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Characterizations of carbonized electrospun mats as diffusion layers for direct methanol fuel cells
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Characterizations of carbonized electrospun mats as diffusion layers for direct methanol fuel cells Xiangyang Zhang, Yuxin Huang, Xuelong Zhou, Fang Wang, Zhongkuan Luo, Qixing Wu * Shenzhen Key Laboratory of New Lithium-ion Batteries and Mesoporous Materials, College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen, 518060, Guangdong, PR China
H I G H L I G H T S
� Application of electrospun mats as the diffusion layers of DMFCs is evaluated. � Morphology, pore size, permeability, resistivity and DMFC performance are studied. � A higher PAN content leads to a higher permeability, conductivity and performance. � Performance variation is low for anode electrospun mats but high for cathode ones. � Though resistivity is high, PAN/SiO2 mat enhances performance by water management. A R T I C L E I N F O
A B S T R A C T
Keywords: Diffusion layer Fuel cell Electrospun mats Polyacrylonitrile Carbonization
This work attempts to evaluate the application of carbonized electrospun mats as the diffusion layers (DL) for direct methanol fuel cells (DMFC). A series of in-house fabricated polyacrylonitrile (PAN) based fibrous mats are systematically investigated in terms of physical morphology, pore size distribution, wettability, permeability, resistivity and DMFC performance. It is found that a higher PAN concentration leads to a larger fiber diameter, mean pore size, permeability and a lower through-plane areal resistance, which contribute to a higher DMFC performance due to enhancement in reactant and electron transport. To create additional pores in the fibrous mat, polylactic acid (PLA) or silicon dioxide (SiO2) are used as the pore formers and results show that the PAN/ SiO2 mat is more effective on improving the DMFC performance than the PAN/PLA mat. The employment of the carbonized electrospun mats as the anode DL results in a relatively small variation in performance as compared with those as the cathode DL, suggesting the oxygen transport issue is more severe than that of methanol. The present work is anticipated to help gain understanding and guide further optimization of the carbonized elec trospun mats for fuel cells and other related application fields, including flow batteries, lithium/sodium-ion batteries and supercapacitors.
1. Introduction Direct methanol fuel cells (DMFC), capable of converting chemical energy of liquid methanol into electrical energy, have been considered as one of next-generation power sources for consumer electronic devices due to their favorable advantages of large theoretical energy density (~4800 Wh L 1), easy fuel maintaining and simple system design [1–4]. One of the most important components in the DMFC is the diffusion layer (DL), which plays a key role in controlling the performance of the DMFC. Typically, the functions of the DL are multiple: i) distributing reactants (i.e., methanol and oxygen) into the catalyst layer to enhance
the electrochemical reactions; ii) facilitating liquid water removal to mitigate or avoid water flooding; iii) providing electron and heat con duction pathway and mechanical support for the electrode. Historically, inherited from proton exchange fuel cells (PEMFC), carbon paper/carbon cloth, fabricated from carbon fibers with a high degree of graphitization, have been widely selected as the DLs for DMFCs to simultaneously satisfy all these three requirements. Over the past several decades, significant efforts have been devoted into ex-situ charactering the physical and chemical features of the commercial DLs [5–9], in-situ investigating their structural and surface wettability ef fects on the complex gas-liquid two phase flow behavior [10–14],
* Corresponding author. E-mail address: [email protected] (Q. Wu). https://doi.org/10.1016/j.jpowsour.2019.227410 Received 5 June 2019; Received in revised form 14 October 2019; Accepted 4 November 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Xiangyang Zhang, Journal of Power Sources, https://doi.org/10.1016/j.jpowsour.2019.227410
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numerical modeling to gain insights of the intrinsic link between structure and performance macroscopically [15–19] and microscopi cally [20–23], and optimizing their morphologies to improve the fuel cell performance [24–26]. Although the performance of the commercial carbon paper or carbon cloth is excellent in terms of electronic con ductivity, gas permeability, mechanical strength and chemical stability, their manufacturing processes are relatively complicated involving multiple carbonization and graphitization stages [27]. In addition, the main structural parameters of the commercial DLs, including fiber diameter and orientation as well as pore size and its distribution, are usually difficult to be modified at a large extent, which may limit a comprehensive understanding on structural effects and the room for design optimization. To remove the constraint of commercial DLs, various novel customdesigned DLs with well controllable structural features have been pro posed and investigated for potential application in PEMFCs and DMFCs [28–36]. To improve the conductivity and increase the pores for reac tant transport, Gao et al. [28] fabricated the carbon nanotube based DL for the DMFC. Yazici [29] proposed to use expended graphite with custom perforation as the DL for the DMFC and systematically studied the open ratio effect. In addition to carbon-based DLs, metal mesh made from stainless steel [30], titanium [31,32], nickel [33] and nickel-chromium alloy [34] have been studied extensively due to their well-connective pore structure for reactant supply, high conductivity for electron conduction and excellent stability for long-term operation. To enhance the through-plane transport, microfabrication techniques have also been introduced to process the thin metal film to form a series of straight pores with controllable pore size and open ratio [35,36]. Another type of DLs received increasing attentions recently is the carbonized electrospun mats synthesized by electrospinning poly acrylonitrile (PAN) solution with certain concentration and subsequent treatment at a relatively low temperature (<1200 � C) [37–42]. As many operating parameters such as composition of the precursor solution, tip-collector distance, rotating rate and collector shape are easy to pre cisely control, the final morphology of the carbonized nonwoven mat can be well tuned at a large degree to meet the specific purposes for DL related studies. Duan et al. [37] applied the carbonized electrospun mats as the micro-porous layers for PEMFCs, an improved performance was achieved due to a higher gas permeability of the electrospun mats as compared to that of conventional microporous layer with carbon par ticles and hydrophobic polytetrafluoroethylene (PTFE). Todd and M�erida [39] performed preliminary characterizations on in-house fabricated carbonized electrospun mats regarding to their electrical re sistances and wettabilities, and identified the effects of fiber alignment on in-plane mass transfer and fuel cell performance [40]. More recently, Chevalier et al. [41,42] presented an in-depth study on the structural effects of carbonized electrospun mats through polarization analysis and synchrotron X-ray radiography, and found that fiber diameter and alignment as well as pore size significantly influence the water transport and thus the cell performance. In addition to being applied in fuel cells, carbonized electrospun mats have also been considered as potential free-standing electrode materials in other electrochemical energy de vices such as all vanadium flow batteries [43–45], lithium-ion or sodium-ion batteries [46–48] and supercapacitors [49–51]. As carbonized electrospun mats are versatile in a wide range of fields, there is a need for systematically characterizing their main properties to understand the structure-performance relationship and to guide further optimization of their psychical and chemical characteristics. This work attempts to evaluate the application of carbonized electrospun mats as the DLs for DMFCs. A series of in-house fabricated polyacrylonitrile (PAN) based carbon nanofibrous mats are systematically investigated in terms of physical morphology, pore size, wettability, permeability, re sistivity and fuel cell performance. In addition, polylactic acid (PLA) or SiO2 are used as the soft and hard pore formers, respectively, to create additional pores in the fibrous mat for the sake of comparative study. The results of the present work might be helpful for gaining insights of
the carbonized electrospun mats and the experimentally determined key parameters could be readily applied in numerical modeling for future structural optimization. 2. Experimental 2.1. Synthesis of carbonized electrospun mats 2.1.1. Carbonized PAN mats As illustrated in Fig. 1, the carbonized PAN mats were prepared though the well-developed electrospinning method and the setup was similar to our previous work [45]. Various amount of PAN (Sigma-Al drich, MW ¼ 150,000) was dissolved in N, N-dimethylformamide (DMF) (Aladdin, anhydrous, 99.8%) to form a precursor solution with a variety of PAN concentrations ranging from 3 to 14 wt %. The needle to col lector distance, flow rate and the rotational speed of the drum were controlled to be 12 cm, 2 mL h 1 and 800 rpm. The obtained electrospun PAN mats were then stabilized in the air at 260 � C for 2 h by a ramping rate of 1 � C min 1, followed by heat treatment in argon atmosphere for 1 h at a ramping rate of 5 � C min 1. Various carbonization temperatures from 700 to 1100 � C were employed to evaluate the temperature effect on the characteristics of carbonized PAN mats. Note that except for studying the effect of carbonization temperatures on the 14 wt % PAN sample, other samples were all carbonized at 900 � C. 2.1.2. Carbonized PAN/PLA and PAN/SiO2 mats To create additional pores in the nanofibrous mats, polylactic acid (PLA, MW ¼ 60,000) or SiO2 (Aladdin) were introduced into the elec trospinning precursor solution and used as the soft and hard pore for mers, respectively, as illustrated in Fig. 1. The PAN/PLA precursor solution was formed by mixing 9 wt % PAN solution and 5 wt % PLA solution with different ratios of 9:1, 8:2, 7:3 and 6:4, which resulted in 0.5 wt %, 1 wt %, 1.5 wt % and 2 wt % PLA contents based on the total weight of PAN/PLA precursor solution, while the corresponding PAN contents were 8.1 wt %, 7.2 wt %, 6.3 wt % and 5.4 wt %, respectively. For the PAN/SiO2 precursor solution, it was prepared by introducing various amount of SiO2 nanoparticles into 9 wt % PAN solution, which resulted in SiO2 contents of 2 wt %, 3 wt %, 4 wt % and 5 wt % and PAN contents of 8.8 wt %, 8.7 wt %, 8.6 wt% and 8.5 wt % based on total weight of PAN/SiO2 solution, respectively. After electrospinning, the PAN/PLA mats were firstly treated by chloroform to remove the PLA, followed by the stabilization at 260 � C under air for 2 h and carboniza tion at 900 � C under argon for 1 h. On the contrary, the electrospun PAN/SiO2 mats were firstly stabilized at 260 � C under air for 2 h and carbonized at 900 � C under argon for 1 h, and then the resulting carbonized samples were treated by HF to remove SiO2 for pore forming. 2.2. Materials characterizations The physical morphology of the carbonized electrospun mats were evaluated by a field emission scanning electron microscope (SEM, JEOL6700F) and a transmission electron microscope (TEM, JEM-2100). The degrees of graphitization of the samples were analyzed by X-ray diffraction (XRD, D8 Advance) and Raman Spectroscopy (inVia). The pore size and its distribution of the samples were characterized by mercury intrusion porosimetry (Auto Pore IV 9500). Water contact an gles were obtained by the sessile drop experiment. The though-plane permeabilities of the carbonized electrospun mats were measured according to the Gurley method [38] by a Gurley densometer (Model 4110 N) equipped with an automated digital timer (Model 4320). To conduct the measurement, an inner cylinder of 567 g was left to slide freely in the vertical outer cylinder so as to create an air pressure difference (Δp, 1.22 kPa). By recording the time (t) for a given volume of air (V, 300 mL) to flow through a circular area (A, 64.52 mm2) of the sample, the permeability (K) can be estimated by Darcy’s law with given thickness (L) of the sample and viscosity of air (μ): 2
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Fig. 1. Schematic of synthesis process for various carbonized electrospun mats.
