Showing papers by "Hong Jin Fan published in 2021"

Atomically Dispersed Co2 -N6 and Fe-N4 Costructures Boost Oxygen Reduction Reaction in Both Alkaline and Acidic Media.

Abstract: Polynary transition-metal atom catalysts are promising to supersede platinum (Pt)-based catalysts for oxygen reduction reaction (ORR). Regulating the local configuration of atomic catalysts is the key to catalyst performance enhancement. Different from the previously reported single-atom or dual-atom configurations, a new type of ternary-atom catalyst, which consists of atomically dispersed, nitrogen-coordinated Co-Co dimers, and Fe single sites (i.e., Co2 -N6 and Fe-N4 structures) that are coanchored on highly graphitized carbon supports is developed. This unique atomic ORR catalyst outperforms the catalysts with only Co2 -N6 or Fe-N4 sites in both alkaline and acid conditions. Density functional theory calculations clearly unravels the synergistic effect of the Co2 -N6 and Fe-N4 sites, which can induce higher filling degree of Fe-d orbitals and favors the binding capability to *OH intermediates (the rate determining step). This ternary-atom catalyst may be a promising alternative to Pt to drive the cathodic ORR in zinc-air batteries.

Tip‐Enhanced Electric Field: A New Mechanism Promoting Mass Transfer in Oxygen Evolution Reactions

Abstract: The slow kinetics of oxygen evolution reaction (OER) causes high power consumption for electrochemical water splitting. Various strategies have been attempted to accelerate the OER rate, but there are few studies on regulating the transport of reactants especially under large current densities when the mass transfer factor dominates the evolution reactions. Herein, Nix Fe1- x alloy nanocones arrays (with ≈2 nm surface NiO/NiFe(OH)2 layer) are adopted to boost the transport of reactants. Finite element analysis suggests that the high-curvature tips can enhance the local electric field, which induces an order of magnitude higher concentration of hydroxide ions (OH- ) at the active sites and promotes intrinsic OER activity by 67% at 1.5 V. Experimental results show that a fabricated NiFe nanocone array electrode, with optimized alloy composition, has a small overpotential of 190 mV at 10 mA cm-2 and 255 mV at 500 mA cm-2 . When calibrated by electrochemical surface area, the nanocones electrode outperforms the state-of-the-art OER electrocatalysts. The positive effect of the tip-enhanced local electric field in promoting mass transfer is also confirmed by comparing samples with different tip curvature radii. It is suggested that this local field enhanced OER kinetics is a generic effect to other OER catalysts.

Progress and Challenge of Amorphous Catalysts for Electrochemical Water Splitting

Abstract: Electrochemical water splitting has been regarded a promising technology to provide a mobile and sustainable energy supply in the form of hydrogen fuel. The key to further development towards indus...

Mechanistic Insights of Mg 2+ ‐Electrolyte Additive for High‐Energy and Long‐Life Zinc‐Ion Hybrid Capacitors

Abstract: An electrolyte cation additive strategy provides a versatile route for developing high-energy and long-life aqueous zinc-ion hybrid capacitors. However, the mechanisms of energy storage and Zn anode protection are still unclear in Zn-based systems with dual-ion electrolytes. Here, a dual charge storage mechanism for zinc-ion hybrid capacitors with both cations and anions adsorption/desorption and the reversible formation of Zn4SO4(OH)6·xH2O enabled by the Mg2+ additive in the common aqueous ZnSO4 electrolyte are proposed. Theoretical calculations verify that the self-healing electrostatic shield effect and the solvation-sheath structure regulation rendered by the Mg2+ additive account for the observed uniform Zn deposition and dendrite suppression. As a result, an additional energy storage capacity of ≈50% compared to that in a pure 2 m ZnSO4 electrolyte and an extended cycle life with capacity retention of 98.7% after 10 000 cycles are achieved. This work highlights the effectiveness of electrolyte design for dual-ion carrier storage mechanism in aqueous devices toward high energy density and long cycle life.

