Directly deposited Nafion/TiO2 composite membranes for high power medium temperature fuel cells

This work presents a simple production method for TiO2 reinforced Nafion® membranes which are stable up to a 120 C operation temperature. The novel TiO2 reinforced membranes yield a maximum power density of 2.02 W cm 2 at 120 C; H2/O2; 0.5/0.5 L min ; 90% RH, 300/300 kPaabs. This is 2.8 times higher than the highest power density for TiO2 reinforced membranes so far published in literature. The described membranes even exceed the maximum power density of a commercial Nafion® HP membrane in an identical measurement setup at 100 C and 120 C. Compared to the commercial Nafion® HP membrane the maximum power density was increased by 27% and 9% at 100 C and 120 C, respectively. The membrane is manufactured by drop-casting a dispersion of Nafion® and TiO2 nanoparticles onto both the anode and cathode gas diffusion electrodes. Furthermore pure Nafion® membranes manufactured by the same method had higher membrane resistances at temperatures >100 C than TiO2-reinforced Nafion® membranes.


Introduction
Medium temperature polymer electrolyte membrane fuel cells (MT-PEMFC), typically operating at temperatures from 100 to 120 C, bear several advantages compared to the more common low temperature PEM fuel cells (LT-PEMFC), operating at 80 C. [1][2][3][4][5][6][7] The fuel cell cooling systems, for instance in automotive applications, can be strongly simplied due to the higher temperature gradient from the fuel cell stack to ambient temperature. 7 Moreover, the kinetics of the anode and cathode reactions are enhanced at elevated temperatures. 3,6 Furthermore, the catalyst poisoning gas CO reacts to form CO 2 more quickly at higher temperatures, effectively enhancing the catalyst poisoning tolerance. 1,5,6 Consequently, operating fuel cells at even higher temperatures would be benecial. However, at temperatures above 120 C the fuel cell stack start-up procedure becomes time-consuming, and thus less suitable for automobile applications. 6 Furthermore, Naon based membranes proton conductivity diminishes at such temperatures. 3 For this reason, MT-PEMFCs, typically operating at 100-120 C are increasingly of interest and subject to signicant recent research. 5,8 Here, major issues are large decits in terms of cell performance and durability. 9,10 Common approaches to meet the requirements of an MT-PEMFC include reinforcement of traditional Naon® membranes by numerous additives such as inorganic nanoparticles, nanobers, zeolites, or temperature-stable polymers. 6,8,[11][12][13][14][15][16][17][18] These additives were found to enhance mechanical and chemical stability at higher temperatures. This enables MT-PEMFC operation above 80 C also with Naon® as membrane ionomer. Furthermore, additives such as TiO 2 nanoparticles are reported to improve membrane hydration at elevated temperatures. 3,15,19 Without additives, Naon® membranes operated above 80 C typically show a strong decrease in proton conductivity with increasing temperature. 9,17,19 In recent work, direct membrane deposition (DMD) was developed as novel fabrication method for LT-PEMFC membrane electrode assemblies (MEA). 20 This method consists in replacing the conventional membrane foil by two Naon® layers deposited directly on top of anode and cathode gas diffusion electrodes (GDEs). The fuel cell is assembled with the GDEs facing each other. Very low membrane resistances of 12.7 mU cm 2 were reported, enabling high current densities of about 5 A cm À2 at 0.6 V and power densities of 4 W cm À2 (operation conditions: H 2 /O 2 ; 0.5/0.5 L min À1 ; 70 C; 100% RH, 300/300 kPa abs ). 20 In this work, a TiO 2 reinforced, Naon®-based MT-PEMFC is presented that was manufactured alike with a directly deposited membrane. By simple pipette drop-casting of a mixture of Naon dispersion and TiO 2 nanoparticles onto gas diffusion electrodes, a thin fuel cell membrane of only 15 mm thickness was realized, yielding a power density of more than 2 W cm À2 at 120 C operation temperature.

