Covalent Organic Framework Ionomer Steering the CO2 Electroreduction Pathway on Cu at Industrial-Grade Current Density.

CO2 electroreduction holds great promise for addressing global energy and sustainability challenges. Copper (Cu) shows great potential for effective conversion of CO2 toward specific value-added and/or high-energy-density products. However, its limitation lies in relatively low product selectivity. Herein, we present that the CO2 reduction reaction (CO2RR) pathway on commercially available Cu can be rationally steered by modulating the microenvironment in the vicinity of the Cu surface with two-dimensional sulfonated covalent organic framework nanosheet (COF-NS)-based ionomers. Specifically, the selectivity toward methane (CH4) can be enhanced to more than 60% with the total current density up to 500 mA cm-2 in flow cells in both acidic (pH = 2) and alkaline (pH = 14) electrolytes. The COF-NS, characterized by abundant apertures, can promote the accumulation of CO2 and K+ near the catalyst surface, alter the adsorption energy and surface coverage of *CO, facilitate the dissociation of H2O, and finally modulate the reaction pathway for the CO2RR. Our approach demonstrates the rational modulation of reaction interfaces for the CO2RR utilizing porous open framework ionomers, showcasing their potential practical applications.

Controllable Crystallization of Two-Dimensional Bi Nanocrystals with Morphology-Boosted CO2 Electroreduction in Wide pH Environments.

Two-dimensional low-melting-point (LMP) metal nanocrystals are attracting increasing attention with broad and irreplaceable applications due to their unique surface and topological structures. However, the chemical synthesis, especially the fine control over the nucleation (reduction) and growth (crystallization), of such LMP metal nanocrystals remains elusive as limited by the challenges of low standard redox potential, low melting point, poor crystalline symmetry, etc. Here, a controllable reduction-melting-crystallization (RMC) protocol to synthesize free-standing and surfactant-free bismuth nanocrystals with tunable dimensions, morphologies, and surface structures is presented. Especially, ultrathin bismuth nanosheets with flat or jagged surfaces/edges can be prepared with high selectivity. The jagged bismuth nanosheets, with abundant surface steps and defects, exhibit boosted electrocatalytic CO2 reduction performances in acidic, neutral, and alkaline aqueous solutions, achieving the maximum selectivity of near unity at the current density of 210 mA cm-2 for formate evolution under ambient conditions. This work creates the RMC pathway for the synthesis of free-standing two-dimensional LMP metal nanomaterials and may find broader applicability in more interdisciplinary applications.

Boosting CO2 Electroreduction on Bismuth Nanoplates with a Three-Dimensional Nitrogen-Doped Graphene Aerogel Matrix.

Electrochemical CO2 reduction reaction (CO2RR), which uses renewable electricity to produce high-value-added chemicals, offers an alternative clean path to the carbon cycle. However, bismuth-based catalysts show great potential for the conversion of CO2 and water to formate, but their overall efficiency is still hampered by the weak CO2 adsorption, low electrical conductivity, and slow mass transfer of CO2 molecules. Herein, we report that a rationally modulated nitrogen-doped graphene aerogel matrix (NGA) can significantly enhance the CO2RR performance of bismuth nanoplates (BiNPs) by both modulating the electronic structure of bismuth and regulating the interface for chemical reaction and mass transfer environments. In particular, the NGA prepared by reducing graphene oxide (GO) with hydrazine hydrate (denoted as NGAhdrz) exhibits significantly enhanced strong metal-support interaction (SMSI), increased specific surface area, strengthened CO2 adsorption, and modulated wettability. As a result, the Bi/NGAhdrz exhibits significantly boosted CO2RR properties, with a Faradaic efficiency (FE) of 96.4% at a current density of 51.4 mA cm-2 for formate evolution at a potential of -1.0 V versus reversible hydrogen electrode (vs RHE) in aqueous solution under ambient conditions.

Ultrathin Dendritic Pd-Ag Nanoplates for Efficient and Durable Electrocatalytic Reduction of CO2 to Formate.

