Carbon Nanotube-Based Catalyst Modification to Improve Proton Exchange Membrane Fuel Cell Interlayer Interactions
Authors: Ai Ling , Zhao Ziyu , Zhou Zeyu , Yang Xiaocheng , Zhai HENG , Holmes Stuart 
Published: August 2024

Proton exchange membrane fuel cells (PEMFC) were identified as a highly viable solution for addressing climate-related issues. Previous theoretical investigations have underscored the significance of Pt nanoparticles as the optimal metal for hydrogen fuel cell catalysts due to their special activity and selectivity. [1] Current works focused on the utilisation, durability, and site activity of Pt particles on support, and most of the schemes achieved performance enhancement by loading Pt onto porous support with different morphology such as graphene, carbon fibre, and carbon black. [2] Some schemes have also incorporated cost considerations to achieve lower Pt loading. [3] However, the design of the catalyst layer (CL) structure in the membrane electrode assembly (MEA) must consider the interactions between the layers.
Addressing the crucial aspects of water management, low contact resistance, and the establishment of effective three-phase boundary for MEA, multi-walled carbon nanotubes (MWCNTs) are a promising CL support due to their intrinsically high hydrophobicity, high axial electrical conductivity, and potential for ordered alignment. [4] However, the drawbacks of MWCNTs such as strong agglomeration, wall surface chemical inertness, and unopened ends are unfavourable for Pt nanoparticle loading, which is detrimental to MEA processing and leads to inhomogeneous CL surfaces. This further deteriorates the utilisation of Pt and increases the contact resistance. Previous efforts have explored methods such as introduction of polar functional groups on MWCNT using strong reflux oxidation condition, or nitrogen doping to construct localised pyridine structure. [5, 6] However, over-functionalisation can cause MWCNT to lose its favourable structure and properties.
In this work, MWCNT was modified based on the operational requirements of the MEA from the viewpoint of interlayer interactions, including the search for the optimal degree of oxidation, N-doping, and micro-arrangement. MWCNT were functionalise by oxidising, N-doping, as well as micro-alignment to achieve lower contact resistance between CL and proton exchange membrane (PEM), better hydrophobicity, and enhanced performance. Furthermore, this work expects to construct a more continuously distributed three-phase boundary by aligning MWCNT to form a locally ordered structure, which is essential for the efficient utilisation of Pt active sites.
Preliminary experiments in this work have tested a range of reflux oxidation conditions for MWCNT treated with concentrated nitric acid ranging from 0 to 10 h. The maximum power density of 979.81 mw cm-2 was achieved by Pt loading on 6h MWCNT oxidation time (Pt-MWCNT 6h). This represented a 59.53% improvement over the commercial Pt/C catalyst of 614.17 (mw cm-2) (Figure 1 (a)). In addition, due to the stronger electrical conductivity, the charge transfer resistance of Pt-MWCNT6h in the electrochemical impedance spectroscopy (EIS) test was 0.09 Ohm cm-2, which was 48.86% lower than that of Pt/C (Figure 1 (b)). This study will discuss the developed catalysts and their efficacy in a working fuel cell system.
In conclusion, this research proposed alternative perspectives on the design of the CL based on the actual operational characteristics of MEA with emphasised on the interlayer interaction between CL and other layer. Additionally, the research will validate the impact of low-functionalization modification of MWCNTs on the performance of PEMFC, which simplifying the preparation challenges of CL and contributing for the widespread commercial application of PEMFCs on a larger scale.

References

Seh, Z.W., et al., Combining theory and experiment in electrocatalysis: Insights into materials design. Science, 2017. 355(6321): p. eaad4998.

Sui, S., et al., A comprehensive review of Pt electrocatalysts for the oxygen reduction reaction: Nanostructure, activity, mechanism and carbon support in PEM fuel cells. Journal of Materials Chemistry A, 2017. 5(5): p. 1808-1825.

Tian, Z.Q., et al., A Highly Order-Structured Membrane Electrode Assembly with Vertically Aligned Carbon Nanotubes for Ultra-Low Pt Loading PEM Fuel Cells. Advanced Energy Materials, 2011. 1(6): p. 1205-1214.

Lehman, J.H., et al., Evaluating the characteristics of multiwall carbon nanotubes. Carbon, 2011. 49(8): p. 2581-2602.

Hernández-Fernández, P., et al., Functionalization of multi-walled carbon nanotubes and application as supports for electrocatalysts in proton-exchange membrane fuel cell. Applied Catalysis B: Environmental, 2010. 99(1): p. 343-352.

Gribov, E.N., et al., Effect of modification of multi-walled carbon nanotubes with nitrogen-containing polymers on the electrochemical performance of Pt/CNT catalysts in PEMFC. Materials for Renewable and Sustainable Energy, 2019. 8(1): p. 7.

Figure 1