Experimental
Drop-casting was used to coat gas diffusion electrodes (GDEs) of a proton exchange membrane fuel cell with a dispersion containing Naon® as ionomer and TiO 2 nanoparticles as additive. The GDEs (5 cm 2 , Paxitech SAS, France) contained a catalyst loading of 0.5 mg Pt cm À2 with 70% Pt/C on each anode and cathode side. The dispersion consisted of a 1 : 3 mixture of Naon® D2020 (Dupont) and 2-propanol. As additive, 5 wt% per Naon® content of TiO 2 nanoparticles were added to the dispersion. This mixture yields a nal 20 : 1 Naon : TiO 2 ratio of solids. The TiO 2 nanoparticles were purchased as nanopowder from Sigma-Aldrich with an average diameter of 21 nm, a surface area of 35-65 m 2 g À1 and a crystal phase ratio of 80/20% anatase/rutile.
The ionomer solution was distributed manually by a pipette (Eppendorf) to cover the 5 cm 2 sample area homogeneously with a 100 mL dosage. A 100 mL deposition of this dispersion resulted in an average deposited membrane loading (weight of membrane ionomer per cm 2 ) of 1.04 AE 0.04 mg Naon per cm 2 on the GDE. This was in agreement with the calculated 1.05 mg of Naon® in the 100 mL solution. From the average increase in mass of the GDE, we calculated an average membrane thickness gain of 5 mm per 100 mL deposition. In this work, the membrane thickness was varied in 5 mm steps, with subsequent annealing steps of 15 min at 80 C in between. Accordingly, each drop-casting of 100 mL dispersion led to an increase of 5 mm membrane thickness. A schematic representation of the MEA investigated in this work is depicted in Fig. 1.
The ionomer-coated GDEs were assembled face to face with a 50 mm thin polytetrauorethylene (PTFE) subgasket (Scribner, USA) in-between, as depicted in Fig. 1. The subgasket had a 2 Â 2 cm 2 opening in order to seal the GDE edges from internal shorting and gas crossover. Thus, the resulting active fuel cell area was 4 cm 2 . Two 200 mm thick rubber gaskets (Paxitech SAS, France) were used to obtain a suitable compression and sealing of the GDEs. We used graphite ow plates with serpentine ow channels (Scribner Associates Inc., USA). The assembly was torqued to 5 N m.
To investigate the benecial impact of TiO 2 reinforcement at temperatures beyond 80 C, we compared the measurements to a reference cell with a pure Naon® membrane that was also manufactured by direct membrane deposition. The reference dispersion consisted of a 1 : 3 mixture of Naon® D2020 (Dupont) and 2-propanol. First experiments with approximately 10 mm thick membranes showed a signicantly reduced open circuit voltage (OCV) for experiments with the reference (pure Naon®) directly deposited membrane at a temperature of 120 C. In order to circumvent this loss in OCV and ensure comparability, membranes with a total thickness of 20 mm were produced and compared in this work.
For further investigations, we reduced the thickness of the directly deposited TiO 2reinforced Naon membrane to 15 mm, as a reduction of the membrane thickness resulted in an improved fuel cell performance. All results from morphology analysis were performed using the 15 mm thick TiO 2 /Naon composite DMD sample. A commercial reference MEA was purchased (Paxitech SAS) comprising a Naon® HP membrane, identical gas diffusion media and equal catalyst loading as used for the DMD fuel cells. The 20 mm thick Naon® HP membrane was the thinnest reinforced membrane commercially available. It is designed for high power densities at lower relative humidity environments and high operating temperatures, in accordance to the demands on the TiO 2 reinforced DMD sample. This allowed a comparison of our novel reinforced directly deposited membrane to a high-performance reinforced commercial membrane.
To investigate cell performance and to evaluate the membrane resistance, a Scribner 850e fuel cell tester with integrated frequency response analyzer was used. The high frequency resistance (HFR) values were measured continuously within the 850e fuel cell tester at a frequency of 3.2 kHz. The fuel cell xture was encased in two layers of 3.2 mm thick ceramic paper (Krager Industrieprodukte GmbH) in order to enable cell temperatures up to 120 C. All measurements performed in this work were conducted with xed ows of 0.5/0.5 L min À1 H 2 /O 2 . Relative humidity (RH) was adjusted to 90%. All experiments were performed at 300/300 kPa abs . These optimized operation conditions were chosen in order to exclude performance limitations due to mass transport issues and to achieve comparability between the different samples. The break in procedure consisted in cycling polarization curves until steady operation conditions were achieved (approximately aer 4 h).
