2. BACKGROUND
2.4 Strategies in the Literature for Overcoming Current Obstacles
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the nanofiber electrode structure using Nafion/PAA as a binder compared to a standard sprayed electrode. Brodt et al. also showed that the addition of PAA to a conventional painted electrode significantly decreased the fuel cell power output due presumably to a decrease in ionic conductivity of the binder. After attempting to remove the PAA in boiling acid and peroxide, he concluded that it was not possible.[27] Brodt showed that nanofiber diameter between 250 nm and 520 nm does not affect power density and that the nanofiber electrode morphology can be used to improve the performance of commercial Pt/C powders.[27] Subsequently in 2016, Brodt et al. showed that Nafion-PVDF binders appear to suppress carbon corrosion in a hydrogen/air PEMFC by increasing the nanofiber hydrophobicity, thus decreasing the concentration of water at the Pt/C catalyst surface.[26]
The work contained in this dissertation builds upon these prior nanofiber electrode works.
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strategies take into consideration the water flux through an MEA and try to mitigate unwanted drying in the anode and unwanted flooding in the cathode. Anode drying occurs because of electroosmotic drag which is the flux of water induced by the movement of protons across the polymer electrolyte membrane.[44] Electroosmotic drag delivers more water to the cathode than what is dictated by stoichiometry through the reduction of oxygen alone: between 1 and 2.5 water molecules are dragged across the membrane with each proton.[45] Thus, for every water molecule generated, there are between 2 and 5 water molecules dragged from the anode to the cathode.[45],[46,47] The electroosmotic drag of water is countered by the back-diffusion of water[44] where back diffusion is dependent on the concentration gradient of water from the cathode to the anode as well as the thickness of the membrane.[35] Operational considerations such as asymmetric humidification of the anode and cathode[8] and recirculation of the anode feed[48] to recover both H2 and water are other ways of improving performance at low relative humidity. Chapter 5 of this dissertation focuses on a nanofiber anode/cathode design that produces high power at low RH.
2.4.2 Electrode Design for Lowered Pt Content and Improved Durability
Electrode design affects the performance of an MEA by affecting three phenomena:
(1) the activation of the oxygen reduction reaction, (2) electronic and ionic conductivity, and (3) the transport of feed gasses to catalyst sites and the expulsion of water away from the catalyst sites.[49] This section will discuss several prominent electrode fabrication techniques and the benefits of each approach.
26 2.4.2.1 Electrosprayed Electrodes
Using a technique to electrospray the cathode catalyst layers to improve performance while using very low amounts of platinum has been utilized by several groups including Uchida[50,51], Elabd[52], and Castillo.[53] These groups observed an improvement in catalyst utilization, an improvement of electrochemically active surface areas over baseline air-sprayed catalyst layers, an increase in catalyst layer uniformity, and an increase in electrode porosity relative to slurry electrodes. Overall, the electrodes showed improvements in power generation over slurry electrodes. Uchida and coworkers observed that at full humidification, a very high max power of 1010 mW/cm2 was generated using a Pt loading of 0.056 mgPt/cm2 with operating conditions of 100 kPa absolute, 80 °C.
However, the electrosprayed cathodes’ current generation show a strong dependence on RH (decreasing by nearly 80% from 100% RH to 40% RH). In the work performed by Elabd and coworkers, they employ a technique of simultaneously electrospinning nanofibers of Nafion and poly(acrylic acid) and electrospraying droplets of Nafion and Pt/C. This combination nanofiber/nanospray electrodes, the maximum power obtained at 0.052 mgPt/cm2 was 656 mW/cm2 in H2/air feed gas at 80 °C, and 272 kPa absolute pressure.
2.4.2.2 Nanostructured Thin Film (NSTF)
The nanostructured thin film (NSTF) catalyst electrodes developed by 3M Company are platinum “nano-whiskers” which contain no carbon or ionomer binder.[54]
These structures exhibited more stable electrochemical surface area compared to their control electrode when an accelerated stress test of voltage cycling from 0.6 V – 1.2 V was applied.[54] The authors conclude that the absence of carbon in the catalyst layer is the
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reason for the improved durability when being cycled at high voltages (> 1.0 V). While the stability improves, this structure struggles to generate high current densities due to an issue with water management.
The following table is a compilation of recent literature for new fuel cell MEA designs along with the power density, catalyst loading, and fuel cell operating conditions for each MEA. Kumaraguru et al.[55] use a high ion exchange capacity ionomer (825EW perfluorosulfonic acid) for the anode and cathode binder and a membrane that is 12 microns thick (half as thick as a standard Nafion 211 membrane) and a PtCo/C catalyst at the cathode to achieve 1.3 W/cm2 at fuel cell operating conditions listed in Table 1.1.
Kongkanand et al.[56] achieve similarly high power by utilizing a new catalyst type with a Pt monolayer shell around a core of Pd supported on carbon. This paper used low catalyst loading (0.05 mgPt/cm2) and observed very good power generation at high current densities.
Table 1.1 List of competitive power densities obtained at given conditions and catalyst loadings.
Reference Strategy Operating Conditions
Cathode Catalyst loading (mgPt/cm2)
Maximum Power Density
(mW/cm2)
Uchida[50]
Electrosprayed electrodes Cathode: Pt/C
Membrane: N/A Anode: Pt/C
P = 100 kPa T = 80 C RH = 100%
Flow Rates = (N/A)
0.056 1010
Kumaraguru[55]
Optimized slurry cast electrodes Cathode: PtCo/HSC
Membrane: 12 µm 825 PFSA Anode: Pt/C
P = 250 kPa T = 80 C RH = 65%
Flow Rates = Stoic.
1.5/2.0
0.1 1300
Kongkanand[56]
Pt-monolayer/Pd/C core−shell cathode
Cathode ionomer: 900 EW PFSA
Membrane: Nafion 211 Anode: Pt/C 900 EW PFSA
P = 150 kPa T = 80 C RH = 100%
Flow Rates = Stoic.
1.5/2.1
0.05 1100
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