Carbon xerogel as gas diffusion layer in PEM fuel cells

Carbon xerogel as gas diffusion layer in PEM fuel cells

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Short Communication

Carbon xerogel as gas diffusion layer in PEM fuel cells Alexandra M.I. Trefilov, Athanasios Tiliakos*, Elena C. Serban,  ta  lin Ceaus, Stefan M. Iordache, Sanda Voinea, Adriana Balan** Ca University of Bucharest, Faculty of Physics, 3Nano-SAE Research Center, 405 Atomistilor Str., Bucharest-Magurele MG-38, 077125, Romania

article info

abstract

Article history:

We investigate a novel gas diffusion layer/catalyst/membrane assembly based on low-cost

Received 29 November 2016

carbon xerogels replacing the expensive plasma-pyrolyzed carbon black microporous layer

Received in revised form

(MPL). The tri-layer structure (carbon paper/carbon xerogel/carbon black) of the gas

19 February 2017

diffusion layer (GDL) was tailored to enhance the bi-phase (gaseliquid) flow dynamics. GDL

Accepted 4 March 2017

synthesis was performed by immersing carbon paper in a resorcinol-formaldehyde-

Available online xxx

graphene oxide precursor solution, followed by thermal treatment. The end product is a carbon xerogel structure with incorporated graphene layers that counter the additional

Keywords:

overpotential losses due to conductivity and mass-transfer limitations. MPL structure and

Carbon xerogel

porosity were controlled by performing xerogel crosslinking in a centrifugal field at

Graphene

different G-Forces, resulting in a highly conductive material with structural properties

Gas diffusion layer

tailored to the application of interest.

Microporous layer

© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

PEM fuel cell

Introduction With high energy demand looming as one of the key issues of our century, new approaches in energy storage and conversion require a strong initiative focusing on renewable and alternative energy sources [1,2]. This has placed a demand on designing and manufacturing customized materials with enhanced physical properties [3,4]. Porous carbonic nanomaterials are emerging as promising catalyst supports and microporous layers (MPLs) - provided they exhibit high material stability and efficient water and gas management to

reach adequate power densities in proton exchange membrane (PEM) fuel cells (FCs) [5,6]. Such materials (e.g. plasmapyrolyzed carbon black, carbon nanotubes, graphene) are combined with hydrophobic binders to create MPLs at the interface between the catalyst and the macroporous backing layer (e.g. carbon paper, carbon cloth) [7e12]. However, their associated high costs keep FC prices to non-competitive levels. By employing lower cost materials, such as Ketjenblack®, other problems emerge. The erratic structure of such materials results in poor MPL stability that inhibits the electron flow through the network and impedes the bi-phase

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (A. Tiliakos), [email protected] (A. Balan). http://dx.doi.org/10.1016/j.ijhydene.2017.03.016 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Trefilov AMI, et al., Carbon xerogel as gas diffusion layer in PEM fuel cells, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.016

