Electrolysis will play an important role for energy storage in a society based on power from solar and wind. For this to happen, electrolysis cells need to be efficient and cheap. On 13. October 2017 Mikkel Rykær Kraglund defended his PhD thesis ‘Alkaline membrane water electrolysis with non-noble catalysts' which aims to solve this challenge.
Electrolysis cells use power to split water molecules (H2O) into hydrogen (H2) and oxygen (O2). This is an old, well-established technology but it has experienced something of a renaissance after the realization that it can be used to store surplus power from, e.g., wind turbines as chemical energy in hydrogen. When the power is needed again, the hydrogen can be converted in a gas turbine or a fuel cell to generate electricity. It is also possible to go a step further and use hydrogen in combination with CO2 to make synthetic methane or gasoline.
A traditional alkaline electrolysis cell consists of two electrodes submerged in an aqueous solution of potassium hydroxide (KOH). When a sufficient voltage difference is applied to the electrodes, hydrogen begins to bubble up at one electrode and oxygen at the other. To avoid mixing of the two gasses, the electrodes are separated by a porous membrane which only allows OH– ions and water to pass. Such electrolysis cells have been in commercial use for more than a century. However, they are not very efficient and lack the required flexibility in operation to make them suited to store power from fluctuating energy sources such as solar and wind.
A promising alternative is PEM electrolysis, where the cells resemble the corresponding PEM fuel cells. Here the membrane is replaced by a dense polymer membrane (PEM means polymer electrolyte membrane) which only conducts hydrogen ions. Such electrolysis cells have much higher efficiency than the traditional alkaline electrolysis. But the electrodes of a PEM electrolysis cell contains the expensive and rare noble metals platinum and iridium. So even if the cell is more efficient, it is also too expensive.
This is where Mikkel Rykær Kraglund enters. He aims to combine the best of both worlds: The cheap materials from alkaline electrolysis and the high efficiency from PEM electrolysis. Like PEM cells, Mikkel's cells contain a dense membrane but his membranes consist of a different kind of polymer (PBI). By soaking it with KOH, you can make the membrane conduct OH– ions and in this way let it function as an electrolyte. The advantage of using KOH is that you don't need noble metals in the electrodes. Mikkel has tested a number of electrode materials based on nickel and has been able to show that the efficiency of the new type of alkaline cell approaches that of PEM cells, without the use of noble metals.
“We have shown that by the use of suitable membranes it is possible to run an alkaline electrolysis cell, containing no noble metals, at a current level four times higher than that of conventional systems, and with an efficiency equivalent to that of PEM cells", Mikkel explains.
However, the cells he has developed are not stable enough yet. Until now the cells last just under two weeks under the harsh operating conditions, but work is ongoing to improve the lifetime. After his defense, Mikkel has been hired as a postdoc at DTU Energy to continue the development and test of the new alkaline electrolysis cells.