K¼
V μL tAΔp
2.3. Preparation of membrane electrode assembly (MEA)
(1)
For the sake of comparison, the thicknesses of all the tested DL samples in this work were controlled to be around 200 μm, nearly the same as the Toray 060 carbon paper. When evaluating the performance of a carbonized mat as the anode DL, the anode catalyst layer was inhouse fabricated by the decal method [53,54], while the cathode elec trode (consisting of a catalyst layer and a diffusion layer) was a com mercial one from Johnson Matthey (#45375, 2 mgPt cm 2). On the contrary, when investigating a carbonized mat as the cathode DL, the cathode catalyst layer was in-house fabricated and the anode was a commercial one from Johnson Matthey (#45374, 2.7 mgPt cm 2 and 1.35 mgRu cm 2). The decal method included ultrasonic spraying the catalyst ink onto a PTFE blank sheet and transferring the catalyst layer from the PTFE sheet onto the membrane during hot pressing [53,54]. The purpose of using decal method was to ensure various DLs share the same anode or cathode catalyst layer. Pt–Ru/C (50% Pt, 25% Ru, Hispec 12100, Johnson Matthey) and Pt (60% Pt, Hispec 9100, Johnson Mat they) were used as catalysts and the metal loadings in the decal catalyst layers were 4 mg cm 2 and 2 mg cm 2 for the anode and cathode, respectively. The Nafion content was maintained to be about 20 wt %. A 50 μm thick Nafion 212 membrane was used to conduct proton and was sandwiched between anode and cathode electrode by hot pressing at 135 � C and 4.0 MPa for 3 min. The nominal active area of the MEA was 5.0 cm2.
In this work, the measured permeabilities of samples are also compared with those calculated by Carman-Kozeny equation for fibrous mats: K¼
d2f ε3 16kck ð1
εÞ2
(2)
where df is the fiber diameter, ε is the porosity of the sample and kck is a coefficient accounting for the fiber structure and alignment. As sug gested by Tomadakis et al. [52], for fibrous mats consisting of randomly aligned fibers, a value of 4 for kck can be used for estimation of K. Detailed calculation of K are shown in Table S1 and Table S2 in the Supplementary data. The through-plane areal resistances of the carbonized electrospun mats were measured through an in-house setup shown in Fig. 2. It consisted of a piston for supplying and controlling the contact pressure, two PTFE supporting plates and two gold-coated copper plates for clamping the samples [8,39]. With such a setup and a micro-ohm meter (Keysight 34420A), through-plane areal resistances (including the con tact resistances at the interfaces of the sample and copper plate) can be studied under various clamping pressures. In addition to through-plane, the in-plane resistivities of carbonized electrospun mats were also characterized at room temperature through a four-probe resistivity meter (RTS-9).
2.4. Measurement of methanol crossover rate and fuel efficiency To determine the fuel efficiency, the methanol crossover rate under open circuit condition was measured by voltammetric method originally proposed by Ren et al. [2]. During this measurement, the DMFC anode was fed with 2 M methanol solution at a flow rate of 2.5 mL min 1 while the cathode was fed by nitrogen (humidified at 60 � C) at a flow rate of 200 sccm to create an inert environment and to supply water for the methanol oxidation reaction on the cathode. Then, the cell was charged at 0.85 V to ensure the oxidation of permeated methanol from the anode and the charge current, known as limiting methanol crossover current density [2], ic,lim, was recorded until the current was stable. Note that to obtain the actual methanol crossover current density under open circuit, ic,oc, the ic,lim should be corrected by a factor of (1 þ 56.38% [2]) to account for the electro-osmosis of methanol from cathode to anode at 2 M methanol concentration. By obtaining the anode limiting mass transport current density, ia,lim, through polarization tests, the methanol-crossover current density, ic, under various operating current
Fig. 2. Schematic of the in-house setup for through-plane electrical areal resistance measurement. 3
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density, i, can be approximated by Ref. [63]: � � i ic ¼ ic;oc 1 ia;lim
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anode as the counter and reference electrodes while the DMFC cathode as the working electrode, respectively. The measurement was performed under potentiostatic mode at 0.4 V with a perturbation amplitude of 10 mV and the impedance data were recorded from 10 2 to 104 Hz with 8 steps per decade. All the polarization tests were acquired by a DMFC testing system (Arbin BT-5HC). During polarization measurement, the DMFC was first kept at open circuit for 15 min, followed by stepwise discharging at a series of preset currents for 1 min. To investigate the short-term stability of carbonized mats, the constant-current discharge tests were performed for 10 h at the current densities of 50 mA cm 2 and 200 mA cm 2 by using Toray 060 carbon paper (without hydrophobic treatment), 14 wt % PAN mat, PAN/PLA (0.5 wt % PLA) mat and PAN/SiO2 (5 wt % SiO2) mat as the cathode DLs. For all the polarization and short-term stability tests, the anode and cathode of the DMFC were actively supplied by 2 M methanol solution at a flow rate of 2.5 mL min 1 and dry air at 200 sccm without back pressure, respec tively. The operating temperature of the cell was maintained at 60 � C.
(3)
Neglecting the evaporative loss of methanol, the fuel efficiencyηfuel , can be determined as:
ηfuel ¼
i i þ ic
(4)
The measured ic,lim at 2 M methanol and ia,lim at 0.5 M methanol with various anode DLs were shown in Fig. S5 and summarized in Table S3 in the Supplementary data. 2.5. Single DMFC test The prepared MEA was assembled in an in-house DMFC fixture re ported in previous work [55]. It consisted of two graphite blocks (POCO) for distributing reactants, two gold-coated current collectors for con ducting electron and two aluminum end plates for mechanical support. Single-pass serpentine flow field with a channel depth of 1 mm and width of 0.6 mm was grooved in the graphite block. The electrochemical impedance spectra (EIS) of the single-cell DMFC were attained by the Zahner Zennium workstation in two-electrode mode with the DMFC
3. Results and discussion 3.1. Physical morphology The physical morphologies of the carbonized PAN mats with various
Fig. 3. SEM images of carbonized electrospun mats with various PAN concentrations (3–14 wt %) and with pore-forming agents of PLA (from 0.5 to 2 wt %) and SiO2 (from 2 to 5 wt %). 4
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precursor concentrations are shown in Fig. 3. The mean fiber diameter of the carbonized PAN mat increases with an increase of PAN concentra tion in the electrospinning solution; with increasing PAN concentration from 3 wt % to 14 wt %, the mean diameter increases from about 60 nm to 1100 nm (Fig. 4a). This phenomenon is in agreement with previous works [39,41,43] and could be explained by the increased viscosity with high PAN concentrations, which favors entangling of polymer chains to form larger fibers [56]. In addition, it is noted in Fig. 4a that the pre cursor fiber undergoes a decrease in diameter due to loss of mass after carbonization; the reduction in the diameter can reach 50% for high 14 wt % PAN precursor mat and can be further decreased at higher carbonization temperatures (Fig. 4b). In addition to fiber diameter, the pore size is also observed to increase with PAN concentrations. Princi pally, the pore size depends highly on the packing density of fibers: for randomly packed fibers, a higher packing density is expected to increase the probability of fiber intersecting to shorten the average distance be tween intersections, which defines the boundary of a pore. If constant volume fraction of PAN is assumed, a larger fiber diameter (high PAN concentration) will lead to a shorter total length of PAN fiber at given area (or volume), which translates to a lower packing density and thus a larger pore size [57]. It worth mentioning that the assumption of con stant volume fraction of PAN is roughly valid considering that the porosity of randomly orientated PAN mats is typically around 90% [39, 41,43] and the change in pore size is significant (Figs. 3 and 5) as more than one order increase in fiber diameter is achieved when increasing PAN concentration from 3 wt % to 14 wt %. The morphologies of PAN/PLA and PAN/SiO2 mats are shown in Fig. 3. In contrast to PAN mats, some surface pores are created on the fibers by removing pore formers of PLA or SiO2, respectively. The
nanopores are only sparsely distributed on the surface of PAN/PLA fiber even at high PLA contents, whereas the distribution of surface pores on PAN/SiO2 fiber tends to become uniform with increasing SiO2 contents, rendering a hollow fiber structure as shown in Fig. S1. More impor tantly, the surface pore volume of PAN/PLA fiber is substantially lower than that of PAN/SiO2 fiber especially for higher SiO2 contents, sug gesting PAN/SiO2 mats may be more effective in creating additional transport pathway for reactant transfer. Similar to PAN mats, the di ameters of PAN/PLA and PAN/SiO2 mats also reduce after carbonization as shown in Fig. 4c and d. Additionally, the fiber diameter decreases with increasing PLA content while it somehow increases with SiO2 content. This is because a higher PLA content corresponds to a lower PAN content in the precursor solution (increasing PLA content from 0.5 wt % to 2 wt %, resulting in a decreased PAN content from 8.1 wt % to 5.4 wt %), which contributes to a smaller fiber diameter as mentioned previously; for the PAN/SiO2 mat, the increased fiber diameter is pri marily attributed to the local aggregation of SiO2 solid particles, which is exacerbated by a higher SiO2 content. 3.2. Pore size distribution To evaluate the detailed pore size distribution of carbonized PAN, PAN/PLA and PAN/SiO2 mats, the results of porosimetry tests are shown in Fig. 5. Consistent with the SEM results in Fig. 3, the peak of pore diameter shifts gradually from 200 nm to 8 μm with increasing PAN concentration from 3 wt % to 14 wt % due to increased average distance between intersections for larger fibers. It should be mentioned that the mean pore size of carbonized 14 wt % PAN mat is still lower than that of the commercial Toray 060 carbon paper with a mean diameter of 7 μm
Fig. 4. Mean fiber diameters of the carbonized electrospun mats: a) effect of PAN concentration, b) effect of carbonation temperature, c) effect of PLA content and d) effect of SiO2 content. 5
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[27], which shows a peak of pore diameter at around 30 μm. Moreover, Fig. 5b shows that the peak shifts negatively from 850 nm to 560 nm with increasing PLA content from 0.5 wt % to 2 wt %, in good agreement with SEM results in Fig. 3 and can be attributed to the decreased average distance between intersections for smaller fibers at high PLA contents (low PAN content in the precursor). In contrast to PAN/PLA mats, two peaks are observed for PAN/SiO2 mats: a small peak at around ~100 nm and a large and broad peak at 600–700 nm, which possibly correspond to the pores formed by removal of SiO2 particles and fiber intersecting, respectively. Although no obvious peak shift is observed for changing SiO2 contents, the pore volume seems to increase with SiO2 contents probably because a large amount of SiO2 are locally aggregated to expand the pore volume. 3.3. Through-plane permeability The though-plane permeability of the DL is a critically important parameter as it closely links to the mass transfer of reactants (methanol and oxygen), which influences the concentration polarization of the electrochemical reactions especially at high-current region. In this work, the permeability of various carbonized electrospun mats are measured by the standard Gurley method for thin porous paper [38]. Typically, the Gurley second is experimentally measured and it used as an indicator to reflect the air permeability. For ease of comparison, the measured Gurley second is converted to the intrinsic permeability according to Darcy’s law (Eq. (1)) by excluding the influences of pressure gradient, air viscosity and volumetric flow rate. On the other hand, the perme ability of a fibrous mat can be estimated by Carman-Kozeny relation (Eq. (2)) with known mean fiber diameter, porosity and kck (assuming kck ¼ 4 for randomly packed fibers [52]). The detailed calculations can be found in Table S1 and Table S2 in the Supplementary data. Fig. 6a shows that the measured permeability of the carbonized PAN mat increases rapidly with PAN concentrations; the permeability in creases from 4.34 � 10 14 m2 to 1.57 � 10 12 m2, nearly 2 orders in crease, when changing PAN concentrations from 3 wt % to 14 wt %. The measured permeabilities of carbonized PAN mats are in agreement with pervious work by Kok et al. [44]. The increase in the permeability with a higher PAN concentration is qualitatively expected as the pore size is expended with PAN concentrations (Figs. 3 and 5a). For comparison, the permeability of Toray 060 carbon paper is also measured by Gurley method and it found to be 6.09 � 10 12 m2, which is well consistent with previously reported values of 6.15 � 10 12 m2 [8] and 8 � 10 12 m2 [58]. Note that the permeability of 14 wt % PAN mat still lower than that of Toray 060 carbon paper mainly due to relatively small pore size of the PAN mat. In addition, the predicted permeabilities seem to be in excellent agreement with experimentally measured values for porous mats with large fiber diameters (i.e., 11 wt % PAN, 14 wt % PAN and carbon paper), suggesting a good applicability of Carman-Kozeny equation for micro-fibrous mats. As shown in Fig. 6a, when the mean fiber diameter is smaller than 306 nm (corresponding to 9 wt % PAN), a large deviation between measured and predicted permeabilities is observed: measured ones are always higher than the predicted ones. One possible reason for such a deviation might be the breakdown of no-slip assumption for flow through nanopores; under slip flow condition, the drag force on the nanofiber surface becomes smaller as compared with the one under no-slip condition, which increases the flow rate through the nanopores to result in a higher permeability [59,60]. The Knudsen number, the ratio of mean free path of air to mean radius of nanofiber, is estimated to be 0.43 at standard conditions for 9 wt % PAN mats, indi cating the significance of slip flow [59]. Another explanation for this deviation is likely to be the imperfect straightness and inconstant diameter of the real electrospun fiber such that the actual kck is not equal to 4 [60]. The measured permeabilities of carbonized PAN/PLA and PAN/SiO2 mats are shown in Fig. 6b and Table S1. As expected, an increased permeability is observed with a higher SiO2 content due to an
Fig. 5. Mercury intrusion porosimetry results of various carbonized electro spun mats at 900 � C: a) effect of PAN concentrations, b) effect of PLA contents and c) effect of SiO2 contents.
