(Invited) Alkaline Fuel Cells (AFCs) and Alkaline Membrane Fuel Cells ( AMFCs)

Monday, 29 May 2017: 10:40
Grand Salon B - Section 9 (Hilton New Orleans Riverside)
S. Gottesfeld (Fuel Cell Consulting, inc.)
I was fortunate to have many technical discussions with Russ Kunz over the years and learnt a lot from him. Russ was a rarely encountered combination of scientist and technologist , a combination which is prerequisite for being a leader, as he was, in the field of fuel cell development. I chose the subject of this talk having been a common theme in our technical discussions, as Russ was the house expert at UTC for the alkaline fuel cells (AFCs) used in space applications, whereas I got intrigued in recent years by the potential of alkaline membrane fuel cells ( AMFCs) which share key features with the AFC while aiming to operate with no liquid electrolyte.

The talk will accordingly review first some interesrting key features of materials and processes in AFCs about which I learnt to large degree from discussions with Russ. The first subject is the ORR in highly alkaline electrolytes and why gold is actually the active catalyst component in AFCs , being used as a AuPt alloy with the Pt serving basically just to harden the metal catalyst particles. The high activity of gold as ORR catalyst in alkaline media, does not fit the extensively taught volcano shaped relationship between M-O bond strength and ORR rate at a metal M. Gold is way down on the descending branch of the volcano plot in acid electrolyte, but is more active than Pt in alkaline media. This has been explained by the stabilization of a peroxide anion intermediate formed in alkaline electrolytes (HO2- or O2=) on the electrolyte side of the metal electrolyte interface, thereby removing the need of significant M-O interaction to stabilize the HOOads intermediate formed in acid. This opens the door for metals of very low affinity to oxygen to become very active ORR electrocatalysts thanks to the low overpotential they require to generate oxide free metal sites.

In the case of the AMFCs, this high activty of coinage metals in alkaline environment is the basis for the possible choice of silver as ORR electrocatalyst, having activity in alkaline electrolyte comparable to Pt, at a market price which is about 1% that of Pt.

One central aspect which is quite different in AFCs and AMFCs, is water managment. The AMFC presents a significant challenge of combined tendencies for cathode dry-out and anode flooding, originating form the cell water product forming in the anode ( fuel side) and consumed by the electrode process in the cathode ( air side). The resulting demands of water diffusivity through the cell membrane as well as electrode structures required to achieve passive control of the water profile in the AMFCs , staisfying the ORR consumption and ionomer hydration demands, will be described.

Finally, the HOR in alkaline media in general and in the AMFC in particular has received significant attention recently. The rate of the HOR at Pt in 0.1-1MKOH solutions is significantly lower vs. the rate in acid. However, AMFC polarization curves to be presented show a performance that is not much below that of PEMFC in operation on hydrogen and air. This is achieved with HOR catalyzed by very low loadings of Pt in the form of alloy ( or other combination ) with an oxophilic metal component which facilitates completion of the HOR process and, securing optimized cell hydration under current.