Fuel cell technologies and operating characteristics

There are over 20 categories and subcategories of fuel cell technologies. This article will review several of the most common fuel cell technologies, especially as related to commercialization. This is the first of a three-part FAQ series. Part two will look into “Fuel Cells for Backup Power, Microgrids and Renewable Energy” and part three will look into how “Fuel Cells Power Buses, Trucks, Trains, Planes, and Ships.” Hydrogen is the most common fuel used in fuel cells. Hydrogen’s atomic weight is 1.008, and 10/08 (October 8) is National Hydrogen and Fuel Cell Day in the U.S.

All fuel cells consist of three basic elements: an anode and a cathode separated by an electrolyte. A typical cell produces 0.6V to 0.7V. Current generation is proportional to surface area; a fuel cell with a larger surface area will produce more current. Fuel cells are typically used in stacks of multiple cells. A fuel cell stack is a combination of series and parallel cells to deliver the desired voltage and increase the available current, thereby increasing the stack’s output power. In addition to conventional fuel cells of various types, reversible fuel cells are being developed to support renewable energy generation.

The electrolyte used determines the fuel cell type. Typical electrolytes include alkalines, various acids, potassium hydroxide, salt carbonates, and others. The anode catalyst breaks down the fuel. Platinum is a common anode catalyst material. The efficiency of platinum use is a key factor in fuel cell cost. The cathode catalyst is usually another metal such as nickel and converts the ions produced by the fuel cell into a stable chemical compound, usually water.

Low-temperature fuel cells

The alkaline fuel cell (AFC) is one of the most developed fuel cell technologies. Alkaline fuel cells consume hydrogen and pure oxygen to produce potable water, heat, and electricity. They are among the most efficient fuel cells, having the potential to reach 70% efficiency. The National Aeronautics and Space Administration (NASA) has used alkaline fuel cells since the mid-1960s, starting on the Apollo-series moon missions and continuing on the Space Shuttle. Today, AFCs are primarily used in off-grid and backup power (both stationary and mobile) applications.

Alkaline fuel cells operate between ambient temperature and 90°C with an electrical efficiency higher than fuel cells with acidic electrolytes, such as proton exchange membrane (PEM) fuel cells, solid oxide fuel cells, and phosphoric acid fuel cells. Because of the alkaline chemistry, oxygen reduction reaction kinetics at the cathode is less aggressive than in acidic cells, allowing the use of non-noble metals, such as iron, cobalt, or nickel, at the anode (where fuel is oxidized); and cheaper catalysts such as silver or iron phthalocyanines at the cathode.

An alkaline medium also accelerates the oxidation of fuels like methanol, making them more attractive. Less pollution results compared to acidic fuel cells. AFCs are the cheapest of fuel cells to manufacture. The catalyst required for the electrodes can be any of a number of different chemicals that are inexpensive compared to those required for other types of fuel cells. Among the companies offering AFCs are AFC Energy and Gencell Energy.

Proton Exchange Membrane (PEM) fuel cells use an electrolyte, a proton (H+) conducting solid polymer membrane. They are also known as PEFC (polymer electrolyte fuel cell) or SPFC (solid polymer fuel cell). PEM fuel cells claim the highest power densities of any of the fuel cell types, making them particularly attractive for transportation and portable applications where minimum size and weight are required. They contain no corrosive liquid electrolyte, are robust in construction, and are modular and scalable in design. They are low-temperature fuel cells which usually operate below 100°C.

This means that unlike high-temperature fuel cells such as solid oxide, which operate at about 600°C, they can be fabricated from cheaper, less exotic materials. The low temperature of PEM fuel cells can also be an advantage when a low thermal signature is desired. PEM fuel cells also have potential applications across a very wide range, from portable power or powering drones at a few watts to hundreds of kilowatts for vehicular and stationary power.

Mid-temperature fuel cells

Phosphoric acid fuel cells (PAFC) are fuel cells that use liquid phosphoric acid as an electrolyte. They have been commercialized since the 1970s. The electrolyte is highly concentrated or pure liquid phosphoric acid (H3PO4) saturated in a silicon carbide (SiC) matrix. Operating range is about 150 to 200 °C. The electrodes are made of carbon paper coated with a finely dispersed platinum catalyst.

At an operating range of 150°C and higher, the expelled water can be converted to steam for air and water heating (combined heat and power). This potentially allows efficiency increases of up to 70%. PAFCs are CO2-tolerant and even can tolerate a CO concentration of about 1.5%, which broadens the choice of fuels they can use. At lower temperatures, phosphoric acid is a poor ionic conductor, and CO poisoning of the platinum electro-catalyst in the anode becomes severe. However, they are much less sensitive to CO than PEMFCs and AFCs. A disadvantage of PAFCs is their relatively low power density.