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Fig. 6. Through-plane permeabilities of various carbonized electrospun mats: effects of a) PAN concentration and b) SiO2 and PLA contents.
introduction of additional pores on the fiber surface, consistent with porosimetry results in Fig. 5c. In contrast, an increase in the PLA content lead to a reduction in permeability of PAN/PLA mats. This is because negligibly few pores are created by removal of PLA (Fig. 3) and the pore size shifts to a smaller value with increasing the PLA content, as evident in Fig. 5b, which translates to a lower permeability.
seen in Fig. 7e that increasing the PAN concentration from 3 wt % to 9 wt % results in a decrease in the in-plane resistivity from 0.0094 Ω m to 0.00275 Ω m, while further increasing the PAN concentration to 14 wt % leads to an increase to 0.0067 Ω m. Since the carbonized mat is not an ideal planar structure, the electrons may not only transport though inplane fibers located near the probes of resistivity meter (upper fiber layer) but also through the thickness direction to lower fiber layers. When PAN concentration is low (<9 wt %), the relatively large contact resistance between upper and lower fibers, due to more interfacial contact for smaller fibers, might be a dominant factor that hinders the electron transport, thereby resulting in a lower in-plane resistivity at a higher PAN concentration (less interfacial contact). At high PAN con centrations (>9 wt %), however, the relative importance of contact resistance between fiber layers might become minor as compared with the in-plane conduction pathway formed by the intersection of in-plane fibers and hence a higher resistivity is found for the mats with a larger PAN concentration (less intersection points) [41,43]. Similar to the through-plane areal resistances, the in-plane resistivity decreases significantly with increasing the carbonization temperature (Fig. 7e) due to a higher degree of graphitization, whereas it increases with PLA and SiO2 contents (Fig. 7f) as the results of a larger contact resistance for a smaller fiber diameter (higher PLA content) and a more hollow structure of the fiber with a larger SiO2 content, respectively. In addi tion, the measured in-plane resistivity of the Toray 060 carbon paper by the same apparatus is 7.53 � 10 5 Ω m, in agreement with previous re ported values [8,43]. It worth mentioning that although the reported resistivities of carbonized PAN, PAN/PLA and PAN/SiO2 mats are generally higher than that of the commercial carbon paper mainly due to a low-temperature (<1200 � C) heat treatment, they are still well below the resistivity of a fully hydrated Nafion 117 membrane at room tem perature (~0.1 Ω m [2]) such that the electrical resistance may contribute to a relatively trivial impact on the ohmic loss as compared with the ionic resistance.
3.4. Through-plane and in-plane electrical resistances Another key parameter of the DL is the electrical resistance, which directly related to ohmic loss of the DMFC. It is found in Fig. 7 that with an increased clamping pressure, the areal resistances of all the tested samples decrease rapidly at low-pressure region and slightly at highpressure region. This trend is expected as the contact resistance domi nates at low clamping pressures and it decreases significantly when a sufficiently high pressure is applied to ensure an intimate contact be tween the sample and gold-coated copper plates (Fig. 2). In addition, Fig. 7a shows that a higher PAN concentration leads to a lower areal resistance; the areal resistance decreases from 45.04 mΩ cm 2 to 12.07 mΩ cm 2 at 4.71 MPa by increasing PAN concentration from 3 wt % to 14 wt %. This might be because for a given thickness of a fibrous mat, a larger fiber diameter (higher PAN concentration) will result in less interfacial contact between successive fibers in the thickness di rection, giving rise to a smaller total contact resistance for electrons to transfer though the plane direction. Fig. 7b shows the effect of carbon ization temperature on the areal resistance. As expected, the areal resistance increases substantially with carbonization temperature due to a higher degree of crystallinity (XRD results in Fig. S2) and graphitiza tion (Raman spectra in Fig. S2). This result indicates that although areal resistance of the 14 wt % PAN mat carbonized at 900 � C is still higher than that of the carbon paper (Fig. 7a), increasing the carbonization temperature would be an effective approach to enhance the electrical conductivity to reduce ohmic loss. The effects of PLA and SiO2 contents on the though-plane areal re sistances are presented in Fig. 7c and d, which show that increasing the content of PLA or SiO2 leads to a larger resistance. It has been known that a higher PLA content corresponds to a smaller mean fiber diameter, which leads to a higher interfacial resistance across the thickness di rection and thus a larger though-plane areal resistance. For the PAN/ SiO2 mats, a higher SiO2 content can lead to more hollow fibers (Fig. 3). As compared with the smooth and solid fiber, the hollow fibers not only present less conduction pathway for electrons but also probably reduce the contact area between successive fibers in the thickness direction, thereby increasing the areal resistance. In addition to through-plane resistance, the in-plane resistance of the DL also plays an important role in the DMFC as it governs the in-plane uniformity of electric field and thus the electrochemical reaction. It is
3.5. DMFC performance The feasibility of applying carbonized PAN, PAN/PLA and PAN/SiO2 mats as the DLs of DMFCs is investigated in a single-cell DMFC. As all the carbonized mats show contact angles larger than 90� (Fig. S3), the samples are evaluated without an additional hydrophobic treatment and a microporous layer, the effect of which on the DMFC performance would be an interesting topic for future research. For ease of compari son, the commercial Toray 060 carbon paper without a hydrophobic treatment is also tested. Fig. 8a shows the polarization result of the DMFC with various carbonized PAN mats as the anode DLs. With an increase of the PAN concentration from 5 wt % to 14 wt %, the peak power density increases by only 17.2% from 73.1 mW cm 2 to 7
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Fig. 7. Effects of a) PAN concentration, b) carbonization temperature, c) PLA content and d) SiO2 content on through-plane areal resistances of carbonized elec trospun mats; effects of e) PAN concentration and carbonization temperature and f) SiO2 and PLA contents on in-plane resistivities.
85.7 mW cm 2, the latter of which is nearly the same as the carbon paper. Such an improvement is somewhat less than expected as the areal resistance decreases by 67.8% and the permeability increases by 17 times when increasing PAN concentration from 5 wt % to 14 wt %. One possible reason may be that the contribution of electrical resistance to the total cell internal resistance is small as discussed before and its effect on the ohmic loss is unpronounced at relatively low current density (<500 mA cm 2). Another reason can be the use of relatively high methanol concentration (2 M) so that the anode concentration polari zation is not dominated even for the DL with a low permeability [61]. To further elucidate the effects of PAN mats as the anode DLs, EIS results are presented in Fig. S4a. It shows that the ohmic resistances (impedances at high frequencies of ~1000 Hz) of carbon papers are lower than those of
carbonized PAN mats, in good agreement with ex-situ characterizations shown in Fig. 7. In addition, at medium and low frequencies, the cell impedance of DMFC increases with an increase of PAN concentration. This is because a higher PAN concentration leads to a larger perme ability (Fig. 6), which results in a higher methanol concentration in the anode catalyst layer (consistent with measured methanol crossover currents in Fig. S5a and the anode limiting mass transport currents in Fig. S5b). Consequently, methanol oxidation at the anode and oxygen reduction at the cathode (due to methanol crossover) is hindered, accompanied by an enlarged inductive loop caused by adsorbed in termediates [64]. In contrast to the anode, the effect of carbonized mats with various PAN concentrations as the cathode DL is more pronounced: as shown in 8
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Fig. 8. DMFC performances with carbonized electrospun mats: a) carbonized PAN mats as anode DLs, b) as cathode DLs, c) effect of carbonization temperature, d) effect of pore formers; e) short-term stability tests at 50 mA cm 2 and f) at 200 mA cm 2. The inset figure in c) shows the performance of anode PAN mats carbonized at various temperatures.
Fig. 8b, the peak power density improves by 32.5% from 50.1 mW cm 2 to 66.4 mW cm 2 when increasing the PAN concentration from 5 wt % to 14 wt %. The improved performance could be attributed mainly to a larger permeability at a larger PAN content to accelerate oxygen trans fer. This point is further confirmed by the EIS with various cathode DLs shown in Fig. S4b, which shows that a higher PAN concentration leads to a smaller cell impedance at medium and low frequencies. Since the same commercial anode electrode is used, the variation among impedances should be originated from cathode, i.e., oxygen reduction reaction. These results suggest the supply of oxygen on the cathode is more demanding than that of methanol on the anode. Therefore, as shown in Fig. 8b, the performance can be further improved by changing the cathode DL to a commercial carbon paper with a much higher permeability. Fig. 8c shows the polarization curves of 14 wt % PAN mats
carbonized at various temperatures. When being used as the anode DLs, the carbonization temperature reveals a negligible effect on the DMFC performance (the inset figure of Fig. 8c), indicating again the contri bution of electrical resistance play a minor role in the ohmic loss. Regarding to the application of the cathode DLs, a higher carbonization temperature seems to slightly increase the DMFC performance, which can attributed to an enlarged mean pore size for oxygen transport as the result of a reduced fiber diameter at elevated temperatures (Fig. 4b). Since the oxygen transport issue is more severe than that of methanol in the anode and various anode DLs result in a relatively small variation in performance, we have only tested the DMFCs with PAN/PLA and PAN/SiO2 mats as the cathode DLs while keeping the same commercial anode electrodes. The performances of carbonized PAN/PLA (0.5 wt % PLA) and PAN/SiO2 (5 wt % SiO2) mats as the cathode DLs are shown in Fig. 8d. Clearly, the PAN/PLA mat yields the lowest performance among 9
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all the tested samples primarily due to its smallest through-plane permeability as shown in Fig. 6 and Table S1. In addition, it is surpris ing to see that the DMFC with the PAN/SiO2 mat yields not only a nearly the same peak power density as the one with commercial carbon paper but also a higher maximum current density. This is somehow unex pected considering the higher electrical resistance and lower throughplane permeability of the PAN/SiO2 mat. To understand this phenom enon, the stoichiometry of air is calculated. At the maximum current density of 456 mA cm 2 with respective to the PAN/SiO2 mat, the stoi chiometry of air is ~5.3 without considering methanol crossover effect and is estimated to be ~2.9 if considering oxygen consumption by the oxidation of permeated methanol (ic ¼ 367 mA cm 2 at 456 mA cm 2 estimated by Eq. (3), oxygen consumption increases by 80.5%). Hence, the stoichiometry of air might not be high enough to rule out the pos sibility of water flooding especially at high current densities. We therefore speculate that the high performance achieved by using PAN/ SiO2 mat is likely related to the improved water management. Compared to the carbon paper without PTFE coating, the PAN/SiO2 mat shows a highly hydrophobic feature with a contact angle of ~126� (Fig. S3), possibly arising from the micro-roughness of the fiber surface (Fig. 3). As noted by Biesdorf et al. [62], water accumulates in the form of film on a hydrophilic surface, whereas it changes to droplets on a hydrophobic surface to facilitate its removal by the flowing gas due to its increased total surface area. Consequently, the PAN/SiO2 mat shows a higher performance at the large-current region, where the need for oxygen is more demanding. With respective to the short-term stability of the carbonized mats, Fig. 8e and f shows that the transient voltages of all the tested samples drop rapidly at the beginning of tests, followed by slight declines without severe fluctuations. In addition, it is found that cell voltage of PAN/SiO2 mat is slightly lower than that of carbon paper at 50 mA cm 2, whereas the former one shows a much smaller decline rate of voltage and is ~30 mV higher than the latter one at 200 mA cm 2. At the current density of 200 mA cm 2, the stoichiometry of air is ~12.1 without considering methanol crossover but it drops largely to ~3.2 by taking account on methanol crossover effect (ic ¼ 553 mA cm 2 at 200 mA cm 2 estimated by Eq. (3), oxygen consumption increases by 276.5%), indicating a possible occurrence of water flooding even at 200 mA cm 2. Therefore, considering the higher ohmic resistance and lower through-plane permeability of the PAN/SiO2 mat, one possible reason to account for the high performance of PAN/SiO2 mat may be its improved water management arisen from its highly hydrophobic feature. In addition, since all the cells share the same commercial anode electrode, the fuel efficiencies at 50 mA cm 2 and 200 mA cm 2 are estimated to be 7% and 27%, respectively, according to Eq. (4). It should be mentioning that the fuel efficiency can be readily improved by increasing the operating current or using thicker membranes to reduce the fuel loss by methanol crossover.
