Academic Appointments


  • Physical Science Research Scientist, T. H. Geballe Laboratory for Advanced Materials

All Publications


  • Sulfur and Wavy-Stacking Boosted Superior Lithium Storage in 2D Covalent Organic Frameworks. Small (Weinheim an der Bergstrasse, Germany) Li, N., Zhu, J., Yang, C., Huang, S., Jiang, K., Zheng, Q., Yang, Y., Mao, H., Han, S., Zhu, L., Zhuang, X. 2024: e2405974

    Abstract

    2D conjugated covalent organic frameworks (c-COFs) provide an attractive foundation as organic electrodes in energy storage devices, but their storage capability is long hindered by limited ion accessibility within densely pi-pi stacked interlayers. Herein, two kinds of 2D c-COFs based on dioxin and dithiine linkages are reported, which exhibit distinct in-plane configurations-fully planar and undulated layers. X-ray diffraction analysis reveals wavy square-planar networks in dithiine-bridged COF (COF-S), attributed to curved C─S─C bonds in the dithiine linkage, whereas dioxin-bridged COF (COF-O) features densely packed fully planar layers. Theoretical and experimental results elucidate that the undulated stacking within COF-S possesses an expanded layer distance of 3.8 A and facilitates effective and rapid Li+ storage, yielding a superior specific capacity of 1305 mAh g-1 at 0.5 A g-1, surpassing that of COF-O (1180 mAh g-1 at 0.5 A g-1). COF-S also demonstrates an admirable cycle life with 80.4% capacity retention after 5000 cycles. As determined, self-expanded wavy-stacking geometry, S-enriched dithiine in COF-S enhances the accessibility and redox activity of Li storage, allowing each phthalocyanine core to store 12 Li+ compared to 8 Li+ in COF-O. These findings underscore the elements and stacking modes of 2D c-COFs, enabling tunable layer distance and modulation of accessible ions.

    View details for DOI 10.1002/smll.202405974

    View details for PubMedID 39148200

  • Dynamic Bubbling Balanced Proactive CO2Capture and Reduction on a Triple-Phase Interface Nanoporous Electrocatalyst. Journal of the American Chemical Society Zhang, W., Yu, A., Mao, H., Feng, G., Li, C., Wang, G., Chang, J., Halat, D., Li, Z., Yu, W., Shi, Y., Liu, S., Fox, D. W., Zhuang, H., Cai, A., Wu, B., Joshua, F., Martinez, J. R., Zhai, L., Gu, M. D., Shan, X., Reimer, J. A., Cui, Y., Yang, Y. 2024

    Abstract

    The formation and preservation of the active phase of the catalysts at the triple-phase interface during CO2 capture and reduction is essential for improving the conversion efficiency of CO2 electroreduction toward value-added chemicals and fuels under operational conditions. Designing such ideal catalysts that can mitigate parasitic hydrogen generation and prevent active phase degradation during the CO2 reduction reaction (CO2RR), however, remains a significant challenge. Herein, we developed an interfacial engineering strategy to build a new SnOx catalyst by invoking multiscale approaches. This catalyst features a hierarchically nanoporous structure coated with an organic F-monolayer that modifies the triple-phase interface in aqueous electrolytes, substantially reducing competing hydrogen generation (less than 5%) and enhancing CO2RR selectivity (90%). This rationally designed triple-phase interface overcomes the issue of limited CO2 solubility in aqueous electrolytes via proactive CO2 capture and reduction. Concurrently, we utilized pulsed square-wave potentials to dynamically recover the active phase for the CO2RR to regulate the production of C1 products such as formate and carbon monoxide (CO). This protocol ensures profoundly enhanced CO2RR selectivity (90%) compared with constant potential (70%) applied at -0.8 V (V vs RHE). We further achieved a mechanistic understanding of the CO2 capture and reduction processes under pulsed square-wave potentials via in situ Raman spectroscopy, thereby observing the potential-dependent intensity of Raman vibrational modes of the active phase and CO2RR intermediates. This work will inspire material design strategies by leveraging triple-phase interface engineering for emerging electrochemical processes, as technology moves toward electrification and decarbonization.