PAFC have been used for stationary power generators with output in the 100kW to 400kW range and are also finding application in large vehicles such as buses. Major manufacturers of PAFC technology include Doosan Fuel Cell and Fuji Electric. India’s DRDO has developed PAFC based air-independent propulsion for integration into their Kalvari-class submarines.

Solid acid fuel cells (SAFCs) are a class of fuel cells characterized by the use of a solid acid material as the electrolyte. Similar to proton exchange membrane fuel cells and solid oxide fuel cells, they extract electricity from the electrochemical conversion of hydrogen- and oxygen-containing gases, leaving only water as a byproduct. Current SAFC systems use hydrogen gas obtained from a range of different fuels, such as industrial-grade propane and diesel. They operate at mid-range temperatures, from 200 to 300 °C.

High-temperature fuel cells

Solid oxide fuel cells (SOFC) are operated under high temperatures and form a high-efficiency power generating system when combined with micro gas turbines. A SOFC has a solid oxide or ceramic electrolyte. The SOFC generates power between 700°C and 1000°C by being supplied fuel gas (hydrogen, carbon monoxide, etc.) to the fuel electrodes and air (oxygen) to the air electrodes.

The advantages of SOFCs include high combined heat and power efficiency, long-term stability, fuel flexibility, low emissions, and relatively low cost. The high operating temperature is the largest disadvantage and results in longer start-up times and mechanical and chemical compatibility issues. SOFCs are used as auxiliary power units in vehicles and stationary power generation with outputs from 100W to 2MW. Makers of SOFCs include Elcogen, Nexceris, and SOLIDpower.

Reversible SOFCs and renewable energy systems

FuelCell Energy, Inc. is working on performance improvements to advance the commercialization of its reversible solid oxide fuel cell (RSOFC) systems. An RSOFC system is a hybrid operation system that performs water electrolysis for hydrogen production, stores the hydrogen, and then produces power by using the produced hydrogen.

FuelCell Energy’s solution converts intermittent and excess power from renewable energy sources during periods of low power demand into hydrogen stores the hydrogen on-site for long periods of time. It then uses this as a fuel source to generate clean power when needed during high power demand. This megawatt scalable solution is expected to provide long-duration storage and compares very favorably against other technologies.

Also targeting renewable applications, Ballard Power Systems is working with Hydrogene de France (HDF Energy), an independent power producer dedicated to renewable power generation, for the development and integration of a multi-megawatt (MW) scale fuel cell system into HDF Energy’s Renewstable® power plant designed for stationary power applications.

HDF Energy’s Renewstable® power plant is a multi-MW baseload system enabling large-scale storage of intermittent renewable wind or solar energy in the form of hydrogen – through the process of electrolysis – as well as electricity generation using that hydrogen feedstock together with a fuel cell system. This power plant can produce zero-emission power on a 24/7 basis from intermittent renewable energy to support electrical grids.

The integrated architecture of renewables-electrolysis-hydrogen storage fuel cell systems can deliver reduced energy costs, improved grid stability and resiliency, increased penetration of renewables, and greater energy independence. HDF’s Renewstable® power plant may also be used to store green hydrogen for use in other applications, such as zero-emission fuel for fuel cell electric vehicles.

REFLEX project in the EU aims for reversible fuel cells

The European Union’s REFLEX project aims at developing an innovative renewable energies storage solution, the “Smart Energy Hub”, based on reversible Solid Oxide Cell (rSOC) technology, that is to say, able to operate either in electrolysis mode as a solid oxide electrolysis cell (SOEC) to store excess electricity to produce H2 or in fuel cell mode as a solid oxide fuel cell (SOFC) when energy needs exceed local production, to produce electricity and heat again from H2 or any other fuel locally available.

In a real environment, it will demonstrate the high power-to-power round-trip efficiency of this technology and its flexibility in dynamic operation, thus moving the technology from Technology Readiness Level (TRL) 3 to 6. The Smart Energy Hub being modular, made of multistacks/multimodules arrangements, scale-up studies will be performed to evaluate the technology’s techno-economic performance to address different scales of products for different markets.

This was the first of a three-part FAQ series. More details about the EU’s REFLEX project will be discussed in part two, “Fuel Cells for Backup Power, Microgrids and Renewable Energy.” Part three will look into how “Fuel Cells Power Buses, Trucks, Trains, Planes, and Ships.”

References

Ballard and HDF Energy Sign Development Agreement For Multi-Megawatt Fuel Cell Systems, Ballard Power Systems
EU’s REFLEX project – Reversible solid oxide Electrolyzer and Fuel cell for optimized Local Energy miX, REFLEX
Fuel cell, Wikipedia
FuelCell Energy’s Reversible Solid Oxide Fuel Cell Systems, FuelCell Energy