DMFC performance by introducing additional pores for oxygen transport and enhancing the surface hydrophobicity for water management. The reported values of the key parameters in this work is anticipated to be helpful in future numerical modeling and DMFC structural design, which would in turn gain deeper insights of the carbonized electrospun mats to facilitate its application in other relevant fields such as flow batteries, lithium-ion batteries and supercapacitors. 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. Acknowledgements This work was financially supported by the Shenzhen Science and Technology Fund (JCYJ20170818093905960 and JCYJ20180305125 604361), Natural Science Foundation of Guangdong Province (2018A030313194 and 2018A030310618) and Natural Science Foun dation of SZU (No. 2018039 and 827-000015). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227410. References [1] K. Scott, W.K. Taama, P. Argyropoulos, J. Power Sources 79 (1999) 43–59. [2] X. Ren, T.E. Springer, T.A. Zawodzinski, S. Gottesfeld, J. Electrochem. Soc. 147 (2000) 466–474. [3] F. Liu, G. Lu, C.Y. Wang, J. Electrochem. Soc. 153 (2006) A543–A553. [4] T.S. Zhao, C. Xu, R. Chen, W.W. Yang, Prog. Energy Combust. Sci. 35 (2009) 275–292. [5] M.V. Williams, E. Begg, L. Bonville, H.R. Kunza, J.M. Fenton, J. Electrochem. Soc. 151 (2004) A1173–A1180. [6] J.P. Feser, A.K. Prasad, S.G. Advani, J. Power Sources 162 (2006) 1226–1231. [7] A. Tamayol, M. Bahrami, J. Power Sources 196 (2011) 6356–6361. [8] A. El-kharouf, T.J. Mason, D.J.L. Brett, B.G. Pollet, J. Power Sources 218 (2012) 393–404. [9] G. Xu, J.M. LaManna, J.T. Clement, M.M. Mench, J. Power Sources 256 (2014) 212–219. [10] J. Eller, T. Ros� en, F. Marone, M. Stampanoni, A. Wokaun, F.N. Büchi, J. Electrochem. Soc. 158 (2011) B963–B970. [11] E.E. Kimball, J.B. Benziger, Y.G. Kevrekidis, Fuel Cells 10 (2010) 530–544. [12] S. Park, J.W. Lee, B.N. Popov, J. Power Sources 177 (2008) 457–463. [13] K.S.S. Naing, Y. Tabe, T. Chikahisa, J. Power Sources 196 (2011) 2584–2594. [14] K. Jiao, J. Park, X. Li, Appl. Energy 87 (2010) 2770–2777. [15] W. Liu, C.Y. Wang, J. Power Sources 164 (2007) 189–195. [16] B. Xiao, A. Faghri, Int. J. Heat Mass Transf. 51 (2008) 3127–3143. [17] W.W. Yang, T.S. Zhao, Electrochim. Acta 52 (2007) 6125–6140. [18] J.J. Garvin, J.P. Meyers, J. Electrochem. Soc. 158 (2011) B1119–B1127. [19] M. Zago, A. Casalegno, F. Bresciani, R. Marchesi, Int. J. Hydrogen Energy 39 (2014) 21620–21630. [20] A. Bazylak, V. Berejnov, B. Markicevic, D. Sinton, N. Djilali, Electrochim. Acta 53 (2008) 7630–7637. [21] P.K. Sinha, P.P. Mukherjee, C.Y. Wang, J. Mater. Chem. 17 (2017) 3089–3103. [22] N. Zhan, W. Wu, S. Wang, Electrochim. Acta 306 (2019) 264–276. [23] D. Zhang, Q. Cai, S. Gu, Electrochim. Acta 262 (2018) 282–296. [24] D. Gerteisen, T. Heilmann, C. Ziegler, J. Power Sources 177 (2008) 348–354. [25] M. Han, S.H. Chan, S.P. Jiang, J. Power Sources 159 (2006) 1005–1014. [26] Q.X. Wu, L. An, X.H. Yan, T.S. Zhao, Electrochim. Acta 133 (2014) 8–15. [27] N. Zamel, X. Li, Prog. Energy Combust. Sci. 39 (2013) 111–146. [28] Y. Gao, G.Q. Sun, S.L. Wang, S. Zhu, Energy 35 (2010) 1455–1459. [29] M.S. Yazici, J. Power Sources 166 (2007) 424–429. [30] E.H. Yu, K. Scott, Electrochem. Commun. 6 (2004) 361–365. [31] Z.G. Shao, F.Y. Zhu, W.F. Lin, P.A. Christensen, H.M. Zhang, J. Power Sources 160 (2006) 1003–1008. [32] C. Lim, K. Scott, R.G. Allen, S. Roy, J. Appl. Electrochem. 34 (2004) 929–933. [33] P. Liu, G.P. Yin, Q.Z. Lai, Int. J. Energy Res. 33 (2009) 1–7. [34] R. Chen, T.S. Zhao, Electrochem. Commun. 9 (2007) 718–724. [35] K. Fushinobu, D. Takahashi, K. Okazaki, J. Power Sources 158 (2006) 1240–1245. [36] F.Y. Zhang, S.G. Advani, A.K. Prasad, J. Power Sources 176 (2008) 293–298. [37] Q. Duan, B. Wang, J. Wang, H. Wang, Y. Lu, J. Power Sources 195 (2010) 8189–8193.
4. Concluding remarks To evaluate the feasibility of carbonized electrospun mats as the DLs for DMFCs and understand their structure-performance relationships, a series of in-house fabricated PAN, PAN/PLA and PAN/SiO2 mats are systematically investigated regarding to their physical morphology, pore size distribution, wettability, permeability, resistivity and DMFC performance. It is found that increasing the PAN concentration leads to increases of mean fiber diameters, mean pore size and through-plane permeability and a decrease of through-plane areal resistance, which contribute to a better DMFC performance due to enhancement in reac tant and electron transport. Meanwhile, it is demonstrated the employment of the carbonized electrospun mats as the anode DL results in a relatively small variation in cell performance as compared with those as the cathode DL, suggesting the oxygen transport issue is more severe than that of methanol in the DMFC. Furthermore, compared with the PAN/PLA mat, the PAN/SiO2 mat is more effective on improving the 10
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[38] C.J. Hung, C.H. Liu, T.H. Ko, W.H. Chen, S.H. Cheng, W.S. Chen, A. Yu, A. M. Kannan, J. Power Sources 221 (2013) 134–140. [39] D. Todd, W. M� erida, J. Power Sources 273 (2015) 312–316. [40] D. Todd, W. M� erida, J. Power Sources 311 (2016) 182–187. [41] S. Chevalier, N. Lavielle, B.D. Hatton, A. Bazylak, J. Power Sources 352 (2017) 272–280. [42] S. Chevalier, N. Ge, J. Lee, M.G. George, H. Liu, P. Shrestha, D. Muirhead, N. Lavielle, B.D. Hatton, A. Bazylak, J. Power Sources 352 (2017) 281–290. [43] S. Liu, M. Kok, Y. Kim, J.L. Barton, F.R. Brushett, J. Gostick, J. Electrochem. Soc. 164 (2017) A2038–A2048. [44] M. Kok, R. Jervis, D. Brett, P.R. Shearing, J. Gostick, Small 14 (2018) 1703616. [45] Q.X. Wu, Y. Lv, L. Lin, X. Zhang, Y. Liu, X. Zhou, J. Power Sources 410–411 (2019) 152–161. [46] B. Zhao, R. Cai, S. Jiang, Y. Sha, Z. Shao, Electrochim. Acta 85 (2012) 636–643. [47] Y. Liu, N. Zhang, L. Jiao, J. Chen, Adv. Mater. 27 (2015) 6702–6707. [48] L. Li, P. Liu, K. Zhu, J. Wang, G. Tai, J. Liu, Electrochim. Acta 235 (2017) 79–87. [49] J.G. Wang, Y. Yang, Z.H. Huang, F. Kang, Electrochim. Acta 56 (2011) 9240–9247. [50] M. Tebyetekerwa, S. Yang, S. Peng, Z. Xu, W. Shao, D. Pan, S. Ramakrishna, M. Zhu, Electrochim. Acta 247 (2017) 400–409.
[56] [57] [58] [59] [60] [61] [62] [63] [64]
11
C. Tran, V. Kalra, J. Power Sources 235 (2013) 289–296. M.M. Tomadakis, T.J. Robertson, J. Compos. Mater. 39 (2005) 163–188. Q.X. Wu, T.S. Zhao, W.W. Yang, Int. J. Heat Mass Transf. 54 (2011) 1132–1143. Q.X. Wu, L. An, X.H. Yan, T.S. Zhao, Electrochim. Acta 133 (2014) 8–15. R.G. Mei, J.J. Xi, Lei Ma, L. An, F. Wang, H.Y. Sun, Z.K. Luo, Q.X. Wu, J. Electrochem. Soc. 164 (2017) F1556–F1565. A.L. Andrady, Science and Technology of Polymer Nanofibers, John Wiley & Sons, Hoboken, NJ, 2008. R. Wang, Y. Liu, B. Li, B.S. Hsiao, B. Chu, J. Membr. Sci. 392–393 (2012) 167–174. W. Lehnert, H.A. Gasteiger Vielstich, A. Lamm, Handbook of Fuel Cells—Fundamentals, Technology and Applications, vol. 3, John Wiley & Sons, New York, 2003, pp. 517–537. R.S. Barhate, C.K. Loong, S. Ramakrishna, J. Membr. Sci. 283 (2006) 209–218. M. Kok, J. Gostick, J. Membr. Sci. 473 (2015) 237–244. Q.X. Wu, T.S. Zhao, R. Chen, W.W. Yang, J. Power Sources 191 (2009) 304–311. J. Biesdorf, A. Forner-Cuenca, T.J. Schmidt, P. Boillat, J. Electrochem. Soc. 162 (2015) F1243–F1252. C.Y. Wang, Chem. Rev. 104 (2004) 4727–4766. P. Piela, R. Fields, P. Zelenay, J. Electrochem. Soc. 153 (2006) A1902–A1913.