    View details for DOI 10.1021/jacs.4c02786

    View details for PubMedID 39049158

  • Multivariate Machine Learning Models of Nanoscale Porosity from Ultrafast NMR Relaxometry. Angewandte Chemie (International ed. in English) Fricke, S. N., Salgado, M., Menezes, T., Costa Santos, K. M., Gallagher, N., Song, A. Y., Wang, J., Engler, K., Wang, Y., Mao, H., Reimer, J. A. 2024: e202316664

    Abstract

    Nanoporous materials are of great interest in many applications, such as catalysis, separation, and energy storage. The performance of these materials is closely related to their pore sizes, which are inefficient to determine through the conventional measurement of gas adsorption isotherms. Nuclear magnetic resonance (NMR) relaxometry has emerged as a technique highly sensitive to porosity in such materials. Nonetheless, streamlined methods to estimate pore size from NMR relaxometry remain elusive. Previous attempts have been hindered by inverting a time domain signal to relaxation rate distribution, and dealing with resulting parameters that vary in number, location, and magnitude. Here we invoke well-established machine learning techniques to directly correlate time domain signals to BET surface areas for a set of metal-organic frameworks (MOFs) imbibed with solvent at varied concentrations. We employ this series of MOFs to establish a correlation between NMR signal and surface area via partial least squares (PLS), following screening with principal component analysis, and apply the PLS model to predict surface area of various nanoporous materials. This approach offers a high-throughput, non-destructive way to assess porosity in c.a. one minute. We anticipate this work will contribute to the development of new materials with optimized pore sizes for various applications.

    View details for DOI 10.1002/anie.202316664

    View details for PubMedID 38290006

  • Unveiling the complexity of nanodiamond structures. Proceedings of the National Academy of Sciences of the United States of America Zheng, Q., Shi, X., Jiang, J., Mao, H., Montes, N., Kateris, N., Reimer, J. A., Wang, H., Zheng, H. 2023; 120 (23): e2301981120

    Abstract

    Understanding nanodiamond structures is of great scientific and practical interest. It has been a long-standing challenge to unravel the complexity underlying nanodiamond structures and to resolve the controversies surrounding their polymorphic forms. Here, we use transmission electron microscopy with high-resolution imaging, electron diffraction, multislice simulations, and other supplementary techniques to study the impacts of small sizes and defects on cubic diamond nanostructures. The experimental results show that common cubic diamond nanoparticles display the (200) forbidden reflections in their electron diffraction patterns, which makes them indistinguishable from new diamond (n-diamond). The multislice simulations demonstrate that cubic nanodiamonds smaller than 5 nm can present the d-spacing at 1.78 Å corresponding to the (200) forbidden reflections, and the relative intensity of these reflections increases as the particle size decreases. Our simulation results also reveal that defects, such as surface distortions, internal dislocations, and grain boundaries can also make the (200) forbidden reflections visible. These findings provide valuable insights into the diamond structural complexity at nanoscale, the impact of defects on nanodiamond structures, and the discovery of novel diamond structures.

    View details for DOI 10.1073/pnas.2301981120

    View details for PubMedID 37253001

  • Reflections in search of faculty positions MATTER Mao, H., Rosen, A., Sanchez, D., Sanchez, V., Cranford, S. 2023; 6 (2): 300-307
  • A scalable solid-state nanoporous network with atomic-level interaction design for carbon dioxide capture. Science advances Mao, H., Tang, J., Day, G. S., Peng, Y., Wang, H., Xiao, X., Yang, Y., Jiang, Y., Chen, S., Halat, D. M., Lund, A., Lv, X., Zhang, W., Yang, C., Lin, Z., Zhou, H. C., Pines, A., Cui, Y., Reimer, J. A. 2022; 8 (31): eabo6849

