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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

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