Electrochemical Reduction of N2 to Ammonia Promoted by Hydrated Cation Ions: Mechanistic Insights from a Combined Computational and Experimental Study.

Alkali ions, major components at the electrode-electrolyte interface, are crucial to modulating reaction activity and selectivity of catalyst materials. However, the underlying mechanism of how the alkali ions catalyze the N2 reduction reaction (NRR) into ammonia remains elusive, posing challenges for experimentalists to select appropriate electrolyte solutions. In this work, by employing a combined experimental and computational approach, we proposed four essential roles of cation ions at Fe electrodes for N2 fixation: (i) promoting NN bond cleavage; (ii) stabilizing NRR intermediates; (iii) suppressing the competing hydrogen evolution reaction (HER); and (iv) modulating the interfacial charge distribution at the electrode-electrolyte interface. For N2 adsorption on an Fe electrode with cation ions, our constrained ab initio molecular dynamic (c-AIMD) results demonstrate a barrierless process, while an extra 0.52 eV barrier requires to be overcome to adsorb N2 for the pure Fe-water interface. For the formation of *NNH species within the N2 reduction process, the calculated free energy barrier is 0.50 eV at the Li+-Fe-water interface. However, the calculated barrier reaches 0.81 eV in pure Fe-water interface. Furthermore, experiments demonstrate a high Faradaic efficiency for ammonia synthesis on a Li+-Fe-water interface, reaching 27.93% at a working potential of -0.3 V vs RHE and pH = 6.8. These results emphasize how alkali metal cations and local reaction environments on the electrode surface play crucial roles in influencing the kinetics of interfacial reactions.

Electrochemical Nitrate-to-Ammonia Conversion Enabled by Carbon-Decoration of Ni─GaOOH Synthesized via Plasma-Assisted CO2 Reduction.

The release of nitrates into the environment leads to contaminated soil and water that poses a health risk to humans and animals. Due to the transition to renewable energy-based technologies, an electrochemical approach is an emerging option that can selectively produce valuable ammonia from nitrate sources. However, traditional metal-based electrocatalysts often suffer from low nitrate adsorption that reduces NH3  production rates. Here, a Ni-GaOOH-C/Ga electrocatalyst for electrochemical nitrate conversion into NH3 is synthesized via a low energy atmospheric-pressure plasma process that reduces CO2  into highly dispersed activated carbon on dispersed Ni─GaOOH particles produced from a liquid metal Ga─Ni alloy precursor. Nitrate conversion rates of up to 26.3 µg h-1  mg-1 cat  are achieved with good stability of up to 20 h. Critically, the presence of carbon centers is central to improved performance where both Ni─C and NiO─C interfaces act as NO3-  adsorption and reduction centers during the reaction. Density functional theory (DFT) calculations indicate that the NiO─C and Ni─C reaction sites reduce the Gibbs free energy required for NO3-  reduction to NH3  compared to NiO and Ni. Importantly, catalysts without carbon centers do not produce NH3 , emphasizing the unique effects of incorporating carbon nanoparticles into the electrocatalyst.

Computational Design and Experimental Validation of Enzyme Mimicking Cu-Based Metal-Organic Frameworks for the Reduction of CO2 into C2 Products: C-C Coupling Promoted by Ligand Modulation and the Optimal Cu-Cu Distance.

While extensive research has been conducted on the conversion of CO2 to C1 products, the synthesis of C2 products still strongly depends on the Cu electrode. One main issue hindering the C2 production on Cu-based catalysts is the lack of an appropriate Cu-Cu distance to provide the ideal platform for the C-C coupling process. Herein, we identify a lab-synthesized artificial enzyme with an optimal Cu-Cu distance, named MIL-53 (Cu) (MIL= Materials of Institute Lavoisier), for CO2 conversion by using a density functional theory method. By substituting the ligands in the porous MIL-53 (Cu) nanozyme with functional groups from electron-donating NH2 to electron-withdrawing NO2, the Cu-Cu distance and charge of Cu can be significantly tuned, thus modulating the adsorption strength of CO2 that impacts the catalytic activity. MIL-53 (Cu) decorated with a COOH-ligand is found to be located at the top of a volcano-shaped plot and exhibits the highest activity and selectivity to reduce CO2 to CH3CH2OH with a limiting potential of only 0.47 eV. In addition, experiments were carried out to successfully synthesize COOH-decorated MIL-53(Cu) to prove its high catalytic performance for C2 production, which resulted in a -55.5% faradic efficiency at -1.19 V vs RHE, which is much higher than the faradic efficiency of the benchmark Cu electrode of 35.7% at -1.05 V vs RHE. Our results demonstrate that the biologically inspired enzyme engineering approach can redefine the structure-activity relationships of nanozyme catalysts and can also provide a new understanding of the catalytic mechanisms in natural enzymes toward the development of highly active and selective artificial nanozymes.

