Research Papers

J. Electrochem. En. Conv. Stor.. 2018;16(1):011001-011001-12. doi:10.1115/1.4040056.

This study investigated the combination of the direct and indirect hybrid systems in order to develop a combined hybrid system. In the proposed system, a direct solid oxide fuel cell (SOFC) and gas turbine (GT) hybrid system and an indirect fuel cell cycle were combined and exchanged the heat through a heat exchanger. Several electrochemical, thermal, and thermodynamic calculations were performed in order to achieve more accurate results; then, beside the parametric investigation of the abovementioned hybrid system, the obtained results were compared to the results of direct and indirect hybrid systems and simple GT cycle. Results indicate that the efficiency of the combined hybrid system was between those of the direct and indirect hybrid systems. The electrical efficiency and the overall efficiency of the combined hybrid system were 43% and 59%, respectively. The generation power in the combined hybrid system was higher than that of both other systems, which was the only advantage of using the combined hybrid system. The generation power in the combined hybrid system was higher than that of the direct hybrid system by 16%; accordingly, it is recommended to be used by the systems that are supposed to have high generation power.

Commentary by Dr. Valentin Fuster
J. Electrochem. En. Conv. Stor.. 2018;16(1):011002-011002-10. doi:10.1115/1.4040057.

In this paper, we study a solid oxide fuel cell (SOFC) controlled by a multi-input multi-output (MIMO) compensator, which uses the blower/fan power and cathode inlet temperature as actuators. The usable power of the fuel cell (FC) is maximized by limiting the air flow rate deliberately when an increase in power is demanded. Possible rate bounds on the cathode inlet temperature are also modeled. These bounds could represent the physical limitations (due to slow dynamics of heat exchangers) and/or a control concept for accommodating the power saving objective. Applying proper limits to the amplitude and rate of the actuator signals, and incorporating antiwindup (AW) techniques, can raise the net power of the FC by 16% with negligible effects on the spatial temperature profile.

Commentary by Dr. Valentin Fuster
J. Electrochem. En. Conv. Stor.. 2018;16(1):011003-011003-6. doi:10.1115/1.4040077.

Direct methanol fuel cells (DMFC) are typically supplied under pressure or capillary action with a solution of methanol in water optimized for the best specific power and power density at an operating temperature of about 60 °C. Methanol and water consumption at the anode together with water and methanol losses through membrane due to crossover create an imbalance over time so the fuel concentration at the anode drifts from the optimal ratio. In the present study, we demonstrate a DMFC with a means for continuous adjustment of water and methanol content in the anode fuel mixture of an air-breathing DMFC to maintain the optimal concentration for maximum and continuous power. Two types of piezoelectric micropumps were programmed to deliver the two liquids at the designated rate to maintain optimal concentration at the anode during discharge. The micropumps operate over a wide range of temperature, can be easily reprogrammed and can operate in any orientation. A study of performance at different current densities showed that at 100 mA/cm2, the self-contained, free convection, air-breathing cell delivers 31.6 mW/cm2 of electrode surface with thermal equilibrium reached at 52 °C. The micropumps and controllers consume only 2.6% of this power during 43 h of continuous unattended operation. Methanol utilization is 1.83 Wh cm−3.

Commentary by Dr. Valentin Fuster
J. Electrochem. En. Conv. Stor.. 2018;16(1):011004-011004-8. doi:10.1115/1.4040203.