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(gaseliquid) flow dynamics, thus deteriorating the performance of the device [13,14]. After extended operation, carbon thermodynamically destabilizes and the MPL becomes degraded by oxidation, leading to dramatic drops in the mass transport rates through the Membrane Electrode Assembly (MEA) [15]. Solvent elimination leads to shrinkage and eventual cracks; these expand under the influence of fuel starvation and repeated operational cycles, thus accelerating carbon degradation [15e18]. Thus we need to design low-cost carbonic materials with structural, morphological, and electrical properties that satisfy the MPL requirements for robustness, stability, and efficiency. Nanostructured carbons with controllable pore size, specific surface area, and composition, provide for interesting alternatives to carbon powders, as they can be cast directly on the backing substrate [18e20]. Carbon xerogels are synthesized by drying and carbonizing resorcinol-formaldehyde wet gels, their properties tailored by modifying the synthesis parameters [21]. Hydrophobicity can be enhanced by tailoring the pore size distribution within the material: high mesoporous and low microporous volumes are advantageous, as the large pores transport the water away from the catalyst layer, while the small hydrophobic pores remain free for gas distribution [10]. A facile method for reducing the electrical resistivity of a carbonic material is to introduce highly conductive materials (e.g. graphene) in its structure. By strengthening carbon xerogels with graphene sheets, we can create carbonic nanocomposites with high surface area and porosity, improved electrical and thermal conductivity, and better mechanical strength [22]. In our work, we develop a custom gas diffusion layer (GDL) by merging carbon paper (CP) and carbon xerogel (CX): condensation and crosslinking of CX sol particles is performed within the porous network of the CP, resulting in a firm contact between the xerogel-based MPL and the CP support. More flexible support materials, such as carbon cloth, were deemed not appropriate for infusing with xerogels, which present very rigid structures. The starting sol gel precursor of the CX is based on a resorcinol, formaldehyde, and graphene oxide mixture, with graphene oxide serving as the crosslinking catalyst [23e25]. Porosity of the composite xerogel is controlled by performing the crosslinking process in a centrifugal field. Thermal treatment converts the starting resin to xerogel, and reduces the unreacted graphene oxide to graphene sheets impregnated in the xerogel structure. We compare key properties of the prepared GDLs (surface morphology, electrical conductivity, and hydrophobicity), and we employ single-cell tests to measure their performance in membrane assemblies for PEM-FCs.

Experimental Precursor materials, gas diffusion layers, and membrane electrode assemblies Xerogel synthesis was based on a solegel mixture of resorcinol (99%, pellets, SigmaeAldrich), formaldehyde (37wt% in H2O, SigmaeAldrich), and graphene oxide produced according to a modified Hummers Method [26].

Our first reference gas diffusion layer (GDL) consists of a commercial carbon paper substrate (CP, B 2120 Torray Carbon Paper TGPH-120) coated with a microporous layer (MPL, carbon content: 1 mg/cm2) composed of 90%wt. plasmapyrolyzed Carbon Black nanopowder (CB, CB13, BET surface: 570 m2/g, APS: 13 nm, PlasmaChem) and 10%wt. polytetrafluoroethylene (PTFE). We have chosen the minimal loading of 10% PTFE (from an optimal range of 10%e23%) to arrive at an equilibrium between mass transport resistance, FC electrical resistance, and hydrophobicity for both cathode and anode having the same synthesis parameters [27,28]. The carbon slurry was prepared by dispersing CB powder and PTFE resin in isopropanol, and sonicating for 1 h for homogenization; the mixture was uniformly deposited on the CP surface by drop-casting. The coated CP was dried at 150  C for 30 min, and placed in an oven at 100  C for 24 h - this procedure is not related to the classic PTFE sintering method, where higher temperatures (350  C) are employed. PTFE sintering decreases the MPL specific area and inhibits the uniform distribution of fuel to the catalyst - in this lower-temperature procedure, PTFE coating is used primarily to render the MPL hydrophobic. This reference GDL is addressed as GDL-CB in the text. The same procedure was used to produce two more reference GDLs, where the MPLs consisted of: i) Ketjenblack® (KjB, EC-600JD, BET surface: 1400 m2/g, AkzoNobel), addressed as GDL-KjB; and ii) graphene, addressed as GDL-Graph. The graphene MPL was prepared by graphite exfoliation in supercritical conditions [29] with the following properties: 8 nm average flake thickness (20e30 monolayers), 550 nm average flake size (150e3000 nm), and 350 m2/g average BET surface area (100e600 m2/g). The custom GDLs containing carbon xerogel (CX) were synthesized by immersing the CP in a 65%wt. aqueous solution of resorcinol and formaldehyde (R:F ¼ 1:2) in the presence of graphene oxide (1% of R) as the acidic catalyst (precursor solution pH ¼ 2.5) [23e25]. Condensation and crosslinking of sol particles within the CP pores were performed with and without centrifugation at different G-forces: 0G (no centrifugation), 125G, and 250G. The gels were placed in sealed recipients at 50  C for three days to complete the gelation process, and were dried for five days in ambient atmosphere at a constant temperature. The resulting xerogel-infused CPs were pyrolyzed under a nitrogen atmosphere at 850  C for 9 h. The centrifuged GDLs presented microporosity on the side facing the centrifuge exterior (mainly composed of CX), and macroporosity on the side facing the centrifuge interior (mainly composed of CP). The same MPL deposition procedure was used as in the reference GDLs, except that the support materials were coated with 0.1 mg/cm2 CB instead. The obtained tri-layer (CP-CX-CB) GDLs will be referred as GDL-nG in the text, where n is the measure of the applied centrifugal force. For all membrane electrode assemblies (MEAs), the catalyst layers were fabricated by drop-casting a Pt/C (Alfa Aesar, 60% Pt) and 5% Nafion solution (Dupont®) on the GDLs; the catalyst loadings were 0.3 mg/cm2 for the anodes and 0.6 mg/cm2 for the cathodes. MEAs were constructed by hot-pressing the GDL-catalyst plates against proton exchange membranes (PEM, Fumapem F-1050, Quintech) at 133  C and at a pressure of 50 kg/cm2 for 15 min.

