Binder-Free Growth of Nickel-Doped Iron Sulfide on Nickel Foam via Electrochemical Deposition for Electrocatalytic Water Splitting

Iron–sulfur-based materials are advantageous for electrocatalytic activity owing to their high natural abundance and lesser toxicity. A few investigations on the hydrogen evolution reaction (HER) catalyzing activity of Fe–S materials were performed. However, the oxygen evolution reaction (OER) catalyzing activity or overall water splitting activity of Fe–S materials has not been studied extensively till date. Another technical aspect that suppresses the activity of the electrocatalyst is related to the usage of polymeric binders for electrode fabrication. Keeping these aspects in mind, iron sulfide was directly electrodeposited on nickel foam by varying the deposition potentials and duration of deposition. Ni-doped O-incorporated iron sulfide having the FeS2 lattice domains was obtained as the deposition product. The morphology, electronic structure, and charge carrier density in the valence band of the electrodeposits changed with the change in duration of electrodeposition, which in turn modulated the electrocatalytic activity. The electrode fabricated at −0.9 V potential after 30 min was found to be superior toward HER and OER. The electrodeposit obtained after 45 min showed comparable HER catalyzing activity. An asymmetric electrolyzer constructed with these electrodes showed a comparable water splitting activity to that of the RuO2(+)||Pt/C(−) electrolyzer and also surpassed its activity at higher potential

Understanding the Synergistic Effect in Oxygen Evolution Reaction Catalysis from Chemical Kinetics Point of View: An Iron Oxide/Nickel Oxide Case Study

The Oxygen Evolution reaction (OER) is still an enigmatic process, and a few research efforts have been deployed to understand its thermodynamic aspects. However, to date, no significant attention has been given to understand the chemical kinetics of the OER. Herein, the oxides of nickel and iron, and their heterostructure were chosen for the investigation. The electrocatalytic activity was found to augment synergistically when a heterostructure was formed between iron oxide and nickel oxide. The metal oxide catalyzed OER was an entropy-driven process and followed the peroxide linkage formation pathway. The rate-determining step was found to be different for the reactions catalyzed by different oxides. For the first time, Distribution of relaxation time (DRT) plots were utilized to study the chemical kinetics of OER, and the inference obtained from this analysis agreed well with the reaction mechanism. The kinetic barrier for the charge transfer process was found to decrease, and the surface group formation attained a moderate value after the heterostructure formation. These aspects had been the key player behind the synergistic increment in OER catalytic activity of the heterostructure. This investigation will lead a new pathway towards the development of strategies to understand the kinetics of an electrochemical reaction

Effect of Ion Diffusion in Cobalt Molybdenum Bimetallic Sulfide toward Electrocatalytic Water Splitting

The electrocatalyst comprising two different metal atoms is found suitable for overall water splitting in alkaline medium. Hydrothermal synthesis is an extensively used technique for the synthesis of various metal sulfides. Time-dependent diffusion of the constituting ions during hydrothermal synthesis can affect the crystal and electronic structure of the product, which in turn would modulate its electrocatalytic activity. Herein, cobalt molybdenum bimetallic sulfide was prepared via hydrothermal method after varying the duration of reaction. The change in crystal structure, amount of Co–S–Mo moiety, and electronic structure of the synthesized materials were thoroughly investigated using different analytical techniques. These changes modulated the charge transfer at the electrode–electrolyte interface, as evidenced by electrochemical impedance spectroscopy. The Tafel plots for the prepared materials were investigated considering a less explored approach and it was found that different materials facilitated different electrocatalytic pathways. The product obtained after 12 h reaction showed superior catalytic activity in comparison to the products obtained from 4, 8, and 16 h reaction, and it surpassed the overall water splitting activity of the RuO2–Pt/C couple. This study demonstrated the ion diffusion within the bimetallic sulfide during hydrothermal synthesis and change in its electrocatalytic activity due to ion diffusion

Effect of the Solvent Ratio (Ethylene Glycol/Water) on the Preparation of an Iron Sulfide Electrocatalyst and Its Activity towards Overall Water Splitting