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Characterizations of carbonized electrospun mats as diffusion layers for direct methanol fuel cells Xiangyang Zhang, Yuxin Huang, Xuelong Zhou, Fang Wang, Zhongkuan Luo, Qixing Wu * Shenzhen Key Laboratory of New Lithium-ion Batteries and Mesoporous Materials, College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen, 518060, Guangdong, PR China
H I G H L I G H T S
� Application of electrospun mats as the diffusion layers of DMFCs is evaluated. � Morphology, pore size, permeability, resistivity and DMFC performance are studied. � A higher PAN content leads to a higher permeability, conductivity and performance. � Performance variation is low for anode electrospun mats but high for cathode ones. � Though resistivity is high, PAN/SiO2 mat enhances performance by water management. A R T I C L E I N F O
A B S T R A C T
Keywords: Diffusion layer Fuel cell Electrospun mats Polyacrylonitrile Carbonization
This work attempts to evaluate the application of carbonized electrospun mats as the diffusion layers (DL) for direct methanol fuel cells (DMFC). A series of in-house fabricated polyacrylonitrile (PAN) based fibrous mats are systematically investigated in terms of physical morphology, pore size distribution, wettability, permeability, resistivity and DMFC performance. It is found that a higher PAN concentration leads to a larger fiber diameter, mean pore size, permeability and a lower through-plane areal resistance, which contribute to a higher DMFC performance due to enhancement in reactant and electron transport. To create additional pores in the fibrous mat, polylactic acid (PLA) or silicon dioxide (SiO2) are used as the pore formers and results show that the PAN/ SiO2 mat is more effective on improving the DMFC performance than the PAN/PLA mat. The employment of the carbonized electrospun mats as the anode DL results in a relatively small variation in performance as compared with those as the cathode DL, suggesting the oxygen transport issue is more severe than that of methanol. The present work is anticipated to help gain understanding and guide further optimization of the carbonized elec trospun mats for fuel cells and other related application fields, including flow batteries, lithium/sodium-ion batteries and supercapacitors.
1. Introduction Direct methanol fuel cells (DMFC), capable of converting chemical energy of liquid methanol into electrical energy, have been considered as one of next-generation power sources for consumer electronic devices due to their favorable advantages of large theoretical energy density (~4800 Wh L 1), easy fuel maintaining and simple system design [1–4]. One of the most important components in the DMFC is the diffusion layer (DL), which plays a key role in controlling the performance of the DMFC. Typically, the functions of the DL are multiple: i) distributing reactants (i.e., methanol and oxygen) into the catalyst layer to enhance
the electrochemical reactions; ii) facilitating liquid water removal to mitigate or avoid water flooding; iii) providing electron and heat con duction pathway and mechanical support for the electrode. Historically, inherited from proton exchange fuel cells (PEMFC), carbon paper/carbon cloth, fabricated from carbon fibers with a high degree of graphitization, have been widely selected as the DLs for DMFCs to simultaneously satisfy all these three requirements. Over the past several decades, significant efforts have been devoted into ex-situ charactering the physical and chemical features of the commercial DLs [5–9], in-situ investigating their structural and surface wettability ef fects on the complex gas-liquid two phase flow behavior [10–14],
* Corresponding author. E-mail address: [email protected] (Q. Wu). https://doi.org/10.1016/j.jpowsour.2019.227410 Received 5 June 2019; Received in revised form 14 October 2019; Accepted 4 November 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Xiangyang Zhang, Journal of Power Sources, https://doi.org/10.1016/j.jpowsour.2019.227410
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numerical modeling to gain insights of the intrinsic link between structure and performance macroscopically [15–19] and microscopi cally [20–23], and optimizing their morphologies to improve the fuel cell performance [24–26]. Although the performance of the commercial carbon paper or carbon cloth is excellent in terms of electronic con ductivity, gas permeability, mechanical strength and chemical stability, their manufacturing processes are relatively complicated involving multiple carbonization and graphitization stages [27]. In addition, the main structural parameters of the commercial DLs, including fiber diameter and orientation as well as pore size and its distribution, are usually difficult to be modified at a large extent, which may limit a comprehensive understanding on structural effects and the room for design optimization. To remove the constraint of commercial DLs, various novel customdesigned DLs with well controllable structural features have been pro posed and investigated for potential application in PEMFCs and DMFCs [28–36]. To improve the conductivity and increase the pores for reac tant transport, Gao et al. [28] fabricated the carbon nanotube based DL for the DMFC. Yazici [29] proposed to use expended graphite with custom perforation as the DL for the DMFC and systematically studied the open ratio effect. In addition to carbon-based DLs, metal mesh made from stainless steel [30], titanium [31,32], nickel [33] and nickel-chromium alloy [34] have been studied extensively due to their well-connective pore structure for reactant supply, high conductivity for electron conduction and excellent stability for long-term operation. To enhance the through-plane transport, microfabrication techniques have also been introduced to process the thin metal film to form a series of straight pores with controllable pore size and open ratio [35,36]. Another type of DLs received increasing attentions recently is the carbonized electrospun mats synthesized by electrospinning poly acrylonitrile (PAN) solution with certain concentration and subsequent treatment at a relatively low temperature (<1200 � C) [37–42]. As many operating parameters such as composition of the precursor solution, tip-collector distance, rotating rate and collector shape are easy to pre cisely control, the final morphology of the carbonized nonwoven mat can be well tuned at a large degree to meet the specific purposes for DL related studies. Duan et al. [37] applied the carbonized electrospun mats as the micro-porous layers for PEMFCs, an improved performance was achieved due to a higher gas permeability of the electrospun mats as compared to that of conventional microporous layer with carbon par ticles and hydrophobic polytetrafluoroethylene (PTFE). Todd and M�erida [39] performed preliminary characterizations on in-house fabricated carbonized electrospun mats regarding to their electrical re sistances and wettabilities, and identified the effects of fiber alignment on in-plane mass transfer and fuel cell performance [40]. More recently, Chevalier et al. [41,42] presented an in-depth study on the structural effects of carbonized electrospun mats through polarization analysis and synchrotron X-ray radiography, and found that fiber diameter and alignment as well as pore size significantly influence the water transport and thus the cell performance. In addition to being applied in fuel cells, carbonized electrospun mats have also been considered as potential free-standing electrode materials in other electrochemical energy de vices such as all vanadium flow batteries [43–45], lithium-ion or sodium-ion batteries [46–48] and supercapacitors [49–51]. As carbonized electrospun mats are versatile in a wide range of fields, there is a need for systematically characterizing their main properties to understand the structure-performance relationship and to guide further optimization of their psychical and chemical characteristics. This work attempts to evaluate the application of carbonized electrospun mats as the DLs for DMFCs. A series of in-house fabricated polyacrylonitrile (PAN) based carbon nanofibrous mats are systematically investigated in terms of physical morphology, pore size, wettability, permeability, re sistivity and fuel cell performance. In addition, polylactic acid (PLA) or SiO2 are used as the soft and hard pore formers, respectively, to create additional pores in the fibrous mat for the sake of comparative study. The results of the present work might be helpful for gaining insights of
the carbonized electrospun mats and the experimentally determined key parameters could be readily applied in numerical modeling for future structural optimization. 2. Experimental 2.1. Synthesis of carbonized electrospun mats 2.1.1. Carbonized PAN mats As illustrated in Fig. 1, the carbonized PAN mats were prepared though the well-developed electrospinning method and the setup was similar to our previous work [45]. Various amount of PAN (Sigma-Al drich, MW ¼ 150,000) was dissolved in N, N-dimethylformamide (DMF) (Aladdin, anhydrous, 99.8%) to form a precursor solution with a variety of PAN concentrations ranging from 3 to 14 wt %. The needle to col lector distance, flow rate and the rotational speed of the drum were controlled to be 12 cm, 2 mL h 1 and 800 rpm. The obtained electrospun PAN mats were then stabilized in the air at 260 � C for 2 h by a ramping rate of 1 � C min 1, followed by heat treatment in argon atmosphere for 1 h at a ramping rate of 5 � C min 1. Various carbonization temperatures from 700 to 1100 � C were employed to evaluate the temperature effect on the characteristics of carbonized PAN mats. Note that except for studying the effect of carbonization temperatures on the 14 wt % PAN sample, other samples were all carbonized at 900 � C. 2.1.2. Carbonized PAN/PLA and PAN/SiO2 mats To create additional pores in the nanofibrous mats, polylactic acid (PLA, MW ¼ 60,000) or SiO2 (Aladdin) were introduced into the elec trospinning precursor solution and used as the soft and hard pore for mers, respectively, as illustrated in Fig. 1. The PAN/PLA precursor solution was formed by mixing 9 wt % PAN solution and 5 wt % PLA solution with different ratios of 9:1, 8:2, 7:3 and 6:4, which resulted in 0.5 wt %, 1 wt %, 1.5 wt % and 2 wt % PLA contents based on the total weight of PAN/PLA precursor solution, while the corresponding PAN contents were 8.1 wt %, 7.2 wt %, 6.3 wt % and 5.4 wt %, respectively. For the PAN/SiO2 precursor solution, it was prepared by introducing various amount of SiO2 nanoparticles into 9 wt % PAN solution, which resulted in SiO2 contents of 2 wt %, 3 wt %, 4 wt % and 5 wt % and PAN contents of 8.8 wt %, 8.7 wt %, 8.6 wt% and 8.5 wt % based on total weight of PAN/SiO2 solution, respectively. After electrospinning, the PAN/PLA mats were firstly treated by chloroform to remove the PLA, followed by the stabilization at 260 � C under air for 2 h and carboniza tion at 900 � C under argon for 1 h. On the contrary, the electrospun PAN/SiO2 mats were firstly stabilized at 260 � C under air for 2 h and carbonized at 900 � C under argon for 1 h, and then the resulting carbonized samples were treated by HF to remove SiO2 for pore forming. 2.2. Materials characterizations The physical morphology of the carbonized electrospun mats were evaluated by a field emission scanning electron microscope (SEM, JEOL6700F) and a transmission electron microscope (TEM, JEM-2100). The degrees of graphitization of the samples were analyzed by X-ray diffraction (XRD, D8 Advance) and Raman Spectroscopy (inVia). The pore size and its distribution of the samples were characterized by mercury intrusion porosimetry (Auto Pore IV 9500). Water contact an gles were obtained by the sessile drop experiment. The though-plane permeabilities of the carbonized electrospun mats were measured according to the Gurley method [38] by a Gurley densometer (Model 4110 N) equipped with an automated digital timer (Model 4320). To conduct the measurement, an inner cylinder of 567 g was left to slide freely in the vertical outer cylinder so as to create an air pressure difference (Δp, 1.22 kPa). By recording the time (t) for a given volume of air (V, 300 mL) to flow through a circular area (A, 64.52 mm2) of the sample, the permeability (K) can be estimated by Darcy’s law with given thickness (L) of the sample and viscosity of air (μ): 2
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Fig. 1. Schematic of synthesis process for various carbonized electrospun mats.
K¼
V μL tAΔp
2.3. Preparation of membrane electrode assembly (MEA)
(1)
For the sake of comparison, the thicknesses of all the tested DL samples in this work were controlled to be around 200 μm, nearly the same as the Toray 060 carbon paper. When evaluating the performance of a carbonized mat as the anode DL, the anode catalyst layer was inhouse fabricated by the decal method [53,54], while the cathode elec trode (consisting of a catalyst layer and a diffusion layer) was a com mercial one from Johnson Matthey (#45375, 2 mgPt cm 2). On the contrary, when investigating a carbonized mat as the cathode DL, the cathode catalyst layer was in-house fabricated and the anode was a commercial one from Johnson Matthey (#45374, 2.7 mgPt cm 2 and 1.35 mgRu cm 2). The decal method included ultrasonic spraying the catalyst ink onto a PTFE blank sheet and transferring the catalyst layer from the PTFE sheet onto the membrane during hot pressing [53,54]. The purpose of using decal method was to ensure various DLs share the same anode or cathode catalyst layer. Pt–Ru/C (50% Pt, 25% Ru, Hispec 12100, Johnson Matthey) and Pt (60% Pt, Hispec 9100, Johnson Mat they) were used as catalysts and the metal loadings in the decal catalyst layers were 4 mg cm 2 and 2 mg cm 2 for the anode and cathode, respectively. The Nafion content was maintained to be about 20 wt %. A 50 μm thick Nafion 212 membrane was used to conduct proton and was sandwiched between anode and cathode electrode by hot pressing at 135 � C and 4.0 MPa for 3 min. The nominal active area of the MEA was 5.0 cm2.
