Xiong Peng

Date of Award

Spring 2019

Document Type

Open Access Dissertation

Department

Chemical Engineering

First Advisor

William E. Mustain

Abstract

Because of environmental issues and national conflicts caused by overuse of fossil fuels, there is a need to seek renewable ways to utilize energy. In the transportation sector, hydrogen-based polymer electrolyte membrane fuel cells, including proton exchange membrane fuel cells (PEMFCs) and anion exchange membrane fuel cells (AEMFCs), have been considered as a promising alternative to conventional combustion engines. However, the widespread commercialization of both PEMFCs and AEMFCs has been hindered by several factors, including cost and performance. In this thesis, a series of topics including electrode fabrication, electrocatalyst development for both PEMFCs and AEMFCs, as well as the water dynamics of AEMFCs, will be investigated and discussed. The central theme behind this thesis was to make materials and performance advances that would allow the cost of low temperature polymer fuel cells to be reduced.

In the chapters of this thesis focusing on PEMFCs, an air-assisted cylindrical liquid jets spraying (ACLJS) system was developed to prepare high-performance catalyst-coated membranes (CCMs). The CCM pore architecture, including size, distribution and volume, can be controlled using various flow parameters, and the impact of spraying conditions on electrode structure and PEMFC performance was investigated. CCMs fabricated in the fiber-type break-up regime by ACLJS achieved very high performance during PEMFC testing, with the top-performing cells having a current density greater than 1900 mA/cm2 at 0.7 V under H2/O2 flows at 1.5 bar(absolute) pressure and 60% gas RH, and 80 °C cell temperature. Additionally, a Ni-rich Pt-Ni alloy was synthesized via a solvothermal method and transformed into a Pt-Ni nanocage (PNC) by applying a two-phase corrosion process. During rotating disk electrode (RDE) testing, the half-wave potential of the PNC was 30 mV higher than a commercial Pt/C catalyst for the oxygen reduction reaction (ORR). The RDE experiments showed that the specific and mass activity of the PNC were 2 and 4 times greater, respectively, than the commercial Pt/C at 0.9 V. PNC CCMs prepared via ACLJS showed no obvious Pt and Ni dissolution and redeposition in the membrane, even after 30,000 cycles. The performance and electrochemically active surface area (ECSA) retention of the PNC was far superior to commercial Pt/C, and just short of the US Department of Energy (DOE) 2020 targets, suggesting that PNC catalysts may be very promising candidates for high-performing commercial PEMFCs.

In the chapters of this thesis focusing on AEMFCs, a B2-type phase Pd-Cu catalyst, supported on Vulcan XC-7R carbon, was synthesized via a solvothermal method. During RDE testing, the half-wave potential of the Pd-Cu/Vulcan catalyst was 50 mV higher compared to that of commercial Pt/C catalyst for the ORR in alkaline media. The Pd-Cu/Vulcan catalyst also showed higher in-situ AEMFC performance, with operating power densities of 1100 mW/cm2 operating on H2/O2 and 700 mW/cm2 operating on H2/Air (CO2-free). Another 2D planar electrocatalyst with CoOx embedded in nitrogen-doped graphitic carbon (CoOx-N-C) was created through the direct pyrolysis of a metal organic complex with a NaCl template. The CoOx-N-C catalyst showed high ORR activity, indicated by excellent half-wave (0.84 V vs. RHE) and onset (1.01 V vs. RHE) potentials. This high intrinsic activity was also observed in operating AEMFCs where the kinetic current density was 100 mA cm-2 at 0.85 V. When paired with a radiation-grafted ETFE powder ionomer, the CoOx-N-C AEMFC cathode was able to achieve extremely high peak power density (1.05 W cm-2) and mass transport limited current (3 A cm-2) for a precious metal free electrode. To further improve the mass transport for precious metal free cathodes in AEMFCs, cobalt ferrite (CF) nanoparticles supported on Vulcan carbon XC-72 (CF-VC) were created through a facile, scalable solvothermal method. When used as the cathode in a single cell 5 cm-2 AEMFC, the CF-VC electrode was able to achieve a peak power density of 1350 mW cm-2 (iR-corrected: 1660 mW cm-2) with a peak current density over 4 A cm-2 operating on H2/O2. The cell was able to achieve a peak power density of 670 mW cm-2 (iR-corrected: 730 mW cm-2) with a mass transport limited current density over 2 A cm-2 operating with H2/Air (CO2-free), which is among the best performing PM-free catalysts reported in the literature to date in an operating AEMFC.

Rights

© 2019, Xiong Peng

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