Fuel cell know-how from the Paul Scherrer Institute at the core of the SBB minibar

The idea saw the light of day about 11 years ago at Bern University of Applied Sciences and it drew on earlier work by PSI and ETH Zurich. The goal was to come up with a compact, simple and reasonably priced fuel cell system for portable applications. The main objective was, therefore, to simplify and scale down the voluminous, complex control units for humidification of the fuel cell membrane and cell stack cooling.

Two of the PSI solutions that have since been patented stem from this research work and have been integrated into the fuel cell system. Firstly, the innovative internal humidification of the fuel cell membrane and secondly the cooling seal concept which was further developed by the Bern University of Applied Sciences.

Internal humidification reduces volume and complexity

Hydrogen fuel cells generate power by splitting hydrogen at the cell’s negative electrode. This creates electrons which flow as power into a circuit and protons (hydrogen nuclei) which react with oxygen at the cell’s positive electrode to form water. The precondition for these reactions is, however, that the protons diffuse through the polymer membrane from the anode to the cathode. For this the membrane must have what experts call good proton conductivity. This conductivity increases with the degree of humidity of the membrane. Too much liquid water can, however, also block the pores through which the gas flows to the membrane. It’s all about ensuring the optimum degree of humidity.

In larger fuel cell systems external humidifiers ensure the necessary degree of humidity. They are not economically viable for a portable system with its strict profitability requirements because of their volume and complexity.

The PSI researchers thus came up with a solution: internal membrane humidification. The idea was to return the excess humidity from the outlet air generated during cell operation directly to the fresh input air.

Sophisticated humidification concept saves on energy

For the purposes of recovering humidity, the used air is channelled through specially integrated canals to what is known as the membrane’s humidification area. In the humidification area humid outlet air flows on the one side and the dry, fresh fed-in air on the rear side. This difference in humidity leads to water diffusing through the membrane from the humid to the dry side thus humidifying the dry fresh air. This is almost entirely passive, i.e. without any control and with minimal energy input. Energy is only needed for the compressor to blow air through the cell.

Internal humidification simplifies the system because an external humidifier is no longer needed. Furthermore, it offers a decisive advantage. As humidification can now be undertaken independently on each cell, the system can in principle be scaled up at will without any need for an increasingly large external humidifier. The trick here is to ensure that the humidification area is the right size: not too big but big enough for adequate humidity transport particularly when the cell stack is working at full load.

Cooling with air instead of water

Another contribution from PSI has to do with cell stack cooling. Water is normally used as the cooling agent. Researchers at Bern University of Applied Sciences, developed a cooling concept based on air. Unlike other concepts where the cooling air flows through coolant sections of the cells, they went for what is known as edge air cooling. The heat from the active part of the cells is efficiently transferred to the edge area and then removed from there by the cooling air flow. Heat is transported efficiently from the cell centre to its edges by a graphite-like material with high thermal conductivity. The dimensions of this cooling area were determined on the basis of calculations using a model developed by PSI researchers.

Furthermore, the graphite-like material is used as a gas seal in the cell stack and prevents hydrogen and oxygen from mixing. This use as a seal has since been patented as a PSI invention.

The market launch of the new SBB minibar with the integrated fuel cell system is proof that with perseverance in technology transfer basic research can provide important contributions towards commercial products. The efforts by PSI on all levels of the research and development of hydrogen fuel cells have once again borne visible fruit. This success would not have been possible without the long-standing commitment of Bern University of Applied Sciences and the company CEKAtek who believed in and fine-tuned this technology.

Text: Paul Scherrer Institute/Leonid Leiva

About PSI

The Paul Scherrer Institute PSI develops, builds and operates large, complex research facilities and makes them available to the national and international research community. The institute's own key research priorities are in the fields of matter and materials, energy and environment and human health. PSI is committed to the training of future generations. Therefore about one quarter of our staff are apprentices, post-graduates or post-docs. Altogether PSI employs 1900 people, thus being the largest research institute in Switzerland. The annual budget amounts to approximately CHF 350 million.