The polarization data was acquired by sweeping the current density in 500 mA cm À2 steps at 5 min pt À1 from OCV to low voltages, with additional steps of 25 mA cm À2 at 1 min pt À1 from OCV to 250 mA cm À2 to resolve the kinetic region. Electrochemical impedance spectroscopy was conducted at a constant cell voltage of 750 mV, with AC frequency swept from 10 kHz to 0.1 Hz. Fig. 1 Scheme of the membrane electrode assembly (MEA) fabricated in this work. A thin TiO 2 -reinforced Nafion® layer is deposited directly on both anode and cathode gas diffusion electrodes. A thin subgasket prevents hydrogen and current crossover through the end faces of the active area.

Results & discussion
Morphology of the TiO 2 reinforced directly deposited membrane Scanning electron microscopy (SEM) was employed to investigate the morphology of the directly deposited TiO 2 reinforced membrane, Fig. 2. The cross-sections were prepared by cryofracturing the sample in liquid nitrogen. A low electron acceleration voltage of 2 kV was used to image the sensitive polymer layer. Fig. 2 shows the cross-section of the reinforced membrane layer between anode and cathode GDEs. The SEM images were taken aer fuel cell operation. As can be seen from the SEM image in Fig. 2a, the membrane had an approximate thickness of 15 mm. An SEM-image of the membrane cross-section with higher resolution is shown in Fig. 2b. Energy dispersive X-ray spectroscopy (EDX) measurements (shown in the ESI †) conrmed that the particles highlighted in white correspond to TiO 2 agglomerates. These nanoparticles tended to form agglomerates in the micrometer range and were distributed inhomogeneously throughout the membrane. Similar agglomerates have been reported for Naon/TiO 2 nanocomposite membranes in the literature. 23 One possible interpretation of this result is that stirring the ionomer dispersion for 24 h is insufficient to reach an agglomerate-free distribution of TiO 2 particles. Alternatively, particle agglomeration may occur during the time-delay between removing the solution from the stirrer and dispensing it on the sample surface.

Impact of TiO 2 reinforcement
Naon® direct deposited membranes with and without TiO 2 additives were compared to prove the effectiveness of the TiO 2 reinforcement. For comparison, we extracted the effective membrane resistance from the measured high frequency resistance. To calculate the effective membrane resistance it is important to take the resistance of the gas diffusion electrode into account. 21 The membrane resistance was calculated by subtracting the electrical resistance of the gas diffusion electrodes and the setup from the measured high frequency resistance (R membrane ¼ HFR À R electrode ). The electrical resistance of the gas diffusion electrode was determined to be 11.3 mU cm 2 , which was in accordance with reported values. 20,22 The membrane resistance over current density depicted in Fig. 3a showed the inuence of the TiO 2 enforcement on the membrane resistance for 80, 100, and 120 C.
The pure Naon® and TiO 2 -reinforced Naon® DMD fuel cells had a similar membrane resistance at 80 C. With increasing temperature, the effect of TiO 2 reinforcement became apparent. As in Fig. 3a, the pure Naon® DMD sample  showed a stronger increase in membrane resistance with increasing temperature compared to the TiO 2 reinforced Naon® DMD cell. Fig. 3b illustrates the membrane resistance at a current density of 1250 mA cm À2 for 80 C, 100 C and 120 C. At this current density, when temperature is increased from 80 C to 120 C, the membrane resistance of the TiO 2 -reinforced DMD increases by 10%, while the membrane resistance of a non-reinforced DMD sample increases by 54%. These results indicate that TiO 2 reinforced Naon® DMD cells have an enhanced temperature stability compared to pure Naon® DMD fuel cells. Furthermore the experiment conrms the benecial effect of TiO 2 on Naon® membrane performance at 100 C and 120 C as it was reported in the literature. [23][24][25] Performance comparison to state-of-the-art reinforced membranes To compare the performance of the TiO 2 reinforced membrane to state of the art commercial material, a Naon® HP membrane was used as reference sample. In Fig. 4, the evolution of cell potential and power density over current density is presented for operation temperatures of 100 C and 120 C. As expected, the peak power densities dropped for both samples with raising temperatures. We obtained a maximum power density of 1.85 W cm À2 for the Naon® HP reference sample at a temperature of 120 C, whereas the TiO 2 reinforced DMD fuel cell shows a peak power density of 2.02 W cm À2 . At 100 C, the Naon® HP fuel cell had a maximum power density of 2.28 W cm À2 , whereas the TiO 2reinforced DMD fuel cell had a maximum power density of 2.90 W cm À2 . To our knowledge, this value is beyond any other published data for MT-PEMFCs in literature so far. These results conrm the high potential of directly deposited membranes with TiO 2 reinforcement for fuel cell application in the temperature range between 100 C and 120 C. In the ESI † an additional comparison of the polarization data is provided, comprising all four fuel cells of this work (Naon HP, 20 mm pure Naon DMD, 20 mm TiO 2 /Naon DMD, 15 mm TiO 2 /Naon DMD) at 120 C operation temperature. Comparing both 20 mm DMD samples, from this representation the benecial impact of the addition of TiO 2 nanoparticles can be observed. Moreover also the 20 mm thick TiO 2 /Naon membrane shows higher performance compared the Naon HP reference in spite of comparable thickness.
