Functional and Environmental Performances of Novel Electrolytic Membranes for PEM Fuel Cells: A Lab-Scale Case Study

Despite being the most employed polymer electrolyte for proton exchange membrane fuel cells (PEMFCs), Nafion® has several limitations: expensiveness, poor performance when exposed to temperatures higher than 80 °C, and its potential as a source of environmentally persistent and toxic compounds (i.e., per- and polyfluoroalkyl substances, known as PFASs) when disposed of. This work explores the functional and environmental performances of three potential PFAS-free alternatives to Nafion® as electrolytic membranes in PEMFCs: sulfonated graphene oxide (SGO), graphene oxide-naphthalene sulfonate (GONS), and borate-reinforced sulfonated graphene oxide (BSGO). Investigated via ATR-FTIR spectroscopy, TGA, and cross-sectional SEM, the membranes show an effective functionalization of GO and good thermal stability. Functional properties are determined via Ion Exchange Capacity (IEC) evaluation, Electrochemical Impedance Spectroscopy, and tensile tests. In terms of IEC, the innovative materials outperform Nafion® 212. Proton conductivities at 80 °C of SGO (1.15 S cm-1) and GONS (1.71 S cm-1) are higher than that of the commercial electrolyte (0.56 S cm-1). At the same time, the membranes are investigated via Life Cycle Assessment (LCA) to uncover potential environmental hotspots. Results show that energy consumption during manufacture is the main environmental concern for the three membranes. A sensitivity analysis demonstrates that the impact could be significantly reduced if the production procedures were scaled up. Among the three alternatives, SGO shows the best trade-off between proton conductivity and environmental impact, even though performance results from real-life applications are needed to determine the actual environmental consequences of replacing Nafion® in PEMFCs

The Digital Museum/Archive of Overseas Memories

The Digital Museum/Archive of Overseas Memorie

“Milano-Città Studi: i luoghi del Museo Politecnico”, in QD, Quaderni del Dipartimento di Progettazione, Facoltà di Architettura del Politecnico di Milano n. 2, febbraio 1985

iin QD, Quaderni del Dipartimento di Progettazione, Facoltà di Architettura del Politecnico di Milano n. 2, febbraio 198

Self-standing sulfonated graphene oxide membranes as alternative proton conductors for PEM fuel cells

State-of-the art proton exchange membrane fuel cells employ Chemours Nafion® as the ionomer of choice. However, Nafion® suffers a rapid performance degradation when the temperature is raised above 80 °C and the relative humidity is reduced below 50%. Huge efforts have been made to develop new materials able to work at such conditions, which would be beneficial for fuel cell operation, in terms of faster reaction kinetics, easier water management and simplified design. Among several approaches, graphene oxide (GO) has gained great interest due to its oxygen-bearing groups, which make it an ideal candidate to prepare self-assembling, hydrophilic membranes, although poor fuel cell durability and performance have been verified. This work presents a novel approach to improve the properties of GO and achieve an efficient operation at elevated temperatures and reduced humidity. Self-standing sulfonated GO membranes are produced by investigating two different functionalization processes (Figure 1), with the aim of introducing the same sulfonic groups (–SO3H) of Nafion® and to enhance proton conductivity and water retention. On one end, sulfonation is performed by means of a reaction of GO with sulfuric acid; on the other, the intercalation of a naphthalene sulfonate-based (NS) molecule is exploited. Different acid-to-GO and GO-to-NS molar ratios are studied to identify an optimal composition, as well as two distinct process temperatures when NS molecules are used. Samples of both membrane types are extensively characterized from the morphological (ATR-FTIR, Raman, SEM-EDX, TG-DTG, XRD, static contact angle) and functional (water uptake, ion exchange capacity, electrochemical impedance spectroscopy) viewpoint, to acquire information on the effect of the employed procedures on GO properties, focusing on water retention and proton conductivity at the desired working conditions of low humidity and high temperature, in light of a future implementation in a running fuel cell

Borate-Reinforced Sulfonated Graphene Oxide Membranes as an Alternative Proton Conductor for PEM Fuel Cells

