Tailoring SOFC electrode microstructures for improved performance

The authors thank EPSRC for support through the research grant EP/M014304/1.The key technical challenges that fuel cell developers need to address are performance, durability, and cost. All three need to be achieved in parallel; however, there are often competitive tensions, e.g., performance is achieved at the expense of durability. Stability and resistance to degradation under prolonged operation are key parameters. There is considerable interest in developing new cathodes that are better able to function at lower temperature to facilitate low cost manufacture. For anodes, the ability of the solid oxide fuel cell (SOFC) to better utilize commonly available fuels at high efficiency, avoid coking and sulfur poisoning or resistance to oxidation at high utilization are all key. Optimizing a new electrode material requires considerable process development. The use of solution techniques to impregnate an already optimized electrode skeleton, offers a fast and efficient way to evaluate new electrode materials. It can also offer low cost routes to manufacture novel structures and to fine tune already known structures. Here impregnation methodologies are discussed, spectral and surface characterization are considered, and the recent efforts to optimize both cathode and anode functionalities are reviewed. Finally recent exemplifications are reviewed and future challenges and opportunities for the impregnation approach in SOFCs are explored.PostprintPeer reviewe

Effect of Elevated PEM Fuel Cell Operating Temperature (120°C and 140°C) and Membrane Thickness on Proton Conductivity for Combat Vehicle Use

Electrical power required to operate vehicles in the U.S. Army is increasing due to expanding mission requirements, such as silent watch, exportable power, and powerful onboard electronics. Proton Exchange Membrane Fuel Cells (PEMFCs) provide a solution, but stack thermal-cycling, electrocatalyst and membrane degradation losses need to be reduced before integration of PEMFCs can be realized. Membrane thermal degradation is exacerbated by poor heat rejection (as ballistic grills impede airflow) which can raise stack temperatures ≥140°C. Commercial PEMFCs operate ~65°C so elevated temperatures could degrade the membrane. Nafion 115 (127 μm), 117 (183 μm) and 1110 (254 μm) membranes submerged in 16 MΩ water were heated between 65-140°C to investigate elevated temperature and membrane thickness on proton conductivity. EIS results showed sample thickness did not statistically impact conductivity overall. Conductivity, however, was impacted for temperatures &amp;gt;100°C with each material. Overall, these materials are not suitable when operating PEMFCs above 100°C.</p

Effect of Elevated PEM Fuel Cell Operating Temperature (120°C and 140°C) and Membrane Thickness on Proton Conductivity for Combat Vehicle Use

Energy required from ground vehicles in the United States (U.S.) Army is increasing due to the need for increased capabilities and mission duties required for an ever changing combat environment. These additional capabilities include silent watch (long term mounted surveillance), advanced radios/jamming devices/sensors, directed energy weapons, exportable power supporting stationary applications, and vehicle-to-grid connectivity. Increasing the combat vehicle energy output, while still fitting into the limited space inside each vehicle, requires an energy source that is compact, power dense and energy efficient. PEM Fuel Cells (PEMFCs) meet those requirements for primary and/or auxiliary power generators placed in vehicles for short or long-term mission roles. While PEMFCs provide a potential solution for the U.S. Army’s vehicle energy requirements, there still exists a number of critical areas which need to be resolved before integration into combat vehicles is realized. These issues include: 1. Stack degradation from thermal-cycling and stack sealing loss, 2. Electrocatalyst degradation and 3. Cell membrane thermal degradation. In addition, specifically for the U.S. Army, vehicles significantly restrict heat rejection (due to low air flow from ballistic grills) to the PEMFC system in addition to vehicles operating in locations with elevated ambient temperatures, which can increase the stack operating temperature up to 140°C. Since cooling requirements for PEMFCs are typically engineered for operating temperatures closer to 65°C, these elevated stack temperatures could result in thermal degradation of the cell membrane. Since the cell membrane is a vital component for stack operation, membrane material formulations have been developed with increased chemical and thermal degradation resistances. One such membrane material is Nafion, which starts to thermally decompose around 300°C. Even when the operating temperature is lower than the decomposition temperature, a small percentage of the Nafion membrane potentially could degrade as temperatures reach between 120°C and 140°C which could change the performance of the PEMFC through changes in proton transport resistance. Another possible method of reducing membrane thermal degradation would be engineering cells with thicker membranes, where the added mass could withstand the increased temperature for longer periods of time. These potential changes to the Nafion membrane at elevated temperatures (120°C and 140°C) using different membrane thicknesses, when compared to stack operation at 65°C, were investigated in this study. Pristine Nafion membranes (Nafion 115, 117 and 1110) were used in the following analysis and were heated at 65°C, 120°C and 140°C, for 2, 8 and 24hrs at each temperature. The membranes all had similar polymer formulations with thicknesses of 127, 183 and 254μm, respectively. All three polymers were heated for 2, 8 and 24hrs in 16MΩ water at 65°C, 120°C and 140°C and were characterized to determine the impact of elevated temperatures and material thickness on proton conductivity and structural changes to each material. Figure 1a shows the calculated proton conductivities, from raw EIS data, at 65°C, 120°C and 140°C for all three membranes heated for 2, 8 and 24hrs. Results from heating at 120°C showed all three membranes had increased conductivities between 14% and 30% after 24hrs, compared to the 65°C results, and were statistically similar to each other. While large variations in the conductivities occurred, the majority of the 120°C measurement standard deviations did not overlap with the 65°C results. Results from heating at 140°C showed the three membranes had increased their conductivities by different amounts, depending on membrane thickness. Nafion 115 had been statistically increased by 30% compared to 115 heated at 65°C for 24hrs, while the 117 and 1110 had returned to a statistically similar value as the 65°C 24hr results. Figures 1b,c,d show internal structural changes to the vibrational modes for each membrane material using FTIR characterization, which support the changes in proton conductivities reported in Figure 1a. All three membranes, after 24hrs at 65°C, showed near identical scans. All three membrane materials, after heating at 120°C and 140°C, showed changes to the vibrational modes observed for the 65°C results and produced additional vibrations modes not originally present. These conductivity and FTIR results show heating Nafion, even at elevated temperatures, was sufficient to dynamically change its operating performance over time. Altering the membrane thickness did not mitigate performance changes over time however, with even the thicker membranes being less stable with time. Based on these results, an alternative approach or material should be used if operating the PEM fuel cell at or above 100°C. Figure 1 <jats:p /

