Understanding How Anion Exchange Membranes and Cationic Polymer/Catalyst Interactions Behave with Time in Alkaline Electrolysis

Abstract

We are leveraging our existing success with anion exchange membranes (AEM) to develop new highly stable and highly conductive membranes and ionomers for AEM electrolysis. Our novel and scalable tri-block co-polymers ABA or BAB, where the A block can be functionalized with an advanced chemically stable cation and the B block is an unsaturated polymer that when hydrogenated results in high anion conduction membranes with exceptional mechanical strength and durability analogous to polyethylene (PE). We have also developed a novel immobilization technology for processable catalyst inks. Synergistically the A and B block lengths and chemistries are tuned to achieve all the properties desirable for an anion exchange membrane for electrolyzer applications. Random polymers of the same A and B chemistries can also be formulated for additional fidelity in chemically compatible ionomers for the membrane electrode assemblies (MEA). These systems are unique in that they achieve very high dissociation of anions beyond OH-, giving CO3 2- σ<100 mS cm-1 ca. 60°C and enhanced chemical stability as the water is separated from the high MW polyethylene backbone that additionally gives the materials unprecedented strength. This will allow us to fabricate thinner membranes for long durability pressure differentiated operation. Because of the unusual and exceptional CO3 2- conductivity that we have designed into our membrane, we are able to optimize these for a carbonate electrolyte electrolyzer. While we lose a 100 mV in activity compared to operation in OH-, the advantages including high current densities enabled by our novel membrane outweigh that disadvantage. Chemical stability of the membrane is not a concern, and the system is less complicated as we do not need to use expensive components to protect against caustic corrosion or protect the hydroxide electrolyte from carbon dioxide present in air to avoid its inevitable conversion to CO3 2-.Not all proposed AEM polymer chemistries are scalable, many involve too many synthetic steps, suffer from low overall yields, and are highly exothermic reactions (thus only safely executed on a small scale), or do not produce sufficiently high molecular weight for film formation. The polymer systems proposed here have already been demonstrated on the 1 kg scale. This novel AEM intellectual property, jointly invented by Mines and UMass, has been licensed to Spark Ionix who are commercializing an early version of this polymer technology. The next generation of tailored polymers proposed are increasingly robust, scalable, efficient and more amenable to high volume manufacture to produce inexpensive hydrocarbon polymer membranes for use in electrolyzers. The block lengths, overall molecular weight and chemistries are completely tunable to fabricate AEMs with all desired properties or compatible ionomer chemistries that allow high levels of catalyst utilization, through both ionic conductivity and chemical transport. We have been investigating this polymer system in 1M K2CO3 at 50°C with great success and have shown good performance and durability in 5cm2cells. We are developing the system for >60°C and 25 cm2 cells. We have begun to understand the science behind ionomer catalyst interactions and initially used a commercial Ag nano-catalyst as a model system. Few ave studied the effect of ionomer chemistry and catalyst or catalyst type and loadings on performance. We have shown durability of > 600 h at 0.5 A cm-2 and continue to see improvements. It should be noted that the voltage is dropping in theses tests, at end of test at –ve 60 µV h-1 is achieved, while this may be interpreted as evidence of membrane thinning when the cell was dissembled the membrane thickness was ca. the same as at the beginning of test (80 µm), indicating slow, but long term conditioning, the cell did not reach the point at which a +ve degradation rate was observed. Further investigation of cell conditioning indicated that these cells currently take 40-50 hs under load to reach steady state operation (this break in time will be understood and significantly shortened) giving 400MV improvement at 1 A cm-2. With a MnO2 catalyst the voltage at 0.5 A cm-2drops to 1.5 V indicating that the target of 2 A cm-2 at 1.8 V will be easily achieved with further investigation.