Enhanced electrochemical double-layer capacitive performance with CO2 plasma treatment on activated carbon prepared from pyrolysis of pistachio shells

This study reports an original approach based on the CO₂ plasma treatment on modification of the chemical or physical properties of activated carbon(AC) from the pistachio shells as a waste for application as electrochemical double-layer capacitors(EDLC). In the AC production experiments, impregnation ratio, impregnation pre-treatment temperature, activation temperature and activation time are investigated. In the AC modification experiments with plasma treatment, the effects of plasma gases, plasma power and plasma time are performed. The results of the different conditions indicated that the structural properties of the obtained AC were significantly dependent on the plasma and pyrolysis parameters. The surface properties of the raw AC and plasma-treated AC (PTAC) with X-ray photoelectron spectroscopy (XPS), nitrogen adsorption technique, and scanning electron microscope (SEM) are characterized. Surface area values for the raw AC and PTAC are 768 and 1250 m² g⁻¹, respectively. A change in the peak positions and an increase in the percentage of oxygen of the AC treated with CO₂ plasma were obtained from XPS results. After 15 min of CO₂ plasma activation, a significant increase in the capacitance of up to about 141% was obtained as a 118.4 F g ⁻¹ compared to 49.98 F g ⁻¹ for untreated AC. The results show that the plasma treatment on the specific surface area and surface functional groups of AC has a significant impact.     

Effect of an oxygen plasma treatment on the specific surface of platinum electrodeposits for fuel cells

Fuel cells involve electrochemical reactions often catalysed by platinum whose surface has to be maximized. In this paper, platinum nano-particles are deposited onto graphite by a potentiostatic reduction of a dilute 1.0 mM solution of hexachloroplatinate acid. Some samples are pre-treated by highly dissociated oxygen plasma and exhibit an increase of their specific surface compared to the untreated ones. The gain factor on specific surface reaches 3.6 and even 4.4 when the plasma treatment is coupled with the impregnation technique. Surface functionalization made by the plasma treatment lead to denser deposits thanks to an organized nucleation and growth of platinum nuclei. XPS analyses suggest the existence of C–O bonds in the platinum clusters. Although the testing conditions were not optimized, we have measured the performances of a microfuel cell made with the optimized catalyst. The Scharifker and Hills electrocrystallisation model was used to fit the current transients. Untreated samples transients were correctly fitted by the model whereas plasma treated samples transients did not have the requested shape for this model. Hence, nucleation mechanism was determined and confirmed by observation on untreated samples only. The calculated value of the diffusion coefficient of the Pt(IV) anion PtCl₆²⁻ determined by the Cottrell theory was close to the literature, i.e. 2.6 ± 0.6 × 10⁻⁶ cm² s⁻¹.     

Enhanced electrocatalytic activity of CoTMPP-based catalysts for PEMFCs by plasma treatment

In this work, we developed a methodology of plasma-enhanced preparation of CoTMPP (tetramethoxyphenylporphyrin)-based electrocatalysts. A series of CoTMPP-based electrocatalysts were deposited on the porous gas diffusion substrate (titanium fibre felt) using plasma-enhanced impregnation method. Impregnated 1.5 mg cm⁻² CoTMPP/Ti catalysts were treated by dielectric barrier discharge (DBD) plasma in Ar or N₂ atmosphere. Additionally, the pretreatment methods were utilized to improve the adhesion of CoTMPP on the diffusion layer surface. The plasma pretreatment methods included the a-C:H-layers deposition followed by an Ar:O₂ radio frequency (RF) plasma functionalization. The latter approach led to the formation of specific oxygen surface groups that influenced the catalysts activity. Obtained catalysts were compared in terms of activity, stability and structure. The catalytic activity for hydrogen peroxide (H₂O₂) reduction was tested in a proton exchange membrane fuel cell (PEMFC) using hydrogen peroxide on the cathode side. Surface elemental analysis and structure of catalysts were examined by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Our contribution shows the potential of a plasma treatment in the preparation of electrocatalysts for hydrogen peroxide reduction reaction in a PEMFC. Under the conditions of this study, improvement of the PEMFC performance up to 30% was achievable by a deposition of CoTMPP on the titanium diffusion substrate followed by plasma treatment. The large differences in catalytic activity of CoTMPP/Ti were observed, depending on the plasma treatment applied to the catalysts during their preparation.     

