Microwave-Initiated Facile Formation of Ni₃Se₄ Nanoassemblies for Enhanced and Stable Water Splitting in Neutral and Alkaline Media

Molecular hydrogen (H₂) generation through water splitting with minimum energy loss has become practically possible due to the recent evolution of high-performance electrocatalysts. In this study, we fabricated, evaluated, and presented such a high-performance catalyst which is the Ni₃Se₄ nanoassemblies that can efficiently catalyze water splitting in neutral and alkaline media. A hierarchical nanoassembly of Ni₃Se₄ was fabricated by functionalizing the surface-cleaned Ni foam using NaHSe solution as the Se source with the assistance of microwave irradiation (300 W) for 3 min followed by 5 h of aging at room temperature (RT). The fabricated Ni₃Se₄ nanoassemblies were subjected to catalyze water electrolysis in neutral and alkaline media. For a defined current density of 50 mA cm^–2, the Ni₃Se₄ nanoassemblies required very low overpotentials for the oxygen evolution reaction (OER), viz., 232, 244, and 321 mV at pH 14.5, 14.0, and 13.0 respectively. The associated lower Tafel slope values (33, 30, and 40 mV dec^–1) indicate the faster OER kinetics on Ni₃Se₄ surfaces in alkaline media. Similarly, in the hydrogen evolution reaction (HER), for a defined current density of 50 mA cm^–2, the Ni₃Se₄ nanoassemblies required low overpotentials of 211, 206, and 220 mV at pH 14.5, 14.0, and 13.0 respectively. The Tafel slopes for HER at pH 14.5, 14.0, and 13.0 are 165, 156, and 128 mV dec^–1, respectively. A comparative study on both OER and HER was carried out with the state-of-the-art RuO₂ and Pt under identical experimental conditions, the results of which revealed that our Ni₃Se₄ is a far better high-performance catalyst for water splitting. Besides, the efficiency of Ni₃Se₄ nanoassemblies in catalyzing water splitting in neutral solution was carried out, and the results are better than many previous reports. With these amazing advantages in fabrication method and in catalyzing water splitting at various pH, the Ni₃Se₄ nanoassemblies can be an efficient, cheaper, nonprecious, and high-performance electrode for water electrolysis with low overpotentials

NiTe₂ Nanowire Outperforms Pt/C in High-Rate Hydrogen Evolution at Extreme pH Conditions

Better hydrogen generation with nonprecious electrocatalysts over Pt is highly anticipated in water splitting. Such an outperforming nonprecious electrocatalyst, nickel telluride (NiTe₂), has been fabricated on Ni foam for electrocatalytic hydrogen evolution in extreme pH conditions, viz., 0 and 14. The morphological outcome of the fabricated NiTe₂ was directed by the choice of the Te precursor. Nanoflakes (NFs) were obtained when NaHTe was used, and nanowires (NWs) were obtained when Te metal powder with hydrazine hydrate was used. Both NiTe₂ NWs and NiTe₂ NFs were comparatively screened for hydrogen evolution reaction (HER) in extreme pH conditions, viz., 0 and 14. NiTe₂ NWs delivered current densities of 10, 100, and 500 mA cm^–2 at the overpotentials of 125 ± 10, 195 ± 4, and 275 ± 7 mV in 0.5 M H₂SO₄. Similarly, in 1 M KOH, overpotentials of 113 ± 5, 247 ± 5, and 436 ± 8 mV were required for the same current densities, respectively. On the other hand, NiTe₂ NFs showed relatively poorer HER activity than NiTe₂ NWs, which required overpotentials of 193 ± 7, 289 ± 5, and 494 ± 8 mV in 0.5 M H₂SO₄ for the current densities of 10 and 100 mA cm^–2 and 157 ± 5 and 335 ± 6 mV in 1 M KOH for the current densities of 10 and 100 mA cm^–2, respectively. Notably, NiTe₂ NWs outperformed the state-of-the-art Pt/C 20 wt % loaded Ni foam electrode of comparable mass loading. The Pt/C 20 wt % loaded Ni foam electrode reached 500 mA cm^–2 at 332 ± 5 mV, whereas NiTe₂ NWs drove the same current density with 57 mV less. These encouraging findings emphasize that a NiTe₂ NW could be an alternative to noble and expensive Pt as a nonprecious and high-performance HER electrode for proton-exchange membrane and alkaline water electrolyzers

Core-Oxidized Amorphous Cobalt Phosphide Nanostructures: An Advanced and Highly Efficient Oxygen Evolution Catalyst

We demonstrated a high-yield and easily reproducible synthesis of a highly active oxygen evolution reaction (OER) catalyst, “the core-oxidized amorphous cobalt phosphide nanostructures”. The rational formation of such core-oxidized amorphous cobalt phosphide nanostructures was accomplished by homogenization, drying, and annealing of a cobalt­(II) acetate and sodium hypophosphite mixture taken in the weight ratio of 1:10 in an open atmosphere. Electrocatalytic studies were carried out on the same mixture and in comparison with commercial catalysts, viz., Co₃O₄-Sigma, NiO-Sigma, and RuO₂-Sigma, have shown that our catalyst is superior to all three commercial catalysts in terms of having very low overpotential (287 mV at 10 mA cm^–2), lower Tafel slope (0.070 V dec^–1), good stability upon constant potential electrolysis, and accelerated degradation tests along with a significantly higher mass activity of 300 A g^–1 at an overpotential of 360 mV. The synergism between the amorphous Co_<i>x</i>P_<i>y</i> shell with the Co₃O₄ core is attributed to the observed enhancement in the OER performance of our catalyst. Moreover, detailed literature has revealed that our catalyst is superior to most of the earlier reports