Amorphous VO2: A Pseudocapacitive Platform for High-Rate Symmetric Batteries

Abstract: Among the various VO2 polymorphs, the layered compound, VO2 (B), has been the most widely investigated lithium-ion battery electrode material. For sodium-ion electrodes, however, an amorphous solid may be more advantageous as a result of the open framework to facilitate ion insertion and the ability to tolerate volumetric changes. Herein, it is shown that the Na+ insertion properties of amorphous VO2 (a-VO2 ) are superior to those of crystalline VO2 (B). Amorphous VO2 exhibits a linear voltage characteristic over a 3 V range (4.0 to 1.0 V vs Na/Na+ ) leading to a reversible capacity as high as 400 mAh g-1 and rapid redox kinetics, which is attributed to its pseudocapacitive nature. The linear voltage characteristic over 3 V affords the opportunity of fabricating a symmetric Na-ion battery in which the a-VO2 material serves as both the positive electrode and the negative electrode. Such a symmetric battery offers safer operation in terms of overcharging, overdischarging, polarity reversal, high charge/discharge current abuse, and long-term usage. The results suggest that amorphous transition metal oxides may offer advantageous attributes for rapid, safe, and energy-dense storage.

Molecule Confined Isolated Metal Sites Enable the Electrocatalytic Synthesis of Hydrogen Peroxide

Abstract: The direct synthesis of hydrogen peroxide (H2 O2 ) through the two-electron oxygen reduction reaction is a promising alternative to the industrial anthraquinone oxidation process. Selectivity to H2 O2 is however limited by the four-electron pathway during oxygen reduction. Herein, it is reported that aminoanthraquinone confined isolated metal sites on carbon supports selectively steer oxygen reduction to H2 O2 through the two-electron pathway. Confining isolated NiNx sites under aminoanthraquinone increases the selectivity to H2 O2 from below 55% to above 80% over a wide potential range. Spectroscopy characterization and density functional theory calculations indicate that isolated NiNx sites are confined within a nanochannel formed between the molecule and the carbon support. The confinement reduces the thermodynamic barrier for OOH* desorption versus further dissociation, thus increasing the selectivity to H2 O2 . It is revealed how tailoring noncovalent interactions beyond the binding site can empower electrocatalysts for the direct synthesis of H2 O2 through oxygen reduction.

Printed Zinc Paper Batteries.

Abstract: Paper electronics offer an environmentally sustainable option for flexible and wearable systems and perfectly fit the available printing technologies for high manufacturing efficiency. As the heart of energy-consuming devices, paper-based batteries are required to be compatible with printing processes with high fidelity. Herein, hydrogel reinforced cellulose paper (HCP) is designed to serve as the separator and solid electrolyte for paper batteries. The HCP can sustain higher strain than pristine papers and are biodegradable in natural environment within four weeks. Zinc-metal (Ni and Mn) batteries printed on the HCP present remarkable volumetric energy density of ≈26 mWh cm-3 , and also demonstrate the feature of cuttability and compatibility with flexible circuits and devices. As a result, self-powered electronic system could be constructed by integrating printed paper batteries with solar cells and light-emitting diodes. The result highlights the feasibility of hydrogel reinforced paper for ubiquitous flexible and eco-friendly electronics.

Enlarging the π-Conjugation of Cobalt Porphyrin for Highly Active and Selective CO2 Electroreduction.

Abstract: Heterogeneous molecular catalysts have attracted considerable attention as carbon dioxide reduction reaction (CO2 RR) electrocatalysts. The π-electron system of conjugated ligands in molecular catalysts may play an important role in determining the activity. In this work, by enlarging π-conjugation through appending more aromatic substituents on the porphyrin ligand, altered π-electron system endows the as-prepared 5,10,15,20-tetrakis(4-(pyren-1-yl)phenyl)porphyrin CoII with high Faradaic efficiency (ca. 95 %) for CO production, as well as high turnover frequency (2.1 s-1 at -0.6 V vs. RHE). Density functional theory calculation further suggests that the improved electrocatalytic performance mainly originates from the higher proportion of Co d z 2 orbital and the CO2 π* orbital in the HOMO of the (Co-porphyrin-CO2 )- intermediate with larger π-conjugation, which facilitates the CO2 activation. This work provides strong evidence that π-conjugation perturbation is effective in boosting the CO2 RR.

