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 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.

Operando Investigation of Silicon Anodes During Electrochemical Cycling in Li-ion Batteries.

Silicon (Si) is recognized as a promising anode material for next-generation anodes due to its high capacity. However, large volume expansion and active particle pulverization during cycling rapidly deteriorate the battery performance. The relationship between Si anode particle size and particle pulverization, and the structure evolution of Si particles during cycling is not well understood. In this study, a quantitative, time-resolved "operando" small angle X-ray scattering (SAXS) investigation into the morphological change of unwrapped and reduced graphene oxide (rGO) wrapped Si nanoparticles (Si@rGO) is conducted with respect to the operating voltage. The results provide a clear picture of Si particle size change and the role of nonrigid rGO in mitigating Si volume expansion and pulverization. Further, this study demonstrates the advantage of "operando" SAXS in electrochemical environments as compared to other approaches.

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.

Theoretical Evaluation of Highly Efficient Nitrate Reduction to Ammonia on InBi.

Electrocatalytic reduction of nitrate to ammonia has become a popular approach for wastewater treatment and ammonia production. However, the development of highly efficient electrocatalysts remains a great challenge. Herein, we systematically studied the potential of InBi for nitrate reduction to ammonia (NRA) based on density functional theory (DFT) calculations. Our results reveal that InBi exhibits high activity for NRA via an O-end pathway, where the free energy evolution of all intermediates is downhill in the most favorable elementary steps. The activation of nitrate originates from the strong orbital hybridization between oxygen and indium atoms, leading to an enhanced charge transfer as well as NO3- adsorption. In particular, the competing hydrogen evolution reaction (HER) is effectively suppressed due to the weak adsorption of proton. Our study not only proves the great electrocatalytic potential of InBi as a novel catalyst for NRA but also points out a new way to design NRA electrocatalysts for practical applications.

Preparation of "Co-Nx Carbon Net" Protected CoFe Alloy on Carbon Nanotubes as an Efficient Bifunctional Electrocatalyst in Zn-Air Batteries.

Herein, Co/Fe bimetallic hydroxide nanosheets (Co3Fe2 BMHs) were densely deposited on polypyrrole nanotubes (PPy NTs), followed by the successive coating of polydopamine (PDA) and zeolitic imidazolate frameworks (ZIF)-67 to form a composite catalyst precursor. Then, Co3Fe2 BMHs, PPy NTs, and ZIF-67/PDA in this precursor were calcined into Co2Fe alloy nanoparticles, nitrogen-doped carbon NTs (NCNTs), and a Co-Nx activated carbon net, respectively, which constituted a novel composite catalyst. In this composite catalyst, the high-density Co2Fe alloy nanoparticles are highly dispersed on the NCNT and coated by the Co-Nx activated carbon net. The Co-Nx activated carbon net protects the alloy particles from agglomerating during calcination and from being corroded by the electrolyte. Moreover, the experimental results demonstrated that the calcination temperature and chemical components of the catalyst precursors greatly affect the morphology, structure, composition, and ultimately electrocatalytic activity of the calcined products. The obtained optimum catalyst material exhibited significant electrocatalytic effects on both the oxygen reduction reaction and oxygen evolution reaction with a small ΔE of 0.715 V. The Zn-air battery utilizing this material as the air electrode catalyst showed a power density of 235.5 mW cm-2, an energy density of 1073.5 Wh kg-1, and a round-trip efficiency of 62.3% after 1000 cycles, superior to the benchmark battery based on the mixed commercial catalyst of Pt/C and RuO2. An all-solid-state battery was also assembled to confirm the practical application prospect of the prepared composite material as the air electrode catalyst. More importantly, both experimental data and density functional theory calculations verified that the superior bifunctional catalytic activity was mainly attributed to the synergy between the Co-Nx activated carbon net and Co2Fe alloy.

Electrochemical degradation and mineralisation of organic dyes in aqueous nitrate solutions.

