C.
• SOFC (solid oxide fuel cell) the electrode used is solid oxide material electrode and proves to be more stable than the DMFC. It operates at high temprature and is a pretty simple two-phase solid-gas system
• AFC (alkaline fuel cell) It is considered to have the highest efficiency. This high efficiency is only obtained when the gases used are pure. This seems to be a drawback of it. It has otherwise used in the Apollo mission, Space shuttle program, and was also in consideration for the European Hermes project.     

Challenges

Although there has been tremendous improvements in the design and development of ideas the fuel cells. The thing that is a point of concern is the fuel used. Taking about the PEM there are sevral issues that need to be addressed before it can become commercially used. For optimal performance, PEMFCs require extremely pure hydrogen, and hydrogen derived from hydrocarbons often contains trace amounts of carbon monoxide (CO), which can significantly decrease the efficiency of the anode reaction. Additionally, storing hydrogen onboard can present challenges. The another factor which is stopping PEM from fully commercialization is the cost. Considering all this but we see that the research is going at a tremendous pace for the to address the deign issues for PEMFCs. Multiple scientific groups have are working across the world on it and we see that by increasing patents every year.^[5]

Conclusion

In the early 1900s, scientists and engineers predicted that fuel cells would become widely used for generating electricity and power within a few years. Today, fuel cells are capable of providing highly efficient and environmentally friendly power generation, with successful operations in power generation systems. They can achieve electrical-generation efficiencies of up to 70% and can recover heat as well. With advancing technology, fuel cell-based power systems will become ideal for distributed power generation, offering reliability, cleanliness, quietness, environmental friendliness, and fuel conservation. Recently, the governing body of the United Nations' Global Environment Facility (GEF) approved a demonstration project aimed at showcasing clean fuel cell city buses in five developing countries, including Brazil, Mexico, Egypt, India, and China^[1].

References