CO2 reduction reactions (CO2 RR) powered by renewable electricity can directly convert CO2 to hydrocarbons and fix the sustainable but intermittent energy (e. g., sunlight, wind, etc.) in stable and portable chemical fuels. Advanced catalysts boosting CO2 RR with high activity, selectivity, and durability at low overpotentials are of great importance but still elusive. Here, we report that the ultrathin Pd-Ag dendritic nanoplates (PdAg DNPs) exhibited boosted activity, selectivity, and stability for producing formate from CO2 at a very low overpotential in aqueous solutions under ambient conditions. As a result, the PdAg DNPs exhibited a Faradaic efficiency (FE) for formate of 91% and a cathodic energy efficiency (EE) of ∼90% at the potential of -0.2 V versus reversible hydrogen electrode (vs. RHE), showing significantly enhanced durability as compared with pure Pd catalysts. Our strategy represents a rational catalyst design by engineering the surface geometrical and electronic structures of metal nanocrystals and may find more applicability in future electrocatalysis.

Controlled Synthesis of Intermetallic Au2 Bi Nanocrystals and Au2 Bi/Bi Hetero-Nanocrystals with Promoted Electrocatalytic CO2 Reduction Properties.

The electrocatalytic properties of metal nanoparticles (NPs) strongly depend on their compositions and structures. Rational design of alloys and/or heterostructures provides additional approaches to modifying their surface geometric and electronic structures for optimized electrocatalytic performance. Here, a solution synthesis of freestanding intermetallic Au2 Bi NPs, the heterostructures of Au2 Bi/Bi hetero-NPs, and their promoted electrocatalytic CO2 reduction reaction (CO2 RR) performances were reported. It was revealed that the formation and in-situ conversion of heterogeneous seeds (e. g., Au) were of vital importance for the formation of intermetallic Au2 Bi and Au2 Bi/Bi hetero-NPs. It was also found that the Au components would act as the structure promoter moderating the binding strength for key intermediates on Bi surfaces. The alloying of Bi with Au and the formation of heterogeneous Au2 Bi/Bi interfaces would create more surface active sites with modulated electronic structures and stronger adsorption strengths for key intermediates, promoting the CO2 -to-HCOOH conversion with high activity and selectivity. This work presents a novel route for preparing intermetallic nanomaterials with modulated surface geometric/electric structures and promoting their electrocatalytic activities with alloying effects and interfacial effects. Such strategy may find wide application in catalyst design and synthesis for more electrocatalytic reactions.

Promoting the Electrocatalytic Reduction of CO2 on Ultrathin Porous Bismuth Nanosheets with Tunable Surface-Active Sites and Local pH Environments.

Electrochemical CO2 reduction reaction (CO2RR) yielding value-added chemicals provides a sustainable approach for renewable energy storage and conversion. Bismuth-based catalysts prove to be promising candidates for converting CO2 and water into formate but still suffer from poor selectivity and activity and/or sluggish kinetics. Here, we report that ultrathin porous Bi nanosheets (Bi-PNS) can be prepared through a controlled solvothermal protocol. Compared with smooth Bi nanoparticles (Bi-NPs), the ultrathin, rough, and porous Bi-PNS provide more active sites with higher intrinsic reactivities for CO2RR. Moreover, such high activity further increases the local pH in the vicinity of the catalyst surfaces during electrolysis and thus suppresses the competing hydrogen evolution reaction. As a result, the Bi-PNS exhibit significantly boosted CO2RR properties, showing a Faradaic efficiency of 95% with an effective current density of 45 mA cm-2 for formate evolution at the potential of -1.0 V versus reversible hydrogen electrode.

Modulating the electrocatalytic CO2 reduction performances of bismuth nanoparticles with carbon substrates with controlled degrees of oxidation.