In this work, the measured permeabilities of samples are also compared with those calculated by Carman-Kozeny equation for fibrous mats: K¼
d2f ε3 16kck ð1
εÞ2
(2)
where df is the fiber diameter, ε is the porosity of the sample and kck is a coefficient accounting for the fiber structure and alignment. As sug gested by Tomadakis et al. [52], for fibrous mats consisting of randomly aligned fibers, a value of 4 for kck can be used for estimation of K. Detailed calculation of K are shown in Table S1 and Table S2 in the Supplementary data. The through-plane areal resistances of the carbonized electrospun mats were measured through an in-house setup shown in Fig. 2. It consisted of a piston for supplying and controlling the contact pressure, two PTFE supporting plates and two gold-coated copper plates for clamping the samples [8,39]. With such a setup and a micro-ohm meter (Keysight 34420A), through-plane areal resistances (including the con tact resistances at the interfaces of the sample and copper plate) can be studied under various clamping pressures. In addition to through-plane, the in-plane resistivities of carbonized electrospun mats were also characterized at room temperature through a four-probe resistivity meter (RTS-9).
2.4. Measurement of methanol crossover rate and fuel efficiency To determine the fuel efficiency, the methanol crossover rate under open circuit condition was measured by voltammetric method originally proposed by Ren et al. [2]. During this measurement, the DMFC anode was fed with 2 M methanol solution at a flow rate of 2.5 mL min 1 while the cathode was fed by nitrogen (humidified at 60 � C) at a flow rate of 200 sccm to create an inert environment and to supply water for the methanol oxidation reaction on the cathode. Then, the cell was charged at 0.85 V to ensure the oxidation of permeated methanol from the anode and the charge current, known as limiting methanol crossover current density [2], ic,lim, was recorded until the current was stable. Note that to obtain the actual methanol crossover current density under open circuit, ic,oc, the ic,lim should be corrected by a factor of (1 þ 56.38% [2]) to account for the electro-osmosis of methanol from cathode to anode at 2 M methanol concentration. By obtaining the anode limiting mass transport current density, ia,lim, through polarization tests, the methanol-crossover current density, ic, under various operating current
Fig. 2. Schematic of the in-house setup for through-plane electrical areal resistance measurement. 3
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density, i, can be approximated by Ref. [63]: � � i ic ¼ ic;oc 1 ia;lim
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anode as the counter and reference electrodes while the DMFC cathode as the working electrode, respectively. The measurement was performed under potentiostatic mode at 0.4 V with a perturbation amplitude of 10 mV and the impedance data were recorded from 10 2 to 104 Hz with 8 steps per decade. All the polarization tests were acquired by a DMFC testing system (Arbin BT-5HC). During polarization measurement, the DMFC was first kept at open circuit for 15 min, followed by stepwise discharging at a series of preset currents for 1 min. To investigate the short-term stability of carbonized mats, the constant-current discharge tests were performed for 10 h at the current densities of 50 mA cm 2 and 200 mA cm 2 by using Toray 060 carbon paper (without hydrophobic treatment), 14 wt % PAN mat, PAN/PLA (0.5 wt % PLA) mat and PAN/SiO2 (5 wt % SiO2) mat as the cathode DLs. For all the polarization and short-term stability tests, the anode and cathode of the DMFC were actively supplied by 2 M methanol solution at a flow rate of 2.5 mL min 1 and dry air at 200 sccm without back pressure, respec tively. The operating temperature of the cell was maintained at 60 � C.
(3)
Neglecting the evaporative loss of methanol, the fuel efficiencyηfuel , can be determined as:
ηfuel ¼
i i þ ic
(4)
The measured ic,lim at 2 M methanol and ia,lim at 0.5 M methanol with various anode DLs were shown in Fig. S5 and summarized in Table S3 in the Supplementary data. 2.5. Single DMFC test The prepared MEA was assembled in an in-house DMFC fixture re ported in previous work [55]. It consisted of two graphite blocks (POCO) for distributing reactants, two gold-coated current collectors for con ducting electron and two aluminum end plates for mechanical support. Single-pass serpentine flow field with a channel depth of 1 mm and width of 0.6 mm was grooved in the graphite block. The electrochemical impedance spectra (EIS) of the single-cell DMFC were attained by the Zahner Zennium workstation in two-electrode mode with the DMFC
3. Results and discussion 3.1. Physical morphology The physical morphologies of the carbonized PAN mats with various
Fig. 3. SEM images of carbonized electrospun mats with various PAN concentrations (3–14 wt %) and with pore-forming agents of PLA (from 0.5 to 2 wt %) and SiO2 (from 2 to 5 wt %). 4
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precursor concentrations are shown in Fig. 3. The mean fiber diameter of the carbonized PAN mat increases with an increase of PAN concentra tion in the electrospinning solution; with increasing PAN concentration from 3 wt % to 14 wt %, the mean diameter increases from about 60 nm to 1100 nm (Fig. 4a). This phenomenon is in agreement with previous works [39,41,43] and could be explained by the increased viscosity with high PAN concentrations, which favors entangling of polymer chains to form larger fibers [56]. In addition, it is noted in Fig. 4a that the pre cursor fiber undergoes a decrease in diameter due to loss of mass after carbonization; the reduction in the diameter can reach 50% for high 14 wt % PAN precursor mat and can be further decreased at higher carbonization temperatures (Fig. 4b). In addition to fiber diameter, the pore size is also observed to increase with PAN concentrations. Princi pally, the pore size depends highly on the packing density of fibers: for randomly packed fibers, a higher packing density is expected to increase the probability of fiber intersecting to shorten the average distance be tween intersections, which defines the boundary of a pore. If constant volume fraction of PAN is assumed, a larger fiber diameter (high PAN concentration) will lead to a shorter total length of PAN fiber at given area (or volume), which translates to a lower packing density and thus a larger pore size [57]. It worth mentioning that the assumption of con stant volume fraction of PAN is roughly valid considering that the porosity of randomly orientated PAN mats is typically around 90% [39, 41,43] and the change in pore size is significant (Figs. 3 and 5) as more than one order increase in fiber diameter is achieved when increasing PAN concentration from 3 wt % to 14 wt %. The morphologies of PAN/PLA and PAN/SiO2 mats are shown in Fig. 3. In contrast to PAN mats, some surface pores are created on the fibers by removing pore formers of PLA or SiO2, respectively. The
nanopores are only sparsely distributed on the surface of PAN/PLA fiber even at high PLA contents, whereas the distribution of surface pores on PAN/SiO2 fiber tends to become uniform with increasing SiO2 contents, rendering a hollow fiber structure as shown in Fig. S1. More impor tantly, the surface pore volume of PAN/PLA fiber is substantially lower than that of PAN/SiO2 fiber especially for higher SiO2 contents, sug gesting PAN/SiO2 mats may be more effective in creating additional transport pathway for reactant transfer. Similar to PAN mats, the di ameters of PAN/PLA and PAN/SiO2 mats also reduce after carbonization as shown in Fig. 4c and d. Additionally, the fiber diameter decreases with increasing PLA content while it somehow increases with SiO2 content. This is because a higher PLA content corresponds to a lower PAN content in the precursor solution (increasing PLA content from 0.5 wt % to 2 wt %, resulting in a decreased PAN content from 8.1 wt % to 5.4 wt %), which contributes to a smaller fiber diameter as mentioned previously; for the PAN/SiO2 mat, the increased fiber diameter is pri marily attributed to the local aggregation of SiO2 solid particles, which is exacerbated by a higher SiO2 content. 3.2. Pore size distribution To evaluate the detailed pore size distribution of carbonized PAN, PAN/PLA and PAN/SiO2 mats, the results of porosimetry tests are shown in Fig. 5. Consistent with the SEM results in Fig. 3, the peak of pore diameter shifts gradually from 200 nm to 8 μm with increasing PAN concentration from 3 wt % to 14 wt % due to increased average distance between intersections for larger fibers. It should be mentioned that the mean pore size of carbonized 14 wt % PAN mat is still lower than that of the commercial Toray 060 carbon paper with a mean diameter of 7 μm
Fig. 4. Mean fiber diameters of the carbonized electrospun mats: a) effect of PAN concentration, b) effect of carbonation temperature, c) effect of PLA content and d) effect of SiO2 content. 5
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[27], which shows a peak of pore diameter at around 30 μm. Moreover, Fig. 5b shows that the peak shifts negatively from 850 nm to 560 nm with increasing PLA content from 0.5 wt % to 2 wt %, in good agreement with SEM results in Fig. 3 and can be attributed to the decreased average distance between intersections for smaller fibers at high PLA contents (low PAN content in the precursor). In contrast to PAN/PLA mats, two peaks are observed for PAN/SiO2 mats: a small peak at around ~100 nm and a large and broad peak at 600–700 nm, which possibly correspond to the pores formed by removal of SiO2 particles and fiber intersecting, respectively. Although no obvious peak shift is observed for changing SiO2 contents, the pore volume seems to increase with SiO2 contents probably because a large amount of SiO2 are locally aggregated to expand the pore volume. 3.3. Through-plane permeability The though-plane permeability of the DL is a critically important parameter as it closely links to the mass transfer of reactants (methanol and oxygen), which influences the concentration polarization of the electrochemical reactions especially at high-current region. In this work, the permeability of various carbonized electrospun mats are measured by the standard Gurley method for thin porous paper [38]. Typically, the Gurley second is experimentally measured and it used as an indicator to reflect the air permeability. For ease of comparison, the measured Gurley second is converted to the intrinsic permeability according to Darcy’s law (Eq. (1)) by excluding the influences of pressure gradient, air viscosity and volumetric flow rate. On the other hand, the perme ability of a fibrous mat can be estimated by Carman-Kozeny relation (Eq. (2)) with known mean fiber diameter, porosity and kck (assuming kck ¼ 4 for randomly packed fibers [52]). The detailed calculations can be found in Table S1 and Table S2 in the Supplementary data. Fig. 6a shows that the measured permeability of the carbonized PAN mat increases rapidly with PAN concentrations; the permeability in creases from 4.34 � 10 14 m2 to 1.57 � 10 12 m2, nearly 2 orders in crease, when changing PAN concentrations from 3 wt % to 14 wt %. The measured permeabilities of carbonized PAN mats are in agreement with pervious work by Kok et al. [44]. The increase in the permeability with a higher PAN concentration is qualitatively expected as the pore size is expended with PAN concentrations (Figs. 3 and 5a). For comparison, the permeability of Toray 060 carbon paper is also measured by Gurley method and it found to be 6.09 � 10 12 m2, which is well consistent with previously reported values of 6.15 � 10 12 m2 [8] and 8 � 10 12 m2 [58]. Note that the permeability of 14 wt % PAN mat still lower than that of Toray 060 carbon paper mainly due to relatively small pore size of the PAN mat. In addition, the predicted permeabilities seem to be in excellent agreement with experimentally measured values for porous mats with large fiber diameters (i.e., 11 wt % PAN, 14 wt % PAN and carbon paper), suggesting a good applicability of Carman-Kozeny equation for micro-fibrous mats. As shown in Fig. 6a, when the mean fiber diameter is smaller than 306 nm (corresponding to 9 wt % PAN), a large deviation between measured and predicted permeabilities is observed: measured ones are always higher than the predicted ones. One possible reason for such a deviation might be the breakdown of no-slip assumption for flow through nanopores; under slip flow condition, the drag force on the nanofiber surface becomes smaller as compared with the one under no-slip condition, which increases the flow rate through the nanopores to result in a higher permeability [59,60]. The Knudsen number, the ratio of mean free path of air to mean radius of nanofiber, is estimated to be 0.43 at standard conditions for 9 wt % PAN mats, indi cating the significance of slip flow [59]. Another explanation for this deviation is likely to be the imperfect straightness and inconstant diameter of the real electrospun fiber such that the actual kck is not equal to 4 [60]. The measured permeabilities of carbonized PAN/PLA and PAN/SiO2 mats are shown in Fig. 6b and Table S1. As expected, an increased permeability is observed with a higher SiO2 content due to an
Fig. 5. Mercury intrusion porosimetry results of various carbonized electro spun mats at 900 � C: a) effect of PAN concentrations, b) effect of PLA contents and c) effect of SiO2 contents.