Electrochemical treatment of organic matter for environmental remediation necessitates the development of cheap and robust electrodes that are chemically and structurally stable. To address this challenging requirement, we demonstrate a new electrochemical approach using a simple copper electrode under cathodic conditions to electrochemically generate reactive nitrosonium ions for the degradation of different classes of synthetic organic dyes. This could be achieved in an aqueous HNO3/KNO3 electrolyte at a relatively low cathodic potential of -0.5 V RHE at room temperature. UV-visible absorption spectroscopy, Raman spectroscopy, liquid chromatography - mass spectrometry and total organic carbon measurements revealed the rapid decolorisation and mineralisation of several dye types such as triarylmethane dyes (crystal violet, cresol red), an azo dye (methyl orange) as well as a sulfur containing thiazine dye (toluidine blue). The total organic carbon content of a 50 mg L-1 methyl orange solution was found to decrease by 83% after 1 h of electrolysis. Promisingly, locally sourced river and creek water samples spiked with 50 mg L-1 methyl orange were also successfully treated for up to 6 cycles at a simple Cu electrode, demonstrating potential for the remediation of polluted waterways.

Volcano relationships and a new activity descriptor of 2D transition metal-Fe layered double hydroxides for efficient oxygen evolution reaction.

Iron (Fe) sites play a critical role in boosting the catalytic activity of transition metal layered double hydroxide (LDH) electrocatalysts for the oxygen evolution reaction (OER), but the contribution of the Fe content to the catalysis of Fe-doped LDHs is still not well understood. Herein, a series of two-dimensional (2D) Fe-doped MFe-LDHs (M = Co, Ni, Cu, and Mn) was synthesized via a general molecular self-assembly method to track the role of Fe in their electrocatalytic OER activities. Besides the revelation of the intrinsic activity trend of NiFe > CoFe > MnFe > CuFe, volcano-shaped relationships among the catalytic activity descriptors, i.e., overpotential, Tafel slope, and turnover frequency (TOF), and the Fe-content in MFe-LDHs, were identified. Specifically, a ∼20% Fe content resulted in the highest OER performance for the LDH, while excess Fe compromised its activity. A similar volcano relationship was determined between the intermediate adsorption and Fe content via operando impedance spectroscopy (EIS) measurements, and it was shown that the intermediate adsorption capacitance (CPEad) can be a new activity descriptor for electrocatalysts. In this work, we not only performed a systematic study on the role of Fe in 2D Fe-doped LDHs but also offer some new insights into the activity descriptors for electrocatalysts.

Using H2O2 as a green oxidant to produce fluorescent GaOOH nanomaterials from a liquid metal.

We report a simple and rapid method for the synthesis of fluorescent gallium oxyhydroxide (GaOOH) nanoparticles from liquid Ga by a probe sonication method in the presence of H2O2 as an oxidant. The aspect ratio of the GaOOH nanoparticles is determined by the concentration of H2O2 and solution pH, as well as the probe energy and sonication time. Further surface modification with cyclodextrin to achieve biocompatibility for potential biomedical applications is reported where an example of cell uptake and fluorescence imaging is shown.

Low Temperature Nano Mechano-electrocatalytic CH4 Conversion.

Transforming natural resources to energy sources, such as converting CH4 to H2 and carbon, at high efficiency and low cost is crucial for many industries and environmental sustainability. The high temperature requirement of CH4 conversion regarding many of the current methods remains a critical bottleneck for their practical uptake. Here we report an approach based on gallium (Ga) liquid metal droplets, Ni(OH)2 cocatalysts, and mechanical energy input that offers low-temperature and scalable CH4 conversion into H2 and carbon. Mainly driven by the triboelectric voltage, originating from the joint contributions of the cocatalysts during agitation, CH4 is converted at the Ga and Ni(OH)2 interface through nanotribo-electrochemical reaction pathways. The efficiency of the system is enhanced when the reaction is performed at an increased pressure. The dehydrogenation of other nongaseous hydrocarbons using this approach is also demonstrated. This technology presents a possible low energy route for CH4 conversion without involving high temperature and harsh operating conditions.

