Selective electrochemical production of hydrogen peroxide from reduction of oxygen on mesoporous nitrogen containing carbon

Unveiling an In Situ H2O2 Production: Rechargeable Zinc-H2O2 Battery Powering 26 LEDs

Looking toward ever-growing energy demand, the advancement in energy storage devices and carrying out electrocatalytic reactions in an efficient way is the need of the hour. Herein, we have employed the two birds-one stone approach, viz. fusing energy conversion and storing system enabling 2e- oxygen reduction reaction (ORR) to value-added hydrogen peroxide (H2O2) product and its utilization in the energy storage devices using MnWO4 catalyst, without any external H2O2 source. The designed catalyst exhibited a remarkable H2O2 production of 98% @ 0.37 V versus RHE. The real-time H2O2 production was monitored by in-situ electrochemical Raman and in-situ infrared spectroscopic measurements. The pivotal influence of electrolyte composition, viz. local pH and the formation of carbonate species on H₂O₂ production was meticulously examined through micro-electrochemical studies using gold (Au) microelectrodes. Further, we have explored an aqueous rechargeable Zn-H2O2 battery utilizing MnWO4 as bifunctional electrocatalyst for sustainable H2O2 production simultaneously producing the electricity. The Zn-H2O2 battery exhibited a remarkable cycle life of 136 h and an practical energy efficiency of 43%. The galvanostatic charge-discharge measurement of Zn-H2O2 battery attained a capacity of 25 mAh cm-2 at 3 mA cm-2. The battery also demonstrated ≥90% cycle efficiency. Interestingly, the designed Zn-H2O2 battery (two connected in series) exhibited a stable open circuit voltage (OCV) with a promising power density of 10.5 mW cm-2. As a proof of concept, we have demonstrated Zn-H2O2 batteries by powering 26 blue LEDs for more than 180 h (7 days) without fading in the illumination of LEDs.

Employing Mesoporous Nitrogen Containing Carbon for Simultaneous Electrochemical Detection of Heavy Metal Ions

Heavy metal ions are major contributors to water pollution, posing significant threats to both ecological balance and human health due to their carcinogenic properties. The increasing need for heavy metal detection highlights the advantages of electrochemical methods, which offer high sensitivity and efficiency. Herein mesoporous nitrogen containing carbon (MNC) was utilized for the simultaneous determination of heavy metals using square wave voltammetry technique in the established conditions of a buffer pH of 5.0. MNC demonstrated low detection limits (1, 10 and 50 μM), wide linear ranges (1 μM-6 mM, 10 μM-7 mM and 50 μM-17 mM), and high sensitivities (2.5 μA μM-1 cm-2, 1.03 μA μM-1 cm-2 and 5.14 mA mM-1 cm-2) for, Pb2+, Cd2+ and Cu2+, respectively. Moreover, the reproducibility, and selectivity of the sensor was investigated in the presence of K+, Mg2+, Zn2+, Ni2+, and Fe3+ which are the possible interferents present in water.

Modulating Core Polarity in Metal-free Covalent Organic Frameworks for Selective Electrocatalytic Hydrogen Peroxide Production

Tuning the charge density at the active site to balance the adsorption ability and reactivity of oxygen is extremely significant for driving a two-electron oxygen reduction reaction (ORR) to produce hydrogen peroxide (H2O2). Herein, we have highlighted the influence of intermolecular polarity in covalent organic frameworks (COFs) on the efficiency and selectivity of electrochemical H2O2 production. Different C3 symmetric building blocks have been utilized to regulate the charge density at the active sites. The benzene-cored COF, which exhibits reduced polarity than the triazine-cored COF, displayed enhanced performance in H2O2 production, achieving 93.1 % selectivity for H2O2 at 0.4 V with almost two-electron transfer and a faradaic efficiency of 90.5 %. In-situ electrochemical Raman spectroscopy and scanning electrochemical microscopy (SECM) were employed to confirm H2O2 generation and analyze spatial reactivity patterns. These techniques provided detailed insights into localized catalytic behavior, emphasizing the influence of core polarity.

Realizing the label-free sensitive detection of carcinoembryogenic antigen (CEA) in blood serum via a MNC-decorated flexible immunosensor

A label-free electrochemical immunosensor utilising nitrogen-rich mesoporous carbon (MNC) as the substrate material was developed for the sensitive quantification of carcinoembryonic antigen (CEA). The synergic interactions between MNC and AbCEA also eliminated the need for coupling agents such as EDC/NHS. The novel immunosensor demonstrated a wide detection range from 500 fM (9.04 pg mL-1) to 50 nM (1 μg mL-1) and a low detection limit (LOD) of 500 fM. Moreover, the immunosensor showed sensitivities of 12.27 mA nM-1 cm-2 and 0.066 mA nM-1 cm-2 for detecting CEA in the linear ranges 10 pM to 1 nM and 2 nM to 50 nM, respectively, while maintaining long-term storage stability of 6 weeks. Analysis of real serum sample analysis yielded highly accurate results with recovery rates ranging from 99.3% to 103.7%. Furthermore, the developed paper-based screen-printed electrode exhibited a similar detection range, suggesting its potential for use in point-of-care detection devices in future applications.