An asymmetric, inorganic ion-conducting membrane was synthesized by depositing a top layer containing silica-immobilized phosphotungstic acid (Si-PWA) over a graphite sheet. Surface morphology, thermal stability, and structure of the top layer of the membrane were studied using scanning electron microscopy (SEM), thermal gravimetric analysis (TGA), X-ray diffraction (XRD), and Fourier transform infrared (FT-IR), respectively. The transport number and specific conductivity of the membrane were measured using membrane potential and impedance measurements, respectively. The composition of the top layer was varied by changing the molar ratio of PWA and tetraethoxy orthosilicate (TEOS) in the casting sol. The transport number and specific conductivity of the membrane increased on increasing PWA fraction in the casting solution. The highest transport number for sodium ion was 0.98 for PWA: TEOS molar ratio of 1.5. Specific conductivity of the membrane, with 0.5 PWA: TEOS, was 0.0082 S cm−1 which was lower compared to the membrane with 1.5 PWA: TEOS of specific conductivity 0.017 S cm−1. The specific conductivity of the membrane increased with increase in the temperature for both 0.5 and 1.5 molar ratio of PWA: TEOS with the calculated activation energy 18.9 and 8.8 kJ/mol, respectively.

Commentary by Dr. Valentin Fuster
J. Electrochem. En. Conv. Stor.. 2018;16(1):011005-011005-9. doi:10.1115/1.4040329.

In this study, a new type of redox flow battery (RFB) named “membrane-less hydrogen-iron RFB” was investigated for the first time. The membrane is a cell component dominating the cost of RFB, and iron is an abundant, inexpensive, and benign material, and thus, this iron RFB without the membrane is expected to provide a solution to the challenging issues of current battery systems such as high cost and safety concerns. The research focus in this study was placed on defining key design parameters to make this new system promising as an RFB. Crossing rate of reactants over carbon porous electrode (CPE) was controlled by modifying its pore structure with Teflon impregnation, and the effects of the Teflon on crossover, kinetic, Ohmic, and mass transfer was investigated by cell-based test and one-dimensional computational model. It was found that the cell performance (i.e., charge and discharge polarization) of the new membrane-less system was equivalent to that of the conventional membrane-system (i.e., RFB having a membrane). Especially, the Ohmic properties of the new system were constant and stable, while in the conventional membrane system, they were significantly varied and deteriorated as cell tests were continued, indicating that degradation or contamination of membrane affecting Ohmic properties could be mitigated effectively in the membrane-less system, which was found first in this research. The modeling analysis provided insight into the system, showing that the effect of reactant crossover on performance decay was not significant, and Teflon impregnation in the CPE caused significant kinetic and Ohmic losses by impeding ion transport and reactant access to reaction sites. From this study, it was found that the membrane-less H2-iron system is feasible and promising in resolving the challenge issues of the conventional systems. And the results of this study are expected to provide guidelines for research and development of flow battery systems without having a membrane.

Commentary by Dr. Valentin Fuster
J. Electrochem. En. Conv. Stor.. 2018;16(1):011006-011006-8. doi:10.1115/1.4040567.

Three types of LiMn2O4 (LMO) microspheres with different pore size are prepared by a facile method, using porous MnCO3–MnO2 and Mn2O3 microspheres as the self-supporting template, for lithium ion batteries (LIBs) cathode material. Briefly, Mn2O3 and MnO2 microspheres are heated in air at 600 °C for 10 h to synthesize porous Mn2O3 spheres. Then the mixture of as-prepared spherical Mn2O3 and LiNO3 is calcined to obtain the LMOs. The morphology and structure of LMOs are characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and nitrogen adsorption/desorption analyses. The result shows that the maximum pore diameters of LMOs are 17 nm, 19 nm, and 11 nm, respectively. All LMOs microspheres are composed of similar sized nanoparticles; however, the surface of these microspheres is strewed with dense tinier pores or sparse larger pores. Generally, the nanoparticles will reduce the path of Li+ ion diffusion and increases the reaction sites for lithium insertion/extraction. Moreover, the pores can provide buffer spaces for the volume changes during charge–discharge process. The electrochemical performances of LMOs are investigated and LMO2 exhibits extremely good electrochemical behavior, especially the rate capability. The as-prepared LMO2 delivers a discharge capacity of 124.3 mAh g−1 at 0.5 C, retaining 79.6 mAh g−1 even at 5 C. The LMO2 sample also shows good capacity retention of 96.9% after 100 cycles at 0.5 C.

