The recent advances in electrocatalysis for oxygen reduction reaction (ORR) for proton exchange membrane fuel cells (PEMFCs) are thoroughly reviewed. This comprehensive Review focuses on the low- and non-platinum electrocatalysts including advanced platinum alloys, core–shell structures, palladium-based catalysts, metal oxides and chalcogenides, carbon-based non-noble metal catalysts, and metal-free catalysts. The recent development of ORR electrocatalysts with novel structures and compositions is highlighted. The understandings of the correlation between the activity and the shape, size, composition, and synthesis method are summarized. For the carbon-based materials, their performance and stability in fuel cells and comparisons with those of platinum are documented. The research directions as well as perspectives on the further development of more active and less expensive electrocatalysts are provided.

http://repository.ust.hk/ir/bitstream/1783.1-77094/1/acs.chemrev.5b00462_withlink.pdf

Recent Advances in Electrocatalysts for Oxygen Reduction Reaction

To enhance and optimize nanocatalyst performance and durability for the oxygen reduction reaction in fuel-cell applications, we look beyond Pt-metal disordered alloys and describe a new class of Pt-Co nanocatalysts composed of ordered Pt(3)Co intermetallic cores with a 2-3 atomic-layer-thick platinum shell. These nanocatalysts exhibited over 200% increase in mass activity and over 300% increase in specific activity when compared with the disordered Pt(3)Co alloy nanoparticles as well as Pt/C. So far, this mass activity for the oxygen reduction reaction is the highest among the Pt-Co systems reported in the literature under similar testing conditions. Stability tests showed a minimal loss of activity after 5,000 potential cycles and the ordered core-shell structure was maintained virtually intact, as established by atomic-scale elemental mapping. The high activity and stability are attributed to the Pt-rich shell and the stable intermetallic Pt(3)Co core arrangement. These ordered nanoparticles provide a new direction for catalyst performance optimization for next-generation fuel cells.

Structurally ordered intermetallic platinum–cobalt core–shell nanoparticles with enhanced activity and stability as oxygen reduction electrocatalysts

Gallium oxide (Ga2O3) is emerging as a viable candidate for certain classes of power electronics, solar blind UV photodetectors, solar cells, and sensors with capabilities beyond existing technologies due to its large bandgap. It is usually reported that there are five different polymorphs of Ga2O3, namely, the monoclinic (β-Ga2O3), rhombohedral (α), defective spinel (γ), cubic (δ), or orthorhombic (e) structures. Of these, the β-polymorph is the stable form under normal conditions and has been the most widely studied and utilized. Since melt growth techniques can be used to grow bulk crystals of β-GaO3, the cost of producing larger area, uniform substrates is potentially lower compared to the vapor growth techniques used to manufacture bulk crystals of GaN and SiC. The performance of technologically important high voltage rectifiers and enhancement-mode Metal-Oxide Field Effect Transistors benefit from the larger critical electric field of β-Ga2O3 relative to either SiC or GaN. However, the absence of clear demonstrations of p-type doping in Ga2O3, which may be a fundamental issue resulting from the band structure, makes it very difficult to simultaneously achieve low turn-on voltages and ultra-high breakdown. The purpose of this review is to summarize recent advances in the growth, processing, and device performance of the most widely studied polymorph, β-Ga2O3. The role of defects and impurities on the transport and optical properties of bulk, epitaxial, and nanostructures material, the difficulty in p-type doping, and the development of processing techniques like etching, contact formation, dielectrics for gate formation, and passivation are discussed. Areas where continued development is needed to fully exploit the properties of Ga2O3 are identified.

A review of Ga2O3 materials, processing, and devices

The effect of Ar plasma treatment on amorphous indium gallium zinc oxide (a-IGZO) thin films was investigated The net electron carrier concentration (1020–1021cm−3) of the a-IGZO thin films dramatically increased upon their exposure to the Ar plasma compared to that (1014cm−3) of the as-deposited thin film The authors attempted to reduce the contact resistance between the Pt∕Ti (source/drain electrode) and a-IGZO (channel) by using the Ar plasma treatment Without the treatment, the a-IGZO thin film transistors (TFTs) with W∕L=50∕4μm exhibited a moderate field-effect mobility (μFE) of 33cm2∕Vs, subthreshold gate swing (S) of 025V∕decade, and Ion∕off ratio of 4×107 The device performance of the a-IGZO TFTs was significantly improved by the Ar plasma treatment As a result, an excellent S value of 019V∕decade and high Ion∕off ratio of 1×108, as well as a high μFE of 91cm2∕Vs, were achieved for the treated a-IGZO TFTs

Improvements in the device characteristics of amorphous indium gallium zinc oxide thin-film transistors by Ar plasma treatment

Reduction Reaction in Polymer Electrolyte Membrane Fuel Cells: Particle Size, Shape, and Composition Manipulation and Their Impact to Activity Yan-Jie Wang,†,‡ Nana Zhao,‡ Baizeng Fang,† Hui Li,* Xiaotao T. Bi,*,† and Haijiang Wang* †Department of Chemical and Biological Engineering, University of British Columbia, 2360 East Mall, Vancouver, BC Canada V6T 1Z3 ‡Vancouver International Clean-Tech Research Institute Inc., 4475 Wayburne Drive, Burnaby, Canada V5G 4X4 Electrochemical Materials, Energy, Mining and Environment, National Research Council Canada, 4250 Wesbrook Mall, Vancouver, BC, Canada V6T 1W5

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Carbon-Supported Pt-Based Alloy Electrocatalysts for the Oxygen Reduction Reaction in Polymer Electrolyte Membrane Fuel Cells: Particle Size, Shape, and Composition Manipulation and Their Impact to Activity