The catalytic performances of metal nanoparticles can be widely tuned and promoted by the metal-support interactions. Here, we report that the morphologies and electrocatalytic CO2 reduction reaction (CO2RR) properties of bismuth nanoparticles (BiNPs) can be rationally modulated by their interactions with carbon black (CB) supports by controlling the degree of surface oxidation. Appropriately oxidized CB supports can provide sufficient oxygen-containing groups for anchoring BiNPs with tunable sizes and surface areas, desirable key intermediate adsorption abilities, appropriate surface wettability, and adequate electron transfer abilities. As a result, the optimized Bi/CB catalysts exhibited a promoted CO2RR performance with a Faradaic efficiency of 94% and a current density of 16.7 mA cm-2 for HCOO- at -0.9 V versus a reversible hydrogen electrode. Our results demonstrate the significance of regulating the interactions between supports and metal nanoparticles for both synthesis of the catalyst and electrolysis applications, which may find broader applicability in more electrocatalyst designs.

Nanoporous Graphene via a Pressing Organization Calcination Strategy for Highly Efficient Electrocatalytic Hydrogen Peroxide Generation.

Nanoporous graphenes (NPGs) have recently attracted huge attention owing to their designable structures and diverse properties. Many important properties of NPGs are determined by their structural regularity and homogeneity. The mass production of NPGs with periodic well-defined pore structures under a solvent-free green synthesis poses a great challenge and is largely unexplored. A facile synthetic strategy of NPGs via pressing organization calcination (POC) of readily available halogenated polycyclic aromatic hydrocarbons is developed. The gram-scale synthesized NPGs have ordered structures and possess well-defined nanopores, which can be easily exfoliated to few layers and oxidized in controllable approaches. After being decorated with oxygen species, the oxidized NPGs with tunable catalytic centers exhibit high activity, selectivity, and stability toward electrochemical hydrogen peroxide generation.

Metal-organic framework membranes with single-atomic centers for photocatalytic CO2 and O2 reduction.

The demand for sustainable energy has motivated the development of artificial photosynthesis. Yet the catalyst and reaction interface designs for directly fixing permanent gases (e.g. CO2, O2, N2) into liquid fuels are still challenged by slow mass transfer and sluggish catalytic kinetics at the gas-liquid-solid boundary. Here, we report that gas-permeable metal-organic framework (MOF) membranes can modify the electronic structures and catalytic properties of metal single-atoms (SAs) to promote the diffusion, activation, and reduction of gas molecules (e.g. CO2, O2) and produce liquid fuels under visible light and mild conditions. With Ir SAs as active centers, the defect-engineered MOF (e.g. activated NH2-UiO-66) particles can reduce CO2 to HCOOH with an apparent quantum efficiency (AQE) of 2.51% at 420 nm on the gas-liquid-solid reaction interface. With promoted gas diffusion at the porous gas-solid interfaces, the gas-permeable SA/MOF membranes can directly convert humid CO2 gas into HCOOH with a near-unity selectivity and a significantly increased AQE of 15.76% at 420 nm. A similar strategy can be applied to the photocatalytic O2-to-H2O2 conversions, suggesting the wide applicability of our catalyst and reaction interface designs.

Metal-Organic Framework Membranes Encapsulating Gold Nanoparticles for Direct Plasmonic Photocatalytic Nitrogen Fixation.

Photocatalytic nitrogen fixation reaction can harvest the solar energy to convert the abundant but inert N2 into NH3. Here, utilizing metal-organic framework (MOF) membranes as the ideal assembly of nanoreactors to disperse and confine gold nanoparticles (AuNPs), we realize the direct plasmonic photocatalytic nitrogen fixation under ambient conditions. Upon visible irradiation, the hot electrons generated on the AuNPs can be directly injected into the N2 molecules adsorbed on Au surfaces. Such N2 molecules can be additionally activated by the strong but evanescently localized surface plasmon resonance field, resulting in a supralinear intensity dependence of the ammonia evolution rate with much higher apparent quantum efficiency and lower apparent activation energy under stronger irradiation. Moreover, the gas-permeable Au@MOF membranes, consisting of numerous interconnected nanoreactors, can ensure the dispersity and stability of AuNPs, further facilitate the mass transfer of N2 molecules and (hydrated) protons, and boost the plasmonic photocatalytic reactions at the designed gas-membrane-solution interface. As a result, an ammonia evolution rate of 18.9 mmol gAu-1 h-1 was achieved under visible light (>400 nm, 100 mW cm-2) with an apparent quantum efficiency of 1.54% at 520 nm.

