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Fabrication Method for Laboratory-Scale High-Performance Membrane Electrode Assemblies for Fuel Cells Custom catalyst-coated membranes (CCMs) and membrane electrode assemblies (MEAs) are necessary for the evaluation of advanced electrocatalysts, gas diffusion media (GDM), ionomers, polymer electrolyte membranes (PEMs), and electrode structures designed for use in next-generation fuel cells, electrolyzers, or flow batteries. This Feature provides a reliable and reproducible fabrication protocol for laboratory scale (10 cm2) fuel cells based on ultrasonic spray deposition of a standard Pt/carbon electrocatalyst directly onto a perfluorosulfonic acid PEM. Megan B. Sassin,*,† Yannick Garsany,‡ Benjamin D. Gould,† and Karen E. Swider-Lyons*,† †
U.S Naval Research Laboratory, Washington, District of Columbia 20375, United States Excet Inc., Springfield, Virginia 22151, United States
‡
S Supporting Information *
electrochemical evaluation of advanced ORR electrocatalysts by rotating disk electrode voltammetry was highly dependent on the electrode morphology.6 Once protocols were established to reliably produce uniform electrodes, the expected catalyst activity was reproducibly observed.6−10 The evaluation of advanced materials in a fuel cell setup is even more challenging as PEMFCs contain multiple inherently complex components and all components are evaluated at once. At the heart of the PEMFC is a membrane electrode assembly (MEA) that is comprised of catalyst layers (CLs), a polymer electrolyte membrane (PEM), and GDM, as shown in Figure 1. The micrometer-thick cathode and anode CLs on opposing
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ext-generation fuel cells, electrolyzers, and metal-air batteries aim to meet the often conflicting demands of high-performance, low-cost, and long life through the use of advanced materials (catalysts, membranes, gas diffusion media (GDM)) optimized for targeted operating conditions. For example, researchers are designing GDM to enhance water management in polymer electrolyte membrane fuel cells (PEMFCs), with the overall goal of significantly improving the performance at high current densities under high or low relative humidity conditions.1 New polymer electrolyte membranes (PEMs) are being designed that are less expensive than the standard perfluorosulfonic membranes or that function as anion exchange membranes (AEMs) that are stable at high pH/temperature, opening the opportunity for a variety of nonprecious metal electrocatalysts.2,3 Many are seeking to replace the standard Pt electrocatalysts with more durable and low cost materials that overcome the significant overpotential of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER).4,5 Elucidating the relationship between electrochemical performance and advanced material attributes (e.g., electrocatalyst kinetics, conductivity, surface functionality) and morphology (e.g., pore structure, interfaces) in ex situ setups is extremely challenging in terms of reproducibility and reliability. For example, we found that obtaining reliable data via ex situ © XXXX American Chemical Society
Figure 1. Illustration of the membrane electrode assembly in fuel cell test fixture (left) and the catalyst-coated membrane (right).
sides of the PEM (e.g., Nafion) together form the catalystcoated membrane (CCM). The CL structure contains a continuous network of ionomer (e.g., Nafion) and catalyst (e.g., Pt/carbon) in contact with the PEM to promote proton transport between the PEM and catalyst; the CL must also contain a through-connected pore network to facilitate mass transport of reactant gases to the catalyst and expulsion of product water from the CL (Figure 1). The CCM is transformed into an MEA by sandwiching it between GDM, which are typically bilayered, containing a macroporous carbon fiber-based layer (gas diffusion layer, GDL) and a carbon microporous layer (MPL). These layers are designed to
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formulation21−25 and based on those results, we down-selected an ink recipe that consisted of 33 wt % ionomer in the dry CL (Nafion/carbon mass ratio of 95:100). First, 0.40 g of Pt/ carbon catalyst powder is added to the glass vial followed by 8.65 g of ultrapure water, added dropwise with a pipet to wet the Pt/carbon catalyst. Then, the vial is gently shaken to ensure all of the Pt/carbon catalyst powder is wetted with water to prevent burning of the catalyst (fire hazard) when the alcoholbased components are added next. To the wetted Pt/carbon catalyst, 7.33 g of isopropyl alcohol (IPA) is added dropwise with a pipet. Next, 1.33 g of Nafion ionomer solution is added to the vial. Finally, an additional 7.97 g of IPA is added to the vial to increase the total volume of the ink for the next step. The freshly prepared ink is then mixed with a high