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Fig. 6. Through-plane permeabilities of various carbonized electrospun mats: effects of a) PAN concentration and b) SiO2 and PLA contents.
introduction of additional pores on the fiber surface, consistent with porosimetry results in Fig. 5c. In contrast, an increase in the PLA content lead to a reduction in permeability of PAN/PLA mats. This is because negligibly few pores are created by removal of PLA (Fig. 3) and the pore size shifts to a smaller value with increasing the PLA content, as evident in Fig. 5b, which translates to a lower permeability.
seen in Fig. 7e that increasing the PAN concentration from 3 wt % to 9 wt % results in a decrease in the in-plane resistivity from 0.0094 Ω m to 0.00275 Ω m, while further increasing the PAN concentration to 14 wt % leads to an increase to 0.0067 Ω m. Since the carbonized mat is not an ideal planar structure, the electrons may not only transport though inplane fibers located near the probes of resistivity meter (upper fiber layer) but also through the thickness direction to lower fiber layers. When PAN concentration is low (<9 wt %), the relatively large contact resistance between upper and lower fibers, due to more interfacial contact for smaller fibers, might be a dominant factor that hinders the electron transport, thereby resulting in a lower in-plane resistivity at a higher PAN concentration (less interfacial contact). At high PAN con centrations (>9 wt %), however, the relative importance of contact resistance between fiber layers might become minor as compared with the in-plane conduction pathway formed by the intersection of in-plane fibers and hence a higher resistivity is found for the mats with a larger PAN concentration (less intersection points) [41,43]. Similar to the through-plane areal resistances, the in-plane resistivity decreases significantly with increasing the carbonization temperature (Fig. 7e) due to a higher degree of graphitization, whereas it increases with PLA and SiO2 contents (Fig. 7f) as the results of a larger contact resistance for a smaller fiber diameter (higher PLA content) and a more hollow structure of the fiber with a larger SiO2 content, respectively. In addi tion, the measured in-plane resistivity of the Toray 060 carbon paper by the same apparatus is 7.53 � 10 5 Ω m, in agreement with previous re ported values [8,43]. It worth mentioning that although the reported resistivities of carbonized PAN, PAN/PLA and PAN/SiO2 mats are generally higher than that of the commercial carbon paper mainly due to a low-temperature (<1200 � C) heat treatment, they are still well below the resistivity of a fully hydrated Nafion 117 membrane at room tem perature (~0.1 Ω m [2]) such that the electrical resistance may contribute to a relatively trivial impact on the ohmic loss as compared with the ionic resistance.
3.4. Through-plane and in-plane electrical resistances Another key parameter of the DL is the electrical resistance, which directly related to ohmic loss of the DMFC. It is found in Fig. 7 that with an increased clamping pressure, the areal resistances of all the tested samples decrease rapidly at low-pressure region and slightly at highpressure region. This trend is expected as the contact resistance domi nates at low clamping pressures and it decreases significantly when a sufficiently high pressure is applied to ensure an intimate contact be tween the sample and gold-coated copper plates (Fig. 2). In addition, Fig. 7a shows that a higher PAN concentration leads to a lower areal resistance; the areal resistance decreases from 45.04 mΩ cm 2 to 12.07 mΩ cm 2 at 4.71 MPa by increasing PAN concentration from 3 wt % to 14 wt %. This might be because for a given thickness of a fibrous mat, a larger fiber diameter (higher PAN concentration) will result in less interfacial contact between successive fibers in the thickness di rection, giving rise to a smaller total contact resistance for electrons to transfer though the plane direction. Fig. 7b shows the effect of carbon ization temperature on the areal resistance. As expected, the areal resistance increases substantially with carbonization temperature due to a higher degree of crystallinity (XRD results in Fig. S2) and graphitiza tion (Raman spectra in Fig. S2). This result indicates that although areal resistance of the 14 wt % PAN mat carbonized at 900 � C is still higher than that of the carbon paper (Fig. 7a), increasing the carbonization temperature would be an effective approach to enhance the electrical conductivity to reduce ohmic loss. The effects of PLA and SiO2 contents on the though-plane areal re sistances are presented in Fig. 7c and d, which show that increasing the content of PLA or SiO2 leads to a larger resistance. It has been known that a higher PLA content corresponds to a smaller mean fiber diameter, which leads to a higher interfacial resistance across the thickness di rection and thus a larger though-plane areal resistance. For the PAN/ SiO2 mats, a higher SiO2 content can lead to more hollow fibers (Fig. 3). As compared with the smooth and solid fiber, the hollow fibers not only present less conduction pathway for electrons but also probably reduce the contact area between successive fibers in the thickness direction, thereby increasing the areal resistance. In addition to through-plane resistance, the in-plane resistance of the DL also plays an important role in the DMFC as it governs the in-plane uniformity of electric field and thus the electrochemical reaction. It is
3.5. DMFC performance The feasibility of applying carbonized PAN, PAN/PLA and PAN/SiO2 mats as the DLs of DMFCs is investigated in a single-cell DMFC. As all the carbonized mats show contact angles larger than 90� (Fig. S3), the samples are evaluated without an additional hydrophobic treatment and a microporous layer, the effect of which on the DMFC performance would be an interesting topic for future research. For ease of compari son, the commercial Toray 060 carbon paper without a hydrophobic treatment is also tested. Fig. 8a shows the polarization result of the DMFC with various carbonized PAN mats as the anode DLs. With an increase of the PAN concentration from 5 wt % to 14 wt %, the peak power density increases by only 17.2% from 73.1 mW cm 2 to 7
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Fig. 7. Effects of a) PAN concentration, b) carbonization temperature, c) PLA content and d) SiO2 content on through-plane areal resistances of carbonized elec trospun mats; effects of e) PAN concentration and carbonization temperature and f) SiO2 and PLA contents on in-plane resistivities.
85.7 mW cm 2, the latter of which is nearly the same as the carbon paper. Such an improvement is somewhat less than expected as the areal resistance decreases by 67.8% and the permeability increases by 17 times when increasing PAN concentration from 5 wt % to 14 wt %. One possible reason may be that the contribution of electrical resistance to the total cell internal resistance is small as discussed before and its effect on the ohmic loss is unpronounced at relatively low current density (<500 mA cm 2). Another reason can be the use of relatively high methanol concentration (2 M) so that the anode concentration polari zation is not dominated even for the DL with a low permeability [61]. To further elucidate the effects of PAN mats as the anode DLs, EIS results are presented in Fig. S4a. It shows that the ohmic resistances (impedances at high frequencies of ~1000 Hz) of carbon papers are lower than those of
carbonized PAN mats, in good agreement with ex-situ characterizations shown in Fig. 7. In addition, at medium and low frequencies, the cell impedance of DMFC increases with an increase of PAN concentration. This is because a higher PAN concentration leads to a larger perme ability (Fig. 6), which results in a higher methanol concentration in the anode catalyst layer (consistent with measured methanol crossover currents in Fig. S5a and the anode limiting mass transport currents in Fig. S5b). Consequently, methanol oxidation at the anode and oxygen reduction at the cathode (due to methanol crossover) is hindered, accompanied by an enlarged inductive loop caused by adsorbed in termediates [64]. In contrast to the anode, the effect of carbonized mats with various PAN concentrations as the cathode DL is more pronounced: as shown in 8
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Fig. 8. DMFC performances with carbonized electrospun mats: a) carbonized PAN mats as anode DLs, b) as cathode DLs, c) effect of carbonization temperature, d) effect of pore formers; e) short-term stability tests at 50 mA cm 2 and f) at 200 mA cm 2. The inset figure in c) shows the performance of anode PAN mats carbonized at various temperatures.
Fig. 8b, the peak power density improves by 32.5% from 50.1 mW cm 2 to 66.4 mW cm 2 when increasing the PAN concentration from 5 wt % to 14 wt %. The improved performance could be attributed mainly to a larger permeability at a larger PAN content to accelerate oxygen trans fer. This point is further confirmed by the EIS with various cathode DLs shown in Fig. S4b, which shows that a higher PAN concentration leads to a smaller cell impedance at medium and low frequencies. Since the same commercial anode electrode is used, the variation among impedances should be originated from cathode, i.e., oxygen reduction reaction. These results suggest the supply of oxygen on the cathode is more demanding than that of methanol on the anode. Therefore, as shown in Fig. 8b, the performance can be further improved by changing the cathode DL to a commercial carbon paper with a much higher permeability. Fig. 8c shows the polarization curves of 14 wt % PAN mats
carbonized at various temperatures. When being used as the anode DLs, the carbonization temperature reveals a negligible effect on the DMFC performance (the inset figure of Fig. 8c), indicating again the contri bution of electrical resistance play a minor role in the ohmic loss. Regarding to the application of the cathode DLs, a higher carbonization temperature seems to slightly increase the DMFC performance, which can attributed to an enlarged mean pore size for oxygen transport as the result of a reduced fiber diameter at elevated temperatures (Fig. 4b). Since the oxygen transport issue is more severe than that of methanol in the anode and various anode DLs result in a relatively small variation in performance, we have only tested the DMFCs with PAN/PLA and PAN/SiO2 mats as the cathode DLs while keeping the same commercial anode electrodes. The performances of carbonized PAN/PLA (0.5 wt % PLA) and PAN/SiO2 (5 wt % SiO2) mats as the cathode DLs are shown in Fig. 8d. Clearly, the PAN/PLA mat yields the lowest performance among 9
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all the tested samples primarily due to its smallest through-plane permeability as shown in Fig. 6 and Table S1. In addition, it is surpris ing to see that the DMFC with the PAN/SiO2 mat yields not only a nearly the same peak power density as the one with commercial carbon paper but also a higher maximum current density. This is somehow unex pected considering the higher electrical resistance and lower throughplane permeability of the PAN/SiO2 mat. To understand this phenom enon, the stoichiometry of air is calculated. At the maximum current density of 456 mA cm 2 with respective to the PAN/SiO2 mat, the stoi chiometry of air is ~5.3 without considering methanol crossover effect and is estimated to be ~2.9 if considering oxygen consumption by the oxidation of permeated methanol (ic ¼ 367 mA cm 2 at 456 mA cm 2 estimated by Eq. (3), oxygen consumption increases by 80.5%). Hence, the stoichiometry of air might not be high enough to rule out the pos sibility of water flooding especially at high current densities. We therefore speculate that the high performance achieved by using PAN/ SiO2 mat is likely related to the improved water management. Compared to the carbon paper without PTFE coating, the PAN/SiO2 mat shows a highly hydrophobic feature with a contact angle of ~126� (Fig. S3), possibly arising from the micro-roughness of the fiber surface (Fig. 3). As noted by Biesdorf et al. [62], water accumulates in the form of film on a hydrophilic surface, whereas it changes to droplets on a hydrophobic surface to facilitate its removal by the flowing gas due to its increased total surface area. Consequently, the PAN/SiO2 mat shows a higher performance at the large-current region, where the need for oxygen is more demanding. With respective to the short-term stability of the carbonized mats, Fig. 8e and f shows that the transient voltages of all the tested samples drop rapidly at the beginning of tests, followed by slight declines without severe fluctuations. In addition, it is found that cell voltage of PAN/SiO2 mat is slightly lower than that of carbon paper at 50 mA cm 2, whereas the former one shows a much smaller decline rate of voltage and is ~30 mV higher than the latter one at 200 mA cm 2. At the current density of 200 mA cm 2, the stoichiometry of air is ~12.1 without considering methanol crossover but it drops largely to ~3.2 by taking account on methanol crossover effect (ic ¼ 553 mA cm 2 at 200 mA cm 2 estimated by Eq. (3), oxygen consumption increases by 276.5%), indicating a possible occurrence of water flooding even at 200 mA cm 2. Therefore, considering the higher ohmic resistance and lower through-plane permeability of the PAN/SiO2 mat, one possible reason to account for the high performance of PAN/SiO2 mat may be its improved water management arisen from its highly hydrophobic feature. In addition, since all the cells share the same commercial anode electrode, the fuel efficiencies at 50 mA cm 2 and 200 mA cm 2 are estimated to be 7% and 27%, respectively, according to Eq. (4). It should be mentioning that the fuel efficiency can be readily improved by increasing the operating current or using thicker membranes to reduce the fuel loss by methanol crossover.