Morphology-dependent electrocatalytic nitrogen reduction on Ag triangular nanoplates.

Electrocatalytic nitrogen reduction reactions (ENRR) can produce ammonia from nitrogen and water under ambient conditions. Here, we report the morphology-dependent electro-catalytic nitrogen reduction on Ag triangular nanoplates. Boosted by potassium cations, Ag triangular nanoplates with sharp edges exhibit a high faradaic efficiency of 25% with an ammonia yield of 58.5 mg gAg-1 h-1 at a low overpotential of -0.25 V vs. RHE. In comparison, rounded Ag nanoparticles mainly enclosed by {111} and {100} surfaces show a much smaller faradaic efficiency of 16% and ammonia yield of 38 mg gAg-1 h-1 at a larger overpotential (-0.35 V vs. RHE).

Promoting photocatalytic nitrogen fixation with alkali metal cations and plasmonic nanocrystals.

Photocatalytic nitrogen fixation can produce ammonia from nitrogen and water under ambient conditions in the presence of sunlight. Here, we report that alkali metal cations (Li+, Na+, and K+) can significantly promote nitrogen activation and plasmonic nanocrystals (Au and Ag) can sensitize photocatalysts under visible light. The ammonia yield and selectivity on Au/P25 under UV-vis irradiation could be increased from 0.085 mmol g-1 h-1 and 75% to 0.43 mmol g-1 h-1 and 94.5% when promoted by K+, showing a visible-light-driven activity of 0.14 mmol g-1 h-1 and an AQE of 0.62% at 550 nm. The activity could be further increased to 1.02 (UV-vis) and 0.32 (visible) mmol g-1 h-1 with AQE of 0.93% at 550 nm with methanol added as the sacrificial agent. This strategy could be applied to a series of photocatalysts (e.g. TiO2, ZnO, and BiOBr) and may represent a general approach for designing efficient nitrogen fixation processes.

Ultrathin cobalt oxide nanostructures with morphology-dependent electrocatalytic oxygen evolution activity.

Engineering compositions, structures, and defects can endow nanomaterials with optimized catalytic properties. Here, we report that cobalt oxide (CoOx) ultrathin nanosheets (UTNS, ∼1.6 nm thick) with a large number of oxygen defects and mixed cobalt valences can be obtained through a facile one-step hydrothermal protocol. The large number of oxygen defects make the ultrathin CoOx nanosheet a superior OER catalyst with low overpotentials of 315 and 365 mV at current densities of 50 and 200 mA cm-2, respectively. The stable framework-like architectures of the UTNS further ensure their high OER activity and durability. Our method represents a facile one-step preparation of CoOx nanostructures with tunable compositions, morphologies, and defects, and thus promotes OER properties. This strategy may find its wider applicability in designing active, robust, and easy-to-obtain catalysts for OER and other electrocatalytic systems.

Porous Pt-M (M = Cu, Zn, Ni) nanoparticles as robust nanocatalysts.

Porous Pt-M (M = Cu, Zn, Ni) nanoparticles (NPs) were obtained by reduction of [Pt(CH3NH2)4][PtCl4] and M(Ac)2 (or MCl2) with oleylamine under mild conditions. The porous Pt-Cu NPs exhibited superior catalytic activities over a Pt/Cu NP mixture and Cu NPs as references for CO oxidation processes.

Time--temperature indicator for perishable products based on kinetically programmable Ag overgrowth on Au nanorods.

Food safety is a constant concern for humans. Besides adulteration and contamination, another major threat comes from the spontaneous spoilage of perishable products, which is basically inevitable and highly dependent on the temperature history during the custody chain. For advanced quality control and assessment, time-temperature indicators (TTIs) can be deployed to document the temperature history. However, the use of TTIs is currently limited by either relatively high cost or poor programmability. Here we describe a general, kinetically programmable, and cost-efficient TTI protocol constructed from plasmonic nanocrystals. We present proof-of-principle demonstrations that our TTI can be specifically tailored and thus used to track perishables, dynamically mimic the deteriorative processes therein, and indicate product quality through sharp-contrast multicolor changes. The flexible programmability of our TTI, combined with its substantially low cost and low toxicity, promises a general applicability to each single packaged item of a plethora of perishable products.

