Phosphoric Acid Fuel Cells (PAFC)

Phosphoric Acid Fuel Cells were the first fuel cells to become commercially viable. According to the U.S. Dept of Energy, more than 270 phosphoric acid fuel cells were deployed in stationary applications in both the United States and abroad. PAFCs were first produced in the 1960s for utilization on the space shuttle. As of 2011, PAFCs had accumulated more than 92,000 hours of operation on the space shuttle.

Function and Reactions of PAFCs

PAFCs use liquid phosphoric acid as an electrolyte and run on hydrocarbon fuel. The reaction process is similar to PEMFCs, with hydrogen being removed from the fuel via the assistance of a platinum catalyst. Hydrogen is able to permeate the phosphoric acid while electrons are not. This is performed at moderately high temperatures, somewhere in between proton electrolyte membrane fuel cells and molten carbonate fuel cells.

Benefits of PAFCs

PAFCs enjoyed three major benefits. The first of these is their ability to run on already available fuels. This is the reason that phosphoric acid fuel cells were the first fuel cells to become commercially viable and why they remain the most popular for remote installations and back-up power generation.

The second major benefit is the scale of electricity production. 200 kW units are quite common and the major manufacturer, United Technologies Corp., Has installed over 75 MW of capacity. PAFCs are able to provide power on a scale that is useful for industrial and commercial applications.

PAFCs are moderately high temperature fuel cells, running at roughly 250-300° C. While their electricity generating efficiency ranges from 37 to 42%, their overall efficiency can reach 80% when they are combined for heat and power applications.

Drawbacks of PAFCs

Despite the fact that PAFCs are currently the most popular stationary fuel cells, they still suffer as they rely upon hydrocarbon fuels. This means greenhouse gas emissions are produced and the potential for catalyst poisoning is of concern. The problem of catalyst poisoning has been overcome in these particular fuel cells by producing electrodes made of carbon paper coated with a finely dispersed platinum catalyst. While this solves the carbon monoxide poisoning issue, it also makes these fuel cells exceptionally expensive to produce.

The greenhouse gas emission created by utilizing fossil fuels has only been partly solved. The gases cannot be completely eliminated, but they can be captured and stored to prevent them from escaping into the atmosphere. The problem with the capture/store approach is that it adds cost to the systems, making them less economical. In the end, eliminating hydrocarbon fuels entirely is the major goal.