Mechanistic insights into the electrolyte effects on the electrochemical nitrogen reduction reaction using copper hexacyanoferrate/f-MWCNT nano-composites.

Developing an efficient, selective, and stable electrocatalysis system for the electrocatalytic N2 reduction reaction (ENRR) is a promising strategy for the green and sustainable production of ammonia. The activity, selectivity, and stability of various electrocatalysts in different electrolyte solvents, mainly acidic and alkaline electrolytes, are commonly compared in the literature. However, a mechanistic insight into the effect of these electrolytes on ENRR activity is lacking. Herein we demonstrate that the acidity or alkalinity of the electrolyte is a key factor in determining the rate-limiting step and, by extension, the ENRR performance of an electrochemical setup for the electroproduction of ammonia. Our results from ex situ X-ray photoelectron, Raman, and FTIR spectroscopy analysis of the fresh and spent Cu-hexacyanoferrate Prussian blue analogue-decorated functionalized carbon nanotube (CuFe PBA/f-CNT) catalyst reveal that NH4+-species are more strongly adsorbed on the catalyst surface during the ENRR in acidic than in alkaline electrolytes. The results of our detailed rotating ring-disc electrode voltammetry studies suggest that the ENRR over CuFe PBA/f-CNT is mostly controlled by surface adsorption in an acidic electrolyte and by mass transport in an alkaline electrolyte. In situ Raman spectroscopy confirms this finding and shows that the leaching of Fe(CN)6 species from the CuFe PBA/f-CNT composite in an alkaline electrolyte greatly affects the ENRR performance. We believe that the work presented herein offers a new insight into the mechanistic aspects of the ENRR in different electrolyte systems and hence can prove very valuable for the development of effective ENRR electrode/electrolyte systems for practical applications.

In Situ Detection of the Adsorbed Fe(II) Intermediate and the Mechanism of Magnetite Electrodeposition by Scanning Electrochemical Microscopy.

Electrodeposition is an important approach that can produce functional compound materials by assembling multiple species at the electrode surface. However, a fundamental understanding of the electrodeposition mechanism has been limited by its complexity and is often gained only through ex situ studies of deposited materials. Here we report on the application of scanning electrochemical microscopy (SECM) to enable the in situ, real-time, and quantitative study of electrodeposition and electrodissolution. Specifically, we electrodeposit magnetite (Fe3O4) from an alkaline solution of Fe(III)-triethanolamine as a robust route that can prepare this magnetic and electrocatalytic compound on various conductive substrates. The powerful combination of SECM with cyclic voltammetry (CV) at a gold substrate reveals that the electrodeposition of magnetite requires the preceding adsorption of Fe(II)-triethanolamine on the substrate surface and, subsequently, is mediated through the highly complicated ECadsCmag mechanism, where both chemical steps occur at the substrate surface rather than in the homogeneous solution. SECM-based CV is obtained under high mass-transport conditions and analyzed by the finite element method to kinetically resolve all steps of the ECadsCmag mechanism and quantitatively determine relevant reaction parameters. By contrast, the adsorbed Fe(II) intermediate is unresolvable from co-deposited magnetite in situ by other electrochemical techniques and is undetectable ex situ because of the facile air oxidation of the Fe(II) intermediate. Significantly, SECM-based CV will be useful for the in situ characterization of various electrodeposited compounds to complement their ex situ characterization.

Effect of Chemical Charging/Discharging on Plasmonic Behavior of Silver Metal Nanoparticles Prepared using Citrate-Stabilized Cadmium Selenide Quantum Dots.

The thermodynamics and kinetics of the chemical and electrochemical charging of a catalyst surface are very important to understand its applicability as a catalyst material, particularly in redox catalysis. Through the present study, we hereby communicate the results obtained from our detailed investigations related to the effect of chemical charging on the plasmonic behavior of silver metal nanoparticles (Ag MNPs) as redox catalysts. Two different batches of Ag MNPs were prepared through thermally assisted chemical reduction of silver ions. The difference in these batches was the use or not of citrate-capped cadmium selenide quantum dots (Q-CdSe) for the reduction of solution-phase silver ions to their colloidal plasmonic phase. The charge on the surfaces of the Ag MNPs was varied by the chemical electron injection method by using BH4- ions from a NaBH4 solution. The processes of charging and discharging were monitored by using UV/Vis absorption spectroscopy. The impact of the concentration of the reductant on the charging and discharging processes was also investigated. The Ag MNPs were also tested for their voltammetric response, wherein it was observed that it was more difficult to oxidize the Ag MNPs prepared with Q-CdSe seeds than to oxidize Ag MNPs prepared without Q-CdSe particles. Our results demonstrate that Q-CdSe seeds not only enhance the redox catalytic activity of Ag MNPs but also provide stability towards polarization of their plasmonic behavior.

Probing the Crystal Structure, Composition-Dependent Absolute Energy Levels, and Electrocatalytic Properties of Silver Indium Sulfide Nanostructures.

The absolute electronic energy levels in silver indium sulfide (AIS) nanocrystals (NCs) with varying compositions and crystallographic phases have been determined by using cyclic voltammetry. Different crystallographic phases, that is, metastable cubic, orthorhombic, monoclinic, and a mixture of cubic and orthorhombic AIS NCs, were studied. The band gap values estimated from the cyclic voltammetry measurements match well with the band gap values calculated from the diffuse reflectance spectra measurements. The AIS nanostructures were found to show good electrocatalytic activity towards the hydrogen evolution reaction (HER). Our results clearly establish that the electronic and electrocatalytic properties of AIS NCs are strongly sensitive to the composition and crystal structure of AIS NCs. Monoclinic AIS was found to be the most active HER electrocatalyst, with electrocatalytic activity that is almost comparable to the MoS2 -based nanostructures reported in the literature, whereas cubic AIS was observed to be the least active of the studied crystallographic phases and compositions. In view of the HER activity and electronic band structure parameters observed herein, we hypothesize that the Fermi energy level of AIS NCs is an important factor that decides the electrocatalytic efficiency of these nanocomposites. The work presented herein, in addition to being the first of its kind regarding the composition and phase-dependence of electrochemical aspects of AIS NCs, also presents a simple solvothermal method for the synthesis of different crystallographic phases with various Ag/In molar ratios.