Novel Materials and Architecture for Cathode of Polymer Electrolyte Fuel Cells
Date
2020-09-18
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Abstract
The widespread adoption of polymer electrolyte fuel cell (PEFC) technology is currently limited by the challenges including catalyst activity, local O2-transport, catalyst durability, and water transport pertinent to high current density operation, specially at low Pt loaded electrodes. In addition to the development of new materials, meticulous understanding of the cathode components such as the catalyst layer (CL) and its constituent materials, and microporous layer (MPL) is required, which is difficult to achieve with the random and uncontrolled microstructure of conventional CL and MPL. This dissertation explores alternative cathode CL and MPL architectures guided by the known technical challenges limiting the wider commercialization of the PEFC technology on one hand and transport processes related scientific questions on the other hand. We introduced a new bottom-up designed spherical carbon support with intrinsic N-doping that permits uniform dispersion of Pt catalyst, which reproducibly exhibits record-high ORR mass activity at 0.9 V of ~ 0.6 A mgPt-1 at 100% relative humidity (RH) and ~ 1.0 A mgPt-1 at 60% RH, higher than any reported Pt catalyst in a membrane electrode assembly (MEA). The uniformly distributed N-functional surface groups on the carbon support surface promotes high ionomer coverage directly evidenced from high-resolution electron microscopy and nearly humidity-independent double layer capacitance. The hydrophilic nature of the carbon surface appears to ensure high activity and performance over a broad RH operation. The paradigm challenging large carbon support (~ 135 nm) combined with favorable ionomer film structure, hypothesized recently to arise from the interactions of ionic moiety of ionomer and N-functional group of the catalyst support, results in an unprecedented low oxygen transport resistance for ultra-low Pt loading (36 gPt cm-2). This thesis also introduced a model PEFC CL architecture by incorporating an electrocatalyst and an ionomer into nanoporous carbon scaffold (NCS – developed in Birss group), a nanostructured model carbon support and investigated in an operational fuel cell. Series of in-situ and ex-situ characterization techniques, including electrochemical and morphological characterization, were performed to investigate the ionomer coverage and connectivity in the CL and to perceive the gravity of ionomer in Pt utilization and PEFC performance. The NCS CL showed similar performance as the conventional CL despite of having almost 3 times lower roughness factor. Furthermore, NCS CL exhibited almost 1.7 times lower local O2-transport resistance compared to the conventional counterpart. In addition, the incorporated catalyst (Pt) showed record durability exhibiting only 15% electrochemical surface area loss after 30,000 cycles (square wave, 0.6 – 1.0 V) and no significant loss in mass activity. Upon recognizing the superior mass transport property of NCS through the CL study, NCS (~ 85 nm pore size, 25 μm thick) with ordered pore structure and controlled wettability was introduced as an MPL in a PEFC. Unlike previous studies, the wettability of the MPL was controllably modified without altering the pore structure through chemical functionalization of the pore surfaces. Ex-situ environmental scanning electron microscopy experiments revealed water sorption in hydrophilic NCS at moderate relative humidity (RH) conditions but not in hydrophobic NCS, wherein water condensation on the surface was noted only at high RH. The influence of structure and wettability of different MPLs on cell performance was gleaned from steady-state cell polarization behavior. For cells operated at dry conditions (≤ 80% RH), the limiting current for cells with hydrophilic NCS MPL was the highest while that for cells with hydrophobic NCS MPL was the lowest regardless of the level of water saturation (RH).
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Keywords
Polymer electrolyte fuel cell, Catalyst layer, Ordered structure, Mass transport, ORR electrocatalyst, Microporous layer
Citation
Islam, M. N. (2020). Novel Materials and Architecture for Cathode of Polymer Electrolyte Fuel Cells (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca.