Hyunchul Ju

A three dimensional, transient, non-isothermal model of all-vanadium redox flow batteries

A three-dimensional (3-D) transient non-isothermal model of VRFBs is developed by rigorously accounting for the electrochemical reactions of four kinds of vanadium ions (V2+, V3+ VO2+ and VO2+) and resultant mass and heat transport processes. Particular emphasis is placed on analyzing various heat generation mechanisms, including irreversible and reversible heat generations due to vanadium redox reactions and joule heating arising from the solid electronic and electrolyte ionic resistances. The 3-D model is first validated against voltage evolution curves measured under charging and discharging processes. The model predictions compare well with the experimental data over a wide range of SOCs, and further reveal key electrochemical and transport phenomena inside VRFBs through multi-dimensional contours of solid /electrolyte potentials, species concentration, and temperature. This full 3-D comprehensive VRFB model can be applied to multi-cell stacks in order to find optimal design and operating conditions.

Electrocatalysts composed of a Co(acetylacetonate)2 molecule and refluxed graphene oxide for an oxygen reduction reaction

Hybridization of organometallic complexes with carbon-based materials has been shown to enhance the catalytic performance in the oxygen reduction reaction (ORR). As chemical interactions between the metal centers and ligands are critical factors determining their tunable catalytic nature, it is important to understand the correlation between the chemical structure and catalytic nature and the correlation between the chemical structure and catalytic performance of the hybrids. In this work, the hybrids are synthesized by the reaction of an organometallic complex, Co(II)(acac)2 (acac = acetylacetonate), with refluxed graphene oxide (Re-G-O) materials containing controlled amounts of oxygen atoms at room temperature. Experimental characterization of the hybrids reveals that Co(II)(acac)2 is coordinated to oxygen containing groups in Re-G-O, generating organometallic species, Co–O4–O. These hybrids showed better electrocatalytic activity for the ORR in alkaline media than Co-free Re-G-O materials. The results show that the Co–O4 species is catalytically active for the ORR. We find that the best Co–O4 catalyst for the ORR is produced by the use of reduced graphene oxide with a medium level of reduction.

A Numerical Investigation of Hydrogen Absorption Reaction Based on ZrCo for Tritium Storage (I)

In this paper, a three-dimensional hydrogen absorption model is applied to a thin double-layered annulus ZrCo hydride bed and validated against the temperature evolution data measured by Kang et al. 1) The present model reasonably captures the bed temperature evolution behavior and the 99% hydrogen charging time. The equilibrium pressure expression for hydrogen absorption on ZrCo is derived as a function of temperature and the H/M atomic ratio based on the pressure-composition isotherm data given by Konishi et al. 2) In addition, this present model provides multi-dimensional contours such as temperature and H/M atomic ratio in the thin double- layered annulus metal hydride region. This numerical study provides fundamental understanding during hydrogen absorption process and indicates that efficient design of the metal hydride bed is critical to achieve rapid hydrogen charging performance. The present three-dimensional hydrogen absorption model is a useful tool for the optimization of bed design and operating conditions.

Evaluation and Fabrication of Composite Bipolar Plate to Develop a Light Weight Direct Methanol Fuel Cell Stack for Small-scale UAV Application (I)

Abstract >> A bipolar plate is a major component of a fuel cell stack, which occupies 50～60% of the totalweight and over 50% of the total cost of a typical fuel cell stack. In this study, a composite bipolar plate isdesigned and fabricated to develop a compact and light-weight direct methanol fuel cell (DMFC) stack for a small-scale Unmanned Aerial Vehicle (UAV) application. The composite bipolar plates for DMFCs are preparedby a compression molding method using resole type phenol resin as a binder and natural graphite and carbonblack as a conductor filler and tested in terms of electrical conductivity, mechanical strength and hydrogen permeability. The flexural strength of 63 MPa and the in-plane electrical conductivities of 191 S cm -1 are achieved under the optimum bipolar plate composition of phenol : 18%; natural graphite : 82%; carbon black : 3%, indicating that the composite bipolar plates exhibit sufficient mechanical strength, electrical conductivity and hydrogen permeability to be applied in a DMFC stack. A DMFC with the composite bipolar plate is tested and shows a similar cell performance with a conventional DMFC with graphite-based bipolar plate.

Effects of porous properties on cold-start behavior of polymer electrolyte fuel cells from sub-zero to normal operating temperatures

In this investigation, a parametric study was performed using the transient cold-start model presented in our previous paper, in which the ice melting process and additional constitutive relations were newly included for transient cold-start simulations of polymer electrolyte fuel cells (PEFCs) from a sub-zero temperature (−20°C) to a normal operating temperature (80°C). The focus is placed on exploring the transient cold-start behavior of a PEFC for different porous properties of the catalyst layer (CL) and gas diffusion layer (GDL). This work elucidates the detailed effects of these properties on key cold-start phenomena such as ice freezing/melting and membrane hydration/dehydration processes. In particular, the simulation results highlight that designing a cathode CL with a high ionomer fraction helps to retard the rate of ice growth whereas a high ionomer fraction in the anode CL is not effective to mitigate the anode dry-out and membrane dehydration issues during PEFC cold-start.

