PhD defense: Too much acid flow in membranes leads to fatal breakdowns of fuel cells

Thursday 15 Mar 18

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PhD Defence

Hans Becker defended his PhD “Phosphoric Acid Issues in High Temperature Polymer Electrolyte Membranes” Tuesday, 27 February.

Principal Supervisor: Professor Qingfeng Li, DTU Energy. Co supervisor: Professor Jens Oluf Jensen, DTU Energy.

Hans Becker has defended his PhD-thesis on “Phosphoric Acid Issues in High Temperature Polymer Electrolyte Membranes” at DTU Energy. In his thesis, he showed why certain fuel cells always have fatal breakdowns and how water can either cure problems or create additional problems.

An essential part of any fuel or electrolysis cell is an electrolyte – a substance which allows specific ions to pass but does not conduct electrons. In High Temperature Polymer Electrolyte Membrane Fuel Cells (HT-PEMFC) the electrolyte is phosphoric acid. It is held in place by being absorbed into a polymeric membrane, and since the electrolyte is liquid, its constituent positively charged cations and negatively charged anions are free to move in opposite directions when an electric potential exists across the membrane, creating a current. This is textbook stuff for chemists working on electrochemical cells. However, often they don’t consider that the acid actually slowly moves toward the negative anode under operation of the fuel cell.

“There is a gradual build-up or gathering of acid in one end of the system, and when I say gradual it is one to four percent of the moving current”, explains Hans Becker, 26 yrs., who recently defended his PhD-thesis on “Phosphoric Acid Issues in High Temperature Polymer Electrolyte Membranes” at DTU Energy.

Normally the slow movement of acid is of no concern, as it is mostly evened out by the natural backflow of the acid inside the system. But during actual operations where higher currents are used, the acid flow can lead to fatal breakdowns, as cells stops working due to imbalance of the acid.

"People stopped using PFSA in fuel cells despite its qualities due to unexplained breakdowns. Now we have for the first time shown them what happens"
Hans Becker, former PhD-student at DTU Energy

15 times higher migration

In Hans Becker’s project he made a setup of a segmented three-layered membrane configuration with thin wires in between, like a sandwich, and by applying and measuring current in the cell, Hans BEcker was able to measure acid migration activity in a working cell. He also explored the influence of different membrane thicknesses, humidities, and temperatures and found out that acid migration took place in systems with polybenzimidazole (PBI) membranes and poly(perfluoro sulfonic acid) (PFSA) membranes. Only a very small fraction of acid migration was found in PBI based system, while he detected up to 15 times higher migration activity in PFSA based system.

PFSA doped with phosphoric acid (PFSA-PA) has been considered ideal for membranes for fuel cell and electrolysis cells, as the system has good conductivity and in theory has a very good performance. In operational reality the PFSA-PA membranes always fails.

“People stopped using PFSA despite its qualities due to unexplained breakdowns. Now we have for the first time shown them what happens, as we measured a 15 percent acid migration in PFSA compared to the 1-4 percent in PBI, and the backflow of the cell can’t keep up with such high acid migration. The performance decays and in the end the cell breaks down due to too much acid in one end of the system and too little in the other”, says Hans Becker. The findings give an explanation of why the PBI based high temperature PEM fuel cells (HT-PEMFC) can operate very long while PFSA based HT-PEMFC malfunctions almost immediately.

Water cures the system

Hans Becker also tested the effectiveness of adding water in a PFSA-PA membrane and it turned out that water can actually cure the PFSA system, raising the possible current of the system tenfold from 20 to 200 milliampere, as water changes the entire chemistry within the cell. This is no big news, as PFSA was originally used with water, but many researchers moved on and dropped the use of water in the system.

Water also enables better performance in membranes based on phosphoric acid as it counters or suppress acid condensation from high airflow. High airflow leads to higher performance of the cell, but the drawback was higher acid condensation, leading to most people just avoiding high airflows.

“In using the water of the system to suppress acid condensation, we get the benefits and not the drawback of high airflow”, explains Hans Becker.

Having defended his thesis, Hans Becker is now contemplating his next career move, and he has talks with a Japanese University about a postdoc position. Himself an Indonesian, a move back to Asia will be fine as it’ll bring Hans Becker closer to his family, but he has liked his stay in Denmark and has learned a lot.

“I took my master in chemical engineering at Hong Kong University of Science and Technology in a different research topic, so I didn’t know much about fuel cells before I arrived to DTU. I have learned quite a lot here, and I would like to continue researching in electrochemical cells, as I think it has a great potential. We’ll see how it turns out.”

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http://www.energy.dtu.dk/english/news/nyhed?id=2F275782-3886-4335-AAB5-D71299C50845
23 APRIL 2018