What is a Fuel Cell?

A fuel cell is an energy conversion device that converts chemical energy into electrical energy. For example, it can convert a hydrocarbon fuel like methanol into electricity, which in turn, can power a light bulb, a cell phone, or even a car. A basic fuel cell consists of an electrolyte sandwiched between two electrode, together making a fuel cell "membrane." Oxygen passes over one electrode and a hydrogen-containing fuel (hydrogen, methanol, ethanol, etc.) passes over the other, and through a series of electrochemical reactions, electricity, water and heat are generated.

A set of Polarized Light Microscope images of the Superprotonic Phase Transition

1. Room temperature

2. Beginning transition on left

3. Superprotonic on left side

4. More superprotonic on left side

5. Fully superprotonic

6. Reverse transitioned

In a simple fuel cell, hydrogen fuel is delivered to the anode of the fuel cell. Oxygen (usually from air) is delivered through the cathode on the other side of the membrane. Passing through a catalyst, hydrogen atoms split into protons and electrons (H₂ → 2H⁺ + 2e^-). The protons are transported through the electrolyte, while the electrons are harnessed and diverted out of the fuel cell to provide electric power to a device. The electrons ultimately reunite with the protons at the cathode in the presence of oxygen gas and a catalyst to generate water, which is then expelled (½O₂ + 2H⁺ + 2e^- → H₂O). A fuel cell system which includes a "fuel reformer" can utilize the hydrogen from any hydrocarbon fuel, i.e., from natural gas to methanol to even gasoline.

Fuel cells thus combine the advantage of battery technology with the advantage of combustion engines: like batteries, they operate by having very well controlled electrochemical reactions (which accounts for their high fuel efficiency); and like combustion engines, they can be refuelled.

Types of Fuel Cells

There are five main types of commercially developed fuel cells, which are distinguishable essentially by the type of electrolyte used in each. Different electrolytes transport ions with varying efficiencies as a function of temperature, so that each of these types operates in a different temperature range. An overview of these five fuel cell types is given in Table 1, as compared to Superprotonic's solid acid fuel cell.

Table 1. Common types of fuel cells, their temperature of operation, and electrolyte used.

Fuel Cell Type	Temperature	Electrolyte
SAFC - Solid Acid	100-300°C	Solid acids, e.g. CsHSO₄
PEMFC - Polymer Electrolyte Membrane	70-100°C	Sulfonated polymers, e.g. Nafion®
AFC - Alkali	100-250°C	Aqueous KOH
PAFC - Phosporic acid	150-220°C	H₃PO₄
MCFC - Molten carbonate	500-700°C	(NA,K)2CO₃
SOFC - Solid oxide	700-1000°C	(Zr,Y)O_2-3

Of the five types of commercially available fuel cells, it is predominately PEMFCs that are being seriously pursued for automotive power systems. PEMFC prototype vehicles have been developed by Honda, Daimler-Chrysler, Ford, and other major automotive companies. Nevertheless, the operational temperatures of PEMFCs cannot much exceed 100°C without complicated and costly auxiliary systems which are required to keep the polymer electrolyte hydrated. Therefore, automotive companies are seeking alternatives to the cost-prohibitive PEMFC. Solids acid fuel cells can operate up to 300°C and therefore may prove to be more economical and efficient than PEMFCs.

What are Solid Acids?

Solid acids are chemical intermediates between normal salts and normal acids. If we take a normal acid such as sulfuric acid and react it with a normal salt such as cesium sulfate, we end up with cesium hydrogen sulfate (cesium bisulfate):

½ Cs₂SO₄(salt) + ½ H₂SO₄(acid)

CsHSO₄(solid acid)

This is the prototypical solid acid used in SAFCs. Physically, these materials are similar to salts, such as household table salt (NaCl). At low temperatures they have ordered structures. However, at warm temperatures some solid acids undergo transitions to highly disordered structures which causes the conductivity to increase dramatically.

In the case of CsHSO₄, the bisulfate (HSO₄^-) group forms a tetrahedron with an oxygen atom at each corner and a hydrogen atom sitting on one of the oxygens. At room temperature, all the sulfate groups have a fixed orientation. When the temperature is raised, disorder sets in and the sulfate groups reorient, changing the positions of the hydrogen atoms as they do so. The time frame for this reorientation is about 10-11 seconds. Occasionally, a proton from one sulfate group transfers over to the next, with a transfer rate on the order of 109 Hz. Essentially, these sulfate groups rotate almost freely - and every 100 reorientations or so, they're in exactly the right position for a proton transfer to happen. As the material goes through this transition, there's a sudden increase in conductivity of several orders of magnitude. Conductivity values for the acid salts are comparable to the conductivity of Nafion and other polymer electrolytes, but at slightly higher temperatures. A number of different solid acid compounds with such behavior have been discovered.