Large-scale cold start simulations for automotive fuel cells

In this study, the three-dimensional (3-D) transient cold start model is applied to the real-scale polymer electrolyte fuel cell (PEFC) geometry and transient cold-start simulations are carried out from subzero to normal temperatures. In order to reduce the computational turnaround time involving a large numerical mesh with millions of grid points, the cold start code is parallelized for parallel computing. The simulation results clearly show the evolution of ice, water content, temperature, and current density contours at different cold start stages characterizing freezing, melting, hydration, and dehydration processes. In addition, the model predictions emphasize beneficial influence of vapor phase diffusion from the cathode catalyst layer (CL) to gas diffusion layer (GDL) during cold start, which can contribute to reducing the amount of ice accumulation in the cathode CL. As the effect of vapor phase diffusion is substantial, more ice is accumulated in the cathode GDL rather than in the cathode CL. Therefore, the total amount of ice accumulated inside a cell is not always proportional to the amount of ice in the cathode CL, depending on the strength of vapor phase diffusion.

A Numerical Investigation of Effects of Methanol Concentration Fluctuation in Active-type Direct Methanol Fuel Cell (DMFC) Systems

In this study, we develop a one-dimensional (1-D), two-phase, transient-thermal DMFC　model to investigate the effect of methanol concentration fluctuation that usually occurs in active-type direct methanol fuel cell (DMFC) systems. 1-D transient simulations are conducted and time-dependent behaviors of DMFCs are analyzed under various DMFC operating conditions such as anode/cathode stoichiometry, cell temperature, and cathode inlet humidification. The simulation results indicate that the effect of methanol concentration fluctuation on DMFC performance can be mitigated by proper control of anode/cathode stoichiometry, providing a guideline to optimize operating conditions of active DMFC systems.

A Numerical Investigation of Hydrogen Desorption Reaction for Tritium Delivery from Tritium Storage Based on ZrCo

In this paper, a three-dimensional hydrogen desorption model is applied to a thin double-layered annulus ZrCo hydride bed and validated against the temperature evolution data measured by Kang et al. 1) The present model reasonably captures the bed temperature evolution behavior and the 90% hydrogen discharging time. In addition, the performance of thin double-layered annulus bed is evaluated by comparing with a simple cylindrical bed using hydrogen desorption model. This study provides multi-dimensional contours such as temperature and H/M atomic ratio in the metal hydride region. This numerical study provides fundamental understanding during hydrogen desorption process and indicates that efficient design of the metal hydride bed is critical to achieve rapid hydrogen discharging performance. The present three-dimensional hydrogen desorption model is a useful tool for the optimization of bed design and operating conditions.

Effect of Ammonia Reaction Time on the Calytic Poperties of Sputtered Iron-Based Electrocatalyst

Fe-based electrocatalysts for polymer electrolyte membrane fuel cells (PEMFCs) were produced using a sputter deposition process involving a carbon paper (CP) and a NH3 heat treatment environment. The Fe–N–CP sample reacted with ammonia for 10 min at 950 °C displayed a greater current density than the samples reacted with ammonia for other reaction times. The impedance of the samples treated for 10 min with ammonia shows the highest value, which means much higher ligand formation and lower electric conductivity, which are consistent with the results of cyclic voltammetry (CV). This is directly due to the formation of ligands between the Fe and CP used when exposed to a nitrogen environment. While the current density of the Fe-based electrocatalysts under review did not exceed that of standard Pt/C electrocatalysts, the results herein suggest that nonprecious metal electrodes may be a viable alternative in PEMFCs.

Develop of advanced bipolar plate using carbon fiber prepreg and graphite/polymer composite materials

In this study, we introduce an advanced composite bipolar plate (BP) to improve mechanical behaviors of BPs as compared to conventional composite BPs. The new BPs are fabricated by compression molding process in which the mixture of natural graphite particles as a major filler, synthetic graphite and carbon blacks (CBs) as the additive are compacted and integrated with epoxy-carbon fiber prepreg. The new BPs are evaluated and the test results clearly demonstrate that the proposed BP is superior to typical carbon composite BPs in terms of key BP properties including electrical conductivity with emphasis on its outstanding flexural strength. The unique feature of this new BP enables to further reduce the thickness of BPs, which directly contributes the reduction in fuel cell stack weight and volume.