Abstract
Anion exchange membrane fuel cells (AEMFC) are potentially very low-cost replacements for proton exchange membrane fuel cells. However, AEMFCs suffer from one very serious drawback: significant performance loss when CO2 is present in the reacting oxidant gas (e.g., air) due to carbonation. Although the chemical mechanisms for how carbonation leads to voltage loss in operating AEMFCs are known, the way those mechanisms are affected by the properties of the anion exchange membrane (AEM) has not been elucidated. Therefore, this work studies AEMFC carbonation using numerous high-functioning AEMs from the literature and it was found that the ionic conductivity of the AEM plays the most critical role in the CO2-related voltage loss from carbonation, with the degree of AEM crystallinity playing a minor role. In short, higher conductivity—resulting either from a reduction in the membrane thickness or a change in the polymer chemistry—results in faster CO2 migration and emission from the anode side. Although this does lead to a lower overall degree of carbonation in the polymer, it also increases CO2-related voltage loss. Additionally, an operando neutron imaging cell is used to show that as AEMFCs become increasingly carbonated their water content is reduced, which further drives down cell performance.
Highlights
Published: 31 January 2021Recently, anion exchange membrane fuel cell (AEMFC) performance and stability have been enhanced to the point where their future deployment in real applications can be seriously considered
We investigate the carbonation of AEMFCs assembled from a series of very high-performing AEMs from the literature with different chemistries and properties, including high-density polyethylene with a benzyltrimethylammonium cation (HDPEBTMA) [35], low-density polyethylene with a benzyltrimethylammonium cation (LDPEBTMA) [35], poly copolymers of GT72-5 [36], GT78-15 [1], GT64-15 [37], poly(aryl piperidinium) copolymer that possesses a terphenyl chain (PAP-TP-85) [38]
In the absence of CO2, AEMFC performance generally increases as the membrane thickness decreases due to a combination of decreased ohmic resistance and increased water diffusivity [34,47,48,49]
Summary
Anion exchange membrane fuel cell (AEMFC) performance and stability have been enhanced to the point where their future deployment in real applications can be seriously considered. Recent studies have identified several pathways by which the CO2 -related voltage penalty can mitigated [9,10,13,19] such as increasing the AEMFC operating current density, cell temperature and hydration level as well as decreasing the cathode flowrate. A tremendous amount of literature has been generated regarding membrane preparation and the investigation of the backbone chemistry [22,23,24], headgroup chemistry [25,26,27], and structure [28,29,30] on their electrochemical and mechanical properties [31,32,33] Those electrochemical properties (especially hydroxide conductivity) are directly related to cell Ohmic resistance as well as the water uptake (WU) and transport. CO2 -related voltage loss—was investigated operando by neutron imaging in order to quantify the effect of adding CO2 to the amount of liquid water in the membrane electrode assembly (MEA), which is the first time that the water content of a carbonated AEMFC has been imaged
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