Figure 1. Proton conduction mechanism for solid acids (CsHSO₄ shown here). Protons (H⁺) attached to sulfate tetrahedra are rapidly repositioned (1011 Hz) by rotations of the tetrahedra (1). Approximately once every one hundred rotations (109 Hz), the proton finds itself in an ideal configuration to hop onto a neighboring tetrahedra (2).

Solid Acid Fuel Cells (SAFCs)

Previously, it was believed that solid acids used as electrolytes in fuel cells would be dissolved by the water produced at the cathode. However, Superprotonic's development team has already shown that operating a SAFC over 100°C (where water is in vapor form) is a sufficient condition to prevent dissolution. Presently, SAFCs have demonstrated reliable stability over days of constant operation and multiple heating/cooling cycles.

Figure 2. Solid acid fuel cell performance utilizing a) hydrogen fuel and b) methanol fuel (40 mol% MeOH) and a CsH₂PO₄ solid acid electrolyte.

Technological Advantages of SAFCs

For automotive purposes especially, a fuel cell must be able to operate under intermediate temperature conditions (150-300° C); and this for two main reasons:

First, powering a vehicle (even with a fuel cell) generates so much heat that these temperatures are unavoidable in an automotive system. Any fuel cell that is compromised by such heat conditions requires auxiliary systems to keep it cool. PEMFCs, for example, must employ expensive gas pressurization, humidification, and waste heat management systems to keep from dehydrating. SAFCs, on the other hand, are inherently heat resilient because of their unique material composition and thermodynamic properties.

Second, these intermediate temperatures are high enough to permit good catalytic activity and reaction kinetics, but not so high as to require special heat resistant construction materials. Consequently, requisite fuel cell components can be made from a wide range of inexpensive materials which also happen to work best in this temperature range. Most importantly, SAFCs operating at intermediate temperatures are expected to require far less, if not altogether eliminate precious metal catalysts (the single most expensive component of a fuel cell).

Aside from being able to operate at elevated temperatures, SAFCs have two other distinct advantages over PEMFCs:

they are impermeable to gases (no loss of gas through unwanted cross-over);
they transport "bare" protons through the membrane rather than inefficiently carrying over excess water molecules, as is the case with PEMFCs.

Overall, SAFCs are anticipated to be more economical and efficient than the most advanced PEMFCs, and therefore promise to be more affordable to the public.

Table 2. Advantages of Superprotonic's solid acid-based fuel cells (SAFCs) over polymer electrolyte fuel cells (PEMFCs)

	PEMFC		SAFC
High Conductivity		0.1 S/cm		0.05 S/cm
No Pressure System	X	3 atm		1 atm
Impermeable to Fuels	X	MeOH, EtOH		None
Operable above 100°C	X	25-90°C		100-300°C
High CO Tolerance	X	<100 ppm		1-2%
Simple Water Management System	X	Water re-circulation under pressure		Water condensation
Easy Waste Heat Management	X	Must keep system temperature below 100°C		Need only maintain between 100-300°C

Economic Advantages of SAFCs

SAFCs offer significant economic advantages over current state-of-the-art polymer electrolyte fuel cells (PEMFCs), and have the potential of competing with internal combustion engines (ICEs) on a cost-basis. Presented in the following table is a comparison of the estimated component and total costs of SAFCs with PEMFCs, as compared to the total cost of ICEs - the economic benchmark for automotive power systems. The potential for inexpensive SAFCs comes from:

A decreased need for expensive precious metal catalysts (platinum) due to the higher operational temperatures of SAFCs;
A less expensive solid acid electrolyte (SAE) as compared to polymer electrolyte membranes (PEMs);
The replacement of expensive graphite-composite bipolar plates used in PEMFCs with inexpensive aluminum plates;
The elimination of the need for a tailgas burner due to the more effecient fuel utilization at SAFC's higher operating temperatures;
A simplified fuel / air supply system due to the operation of SAFCs at atmospheric pressures (1 atm) versus the pressurized system (3 atm) used by PEMFCs; and
The simplification of a SAFC's cooling system due to more efficient heat transfer at elevated operational temperatures.

The combination of these estimated cost savings should result in a SAFC that is cost competitive with the ICE ($35-50/kW) (California Energy Commission, October 2001). Furthermore, because the overall SAFC system is simpler than a PEMFC system and has fewer components, long-term operating costs are anticipated to be lower.