Nitrate-to-Ammonia Conversion at an InSn-Enriched Liquid-Metal Electrode.

The renewable energy driven electrochemical conversion of nitrates to ammonia is emerging as a viable route for the creation of this hydrogen carrier. However, the creation of highly efficient electrocatalysts that show prolonged stability is an ongoing challenge. Here we show that room temperature liquid metal Galinstan can be used as an efficient and stable electrocatalyst for nitrate conversion to ammonia achieving rates of up to 2335 µg h-1 cm-2 with a Faradaic efficiency of 100 %. Density functional theory (DFT) calculations and experimental observation indicated the activity is due to InSn alloy enrichment within the liquid metal that occurs during the electrocatalytic reaction. This high selectivity for NH3 is also due to additional suppression of the competing hydrogen evolution reaction at the identified In3 Sn active site. This work adds to the increasing applicability of liquid metals based on Ga for clean energy technologies.

Bi-Sn Catalytic Foam Governed by Nanometallurgy of Liquid Metals.

Metallic foams, with intrinsic catalytic properties, are critical for heterogeneous catalysis reactions and reactor designs. Market ready catalytic foams are costly and made of multimaterial coatings with large sub-millimeter open cells providing insufficient active surface area. Here we use the principle of nanometallurgy within liquid metals to prepare nanostructured catalytic metal foams using a low-cost alloy of bismuth and tin with sub-micrometer open cells. The eutectic bismuth and tin liquid metal alloy was processed into nanoparticles and blown into a tin and bismuth nanophase separated heterostructure in aqueous media at room temperature and using an indium brazing agent. The CO2 electroconversion efficiency of the catalytic foam is presented with an impressive 82% conversion efficiency toward formates at high current density of -25 mA cm-2 (-1.2 V vs RHE). Nanometallurgical process applied to liquid metals will lead to exciting possibilities for expanding industrial and research accessibility of catalytic foams.

Multiscale Plasma-Catalytic On-Surface Assembly.

Controlled modification of surfaces is one of the key pursuits of the nanoscience and nanotechnology fields, allowing for the fabrication of bespoke materials with targeted functionalities. However, many surface modifications currently require painstakingly precise and/or energy intensive processing to implement, and are thus limited in scope and scale. Here, a concept which can enhance the capacity for control of surfaces is introduced: plasma-assisted nucleation and self-assembly at atomic to nanoscales, scalable at atmospheric pressures.

Utilizing a Photocatalysis Process to Achieve a Cathode with Low Charging Overpotential and High Cycling Durability for a Li-O2 Battery.

The practical applications of non-aqueous lithium-oxygen batteries are impeded by large overpotentials and unsatisfactory cycling durability. Reported here is that commonly encountered fatal problems can be efficiently solved by using a carbon- and binder-free electrode of titanium coated with TiO2 nanotube arrays (TNAs) and gold nanoparticles (AuNPs). Ultraviolet irradiation of the TNAs generates positively charged holes, which efficiently decompose Li2 O2 and Li2 CO3 during recharging, thereby reducing the overpotential to one that is near the equilibrium potential for Li2 O2 formation. The AuNPs promote Li2 O2 formation, resulting in a large discharge capacity. The electrode exhibits excellent stability with about 100 % coulombic efficiency during continuous cycling of up to 200 cycles, which is due to the carbon- and binder-free composition. This work reveals a new strategy towards the development of highly efficient oxygen electrode materials for lithium-oxygen batteries.

Electrodeposition at Highly Negative Potentials of an Iron-Cobalt Oxide Catalyst for Use in Electrochemical Water Splitting.