The polyol method is an efficient procedure for metal sulfide preparation where polyol not only acts as a solvent but also as reducing and morphology‐modulating agent. Herein, iron sulfide particles were prepared via a modified polyol method by changing the ethylene glycol (EG) : water (H2O) ratio in the mixed solvent. Analytical techniques and electronic microscopy studies confirmed that the change in EG : H2O ratio modulated the crystal structure, morphology, and electronic structure of the prepared iron sulfide particles. The electrocatalytic activity of iron sulfide changed owing to these modulations. EG helped in the formation of a sheet‐like structure – a morphology that favours a higher accessibility to the catalytically active sites. As evidenced form electrochemical impedance studies, an increased electron density near the Fermi level, a faster substrate adsorption‐desorption rate at the active sites, and a faster charge transfer at the electrode‐electrolyte interface were the key factors for the amplification in catalytic activity. The prepared iron sulfide particles showed an overall water splitting efficiency that is comparable to that of the state‐of‐the‐art RuO2‐Pt/C couple in alkaline medium. This study shows the potential of the polyol method in the preparation and catalytic‐activity modulation of Fe−S‐based electrocatalysts

Optimization of active surface area of flower like MoS2 using V-doping towards enhanced hydrogen evolution reaction in acidic and basic medium

Two dimensional layered transition metal dichalcogenides (TMDS) have immense potential as inexpensive electro-catalyst for hydrogen evolution reaction (HER). Modification of crystal and electronic structure is a promising strategy to enhance the catalytic performance of TMDS. Herein, a colloquial solvothermal method was used to prepare the vanadium (V) doped MoS2 (VMSd). The structural, morphological and chemical analysis confirmed the formation of highly pure and uniform VMSd nanoflower. Tuning of V content in MoS2 successively improved its catalytic activity towards hydrogen evolution reaction (HER). As, evident from the polarization curve, the VMSd required low overpotential of 194 and 206 mV to achieve benchmarking current density of 10 mA cm−2 in acidic and basic medium, respectively. Mott-Schottky analysis suggested that the flat band potential of MoS2 differed upon V-doping, resulting in alteration of charge transfer ability at the electrode-electrolyte interface. The Fermi level shifted towards the conduction band with optimized V-doping and the band structure got modified effectively

Hierarchical Cobalt Sulfide/Molybdenum Sulfide Heterostructure as Bifunctional Electrocatalyst towards Overall Water Splitting

The development of non‐noble metal based electrocatalysts for overall water splitting is a potent strategy towards a carbon‐neutral and clean energy economy. Herein, hierarchical CoSx@MoS2 was synthesized via a one‐pot solvothermal process. Formation of the heterostructure was confirmed by electron microscopy and spectroscopic techniques. CoSx@MoS2 showed competent electrocatalytic activity towards both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline medium. Superior electrocatalytic activity was attributed to the increase in number of active sites, betterment in charge transfer and facilitation of H‐ and O‐ containing active species adsorption‐desorption at the active sites. Overall water splitting efficiency of CoSx@MoS2 was found to be superior in comparison to the state‐of‐the‐art RuO2‐Pt/C couple. Along with efficiency the heterostructure also exhibited long‐term operational durability. Thus, hierarchical CoSx@MoS2 is a potential non‐noble metal based bifunctional electrocatalyst towards overall water splitting

Synthesis of Tri‐functional Core‐shell CuO@carbon Quantum Dots@carbon Hollow Nanospheres Heterostructure for Non‐enzymatic H2O2 Sensing and Overall Water Splitting Applications

A core‐shell structure with CuO core and carbon quantum dots (CQDs) and carbon hollow nanospheres (CHNS) shell was prepared through facile in‐situ hydrothermal process. The composite was used for non‐enzymatic hydrogen peroxide sensing and electrochemical overall water splitting. The core‐shell structure was established from the transmission electron microscopy image analysis. Raman and UV‐Vis spectroscopy analysis confirmed the interaction between CuO and CQDs. The electrochemical studies showed the limit of detection and sensitivity of the prepared composite as 2.4 nM and 56.72 μA μM−1 cm−2, respectively. The core‐shell structure facilitated better charge transportation which in turn exhibited elevated electro‐catalysis towards hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and overall water splitting. The overpotential of 159 mV was required to achieve 10 mA cm−2 current density for HER and an overpotential of 322 mV was required to achieve 10 mA cm−2 current density for OER in 1.0 M KOH. A two‐electrode system was constructed for overall water splitting reaction, which showed 10 and 50 mA cm−2 current density at 1.83 and 1.96 V, respectively. The prepared CuO@CQDs@CHNS catalyst demonstrated excellent robustness in HER and OER catalyzing condition along with overall water splitting reaction. Therefore, the CuO@CQDs@CHNS could be considered as promising electro‐catalyst for H2O2 sensing, HER, OER and overall water splitting