DMFC performance by introducing additional pores for oxygen transport and enhancing the surface hydrophobicity for water management. The reported values of the key parameters in this work is anticipated to be helpful in future numerical modeling and DMFC structural design, which would in turn gain deeper insights of the carbonized electrospun mats to facilitate its application in other relevant fields such as flow batteries, lithium-ion batteries and supercapacitors. 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. Acknowledgements This work was financially supported by the Shenzhen Science and Technology Fund (JCYJ20170818093905960 and JCYJ20180305125 604361), Natural Science Foundation of Guangdong Province (2018A030313194 and 2018A030310618) and Natural Science Foun dation of SZU (No. 2018039 and 827-000015). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227410. References [1] K. Scott, W.K. Taama, P. Argyropoulos, J. Power Sources 79 (1999) 43–59. [2] X. Ren, T.E. Springer, T.A. Zawodzinski, S. Gottesfeld, J. Electrochem. Soc. 147 (2000) 466–474. [3] F. Liu, G. Lu, C.Y. Wang, J. Electrochem. Soc. 153 (2006) A543–A553. [4] T.S. Zhao, C. Xu, R. Chen, W.W. Yang, Prog. Energy Combust. Sci. 35 (2009) 275–292. [5] M.V. Williams, E. Begg, L. Bonville, H.R. Kunza, J.M. Fenton, J. Electrochem. Soc. 151 (2004) A1173–A1180. [6] J.P. Feser, A.K. Prasad, S.G. Advani, J. Power Sources 162 (2006) 1226–1231. [7] A. Tamayol, M. Bahrami, J. Power Sources 196 (2011) 6356–6361. [8] A. El-kharouf, T.J. Mason, D.J.L. Brett, B.G. Pollet, J. Power Sources 218 (2012) 393–404. [9] G. Xu, J.M. LaManna, J.T. Clement, M.M. Mench, J. Power Sources 256 (2014) 212–219. [10] J. Eller, T. Ros� en, F. Marone, M. Stampanoni, A. Wokaun, F.N. Büchi, J. Electrochem. Soc. 158 (2011) B963–B970. [11] E.E. Kimball, J.B. Benziger, Y.G. Kevrekidis, Fuel Cells 10 (2010) 530–544. [12] S. Park, J.W. Lee, B.N. Popov, J. Power Sources 177 (2008) 457–463. [13] K.S.S. Naing, Y. Tabe, T. Chikahisa, J. Power Sources 196 (2011) 2584–2594. [14] K. Jiao, J. Park, X. Li, Appl. Energy 87 (2010) 2770–2777. [15] W. Liu, C.Y. Wang, J. Power Sources 164 (2007) 189–195. [16] B. Xiao, A. Faghri, Int. J. Heat Mass Transf. 51 (2008) 3127–3143. [17] W.W. Yang, T.S. Zhao, Electrochim. Acta 52 (2007) 6125–6140. [18] J.J. Garvin, J.P. Meyers, J. Electrochem. Soc. 158 (2011) B1119–B1127. [19] M. Zago, A. Casalegno, F. Bresciani, R. Marchesi, Int. J. Hydrogen Energy 39 (2014) 21620–21630. [20] A. Bazylak, V. Berejnov, B. Markicevic, D. Sinton, N. Djilali, Electrochim. Acta 53 (2008) 7630–7637. [21] P.K. Sinha, P.P. Mukherjee, C.Y. Wang, J. Mater. Chem. 17 (2017) 3089–3103. [22] N. Zhan, W. Wu, S. Wang, Electrochim. Acta 306 (2019) 264–276. [23] D. Zhang, Q. Cai, S. Gu, Electrochim. Acta 262 (2018) 282–296. [24] D. Gerteisen, T. Heilmann, C. Ziegler, J. Power Sources 177 (2008) 348–354. [25] M. Han, S.H. Chan, S.P. Jiang, J. Power Sources 159 (2006) 1005–1014. [26] Q.X. Wu, L. An, X.H. Yan, T.S. Zhao, Electrochim. Acta 133 (2014) 8–15. [27] N. Zamel, X. Li, Prog. Energy Combust. Sci. 39 (2013) 111–146. [28] Y. Gao, G.Q. Sun, S.L. Wang, S. Zhu, Energy 35 (2010) 1455–1459. [29] M.S. Yazici, J. Power Sources 166 (2007) 424–429. [30] E.H. Yu, K. Scott, Electrochem. Commun. 6 (2004) 361–365. [31] Z.G. Shao, F.Y. Zhu, W.F. Lin, P.A. Christensen, H.M. Zhang, J. Power Sources 160 (2006) 1003–1008. [32] C. Lim, K. Scott, R.G. Allen, S. Roy, J. Appl. Electrochem. 34 (2004) 929–933. [33] P. Liu, G.P. Yin, Q.Z. Lai, Int. J. Energy Res. 33 (2009) 1–7. [34] R. Chen, T.S. Zhao, Electrochem. Commun. 9 (2007) 718–724. [35] K. Fushinobu, D. Takahashi, K. Okazaki, J. Power Sources 158 (2006) 1240–1245. [36] F.Y. Zhang, S.G. Advani, A.K. Prasad, J. Power Sources 176 (2008) 293–298. [37] Q. Duan, B. Wang, J. Wang, H. Wang, Y. Lu, J. Power Sources 195 (2010) 8189–8193.
4. Concluding remarks To evaluate the feasibility of carbonized electrospun mats as the DLs for DMFCs and understand their structure-performance relationships, a series of in-house fabricated PAN, PAN/PLA and PAN/SiO2 mats are systematically investigated regarding to their physical morphology, pore size distribution, wettability, permeability, resistivity and DMFC performance. It is found that increasing the PAN concentration leads to increases of mean fiber diameters, mean pore size and through-plane permeability and a decrease of through-plane areal resistance, which contribute to a better DMFC performance due to enhancement in reac tant and electron transport. Meanwhile, it is demonstrated the employment of the carbonized electrospun mats as the anode DL results in a relatively small variation in cell performance as compared with those as the cathode DL, suggesting the oxygen transport issue is more severe than that of methanol in the DMFC. Furthermore, compared with the PAN/PLA mat, the PAN/SiO2 mat is more effective on improving the 10
X. Zhang et al.
Journal of Power Sources xxx (xxxx) xxx [51] [52] [53] [54] [55]
[38] C.J. Hung, C.H. Liu, T.H. Ko, W.H. Chen, S.H. Cheng, W.S. Chen, A. Yu, A. M. Kannan, J. Power Sources 221 (2013) 134–140. [39] D. Todd, W. M� erida, J. Power Sources 273 (2015) 312–316. [40] D. Todd, W. M� erida, J. Power Sources 311 (2016) 182–187. [41] S. Chevalier, N. Lavielle, B.D. Hatton, A. Bazylak, J. Power Sources 352 (2017) 272–280. [42] S. Chevalier, N. Ge, J. Lee, M.G. George, H. Liu, P. Shrestha, D. Muirhead, N. Lavielle, B.D. Hatton, A. Bazylak, J. Power Sources 352 (2017) 281–290. [43] S. Liu, M. Kok, Y. Kim, J.L. Barton, F.R. Brushett, J. Gostick, J. Electrochem. Soc. 164 (2017) A2038–A2048. [44] M. Kok, R. Jervis, D. Brett, P.R. Shearing, J. Gostick, Small 14 (2018) 1703616. [45] Q.X. Wu, Y. Lv, L. Lin, X. Zhang, Y. Liu, X. Zhou, J. Power Sources 410–411 (2019) 152–161. [46] B. Zhao, R. Cai, S. Jiang, Y. Sha, Z. Shao, Electrochim. Acta 85 (2012) 636–643. [47] Y. Liu, N. Zhang, L. Jiao, J. Chen, Adv. Mater. 27 (2015) 6702–6707. [48] L. Li, P. Liu, K. Zhu, J. Wang, G. Tai, J. Liu, Electrochim. Acta 235 (2017) 79–87. [49] J.G. Wang, Y. Yang, Z.H. Huang, F. Kang, Electrochim. Acta 56 (2011) 9240–9247. [50] M. Tebyetekerwa, S. Yang, S. Peng, Z. Xu, W. Shao, D. Pan, S. Ramakrishna, M. Zhu, Electrochim. Acta 247 (2017) 400–409.
[56] [57] [58] [59] [60] [61] [62] [63] [64]
11
C. Tran, V. Kalra, J. Power Sources 235 (2013) 289–296. M.M. Tomadakis, T.J. Robertson, J. Compos. Mater. 39 (2005) 163–188. Q.X. Wu, T.S. Zhao, W.W. Yang, Int. J. Heat Mass Transf. 54 (2011) 1132–1143. Q.X. Wu, L. An, X.H. Yan, T.S. Zhao, Electrochim. Acta 133 (2014) 8–15. R.G. Mei, J.J. Xi, Lei Ma, L. An, F. Wang, H.Y. Sun, Z.K. Luo, Q.X. Wu, J. Electrochem. Soc. 164 (2017) F1556–F1565. A.L. Andrady, Science and Technology of Polymer Nanofibers, John Wiley & Sons, Hoboken, NJ, 2008. R. Wang, Y. Liu, B. Li, B.S. Hsiao, B. Chu, J. Membr. Sci. 392–393 (2012) 167–174. W. Lehnert, H.A. Gasteiger Vielstich, A. Lamm, Handbook of Fuel Cells—Fundamentals, Technology and Applications, vol. 3, John Wiley & Sons, New York, 2003, pp. 517–537. R.S. Barhate, C.K. Loong, S. Ramakrishna, J. Membr. Sci. 283 (2006) 209–218. M. Kok, J. Gostick, J. Membr. Sci. 473 (2015) 237–244. Q.X. Wu, T.S. Zhao, R. Chen, W.W. Yang, J. Power Sources 191 (2009) 304–311. J. Biesdorf, A. Forner-Cuenca, T.J. Schmidt, P. Boillat, J. Electrochem. Soc. 162 (2015) F1243–F1252. C.Y. Wang, Chem. Rev. 104 (2004) 4727–4766. P. Piela, R. Fields, P. Zelenay, J. Electrochem. Soc. 153 (2006) A1902–A1913.