Pt Nanoparticle Anchored Molecular Self-Assemblies of DNA: An Extremely Stable and Efficient HER Electrocatalyst with Ultralow Pt Content

An efficient electrocatalytic hydrogen evolution reaction (HER) with ultralow loading of Pt has been under intense investigation to make the state-of-the-art Pt economically affordable for water electrolyzers. Here, colloidally synthesized Pt nanoparticles of average size 3.5 ± 0.3 nm were successfully anchored on molecular self-assemblies of DNA. The synthesized Pt@DNA colloidal solution was directly assessed for the electrochemical hydrogen evolution reaction (HER) in 0.5 M H₂SO₄ with a loading of 5 μL of Pt@DNA colloidal solution that corresponds to a Pt equivalent of 15 μg/cm². The excellent adhesion of DNA onto GC and FTO substrate electrodes, the conductivity of DNA, and its stability upon potentiostatic electrolysis and accelerated degradation have made the synthesized, stable Pt@DNA colloidal solution an advanced HER electrocatalyst. The Pt@DNA–GC interface without binder required overpotentials of −0.026 and −0.045 V for current densities of 10 and 20 mA/cm², respectively. The potentiostatic electrolysis and accelerated degradation tests did not affect the electrocatalytic activity, and the observed increase in overpotential was highly negligible. The extreme stability of the Pt@DNA–GC interface was witnessed during an aging study carried out by keeping the working electrode in the electrolyte solution for more than 10 days and acquiring linear sweep voltammograms (LSVs) at intervals of 24 h. Under the same experimental conditions, the commercial Pt/C 10 wt % catalyst with Nafion binder had failed to compete with our colloidal Pt@DNA. These findings certainly indicate the advantageous use of electrocatalyst-loaded DNA molecular self-assemblies for the HER which has never been observed before

Self-Assembled Molecular Hybrids of CoS-DNA for Enhanced Water Oxidation with Low Cobalt Content

Water oxidation in alkaline medium was efficiently catalyzed by the self-assembled molecular hybrids of CoS-DNA that had 20 times lower Co loading than the commonly used loading. The morphological outcome was directed by varying the molar ratio of metal precursor Co­(Ac)₂ and DNA and three different sets of CoS-DNA molecular hybrids, viz. CoS-DNA(0.036), CoS-DNA(0.06), and CoS-DNA(0.084) were prepared. These morphologically distinct hybrids had shown similar electrocatalytic behavior, because of the fact that they all contained the same cobalt content. The CoS-DNA(0.036), CoS-DNA(0.06), and CoS-DNA(0.084) required very low overpotentials of 350, 364, and 373 mV at a current density of 10 mA cm^–2 (1 M KOH), respectively. The advantages of lower overpotential, lower Tafel slope (42.7 mV dec^–1), high Faradaic efficiency (90.28%), high stability and reproducibility after all, with a lower cobalt loading, have certainly shown the worth of these molecular hybrids in large-scale water oxidation. Moreover, since DNA itself a good binder, CoS-DNA molecular hybrids were directly casted on substrate electrodes and used after drying. It also showed minimum intrinsic resistance as DNA is a good ionic and electronic conductor. Besides, the present method may also be extended for the preparation of other active electrocatalysts for water splitting

Microwave-Assisted Template-Free Synthesis of Ni₃(BO₃)₂(NOB) Hierarchical Nanoflowers for Electrocatalytic Oxygen Evolution

The construction of cost-effective, efficient, and sustainable catalytic systems for electrocatalytic hydrogen generation by water splitting is extremely important for future fuels globally. Herein, we have prepared nickel orthoborate (NOB) via simultaneous oxidation and reduction of nickel precursors and studied their role in oxygen evolution reaction (OER) for water electrolysis. In addition, the specific role of microwave irradiation and conventional stirring in the formation of NOB was also investigated with comparative assessment of their catalytic ability in electrochemical water splitting. It was found that NOB nanoflowers prepared via microwave irradiation exhibited better OER electrocatalyst than the ones prepared by conventional heating. Interestingly, the NOB nanoflowers outperformed the commercial NiO nanopowder under the identical experimental conditions in catalyzing OER. Morphological hierarchy and high Brunauer–Emmett–Teller specific surface area were attributed for their enhanced OER activity. A long run of 6 h chronopotentiometry analysis showed a negligible degradation in activity signified the high stability and endurance of NOB nanoflowers. The numbers of merits from the electrochemical characterizations revealed that NOB nanoflowers could be an alternate, efficient, and abundant OER electrocatalyst for bulk water electrolysis

