Fuel Cells: Working Principle
A fuel cell is a device that uses a fuel and oxygen to create electricity by an electrochemical process. A single fuel cell consists of an electrolyte and two catalyst-coated electrodes (a porous anode and cathode). While there are different fuel cell types, all fuel cells work similarly:
- A fuel (such as hydrogen) is fed to the anode where a catalyst separates hydrogen's negatively charged electrons from positively charged ions (protons).
- At the cathode, oxygen combines with electrons and, in some cases, with species such as protons or water, resulting in water or hydroxide ions, respectively.
- For polymer electrolyte membrane and phosphoric acid fuel cells, protons move through the electrolyte to the cathode to combine with oxygen and electrons, producing water and heat.
- For alkaline, molten carbonate, and solid oxide fuel cells, negative ions travel through the electrolyte to the anode where they combine with hydrogen to generate water and electrons.
- The electrons from the anode cannot pass through the electrolyte to the positively charged cathode; they must travel around it via an electrical circuit to reach the other side of the cell. This movement of electrons is an electrical current.
Fuel Cell Technology
Fuel cells are an critical enabling technology in the world's portfolio of effincient energy solutions and have the potential to drastically improve our utilization of energy resources, offering cleaner, more-efficient alternatives to the combustion of gasoline and other fossil fuels. Fuel cell technology has the potential to replace the internal-combustion engine in vehicles and provide power in stationary and portable power applications because they are energy-efficient, clean, and fuel-flexible. A fuel cell uses the chemical energy of hydrogen to cleanly and efficiently produce electricity with water and heat as byproducts. Fuel cells are unique in terms of the variety of their potential applications; they can provide energy for systems as large as a utility power station and as small as a laptop computer.
The fuel cell technology has not reached yet the maturity stage sufficient to to overcome critical technical barriers to fuel cell commercialization. Current R&D focuses on the development of reliable, low-cost, high-performance fuel cell system components for transportation and buildings applications. Fuel cells have several benefits over conventional combustion-based technologies currently used in many power plants and passenger vehicles. They emit no emissions at the point of operation, including greenhouse gases and air pollutants that create smog and cause health problems. On a life-cycle basis, if pure hydrogen is used as a fuel, fuel cells emit only heat and water as byproducts. Despite technology limitations, the market for fuel cells is growing at the robust double-digit rate, driven by green sentiment and unique fuel cell advantages in specific applications.
Today, fuel cells are being developed to power passenger vehicles, commercial buildings, homes, and even small devices such as laptop computers. Fuel cell systems can be extremely efficient over a large range of sizes (from 1 kW to hundreds of megawatts). Some systems can achieve overall efficiencies of 80% or more when heat production is combined with power generation. Fuel cell systems integrated with hydrogen production and storage can provide fuel for vehicles, energy for heating and cooling, and electricity to power our communities. These clean systems offer a unique opportunity for energy independence, highly reliable energy services, and economic benefits.
Fuel Cell Technology Challenges
Cost and durability are the major challenges to fuel cell commercialization. However, hurdles vary according to the application in which the technology is employed. Size, weight, and thermal and water management are barriers to the commercialization of fuel cell technology. In transportation applications, these technologies face more stringent cost and durability hurdles. In stationary power applications, where cogeneration of heat and power is desired, use of PEM fuel cells would benefit from raising operating temperatures to increase performance. The key challenges include:
Cost - The cost of fuel cell power systems must be reduced before they can be competitive with conventional technologies. Currently, the costs for automotive internal-combustion engine power plants are about $25–$35/kW; for transportation applications, a fuel cell system needs to cost $30/kW for the technology to be competitive. For stationary systems, the acceptable price point is considerably higher ($400–$750/kW for widespread commercialization and as much as $1000/kW for initial applications).
Durability and Reliability - The durability of fuel cell systems has not been established. For transportation applications, fuel cell power systems will be required to achieve the same level of durability and reliability of current automotive engines [i.e., 5,000-hour lifespan (150,000 miles)] and the ability to function over the full range of vehicle operating conditions (40°C to 80°C). For stationary applications, more than 40,000 hours of reliable operation in a temperature at -35°C to 40°C will be required for market acceptance.
System Size - The size and weight of current fuel cell systems must be further reduced to meet the packaging requirements for automobiles. This applies not only to the fuel cell stack, but also to the ancillary components and major subsystems (i.e., fuel processor, compressor/expander, and sensors) making up the balance of power system.
Air, Thermal, and Water Management - Air management for fuel cell systems is a challenge because today's compressor technologies are not suitable for automotive fuel cell applications. In addition, thermal and water management for fuel cells are issues because the small difference between the operating and ambient temperatures necessitates large heat exchangers.
Improved Heat Recovery Systems - The low operating temperature of PEM fuel cells limits the amount of heat that can be effectively utilized in combined heat and power (CHP) applications. Technologies need to be developed that will allow higher operating temperatures and/or more-effective heat recovery systems and improved system designs that will enable CHP efficiencies exceeding 80%. Technologies that allow cooling to be provided from the low heat rejected from stationary fuel cell systems (such as through regenerating dessiccants in a desiccant cooling cycle) also need to be evaluated.