Please cite this article in press as: Trefilov AMI, et al., Carbon xerogel as gas diffusion layer in PEM fuel cells, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.016

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Characterization methods Thermogravimetric Analysis (TGA) measurements were performed on a TGA/SDTA Mettler Toledo operating in the range between 25  C and 1000  C at 10  C/min under an inert Argon atmosphere. Raman spectroscopy was performed on a NRS3100 JASCO using a red laser at 532 nm. X-ray Photoelectron Spectroscopy (XPS) of the GDL principal elements were conducted on an Omicron Multiprobe XPS system with a base pressure of 2∙1010 mbar; high resolution Carbon 1s (C1s) and Oxygen 1s (O1s) core level spectra were recorded at normal emission take-off angles, using an energy step of 0.05 eV and pass energy of 14 eV, giving an overall energy resolution of 0.48 eV, as measured at the Fermi edge of a silver reference sample. Scanning Electron Microscopy (SEM) was performed on a HR-SEM, Zeiss Gemini Ultra-55. Bulk densities, defined as the mass of the material divided by the total volume it occupies, were determined by weighting a number of rectangular or cylindrical samples of known dimensions. BrunauereEmmetteTeller (BET) surface area and porosity measurements were performed by N2 adsorptionedesorption

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isotherms at 77 K with Micrometrics physisorption determination. Static contact angles were measured according to the sessile drop method to determine the hydrophobicity of the GDL. Resistivity was measured on a Voltalab 40 PGZ 301, Analytical Radiometer SAS universal potentiostat. Single-cell tests were carried out using an Agilent 6060B system at 80  C, 80% relative humidity, and 150 kPa pressure; flow rates were fixed at 100 sccm for hydrogen and 500 sccm for air.

Results and discussion Xerogel characterization To gain insight on the properties of the composite GDLs, we initially characterized only the carbon xerogels (at the same centrifugal speeds as in GDL preparation) using Thermogravimetric Analysis (TGA), Raman Spectrometry, and X-ray Photoelectron Spectroscopy (XPS). TGA measurements were used to monitor the xerogel mass loss as a function of temperature: mass loss increased with