The superior performance of the fuel cells presented in this work becomes especially obvious if it is compared to the literature of TiO 2 -Naon-PEMFC research. Table 1 lists recent results from literature. The maximum power density of the fuel cells assembled in this work reach 2.8 times the value of the best references found.
As revealed by electrochemical impedance spectroscopy measurements depicted in Fig. 5, we obtained a high frequency resistance (HFR) below 40 mU cm 2 for the TiO 2 -reinforced DMD fuel cell and the Naon® HP fuel cell at 120 C. The diameter of  Fig. 4 The graphs show a comparison of a directly deposited membrane fuel cell to a commercial Nafion® HP (DuPont) membrane fuel cell. For each membrane the current density characteristics of (a) cell voltage and (b) power density was evaluated at 100 C (black curves) and 120 C (red dashed curves). The operation conditions were: H 2 /O 2 ; 0.5/0.5 L min À1 ; 300/300 kPa abs , 90% RH.
the loop is a measure of the charge transfer resistance. 26 The results showed that the TiO 2 -reinforced DMD fuel cell had a lower charge-transfer resistance compared to the Naon® HP cell. Since identical catalyst layers were used, the improved charge transfer resistance indicated that the ionic connection between the catalyst layers and the membrane was improved. At 120 C an emerging second loop could be observed in the lowfrequency regime. This low-frequency arch is typically attributed to mass transport issues. 26

Conclusion
A simple membrane production method for medium temperature TiO 2 reinforced PEM fuel cells, based on the recently introduced concept of direct membrane deposition, is presented in this work. The membranes manufactured by the presented method enabled stable fuel cell operation with maximum power density of 2.90 W cm À2 at 100 C and 2.02 W cm À2 at 120 C (H 2 /O 2 ; 0.5/0.5 L min À1 ; 90% RH, 300/300 kPa abs ). This exceeds the maximum power density (2.28 W cm À2 at 100 C and 1.85 W cm À2 at 120 C) of a high performance state of the art Naon® HP membrane in an identical measurement setup. It also proves the high potential of directly deposited membranes for fuel cell application in the temperature range between 100 C and 120 C. Furthermore, the maximum power density of the fuel cells assembled in this work reach 2.8 times the value of the best TiO 2 reinforced membrane references found in the literature. By the introduction of TiO 2 nanoparticles into the Naon® membrane ionomer, fuel cell operation in the medium temperature range is signicantly improved. The reinforcement leads to lower membrane resistances at 100 C and 120 C compared to a pure Naon® directly deposited membrane. This behavior was related to improved humidication due to the hydrophilic character of the TiO 2 nanoparticles and conrms the results of several research groups with conventionally casted composite membranes. [23][24][25] The medium temperature range for fuel cell operation is of high interest for automotive and stationary applications. Therefore, direct membrane deposition might open the way for signicantly reduced cost per watt ratio due to lower material consumption and higher cell power densities. Direct membrane deposition enables new possibilities in the design of high performance membranes: due to the easy manufacturing approach, various modications of a single membrane layer can be realized. A next step could be the investigation of alternative hydrophilic additives such as SiO 2 or zeolites.