Effect of pH value on corrosion resistance and surface conductivity of plasma‐nitrided 304L bipolar plate for PEMFC

Low temperature plasma nitriding is developed to meet the requirements for corrosion resistance and interfacial contact resistance (ICR) of stainless steel 304L as the bipolar plate for PEMFC. A dense and supersaturated‐nitrogen nitrided layer has formed on the surface of the stainless steel 304L. Electrochemical behavior for the untreated and plasma‐nitrided 304L was measured in H₂SO₄ (pH=1–5)+2 ppm F^? simulating PEMFC environment, and the ICR was evaluated before and after corrosion tests. The experimental results have shown that the ICR for the plasma nitrided 304L is lower than the requirement of U.S. DOE (<10 mΩ cm² to 2010). Corrosion resistance and the ICR at the compaction force of 150–200 N cm^?2 increase with increasing pH value for the untreated and plasma‐nitrided 304L. The passive current densities for the untreated and plasma‐nitrided 304L are all lower than 16 µA cm^?2. The ICR between passive film and carbon paper are increased markedly because of passive film formed on the surface of both studied 304L. However, the passive current density and the ICR are lower for the plasma nitrided 304L than those for the untreated one at the given pH value, which results from the different composition of the stable passive film formed on the surface. The low temperature plasma nitriding provides a promising method for 304L using as bipolar plate for PEMFC. Further research is needed to evaluate the long‐term stability of passive film and the performance of single fuel cell. Copyright © 2010 John Wiley & Sons, Ltd.     

Low-temperature oxygen plasma treatment of TiO2 film for enhanced performance of dye-sensitized solar cells

The effects of low-temperature O₂ plasma treatment of a TiO₂ film are studied with the objective of improving the performance of dye-sensitized solar cells (DSSCs). X-ray photoelectron spectra (XPS) reveal that the ratio of titanium dioxide to titanium sub-oxides is increased in the O₂ plasma-treated TiO₂ film, compared with that of the untreated TiO₂ film. This increase suggests that the oxygen vacancies in the film are effectively reduced. The near-edge X-ray absorption fine structure (NEXAFS) spectra results agree with the XPS result. It is proposed that there is a correlation between the shifts of the peaks in the NEXAFS spectra and the adsorption of N719 dye on the TiO₂ particles. A DSSC having an O₂ plasma-treated, 4 μm thick TiO₂ film electrode renders a short-circuit photocurrent of 7.59 mA cm⁻², compared with 6.53 mA cm⁻² for a reference cell with an untreated TiO₂ electrode of the same thickness. As a result of these changes, the solar-to-electricity conversion efficiency of the O₂ plasma-treated cell is found to be 4.0% as compared with 3.5% for the untreated cell. This improvement in the performance is rationalized on the basis of increased N719 dye adsorption on to the TiO₂, due to the reduction in the number of oxygen vacancies caused by the oxygen plasma treatment.     

NH3 plasma synthesis of N-doped activated carbon supported Pd catalysts with high catalytic activity and stability for HCOOH dehydrogenation

Plasma is a simple and effective method to prepare N-doped carbon materials and supported metal catalysts. In this work, Pd/C–C(NH₃) and Pd/C–P(NH₃) catalysts are prepared by heat treatment and cold plasma methods using Ar and NH₃ as the working gas. The activity and stability of obtained catalysts are tested by formic acid dehydrogenation reaction. The results show that TOF_initial of Pd/C–P(NH₃) is 527.1 h⁻¹ at 50 °C, and the HCOOH decomposition rate is about 89.2% at 4 h. The hydrogen production of Pd/C–P(NH₃) when used in first and third cycle are 1.14 and 1.14 times than that of Pd/C–C(NH₃), and 1.24 and 13.24 times than that of commercial Pd/C. Various characterization techniques are used to characterize the structure of the prepared Pd/C catalysts. The results indicate that NH₃ plasma is milder than NH₃ thermal treatment. The high activity and stability of Pd/C–P(NH₃) are mainly due to the NH₃ cold plasma effectively achieving N-doping of the carbon support, and Pd nanoparticles with a small size and high dispersion. Atmospheric pressure NH₃ cold plasma provides an effective method to prepare high-performance Pd/C catalysts for HCOOH dehydrogenation and plays a guiding role in the preparation of high-performance carbon-supported noble metal catalysts.     