Commentary by Dr. Valentin Fuster
J. Electrochem. En. Conv. Stor.. 2018;16(1):011007-011007-7. doi:10.1115/1.4040819.

Heat is a by-product of all energy conversion mechanisms. Efforts to utilize and dissipate heat remain a challenge for further development and optimization of energy conversion devices. Stationary thermo-electrochemical cell is a low cost method to harvest heat; however, it suffers from low power density. Flow thermo-electrochemical cell (fTEC) heat sink presents itself as a unique solution as it can simultaneously scavenge and remove heat to maintain devices in the operating range. In this work, multiwalled nanotube (MWNT) electrodes have been used and electrode configuration has been changed to maximize the temperature difference over a small interelectrode separation. As a result, power per unit area of fTEC heat sink has been improved by more than seven-fold to 0.36 W/m2.

Commentary by Dr. Valentin Fuster
J. Electrochem. En. Conv. Stor.. 2018;16(1):011008-011008-7. doi:10.1115/1.4040824.

Lithium-ion batteries (LIBs) are the heart of electric vehicle because they are the main source of its power transmission. The current scientific challenges include the accurate and robust evaluation of battery state such as the discharging capacity so that the occurrence of unforeseen dire events can be reduced. State-of-the-art technologies focused extensively on evaluating the battery states based on the models, whose measurements rely on determination of parameters such as the voltage, current, and temperature. Experts have well argued that these models have poor accuracy, computationally expensive, and best suited for laboratory conditions. This forms the strong basis of conducting research on identifying and investigating the parameters that can quantify the battery state accurately. The unwanted, irreversible chemical and physical changes in the battery result in loss of active metals (lithium ions). This shall consequently result in decrease of capacity of the battery. Therefore, measuring the stack stress along with temperature of the battery can be related to its discharging capacity. This study proposes the evaluation of battery state of health (SOH) based on the mechanical parameter such as stack stress. The objective of this study will be to establish the fundamentals and the relationship between the battery state, the stack stress, and the temperature. The experiments were designed to validate the fundamentals, and the robust models are formulated using an evolutionary approach of genetic programming (GP). The findings from this study can pave the way for the design of new battery that incorporates the sensors to estimate its state accurately.

Commentary by Dr. Valentin Fuster
J. Electrochem. En. Conv. Stor.. 2018;16(1):011009-011009-6. doi:10.1115/1.4040826.

An electrochemically stable hybrid structure material consisting of porous silicon (Si) nanoparticles, carbon nanotubes (CNTs), and reduced graphene oxide (rGO) is developed as an anode material (Si/rGO/CNT) for full cell lithium-ion batteries (LIBs). In the developed hybrid material, the rGO provides a robust matrix with sufficient void space to accommodate the volume change of Si during lithiation/delithiation and a good electric contact. CNTs act as a mechanically stable and electrically conductive support to enhance the overall mechanical strength and conductivity. The developed Si/rGO/CNT composite anode has been first tested in half cell and then in full cell lithium-ion batteries. In half cell, the composite anode shows a high reversible capacity of 1100 mAh g−1 with good capacity retention over 500 cycles when cycled at 1 A g−1. In a full cell lithium-ion battery paired up with LiNi1/3Mn1/3Co1/3O2 (NMC) cathodes, the composite anode shows a specific charge capacity of 161.4 mAh g−1 and a discharge capacity of 152.8 mAh g−1, respectively, with a Coulombic efficiency of 94.7%.

Commentary by Dr. Valentin Fuster
J. Electrochem. En. Conv. Stor.. 2018;16(1):011010-011010-7. doi:10.1115/1.4040760.

