Browsing by Author "Xia, Yang"

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    A Combined Strategy of Catalyst Design and Cell Development for Efficient Electroreduction of O2 to H2O2
    (2023-04-21) Xia, Yang; Wang, Haotian
    As the cost of renewable energy decreases in the past decades, it becomes increasingly attractive to utilize electric energy to produce fundamental chemical feedstocks or fuels. Hydrogen peroxide (H2O2), as an important chemical with a wide range of applications, is currently produced through the industrial anthraquinone process, which is energy and waste intensive. Electrocatalytic oxygen reduction reaction (ORR) towards H2O2 provides an alternative way to produce H2O2 in a green and delocalized H2O2 manner, with the need of only electricity, water, and air. The efficiency of this attractive alternative depends on the choices of cost-effective catalysts as well as well-designed reaction system. In the last decade, a rising number of catalysts have been reported showing promising ORR activity and selectivity. However, to make this green alternative as competitive as the current anthraquinone process in the industrial-relevant scale, there are still three major challenges. Firstly, there are still not many catalytic materials which can selectively and actively catalyze the 2e--ORR towards H2O2 instead of the commonly observed 4e- pathway towards H2O. Moreover, the as-generated H2O2 are typically in a mixture with liquid electrolytes in the traditional cell configurations, which needs further complicated separation process to obtain pure H2O2 solutions for real-world applications. Finally, the cell performance does not meet the requirement for industrial-level production needs, including low stability and low concentration. In this dissertation, we demonstrated a multi-approach solution, from catalyst design, reactor development and interfacial tuning to address the problems listed above. First, we report a direct electrosynthesis strategy combining a cost-effective oxidized carbon catalyst and a brand-new reactor design. The reaction system delivered separate hydrogen (H2)/water (H2O) and oxygen (O2) streams to an anode and cathode separated by a porous solid electrolyte, wherein the electrochemically generated H+ and HO2– recombine to form pure aqueous H2O2 solutions. We achieved over 90% selectivity for pure H2O2 solutions with concentrations up to 20 weight %, and the catalyst retained activity and selectivity for 100 hours. This reactor design was set up as the optimal platform for our later studies (Chapter 3). To further boost the performance of the reaction system, especially under high-current regime, we developed a boron-doped carbon (B-C) catalyst by xxx. Unlike previously reported carbon-based catalyst, it can achieve high selectivity and activity simultaneously under industrial-relevant production rates. Compared to the state-of-the-art oxidized carbon catalyst, B-C catalyst presents enhanced activity (saving more than 210 mV overpotential) under industrial-relevant currents (up to 300 mA cm−2) while maintaining high H2O2 selectivity (Chapter 4). In addition to catalyst development, we tuned the reaction interfacial conditions to enhance the concentration of the H2O2 product that can be obtained in our reaction system and substantially enhanced the stability to a new level (over 1000 hours) (Chapter 5). Finally, the key strategies combining catalyst design, reactor development and interfacial tuning are summarized, and possible future directions are discussed (Chapter 6).
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    Active dielectric antenna on chip for spatial light modulation
    (Nature Publishing Group, 2012-11-14) Qiu, Ciyuan; Chen, Jianbo; Xia, Yang; Xu, Qianfan
    Integrated photonic resonators are widely used to manipulate light propagation in an evanescently-coupled waveguide. While the evanescent coupling scheme works well for planar optical systems that are naturally waveguide based, many optical applications are free-space based, such as imaging, display, holographics, metrology and remote sensing. Here we demonstrate an active dielectric antenna as the interface device that allows the large-scale integration capability of silicon photonics to serve the free-space applications. We show a novel perturbation-base diffractive coupling scheme that allows a high-Q planer resonator to directly interact with and manipulate free-space waves. Using a silicon-based photonic crystal cavity whose resonance can be rapidly tuned with a p-i-n junction, a compact spatial light modulator with an extinction ratio of 9.5 dB and a modulation speed of 150 MHz is demonstrated. Method to improve the modulation speed is discussed.
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    CO2/carbonate-mediated electrochemical water oxidation to hydrogen peroxide
    (Springer Nature, 2022) Fan, Lei; Bai, Xiaowan; Xia, Chuan; Zhang, Xiao; Zhao, Xunhua; Xia, Yang; Wu, Zhen-Yu; Lu, Yingying; Liu, Yuanyue; Wang, Haotian
    Electrochemical water oxidation reaction (WOR) to hydrogen peroxide (H2O2) via a 2e− pathway provides a sustainable H2O2 synthetic route, but is challenged by the traditional 4e− counterpart of oxygen evolution. Here we report a CO2/carbonate mediation approach to steering the WOR pathway from 4e− to 2e−. Using fluorine-doped tin oxide electrode in carbonate solutions, we achieved high H2O2 selectivity of up to 87%, and delivered unprecedented H2O2 partial currents of up to 1.3 A cm−2, which represents orders of magnitude improvement compared to literature. Molecular dynamics simulations, coupled with electron paramagnetic resonance and isotope labeling experiments, suggested that carbonate mediates the WOR pathway to H2O2 through the formation of carbonate radical and percarbonate intermediates. The high selectivity, industrial-relevant activity, and good durability open up practical opportunities for delocalized H2O2 production.
