Fuel cell unlike batteries have unique design to burn fuel mostly hydrogen with air (oxidizer) continuously to sustain the chemical reaction to generate electricity, whereas in a battery the chemical energy usually comes from metals and their ions or oxides that are commonly already present in the battery, except in flow batteries.

A Fuel cell is an electrochemical cell that converts the chemical energy of a fuel requiring a continuous source of the fuel mostly hydrogen and oxygen (usually from air) to create electricity through a pair of redox reactions. The Fuel cells can produce electricity continuously for as long as fuel and oxygen are supplied.

Scheme of Hydrogen Fuel Cell

Design features in a fuel cell

The design of different types of fuel cell depends on the application and comprises of the electrolyte, fuel, anode catalyst, cathode catalyst and Gas diffusion layers.

Key components in a single cell

The electrolyte substance usually defines the type of fuel cell and can be made from a number of substances like potassium hydroxide, salt carbonates, and phosphoric acid; The fuel which is most commonly hydrogen; The anode catalyst, usually fine platinum powder which breaks down the fuel into electrons and ions; The cathode catalyst, often nickel, converts ions into waste chemicals, with water being the most common type of waste; There are gas diffusion layers designed to resist oxidization.

Scheme of fuel cell design

A typical fuel cell produces a voltage from 0.6 to 0.7 V at full rated load. Voltage decreases as current increases, due to several factors. Other loses in the system are Activation loss, Ohmic loss as a result of voltage drop due to resistance of the cell components and interconnections. Mass transport loss from depletion of reactants at catalyst sites under high loads, causing rapid loss of voltage.

To deliver the desired amount of energy, the fuel cells can be combined in series to yield higher voltage, and in parallel to allow a higher current to be supplied. Such a design is called a fuel cell stack. The cell surface area can also be increased, to allow higher current from each cell.

Types and Design of Fuel Cell

Fuel cells come in many types; classified by the kind of electrolyte they employ, each with its own advantages, limitations, and applications. however, all the types work in the same general manner. They include;

• Proton-exchange membrane fuel cells
• Alkaline fuel cell
• Phosphoric acid fuel cell
• Solid acid fuel cell
• Solid oxide fuel cell
• Molten-carbonate fuel cell
• Electric storage fuel cell

All Fuel cells are made up of three adjacent segments: the anode, the electrolyte, and the cathode. Two chemical reactions occur at the interfaces of the three different segments. The net result of the two reactions is that fuel is consumed, water or carbon dioxide is created, and an electric current is created, which can be used to power electrical devices, normally referred to as the load.

At the anode, a catalyst oxidizes the fuel, usually hydrogen, turning the fuel into a positively charged ion and a negatively charged electron. The electrolyte is a specifically designed substance to allow ions pass through it, but the electrons cannot. The freed electrons travel through a wire creating the electric current. The ions travel through the electrolyte to the cathode. Once reaching the cathode, the ions are reunited with the electrons and the two react with a third chemical, usually oxygen, to create water or carbon dioxide.

Proton-exchange membrane fuel cells (PEMFC)

Proton exchange membrane fuel cell is designed to work with a polymer electrolyte (typically nafion) in the form of a thin, permeable sheet that separates the anode and cathode sides. It was formally referred to as a “solid polymer electrolyte fuel cell”. Efficiency is about 40 to 50%, and operating temperature is about 80°C (about 175°F). Cell outputs generally range from 50 to 250 kW. The solid, flexible electrolyte will not leak or crack, and these cells operate at a low temperature enough to make them suitable for homes and cars. But their fuels must be purified, and a platinum catalyst is used on both sides of the membrane, raising costs.

Proton-exchange membrane fuel cells (PEMFC)

On the anode side, hydrogen diffuses to the anode catalyst where it later dissociates into protons and electrons. These protons often react with oxidants causing them to become what are commonly referred to as multi-facilitated proton membranes. The protons are conducted through the membrane to the cathode, but the electrons are forced to travel in an external circuit (supplying power) because the membrane is electrically insulating. On the cathode catalyst, oxygen molecules react with the electrons (which have traveled through the external circuit) and protons to form water.

The components of a PEMFC include

• Bipolar plates,
• Electrodes,
• Catalyst,
• Membrane, and
• The necessary hardware such as current collectors and gaskets.