Fig. 1 e Characterization of carbon xerogels synthesized at different centrifugation settings (0G, 125G, 250G): a) Thermogravimetric analysis, indicating the higher thermal stability of centrifuged xerogels and the possible presence of oxygen functionalities; b) Raman spectra focusing on the D and G peaks, showing declining D to G intensity ratios over centrifugation speed; c) XPS spectra focusing on C1s peaks, marking the primary components of HOPG at 284.5 eV, C1s at 285.0 eV, and carbonate impurities at 289.4 eV; and d) XPS spectra focusing on O1s peaks, marking the primary components of CeOeC at 531.8 eV, CeOH at 533.4 eV, and chemisorbed water at 534.2 eV. Please cite this article in press as: Trefilov AMI, et al., Carbon xerogel as gas diffusion layer in PEM fuel cells, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.016

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Fig. 2 e SEM images of composite GDL/MPL surfaces: a) GDL-Graph, bearing the graphene MPL, presenting a layered surface riddled with graphene flakes; b) GDL-0G, presenting carbon paper fibers covered by xerogel particles; c) GDL-125G, with diminishing particle coverage; and d) GDL-250G, with xerogel particles migrating and revealing greater surfaces of the carbon paper fibers. Scale bar at 5 mm. temperature, while it decreased with centrifugation speed for the same level of temperature (Fig. 1a). TGA curves for all samples reveal no mass loss up to the temperature of 150  C, indicating that the thermal treatment effectively removes all traces of water. The TGA curves notably divert above 220  C, indicating differences in the chemical composition of xerogels under different centrifugal speeds: these can be attributed to oxygen formed during centrifugation. At the high temperatures of 800  C, TGA curve divergence becomes more pronounced, with uncentrifuged xerogels dropping to 32% of their original mass, while xerogels centrifuged over high speeds (250G) reach a plateau of 65% starting at 650  C, indicating better thermal stability. Raman spectra for centrifuged and uncentrifuged xerogels were typical to carbonic aerogels and xerogels (Fig. 1b), showing the D band (1350 cm1) corresponding to sp3-hybridized carbons from amorphous disordered structures, and the G band (1580 cm1) corresponding to sp2-hybridized

carbons derived from the resorcinol aromatic nuclei. The D band, attributed to the A1g mode, does not appear in perfect graphite crystals and is associated with the disorder-induced scattering produced by imperfections or loss of hexagonal symmetry in the carbon structure - the G band, assigned to the Raman active 2E2g mode in 2D network structures, is always observed in carbonic and graphitic materials [30]. The D band presented a lower intensity value and a broader FWHM than the G band; their intensity ratio, R ¼ ID/IG, remained close to unity, with lower R values for higher centrifugation forces, indicating the relative decline of structural imperfections. This effect of lowering the R ratio over increased centrifugation speeds can be regarded as a possible remedy to the observed increase of R values over higher treatment

Table 2 e Tested GDL characteristics: thickness w, static contact angle q, and electrical resistivity r. Material

Table 1 e Xerogel-based composite GDL characteristics: water uptake WA, bulk density rb, specific BET surface SBET, and porosity F. Material

WA [%]

rb [g/cm3]

SBET [m2/g]

F [%]

GDL-0G GDL-125G GDL-250G

89.2 83.5 75.2

0.52 0.59 0.68

522.12 476.08 472.33

88.4 80.9 71.1

Carbon paper (CP) GDL-CB GDL-KjB GDL-Graph GDL-0G GDL-125G GDL-250G

Thickness w [mm]

Contact angle q [ ]

Resistivity r [U$cm]

97 174 168 155 125 117 111

101.1 166.2 158.6 145.3 145.3 149.9 153.7

0.85 0.45 0.79 0.74 0.49 0.67 0.82

Please cite this article in press as: Trefilov AMI, et al., Carbon xerogel as gas diffusion layer in PEM fuel cells, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.016

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temperatures [24]. Increased R values can also signify xerogel particle shrinkage due to thermal treatment, which induces the loss of microporosity [24]; this indicates that centrifugation has the added effect of partially preserving the fine porous structure of the material at high temperatures. The low peak at 1100 cm1, attributed to mixed CeC and C]C bonds with sp2-sp3 hybridization, was present in all three cases. XPS experiments were carried out to obtain information on the surface structure of the xerogels. XPS offered interesting observations for the Carbon C1s spectra (282e292 eV), which can be deconvoluted into: a peak centered at 284.5 eV pertaining to highly ordered pyrolytic graphite (HOPG), and a pure Carbon 1s peak centered at 285.0 eV (Fig. 1c). The FWHM of the C1s peak decreases with the degree of graphitization [31], with