Tunable Metal-Free Imidazole-Benzimidazole Electrocatalysts for Oxygen Reduction in Aqueous Solutions

A series of metal-free imidazole-benzimidazole catalysts (ImBenz-H, ImBenz-NO2 , ImBenz-OCH3 ) for oxygen reduction reaction (ORR) were prepared. We demonstrate that the electrocatalytic O2 reduction by ImBenz-NO2 with the electron-withdrawing group showed high selectivity toward H2 O with the number of electrons transferred (n=3.7) in a neutral aqueous solution. The highest ORR selectivity toward H2 O2 was achieved using ImBenz-H (n=2.4) in an alkaline solution. Electrochemical studies of reaction kinetics disclosed that the highest turnover frequencies were obtained from ImBenz-H in both neutral and alkaline aqueous solutions. The results prove that the ORR selectivity is tunable by modulating the substituent of the ImBenz catalysts. Furthermore, DFT calculations suggested that the ORR mechanism of ImBenz-H involves the electron transfer from imidazole-benzimidazole to O2 resulting in the formation of H2 O2 which supports the redox active properties of the catalysts ImBenz.

Electrochemical production of hydrogen peroxide by non-noble metal-doped g-C3N4 under a neutral electrolyte

Electrochemical oxygen reduction (ORR) for the production of clean hydrogen peroxide (H2O2) is an effective alternative to industrial anthraquinone methods. The development of highly active, stable, and 2e- ORR oxygen reduction electrocatalysts while suppressing the competing 4e- ORR pathway is currently the main challenge. Herein, bimetallic doping was successfully achieved based on graphitic carbon nitride (g-C3N4) with the simultaneous introduction of K and Co, whereby 2D porous K-Co/CNNs nanosheets were obtained. The introduction of Co promoted the selectivity for H2O2, while the introduction of K not only promoted the formation of 2D nanosheets of g-C3N4, but also inhibited the ablation of H2O2 by K-Co/CNNs. Electrochemical studies showed that the selectivity of H2O2 in K-Co/CNNs under neutral electrolyte was as high as 97%. After 24 h, the H2O2 accumulation of K-Co/CNNs was as high as 31.7 g L-1. K-Co/CNNs improved the stability of H2O2 by inhibiting the ablation of H2O2, making it a good 2e- ORR catalyst and providing a new research idea for the subsequent preparation of H2O2.

Review on the Degradation Mechanisms of Metal-N-C Catalysts for the Oxygen Reduction Reaction in Acid Electrolyte: Current Understanding and Mitigation Approaches

One bottleneck hampering the widespread use of fuel cell vehicles, in particular of proton exchange membrane fuel cells (PEMFCs), is the high cost of the cathode where the oxygen reduction reaction (ORR) occurs, due to the current need of precious metals to catalyze this reaction. Electrochemists tackle this issue in the short/medium term by developing catalysts with improved utilization or efficiency of platinum, and in the longer term, by developing catalysts based on Earth-abundant elements. Considerable progress has been achieved in the initial performance of Metal-nitrogen-carbon (Metal-N-C) catalysts for the ORR, especially with Fe-N-C materials. However, until now, this high performance cannot be maintained for a sufficiently long time in an operating PEMFC. The identification and mitigation of the degradation mechanisms of Metal-N-C electrocatalysts in the acidic environment of PEMFCs has therefore become an important research topic. Here, we review recent advances in the understanding of the degradation mechanisms of Metal-N-C electrocatalysts, including the recently identified importance of combined oxygen and electrochemical potential. Results obtained in a liquid electrolyte and a PEMFC device are discussed, as well as insights gained from in situ and operando techniques. We also review the mitigation approaches that the scientific community has hitherto investigated to overcome the durability issues of Metal-N-C electrocatalysts.

Highly Efficient Electrochemical Production of Hydrogen Peroxide Using the GDE Technology

This work examines the role of oxygen supply in the improvement of the hydrogen peroxide (H₂O₂) electrochemical production efficiency and the generation of high H₂O₂ concentrations in electrochemical processes operated in a discontinuous mode. To conduct this study, a highly efficient Printex L6 carbon-based gas diffusion electrode (GDE) as a cathode was employed for the electrogeneration of H₂O₂ in a flow-by reactor and evaluated the effects of lowering the operation temperature (to increase solubility) and increasing the air supply in the system on H₂O₂ electrogeneration. The results obtained in this study show that unlike what is expected in flow-through reactors, the efficiency in the H₂O₂ production is not affected by the solubility of oxygen when GDE is employed in the electrochemical process (using the flow-by reactor); i.e., the efficiency of H₂O₂ production is not significantly dependent on O₂ solubility, temperature, and pressure. The application of the proposed PL6C-based GDE led to the generation of accumulated H₂O₂ of over 3 g L^-1 at a high current density. It should be noted, however, that the application of the electrocatalyst at lower current densities resulted in higher energy efficiency in terms of H₂O₂ production. Precisely, a specific production of H₂O₂ as high as 131 g kWh^-1 was obtained at 25 mA cm^-2; the energy efficiency (in terms of H₂O₂ production) values obtained in this study based on the application of the proposed GDE in a flow-by reactor at low current densities were found to be within the range of values recorded for H₂O₂ production techniques that employ flow-through reactors.