shear mixer. The vial is secured to a ring stand with a three-prong clamp, and the mixing probe is inserted through the vial cap containing a predrilled hole in the center until the two lower holes of the probe are submerged in the ink. To prevent solvent evaporation, the probe fits snugly into the cap hole and the cap screws completely on the vial. The high shear mixer is then turned on and the speed slowly increased to level 4. After 5 h of mixing, the high shear mixer speed is reduced to 1 and then turned off. The vial is removed from the probe, a stir bar is added, capped with a lid to prevent solvent evaporation, and stirred at 750 rpm for 12 h on a stir plate. When ink is not in use, it should be kept stirring and should be used within 4 days. Ultrasonic Spray Deposition of CL onto PEM. The anode and cathode CLs are directly deposited onto the PEM; in our case a Nafion HP membrane, via a layer-by-layer method using the automated ultrasonic spray apparatus shown in Figure 2a. The base of the apparatus comprises a computer numerical control (CNC) router table with an adjustable head drive which serves as the holder for the ultrasonic nozzle. The head is controlled by MACH3 software to set fabrication variables such as spray height, spray speed, and automation of repeated steps. Beneath the head sits a heated vacuum plate retro-fitted with a porous aluminum (Al) plate with alignment pins to guide placement of the PEM (Figure S1). The ultrasonic spray system
facilitate water expulsion from the CL, transport heat, and provide an electronic pathway from the CL to the current collectors. Experimental MEAs are usually surrounded by rigid gaskets that serve as compression stops to prevent damage to the pore structure of the CL and/or GDM while it is compressed between the flow field plates and current collectors The gaskets also mitigate gas leaks from the fuel cell. Before advanced catalysts, PEMs, or ionomers can be electrochemically evaluated in a fuel cell setup with confidence, a reliable, reproducible, and lab-scale CCM fabrication method is necessary. Several CCM fabrication methods exist, including doctor blade/decal transfer, hand painting, air spraying, pulse spray swirl, ultrasonic spray deposition and ink jet printing.11−14 While all of these methods have merit, ultrasonic spray deposition is attractive as a laboratory method as it uses small-to-moderate amounts of material (e.g., catalyst, ionomer) and the process can be automated, greatly improving reproducibility. Recently, several groups reported the use of ultrasonic spray deposition in the literature to directly form Pt/ carbon CLs on PEMs to produce CCMs or onto GDM to produce gas diffusion electrodes (GDEs).15−17 In addition to a reliable and reproducible CCM, proper fuel cell assembly is also critical for the evaluation of advanced materials in a fuel cell setup. Improper assembly can lead to data that does not yield the anticipated performance improvements expected from the inclusion of advanced materials in a fuel cell. For example, we have shown that if during assembly of a Pt/carbon-based PEMFC the cell is overcompressed, damage to the pore structure of the GDM occurs, while if it is undercompressed, the cell has high ohmic resistance. Both circumstances result in low fuel cell performance, despite that a highperformance CCM was utilized and the structural integrity of the CCM was not altered.18 In this Feature, we give the procedural details for the reproducible fabrication of laboratory-scale (10 cm2) Pt/ carbon-based CCMs produced via ultrasonic spray deposition directly onto a perfluorosulfonic acid PEM. Subsequent processing steps are described for the transformation of the CCM into an MEA with commercial GDM and then assembly into a single cell PEMFC. We report the scientific rational behind each processing step and the data to show the direct impact of improper processing/assembly conditions on PEMFC performance. The structure of the CL and GDM are characterized with scanning electron microscopy (SEM) before and/or after electrochemical testing. The CL is characterized with cyclic voltammetry to extract the Pt electrochemical surface area (ECSA).19,20 Characterization of the MEA is done in a single fuel cell setup with constant voltage polarization techniques at a given condition (80 °C cell, 64 °C gases, relative humidity 50%, (RH50), ambient pressure in H2|air 2|2 stoichiometric flow to enable quick determination of the PEMFC performance; more thorough characterization at different conditions will be performed in the future, enabling optimization of the properties and structure of the MEA for targeted operating conditions. Attention is paid to requiring as little custom, expensive equipment as possible and making reproducible Pt/carbon-based PEMFCs. Detailed lists of chemicals, materials, and equipment can be found in the Supporting Information.