Limited lifetime and performance degradation in lithium ion batteries in electrical vehicles and power tools is still a challenging obstacle which results from various interrelated processes, especially under specific conditions such as higher discharging rates (C-rates) and longer cycles. To elucidate these problems, it is very important to analyze electrochemical degradation from a mechanical stress point of view. Specifically, the goal of this study is to investigate diffusion-induced stresses and electrochemical degradation in three-dimensional (3D) reconstructed LiFePO4. We generate a reconstructed microstructure by using a stack of focused ion beam-scanning electron microscopy (FIB/SEM) images combined with an electrolyte domain. Our previous two-dimensional (2D) finite element model is further improved to a 3D multiphysics one, which incorporates both electrochemical and mechanical analyses. From our electrochemistry model, we observe 95.6% and 88.3% capacity fade at 1.2 C and 2 C, respectively. To investigate this electrochemical degradation, we present concentration distributions and von Mises stress distributions across the cathode with respect to the depth of discharge (DoD). Moreover, electrochemical degradation factors such as total polarization and over-potential are also investigated under different C-rates. Further, higher total polarization is observed at the end of discharging, as well as at the early stage of discharging. It is also confirmed that lithium intercalation at the electrode-electrolyte interface causes higher over-potential at specific DoDs. At the region near the separator, a higher concentration gradient and over-potential are observed. We note that higher over-potential occurs on the surface of electrode, and the resulting concentration gradient and mechanical stresses are observed in the same regions. Furthermore, mechanical stress variations under different C-rates are quantified during the discharging process. With these coupled mechanical and electrochemical analyses, the results of this study may be helpful for detecting particle crack initiation.

Commentary by Dr. Valentin Fuster
J. Electrochem. En. Conv. Stor.. 2018;16(1):011011-011011-5. doi:10.1115/1.4040827.

Durability and cost are the two major factors limiting the large-scale implementation of fuel cell technology for use in commercial, residential, or transportation applications. The conditioning cost is usually negligible for making proton exchange membrane fuel cells (PEMFCs) at R&D or demo stage with several tens of stacks each year. However, with industry's focus shifting from component development to commercial high-volume manufacturing, the conditioning process requires significant additional capital investments and operating costs, thus becomes one of the bottlenecks for PEMFC manufacturing, particularly at a high production volume (>1000 stack/year). To understand the mechanisms behind PEMFC conditioning, and to potentially reduce conditioning time or even to eliminate the conditioning process, the conditioning behaviors of commercial Nafion™ XL100 and Nafion® NRE 211 membranes were studied. The potential effects of the membrane additive on fuel cell conditioning were diagnosed using in situ electrochemical impedance spectroscopy (EIS). It was found that the membrane additive led to the significant variation of the charge transfer resistance in EIS during conditioning, which affected the conditioning behavior of the membrane electrode assembly (MEA).

Commentary by Dr. Valentin Fuster
J. Electrochem. En. Conv. Stor.. 2018;16(1):011012-011012-8. doi:10.1115/1.4041013.

The effect of the charge/discharge profile on battery durability is a critical factor for the application of batteries and for the design of appropriate battery testing protocols. In this work, commercial high-power prismatic lithium ion cells for hybrid electric vehicles (HEVs) were cycled using a pulse-heavy profile and a simple square-wave profile to investigate the effect of cycle profile on battery durability. The pulse-heavy profile was designed to simulate on-road conditions for a typical HEV, while the simplified square-wave profile was designed to have the same total charge throughput, but with lower peak currents. The 5 Ah batteries were cycled for 100 kAh with periodic performance tests to monitor the state of the batteries. Results indicate that, for the batteries tested, the capacity fade for the two profiles was very similar and was 11±0.5% compared to beginning of life (BOL). The change in internal resistance of the batteries during testing was also monitored and found to increase 21% and 12% compared to BOL for the pulse-heavy and square-wave profiles, respectively. The results suggest that simplified testing protocols using square-wave cycling may provide adequate insight into capacity fade behavior for more complex hybrid vehicle drive cycles.

Commentary by Dr. Valentin Fuster

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