    Abstract

    Carbon capture and sequestration reduces carbon dioxide emissions and is critical in accomplishing carbon neutrality targets. Here, we demonstrate new sustainable, solid-state, polyamine-appended, cyanuric acid-stabilized melamine nanoporous networks (MNNs) via dynamic combinatorial chemistry (DCC) at the kilogram scale toward effective and high-capacity carbon dioxide capture. Polyamine-appended MNNs reaction mechanisms with carbon dioxide were elucidated with double-level DCC where two-dimensional heteronuclear chemical shift correlation nuclear magnetic resonance spectroscopy was performed to demonstrate the interatomic interactions. We distinguished ammonium carbamate pairs and a mix of ammonium carbamate and carbamic acid during carbon dioxide chemisorption. The coordination of polyamine and cyanuric acid modification endows MNNs with high adsorption capacity (1.82 millimoles per gram at 1 bar), fast adsorption time (less than 1 minute), low price, and extraordinary stability to cycling by flue gas. This work creates a general industrialization method toward carbon dioxide capture via DCC atomic-level design strategies.

    View details for DOI 10.1126/sciadv.abo6849

    View details for PubMedID 35921416

  • Origin of enhanced water oxidation activity in an iridium single atom anchored on NiFe oxyhydroxide catalyst. Proceedings of the National Academy of Sciences of the United States of America Zheng, X., Tang, J., Gallo, A., Garrido Torres, J. A., Yu, X., Athanitis, C. J., Been, E. M., Ercius, P., Mao, H., Fakra, S. C., Song, C., Davis, R. C., Reimer, J. A., Vinson, J., Bajdich, M., Cui, Y. 2021; 118 (36)

    Abstract

    The efficiency of the synthesis of renewable fuels and feedstocks from electrical sources is limited, at present, by the sluggish water oxidation reaction. Single-atom catalysts (SACs) with a controllable coordination environment and exceptional atom utilization efficiency open new paradigms toward designing high-performance water oxidation catalysts. Here, using operando X-ray absorption spectroscopy measurements with calculations of spectra and electrochemical activity, we demonstrate that the origin of water oxidation activity of IrNiFe SACs is the presence of highly oxidized Ir single atom (Ir5.3+) in the NiFe oxyhydroxide under operating conditions. We show that the optimal water oxidation catalyst could be achieved by systematically increasing the oxidation state and modulating the coordination environment of the Ir active sites anchored atop the NiFe oxyhydroxide layers. Based on the proposed mechanism, we have successfully anchored Ir single-atom sites on NiFe oxyhydroxides (Ir0.1/Ni9Fe SAC) via a unique in situ cryogenic-photochemical reduction method that delivers an overpotential of 183 mV at 10 mA cm- 2 and retains its performance following 100 h of operation in 1 M KOH electrolyte, outperforming the reported catalysts and the commercial IrO2 catalysts. These findings open the avenue toward an atomic-level understanding of the oxygen evolution of catalytic centers under in operando conditions.