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the narrower peak collected from the uncentrifuged xerogels signifying a higher graphitic content e the peak's shift towards 284.5 eV also denotes a more ordered graphitic structure. The peaks for the centrifuged xerogels are instead clearly centered at 285.0 eV with broader FWHM, indicating the decline of graphitic ordered structures under high centrifugation speeds. However, in all cases the graphitic content must be relatively low, as revealed by the virtual absence of the shake-up satellite at 291.2 eV associated with increasing graphitization [32]. The broad shoulders above 285.0 eV can be attributed to the presence of oxygen functional groups (C]O, COOR) [33], while the low peaks at 289.4 eV are related to carbonate-forming impurities. The Oxygen O1s XPS spectra (528e540 eV) contain more detailed information about the oxygen functional groups

Fig. 3 e PEM-FC single-cell polarization and power density profiles for the reference MEAs: a) MEA-CB, b) MEA-KjB, c) MEAGraph; and the xerogel-composite MEAs: d) MEA-0G, e) MEA-125G, and f) MEA-250G. The highly-centrifuged MEA-250G is comparable in performance to the optimal MEA-CB reference. Values were recorded at 80  C, 80% relative humidity, 150 kPa pressure, 100 sccm H2 flow and 500 sccm air flow. Please cite this article in press as: Trefilov AMI, et al., Carbon xerogel as gas diffusion layer in PEM fuel cells, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.016

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(Fig. 1d). The major components in the O1s spectra stem from contributions of the following groups: C]O at 530.1 eV, CeOe C at 531.8 eV, and CeOH at 533.4 eV [31], with the peak centers for all samples indicating a predominant CeOH component. The shoulder at 534.2 eV can be attributed to chemisorbed water [34]. In general, we can observe that the level of oxygen functionalization increases with centrifugation speed.

GDL characterization SEM imaging was employed to study the surface morphology of composite GDLs. The graphene microporous layer (MPL) consisted of large exfoliated flakes; xerogel-infused gas diffusion layers (GDLs) presented interconnected particle structures deposited on the carbon paper (CP) fibers, resulting in highly porous surfaces (Fig. 2). Increased centrifugal forces impose a migration effect on xerogel particles towards the exterior of the composite structure, gradually uncovering the underlying CP. Xerogel particles migrate and compact on the exterior side of the structure, resulting in higher densities but lower water uptake, specific BET surfaces, and porosities relative to the centrifugation speed (Table 1). Structures composed of hydrophobic channels of gradual porosity (from micropores to macropores) can easily divert the water from the catalyst layer without blocking the pores, and thus without impeding gas flow. Even though high hydrophobicity is favorable for water removal, it presents the risk of water accumulation at the interface between the catalyst and the GDL, increasing the transfer resistance of gas transportation through the channels, especially in the cathodic electrolyte/electrode interface [10,15,35]. GDL hydrophobicity was determined by measuring the static contact angle q (Table 2). Controlling xerogel density under increased centrifugal forces allows for moderate increases of hydrophobicity, albeit not reaching the higher contact angles of the classic Carbon Black (CB) and Ketjenblack® (KjB) materials. Resistivity for CP-CX-CB composite GDLs is likewise affected by centrifugation (Table 2). Non-centrifuged composites presented resistivity as low as the reference GDL-CB. At higher centrifugal forces, resistivity values rise to the same levels as the KjB and the graphene-coated GDLs.