Vertical geometry Ni/Au-β-Ga2O3 Schottky rectifiers were fabricated on Hydride Vapor Phase Epitaxy layers on conducting bulk substrates, and the rectifying forward and reverse current-voltage characteristics were measured at temperatures in the range of 25–100 °C. The reverse breakdown voltage (VBR) of these β-Ga2O3 rectifiers without edge termination was a function of the diode diameter, being in the range of 920–1016 V (average value from 25 diodes was 975 ± 40 V, with 10 of the diodes over 1 kV) for diameters of 105 μm and consistently 810 V (810 ± 3 V for 22 diodes) for a diameter of 210 μm. The Schottky barrier height decreased from 1.1 at 25 °C to 0.94 at 100 °C, while the ideality factor increased from 1.08 to 1.28 over the same range. The figure-of-merit (VBR2/Ron), where Ron is the on-state resistance (∼6.7 mΩ cm2), was approximately 154.07 MW·cm−2 for the 105 μm diameter diodes. The reverse recovery time was 26 ns for switching from +5 V to −5 V. These results represent another impressive advanc...

High reverse breakdown voltage Schottky rectifiers without edge termination on Ga2O3

$\beta $ -Ga2O3 Schottky barrier diodes were fabricated in a vertical geometry structure consisting of Ni/Au rectifying contacts without edge termination on Si-doped epitaxial layers ( $10~\mu \text{m}$ , $\text{n}\sim 4\times 10^{15}$ cm $^{-3})$ on Sn-doped bulk Ga2O3 substrates with full-area Ti/Au back Ohmic contacts The reverse breakdown voltage, ${V} _ \text {BR}$ , was a function of rectifying contact area, ranging from 1600 V at $31\times 10^{-6}$ cm2 (20- $\mu \text{m}$ diameter) to ~250 V at $22{\times }10^{-3}$ cm $^{-2}$ (053-mm diameter) The current density near breakdown was not strongly dependent on contact circumference but did scale with contact area, indicating that the bulk current contribution was dominant The lowest ON-state resistance, ${R} _\text {on}$ , was 16 $\text{m}\Omega \cdot $ cm2 for the largest diode and 25 $\text{m}\Omega \cdot $ cm2 for the 1600-V rectifier, leading to a Baliga figure-of-merit ( ${V} _\text {BR}^{2}/{R} _\text {on})$ for the latter of approximately 1024 MW $\cdot $ cm $^{-2}$  The ON-OFF ratio was measured at a forward voltage of 13 V and ranged from $3\times 10^{7}$ to $25\times 10^{6}$ for reverse biases from −5 to −40 V and showed only a small dependence on temperature in the range from 25 °C to 100 °C

High Breakdown Voltage (−201) $\beta $ -Ga2O3 Schottky Rectifiers

The studies on enhancing the sensitivities of chemical sensors based on two-dimensional (2D) materials have been focused primarily on surface modifications including defect engineering, chemical doping, and incorporation of metal nanoparticles. Exfoliated black phosphorus (BP), which is one of the 2D materials, has attracted considerable attention because it offers higher sensitivity than other 2D materials (e.g., graphene and MoS2). In this study, for the first time, we attempt to increase the performance of BP chemical sensors to their theoretical limit by floating BP flakes on top of electrode posts in order to provide full (both sides) adsorption sites and avoid interface scattering effects. Our suspended BP gas sensors fabricated via dry transfer showed higher sensing performances than the conventional supported BP gas sensors (gas response was increased by approximately 23% at 200 ppm). In addition, faster response and recovery with high reproducibility were observed in suspended BP chemicals sensors than in the supported ones. Our work reveals the full potential of pristine BP-based chemical sensors and paves the way for the next-generation high performance 2D chemical sensors.

Suspended black phosphorus nanosheet gas sensors

The robust radiation resistance of wide-band gap materials is advantageous for space applications, where the high-energy particle irradiation deteriorates the performance of electronic devices. We report on the effects of proton irradiation of β-Ga2O3 nanobelts, whose energy band gap is ∼4.85 eV at room temperature. Back-gated field-effect transistor (FET) based on exfoliated quasi-two-dimensional β-Ga2O3 nanobelts were exposed to a 10 MeV proton beam. The proton-dose- and time-dependent characteristics of the radiation-damaged FETs were systematically analyzed. A 73% decrease in the field-effect mobility and a positive shift of the threshold voltage were observed after proton irradiation at a fluence of 2 × 1015 cm–2. Greater radiation-induced degradation occurs in the conductive channel of the β-Ga2O3 nanobelt than at the contact between the metal and β-Ga2O3. The on/off ratio of the exfoliated β-Ga2O3 FETs was maintained even after proton doses up to 2 × 1015 cm–2. The radiation-induced damage in the β...

Influence of High-Energy Proton Irradiation on β-Ga2O3 Nanobelt Field-Effect Transistors

An electrodeposition-based protocol for the synthesis of ternary Pt−Fe−Co electrocatalysts for the oxygen reduction reaction (ORR) has been developed. The eletrodeposition method suits the purpose ...

Soohwan Jang

Papers

High reverse breakdown voltage Schottky rectifiers without edge termination on Ga2O3

High Breakdown Voltage (−201) $\beta $ -Ga2O3 Schottky Rectifiers

Suspended black phosphorus nanosheet gas sensors

Influence of High-Energy Proton Irradiation on β-Ga2O3 Nanobelt Field-Effect Transistors

Ternary Pt−Fe−Co Alloy Electrocatalysts Prepared by Electrodeposition: Elucidating the Roles of Fe and Co in the Oxygen Reduction Reaction