As the world produces more renewable energy in the need to transition away from fossil fuels, the use of low-temperature electrochemical reactors is an increasingly important topic of research. An enabling component of electrochemical reactors is an ion exchange membrane, which selectively transports either cations, in a cation exchange membrane (CEM), or anions, in an anion exchange membrane (AEM), from one compartment of a reactor to another. When a CEM and AEM are physically combined they can create a bipolar membrane (BPM), which can generate protons and hydroxide ions from water dissociation at the junction of the AEM and CEM when operated in reverse bias, while also reducing unwanted ion crossover in electrodialysis and electrolysis applications. Maintaining the junction interface is critical to the operation of a BPM, however due to the small size of the junction and the similarities of appearance between the AEM and CEM, it is difficult to obtain spectroscopic information about the BPM junctions. Additionally, in the literature the mechanism behind BPM failures are not well reported. These gaps in knowledge make rational design for improved membranes hard to achieve. Here, we use Confocal Raman Spectroscopy (CRS) to spatially observe spectral data throughout the AEM, CEM and AEM/CEM junction of a novel 3D spun bipolar membrane. CRS uses a monochromatic light to initiate and observe Raman scattering from the polymers in each membrane, showing characteristics of the polymer backbone structure, linkers, and ion exchange functional groups, throughout the membrane thickness. The confocal Raman microscope enables spectrum collection from a micron scale voxel, as opposed to the bulk volume, resulting in a nominally non-destructive technique for making spatiotemporal measurements. Using CRS, we identify the BPM junction thickness and homogeneity through 3-dimensional spatial mapping. We also demonstrate the ability of CRS to analyze the method of BPM failure when used in an electrodialysis reactor by obtaining spectral maps of failed membranes, showing junction delamination, ionic buildup in either the CEM or AEM, and ionic polymer breakdowns of the CEM or AEM due to contaminants. In collaboration with experimental collaborators, 3D BPM fabrication techniques were assessed for their impact on overall BPM performance and durability, while demonstrating the non-destructive usage of the CRS technique.
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