Effect of Elevated PEM Fuel Cell Operating Temperature (120°C and 140°C) and Membrane Thickness on Proton Conductivity for Combat Vehicle Use

Electrical power required to operate vehicles in the U.S. Army is increasing due to expanding mission requirements, such as silent watch, exportable power, and powerful onboard electronics. Proton Exchange Membrane Fuel Cells (PEMFCs) provide a solution, but stack thermal-cycling, electrocatalyst and membrane degradation losses need to be reduced before integration of PEMFCs can be realized. Membrane thermal degradation is exacerbated by poor heat rejection (as ballistic grills impede airflow) which can raise stack temperatures ≥140°C. Commercial PEMFCs operate ~65°C so elevated temperatures could degrade the membrane. Nafion 115 (127 μm), 117 (183 μm) and 1110 (254 μm) membranes submerged in 16 MΩ water were heated between 65-140°C to investigate elevated temperature and membrane thickness on proton conductivity. EIS results showed sample thickness did not statistically impact conductivity overall. Conductivity, however, was impacted for temperatures &gt;100°C with each material. Overall, these materials are not suitable when operating PEMFCs above 100°C

Effect of Elevated PEM Fuel Cell Operating Temperature (120°C and 140°C) and Membrane Thickness on Proton Conductivity for Combat Vehicle Use

Electrical power required to operate vehicles in the U.S. Army is increasing due to expanding mission requirements, such as silent watch, exportable power, and powerful onboard electronics. Proton Exchange Membrane Fuel Cells (PEMFCs) provide a solution, but stack thermal-cycling, electrocatalyst and membrane degradation losses need to be reduced before integration of PEMFCs can be realized. Membrane thermal degradation is exacerbated by poor heat rejection (as ballistic grills impede airflow) which can raise stack temperatures ≥140°C. Commercial PEMFCs operate ~65°C so elevated temperatures could degrade the membrane. Nafion 115 (127 μm), 117 (183 μm) and 1110 (254 μm) membranes submerged in 16 MΩ water were heated between 65-140°C to investigate elevated temperature and membrane thickness on proton conductivity. EIS results showed sample thickness did not statistically impact conductivity overall. Conductivity, however, was impacted for temperatures &gt;100°C with each material. Overall, these materials are not suitable when operating PEMFCs above 100°C.</jats:p

Effect of Water Conductivity on Proton Exchange Membrane (PEM) Catalyst Durability Using Thermal Degradation Resistant Polymer Membranes in Combat Applications