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EXPERIMENTAL METHODS Catalyst-Coated Membrane (CCM) Fabrication. Catalyst Ink. Numerous studies have been conducted on ink
Figure 2. (a) Schematics of ultrasonic spray deposition apparatus and (b) ultrasonic spray pattern showing paths #1−4. B
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Figure 3. Schematic of steps for deposition of CL#1 (a−c) and CL#2 (d−f).
The desired Pt loading is achieved by adjusting the number of layers sprayed. Each spray path (layer) described in Figure 2b deposits ∼0.041 mgpt cm−2. Increasing the number of layers to 8 (2 sets of spray paths #1−4) increases the loading to 0.33 mgpt cm−2. After the desired Pt loading is obtained, the CL#1| PEM is dried for 10 min on the heated vacuum plate and then removed and stored at ambient conditions overnight. We determined that optimal area-normalized current density is obtained at a loading of 0.33 mgptcm−2 and use this loading for all CCMs in this tutorial. Deposition of CL#2 (Cathode) Onto CL#1|PEM. The steps necessary to deposit CL#2 (cathode) to form CL#1| PEM|CL#2 are illustrated in Figure 3d−f and in a video format in the Supporting Information. First, the thick protective backing is removed from the CL#1|PEM, and CL#1|PEM is adhered to the acrylic mask with CL#1 facing away from the mask and centered in the window (Figure 3d). Wrinkles in the CL#1|PEM are gently smoothed out by hand. Next, the thick protective backing is placed back on the heated (80−90 °C) porous Al vacuum plate (Figure 3e) and the acrylic mask + CL#1|PEM is placed onto the vacuum plate with the uncoated side of the PEM facing up toward the ultrasonic spray nozzle for subsequent deposition of CL#2 to form the complete CCM (CL#1|PEM|CL#2) as shown in Figure 3f. After the desired Pt loading is obtained, the complete CCM is dried for 10 min on the heated vacuum plate and then removed and stored at ambient conditions. Estimation of Pt Loading in CL. The CL Pt loading is estimated by depositing the CL onto 4 coupons (5 cm × 5 cm) of preweighed fiber-reinforced Teflon using the same procedure as described for “deposition of CL#1.” The CL-coated fiberreinforced Teflon samples are dried overnight at ambient conditions and then weighed 5 times to obtain an average value. The weight of the CL is the difference of the average weight of the CL + fiber-reinforced Teflon from the weight of the uncoated fiber-reinforced Teflon. The Pt loading (mgPt cm−2) and wt % ionomer in the dry CL are calculated with eqs 1a,1b, and 2, respectively.19,20
consists of a 10 mL PTFE-tipped plunger and dispenser syringe and a syringe pump that controls the ink flow rate delivered to the ultrasonic narrow spray atomizer nozzle. A pinpoint spray shaper is connected to the ultrasonic narrow spray atomizer nozzle, and a gas inlet port at the side of the spray shaper forces the catalyst ink droplet into the pinpoint spray pattern down to 250 μm at the focal point. The catalyst spray pattern is designed with SOLIDWORKS and consists of four separate and distinct paths to ensure a uniform coating of the catalyst layer on the PEM (Figure 2b). Each spray line is 33.6 mm long and 10 mm apart. Spray path #1 corresponds to layer #1, spray path #2 corresponds to layer #2, spray path #3 corresponds to layer #3, and spray path #4 corresponds to layer #4. Spray path #1 and spray path #3 intertwine and are 5 mm apart, while spray path #2 and spray path #4 intertwine and are also 5 mm apart. Once the spray patterns are finalized, they are converted into G-code using FlashCut CNC (see the Supporting Information for code). Deposition of CL#1 (Anode) Onto PEM. A schematic of the procedure for the deposition of CL#1 (anode) to form CL#1|PEM is shown in Figure 3a−c and a video of the process is available in the Supporting Information. Prior to the deposition of CL#1, the PEM is cut into pieces (76.2 mm × 76.2 mm) and alignment holes are added (Figure S2). The thin protective backing on the Nafion HP membrane is removed, but the thick protective backing is left attached. Two pieces of the PEM are placed next to each other on the heated (temperature set between 80 and 90 °C) porous Al vacuum plate (Figure 3a,b). The acrylic mask (Figure S3) is placed on top of the PEM with the two deposition windows centered on the PEM (Figure 3c). The catalyst ink is loaded into the PTFE tipped plunger and dispenser syringe and inserted into the syringe pump, and the flow rate is set to 65 μL min−1. Once the ink reaches the ultrasonic narrow spray atomizer nozzle, the ultrasonic generator is turned on and set to 66% of the full power, the gas solenoid valve is turned on, the pressure delivered to the pinpoint spray shaper is set to 0.5 psi, and spraying commences. With this procedure, the two pieces of PEM are sprayed sequentially. When spray path #1 on PEM #1 is complete, the ultrasonic spray nozzle traverses to PEM #2 and spray path #1 is sprayed. This allows layer #1 sprayed on PEM #1 to dry prior to deposition of path #2.