Earth-abundant transition metal-based catalysts have been extensively investigated for their applicability in water electrolysers to enable overall water splitting to produce clean hydrogen and oxygen. In this study a Fe-Co based catalyst is electrodeposited in 30 seconds under vigorous hydrogen evolution conditions to produce a high surface area material that is active for both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). This catalyst can achieve high current densities of 600 mAcm-2 at an applied potential of 1.6 V (vs RHE) in 1 M NaOH with a Tafel slope value of 48 mV dec-1 for the OER. In addition, the HER can be facilitated at current densities as high as 400 mA cm-2 due to the large surface area of the material. The materials were found to be predominantly amorphous but did contain crystalline regions of CoFe2 O4 which became more evident after the OER indicating interesting compositional and structural changes that occur to the catalyst after an electrocatalytic reaction. This rapid method of creating a bimetallic oxide electrode for both the HER and OER could possibly be adopted to other bimetallic oxide systems suitable for electrochemical water splitting.

Monitoring compositional changes in Ni(OH)2 electrocatalysts employed in the oxygen evolution reaction.

Electrochemical water splitting to generate hydrogen has been identified as a possible solution to the storage of intermittent renewable energy. However there are still challenges remaining in the development of stable electrocatalysts for the oxygen evolution half-reaction. Here we investigate the effects that the oxygen evolution reaction (OER) has on an electrodeposited Ni(OH)2 catalyst operated under alkaline conditions. The electrocatalyst was characterised by established methods including cyclic voltammetry, electrochemical impedance spectroscopy, scanning electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy both before and after the OER to identify changes that may have occurred in the structure and/or composition of the catalyst. In addition, synchrotron X-ray absorption near edge structure mapping was used to generate spatially resolved maps of the species present within the Ni(OH)2 catalyst and how they change in a heterogeneous manner into a NiO species after the OER. When compared to the morphological data it suggests that changes in the morphology after the OER can be correlated to the formation of NiO within the newly formed clusters that were generated across the electrocatalyst.

Density Functional Theory Investigation of Carbon Dots as Hole-transport Material in Perovskite Solar Cells.

Charge transfer in solar cells is crucial, and so is the hole transporting layer (HTL) component in perovskite solar cells (PSCs). Finding a suitable material for this purpose that is inexpensive - either organic or inorganic - is currently one of the prime research objectives to improve the performance, through charge transfer dynamics, of PSCs.< One such recent finding is carbon quantum dots (C-dots), which is a simple and low-cost organic material that could be an alternative option to the currently employed high-cost and complex-structured hole transporting materials (HTMs) utilized in perovskite solar cells. A series of C-dots functionalized with hydrogen, hydroxyl (-OH), and carboxyl (-COOH) groups are considered in this study for their hole-transporting properties. The results reveal that simple hexagonal structured C-dots including -OH and -COOH group substituted C-dots have suitable valance band maximum (VBM) positions, which are suitable for hole transport. It is discovered that the position of the functional moieties on the C-dots would impact the band-edge positions of the C-dots. This implies that tuning the band position is possible so that these two-dimensional C-dots could, in principle, be used for other solar-cell applications that may require different band positions for optimal performance. As a representative example, we studied the perovskite/C-Dot interface of two different possible surfaces (i. e. MAI and PbI2 terminated perovskites) combined with a hexagonal C-Dot layer and found that there is a good probability of charge transfer between the perovskite layer and the C-dots, which promotes hole transfer between the perovskite and the C-dots.Introduction.

Room Temperature Electrochemical Synthesis of Crystalline GaOOH Nanoparticles from Expanding Liquid Metals.

