Non-Metal-Doped Porous Carbon Nitride Nanostructures for Photocatalytic Green Hydrogen Production

Scalable One-Pot Fabrication of Carbon-Nanofiber-Supported Noble-Metal-Free Nanocrystals for Synergetic-Dependent Green Hydrogen Production: Unraveling Electrolyte and Support Effects

Electrocatalytic hydrogen evolution reactions (HER) are envisaged as the most promising sustainable approach for green hydrogen production. However, the considerably high cost often associated with such reactions, particularly upon scale-up, poses a daunting challenge. Herein, a facile, effective, and environmentally benign one-pot scalable approach is developed to fabricate MnM (M═Co, Cu, Ni, and Fe) nanocrystals supported over in situ formed carbon nanofibers (MnM/C) as efficient noble-metal-free electrocatalysts for HER. The formation of carbon nanofibers entails impregnating cellulose in an aqueous solution of metal precursors, followed by annealing the mixture at 550 °C. During the impregnation process, cellulose acts as a reactor for inducing the in situ reductions of MnM salts with the assistance of ether and hydroxyl groups to drive the mass production (several grams) of ultralong (5 ± 1 μM) carbon nanofibers ornamented with MnM nanoparticles (10-14 nm in size) at an average loading of 2.87 wt %. For better electrocatalytic HER benchmarking, the fabricated catalysts were tested over different working electrodes, i.e., carbon paper, carbon foam, and glassy carbon, in the presence of different electrolytes. All the fabricated MnM/C catalysts have demonstrated an appealing synergetic-effect-dependent HER activity, with MnCo/C exhibiting the best performance over carbon foam, close to that of the state-of-the-art commercial Pt/C (10 wt % Pt), with an overpotential of 11 mV at 10 mA cm-2, a hydrogen production rate of 2448 mol g-1 h-1, and a prolonged stability of 2 weeks. The HER performance attained by MnCo/C nanofibers is among the highest reported for Pt-free electrocatalysts, thanks to the mutual alloying effect, higher synergism, large surface area, and active interfacial interactions over the nanofibers. The presented findings underline the potential of our approach for the large-scale production of cost-effective electrocatalysts for practical HER.

Investigating the thermo-physical properties of a new kind of graphitic carbon nitride included ternary hybrid nanofluids and the property correlations

In this work, a simple and facile approach was employed for the preparation of the ternary hybrids comprising of titanium dioxide, zinc oxide and graphitic nitride (designated as TZG-TH) with varying compositions of the components. In the context of complex and multi-stages involved for preparation of many of the THs in the literature, the present work uses the much simpler mythology for the preparation of TH. Nanofluids (NF) were formulated in ethylene glycol: water base fluid using TZG TH as the solid particles. Scanning electron microscope of TZG TH informs that the particles are agglomerated. High resolution transmission electron microscopy image of TZG-TH reveals the presence narrowly distributed spherical particles (having the sizes in the range 40 nm-100 nm) in sheet like structure The core level X-ray photoelectron spectrum of carbon and nitrogen elements reveal the existence of sp2 -bonded C in the C[bond, double bond]N and pyridinic and graphitic nitrogen in TZG-TH. X-ray diffraction patterns of TZG TH show the existence of anatase and hexagonal phase wurtzite crystalline structure in TH. The thermo-physical properties were determined for of the THNFs in order to elucidate the influence of compositions of the component and concentration ofof TZG-TH on the thermophysical properties. The TZG TH containing larger proportions of ZnO showed the maximum of 9.11 % and 12.1 % higher increase in viscosity than the binary and base fluid, respectively. The density of TZG THs varies from 1.079 to 1.095 cp, which is closer to the base fluid. The influence of TZG TH composition on refractive index and ultrasonic velocity indicates the existence of molecular level interactions between the nanoparticles in the TH and base fluid. The ∼210 % thermal conductivity enhancement was witnessed for the TZG TH, which is significantly higher than that of ZnO mono NF (26.9%) and TiO2 mono NF (33.0%). The influence of composition and concentration of TZG- TH on molecular interaction parameters like adiabatic compressibility, intermolecular free length, free volume, internal pressure and specific acoustic impedance are reported. The TZG TH based NF showed adequate dispersion stability as inferred from dynamic light scattering and UV-visible spectroscopy results. The results on TZG TH included THNF are new to the literature and would be helpful in exploring multifunctional properties with heat transfer capabilities for applications.

Fabricating carbon quantum dots of graphitic carbon nitride vis ultrasonic exfoliation for highly efficient H2O2 production

A promising and sustainable approach for producing hydrogen peroxide is the two-electron oxygen reduction reaction (2e- ORR), which uses very stable graphitic carbon nitride (g-C3N4). However, the catalytic performance of pristine g-C3N4 is still far from satisfactory. Here, we demonstrate for the first time the controlled fabrication of carbon quantum dots (CQDs)-modified graphitic carbon nitride carbon (g-C3N4/CQDs-X) by ultrasonic stripping for efficient 2e- ORR electrocatalysis. HRTEM, UV-vis, EPR and EIS analyses are in good consistent which prove the in-situ generation of CQDs. The effect of sonication time on the physical properties and ORR activity of g-C3N4 is discussed for the first time. The g-C3N4/CQDs-12 catalyst shows a selectivity of up to 95% at a potential of 0.35 V vs. RHE, which is much higher than that of the original g-C3N4 catalyst (88%). Additionally, the H2O2 yield is up to 1466.6 mmol g-1 in 12 h, which is twice as high as the original g-C3N4 catalyst. It is discovered that the addition of CQDs through ultrasonic improves the g-C3N4 catalyst's electrical conductivity and electron transfer capability in addition to its high specific surface area and distinctive porous structure, speeding up the reaction rate. This research offers a green method for enhancing g-C3N4 activity.