Simplified edge isolation of buried contact solar cells

The aim of this work is to implement simple edge isolation techniques in buried contact solar cell (BCSC) process by preserving the active cell area. Here we present results of two simplified edge isolation techniques for BCSC and they are compared with the standard process incorporating mechanical edge isolation using a dicing saw. The first technique is chemical wet etching of the solar cell's rear side in an inline system recently developed by University of Konstanz and Rena. The second technique is edge removal carried out in a fluoride/oxide radicals environment of a Asyntis plasma etcher. While the shunt resistance R_sh obtained with wet etching is between 1500 and 7000 Ω cm², the standard process shows R_sh values ranging from 2100–6300 Ω cm². The R_sh after plasma processing is between 1000 and 3600 Ω cm². These cell results show that both wet and plasma etching achieve results close to mechanical edge isolation.However, a slight reduction of short circuit current is observed for the cells undergone standard as well as plasma processing. This is due to the presence of floating volume shunts formed at the rear n–p⁺ junction, which are not removed by either the standard or plasma process. These shunts do not influence the IV-curve of the solar cells and are nearly invisible with conventional thermography, as they are not connected to the front side emitter grid. Hence, light-modulated lock-in thermography measurements were carried out to analyse these shunts.     

New nanostructured silicon films grown by PECVD technique under controlled powder formation conditions

In this paper the influence of the DC grid bias on the plasma impedance and the I–V behaviour of silane plasmas used to grow undoped amorphous silicon films by plasma enhanced chemical vapour deposition technique using a triode configuration at or close to the powder regime is studied. The aim is to determine the correlation between the r.f. power and the DC grid voltage with the plasma parameters, under isothermal gas conditions. The results should lead to the production of nanostructured films, with the required optoelectronic characteristics for photovoltaic applications. The results achieved show the existence of a boundary region close to the γ-regime (powder formed) where nanoparticles can be formed by moderated ion bombardment of the growing surface. This is characterised by the plasma resistance of the same order of magnitude of the plasma reactance. Under this condition, it is possible to grow amorphous silicon films that can incorporate nanoparticles, exhibiting photosensitivities of about 10⁷ (two orders of magnitude larger than the one exhibited by films grown under conventional conditions) with densities of states determined by the constant photocurrent method below 3×10¹⁵ cm⁻³. Apart from that, the growth of the films is less affected by light soaking than the conventional films grown by standard techniques.     

Plasma dry reforming of methane in an atmospheric pressure AC gliding arc discharge: Co-generation of syngas and carbon nanomaterials

An alternating-current (AC) gliding arc reactor has been developed offering a new route for the co-generation of syngas and value-added carbon nanomaterials by plasma dry reforming of methane. Different carbon nanostructures including spherical carbon nanoparticles, multi-wall carbon nanotubes and amorphous carbon have been obtained as by-products of syngas generation in the plasma system. Optical emission spectra of the discharge demonstrate the formation of different reactive species (Al, CO, CH, C₂, H_α, H_β and O) in the plasma dry reforming reaction. The effect of different operating parameters (feed flow rate, input power and CH₄/CO₂ molar ratio) on the performance of the plasma process has been evaluated in terms of the conversion of feed gas, product selectivity and energy conversion efficiency. It is interesting to note that gliding arc plasma can be used to generate much cleaner gas products of which syngas is the main one. The results also show that the energy efficiency of dry reforming using gliding arc plasma is an order of magnitude higher than that for processing using dielectric barrier or corona discharges. Both of these can be attributed to the higher electron density in the order of 10²³ m⁻³ generated in the gliding arc plasma.     

Numerical modeling of a downdraft plasma coal gasifier with plasma reactions

In this paper, a 3D numerical simulation of a downdraft plasma gasifier with plasma reactions is conducted. The effects of the equivalence ratio (ER) on the syngas properties in the presence of the plasma reactions are investigated. The boundary conditions for the air plasma inlet of the gasifier are obtained from the outlet of a 10 kW microwave plasma generator. A conventional gasification analysis is carried out to validate the model. In the second part of the study, plasma reactions are added to conventional gasification equations. Mole fractions of the constituents of the syngas and temperature contours are obtained for different ER values. According to the results, with the increase of ER from 0.20 to 0.45 the lower heating value of the produced syngas decreased from 1536.6 kcal/m³ to 751.8 kcal/m³.     

Boosting the efficiency of solar cells fabricated on electromagnetic cold crucible cast multicrystalline silicon by means of hydrogen passivation

In this work the improvement of the quality of the electromagnetic cold crucible cast multicrystalline silicon (EMC) material produced by Sumitomo Sitix Co. (SCC, previously Osaka Titanium Co.) by hydrogen plasma is investigated with the final goal of realizing solar cells with the maximum posible efficiency. Two different hydrogen passivation techniques are implemented: hydrogen passivation by means of rf (radio frequency) plasma treatment and hydrogen passivation using microwave induced remote plasma treatment. By combining the oxide surface passivation and hydrogen passivation by remote plasma from the front side and by rf plasma from the back side, a significant improvement in short-circuit current, in open-circuit voltage, and in fill factor is obtained. A maximum efficiency of 16% on 2 × 2 cm² cells and of 14.5% on 10 × 10 cm² cells is achieved. This 16% efficiency is the highest ever reported on EMC multicrystalline silicon.     

