Elsevier

Electrochimica Acta

Volume 406, 20 February 2022, 139812
Electrochimica Acta

Spatially resolved performance and degradation in a perfluorinated anion exchange membrane fuel cell

https://doi.org/10.1016/j.electacta.2021.139812Get rights and content

Abstract

Anion exchange membrane fuel cells may enable future operation with non-precious metal-based catalysts. These systems have a delicate sensitivity to operating conditions such as humidification levels and the presence of CO2 in the air oxidant stream. We present spatially resolved in-situ performance results that shed light on phenomena that are unique to anion exchange membrane fuel cells. For cell construction, a highly conductive perfluorinated anion exchange polymer was used as the membrane and the material in powder form as the ionomer. Experiments were conducted to investigate the effects of humidification, fuel/oxidant concentration, and carbonation effects on the performance and its distribution in the cell. The results indicated that (i) dry conditions at the cathode have a stronger effect than at the anode on overall cell performance, (ii) performance significantly suffered when humidification was below 90%, (iii) fuel and oxidant dilution effects lead mass-transport losses and were stronger than flow rate effects, (iv) CO2 in the cathode feed stream creates an equilibration disparity between the inlet and outlet sections and CO2 purging is affected by flooding conditions, and (v) after >500 h of operation, performance deteriorates predominantly at the inlet.

Introduction

Ionic polymers used for low-temperature fuel cell applications have emerged as a promising material for energy and transportation applications over the past few decades [1,2]. The ionic polymers are categorized as proton or anion exchange membranes (PEMs, AEMs) [3] and PEM materials are currently used in commercially available fuel cell vehicles. The PEM fuel cell systems have to rely on expensive platinum-based catalysts, while the use of non-platinum cathode catalysts is feasible in AEM fuel cells (AEMFCs). This is due to operation at high pH which allows the OH to act as a charge carrier [4]. However, the AEMFC technology must overcome several challenges before it can compete with PEMFC technology, such as cell durability due to lower cationic stability and loss in performance due to carbon-dioxide reaction with the OH ion [5,6].

Automotive fuel cells require at least 5000 h of durability [4], which is very challenging for AEMFCs. To date the highest AEM fuel cell durability was reported for a constant current density of 600 mA/cm2 over 2000 h [7]. Two common loss mechanisms are known. On the one hand, the OH ion is a strong nucleophile and attacks polar functionalities in the AEM. For example, reactions such as the Hoffman elimination directly remove the polymer's functionality to conduct ions which leads to a drop in performance [3]. This can be mitigated by tethering advanced cations to the polymer which results in better alkaline stability than conventional trimethyl ammonium cations under certain circumstances [8]. On the other hand, CO2 from the ambient air, which has a concentration of about 400 ppm, creates a major challenge for long term operation. It reacts with the OH anion to form HCO3 and CO32− anions, a process which results in the reduction of ionic conductivity [9]. The CO2 reaction affects the membrane properties progressively and hence the fuel cell performance is eventually lost [10].

Improved AEMs for fuel cells were first conceptualized and tested in early 2000s [4]. Until 2008, the AEM studies reported OH conductivity below 40 mScm−1 at room conditions. Beyond 2008, researchers reported conductivity values <100 mScm−1 at elevated temperatures, but after 2014, the conductivity values of 100 mScm−1 were achieved. Very recently, some groups have even reported conductivity values close to ∼200 mScm−1 [11], [12], [13]. The significant improvements in the ionic conductivity values over the years have resulted in higher peak power performances of up to 3.4 Wcm−2 [14]. AEMs have a much wider range of chemistries than PEMs and several approaches have been shown to be successful such as synthesis and processing of block co-polymers, functionalizing perfluorinated ionomers, or optimizing radiation grafting of engineering plastics. It was realized that simply increasing IEC of the polymer does not lead to higher performance and not only leads to excessive dimensional swelling but also the cationic groups condense with each other and the conductivity plateaus [15]. Various cross-linking strategies were employed to induce a phase segregation of ionic domains in the polymer. This enabled an optimized morphology at a higher IEC and also inhibited the dimensional swelling [13,16]. Radiation grafted polymers begin with an unfunctionalized substrate like ethylene tetrafluroethylene (ETFE) exposed to e-beam radiation to graft ionic moieties such as vinyl benzyl chloride (VBC) onto the polymer in a controlled manner [17], which has been optimized [18], [19], [20], [21].

