Soumen Saha

Enhanced hydrogen/oxygen evolution and stability of nanocrystalline (4–6 nm) copper particles

The exploration of nanomaterials as catalysts for hydrogen and oxygen evolution is highly desirable for renewable and clean energy applications. We have obtained highly efficient and stable copper nanoparticles (4–6 nm) by thermal decomposition of aligned copper oxalate nanorods (obtained by a microemulsion method) in an argon atmosphere. Hydrogen and oxygen evolution reactions (HER and OER) were carried out on glassy carbon as well as platinum as working electrodes in KOH solution. In the case of HER the current densities were found to be 12 mA cm−2 (glassy carbon electrode) and 46 mA cm−2 (Pt electrode) which are significantly higher than reported values (maximum 1 mA cm−2). In the case of OER the current density was found to be 1.6 mA cm−2 (which is slightly higher) for the glassy carbon electrode and 15 mA cm−2 for Pt as the working electrode which is 4–30 times higher than earlier reports. The high efficiency can be related to the high surface area (34 m2 g−1) of these tiny well-crystalline copper nanoparticles obtained by the microemulsion mediated synthesis. The copper nanoparticles obtained by us show excellent stability as electrocatalysts and retain their activity even after 50 cycles.

Controlling the Morphology and Efficiency of Nanostructured Molybdenum Nitride Electrocatalysts for the Hydrogen Evolution Reaction

Among catalysts based on non‐noble metals, Mo‐based materials are important for hydrogen evolution because of their low cost, good conductivity, and catalytic efficiency. This study demonstrates a facile two‐step synthesis of Mo2N nanostructures assembled from 5–8 nm particles with graphitic carbon nitride as the nitrogen source. These Mo2N nanostructures of various morphologies (hexagons, triangles, and nanowires) show a very high activity and stability in acidic media during water electrolysis. Their nanostructures were characterized by using powder XRD, electron microscopy, N2 gas adsorption analysis, and X‐ray photoelectron spectroscopy. Hydrogen evolution reaction parameters, which include the Tafel slope, charge transfer resistance, and stability, were analyzed by using linear sweep voltammetry and electrochemical impedance spectroscopy. Thin hexagonal sheets of Mo2N show the highest apparent electrocatalytic activity (current density of 197 mA cm−2geometric at −400 mV vs. the reversible hydrogen electrode) and excellent stability in an acidic medium with a small onset potential of 90 mV and a Tafel slope of 145 mV decade−1. The lowest Tafel slope was observed for Mo2N nanowires.

A facile low temperature (350 °C) synthesis of Cu2O nanoparticles and their electrocatalytic and photocatalytic properties

We have synthesized Cu2O nanoparticles (∼25 nm) starting from CuO (∼107 nm) and copper oxalate nanorods in an inert atmosphere (Ar) at a very low temperature of 350 °C. The process in the absence of the oxalate nanorods yields Cu2O at a much higher temperature of 850 °C. The Cu2O synthesized at lower temperature (350 °C) has smaller particles than Cu2O synthesized at 850 °C. We explored these nanomaterials as electrocatalysts for hydrogen and oxygen evolution which are highly desirable for renewable and clean energy applications. Electrochemical studies such as the hydrogen evolution reaction and oxygen evolution reaction (HER & OER) were carried out on glassy carbon as well as on platinum as the working electrode in KOH solution. Cu2O synthesized at lower temperature (350 °C) has 14 times higher current density during HER and 2 times higher current density during OER. These electrocatalysts were stable for 50 cycles. However, the Cu2O synthesized at higher temperature (850 °C) showed very efficient (∼98%) degradation of methylene blue (MB) in 120 min and the catalyst is stable up to the 4th cycle whereas Cu2O (350 °C) shows only 40% degradation.

