A review on recent developments in electrochemical hydrogen peroxide synthesis with a critical assessment of perspectives and strategies

Electrochemical hydrogen peroxide synthesis using two-electron oxygen electrochemistry is an intriguing alternative to currently dominating environmentally unfriendly and potentially hazardous anthraquinone process and noble metals catalysed direct synthesis. Electrocatalytic two-electron oxygen reduction reaction (ORR) and water oxidation reaction (WOR) are the source of electrochemical hydrogen peroxide generation. Various electrocatalysts have been used for the same and were characterized using several electroanalytical, chemical, spectroscopic and chromatographic tools. Though there have been a few reviews summarizing the recent developments in this field, none of them have unified the approaches in catalysts' design, criticized the ambiguities and flaws in the methods of evaluation, and emphasized the role of electrolyte engineering. Hence, we dedicated this review to discuss the recent trends in the catalysts' design, performance optimization, evaluation perspectives and their appropriateness and opportunities with electrolyte engineering. In addition, particularized discussions on fundamental oxygen electrochemistry, additional methods for precise screening, and the role of solution chemistry of synthesized hydrogen peroxide are also presented. Thus, this review discloses the state-of-the-art in an unpresented view highlighting the challenges, opportunities, and alternative perspectives

In Situ Mn-Doping-Promoted Conversion of Co(OH)2 to Co3O4 as an Active Electrocatalyst for Oxygen Evolution Reaction

Revealing the intrinsic catalytic properties and their dependence on other factors is always a desired challenge in the field of catalysis. Finding and fine-tuning of the inherent electrocatalytic properties of 3D nonprecious metal hydroxides and oxides by a foreign element doping is an attractive domain in the electrocatalysis of water oxidation. This report reveals such an effect of Mn doping on the electrocatalytic oxygen evolution reaction (OER) of Co3O4 nanosheet arrays grown on nickel foam (NF) by the ammonia evaporation technique. The Mn doping induces the in situ conversion of Co(OH)2 into Co3O4 during deposition. The strain generation during doping and the presence of metallic Mn are important factors for enhanced OER performance. Such a unique way of doping as well as in situ conversion of hydroxides into oxides offer an excellent electrocatalyst with superior performance compared to pristine Co3O4 or Co(OH)2 nanoflakes

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

The upsurge of photocatalysts in antibiotic micropollutants treatment: Materials design, recovery, toxicity and bioanalysis

The excessive use of antimicrobial agents such as antibiotics and disinfectants for domestic purposes and industries polluted the water bodies severely in the recent past. Thus released antimicrobial agents negatively impact the environment and human health as it induce antimicrobial resistance (AMR) to microbes in the environment. Conventional biodegradation routes showed feasible antibiotics pollutants degradation. Nonetheless, they often demand a long time of operation (usually in days) and a major portion of the antimicrobial agents is left untreated unlike the complete oxidation with advanced oxidation processes. The residues of antibiotics left in the water bodies accelerate growth of microorganisms (bacterial, fungal, and viral) with AMR. In virtue of avoiding the catastrophe of widespread AMR, photocatalysis assisted antibiotic pollutant treatment is recently gaining a great popularity as an advanced oxidation process and has shown to be useful for the removal of antimicrobial compounds, mainly antibiotics. Recent review reports on photocatalytic antibiotic degradation focus on summarizing materials progress and antibiotics pollutants in chronological viewpoints. However, the relationship between photocatalytic materials and antibiotics oxidation reaction pathways and the toxicity of by-products are needed to be shown with better clarity to transfer the photocatalysis technique from lab to market in a safe way. This review critically analyzes the insights of energetic semiconductor structure lacking to achieve hydroxyl and superoxide radicals mediated antibiotics degradation, recommends new materials design (Z scheme) and standardization in the experimental designs, and also informs the influencing parameters on antibiotic degradation. It further assesses the possibility of recovering value-added chemicals from the photocatalytic treatment process and highlights the importance of environmental toxicity analysis. Overall, this review will be a resourceful guide for interdisciplinary researchers working on advanced photocatalysis and pharmaceutical pollutant treatment for achieving a sustainable ecology and initiating a circular economy in chemical industries

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