The fabrication of MEAs with existing and novel AEM materials requires the successful integration of ionomers that are compatible with conventional ink-based electrodes [22]. Recently, Mustain and co-workers have demonstrated that using the ground powder form of an AEM material in the electrode can produce high performing membrane electrode assemblies (MEAs) [7]. With the availability of high performing MEAs practical aspects of fuel cell operation can move into the focus to improve AEMFC performance and lifetime. It is for example informative to understand the spatial performance of the cell. Such studies can shed light on effects of water management, ionomer carbonation, hydrogen and oxygen concentration, and degradation mechanisms.

Researchers have used segmented fuel cells (SFC) since decades to study the spatial performance in PEMFCs. The diagnostic tool was introduced by Cleghorn et al. and Stumper et al., and further refined and customized over the years to address specific research questions [23], [24], [25], [26], [27]. Researchers have gained valuable insights using SFC systems to understand the effect of humidification, electro-osmosis, back-diffusion, temperature, inlet pressure, clamping pressure, flow rate, flow-field design, flow configuration, and contaminant species on the spatial PEMFC performance [28].

For the benefit of the reader a short review of the SFC research on PEMFC is given in the following paragraphs. Most of the SFC are capable to quantify the current distribution of the cell at various humidification levels. In PEMFC, at a lower inlet humidification, the cell is required to intrinsically hydrate using product water which leads to a performance gradient from this operating condition [29], [30], [31], [32]. At a sufficiently high humidification the distribution becomes more uniform but may also lead to a higher presence of liquid water in the cell which may result in flooding and mass-transfer limitations near the end of the flow channel [25,31,33]. Increases in the gas flow resistance due to flooding results in the reduction of cell performance downstream and also develops shortcut gas flows [34]. Strong performance gradients may further lead to local temperature increases [35]. In PEMFCs, the SFC has been used to study the transition of water management dominance from electro-osmotic drag of water (anode to the cathode due to the transport of protons) to the back-diffusion of water (cathode to the anode driven by a concentration gradient) [36]. Dong et al. have reported that a higher humidification leads to improved cell performance and that sufficient anode humidification avoids local dry out and performance losses [37]. In contrast to PEMFC systems, the direction of electro-osmotic drag and back-diffusion is reversed in AEMFC: electro-osmotic drag occurs from the cathode to the anode with the transport of hydroxide ions and the back-diffusion of water from the anode to the cathode, driven by the resulting water gradient. Results of the spatial effects of these water management governing processes on the AEMFC system are presented in the results and discussion section of this work.

Flow-rates and gas composition effects have also been studied on PEMFC systems. The current density distribution in the SFC is more dependent on the oxygen utilization rather than the fuel and is non-uniform at low airflows due to the O2 starvation effect [31,38,39]. On the one hand, at low air flows, the current density declines in the downstream segments due to oxidant depletion and flooding of liquid water generated upstream in the channels [40], [41], [42], [43]. On the other hand, higher airflow rates improve liquid water drainage and reduce MEA flooding [44], but may also dry out the oxidant inlet [45]. Factors such as low pressure conditions, gasket material, clamping pressure affecting the flooding and gas flow resistance are studied using SFC [34,44].

SFC systems can further assist in understanding and optimizing effects of the cell architecture such as the type of flow field [46]. Another critical loss mechanism is the introduction of contaminant species into the feed stream of the cell. For example, in AEMFCs the CO2 that is present in air significantly reduces the performance by impacting the conductivity of the alkaline electrolyte membrane. This process is expected to show a time and spatial dependency along the flow-field, similar to most of the reported PEMFC contamination processes. One critical contaminant for mobile PEMFC applications is CO, which was extensively studied using SFC systems [47], [48], [49].