Efficient Electrocatalytic Hydrogen Evolution from MoS2-Functionalized Mo2N Nanostructures

Molybdenum-based compounds and their composites were investigated as an alternative to Pt for hydrogen evolution reactions. The presence of interfaces and junctions between Mo2N and MoS2 grains in the composites were investigated to understand their role in electrochemical processes. Here we found that the electrocatalytic activity of Mo2N nanostructures was enhanced remarkably by conjugation with few-layer MoS2 sheets. The electrocatalytic performance of Mo2N-MoS2 composites in the hydrogen evolution reaction (HER) was revealed from the high catalytic current density of ∼175 mA cm-2 (at 400 mV) and good electrochemical stability (more than 18 h) in acidic media. Increasing the amount of MoS2 in the composite, decreases the HER activity. The mechanism and kinetics of the HER process on the Mo2N-MoS2 surface were analyzed using Tafel slopes and charge transfer resistance.

Cu-Co-Ni alloys: an efficient and durable electrocatalyst in acidic media

We have developed efficient nanostructures of Cu–Co–Ni alloy with varied stoichiometry as an alternative to the costly Pt-based alloys for hydrogen evolution reaction (HER). These nanoparticles were synthesized using the reverse micellar method. The size of the alloy nanoparticles varied from 40 to 70 nm. An enhanced catalytic activity as evident from high current density was observed for these Cu–Co–Ni (111) alloys which follows the Volmer–Heyrovsky mechanism. They have excellent stability (up to 500 cycles) and significant activity in acid media which might be due to the low hydrogen binding energy.

Cu-Based Nanocomposites as Multifunctional Catalysts.

Herein, we report the synthesis of Cu/Cu2 O nanocomposites by a one-step hydrothermal process at 180 °C, for which the resulting morphology is dependent on the hydrothermal reaction time (24, 72, and 120 h). With a longer reaction time of 120 h, a rod-shape morphology is obtained, whereas at 72 and 24 h assemblies of nanoparticles are obtained. The rod-shaped (120 h) particles of the Cu/Cu2 O nanocomposites show a much higher efficiency (6.3 times) than the agglomerates and 2.5 times more than the assemblies of nanoparticles for the hydrogen-evolution reaction. During the oxygen-evolution reaction, the nanorods produce a current that is 5.2 and 3.7 times higher than that produced by the agglomerated and assembled nanoparticles, respectively. The electrocatalysts are shown to be highly stable for over 50 cycles. As catalysts for organic synthesis, a 100 % yield is achieved in the Sonogashira cross-coupling reaction with the nanorods, which is higher than with the other nanocomposite particles. This result demonstrates the significant enhancement of yield obtained with the nanorods for cross-coupling reactions.

Ternary alloy nanocatalysts for hydrogen evolution reaction

Cu–Fe–Ni ternary alloys (size ∼55–80 nm) with varying compositions viz. CuFeNi (A1), CuFe2Ni (A2) and CuFeNi2 (A3) were successfully synthesized using microemulsion. It is to be noted that synthesis of nanocrystalline ternary alloys with precise composition is a big challenge which can be overcome by choosing an appropriate microemulsion system. High electrocatalytic activity towards HER in alkaline medium was achieved by the formation of alloys of metals with low and high binding energies. A high value of current density (228 mA cm2) at an overpotential of 545 mV was obtained for CuFeNi (A1), which is significantly high as compared to the previously reported Ni59Cu41 alloy catalyst.

Ni3Co/G alloy as an earth-abundant robust and stable electrocatalyst for the hydrogen evolution reaction

Herein, a noble-metal-free robust and efficient graphene-supported Ni3Co alloy electrocatalyst was fabricated for the hydrogen evolution reaction. The synthesized nanoalloy was constructed from earth-abundant and low-cost components; it exhibited excellent performance for the hydrogen evolution reaction in an alkaline medium with a small overpotential of 95 mV to achieve a current density of 10 mA cm−2 and a Tafel slope of 104 mV dec−1, which are quite impressive. Moreover, the extraordinary durability of the current shown by the Ni3Co/G alloy during 16 h chronopotentiometric study without any decay deserves notable attention.