Deuterium retention and surface morphology modification in oxidized tungsten exposed to D plasma

Tungsten (W) is the most promising plasma facing material in future fusion reactor. However, oxidation occurs readily on W and might influence the hydrogen isotopes retention behavior in W. To explore the mechanism of oxidation on the hydrogen isotopes retention behavior in W, oxidized W samples have been exposed to D plasma (32.3 eV, 2 × 10²⁰ Dm⁻²s⁻¹) in a linear plasma device. Surface morphology and deuterium (D) retention behavior in W have been characterized after exposure to D plasma with fluence of 10²³ D/m². After D plasma exposure, a porous layer has been found on the surface of oxidized W. The main chemical content of the porous layer is pure W. The existence of pre-oxide layer contributes the increase of retention amount of D in the form of D₂O and may contribute to the decrease of the retention amount of D in the form of HD/D₂.     

Effect of PBI-HFA surface treatments on Pd/PBI-HFA composite gas separation membranes

For pure hydrogen separation, palladium was deposited on surface-treated polybenzimidazole (PBI-HFA, 4,4′-(hexafluroisopropylidene)bis(benzoic acid)) via the vacuum electroless plating technique (VELP). Since the hydrophobic characteristics of the polymer surface restrict strong adhesion of Pd on it and cause the peel-off of Pd film, various surface treatments have been employed. To increase the number of Pd anchoring sites on the PBI-HFA surface, mechanical abrasion (polishing) was applied, and to increase the hydrophilicity of the PBI-HFA surface, wet-chemical and O₂ plasma treatment (dry etching) were used. The thickness and effective permeating area of the deposited Pd films on the PBI-HFA membranes were estimated to be in the range of 160–340 nm and 8.3 cm², respectively. Among the tested membranes, membranes with Pd layers deposited on O₂ plasma treated PBI-HFA surfaces had the most uniform microstructure and the least number of defects compared to the other membranes. Gas permeation experiments were performed as a function of temperature and pressure. The gases used in the permeation measurements were H₂, N₂, CO₂, and CO (99.9% purity). A Pd-O₂30 m membrane, fabricated by O₂ plasma surface treatment during 30 min, exhibited superior gas separation performance (H₂ permeability of 275.5 Barrer), and proved to be impermeable to carbon monoxide. Enhancement of H₂ permselectivity of Pd/PBI-HFA composite membrane treated by O₂ plasma shows promising hydrogen separation membrane.     

Synthesis of metallic copper nanowires using dielectric barrier discharge plasma and their application in hydrogen evolution reaction

In this study, metallic copper (Cu) nanowires are synthesized by reducing thermally synthesized CuO nanowires under an indigenously developed hydrogen plasma system. The X-ray diffraction (XRD) results of the plasma-synthesized nanowires indicate the presence of metallic copper [(111) and (200)] and the field emission scanning electron microscopy (FESEM) further affirms the findings by presenting a stark difference in contrast of the nanowires before and after plasma treatment with diameters of 50 and 100 nm, respectively. The nanowires are studied for hydrogen evolution reaction in a neutral medium and they show excellent performance than the previously reported studies on bulk copper, with an overpotential of 210 mV at a current density of 10 mA/cm² and an exchange current density of 60 exp-5 A/cm² which is an order of magnitude larger than the reported values on bulk copper. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy indicates that the surface of the nanowires is highly rich in metallic copper resulting in better electrochemical performance of the metallic Cu nanowires in a neutral environment.     

Flame propagation enhancement by plasma excitation of oxygen. Part II: Effects of O2(aΔg)

The isolated effect of O₂(a¹Δ_g) on the propagation of C₂H₄ lifted flames was studied at reduced pressures (3.61 kPa and 6.73 kPa). The O₂(a¹Δ_g) was produced in a microwave discharge plasma and was isolated from O and O₃ by NO addition to the plasma afterglow in a flow residence time on the order of 1 s. The concentrations of O₂(a¹Δ_g) and O₃ were measured quantitatively through absorption by sensitive off-axis integrated-cavity-output spectroscopy and one-pass line-of-sight absorption, respectively. Under these conditions, it was found that O₂(a¹Δ_g) enhanced the propagation speed of C₂H₄ lifted flames. Comparison with the results of enhancement by O₃ found in part I of this investigation provided an estimation of 2-3% of flame speed enhancement for 5500 ppm of O₂(a¹Δ_g) addition from the plasma. Numerical simulation results using the current kinetic model of O₂(a¹Δ_g) over-predicts the flame propagation enhancement found in the experiments. However, the inclusion of collisional quenching rate estimations of O₂(a¹Δ_g) by C₂H₄ mitigated the over-prediction. The present isolated experimental results of the enhancement of a hydrocarbon fueled flame by O₂(a¹Δ_g), along with kinetic modeling results suggest that further studies of C_nH_m + O₂(a¹Δ_g) collisional and reactive quenching are required in order to correctly predict combustion enhancement by O₂(a¹Δ_g). The present experimental results will have a direct impact on the development of elementary reaction rates with O₂(a¹Δ_g) at flame conditions to establish detailed plasma-flame kinetic mechanisms.     