Aqueous Zn2+/Na+ dual-salt batteries with stable discharge voltage and high columbic efficiency by systematic electrolyte regulation

Abstract: While aqueous Zn-Na hybrid batteries have garnered widespread attention because of their low cost and high safety, it is still challenging to achieve long cycle-life and stable discharge-voltage due to sluggish reaction kinetics, zinc dendrite formation, and side reactions. Herein, we design a Zn2+/Na+ dual-salt battery, in which sodiation of the NVP cathode favors zinc intercalation under an energy threshold, leading to decoupled redox reactions on the cathode and anode. Systematic investigations of the electrolyte effects show that the ion intercalation mechanism and the kinetics in the mixture of triflate- and acetate-based electrolytes are superior to those in the common acetate-only electrolytes. As a result, we have achieved fast discharging capability, suppressed zinc dendrites, a stable discharge voltage at 1.45 V with small polarization, and nearly 100% Columbic efficiency in the dual-salt mixture electrolyte with optimized concentration of 1 M Zn(OAc)2 + 1 M NaCF3SO3. This work demonstrates the importance of electrolyte regulation in aqueous dual-salt hybrid batteries for the energy storage.

Room-temperature continuous-wave vertical-cavity surface-emitting lasers based on 2D layered organic-inorganic hybrid perovskites

Abstract: Two-dimensional (2D) layered lead halide perovskites with large exciton binding energies, efficient radiative recombination, and outstanding environmental stability are regarded as supreme candidates for realizing highly compact and ultralow threshold lasers. However, continuous-wave (CW) pumped lasing of 2D lead halide perovskites, as the precondition for the electrically pumped lasing, is still challenging. Here, we tackled this challenge by demonstrating lasing emission in phenylethylammonium lead iodide [(PEA)2PbI4] embedded in a vertical microcavity under continuous pumping at room temperature. The millimeter-sized (PEA)2PbI4 single crystal was obtained from a two-step seed-growth method, showing high crystallization, excellent thermal stability, and outstanding optical properties. We used the exfoliated (PEA)2PbI4 thin flake as the gain medium to construct a vertical-cavity surface-emitting laser (VCSEL), showing robust single-mode CW lasing operation with an ultra-low threshold of 5.7 W cm−2 at room temperature, attributed to strong optical confinement in the high-Q cavity. Our findings provide a strategy to design and fabricate solution-based 2D perovskite VCSELs and mark a significant step toward the next-generation of coherent light sources.

Spectrum-shaped Si-perovskite hybrid photodetectors for hyperspectral bioimaging

Abstract: Hyperspectral imaging (HSI) with rich spectral and spatial information holds potential for applications ranging from remote sensing to biomedicine. However, charge-coupled device (CCD) detectors used in conventional HSI systems suffer from inferior and unbalanced responsivity in the visible region, which is not a perfect choice for high-performance visible HSI. That is, conventional Si-based CCDs exhibit poor responsivity at short wavelengths (e.g., 400–600 nm) compared with that at longer wavelengths due to the nature of the indirect bandgap in silicon of around 1.1 eV. To solve this challenge, we introduce a CsPbBr3 perovskite layer to shape the spectrum of a Si/PEDOT:PSS heterojunction photodetector (PD), resulting in a fabricated Si-CsPbBr3 hybrid PD with enhanced responsivity at 400–600 nm. This results in an approximately flat spectral responsivity curve in the visible region (400–800 nm). Therefore, the stable Si−CsPbBr3 hybrid PD with a flat spectrum overcomes the shortcomings of traditional Si-based PDs and makes it more suitable for HSI. Further, we set up a first perovskite HSI system with high spectrum resolution and demonstrate potential applications for tumor detection and tissue identification. We believe that this perovskite optimization can be integrated into modern CCD, thus becoming a step in future CCD fabrication processes, which is a milestone for high-performance HSI systems.