High-Performance Oxygen Evolution Anode from Stainless Steel via Controlled Surface Oxidation and Cr Removal

Improving the water oxidation performance of abundantly available materials, such as stainless steel (SS), with notable intrinsic electrocatalytic oxygen evolution reaction (OER) activity due to the presence of Ni and Fe is highly anticipated in water splitting. A new method for promoting the corrosion of stainless steel (304) was found which assisted the uniform formation of oxygen evolution reaction (OER) enhancing NiO incorporated Fe₂O₃ nanocrystals with the simultaneous reduction in the surface distribution of OER inactive Cr. An equimolar combination of KOH and hypochlorite was used as the corroding agent at 180 °C. The effect of corrosion time on the OER activity was studied and found that better water oxidation performance was observed when the corrosion time was 12 h (SS-12). The SS-12 showed an abnormal enhancement in OER activity compared to the untreated SS and other optimized versions of the same by requiring very low overpotentials of 260, 302, and 340 mV at the current densities of 10, 100, and 500 mA cm^–2 along with a very low Tafel slope in the range of 35.6 to 43.5 mV dec^–1. These numbers have certainly shown the high-performance electrocatalytic water oxidizing ability of SS-12. The comparative study revealed that the state-of-the-art IrO₂ had failed to compete with our performance improved catalytic water oxidation anode “the SS-12”. This fruitful finding indicates that the SS-12 has the potential to be an alternate anode material to precious IrO₂/RuO₂ for alkaline water electrolyzers in future

Stainless Steel Scrubber: A Cost Efficient Catalytic Electrode for Full Water Splitting in Alkaline Medium

Sometimes, searching for a cost efficient bifunctional catalytic material for water splitting can be accomplished from a very unlikely place. In this work, we are reporting such a discovery of utilizing the stainless steel (SS) scrubber directly as a catalytic electrode for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) of water electrolysis in 1 M KOH. The <i>iR</i> corrected overpotential calculated at an areal current density of 10 mA cm^–2 for a SS scrubber in HER is 315 mV which is 273 mV higher than Pt/C. Similarly, the SS scrubber required 418 mV at 10 mA cm^–2 which is just 37 and 98 mV higher than Ni­(OH)₂ and RuO₂. Interestingly, the kinetic analysis revealed that the SS scrubber had facile kinetics for both HER and OER in 1 M KOH as reflected by their corresponding Tafel slope values viz., 121 and 63 mV dec^–1, respectively. In addition, the two electrode cell fabricated using the same SS scrubber electrode delivered 10 mA cm^–2 at 1.98 V. Beyond everything, the SS scrubber had shown ultrahigh stability in both half-cell and full-cell studies for total water splitting. Further, as far as the cost of an electrode material per gram is concerned, the SS scrubber defeats all the best electrocatalysts of water splitting by having a price of just $0.012 USD which is$2.228 USD lower than pure Ni, $59.658 USD lower than RuO₂ and$158.028 USD lower than Pt/C 20 wt % catalyst. The overall study specified that the SS scrubber can be adapted for cost-efficient large scale water electrolysis for bulk hydrogen production

Shrinking the Hydrogen Overpotential of Cu by 1 V and Imparting Ultralow Charge Transfer Resistance for Enhanced H₂ Evolution

Copper and its oxides are among the best electrocatalysts for the electrochemical conversion of CO₂ to value-added small organics because of its high hydrogen overvoltage, making the hydrogen evolution reaction (HER) a poor side reaction. Here we report an interesting finding that turned the nature of surface-oxidized Cu upside down in electrochemical H₂ evolution. It is commonly known that the electrochemical reactivity of a metal ion is highly sensitive to the anion to which it is coordinated in the electrolyte. In the case of Cu, when it is in the form of copper oxide, the hydrogen overvoltage is huge. Nonetheless, we found that when Cu is in coordination with Se^2– ions as Cu₂Se, the hydrogen overvoltage was shrunken by ∼1 V, imparting ultralow charge transfer resistance (<i>R</i>_CT) that varied from 0.32 to 0.61 Ω depending on the means by which selenization was carried out. Selenization was done by two different methods. In one method, conventional stirring was employed to selenize Cu foam in a preheated NaHSe solution at 90 °C for 20 min. In another method, hydrothermal treatment was employed to selenize Cu foam with NaHSe solution at 120 °C for 1 h. The wet-chemical method yielded honeycomb-like hierarchical arrays of Cu₂Se sheets on Cu foam (designated as Cu₂Se-ch/Cu), and the hydrothermal method yielded a uniform array of spiky rods of Cu₂Se (designated as Cu₂Se-ht/Cu). The HER electrocatalytic studies carried out in 0.5 M H₂SO₄ showed that Cu₂Se-ch/Cu and Cu₂Se-ht/Cu had similar kinetics, with Tafel slopes of 32 to 35 mV dec^–1, which is closer to the state-of-the-art Pt/C. Interestingly, the Cu₂Se-ch/Cu delivered a total kinetic current density of −1200 mA cm^–2 when polarized up to −0.85 V vs RHE, whereas Cu₂Se-ht/Cu delivered a maximum of −780 mA cm^–2 only