Browsing by Author "Choi, Changhyeok"

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    Electrochemical Ammonia Synthesis: Mechanistic Understanding and Catalyst Design
    (Chem,, 2021) Shen, Huidong; Choi, Changhyeok; Masa, Justus; Qiu, Jieshan; Jung, Yousung; Sun, Zhenyu
    NH3 production is dependent on the century-old Haber-Bosch process, which is energy and capital intensive and relies on H2 from steam reforming, hence, contributing to greenhouse gas emissions. Electrochemical NH3 synthesis can be realized by reaction of N2 and a proton source under mild conditions powered by renewable electricity, which offers a promising carbon-neutral and sustainable strategy. However, N2 has remarkable thermodynamic stability and requires high energy to be activated. Implementation of this “clean” NH3 synthesis route therefore still requires significant enhancement in energy efficiency, conversion rate, and durability, which is only achievable through the design of efficient electrocatalysts. This article provides a timely theoretical and experimental overview of recent advances in the electrocatalytic conversion of N2 to NH3 underlining the development of novel electrocatalysts. Advances of in situ and operando studies for mechanistic understanding of the reaction and the main challenges and strategies for improving electrocatalytic N2 reduction are highlighted.
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    Highly Stable Two-Dimensional Bismuth Metal-Organic Frameworks for Efficient Electrochemical Reduction of CO2
    (Applied Catalysis B: Environmental, 2020) Li, Fang; Gu, Geun Ho; Choi, Changhyeok; Masa, Justus; Mukerjee, Sanjeev; Jung, Yousung; Qiu, Jieshan
    We report a unique 2D bismuth metal-organic framework (Bi-MOF) that possesses permanent accessible porosity for efficient electrochemical CO2 reduction (ECR) to HCOOH. The 2D open-framework structure with helical Bi-O rods bridged by tritopic carboxylate ligands exhibits a remarkable Faradaic efficiency for HCOOH formation over a broad potential window, reaching 92.2 % at ∼ –0.9 V (vs. reversible hydrogen electrode, RHE) with excellent durability over 30 h. The mass-specific HCOOH partial current density is up to 41.0 mA mgBi−1, exceeding 4 times higher than that of commercial Bi2O3 and Bi sheets at ∼ –1.1 V (vs. RHE). Operando and ex-situ X-ray absorption fine structure spectroscopy revealed a structural feature associated with Bi-MOF to preserve Bi(3+) during and after long-term ECR. Theoretical calculations further showed that the crystallographically channels with abundant Bi active sites in the MOF structure favor the formation of *HCOO while suppressing the side-reaction of hydrogen evolution, thereby leading to the high selectivity for HCOOH.
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    Stabilization of Cu+ by tuning a CuO–CeO2 interface for selective electrochemical CO2 reduction to ethylene
    (Green Chemistry, 2020) Chu, Senlin; Yan, Xupeng; Choi, Changhyeok; Masa, Justus; Han, Buxing; Jung, Yousung; Sun, Zhenyu
    Electrochemical conversion of carbon dioxide (CO2) into multi-carbon fuels and chemical feedstocks is important but remains challenging. Here, we report the stabilization of Cu+ within a CuO–CeO2 interface for efficient and selective electrocatalytic CO2 reduction to ethylene under ambient conditions. Tuning the CuO/CeO2 interfacial interaction permits dramatic suppression of proton reduction and enhancement of CO2 reduction, with an ethylene faradaic efficiency (FE) as high as 50.0% at −1.1 V (vs. the reversible hydrogen electrode) in 0.1 M KHCO3, in stark contrast to 22.6% over pure CuO immobilized on carbon black (CB). The composite catalyst presents a 2.6-fold improvement in ethylene current compared to that of CuO/CB at similar overpotentials, which also exceeds many recently reported Cu-based materials. The FE of C2H4 remained at over 48.0% even after 9 h of continuous polarization. The Cu+ species are believed to be the adsorption as well as active sites for the activation of CO2 molecules, which remain almost unchanged after 1 h of electrolysis. Further density functional theory calculations demonstrate the preferred formation of Cu+ at the CuO–CeO2 interface. This work provides a simple avenue to convert CO2 into high-value hydrocarbons by rational stabilization of Cu+ species.
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    Surface-engineered Oxidized Two-dimensional Sb for Efficient Visible Light-Driven N2 Fixation
    (Nano Energy, 2020) Zhao, Zhenqing; Choi, Changhyeok; Hong, Song; Masa, Justus; Jung, Yousung
    Solar N2 fixation under visible light offers a promising method toward sustainable NH3 production at benign conditions. However, it still remains a formidable challenge to activate and cleave Ntriple bondN bonds and promote the separation and transport of electrons and holes during photocatalysis. To address these issues, the discovery and design of high-performance and robust photocatalysts is imperative. Here, we report the defect engineering of two-dimensional oxidized Sb nanosheets to activate intrinsically inactive Sb for efficient visible light-driven N2 reduction to NH3. Impressively, the Sb nanosheets rich in Sb and oxygen vacancies afford a remarkable NH3 formation rate of up to 388.5 μgNH3 h−1 gcat.−1 without cocatalyst in visible light, 8 times higher than that for bulk Sb and also significantly outperforming many previously reported photocatalysts. The defective Sb nanosheets exhibit excellent stability after five successive reaction cycles. Further density functional theory calculations reveal a considerably strong interaction between N2 and defects on the surface and edge of Sb nanosheets, which facilitates the formation of *NNH (N2 + (H+ + e-) → *NNH, where * denotes an adsorption site), thus promoting photocatalytic N2 reduction. This finding opens a novel avenue to enhancing N2 photofixation over inherently inactive surfaces by synergistically engineering defect sites.