3 May 2019

About Fuel Cells

Fuel cells (FCs) are electro-chemical devices that convert hydrogen into electricity. For most of them, water is the only byproduct. Among the various fuel cell types that have been developed over the last decades, we are interested in proton exchange membrane fuel cells (PEMFC), which represent 75% of the total power deployed. Their very basic principle relies on a semi-permeable membrane covered with a platine catalyst, whose pores are too small to allow hydrogen atoms to go through. When hydrogen is provided on the anode, only the positive H+ ions can cross the membrane, while electrons are collected to generate current. On the cathodic side, oxygen recombines with the electron and the H+ ion to form pure water, the only byproduct of the reaction.

Each membrane is clamped between plates (called bipolar plates or flow plates) that uniformly distribute oxygen – from air – and hydrogen onto the membrane’s surface. They also ensure the transportation of electrons that are generated at its interface.

Fuel cell basic principle
Fuel cell basic principle – By CFA213FCECC BY-SA 3.0, Link

THE LOW TEMPERATURE PEMFC (LT-PEMFC)

The LT-PEMFC is a mature technology used in several industries, like backup power generation, forklift, trains or remote power. But they now also reach consumers, with the growing mobility market. The technology however suffers of some drawbacks that undermine the overall picture.

LT-PEMFC performances are really dependant on hydrogen purity.

Indeed, the LT-PEMFC’ membranes can be poisoned by impurities (like carbon monoxide) that durably bond with the catalyst and durably degrades the overall stack performance. For this reason, this technology can only use ultra-pure hydrogen that must reach at least 99.995% of purity. As of today, hydrogen produced by water electrolysis remains the only acceptable source of fuel for those systems.

LT-PEMFC are complex systems

The chemistry of the membrane implies that the hydrogen humidification must be carefully controlled for the stack to work properly. That adds many complex and bulky auxiliaries like humidifiers and blowers, which reduce the overall stack efficiency. Even if significant milestones have been achieved in that respect, the complexity of humidification system is still a major issue for most industrial. Finally, when hydrogen and oxygen recombine to produce water, droplets block the tiny channels used for fuel admission. That leads to complex and expensive flow plate design in order to help the water removal.

THE HIGH TEMPERATURE PEMFC (HT-PEMFC)  

To mitigate the LT-PEMFC drawbacks described above, simpler systems that are more tolerant to impurities have emerged. They use phosphoric acid doped membranes (PBI for Polybenzimidazole membrane) and work in the range of 160-180°C.

Simplified system…

The humidification system present on LT-PEMFC are not relevant anymore, as the membrane uses phosphoric acid as conductive media so uncontrolled hydrogen humidity is just fine. Still talking about water, high temperature systems (well above boiling point) have no more channel blockage effect like the one described above.

… that can use various gas sources

HT-PEMFC also have the huge advantage of being tolerant to massive amount of impurities. A thousand time more tolerant than LT-PEMFC! HT-PEMFC can then run on many different fuels, from electrolyser to biogaz, through readily available methanol or natural gas.

Sources of gas for HT-PEMFC

A higher efficiency

Because HT-PEMFCs run at higher temperature, the generated heat can be valued in micro-combined heat and power systems (micro-CHP). Thus, they are perfect candidates for domestic use, with an overall efficiency up to 95%.