Acid Modified Carbon Support with Proton Conductive Properties for Polymer Electrolyte Membrane Fuel Cell Applications
Authors: Wu Dan , Fujigaya Tsuyohiko 
Published: December 2024

Proton exchange membrane fuel cell (PEMFC) are one of highly efficient electrochemical energy conversion technologies, with a potential to replace conventional fossil-fueled power generators for vehicles and stationary applications. One of the challenges for widespread use of PEMFC is to minimize the amount of platinum (Pt) to lower the cost of PEFCs. Therefore, optimizing electrocatalyst design to improve Pt mass activity is required. In the conventional PEMFC electrocatalyst, the proton conductive polymers such as Nafion are contained to transport protons to the Pt surface. However, the main problem of Nafion in catalyst layer (CL) is the inhomogeneously covered on commercial catalyst of Vulcan/Pt (CB/Pt) surface leads a low proton conductivity or high O2 diffusion resistance. Furthermore, the SO3
− groups of the polymers are strongly adsorbed onto the Pt surface and decreasing Pt active sites to lower the catalytic activity. Hence, it’s necessary to develop a new CL without using ionomer (Nafion).
An acid modified carbon support with proton conductive properties was considered for PEMFC applications. In our previous study, we synthesized a high-density -SO3H modified carbon support (named SCB) by covalent bonding and loaded Pt on SCB (SCB/Pt) to conductive proton. Attained a higher O2 diffusivity at high current density range because without Nafion added [1]. However, this method of destroying the sp2 structure of carbon has the problem of reducing the durability and electrical conductivity of carbon. In this work, we changed the acid modify carbon method from chemical modification to physical modification which using proton conductivity polymer adsorb on carbon surface to conductive proton. Polybenzimidazole (PBI) having a side chain of trifluorosulfonimide (PBI-TFSI) was chosen as a proton conductor adsorbed on carbon surface because PBI-TFSI was reported to have high proton conductivity [2].
In this study, instead of adding Nafion as a proton conductor after loading Pt on carbon support, Pt is directly loaded on carbon support coated with PBI-TFSI to fabricate novel catalyst, CB/PBI-TFSI/Pt, and CL was fabricated. Absence of ionomer is expected to improve O2 diffusivity in the CL and also avoid specific adsorption of SO3
− groups on the Pt surface. Principal experiment of this study is synthesis composite, characterize the catalyst properties and evaluate of electrochemical cell performance. As the control catalyst, commercial CB/Pt (TKK10V30E) was used.
The PBI-TFSI content of CB/PBI-TFSI was estimated by CHN elemental analysis, which revealed that its C: H: N atomic ratio was 0.37: 95.59: 0.61. Considering that all the nitrogen was originating from PBI-TFSI, and side chain of graft ratio is 73.25%, the PBI-TFSI content of CB/PBI-TFSI was determined to be 4.5%. Thermal stability of the catalysts was compared by thermogravimetric analysis (TGA). As the results, CB/PBI-TFSI displayed a one-step weight loss similar to that of CB with a degradation temperature around 650°C, while SCB started to drop at 550°C [1]. This superior thermal stability of CB/PBI-TFSI compared to SCB was because sp3 defect was not introduced by using polymer wrapping method, while chemical modification method used to SCB introduced sp3 defects. Such a good thermal stability is expected to offer the high electrochemical durability in the PEMFC operation. The Pt loading of CB/PBI-TFSI/Pt determined from the TGA weight residue at 900 °C was 30 wt% comparable with CB/Pt. The size of the Pt nanoparticles in both CB/PBI-TFSI/Pt and CB/Pt was analyzed by TEM. The determined sizes of the Pt nanoparticles in CB/PBI-TFSI/Pt and CB/Pt were 2.04 ± 0.3 and 2.10 ± 0.2 nm, respectively. The comparable Pt loading and size of Pt nanoparticles in CB/Pt and CB/PBI-TFSI/Pt mean that these samples can be used in comparative studies.
For the electrochemical characterization, the electrochemical active surface area (ECSA) was determined using the hydrogen adsorption/desorption (HAD) method and the polarization curves are operated using H2/air. The results indicated that the cell of CB/PBI-TFSI/Pt exhibits activity over 1.0 A/cm2 current density even without ionomer. However, compare with commercial catalyst of CB/Pt with Nafion, CB/PBI-TFSI/Pt still shows a lower cell performance because the higher resistance between Nafion membrane and CL. Additional improvements to the proton conductivity of the acid-grafted carbon support and the reduction of the interfacial resistance between PEM and CL could potentially further increase the PEMFC performance. O2 diffusivity as well as the fuel cell operation under the low humidity will also discuss.
[1] Yoshihara R, Wu D, Phua YK, Nagashima A, Choi E, Jayawickrama SM, et al. Journal of Power Sources. 2022;529:231192.
[2] Han H, Miura H, Motoishi Y, Tanaka N, Fujigaya T. Polymer Journal. 2021;53:1403-11.

Figure 1