Browsing by Author "Wang, Haotian"

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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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    Catalysts Design and Reactor Engineering for Electrochemical CO2 Capture and Utilization
    (2023-08-11) Zhu, Peng; Wang, Haotian
    The rapidly increasing concentration of carbon dioxide (CO2) in the atmosphere has raised serious concerns regarding global climate change. In response to this challenge, the Paris Agreement has set ambitious targets to reduce global greenhouse gas emissions and limit the global temperature increase to no more than 1.5˚C above pre-industrial levels. While international communities have announced ambitious goals for carbon emission reduction during the 2021 Leaders’ Summit on Climate, there is an urgent need for advanced technologies, including carbon capture, conversion, and storage, to effectively neutralize or even reduce CO2 emissions. Significant advancements have been made in renewable grid technologies, enabling the efficient harnessing of green electricity from sources such as solar and wind power. By leveraging these developments, electrochemical CO2 capture and utilization (CCU) have become increasingly attractive as a sustainable and economically viable approach for utilizing CO2 as a valuable resource. The decreasing cost of renewable electricity has made the electrochemical conversion of CO2 into useful chemical feedstocks more economically feasible, opening up new possibilities for mitigating CO2 emissions and advancing the circular carbon economy. In this dissertation, I focus on coupling catalysts design and cell engineering to develop CO2 capture technologies and CO2 electrochemical reduction methods. Our research aims to contribute to the mitigation of CO2 emissions, the capture of carbon as a valuable resource, and the promotion of a more sustainable and low-carbon future. Through the development of various catalysts such as bismuth (Bi), copper (Cu) and single atom catalysts (SACs), as well as novel solid electrolyte reactors, I have successfully achieved the continuous generation of CO gas, pure liquid fuels such as formic acid and acetic acid, and CO2 capture. These advancements offer promising solutions for addressing CO2 emissions and advancing the utilization of CO2 as a valuable feedstock.
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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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    Electrochemical CO 2 reduction to high-concentration pure formic acid solutions in an all-solid-state reactor
    (Springer Nature, 2020) Fan, Lei; Xia, Chuan; Zhu, Peng; Lu, Yingying; Wang, Haotian
    Electrochemical CO2 reduction reaction (CO2RR) to liquid fuels is currently challenged by low product concentrations, as well as their mixture with traditional liquid electrolytes, such as KHCO3 solution. Here we report an all-solid-state electrochemical CO2RR system for continuous generation of high-purity and high-concentration formic acid vapors and solutions. The cathode and anode were separated by a porous solid electrolyte (PSE) layer, where electrochemically generated formate and proton were recombined to form molecular formic acid. The generated formic acid can be efficiently removed in the form of vapors via inert gas stream flowing through the PSE layer. Coupling with a high activity (formate partial current densities ~450 mA cm−2), selectivity (maximal Faradaic efficiency ~97%), and stability (100 hours) grain boundary-enriched bismuth catalyst, we demonstrated ultra-high concentrations of pure formic acid solutions (up to nearly 100 wt.%) condensed from generated vapors via flexible tuning of the carrier gas stream.
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    Electrochemical Synthesis of Green Hydrogen and Ammonia via Catalyst Design and Electrolyzer Engineering
    (2024-08-06) Chen, Feng-Yang; Wang, Haotian
    The rapid increase in atmospheric carbon dioxide levels has become a pressing concern for global climate change. Electrocatalysis has emerged as a critical pathway for decarbonizing chemicals and fuels, particularly in the production of hydrogen and ammonia, given the intensive carbon emissions associated with conventional chemical engineering plants. In this thesis, we systematically address the current challenges within electrocatalytic water splitting and nitrate reduction reactions, which are critical processes for green hydrogen and ammonia synthesis. We first investigated mechanistic insights into the stability challenges of oxygen evolution reaction catalysts, alongside practical considerations for reactor design. A non-iridium-based electrocatalyst was then developed to reduce costs and enhance durability for the acidic oxygen evolution reaction, integrated into a proton exchange membrane electrolyzer to facilitate efficient green hydrogen production. Additionally, we investigated an oxide alloy catalyst system aimed at further reducing noble metal loading while enhancing catalyst activity. Furthermore, we examined electrochemical nitrate reduction as an alternative pathway for green ammonia production, focusing on the design and synthesis of catalysts for efficient conversion. Moreover, we designed a solid electrolyte reactor and coupled it with a cation shuttling process to advance the direct conversion of waste nitrate streams into green ammonia. The catalyst design and electrolyzer engineering strategies proposed in this dissertation contribute meaningfully to the development of electrochemical technologies crucial for sustainable energy and resource management.