INTRODUCTION Graphene oxide (GO) and its sulfonated versions (SGO), bearing sulfonic groups (-SO3H), have been extensively studied by researchers to be used as a potential alternative to Nafion® as the electrolyte of proton exchange membrane fuel cells (PEMFCs). Their main point of interest is the combination of a noteworthy proton conductivity with an excellent self-assembling ability, enabling the production of freestanding membranes1,2. Nevertheless, issues of poor structural stability have emerged, in spite of the excellent mechanical properties of the individual flakes, because they are kept together by weak hydrogen-bonding and van der Waals interactions, which reduce the overall strength. A possible remedy to this flaw, borrowed from nature, is the exploitation of borate ions to improve the cohesion of the membrane, as they might promote the linkage among the oxygenated groups of different GO sheets3. Therefore, this work proposes an effective method to manufacture standalone borate-reinforced membranes (SGO+B), based on a GO matrix adequately functionalized by both sulfonic acid groups and sodium tetraborate decahydrate, with the aim of enhancing their proton-conducting properties and structural stability. EXPERIMENTAL/THEORETICAL STUDY The borate-modified membranes were produced starting from a commercial aqueous dispersion of GO (4 mg mL-1, Graphenea Inc.), 25 mL of which were put into a round-bottomed flask and underwent 10 min of a mild ultrasound bath. Sulfonation was carried out after sonication by introducing precise amounts of sulfuric acid, in order to achieve different acid-to-GO molar ratios. The mixture was then subjected to 6 h of magnetic stirring at 50 °C, in order to favor the sulfonation reaction. Afterwards, it was diluted with deionized water to a concentration of 0.75 mg mL-1 and mixed under vigorous stirring to controlled volumes of a 0.1 M solution of sodium tetraborate decahydrate3. The resulting dispersion was then vacuum filtered on a PVDF filter and dried in oven, so as to remove the excess of water and to favor the formation of the expected borate-mediated bonds among the GO sheets. Reference membranes with no sulfonation (GO+B), no borate addition (SGO) or neither of them (GO) were prepared as well as a basis for comparison. The previous samples were characterized by employing ATR-FTIR and SEM-EDX spectroscopies, TG-DTG and static contact angle analyses, XRD, and by measuring their ion exchange capacity (IEC) and degree of sulfonation. Selected samples underwent water uptake tests at different conditions of temperature and humidity, while their proton conductivity was assessed by electrochemical impedance spectroscopy. A preliminary evaluation of their performance was eventually conducted in a lab-scale hydrogen-fed fuel cell. RESULTS AND DISCUSSION The main aim of the applied characterization procedures was to evaluate the successful introduction of both sulfonic acid and borate-based groups into the membranes, as well as to assess their practical performance, in view of their possible application as an electrolyte in PEMFCs. The effectiveness of the sulfonation reaction is demonstrated in ATR-FTIR spectra by the rising of bands that can be ascribed to the stretching vibrations of O=S=O and S-O bonds in -SO3H groups. These results are also in agreement with those of SEM-EDX spectra, displaying an increase in the contents of oxygen and sulfur with respect to bare GO. The effect of borate functionalization seems clearer in non-sulfonated (GO+B) membranes, whose ATR-FTIR spectra exhibit a reduction in C-OH and O-H signals referred to hydroxyl and carboxyl groups. This is a potential symptom of the formation of interplanar bonds among the oxygen-bearing functions of different GO layers3. Then, GO+B membranes also exhibit a higher thermal stability in the typical operating range of PEMFCs, as witnessed from TG-DTG analyses. On the other hand, the consequences of borate addition are of less clear interpretation in the case of SGO+B specimens, because there might be a competition between -SO3H and borate groups in substituting GO’s functionalities. However, these samples seem to exhibit a superior swelling resistance compared to both Nafion® 212 and pristine GO, while keeping a good water sorption. This may result in a higher resistance to mechanical degradation caused by wet/dry cycles during cell operation. In addition, the presence of borates seems to influence positively the value of the IEC, which is improved as against Nafion® 212, GO and SGO samples. CONCLUSION We presented a straightforward and efficient procedure to fabricate self-standing, borate-treated SGO membranes, whose characterization evidenced a promising structural stability coupled to a fair proton conductivity, which make them an interesting candidate for a successful application as a novel electrolyte for PEMFCs. REFERENCES 1. T. Bayer et. al, J. Power Sources 272, 239 (2014) 2. R. Kumar et al, Chem. Commun. 48, 5584 (2012) 3. Z. An et al, Adv. Mater. 23, 3842 (2011