Effect of compression on the water management of polymer electrolyte fuel cells An in operando neutron radiography study

In-depth understanding of the effect of compression on the water management in polymer electrolyte fuel cells (PEFCs) is indispensable for optimisation of performance and durability. Here, in-operando neutron radiography is utilised to evaluate the liquid water distribution and transport within a PEFC under different levels of compression. A quantitative analysis is presented with the influence of compression on the water droplet number and median droplet surface area across the entire electrode area. Water management and performance of PEFCs is strongly affected by the compression: the cell compressed at 1.0 MPa demonstrates ∼3.2% and ∼7.8% increase in the maximum power density over 1.8 MPa and 2.3 MPa, respectively. Correlation of performance to neutron radiography reveals that the performance deviation in the mass transport region is likely due to flooding issues. This could be ascribed to the loss of the porosity and increased tortuosity factor of the gas diffusion layer under the land at higher compression pressure. The size and number of droplets formed as a function of cell compression was examined: with higher compression pressure, water droplet number and median droplet surface area rapidly increase, showing the ineffective water removal, which leads to fuel starvation and the consequent performance decay

Correlating electrochemical impedance with hierarchical structure for porous carbon-based supercapacitors using a truncated transmission line model

This work considers the relationship between the morphology of porous carbon materials used for supercapacitors and the electrochemical impedance spectroscopy (EIS) response. EIS is a powerful tool that can be used to study the porous 3-dimensional electrode behavior in different electrochemical systems. Porous carbons prepared by treatment of cellulose with different compositions of potassium hydroxide (KOH) were used as model systems to investigate the form vs. electrochemical function relationship. A simple equivalent circuit that represents the electrochemical impedance behavior over a wide range of frequencies was designed. The associated impedances with the bulk electrolyte, Faradaic electrode processes and different pore size ranges were investigated using a truncated version of the standard transmission line model. The analysis considers the requirements of porous materials as electrodes in supercapacitor applications, reasons for their non-ideal performance and the concept of ‘best capacitance’ behavior in different frequency ranges

Multi length scale characterization of compression on metal foam flow field based fuel cells using X ray computed tomography and neutron radiography

The mechanical compression of metal foam flow field based polymer electrolyte fuel cells PEFCs is critical in determining the interfacial contact resistance with gas diffusion layers GDLs , reactant flow and water management. The distinct scale between the pore structure of metal foams and the entire flow field warrant a multilength scale characterization that combines ex situ tests of compressed metal foam samples and in operando analysis of operating PEFCs using X ray computed tomography CT and neutron radiography. An optimal medium compression was found to deliver a peak power density of 853 mW cm2. The X ray CT data indicates that the compression process significantly decreases the mean pore size and narrows the pore size distribution of metal foams. Simulation results suggest compressing metal foam increases the pressure drop and gas velocity, improving the convective liquid water removal. This is in agreement with the neutron imaging results that demonstrates an increase in the mass of accumulated liquid water with minimum compression compared to the medium and maximum compression cases. The results show that a balance between Ohmic resistance, water removal capacity and parasitic power is imperative for the optimal performance of metal foam based PEFC

Integration of supercapacitors into printed circuit boards

Physically integrated energy storage devices are gaining increasing interest due to the rapid development of flexible, wearable and portable electronics technology. For the first time, supercapacitor components have been integrated into a printed circuit board (PCB) construct. This proof-of-concept study paves the way for integrating supercapacitors into power electronics devices and hybridising with PCB fuel cells. Commercial Norit activated carbon (NAC) was used as the electrode material and was tested in two types of electrolytes, sodium sulfate (Na2SO4) aqueous electrolyte, and Na2SO4-polyvinyl alcohol (Na2SO4-PVA) gel electrolyte. Electrochemical measurements compare the SC-PCBs to standard two-electrode button-cell supercapacitors. A volumetric energy density of 0.56 mW h cm−3 at a power density of 26 mW cm−3 was obtained in the solid-state SC-PCB system, which is over twice the values acquired in the standard cell configuration. This is due to the removal of bulky components in the standard cell, and/or decreased thickness of the overall device, and thus a decrease in the total volume of the SC-PCB configuration. The results show great potential for embedding supercapacitors into PCBs for a broad range of applications. In addition, further advantages can be realised through close physical integration with other PCB-based electrochemical power systems such as fuel cells

Investigation of water generation and accumulation in polymer electrolyte fuel cells using hydro electrochemical impedance Imaging

In depth understanding of water management is essential for the optimization of the performance and durability of polymer electrolyte fuel cells PEFCs . Neutron imaging of liquid water has proven to be a powerful diagnostic technique, but it cannot distinguish between legacy water that has accumulated in the system over time and nascent water recently generated by reaction. Here, a novel technique is introduced to investigate the spatially resolved water exchange characteristics inside PEFCs. Hydro electrochemical impedance imaging HECII involves making a small AC sinusoidal perturbation to a cell and measuring the consequential water generated, using neutron radiographs, associated with the stimulus frequency. Subsequently, a least squares estimation LSE analysis is applied to derive the spatial amplitude ratio and phase shift. This technique provides a complementary view to conventional neutron imaging and provides information on the source and history of water in the system. By selecting a suitable perturbation frequency, HECII can be used to achieve an alternative image contrast and identify different features involved in the water dynamics of operational fuel cell

Characterization of water management in metal foam flow field based polymer electrolyte fuel cells using in operando neutron radiography

Metal foam flow fields have shown great potential in improving the uniformity of reactant distribution in polymer electrolyte fuel cells PEFCs by eliminating the land channel geometry of conventional designs. However, a detailed understanding of the water management in operational metal foam flow field based PEFCs is limited. This study aims to provide the first clear evidence of how and where water is generated, accumulated and removed in the metal foam flow field based PEFCs using in operando neutron radiography, and correlate the water maps with electrochemical performance and durability. Results show that the metal foam flow field based PEFC has greater tolerance to dehydration at 1000 mA cm amp; 8722;2, exhibiting a 50 increase in voltage, 127 increase in total water mass and 38 decrease in high frequency resistance HFR than serpentine flow field design. Additionally, the metal foam flow field promotes more uniform water distribution where the standard deviation of the liquid water thickness distribution across the entire cell active area is almost half that of the serpentine. These superior characteristics of metal foam flow field result in greater than twice the maximum power density over serpentine flow field. Results suggest that optimizing fuel cell operating condition and foam microstructure would partly mitigate flooding in the metal foam flow field based PEF