Seungbum Ha

Combinatorial deposition of a dense nano-thin film YSZ electrolyte for low temperature solid oxide fuel cells

This paper presents a combinatorial deposition of a nano-thin film yttria-stabilized zirconia (YSZ) electrolyte deposited on a nanoporous anodic aluminum oxide (AAO) membrane, which serves as a supporting and gas-permeable substrate. The dense YSZ electrolyte was deposited by combining atomic layer deposition (ALD) and sputtering in an attempt to realize a pinhole-free electrolyte on a porous structure with a minimum electrolyte thickness. The YSZ electrolyte with an overall thickness of about 390 nm was successfully fabricated on porous AAO with this combined deposition method, and the open circuit voltage (OCV) of the cell with this electrolyte was verified to reach a high value of 1.14 V at 350 °C under an atmosphere of pure hydrogen and ambient air. The fuel cell performance reached power densities of 88, 129, and 180 mW cm−2 at 350, 400, and 450 °C, respectively, and has been so far the highest attainable for a nanoscale thin film electrolyte on a porous substrate at such low temperatures. The addition of the pre-ALD YSZ layer also serves to confine the anode morphology and thus enhance the thermal stability of the metal electrodes.

Sufficient Conditions for Optimal Energy Management Strategies of Fuel Cell Hybrid Electric Vehicles Based on Pontryagin’s Minimum Principle

Although an onboard fuel cell system is able to provide sufficient electricity to power a vehicle, a secondary energy storage system such as an electrical battery is desirable because the fuel cell system is generally not capable of braking energy recuperation, and the efficiency of the fuel cell system can be improved by using the secondary energy system. In this paper, sufficient conditions are obtained for an optimal control idea for fuel cell hybrid electric vehicles when the fuel cell system is combined with a rechargeable battery. The optimal control is based on Pontryagin’s minimum principle, and this study demonstrates that it results in total hydrogen consumptions if the properties of the fuel cell hybrid system satisfy two assumptions: first, the hydrogen consumption rate of the fuel cell system is convex and, second, the time derivative of the state of charge is concave. In order to validate optimality of the control concept, a vehicle simulation model is developed for a fuel cell hybrid electric vehicle, and control based on Pontryagin’s minimum principle is applied in a backward-looking simulator. In the simulation results, control based on Pontryagin’s minimum principle produces an optimal solution, which is very close to the global optimal solution obtained by dynamic programming. In addition, control based on Pontryagin’s minimum principle simplifies the control organization; the control concept should be considered as a promising solution for fuel cell hybrid electric vehicles because it really achieves results which are near the global optimal results.

Fabrication of gated diamond field emitter array using a selective diamond growth process

Abstract A novel processing sequence for the formation of gated diamond field emitter array (triode system) is proposed and the feasibility is tested by investigating the field emission property. The processing scheme is based on the selective deposition of diamond by using the well established nucleation enhanced process on substrate, so callled bias enhanced nucleation (BEN). The structure of substrate was patterned [SiO 2 /Mo(gate)SiO 2 (insulator)Si(100). Our preliminary results show that the diamond field emitter is turned on at around 87 V/μm with the current level of about several μA.