Reduced graphene oxide (rGO) nanosheets decorated with gold nanoparticles (Au NPs/rGO), palladium nanoparticles (Pd NPs/rGO), and gold-palladium bimetallic nanoparticles (Au-Pd NPs)/rGO are synthesized by a simple solution chemistry approach using ascorbic acid as ecofriendly reducing agent. These materials are characterized by high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA). The as-prepared nanocomposites are tested as electrocatalysts for efficient hydrogen evolution in deaerated 0.5 M H 2 SO 4 aqueous solution using polarization and impedance measurements. Experimental findings show that the tested catalysts exhibit fast hydrogen evolution kinetics with onset potentials as low as −17, −7.2, and −0.8 mV vs. RHE for Au NPs/rGO, Pd NPs/rGO, and Au-Pd NPs/rGO, respectively. In addition, Tafel slopes of 39.2, 33.7 and 29.0 mV dec -1 and exchange current densities of 0.09, 0.11, and 0.47 mA cm -2 are measured for Au NPs/rGO, Pd NPs/rGO, and Au-Pd NPs/rGO, respectively. The tested materials not only maintain their high performance after 5000 sweep cycles, but their activity is simultaneously enhanced after this aging process. These findings reveal that the tested catalysts, particularly Au-Pd NPs/rGO, are promising candidates among other noble metal catalysts for hydrogen evolution, approaching the commercial Pt/C catalyst (onset potential: 0.0 mV, Tafel slope: 31 mV dec -1 , and exchange currrent density: 0.78 mA cm -2 ). The high hydrogen evolution reaction (HER) activity of such materials is likely due to the abundance of active catalytic sites, the increased electrochemically accessible surface area and significantly improved electrochemical conductivity.
In this paper, we demonstrated, for the first time, aluminum titania nanoparticle (Al-TiO2 NP) composites with variable amounts of TiO2 NPs as nonprecious active catalysts for the electrochemical generation of H2. These materials were synthesized by mixing desired amounts of hydrogen titanate nanotubes (TNTs), fabricated here by a cost-effective approach at moderate hydrothermal conditions, with aluminum powder (purity 99.7%; size 35 μm). The mixture was compacted under an applied uniaxial stress of 300 MPa followed by sintering at 500 °C for 1 h. After sintering had been completed, all TNTs were found to convert to TiO2 NPs (average particle size 15 nm). Finally, Al-xTiO2 NP nanocomposites (x = 1, 3, 5, and 10) were obtained and characterized by scanning electron microscopy/energy-dispersive X-ray, X-ray diffraction, and X-ray photoelectron spectroscopy. The hydrogen evolution reaction (HER) activity of these materials was studied in 0.5 M H2SO4 at 298 K using polarization and impedance measurements. The nanocomposite of chemical composition Al-5% TiO2 NPs showed the best catalytic performance for the HER, with an onset potential (EHER), a Tafel slope (βc), and an exchange current density (j0) of -100 mV (RHE), 59.8 mV decade(-1), and 0.14 mA cm(-2), respectively. This HER activity is not far from that of the commercial platinum/carbon catalyst (EHER = 0.0 mV, βc = 31 mV dec(-1), and j0 = 0.78 mA cm(-2)). The best catalyst also exhibited good stability after 10000 repetitive cycles with negligible loss in current.
The paper presents a new approach based on the appearance of negative capacitance (NC) at high and low frequencies; previously, researchers agreed that NC only occurred at high or low frequencies. For the first time, we synthesized Au/polypyrrole/MWCNT composite/TiO2/Al2O3/n-Si/Al structure for use in electronic and supercapacitor applications. The structural, electrical, and dielectric properties were investigated by x-ray diffraction, FTIR, Raman spectroscopy, I-V, and C-V measurements. The results revealed that for all working voltages and temperatures, negative capacitance and conductance occurred at high frequencies between 2 × 107 and 107 Hz. Similarly, the phenomena of negative capacitance occurred at low frequencies (100, 10) Hz. The capacitance-voltage experiments were used to perform the analysis of the variation of capacitance and conductance with frequency, voltage, and temperature. The polypyrrole, MWCNT composite /TiO2/Al2O3/n-Si structure exhibits diode behavior and has a high rectification ratio. I-V measurements were used to investigate ideality factors, barrier height, series and shunt resistance#, and rectification ratio.
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