Joon Hyung Shim

Solid oxide fuel cell with corrugated thin film electrolyte

A low temperature micro solid oxide fuel cell with corrugated electrolyte membrane was developed and tested. To increase the electrochemically active surface area, yttria-stabilized zirconia membranes with thickness of 70 nm were deposited onto prepatterned silicon substrates. Fuel cell performance of the corrugated electrolyte membranes released from silicon substrate showed an increase of power density relative to membranes with planar electrolytes. Maximum power densities of the corrugated fuel cells of 677 mW/cm2 and 861 mW/cm2 were obtained at 400 and 450 degrees C, respectively.

Low-frequency noise in multilayer MoS2 field-effect transistors: the effect of high-k passivation

Diagnosing of the interface quality and the interactions between insulators and semiconductors is significant to achieve the high performance of nanodevices. Herein, low-frequency noise (LFN) in mechanically exfoliated multilayer molybdenum disulfide (MoS2) (~11.3 nm-thick) field-effect transistors with back-gate control was characterized with and without an Al2O3 high-k passivation layer. The carrier number fluctuation (CNF) model associated with trapping/detrapping the charge carriers at the interface nicely described the noise behavior in the strong accumulation regime both with and without the Al2O3 passivation layer. The interface trap density at the MoS2-SiO2 interface was extracted from the LFN analysis, and estimated to be Nit ~ 10(10) eV(-1) cm(-2) without and with the passivation layer. This suggested that the accumulation channel induced by the back-gate was not significantly influenced by the passivation layer. The Hooge mobility fluctuation (HMF) model implying the bulk conduction was found to describe the drain current fluctuations in the subthreshold regime, which is rarely observed in other nanodevices, attributed to those extremely thin channel sizes. In the case of the thick-MoS2 (~40 nm-thick) without the passivation, the HMF model was clearly observed all over the operation regime, ensuring the existence of the bulk conduction in multilayer MoS2. With the Al2O3 passivation layer, the change in the noise behavior was explained from the point of formation of the additional top channel in the MoS2 because of the fixed charges in the Al2O3. The interface trap density from the additional CNF model was Nit = 1.8 × 10(12) eV(-1) cm(-2) at the MoS2-Al2O3 interface.

Proton conduction in thin film yttrium-doped barium zirconate

The proton conductivity of yttrium-doped barium zirconate (BYZ) films epitaxially grown on MgO(100) has been studied in the range of 140–290°C as a function of film thickness (60–670nm) in relation to their crystal and morphological structure at the nanoscale. Highly textured 60nm BYZ film epitaxially grown on MgO(100) showed high ionic conductivity, close to its bulk value. In contrast, thicker polycrystalline samples with rougher surfaces, caused by grain boundary formation, exhibited lower conductivity.

Catalysts with Pt Surface Coating by Atomic Layer Deposition for Solid Oxide Fuel Cells

We tested the atomic layer deposition of platinum layers onto various sputtered porous metals including silver, palladium, ruthenium, and gold as catalysts for solid oxide fuel cells. We investigated thermal and chemical stability against oxidation using X-ray photoelectron spectroscopy. Fuel cell tests were conducted using the metal-atomic layer deposition platinum cathodes with 200 μm thick, 8% yttria-stabilized zirconia electrolytes and sputtered platinum anodes. Performance of the fuel cells was measured by comparing the current-voltage curves and maximum power densities with varying temperature.

Demonstrating the potential of yttrium-doped barium zirconate electrolyte for high-performance fuel cells

In reducing the high operating temperatures (≥800 °C) of solid-oxide fuel cells, use of protonic ceramics as an alternative electrolyte material is attractive due to their high conductivity and low activation energy in a low-temperature regime (≤600 °C). Among many protonic ceramics, yttrium-doped barium zirconate has attracted attention due to its excellent chemical stability, which is the main issue in protonic-ceramic fuel cells. However, poor sinterability of yttrium-doped barium zirconate discourages its fabrication as a thin-film electrolyte and integration on porous anode supports, both of which are essential to achieve high performance. Here we fabricate a protonic-ceramic fuel cell using a thin-film-deposited yttrium-doped barium zirconate electrolyte with no impeding grain boundaries owing to the columnar structure tightly integrated with nanogranular cathode and nanoporous anode supports, which to the best of our knowledge exhibits a record high-power output of up to an order of magnitude higher than those of other reported barium zirconate-based fuel cells.