wt%Pt CCM = C
(Pt wt%catalyst × catalyst wt) + ionomer dry wt catalyst wt + ionomer dry wt
(1a)
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the flow field plate with Power Duster. One or more gaskets with half the total thickness required (from eq 3) are placed on the anode flow field plate (Figure S8, step 2). Then, one piece of Sigracet 25BC GDM is placed with the MPL side up in the window of the gaskets (Figure S8, step 3) and the CCM is placed with CL#1 (anode) side down on top of the GDM (Figure S8, step 4). Next, the remaining gaskets are placed on top of the CCM (Figure S8, step 5), followed by the second Sigracet 25BC GDM with the MPL side facing down toward the CL (Figure S8, step 6). The cathode flow field plate and end plate are placed on the MEA assembly (Figure S8, step 7) and the end plate bolts are hand screwed to secure the fixture (Figure S8, step 8). The bolts are torqued with an electronic torque wrench using the following sequence 1 N m, 3 N m, 6 N m, 9 N m, and 10 N m. A schematic of the components in the assembled cell and details of a flow field plate are shown in Figure S9. Electrochemical Evaluation. We used the following test conditions for a cursory evaluation of the PEMFC performance. The cell is evaluated at ambient pressure and the cell temperature is set to 80 °C with the gases set to 64 °C (RH50). Reactants can be fed to the fuel cell either at fixed flow rate or at a flow rate that is dependent on the cell current. The latter is called load-based flow control (L/min/amp/cell). We choose to test our MEAs using load-based flow control in which the H2 and air will be supplied at rate such that the stoichiometry will be 2x and 2x, respectively (stoichiometric flow conditions of 2|2 for H2|air). The cell is subjected to the following “break-in” sequence in H2|air: 0.60 V for 2 h, followed by 20 cycles alternating between 0.70 and 0.40 V with each voltage held for 10 min. Polarization Curve. The same cell and gas temperatures of 80 °C and 54 °C, respectively, are used to collect the polarization curve. The polarization curves are obtained by measuring the current at applied voltage (0.90, 0.88, 0.85, 0.80, 0.74, 0.70, 0.64, 0.60, 0.55, 0.50, 0.45, and 0.40 V); each voltage is held for 15 min, collecting 6 points min−1. The last 30 points are averaged for analysis purposes. Cell resistance is measured by the current interrupt method for currents above 100 mA cm−2. If data in H2|O2 is desired, then the polarization curve sequence should be repeated using a stoichiometric flow of 2| 10, respectively. Cyclic Voltammetry. After polarization curves are collected, the cell temperature is decreased to 30 °C, the gas temperatures to 50 °C, and the gases are changed to H2|N2 at fixed flow rates of 0.2 LH2 min−1 and 0.05 LN2 min−1. The load leads are removed from the test stand, and the leads from the potentiostat are connected to the test fixture, with the anode serving as both the reference and counter electrode. The cathode voltage is scanned from 0.06 to 1.15 V at a potential scan rate of 20 mV s−1, and a total of 3 scans are collected. The total charge for hydrogen adsorption is determined by integrating between a straight baseline drawn from the double layer capacitance region (i.e., 0.40−0.50 V) and the final minimum voltage (Figure S10). A correction for double layer charging is done by subtracting the current observed at 0.40 V from the total current. The electrochemical surface area (ECSA) of the Pt is determined from the hydrogen adsorption charge and the characteristic value of charge density associated with a monolayer of hydrogen adsorbed on polycrystalline platinum, 210 μC cm−2Pt, as shown in eq 4:
mdry‐CL × wt%Pt CCM geometric area
(1b)
where mdry‑CL is the weight of the dry CL (mg), wt % Pt is the weight percentage of Pt in the dry CL, and CCM geometric area is the CL geometric surface area (cm2). The weight % of ionomer in the dry CL is calculated: wt%ionomer =