Gallium oxyhydroxide (GaOOH) is a wide band gap semiconductor of interest for a variety of applications in electronics and catalysis where the synthesis of the crystalline form is usually achieved via hydrothermal routes. Here we synthesize GaOOH via the electrochemical oxidation of gallium based liquid metals in solutions of 0.1 M NaNO3 electrolyte with pH adjusted over the range of 7-8.4 with NaOH. This electrochemical approach employed under ambient conditions results in the formation of crystalline oblong shaped α-GaOOH nanoparticles from both liquid gallium and liquid galinstan which is a eutectic based on Ga, In, and Sn. The size and shape of the GaOOH particles could be controlled by the solution pH. The product is characterized with scanning electron microscopy, transmission electron microscopy, X-ray diffraction, UV-visible spectroscopy, and photoluminescence spectroscopy. During the electrochemical oxidation process, the liquid metal drop was found to expand significantly in the case of galinstan due to a constant electrowetting effect which resulted in the continuous expulsion of nanomaterial from the expanding liquid metal droplet. This electrochemical approach may be applicable to other liquid metals for the fabrication of metal oxide nanomaterials and also demonstrates that significant chemical reactions may be occurring at the surface of liquid metals that are actuated under an applied electric field in aqueous electrolytes.

Galvanic Replacement of the Liquid Metal Galinstan.

The galvanic replacement reaction is a highly versatile approach for the creation of a variety of nanostructured materials. However, the majority of reports are limited to the replacement of metallic nanoparticles or metal surfaces. Here we extend this elegant approach and describe the galvanic replacement of the liquid metal alloy galinstan with Ag and Au. This is achieved at a macrosized droplet to create a liquid metal marble that comprises a liquid metal core and a solid metal shell, whereby the morphology of the outer shell is determined by the concentration of metallic ions used in the solution during the galvanic replacement process. In principle, this allows one to recover precious metal ions from solution in their metallic form, which are immobilized on the liquid metal and therefore easy to recover. The reaction is also undertaken at liquid metal microdroplets created via sonication to produce Ag- and Au-based galinstan nanorice particles. These materials are characterized with SEM, XRD, TEM, SAED, EDX, XPS, UV-visible spectroscopy, and open-circuit potential versus time experiments to understand the galvanic replacement process. Finally, the nanosized materials are investigated for their catalytic activity toward the reduction of methylene blue in the presence of sodium borohydride. This approach illustrates a new avenue of research for the galvanic replacement process and, in principle, could be applied to many more systems.

Investigating the effect of ionic strength on the suppression of dendrite formation during metal electrodeposition.

The effect of ionic strength on the electrodeposition of silver has been investigated in acetonitrile (MeCN) containing TBAPF6 or in the ionic liquid [EMIm][OTf]. The use of an ionic liquid allows a greater ionic strength to be investigated as the solubility limits of supporting electrolytes in organic solvents can be overcome using neat ionic liquid. The SEM and XRD data show that polycrystalline silver is deposited in a fcc structure and that dendrite formation is retarded at high ionic strength. Electrochemical measurements undertaken in electrolytes of low ionic strength indicate that the deposition and growth of a few nuclei is preferred and leads to dendrite formation. However, at higher ionic strength, the deposition and growth of significantly more nuclei is observed and therefore dendrite growth rates and tip currents are lower leading to the deposition of spherical particulates. Crucially, the data shows that if the ionic strength of the electrolyte is controlled there are no differences between ionic liquids and molecular solvents for the electrodeposition of silver.

Liquid metal enabled pump.

Small-scale pumps will be the heartbeat of many future micro/nanoscale platforms. However, the integration of small-scale pumps is presently hampered by limited flow rate with respect to the input power, and their rather complicated fabrication processes. These issues arise as many conventional pumping effects require intricate moving elements. Here, we demonstrate a system that we call the liquid metal enabled pump, for driving a range of liquids without mechanical moving parts, upon the application of modest electric field. This pump incorporates a droplet of liquid metal, which induces liquid flow at high flow rates, yet with exceptionally low power consumption by electrowetting/deelectrowetting at the metal surface. We present theory explaining this pumping mechanism and show that the operation is fundamentally different from other existing pumps. The presented liquid metal enabled pump is both efficient and simple, and thus has the potential to fundamentally advance the field of microfluidics.

Electrochemical Tailoring of Fibrous Polyaniline and Electroless Decoration with Gold and Platinum Nanoparticles.

Presented in this work is a facile and quick electrochemical method for controlling the morphology of thick polyaniline (PANi) films, without the use of templates. By stepping the polymerization potential from high voltages to a lower (or series of lower) voltage(s), we successfully controlled the morphology of the polymer, and fibrous structures, unique to each potential step, were achieved. In addition, the resultant film was tested electrochemically for its viability as an electrode material for flexible batteries and supercapacitors. Furthermore, the PANi film was decorated with gold and platinum nanoparticles via an electroless deposition process for possible electrocatalytic applications, whereby the oxidation of hydrazine at the composite was investigated.