All this work indicates how useful the SFC diagnostic tool is for understanding the processes within a fuel cell that cannot be observed through single cell experiments. In this work the SFC diagnostic is applied to an AEMFC system. The work employs a previously introduced novel perfluorinated anionic membrane [50,51]. The AEM is synthesized from a perfluorinated sulfonyl fluoride ionomer precursor (EW 798) developed by 3M (USA). It is a copolymer of tetrafluoroethylene (PTFE) and a trifluoroethylene functionalized with a perfluorinated sulfonyl fluoride carbon chain. The trimethylammonium cation is tethered to the sulfonamide through a six-carbon alkyl spacer chain [51]. We present spatial experimental results that demonstrate the performance effects of hydration, fuel or oxidant starvation, CO2 poisoning, and degradation. For analysis, we compare the results to a modeling study that highlights the physics of the occurring processes.

The employed SFC features 121 segments within a 50 cm2 active area [52], and is used to study the dependence of the spatial current distribution in an AEMFC on various operating conditions and the flow-field architecture. For PEMFCs, water is generated at the cathode whereas for the AEMFCs it is a reactant at the cathode as well as a product at the anode. Therefore water management is complicated and needs to be understood thoroughly for optimal performance [53].

In summary, most of the studies on AEM that are available in the literature are focused on developing new membrane material, developing catalyst inks for higher performance and understanding the carbonation phenomenon [4,53,54]. This work employs for the first time a SFC diagnostic to study fundamental processes associated with AEMFCs with the intention to increase understanding of the effect of various operating conditions on the spatial current distribution and shed more light on MEA durability in AEMFC.

Section snippets

AEM materials

Perfluorinated anion exchange membrane was prepared in two thicknesses of 70 and 35 μm, using PFAEM_CH3_C6 polymer synthesized from perfluorinated sulfonyl fluoride ionomer precursor (EW 798, 3 M USA) as described previously [50,51]. Powder of the same material were used as ionomer in the electrode.

MEA fabrication

To create an electrode ink, the polymer from the previous section was ground to a fine powder in a mortar and pestle and mixed with 46% Pt on Vulcan® carbon (Alfa Aesar HiSPEC 4000) catalyst. A small

Results and discussion

Generally, this study can be separated into two high-level categories that investigate (i) the effects of key operating parameters and (ii) the chemical phenomena leading to loss in conductivity and thus performance. The key operating parameters all have their own subsection addressing inlet gas humidification levels, flow rate, and concentration. The two chemical phenomena, i.e., hydroxide attack and carbonation are also described in their own subsections. Note that the typical performance

Conclusion

A SFC hardware was used to shed light on two high-level phenomena: 1. The effect of key operating conditions such as hydration, flow rate, and concentration, and 2. the loss in cell performance due to OH attack and carbonation observed in a perfluorinated anion exchange membrane fuel cell. The fuel cell was tested using two kinds of flow-field architectures: serpentine and straight channel. The straight channel cell study eliminates the non-uniformity of current distribution observed in the

CRediT authorship contribution statement

Ashutosh G. Divekar: Conceptualization, Validation, Formal analysis, Investigation, Writing – original draft, Visualization. Michael R. Gerhardt: Methodology, Validation, Formal analysis, Investigation, Visualization. Christopher M. Antunes: Validation, Resources, Investigation. Luigi Osmieri: Methodology, Validation, Formal analysis, Investigation. Ami C. Yang-Neyerlin: Validation, Formal analysis, Investigation. Adam Z. Weber: Supervision. Bryan S. Pivovar: Funding acquisition, Project

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The work was supported by the U.S. Department of Energy under Contract No. DE-AC36-08GO28308 with Alliance for Sustainable Energy, LLC, the Manager and Operator of the National Renewable Energy Laboratory. Funding provided by the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy Fuel Cell Technologies Office. The views and opinions of the authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. Neither the

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