Types of Fuel Cells

Fuel cells, in general, are units that convert chemical energy found within various fuel sources along with an oxidizing agentinto electricity through a pair of redox reactions. Some are capable of taking electricity, water, and gas and converting these inputs into hydrogen. This paper covers the fundamentals of the various fuel cells in use today. It also mentions some of the places they may be found based on their operating parameters and capabilities.

Types of fuel cells currently on the market:

Polymer Electrolyte Membrane Fuel Cells (PEMFC) AKA Proton Exchange Membrane Fuell Cells

– Consists of (2) catalyst electrodes separated by polymer electrolyte
– These involve a platinum nanoparticle cathode and anode catalysts which are deposited onto a carbon support material
– Operate at lower temperatures which results in less water on system components, increasing durability
– Fast start-up time
– Short response time
– Low gas cross permeation
– Higher purity of hydrogen produced
– Higher thermodynamic voltage
– Can withstand higher operating pressure across the internal membrane
– Low ohm loss
– Operate over a wide range of power inputs
– Lower operational costs
– Electrical efficiency is 40%–50%,
– Power capacity at output is up to 250 kW.
Disadvantages

– High cost
– Limited choices of stable earth-abundant electrocatalysts
– Easily damaged by inappropriate operation
– Sensitive to imperfections, dust, and impurities
– Low-temperature water for heating
– Critical on gas quality
Where might these be seen:

– Great candidate for powering automobiles
o passenger vehicles, forklift and material handling, trains
– portable power supply applications
o laptops, cell phones
– local power generation to support homes and businesses
– Replacement of portable diesel generators

Diagram of a Proton exchange membrane fuel cell.

Albris, Public domain, via Wikimedia Commons

Direct Methanol Fuel Cells (DMFC)

– Same as PEM fuel cells in using a polymer membrane as an electrolyte

– Platinum-ruthenium catalyst on the anode can draw hydrogen from liquid methanol

– Doesn’t have a fuel reformer like the PEM fuel cells

– Developed to combat the problem of hydrogen fuel storage

– Methanol is generally considered a “safer” and more abundant fuel source

Disadvantages

– Efficiency due to the following:

o High polarization of the anode for the oxidation of methanol

o Methanol crossover from the anode to the cathode across the membrane separator which causes:

▪ Carbon monoxide poising from intermediate methanol oxidation

▪ Inefficient cathode reaction

o Slow anode reaction = slow response time

o methanol needs to be watered down for a full reaction

Where might these be seen:

– Off-grid mobile applications

– Portable electronic devices such as phones and computers

– Sensors

– Auxiliary power

– Optronics

– Mission-critical communication systems

DMFC type fuel cell diagram

Amalia1983, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons

Alkaline Fuel Cells (AFC)

– Highest electrical efficiency

– One of the first fuel cells systems developed and used in the U.S. space program

– High–performance fuel cells due to the rate at which chemical reactions take place inside the cell

– High efficiency – up to 60%

– Use an alkaline electrolyte – lower cost

– Compatible with a larger variety of non–precious metals as catalysts – lower cost

– Operate at atmospheric pressure and low temperature

– Low weight and volume

Disadvantages

– Very sensitive to CO2 poising, even small amounts in the air

o This requires hydrogen and oxygen inputs to be purified = larger cost

– Short operating life due to the use of highly corrosive potassium hydroxide electrolyte

Where might these be seen:

– Spacecraft power for electrical onboard systems

– Submarines and other underwater vehicles

Alkaline Fuell Cell.

1: Hydrogen 2:Electron flow 3:Charge 4:Oxygen 5:Cathode 6:Electrolyte 7:Anode 8:Water 9:Hydroxyl Ions

Created by Darryl Ring, Vectorization: Chabacano, CC BY-SA 3.0 http://creativecommons.org/licenses/by-sa/3.0/>, via Wikimedia Commons

Phosphoric Acid Fuel Cells (PAFC)

– Uses liquid phosphoric acid as an electrolyte

– One of the first fuel cells commercially used

– Higher tolerance to carbon monoxide poising

– Tolerant to CO2 exposure

– Capable of cogeneration (electricity – heat)

Disadvantages

– Needs to be kept at temperatures above 40 deg. Celcius to prevent the electrolyte from solidifying

– Consists of a highly corrosive solution which leads to the deterioration of the electrodes

– Susceptible to carbon monoxide poising

– The platinum catalyst is expensive with significant lead times

– Highly sensitive to sulfur

Where might these be seen:

– Stationary power generation – independent grid electricity

– Powering large vehicles such as buses, locomotives, etc

– Output in the 100 kW to 400 kW range

Phosphoric acid fuel cell

U.S. Department of Energy, USA.gov, Public domain, via Wikimedia Commons

Molton Carbonate Fuel Cells (MCFC)

– Use molten carbonate salt as the electrolyte that is suspended in a porous inert ceramic lithium aluminum oxide matrix

– Unlike AFC, PAFC, and PEMFCs MCFCs don’t require external reformers to convert more energy-dense fuels to hydrogen – uses an internal reforming instead

– Can be fueled with coal-derived gases, methane, or natural gas

– High efficiency – up to 60%

– Corrosion-resistant materials which increase cell life without decreasing resistance

– Capture CO2 from flue gases

– Economic at 100kW of larger applications

– Covert methane to syngas via catalytic steam reforming process

Disadvantages

– Durability due to the high temperature at which fuel cells operate

– Requires a continuous supply of CO2 to the cathode

– Sulfur poisoning

– Slow start-up time with a range of 5–10 h

Where might these be seen:

– Applications from 100kW to several MW

– Areas where electricity, water, and natural gas are available in large quantities

– Stationary power generation

– Hydrogen supply locations

– Carbon capture projects

Output in the 100 kW to 400 kW range

MCFC type fuel cell diagram

Amalia1983, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons

Solid Oxide Fuel Cells (SOFC)

– Hydrogen, methane, natural gas, or propane fuel inputs

– Being of “solid” construction, these fuel cells do not needelectrolyte loss maintenance

– No electrode corrosion

– Produce more heat than MCFCs

– When combined with gas and steam turbine systems (tri-gen) as well as operating pressures above 9 atm, these fuel cells boost the highest efficiencies without resulting in increased cathode corrosion.

– A SOFC can be combined with a solid oxide electrolysis cell(SOEC) to produce both hydrogen or electricity depending on the available inputs

Disadvantages

– The high operating temperature necessary for these fuel cells to operate leads to long start–up times (more than 2 ½ hours)

– Made using ceramic materials which leads to higher costs to manufacture

Where might these be seen:

– Both large and small power generation

– Stationary power for homes and businesses

– Backup power for emergency and standby power systems (NFPA 110)

o Failure of these systems could result in the loss of human life or serious injury

▪ Hospitals, communications, fire alarms, etc.

SOFC type fuel cell diagram

Amalia1983, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons

If you are considering implementing fuel cells to meet your energy needs, Ryanco Construction can help you choose, design,and install the right system. We work with suppliers of various fuel cells to fit your project needs and constraints.

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Want to work together? Email us at ryan@ryancoconstruction.com

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