Ferroelastic-switching-driven large shear strain and piezoelectricity in a hybrid ferroelectric

Abstract: Materials that can produce large controllable strains are widely used in shape memory devices, actuators and sensors1,2, and great efforts have been made to improve the strain output3–6. Among them, ferroelastic transitions underpin giant reversible strains in electrically driven ferroelectrics or piezoelectrics and thermally or magnetically driven shape memory alloys7,8. However, large-strain ferroelastic switching in conventional ferroelectrics is very challenging, while magnetic and thermal controls are not desirable for practical applications. Here we demonstrate a large shear strain of up to 21.5% in a hybrid ferroelectric, C6H5N(CH3)3CdCl3, which is two orders of magnitude greater than that in conventional ferroelectric polymers and oxides. It is achieved by inorganic bond switching and facilitated by structural confinement of the large organic moieties, which prevents undesired 180° polarization switching. Furthermore, Br substitution can soften the bonds, allowing a sizable shear piezoelectric coefficient (d35 ≈ 4,830 pm V−1) at the Br-rich end of the solid solution, C6H5N(CH3)3CdBr3xCl3(1−x). The electromechanical properties of these compounds suggest their potential in lightweight and high-energy-density devices, and the strategy described here could inspire the development of next-generation piezoelectrics and electroactive materials based on hybrid ferroelectrics. Reversible strains are widely used in high-technology systems, with piezoelectrics showing fast response but low strain. Here, ferroelectric C6H5N(CH3)3CdCl3 is shown to produce a strain of 21.5%, two orders of magnitude larger than other piezoelectrics, due to organic molecules preventing 180° polarization switching.

Correction to: High-Index-Faceted Ni3S2 Branch Arrays as Bifunctional Electrocatalysts for Efficient Water Splitting.

Abstract: In the original publication Figure S4 is an ancillary image to compare the specific surface areas of TiO2/Ni3S2 and Ni3S2 samples was incorrectly published.

UV soaking for enhancing the photocurrent and response speed of Cs2AgBiBr6-based all-inorganic perovskite photodetectors

Abstract: The response speed of the reported Cs2AgBiBr6-based photodetectors exhibits a wide variation ranging from microseconds to nanoseconds, while the reason is still unclear. Apart from the conventional approaches such as reducing effective area, new regulating approaches for response speed improvement have rarely been reported. On the other hand, it is generally believed that ultraviolet (UV) light has negative impact on perovskite devices resulting in performance degradation. In this work, we demonstrated that the response speed of the photodetector with FTO/Cs2AgBiBr6/Au structure can be effectively regulated by utilizing UV light-soaking effect without reducing the device area. Particularly, the decay time is efficiently modulated from 30.1 µs to 340 ns. In addition, the −3 dB bandwidth of the device is extended from 5 to 20 kHz. It is worth mentioning that the light current is remarkably boosted by 15 times instead of any attenuation. Furthermore, we prove the universality of UV soaking treatment on Cs2AgBiBr6-based photodetectors with other all-inorganic structures, i.e., FTO/TiO2/Cs2AgBiBr6/Au, FTO/Cs2AgBiBr6/TiO2/Au and FTO/TiO2/Cs2AgBiBr6/CuSCN/Au. Our results demonstrate a new method to improve the response speed and light current of Cs2AgBiBr6-based perovskite all-inorganic photodetectors.

Facile Preparation of Cobalt Hydroxide Based Supercapacitor with High Volumetric Energy Density at High Volumetric Power Density

Abstract: Developing supercapacitor electrodes with an ultra-high specific energy density at high power density and long cycle life is critical to future energy storage devices. However, it is still challenging to fabricate highperformance supercapacitors in a facile and scalable process. In this work, the cobalt nanocone arrays (CNAs) were plated on the copper foil (CF) within 40-second electrodeposition and were transferred into cobalt (Co)/cobalt hydroxide [Co(OH) 2 ] in the alkaline solution after activation of several CV cycles. The flexible Co/Co(OH) 2 @CF electrode can deliver an ultrahigh specific capacitance of 1043 F cm−3 at 1 mA cm−2 and excellent cycle stability with a retention of 98% after 20000 cycles in the three-electrode test. Co/Co(OH) 2 @CF and active carbon were used as the cathode and anode to assemble asymmetric supercapacitors, respectively, in the form of coin and soft package. The soft-package supercapacitor shows an energy density of 28 mWh cm−3 at 62 W cm−3 with 82 % retention after 5000 cycles. And the coin supercapacitor shows a larger energy density of 69 mWh cm−3 at 100 W cm−3 with retention of 123% after 10000 cycles. Good electrical and mass transport and stable structure contribute to its excellent electrochemical performance. Considering the advantages of the facile and scalable preparation process, low cost, flexibility, and excellent capacitance and stability, this electrode is promising in application to high-performance flexible supercapacitors.