Conversion of Catalytically Inert 2D Bismuth Oxide Nanosheets for Effective Electrochemical Hydrogen Evolution Reaction Catalysis via Oxygen Vacancy Concentration Modulation.

Oxygen vacancies (Vo) in electrocatalysts are closely correlated with the hydrogen evolution reaction (HER) activity. The role of vacancy defects and the effect of their concentration, however, yet remains unclear. Herein, Bi2O3, an unfavorable electrocatalyst for the HER due to a less than ideal hydrogen adsorption Gibbs free energy (ΔGH*), is utilized as a perfect model to explore the function of Vo on HER performance. Through a facile plasma irradiation strategy, Bi2O3 nanosheets with different Vo concentrations are fabricated to evaluate the influence of defects on the HER process. Unexpectedly, while the generated oxygen vacancies contribute to the enhanced HER performance, higher Vo concentrations beyond a saturation value result in a significant drop in HER activity. By tunning the Vo concentration in the Bi2O3 nanosheets via adjusting the treatment time, the Bi2O3 catalyst with an optimized oxygen vacancy concentration and detectable charge carrier concentration of 1.52 × 1024 cm-3 demonstrates enhanced HER performance with an overpotential of 174.2 mV to reach 10 mA cm-2, a Tafel slope of 80 mV dec-1, and an exchange current density of 316 mA cm-2 in an alkaline solution, which approaches the top-tier activity among Bi-based HER electrocatalysts. Density-functional theory calculations confirm the preferred adsorption of H* onto Bi2O3 as a function of oxygen chemical potential (∆µO) and oxygen partial potential (PO2) and reveal that high Vo concentrations result in excessive stability of adsorbed hydrogen and hence the inferior HER activity. This study reveals the oxygen vacancy concentration-HER catalytic activity relationship and provides insights into activating catalytically inert materials into highly efficient electrocatalysts.

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.

Applications of liquid metals in nanotechnology.

Post-transition liquid metals (LMs) offer new opportunities for accessing exciting dynamics for nanomaterials. As entities with free electrons and ions as well as fluidity, LM-based nanomaterials are fundamentally different from their solid counterparts. The low melting points of most post-transition metals (less than 330 °C) allow for the formation of nanodroplets from bulk metal melts under mild mechanical and chemical conditions. At the nanoscale, these liquid state nanodroplets simultaneously offer high electrical and thermal conductivities, tunable reactivities and useful physicochemical properties. They also offer specific alloying and dealloying conditions for the formation of multi-elemental liquid based nanoalloys or the synthesis of engineered solid nanomaterials. To date, while only a few nanosized LM materials have been investigated, extraordinary properties have been observed for such systems. Multi-elemental nanoalloys have shown controllable homogeneous or heterogeneous core and surface compositions with interfacial ordering at the nanoscale. The interactions and synergies of nanosized LMs with polymeric, inorganic and bio-materials have also resulted in new compounds. This review highlights recent progress and future directions for the synthesis and applications of post-transition LMs and their alloys. The review presents the unique properties of these LM nanodroplets for developing functional materials for electronics, sensors, catalysts, energy systems, and nanomedicine and biomedical applications, as well as other functional systems engineered at the nanoscale.

Liquid-Metal-Enabled Mechanical-Energy-Induced CO2 Conversion.