Ru nanocrystals with shape-dependent surface-enhanced Raman spectra and catalytic properties: controlled synthesis and DFT calculations.

Despite its multidisciplinary interests and technological importance, the shape control of Ru nanocrystals still remains a great challenge. In this article, we demonstrated a facile hydrothermal approach toward the controlled synthesis of Ru nanocrystals with the assistance of first-principles calculations. For the first time, Ru triangular and irregular nanoplates as well as capped columns with tunable sizes were prepared with high shape selectivity. In consistency with the experimental observations and density functional theory (DFT) calculations confirmed that both the intrinsic characteristics of Ru crystals and the adsorption of certain reaction species were responsible for the shape control of Ru nanocrystals. Ultrathin Ru nanoplates exposed a large portion of (0001) facets due to the lower surface energy of Ru(0001). The selective adsorption of oxalate species on Ru(10-10) would retard the growth of the side planes of the Ru nanocrystals, while the gradual thermolysis of the oxalate species would eliminate their adsorption effects, leading to the shape evolution of Ru nanocrystals from prisms to capped columns. The surface-enhanced Raman spectra (SERS) signals of these Ru nanocrystals with 4-mercaptopyridine as molecular probes showed an enhancement sequence of capped columns > triangle nanoplates > nanospheres, probably due to the sharp corners and edges in the capped columns and nanoplates as well as the shrunk interparticle distance in their assemblies. CO-selective methanation tests on these Ru nanocrystals indicated that the nanoplates and nanospheres had comparable activities, but the former has much better CO selectivity than the latter.

Chemoaffinity-mediated synthesis of NaRES2-based nanocrystals as versatile nano-building blocks and durable nano-pigments.

In this article, we present a chemoaffinity-mediated synthetic strategy toward trivalent rare earth (RE) sulfides-based nanocrystals with poor affinity between cation and anion (i.e., RE(3+) and S(2-)). With the affinity mediation among multiple constituents based on hard and soft acid and base theory, we synthesized a series of monodisperse NaRES(2) nanocrystals (RE = La to Lu, Y). The revelation of the nanocrystal growth mechanism from both experimental evidence and crystal structure modeling has enabled a robust control over the sizes and morphologies of the nanocrystals. This principle of chemoaffinity has also promised the synthesis of well-defined but even more complex RE-based hetero-nanostructures (i.e. NaLaS(2)-Au, Au@NaLaS(2), NaLaS(2)@Ag(2)S, Au@NaLaS(2)@Ag(2)S) with tunable optical properties. Furthermore, this synthetic method has yielded durable NaCeS(2)-based red nano-pigments under ambient conditions, with superior brightness and permeability in polydimethylsiloxane.

Pt-Cu and Pt-Pd-Cu concave nanocubes with high-index facets and superior electrocatalytic activity.

Pt-Cu nanostructures: Pt-Cu nanocubes (NCs), concave nanocubes (CNCs), and Pt-Pd-Cu CNCs with high-index facets (HIFs) were prepared through progressive galvanic replacements in a one-pot hydrothermal approach. The HIF-enclosed CNCs showed superior activities to (100)-enclosed NC catalysts for methanol oxidations owing to the modification of both the surface electronic structures and the surface atomic arrangements (see figure).

Multiply twinned Pt-Pd nanoicosahedrons as highly active electrocatalysts for methanol oxidation.

Bimetallic Pt-Pd nanoicosahedrons (NIs) with multiple {111} twins were obtained through a facile one-pot hydrothermal synthesis in a high shape selectivity of 82%. The {111}-enclosed NIs exhibited superior electrocatalytic activities to {111}-enclosed Pt-Pd nanotetrahedrons as well as commercial Pt catalysts (Pt black and Pt/C) for methanol oxidation.