The U.S. Army is taking an increasing interest in fuel cell technology as vehicles require more electrical power to support additional capabilities for different combat roles and adversarial threats. These additional power requirements include silent watch (long term mounted surveillance), advanced radios/jamming devices and exportable power supporting stationary applications and vehicle-to-grid connectivity. Fuel cells are attractive in these roles as they generate a lower thermal and acoustic signature while having the capability of being more efficient than current internal combustion engines. The goal is to have increased energy savings and decreased fuel transport logistic burdens. Despite these advantages, significant work needs to be completed before fuel cell technology can be integrated into combat vehicles. In addition to the thermal-cycling and stack sealing issues leading to stack power degradation, the U.S. Army has unique challenges engineering fuel cells into vehicles. One challenge is that air flow for prime mover heat rejection for combat vehicles is significantly restricted (due to many components competing for the same space and the use of ballistic grills) compared to civilian automobiles. This can increase the internal temperature inside the stack to 140°C when operating in hot environments. One commercial market approach the U.S. Army is analyzing to address increased stack internal temperature is the incorporation of silicon (as quartz or silica) into the polymer membrane, which literature studies have shown can reduce polymer thermal degradation. Even though polymer thermal degradation has been shown to be reduced when silicon is incorporated, it is likely not completely eliminated, and the elevated temperatures the U.S. Army expects to experience could easily degrade membranes, even with the added silicon, and produce side products which are rejected into the exhaust water generated by fuel cells. This is a concern since many polymer materials, such as Perfluorosulfonic Acid and other novel proprietary membranes, incorporate acid side chains into the polymer chemistry to enhance their material properties for increased fuel cell power and durability. These acid side chains, or smaller compounds which contain these side chains, have the potential to be removed under thermal degradation. The removed compounds would contaminate the water produced by the stack, making it weakly acidic and electrically conductive. Studies have shown that strong acids in contact with PEM catalysts, with current cycling remove platinum electrocatalyst materials through dissolution and lower the overall stack power. Since there is the potential for PEM stack catalyst degradation and overall loss of vehicle capabilities due to elevated operating temperatures, this effect of weakly acidic and electrically conductive water was investigated in this study. Quartz containing Polybenzimidazole polymer membranes were sputter-coated with a platinum layer 6nm thick, on a single side of each sample, to simulate the boundary layer between the nano-particle electrocatalysts coated on carbon supports and polymer membrane in fuel cells. These 6nm thick sputter-coatings were applied to polymer membrane samples as thin strips (0.635cm in length and 0.238cm in width) to direct the applied electrical current, which simulated fuel cell operating current passing through the electrocatalysts generated by electrochemical reactions inside the fuel cell. Water, combined with various amounts of acetic acid, was placed in contact with the platinum coated membrane samples while electrical current cycles, between 50 and 400, were applied through sample coatings. Each cycle used a square wave profile which consisted of current being applied for 5s and then removed for 5s. Figure 1 shows Energy Dispersive Spectroscopy (EDS) scans (1a,b) and optical microscopy images (1c,d) of platinum coating levels before and after electrical currents (1a) 0.7mA and (1b,c,d) 30mA were applied through sample coatings. EDS results show that both 0.7mA and 30mA currents were sufficient to remove platinum when in contact with water containing 5 vol% acetic acid. Results showed a 38% loss of platinum when 400 cycles of 0.7mA electrical current was applied and platinum degradation was even more sever, only requiring optical microscopy images to observe the platinum loss, when using a 30mA current and a minimum of 50 cycles. Removal of platinum from membranes was also a fast process that was completed in ~8 minutes for 50 cycles and ~66 minutes for 400 cycles. Overall sample parameter testing indicated water electrical conductivity was the primary driving force for catalyst degradation. Figure 1 <jats:p /

Tailoring Mixed Ionic Electronic Conducting Nano-Particle Size through Desiccation and/or Doped Ceria Oxide Pre-Infiltration

Mixed Ionic Electronic Conducting (MIEC) multi-cation oxide nano-particles are commonly used to enhance the performance of Solid Oxide Fuel Cell (SOFC) Nano-Composite Cathodes (NCCs) electrodes. The average MIEC NCC nano-particle size produced by the infiltration method are typically 50-60 nm in diameter and result in SOFC operating temperatures in excess of 600°C. Since MIEC particle size directly correlates with the surface area available for oxygen incorporation and hence NCC performance, the objective of this study was to determine if MIEC infiltrate particle size could be reduced through nitrate gel desiccation and/or the pre-infiltration of a nitrate decomposition catalyst (in this case nano-Gd0.1Ce0.9O1.95 (GDC)). For the La0.6Sr0.4Co0.8Fe0.2O3-δ (LSCF) infiltrate particles examined here, average nano-particle sizes were reduced from 50 nm to 20 nm using both desiccation and/or pre-infiltration processing techniques. These reduced LSCF nano-particle sizes lowered the 550°C NCC polarization resistance from 0.33 Ωcm2 to 0.16 Ωcm2 with desiccation and from 0.33 Ωcm2 to 0.11 Ωcm2 with nitrate decomposition catalyst pre-infiltration.</jats:p