Self-Standing Pd-Based Nanostructures for Electrocatalytic CO Oxidation: Do Nanocatalyst Shape and Electrolyte pH Matter?

Tailoring the shape of Pd nanocrystals is one of the main ways to enhance catalytic activity; however, the effect of shapes and electrolyte pH on carbon monoxide oxidation (CO_Oxid) is not highlighted enough. This article presents the controlled fabrication of Pd nanocrystals in different morphologies, including Pd nanosponge via the ice-cooling reduction of the Pd precursor using NaBH₄ solution and Pd nanocube via ascorbic acid reduction at 25 °C. Both Pd nanosponge and Pd nanocube are self-standing and have a high surface area, uniform distribution, and clean surface. The electrocatalytic CO oxidation activity and durability of the Pd nanocube were significantly superior to those of Pd nanosponge and commercial Pd/C in only acidic (H₂SO₄) medium and the best among the three media, due to the multiple adsorption active sites, uniform distribution, and high surface area of the nanocube structure. However, Pd nanosponge had enhanced CO_Oxid activity and stability in both alkaline (KOH) and neutral (NaHCO₃) electrolytes than Pd nanocube and Pd/C, attributable to its low Pd-Pd interatomic distance and cleaner surface. The self-standing Pd nanosponge and Pd nanocube were more active than Pd/C in all electrolytes. Mainly, the CO_Oxid current density of Pd nanocube in H₂SO₄ (5.92 mA/cm²) was nearly 3.6 times that in KOH (1.63 mA/cm²) and 10.3 times that in NaHCO₃ (0.578 mA/cm²), owing to the greater charge mobility and better electrolyte-electrode interaction, as evidenced by electrochemical impedance spectroscopy (EIS) analysis. Notably, this study confirmed that acidic electrolytes and Pd nanocube are highly preferred for promoting CO_Oxid and may open new avenues for precluding CO poisoning in alcohol-based fuel cells.

Unveiling the effect of shapes and electrolytes on the electrocatalytic ethanol oxidation activity of self-standing Pd nanostructures

Morphologically controlled Pd-based nanocrystals are the most efficient strategies for improving the electrocatalytic ethanol oxidation reaction (EOR) performance; however, their morphological-EOR activity relationship and effect of electrolytes at a wide pH range are still ambiguous. Here, we have synthesized porous self-standing Pd clustered nanospheres (Pd-CNSs) and Pd nanocubes (Pd-NCBs) for the EOR in acidic (H₂SO₄), alkaline (KOH), and neutral (NaHCO₃) electrolytes compared to commercial spherical-like Pd/C catalysts. The fabrication process comprises the ice-cooling reduction of Pd precursor by sodium borohydride (NaBH₄) and l-ascorbic acid to form Pd-CNSs and Pd-NCBs, respectively. The EOR activity of Pd-CNSs significantly outperformed those of Pd-NCBs, and Pd/C in all electrolytes, but the EOR activity was better in KOH than in H₂SO₄ and NaHCO₃. This is due to the 3D porous clustered nanospherical morphology that makes Pd active centers more accessible and maximizes their utilization during EOR. The EOR specific/mass activities of Pd-CNSs reached (8.51 mA/cm²/2.39 A/mg_Pd) in KOH, (2.98 mA/cm²/0.88 A/mg_Pd) in H₂SO₄, and (0.061 mA/cm²/0.0083 A/mg_Pd) in NaHCO₃, in addition to stability after 1000 cycles. This study affirms that porous 3D spherical Pd nanostructures are preferred for the EOR than those of 0D spherical-like and multi-dimensional cube-like nanostructures.

Reasonable Design of MXene-Supported Dual-Atom Catalysts with High Catalytic Activity for Hydrogen Evolution and Oxygen Evolution Reaction: A First-Principles Investigation

MXene-supported single-atom catalysts (SACs) for water splitting has attracted extensive attention. However, the easy aggregation of individual metal atoms used as catalytic active centers usually leads to the relatively low loading of synthetic SACs, which limits the development and application of SACs. Herein, by performing first-principles calculations for Pt and 3d transition metal single atoms immobilized on a two-dimensional (2D) Mo2TiC2O2 MXene surface, we systematically studied the performance of heterogeneous dual-atom catalysts (h-DACs) in hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Significantly, h-DACs exhibit higher metal atom loading and more flexible active sites compared to SACs. Benefiting from these features, we found that Pt/Cu@Mo2TiC2O2 heterogeneous DACs exhibits excellent HER activity with ultra-low overpotential |ΔGH∗| (0.04 eV), lower than the corresponding Pt@Mo2TiC2O2 (0.14 eV) and Cu@Mo2TiC2O2 (0.33 eV) SACs, and even lower than that of Pt (0.09 eV). Meanwhile, Pt/Ni@Mo2TiC2O2 exhibits superior OER activity with ultra-low overpotential ηOER (0.38 V), lower than that of Pt@Mo2TiC2O2 (1.11 V) and Ni@Mo2TiC2O2 (0.57 V) SACs, and even lower than that of RuO2 (0.42 V) and IrO2 (0.56 V). Our finding paves the way for the rational design of h-DACs for HER and OER with excellent activity, which provides guidance for other catalytic reactions.