The materials used for different parts of the fuel cells differ by type. The bipolar plates may be made of different types of materials, such as, metal, coated metal, graphite, flexible graphite, C–C composite, carbon–polymer composites etc. The membrane electrode assembly (MEA) is referred to as the heart of the PEMFC and is usually made of a proton-exchange membrane sandwiched between two catalyst-coated carbon papers. Platinum and/or similar type of noble metals is usually used as the catalyst for PEMFC, and these can be contaminated by carbon monoxide, necessitating a relatively pure hydrogen fuel. The electrolyte could be a polymer membrane.

Alkaline fuel cell

The alkaline fuel cell (AFC) operates on compressed hydrogen and oxygen. It was used as a primary source of electrical energy in the Apollo space program on the spacecraft to provide both electricity and drinking water. The cell consists of two porous carbon electrodes impregnated with a suitable catalyst such as Pt, Ag, CoO, etc. The space between the two electrodes is filled with a concentrated solution of KOH or NaOH which serves as an electrolyte. H2 gas and O2 gas are bubbled into the electrolyte through the porous carbon electrodes.

Alkaline fuel cell diagram

Thus the overall reaction involves the combination of hydrogen gas and oxygen gas to form water. The cell runs continuously until the reactant’s supply is exhausted. This type of cell operates at efficiency of about 70 percent in the temperature range of 343–413 K and provides a potential of about 0.9 V. Alkaline anion exchange membrane fuel cell (AAEMFC) is a type of AFC which employs a solid polymer electrolyte instead of aqueous potassium hydroxide (KOH) and it is superior to aqueous AFC.

Phosphoric acid fuel cell

Phosphoric Acid fuel cells (PAFC) use phosphoric acid as the electrolyte. Efficiency ranges from 40 to 80 percent, and operating temperature is between 150 to 200 degrees C (about 300 to 400 degrees F). Existing phosphoric acid cells have outputs up to 200 kW, and 11 MW units have been tested. PAFCs tolerate a carbon monoxide concentration of about 1.5 percent, which broadens the

Drawing of a solid oxide cell choice of fuels they can use. If gasoline is used, the sulfur must be removed. Platinum electrode-catalysts are needed, and internal parts must be able to withstand the corrosive acid.

Solid acid fuel cell

Solid acid fuel cells (SAFCs) are characterized by the use of a solid acid material as the electrolyte. At low temperatures, solid acids have an ordered molecular structure like most salts. At warmer temperatures (between 140 and 150 °C for CsHSO4), some solid acids undergo a phase transition to become highly disordered “superprotonic” structures, which increases conductivity by several orders of magnitude. The first proof-of-concept SAFCs were developed in 2000 using cesium hydrogen sulfate (CsHSO4). Current SAFC systems use cesium dihydrogen phosphate (CsH2PO4) and have demonstrated lifetimes in the thousands of hours.

Solid oxide fuel cell

Solid Oxide fuel cells (SOFC) are high-temperature fuel cells that use a hard ceramic compound of metal (like calcium or zirconium) oxides as electrolyte, most commonly yttria-stabilized zirconia (YSZ). Efficiency is about 60 percent and Cells output is up to 100 kW. They require high operating temperatures (800–1000 °C) and can be run on a variety of fuels including natural gas. At such high temperatures a reformer is not required to extract hydrogen from the fuel, and waste heat can be recycled to make additional electricity.

Solid oxide fuel cell diagram

However, the high temperature limits applications of SOFC units and they tend to be rather large. While solid electrolytes cannot leak, they can crack. Other challenges exist in SOFC systems due to their high operating temperatures. One such challenge is the potential for carbon dust to build up on the anode, which slows down the internal reforming process. Another disadvantage of SOFC systems is slow start-up time, making SOFCs less useful for mobile applications.

Despite these disadvantages, a high operating temperature provides an advantage by removing the need for a precious metal catalyst like platinum, thereby reducing cost. Additionally, waste heat from SOFC systems may be captured and reused, increasing the theoretical overall efficiency to as high as 80–85%.

SOFCs are unique since in those, negatively charged oxygen ions travel from the cathode (positive side of the fuel cell) to the anode (negative side of the fuel cell) instead of positively charged hydrogen ions travelling from the anode to the cathode, as is the case in all other types of fuel cells. Oxygen gas is fed through the cathode, where it absorbs electrons to create oxygen ions. The oxygen ions then travel through the electrolyte to react with hydrogen gas at the anode. The reaction at the anode produces electricity and water as by-products. Carbon dioxide may also be a by-product depending on the fuel, but the carbon emissions from an SOFC system are less than those from a fossil fuel combustion plant.