Integrated FC testing PEM-FC single-cell polarization and power density curves presented open circuit voltages (OCV) with increased variability in registered values, ranging from 530 mV to 800 mV (Fig. 3). These represent activation voltage drops that limit the OCV below the theoretical value, and can be associated with low exchange current densities, as described by the Tafel equation: DVact ¼ A lnði=i0 Þ where A is a constant, i is the current density, and i0 is the exchange current density, defined as the current density in equilibrium (when the overpotential is zero by definition). All else considered equal, low exchange current densities leading to large activation losses in tested MEAs can be attributed to insufficient electrode activation, pertaining to electrode

surface roughness, or low reactant pressures/concentrations, pertaining to catalyst site occupancy [36]. Power density performances increased gradually with centrifugation speed, from 68.4 mW/cm2 for MEA-0G (Fig. 3d), to 111.0 mW/cm2 for MEA-125G (Fig. 3e), to 135.6 mW/cm2 for MEA-250G (Fig. 3f). This performance variation with centrifugation speed can be seen as a composite effect of material density, resistivity, and porosity: xerogel-based MPLs with higher porosity and low density (MEA-0G) favor water accumulation in their sponge-like porous network, thus impeding gas transportation to the catalyst, while MPLs with lower porosity (MEA-250G) favor the gas/water management and enhance the overall fuel cell specific parameters. Overall, the power performance of the xerogel-based high-centrifugation MEA-250G was higher than both the reference MEA-KjB with 123.2 mW/cm2 (Fig. 3b) and MEA-Graph with 125.0 mW/cm2 (Fig. 3c), which register as low-cost, but did not reach the high power density of the expensive MEA-CB with 190.0 mW/cm2 (Fig. 3a).

Conclusions Investigations of low-cost carbonic nanomaterials were carried out to replace the expensive plasma-pyrolyzed Carbon Black microporous layer. We prepared carbonic materials with tailored structural, morphological, and electrical properties, to address the key GDL parameters: stability, electrical conductivity, hydrophobicity, thickness, and porosity. We compared the performances of different GDLs bearing: industrial Carbon Black, Ketjenblack®, graphene, and composite carbon xerogels. We created a tri-layer GDL with gradually decreasing porosity through its profile (from carbon paper macroporosity, to xerogel mesoporosity, to MPL microporosity), to enhance the water-gas management during FC operation. Electrical resistivity was lowered due to the addition of graphene, which augmented the conductive pathways inside the carbon xerogel structure. Stability was addressed by casting the xerogels directly on the carbon paper, thus preventing MPL detachment due to cracks formed during its deposition, and enhancing the contact between tri-layers. MPL resistance shedding was considerably reduced, as the xerogel cross-linking process was performed within the pores of the carbon paper, producing a firm contact between the carbon xerogel and the support. Porosity and density were adjusted by manipulating the centrifugation parameters of the synthesis method. At the same cost level, the performance of xerogel-based GDLs of high centrifugation (MEA-250G) were higher than both the Ketjenblack®-based (MEA-KjB) and graphene-based (MEA-Graph) GDLs. However, they did not reach the high power density of the expensive reference Carbon Black MEACB, albeit they displayed comparable optimum potential at the maximum power density. This marks the xerogel-based centrifuged GDLs as alternatives offering reasonable performance at a less demanding power output for a lower cost. Future research needs to address activation losses in our xerogel-composite MEAs to improve their performance, rendering this alternative material as truly comparable to higher cost and higher efficiency options.

Please cite this article in press as: Trefilov AMI, et al., Carbon xerogel as gas diffusion layer in PEM fuel cells, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.016

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Acknowledgements The authors were supported by grants CNCS/CCCDI-UEFISCDI PCCA-24/2014, 46/2014, and 142/2014; and PN-III-P2-2.1-PED2016-0630 73/2017 of the Romanian National Authority for Scientific Research.

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Please cite this article in press as: Trefilov AMI, et al., Carbon xerogel as gas diffusion layer in PEM fuel cells, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.016