Enhanced Oxygen Exchange on Surface-Engineered Yttria-Stabilized Zirconia

Ion conducting oxides are commonly used as electrolytes in electrochemical devices including solid oxide fuel cells and oxygen sensors. A typical issue with these oxide electrolytes is sluggish oxygen surface kinetics at the gas-electrolyte interface. An approach to overcome this sluggish kinetics is by engineering the oxide surface with a lower oxygen incorporation barrier. In this study, we engineered the surface doping concentration of a common oxide electrolyte, yttria-stabilized zirconia (YSZ), with the help of atomic layer deposition (ALD). On optimizing the dopant concentration at the surface of single-crystal YSZ, a 5-fold increase in the oxygen surface exchange coefficient of the electrolyte was observed using isotopic oxygen exchange experiments coupled with secondary ion mass spectrometer measurements. The results demonstrate that electrolyte surface engineering with ALD can have a meaningful impact on the performance of electrochemical devices.

Separation of interlayer resistance in multilayer MoS2 field-effect transistors

We extracted the interlayer resistance between two layers in multilayer molybdenum disulfide (MoS2) field-effect transistors by confirming that contact resistances (Rcontact) measured using the four-probe measurements were similar, within ∼30%, to source/drain series resistances (Rsd) measured using the two-probe measurements. Rcontact values obtained from gated four-probe measurements exhibited gate voltage dependency. In the two-probe measurements, the Y-function method was applied to obtain the Rsd values. By comparing those two Rcontact (∼9.5 kΩ) and Rsd (∼12.3 kΩ) values in strong accumulation regime, we found the rationality that those two values had nearly the same properties, i.e., the Schottky barrier resistances and interlayer resistances. The Rsd values of devices with two-probe source/drain electrodes exhibited thickness dependency due to interlayer resistance changes. The interlayer resistance between two layers was also obtained as ∼2.0 Ω mm.

Ion conduction in nanoscale yttria-stabilized zirconia fabricated by atomic layer deposition with various doping rates

The ion conduction of yttria-stabilized zirconia (YSZ) was studied by varying the doping ratios during atomic layer deposition (ALD). The ALD cycle ratio for the yttria and zirconia depositions was varied from 1:1 to 1:6, which corresponded to the doping ratios from 28.8% to 4.3%. The in-plane conductivity of ALD YSZ was enhanced by up to 2 orders of magnitude; the optimal ALD doping ratio (10.4%) was found to differ from that of bulk YSZ (8%). This different relationship between the doping ratio and the ion conduction for ALD YSZ versus bulk YSZ is due to the inhomogeneous doping in the vertical direction of the ALD YSZ films, as opposed to the homogenous doping of bulk YSZ.

High-Performance Protonic Ceramic Fuel Cells with Thin-Film Yttrium-Doped Barium Cerate–Zirconate Electrolytes on Compositionally Gradient Anodes

In this study, we used a compositionally gradient anode functional layer (AFL) consisting of Ni-BaCe(0.5)Zr(0.35)Y(0.15)O(3-δ) (BCZY) with increasing BCZY contents toward the electrolyte-anode interface for high-performance protonic ceramic fuel cells. It is identified that conventional homogeneous AFLs fail to stably accommodate a thin film of BCZY electrolyte. In contrast, a dense 2 μm thick BCZY electrolyte was successfully deposited onto the proposed gradient AFL with improved adhesion. A fuel cell containing this thin electrolyte showed a promising maximum peak power density of 635 mW cm(-2) at 600 °C, with an open-circuit voltage of over 1 V. Impedance analysis confirmed that minimizing the electrolyte thickness is essential for achieving a high power output, suggesting that the anode structure is important in stably accommodating thin electrolytes.

High-performance thin-film protonic ceramic fuel cells fabricated on anode supports with a non-proton-conducting ceramic matrix

A novel strategy to fabricate high-performance thin-film protonic ceramic fuel cells (PCFCs) is introduced by building thin-film PCFC components, including BaCe0.55Zr0.3Y0.15O3−δ (BCZY) electrolytes (1.5 μm) over anode supports consisting of non-proton-conducting ceramic and metal catalytic phases. Ni–yttria-stabilized zirconia (YSZ) was used as supports in this study, which is superior in terms of its well-established facile fabrication process, along with physical and chemical stability, compared to proton-conducting materials. The Ni–YSZ supports provided a flat and smooth deposition surface that facilitates the deposition of the thin film components. A Ni–BCZY anode (∼3 μm), a dense BCZY electrolyte layer (∼1.5 μm), and a porous Ba0.5Sr0.5Co0.8Fe0.2O3−δ cathode (∼2 μm) were sequentially fabricated over the Ni–YSZ substrates using pulsed laser deposition, followed by post-annealing, and the process was optimized for each component. A fully integrated thin-film PCFC microstructure was confirmed, resulting in high open circuit voltages exceeding 1 V at operating temperatures in the range of 450–650 °C. A promising fuel cell performance was obtained using the proposed fuel cell configuration, reaching a peak power density of 742 mW cm−2 at 650 °C.