ionomer dry wt × 100 catalyst wt +ionomer dry wt
(2)
The average Pt loading and resulting standard deviation for 4 different CLs produced by spraying 4, 8, and 12 layers are 0.17 ± 0.02 mgpt cm−2, 0.33 ± 0.02 mgpt cm−2, and 0.50 ± 0.01 mgpt cm−2, respectively. Postprocessing of the CCM. Hot-Press of CCM. Two inhouse fabricated Al platens (Figure S4) and two fiberreinforced Teflon sheets are cleaned with a lint-free wipe wetted with isopropanol (IPA) and dried with Power Duster to remove debris (Figure S5, step 1). One fiber-reinforced Teflon sheet is placed on the bottom Al platen (Figure S5, step 2) and the CCM is placed with CL#1 (anode) facing down on the fiber-reinforced Teflon sheet (Figure S5, step 3). The other fiber-reinforced Teflon sheet is placed on top of the CCM (Figure S5, step 4) and the second Al platen in placed on top (Figure S5, step 5). The assembly is inserted into the hot-press set at 135 °C, a type J thermocouple is inserted into the hole on the bottom Al platen, and the assembly is pressed until the hotpress plates touch the Al platens (Figure S5, step 6, pressure on entire CCM is ∼0.068 MPa). Once the thermocouple reads 135 °C, the timer is set for 2 min. The assembly is then removed from the press and cooled to room temperature before removing the CCM. Hot-Press of MEA. The same steps are used as in the hotpressing procedure of the CCM, except that the GDM are placed on either side of the CLs prior to hot-pressing. Assembly of a Single Cell. Others have described similar procedures on cell assembly,26,27 but the protocol is described here in detail for clarity. Setting Appropriate MEA Compressive Stress with Rigid Gaskets. The thicknesses of the CCM and GDM are measured with a digital micrometer at 9 evenly spaced locations over the component area and these measurements are averaged to calculate the component thickness and used to determine the required gasket thickness (Figure S6). We have previously reported that an MEA compression of 14% yields the highest performance for the materials described in this tutorial.18 This percent compression is met in experimental cells through using rigid gaskets with thicknesses according to eq 3: gasket thickness (mil) =
total MEA thickness − (total MEA thickness × 0.14) 25.4 (3)
where the total MEA thickness is the CCM thickness + the GDM thicknesses (μm). Teflon gaskets are designed in SOLIDWORKS (Figure S7) and cut with a laser engraver/ printer set at 70% speed and 40% power. Assembly of Single Cell Test Fixture. The test fixture is mounted on a vice with the anode end plate on the bottom (Figure S8, step 1). The graphite flow field plate is cleaned with a lint-free wipe wetted with IPA and debris are removed from D
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Q Hads 210LCathAgeo
evaporate before it reaches the PEM, leading to crack-free CLs. The thickness of the CL is independent of the nozzle height and scanning electron micrographs reveal that porosity persists throughout the thickness of all CLs, regardless of nozzle height. The structure of the CL is also similar for all nozzle heights and is comprised of an interconnected network of ∼100 nm carbon particles with 2-to-5 nm Pt particles and 10−200 nm pores. The lack of change in the CL structure is expected, as the ink formulation was not changed (e.g., ionomer/carbon ratio). Figure 4b shows the impact of ultrasonic nozzle on PEMFC performance. The current density at 0.85 V (kinetic region) shows minimal dependence on ultrasonic nozzle height and ranges from 87 ± 2 mA cm−2 to 80 ± 4 mA cm−2 for CLs deposited at 3.5 and 6.4 cm, respectively. This is expected as the ultrasonic nozzle heights used in this study do not result in drastic differences in the CL structure. However, differences in electrochemical performance as a function of nozzle height become apparent at voltages