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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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    Highly selective oxygen reduction to hydrogen peroxide on transition metal single atom coordination
    (Springer Nature, 2019) Jiang, Kun; Back, Seoin; Akey, Austin J.; Xia, Chuan; Hu, Yongfeng; Liang, Wentao; Schaak, Diane; Stavitski, Eli; Nørskov, Jens K.; Siahrostami, Samira; Wang, Haotian
    Shifting electrochemical oxygen reduction towards 2e– pathway to hydrogen peroxide (H2O2), instead of the traditional 4e– to water, becomes increasingly important as a green method for H2O2 generation. Here, through a flexible control of oxygen reduction pathways on different transition metal single atom coordination in carbon nanotube, we discovered Fe-C-O as an efficient H2O2 catalyst, with an unprecedented onset of 0.822 V versus reversible hydrogen electrode in 0.1 M KOH to deliver 0.1 mA cm−2 H2O2 current, and a high H2O2 selectivity of above 95% in both alkaline and neutral pH. A wide range tuning of 2e–/4e– ORR pathways was achieved via different metal centers or neighboring metalloid coordination. Density functional theory calculations indicate that the Fe-C-O motifs, in a sharp contrast to the well-known Fe-C-N for 4e–, are responsible for the H2O2 pathway. This iron single atom catalyst demonstrated an effective water disinfection as a representative application.
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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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    Optimizing Carbon Dioxide Electrolyzers Towards Industrialization via Local Environment and Device Engineering
    (2024-08-09) Kim, Timothy Yoon; Wang, Haotian
    Electrochemical CO2 reduction reaction (CO2RR) holds promise for storing and utilizing renewable electricity, contributing to carbon neutralization efforts. However, significant challenges persist in transitioning CO2RR devices to industrial applications. This research addresses key hurdles, focusing on enhancing carbon utilization, versatility, and controllability of CO2 electrolysis through device engineering and local environment adjustments. Specifically, our investigations encompass: CO2 recovery using solid electrolytes, investigating selectivity differences between CO2 and CO within local environments, and designing novel covalent framework-based solid electrolytes. The findings from our investigations not only provide insights into fundamental aspects of CO2 reduction but also offer practical pathways for realizing more efficient and sustainable CO2 utilization technologies.
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    Revealing the Dual-Layered Solid Electrolyte Interphase on Lithium Metal Anodes via Cryogenic Electron Microscopy
    (American Chemical Society, 2023) Wi, Tae-Ung; Park, Sung O; Yeom, Su Jeong; Kim, Min-Ho; Kristanto, Imanuel; Wang, Haotian; Kwak, Sang Kyu; Lee, Hyun-Wook
    It is crucial to comprehend the effect of the solid electrolyte interphase (SEI) on battery performance to develop stable Li metal batteries. Nonetheless, the exact nanostructure and working mechanisms of the SEI remain obscure. Here, we have investigated the relationship between electrolyte components and the structural configuration of interfacial layers using an optimized cryogenic transmission electron microscopy (Cryo-TEM) analysis and theoretical calculation. We revealed a unique dual-layered inorganic-rich nanostructure, in contrast to the widely known simple specific component-rich SEI layers. The origin of stable Li cycling is closely related to the Li-ion diffusion mechanism via diverse crystalline grains and numerous grain boundaries in the fine crystalline-rich SEI layer. The results can elucidate a particular issue pertaining to the chemical structure of SEI layers that can induce uniform Li diffusion and rapid Li-ion conduction on Li metal anodes, developing stable Li metal batteries.