The high operating temperature is largely due to the physical properties of the YSZ electrolyte. As temperature decreases, so does the ionic conductivity of YSZ. Therefore, to obtain optimum performance of the fuel cell, a high operating temperature is required. Ceres Power, a UK SOFC fuel cell manufacturer, has developed a method of reducing the operating temperature of their SOFC system to 500–600 degrees Celsius. They replaced the commonly used YSZ electrolyte with a CGO (cerium gadolinium oxide) electrolyte. The lower operating temperature allows them to use stainless steel instead of ceramic as the cell substrate, which reduces cost and start-up time of the system

Molten-carbonate fuel cell

Molten Carbonate fuel cells (MCFC) use high temperature compounds of salt like sodium or magnesium carbonates as the electrolyte. Efficiency ranges from 60 to 80 percent, and operating temperature is about 650 degrees C (1,200 degrees F). Units with output up to 2 megawatts (MW) have been constructed, and designs exist for units up to 100 MW.

Molten-carbonate fuel cell diagram

The high temperature limits damage from carbon monoxide “poisoning” of the cell and waste heat can be recycled to make additional electricity. Their nickel electrode-catalysts are inexpensive compared to the platinum used in other cells. But the high temperature also limits the materials and safe uses of MCFCs– they would probably be too hot for home use. Carbonate ions from the electrolyte are used up in the reactions, making it necessary to inject carbon dioxide to compensate.

Electric storage fuel cell

The electric storage fuel cell is a conventional battery chargeable by electric power input, using the conventional electro-chemical effect. However, the battery further includes hydrogen (and oxygen) inputs for alternatively charging the battery chemically. It can be viewed as a hybrid cell.

Applications of Fuel Cells

Fuel Cells commonly find applications in power generation including cogeneration and in Fuel cell electric Vehicles like in automobile, buses, trucks, forklifts, Motorcycles and bicycles, airplanes, boats and even in submarines.

Hydrogen Fuel Cell Electric Vehicle

Hydrogen Fuel cell is making waves in electric vehicle paradigm transforming the face of how electric vehicles operate generally. By year-end 2019, about 18,000 FCEVs had been leased or sold worldwide. Three fuel cell electric vehicles have been introduced for commercial lease and sale: the Honda Clarity, Toyota Mirai and the Hyundai ix35 FCEV.

Hydrogen Fuel Cell Electric Vehicle Layout diagram for Toyota Mirai.

Additional demonstration models include the Honda FCX Clarity, and Mercedes-Benz F-Cell. As of June 2011 demonstration FCEVs had driven more than 4,800,000 km (3,000,000 mi). Fuel cell electric vehicles feature an average range of 314 miles between refuelings. Fuel cell electric vehicles  can be refueled in less than 5 minutes.

Markets and economics

In 2012, fuel cell industry revenues exceeded $1 billion market value worldwide, with Asian pacific countries shipping more than 3/4 of the fuel cell systems worldwide. However, as of January 2014, no public company in the industry had yet become profitable. 140,000 fuel cell stacks were shipped globally in 2010, up from 11,000 shipments in 2007, and from 2011 to 2012 worldwide fuel cell shipments had an annual growth rate of 85%. Tanaka Kikinzoku expanded its manufacturing facilities in 2011.

Approximately 50% of fuel cell shipments in 2010 were stationary fuel cells, up from about a third in 2009, and the four dominant producers in the Fuel Cell Industry were the United States, Germany, Japan and South Korea. The Department of Energy Solid State Energy Conversion Alliance found that, as of January 2011, stationary fuel cells generated power at approximately $724 to $775 per kilowatt installed. In 2011, Bloom Energy, a major fuel cell supplier, said that its fuel cells generated power at 9 to 11 cents per kilowatt-hour, including the price of fuel, maintenance, and hardware.

Industry groups predict that there are sufficient platinum resources for future demand, and in 2007, research at Brookhaven National Laboratory suggested that platinum could be replaced by a gold-palladium coating, which may be less susceptible to poisoning and thereby improve fuel cell lifetime. Another method would use iron and sulpur instead of platinum. This would lower the cost of a fuel cell (as the platinum in a regular fuel cell costs around US$1,500, and the same amount of iron costs only around US$1.50). Fuel cell technology concept was being developed by a coalition of the John Innes Centre and the University of Milan-Bicocca. PEDOT cathodes are immune to monoxide poisoning.

In 2016, Samsung “decided to drop fuel cell-related business projects, as the outlook of the market isn’t good”.