A green carbon capture and conversion technology offering scalability and economic viability for mitigating CO2 emissions is reported. The technology uses suspensions of gallium liquid metal to reduce CO2 into carbonaceous solid products and O2 at near room temperature. The nonpolar nature of the liquid gallium interface allows the solid products to instantaneously exfoliate, hence keeping active sites accessible. The solid co-contributor of silver-gallium rods ensures a cyclic sustainable process. The overall process relies on mechanical energy as the input, which drives nano-dimensional triboelectrochemical reactions. When a gallium/silver fluoride mix at 7:1 mass ratio is employed to create the reaction material, 92% efficiency is obtained at a remarkably low input energy of 230 kWh (excluding the energy used for dissolving CO2 ) for the capture and conversion of a tonne of CO2 . This green technology presents an economical solution for CO2 emissions.

Antibody coated conductive polymer for the electrochemical immunosensing of Human Cardiac Troponin I in blood plasma.

Cardiac troponin I (cTnI) is a sensitive biomarker for cardiovascular disease (CVD). Rapid determination of cTnI concentration in blood can greatly reduce the potential of significant heart damage and heart failure. Herein, we demonstrate a new electrochemical immunosensor for selective affinity binding and rapid detection of cTnI in blood plasma by an electrochemical method. A conductive film of "poly 2,5-bis(2-thienyl)3,4-diamine-terthiophene (PDATT)" was deposited onto an Indium Tin Oxide (ITO) electrode using chronoamperometry. Anti-cardiac troponin I antibody was then attached to the two amine (NH2) groups substituted on the central thiophene of terthiophene repeating unit of the polymer chain via amide bond formation. The gaps on the surface of the antibody coated immunosensor were backfilled with bovine serum albumin (BSA) to prevent nonspecific binding of interfering molecules. Differential pulse voltammetry (DPV) was used to determine cTnI upon the formation of cTnI immunocomplex on the sensing surface, appearing a peak at 0.27 V. The response range was 0.01-100 ng mL-1 with limit of quantification down to 0.01 ng mL-1. The developed immunosensor was used to determine cTnI in spiked blood plasma without interference from cardiac troponin T (cTnT). Therefore, this new sensor can be utilised for the detection of cTnI biomarker in pathological laboratories and points of care in less than 15 min.

Liquid metal assisted sonocatalytic degradation of organic azo dyes to solid carbon particles.

Room temperature liquid metals are an emerging class of materials for a variety of heterogeneous catalytic reactions. In this work we explore the use of Ga based liquid metals as a sonochemical catalyst for the degradation of organic azo dyes such as methyl orange, congo red and eriochrome black T. Rapid degradation to non toxic solid carbon particles was achieved over a large dye concentration range to produce differently sized particles via both bath and probe sonication which could be repeated multiple times with the same catalyst.

Electrochemical Capture and Storage of CO2 as Calcium Carbonate.

A carbon dioxide capture, conversion, and utilization technology has been developed that can be powered by renewable energy with the potential to mitigate CO2 emissions. This relies on an electrochemical process whereby the dissolution of carbon dioxide into carbonate ions is accelerated by a locally induced pH change at the cathode. The carbonate ions can then complex with metal cations, such as Ca2+ , Sr2+ , or Mn2+ , present in solution to form their respective metal carbonates, which precipitate out of solution. To ensure the cathode is not fouled by deposition of the insulating metal carbonate, the process is operated under hydrogen evolution conditions, thereby alleviating any significant attachment of the solid to the electrode. This process is demonstrated in CO2 -saturated solutions while the possibility of direct air capture is also shown, where the precipitation of CaCO3 from atmospherically dissolved CO2 during electrolysis is observed. The latter process can be significantly enhanced by using 5 vol.% of monoethanolamine (MEA) in the electrochemical cell. Finally, the process is investigated using seawater, which is also successful after the initial precipitation of metal sulfates from solution. In particular, the use of renewable energy to capture CO2 and create CaCO3 while also generating hydrogen may be of particular interest to the cement industry, which has a significant CO2 footprint.

Activating Iron Based Materials for Overall Electrochemical Water Splitting via the Incorporation of Noble Metals.