High-throughput approach for the identification of anilinium-based ionic liquids that are suitable for electropolymerisation.

We report the synthesis of new protic ionic liquids (PILs) based on aniline derivatives and the use of high-throughput (HT) techniques to screen possible candidates. In this work, a simple HT method was applied to rapidly screen different aniline derivatives against different acids in order to identify possible combinations that produce PILs. This was followed by repeating the HT process with a Chemspeed robotic synthesis platform for more accurate results. One of the successful combinations were then chosen to be synthesised on a larger scale for further analysis. The new PILs are of interest to the fields of ionic liquids, energy storage and especially, conducting polymers as they serve as solvents, electrolytes and monomers at the same time for possible electropolymerisation (i.e. a self-contained polymer precursor).

Computational electrocatalysis beyond conventional hydrogen electrode model: CO2 reduction to C2 species on copper facilitated by dynamically formed solvent halide ions at the solid-liquid interface.

The reduction of CO2 into value-added chemicals and fuels has been actively studied as a promising strategy for mitigating carbon dioxide emissions. However, the dilemma for the experimentalist in choosing an appropriate reaction medium and neglecting the effect of solvent ions when using a simple thermochemical model, normally leads to the disagreement between experimental observations and theoretical calculations. In this work, by considering the effects of both the anion and cation, a more realistic CO2 reduction environment at the solid-liquid interface between copper and solvent ions has been systematically studied by using ab initio molecular dynamics and density functional theory. We revealed that the co-occurrence of alkali ions (K+) and halide ions (F-, Cl-, Br-, and I-) in the electric double layer (EDL) can enhance the adsorption of CO2 by more than 0.45 eV compared to that in pure water, and the calculated energy barrier for CO-CO coupling also decreases 0.32 eV in the presence of I ion on a negatively charged copper electrode. The hydrated ions can modulate the distribution of the charge near the solid-liquid interface, which significantly promotes CO2 reduction and meanwhile impedes the hydrogen evolution reaction. Therefore, our work unveils the significant role of halide ions at the electrode-electrolyte interface for promoting CO2 reduction on copper electrode.

Effect of imidazolium-based ionic liquids on the nanoscale morphology of CuTCNQ (TCNQ = 7,7,8,8-tetracyanoquinodimethane) metal-organic semiconductors.

We demonstrate for the first time the ionic-liquid-mediated synthesis of nanostructured CuTCNQ by the simple immersion of copper in a solution of TCNQ where the viscosity of the medium significantly impacts the corrosion-crystallization process and the final morphology of the material.

Probing the effect of charge transfer enhancement in off resonance mode SERS via conjugation of the probe dye between silver nanoparticles and metal substrates.

The charge transfer-mediated surface enhanced Raman scattering (SERS) of crystal violet (CV) molecules that were chemically conjugated between partially polarized silver nanoparticles and optically smooth gold and silver substrates has been studied under off-resonant conditions. Tyrosine molecules were used as a reducing agent to convert silver ions into silver nanoparticles where oxidised tyrosine caps the silver nanoparticle surface with its semiquinone group. This binding through the quinone group facilitates charge transfer and results in partially oxidised silver. This establishes a chemical link between the silver nanoparticles and the CV molecules, where the positively charged central carbon of CV molecules can bind to the terminal carboxylate anion of the oxidised tyrosine molecules. After drop casting Ag nanoparticles bound with CV molecules it was found that the free terminal amine groups tend to bind with the underlying substrates. Significantly, only those CV molecules that were chemically conjugated between the partially polarised silver nanoparticles and the underlying gold or silver substrates were found to show SERS under off-resonant conditions. The importance of partial charge transfer at the nanoparticle/capping agent interface and the resultant conjugation of CV molecules to off resonant SERS effects was confirmed by using gold nanoparticles prepared in a similar manner. In this case the capping agent binds to the nanoparticle through the amine group which does not facilitate charge transfer from the gold nanoparticle and under these conditions SERS enhancement in the sandwich configuration was not observed.