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    Electrochemical ammonia synthesis via nitrate reduction on Fe single atom catalyst
    (Springer Nature, 2021) Wu, Zhen-Yu; Karamad, Mohammadreza; Yong, Xue; Huang, Qizheng; Cullen, David A.; Zhu, Peng; Xia, Chuan; Xiao, Qunfeng; Shakouri, Mohsen; Chen, Feng-Yang; Kim, Jung Yoon (Timothy); Xia, Yang; Heck, Kimberly; Hu, Yongfeng; Wong, Michael S.; Li, Qilin; Gates, Ian; Siahrostami, Samira; Wang, Haotian
    Electrochemically converting nitrate, a widespread water pollutant, back to valuable ammonia is a green and delocalized route for ammonia synthesis, and can be an appealing and supplementary alternative to the Haber-Bosch process. However, as there are other nitrate reduction pathways present, selectively guiding the reaction pathway towards ammonia is currently challenged by the lack of efficient catalysts. Here we report a selective and active nitrate reduction to ammonia on Fe single atom catalyst, with a maximal ammonia Faradaic efficiency of ~ 75% and a yield rate of up to ~ 20,000 μg h−1 mgcat.−1 (0.46 mmol h−1 cm−2). Our Fe single atom catalyst can effectively prevent the N-N coupling step required for N2 due to the lack of neighboring metal sites, promoting ammonia product selectivity. Density functional theory calculations reveal the reaction mechanisms and the potential limiting steps for nitrate reduction on atomically dispersed Fe sites.
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    Highly active and selective oxygen reduction to H2O2 on boron-doped carbon for high production rates
    (Springer Nature, 2021) Xia, Yang; Zhao, Xunhua; Xia, Chuan; Wu, Zhen-Yu; Zhu, Peng; Kim, Jung Yoon (Timothy); Bai, Xiaowan; Gao, Guanhui; Hu, Yongfeng; Zhong, Jun; Liu, Yuanyue; Wang, Haotian
    Oxygen reduction reaction towards hydrogen peroxide (H2O2) provides a green alternative route for H2O2 production, but it lacks efficient catalysts to achieve high selectivity and activity simultaneously under industrial-relevant production rates. Here we report a boron-doped carbon (B-C) catalyst which can overcome this activity-selectivity dilemma. Compared to the state-of-the-art oxidized carbon catalyst, B-C catalyst presents enhanced activity (saving more than 210 mV overpotential) under industrial-relevant currents (up to 300 mA cm−2) while maintaining high H2O2 selectivity (85–90%). Density-functional theory calculations reveal that the boron dopant site is responsible for high H2O2 activity and selectivity due to low thermodynamic and kinetic barriers. Employed in our porous solid electrolyte reactor, the B-C catalyst demonstrates a direct and continuous generation of pure H2O2 solutions with high selectivity (up to 95%) and high H2O2 partial currents (up to ~400 mA cm−2), illustrating the catalyst’s great potential for practical applications in the future.
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    Hydrogen Peroxide Electrosynthesis in a Strong Acidic Environment Using Cationic Surfactants
    (American Chemical Society, 2024) Adler, Zachary; Zhang, Xiao; Feng, Guangxia; Shi, Yaping; Zhu, Peng; Xia, Yang; Shan, Xiaonan; Wang, Haotian
    The two-electron oxygen reduction reaction (2e–-ORR) can be exploited for green production of hydrogen peroxide (H2O2), but it still suffers from low selectivity in an acidic electrolyte when using non-noble metal catalysts. Here, inspired by biology, we demonstrate a strategy that exploits the micellization of surfactant molecules to promote the H2O2 selectivity of a low-cost carbon black catalyst in strong acid electrolytes. The surfactants near the electrode surface increase the oxygen solubility and transportation, and they provide a shielding effect that displaces protons from the electric double layer (EDL). Compared with the case of a pure acidic electrolyte, we find that, when a small number of surfactant molecules were added to the acid, the H2O2 Faradaic efficiency (FE) was improved from 12% to 95% H2O2 under 200 mA cm–2, suggesting an 8-fold improvement. Our in situ surface enhanced Raman spectroscopy (SERS) and optical microscopy (OM) studies suggest that, while the added surfactant reduces the electrode’s hydrophobicity, its micelle formation could promote the O2 gas transport and its hydrophobic tail could displace local protons under applied negative potentials during catalysis, which are responsible for the improved H2O2 selectivity in strong acids.
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    Suspended Si Ring Resonator for Mid-IR Application
    (2013-11-22) Xia, Yang; Xu, Qianfan; Mittleman, Daniel M.; Kono, Junichiro
    Suspended ring resonators formed by both single-mode waveguide (SMW) and multi-mode waveguide (MMW) are designed, fabricated and characterized near 3.4 μm by thermal tuning and near 4.5 μm and 5.2 μm by tunable quantum cascade lasers. The dispersion property is analyzed by simulation in regards to frequency comb generation. The taper width is optimized for maximum coupling. Measurement setup is built up and described. For the SMW ring resonator, the intrinsic quality factor is fitted to be 6,800 and 16,000 near 5.2 μm and 4.5 μm, respectively. For the MMW ring resonator, it rises to 35,000 near 4.5 μm. Transmission spectrum distortion is observed at high input power, and is modeled as heat effect. Thermal tuning rate is experimentally confirmed at 0.21 nm/°C. Based on the measured distortion and heat simulation, absorption loss is estimated. All-optical modulation is conducted to estimate the response time of this process. It can be shown that main loss is from surface thus is reducible by improving surface quality. On-chip electrical heater is designed and preliminary experiment indicate the feasibility to pattern it with our Electron Beam Lithography system.