Although the presence of iron in mixed metal oxide based catalysts has shown significant performance improvement in the oxygen evolution reaction (OER), iron oxides themselves demonstrate much poorer activity. In this study, we investigate improving the performance of iron catalysts via surface decoration with gold or platinum for not only the OER but also the hydrogen evolution reaction (HER) for overall water splitting in an alkaline electrolyte. Two types of iron catalysts were synthesised, iron nanocubes and iron oxide via electrochemical deposition methods which were decorated with either Au or Pt via galvanic replacement. It was found that the presence of Au significantly enhanced the OER performance of iron oxide and the HER performance of iron nanocubes. The presence of Pt resulted in moderate improvement in the OER but significant improvement for the HER but did not surpass the performance of gold decorated iron nanocubes. This indicates that the speciation of the iron catalyst and the decorating metal was important for tuning the activity to the OER and the HER. For the OER, the formation of iron oxide/Au interfaces was determined to be an important component for high activity whereas the metallic nature of metal decorated iron nanocubes was important for the HER. Therefore, iron based catalysts can be modified to demonstrate bifunctional behaviour for overall water splitting via the inclusion of gold nanoparticles.

Pulsing Liquid Alloys for Nanomaterials Synthesis.

Although it remains unexplored, the direct synthesis and expulsion of metals from alloys can offer many opportunities. Here, such a phenomenon is realized electrochemically by applying a polarizing voltage signal to liquid alloys. The signal induces an abrupt interfacial perturbation at the Ga-based liquid alloy surface and results in an unrestrained discharge of minority elements, such as Sn, In, and Zn, from the liquid alloy. We show that this can occur by either changing the surface tension or inducing a reversible redox reaction at the alloys' interface. The expelled metals exhibit nanosized and porous morphologies, and depending on the cell electrochemistry, these metals can be passivated with oxide layers or fully oxidized into distinct nanostructures. The proposed concept of metal expulsion from liquid alloys can be extended to a wide variety of molten metals for producing metallic and metallic compound nanostructures for advanced applications.

Correlating the Local Electrocatalytic Activity of Amorphous Molybdenum Sulfide Thin Films with Microscopic Composition, Structure, and Porosity.

Thin-film electrodes, produced by coating a conductive support with a thin layer (nanometer to micrometer) of active material, retain the unique properties of nanomaterials (e.g., activity, surface area, conductivity, etc.) while being economically scalable, making them highly desirable as electrocatalysts. Despite the ever-increasing methods of thin-film deposition (e.g., wet chemical synthesis, electrodeposition, chemical vapor deposition, etc.), there is insufficient understanding on the nanoscale electrochemical activity of these materials in relation to structure/composition, particularly for those that lack long-range order (i.e., amorphous thin-film materials). In this work, scanning electrochemical cell microscopy (SECCM) is deployed in tandem with complementary, colocated compositional/structural analysis to understand the microscopic factors governing the electrochemical activity of amorphous molybdenum sulfide (a-MoSx) thin films, a promising class of hydrogen evolution reaction (HER) catalyst. The a-MoSx thin films, produced under ambient conditions by electrodeposition, possess spatially heterogeneous electrocatalytic activity on the tens-of-micrometer scale, which is not attributable to microscopic variations in elemental composition or chemical structure (i.e., Mo and/or S bonding environments), shown through colocated, local energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) analysis. A new SECCM protocol is implemented to directly correlate electrochemical activity to the electrochemical surface area (ECSA) in a single measurement, revealing that the spatially heterogeneous HER response of a-MoSx is predominantly attributable to variations in the nanoscale porosity of the thin film, with surface roughness ruled out as a major contributing factor by complementary atomic force microscopy (AFM). As microscopic composition, structure, and porosity (ECSA) are all critical factors dictating the functional properties of nanostructured materials in electrocatalysis and beyond (e.g., battery materials, electrochemical sensors, etc.), this work further cements SECCM as a premier tool for structure-function studies in (electro)materials science.