    View details for DOI 10.1073/pnas.2101817118

    View details for PubMedID 34465618

  • Origin of enhanced water oxidation activity in an iridium single atom anchored on NiFe oxyhydroxide catalyst PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Zheng, X., Tang, J., Gallo, A., Torres, J., Yu, X., Athanitis, C. J., Been, E., Ercius, P., Mao, H., Fakra, S. C., Song, C., Davis, R. C., Reimer, J. A., Vinson, J., Bajdich, M., Cui, Y. 2021; 118 (36)
  • Revealing Molecular Mechanisms in Hierarchical Nanoporous Carbon via Nuclear Magnetic Resonance MATTER Mao, H., Tang, J., Xu, J., Peng, Y., Chen, J., Wu, B., Jiang, Y., Hou, K., Chen, S., Wang, J., Lee, H., Halat, D. M., Zhang, B., Chen, W., Plantz, A. Z., Lu, Z., Cui, Y., Reimer, J. A. 2020; 3 (6): 2093–2107
  • Anion Etching for Accessing Rapid and Deep Self-Reconstruction of Precatalysts for Water Oxidation MATTER Wang, Y., Zhu, Y., Zhao, S., She, S., Zhang, F., Chen, Y., Williams, T., Gengenbach, T., Zu, L., Mao, H., Zhou, W., Shao, Z., Wang, H., Tang, J., Zhao, D., Selomulya, C. 2020; 3 (6): 2124–37
  • Dynamic Covalent Synthesis of Crystalline Porous Graphitic Frameworks CHEM Li, X., Wang, H., Chen, H., Zheng, Q., Zhang, Q., Mao, H., Liu, Y., Cai, S., Sun, B., Dun, C., Gordon, M. P., Zheng, H., Reimer, J. A., Urban, J. J., Ciston, J., Tan, T., Chan, E. M., Zhang, J., Liu, Y. 2020; 6 (4): 933–44
  • Designing hierarchical nanoporous membranes for highly efficient gas adsorption and storage. Science advances Mao, H. n., Tang, J. n., Chen, J. n., Wan, J. n., Hou, K. n., Peng, Y. n., Halat, D. M., Xiao, L. n., Zhang, R. n., Lv, X. n., Yang, A. n., Cui, Y. n., Reimer, J. A. 2020; 6 (41)

    Abstract

    Nanoporous membranes with two-dimensional materials such as graphene oxide have attracted attention in volatile organic compounds (VOCs) and H2 adsorption because of their unique molecular sieving properties and operational simplicity. However, agglomeration of graphene sheets and low efficiency remain challenging. Therefore, we designed hierarchical nanoporous membranes (HNMs), a class of nanocomposites combined with a carbon sphere and graphene oxide. Hierarchical carbon spheres, prepared following Murray's law using chemical activation incorporating microwave heating, act as spacers and adsorbents. Hierarchical carbon spheres preclude the agglomeration of graphene oxide, while graphene oxide sheets physically disperse, ensuring structural stability. The obtained HNMs contain micropores that are dominated by a combination of ultramicropores and mesopores, resulting in high VOCs/H2 adsorption capacity, up to 235 and 352 mg/g at 200 ppmv and 3.3 weight % (77 K and 1.2 bar), respectively. Our work substantially expands the potential for HNMs applications in the environmental and energy fields.

    View details for DOI 10.1126/sciadv.abb0694

    View details for PubMedID 33028517

  • Three-Dimensional Hierarchical Porous Nanotubes Derived from Metal-Organic Frameworks for Highly Efficient Overall Water Splitting. iScience Wang, Y. n., Zhao, S. n., Zhu, Y. n., Qiu, R. n., Gengenbach, T. n., Liu, Y. n., Zu, L. n., Mao, H. n., Wang, H. n., Tang, J. n., Zhao, D. n., Selomulya, C. n. 2019; 23 (1): 100761

    Abstract

    Effective design of bifunctional catalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is important but remains challenging. Herein, we report a three-dimensional (3D) hierarchical structure composed of homogeneously distributed Ni-Fe-P nanoparticles embedded in N-doped carbons on nickel foams (denoted as Ni-Fe-P@NC/NF) as an excellent bifunctional catalyst. This catalyst was fabricated by an anion exchange method and a low-temperature phosphidation of nanotubular Prussian blue analogue (PBA). The Ni-Fe-P@NC/NF displayed exceptional catalytic activity toward both HER and OER and delivered an ultralow cell voltage of 1.47 V to obtain 10 mA cm-2 with extremely excellent durability for 100 h when assembled as a practical electrolyser. The extraordinary performance of Ni-Fe-P@NC/NF is attributed to the abundance of unsaturated active sites, the well-defined hierarchical porous structure, and the synergistic effect between multiple components. Our work will inspire more rational designs of highly active non-noble electrocatalysts for industrial energy applications.

    View details for DOI 10.1016/j.isci.2019.100761

    View details for PubMedID 31887660