Phosphoric acid fuel cell

The phosphoric acid fuel cell ( engl. Phosphoric Acid Fuel Cell , PAFC ) is a medium temperature fuel cell .

principle

Section through a PAFC

The PAFC differs from other fuel cells in that it works with phosphoric acid as the electrolyte . The highly concentrated phosphoric acid ( 90-100 %) is fixed in a PTFE fiber structure. Like all fuel cells, it produces electricity by oxidizing a fuel gas. Is used as the fuel gas is hydrogen . Air or pure oxygen can be used as the oxidizing agent . No clean gases have to be used. In contrast to the AFC, traces of carbon dioxide in the gases are not a problem with the PAFC . The phosphoric acid fuel cell operates in a temperature range of 135 to 200 ° C.

Due to the temperature and the use of phosphoric acid, high demands are placed on the quality and resistance of the components. Usually carbon and graphite parts are used for the electrodes. The advantage of this type is that it is relatively insensitive to contamination of the fuel gas.

Phosphoric acid H ₃ PO _{4 is} used as the electrolyte . A particular advantage of phosphoric acid is the low vapor pressure , which however cannot be ignored, i. H. liquid phosphoric acid nevertheless evaporates slowly. For this reason, the exhaust gas is normally cooled in a cooling zone to 160 - 180 ° C and thus to around 30 ° C below operating temperature. The electrolyte thus condenses before it emerges from the cell and can be fed back again. The phosphoric acid is pressed into a matrix made of silicon carbide, which is attached between the anode and cathode and through which the protons migrate.

Mostly platinum or a platinum alloy is used as a catalyst, as these are stable in hot phosphoric acid H ₃ PO ₄ , have catalytic properties and are present in sufficient quantities on earth to be used industrially for electrode construction. In more recent developments z. B. used a platinum-ruthenium mixture as a catalyst, which increased the carbon monoxide tolerance of the electrode. Nowadays you need around 0.25 mg platinum per cm² for the anode and 0.5 mg platinum per cm² for the cathode.

Different cooling methods include liquid cooling, air cooling, and water cooling, with water cooling being the most widely used. The water cooling basically differs in two ways. In both types, the water is introduced into the cooling system in liquid form. With one of them, the water comes out again in liquid form, which, however, requires large amounts of water, as only a small amount of heat can be absorbed. With the other, water emerges from the cooling system in two phases (liquid / gaseous), which means that the construction of the cooling system is more complex, but less water is required for cooling.

Reaction equations

	equation
anode	${\ displaystyle \ mathrm {2H_ {2} \ to \ 4H ^ {+} + 4 \ e ^ {-}}}$ Oxidation / electron donation
cathode	${\ displaystyle \ mathrm {O_ {2} + 4H ^ {+} + 4 \ e ^ {-} \ to \ 2H_ {2} O}}$ Reduction / electron uptake
Overall response	${\ displaystyle \ mathrm {2H_ {2} + O_ {2} \ to 2H_ {2} O}}$ Redox reaction / cell reaction

The fuel in a PAFC is hydrogen or a hydrogen-rich gas. Hydrogen is fed to the anode, where the electrochemical reaction takes place. Hydrogen is oxidized to hydrogen ions (H ⁺ ), the electrons (e ^- ) are captured by a collecting electrode and fed to the cathode via a consumer. The protons migrate through the electrolyte from the anode to the cathode. Oxygen is supplied to the cathode, causing an electrochemical reaction between oxygen, hydrogen ions and electrons; this creates water and heat.

Advantages and disadvantages compared to other fuel cells

Advantages of a PAFC are:

Robust
Increased tolerance to contamination of the fuel gas
Air can be used on the cathode side (no clean gas required)
Good dynamic behavior
CO ₂ tolerant

Disadvantages are:

Low power density
Short service life (due to the extremely aggressive electrolyte )
CO is a catalyst poison , the tolerance to carbon monoxide is between (1–3%) and increases with increasing operating temperature.
Plant efficiency rather low

SAFC

In solid acid fuel cells (SAFC), solid phosphate salts form the electrolyte. Above 220 ° C, the proton conductivity is above 0.05 (Ωcm) ⁻¹ and almost reaches the values of PEFC (around 0.1 (Ωcm) ⁻¹ ). The salt formation of phosphoric acid is initiated by adding alkali phosphates:

Cs ₃ PO ₄ + 2 H ₃ PO ₄ → 3 CsH ₂ PO ₄

Currently (2011) it is being investigated whether to operate SAFC with hydrogen, which is obtained from gasoline through reformation.

Individual evidence

↑ Kevin Bullis: Fuel cells run on diesel. In: Technology Review . October 27, 2010, accessed February 25, 2017 .

[1] Kevin Bullis: Fuel cells run on diesel. In: Technology Review . October 27, 2010, accessed February 25, 2017 .