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 natural gas 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 electrodes, together making a fuel cell "membrane." Oxygen passes over one electrode and a hydrogen-containing fuel (hydrogen, natural gas, diesel, alcohols, etc.) passes over the other, and through a series of electrochemical reactions, electricity, water and heat are generated.

Figure 1. Simple fuel cell diagram

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 SAFCell'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	200-300°C	Solid acids, e.g. CsHSO₄
PEMFC - Polymer Electrolyte Membrane	70-100°C	Sulfonated polymers, e.g. Nafion®
AFC - Alkaline	100-200°C	Aqueous KOH
PAFC - Phosporic acid	150-200°C	H₃PO₄
MCFC - Molten carbonate	600-700°C	(NA,K)2CO₃
SOFC - Solid oxide	700-1000°C	(Zr,Y)O_2-3

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.

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 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^-12 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 1000 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 moderately higher temperatures. A number of different solid acid compounds with such behavior have been discovered.

Figure 2. Proton conduction mechanism for solid acids (CsHSO₄ shown here). Protons (H⁺) attached to sulfate tetrahedra are rapidly repositioned (10¹¹Hz) by rotations of the tetrahedra (1). Approximately once every one hundred rotations (10⁹Hz), 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, SAFCell'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 thousands of hours of constant operation on various reformed fuels.

Figure 3. SAFC stack performance utilizing a) internally reformed methanol, b) commercial propane (Home Depot brand) and c) diesel (STATOIL fuel station, tested at Nordic Power Systems) all versus Hydrogen baseline