Plasma-chemical reduction of iron oxide photoanodes for efficient solar hydrogen production

We demonstrate the effect of hydrogen plasma treatment on hematite films as a simple and effective strategy for modifying the existing substrate to improve significantly the band edge positions and photoelectrochemical (PEC) performance. Plasma treated hematite films were consist of mixed phases (Fe₃O₄:α-Fe₂O₃) which was confirmed by XPS and Raman analysis, treated films also showed higher absorption cross-section and were found to be a promising photoelectrode material. The treated samples showed enhance photocurrent densities with maximum of 3.5 mA/cm² at 1.8 V/RHE and the photocurrent onset potentials were shifted from 1.68 V_RHE (untreated) to 1.28 V_RHE (treated). Hydrogen plasma treatment under non-equilibrium conditions induced a valence dynamics among Fe centers in the sub-surface region that was sustained by the incorporation of hydrogen in the hematite lattice as supported by the density functional theory calculations.     

Effect of plasma nitriding on behavior of austenitic stainless steel 304L bipolar plate in proton exchange membrane fuel cell

A dense and supersaturated nitrogen layer with higher conductivity is obtained on the surface of austenitic stainless steel 304L by the low temperature plasma nitriding. The effect of plasma nitriding on the corrosion behavior and interfacial contact resistance (ICR) for the austenitic stainless steel 304L was investigated in 0.05 M H₂SO₄ + 2 ppm F⁻ simulating proton exchange membrane fuel cell (PEMFC) environment using electrochemical and electric resistance measurements. The experiment results show that the stable passive film is formed after the potentiostatic polarization at the specified anodic or cathodic potentials under PEMFC operation condition, and the plasma nitriding improves slightly the corrosion resistance and decreases markedly the ICR of 304L. The ICR of the plasma nitrided 304L increases after the potentiostatic polarizations for 4 h, and lower than 100 mΩ cm² at the compaction force of 150 N cm⁻².     

Effect of pre-damage induced by argon ions on the deuterium blister formation in tungsten-tantalum alloys exposed to deuterium plasma

This work aims to investigate the effects of 800 keV argon ions pre-damage on the following deuterium plasma irradiation behavior in tungsten-tantalum alloy (5 wt% tantalum dopant). The un-damaged and pre-damaged W–5%Ta were exposed to 60 eV deuterium plasma at a fluence of 1.944 × 10²⁶ D/m² in the linear plasma device. The results suggest that pre-damaged induced by Ar⁸⁺ ions can effectively mitigate deuterium-induced blistering, while the deuterium retention is significantly higher as compared to the un-damaged W–5%Ta. According to the transmission electron microscopic (TEM) analysis, a large number of defects can be observed clearly after irradiation by 800 keV Ar⁸⁺ ions, such as vacancies and dislocation loops, which eventually affect the surface blistering and deuterium retention in W–5%Ta. Furthermore, we also compared the deuterium retention in pre-damaged pure W, which was significantly increased by a factor of 15 than in pre-damaged W–5%Ta.     

In situ hydrogen plasma treatment for improved transport of (4 0 0) oriented polycrystalline silicon films

Polycrystalline silicon (poly-Si) films were deposited on glass by very high-frequency (100 MHz) plasma enhanced chemical vapor deposition from a gaseous mixture of SiF₄ and H₂ with small amounts of SiH₄. (2 2 0) oriented films prepared at small SiF₄/H₂ ratios (<30/40 sccm) showed intrinsic transport properties of poly-Si. However, the room temperature dark conductivity (σ_d) of the (4 0 0) oriented film was very high for intrinsic poly-Si, 7.2×10⁻⁴S/cm. This conductivity exhibited a T^−1/4 behavior, suggesting a high defect density at the grain boundaries. It was found that in situ hydrogen plasma treatment successfully produced (4 0 0) oriented poly-Si with a reasonably low σ_d of 4.5×10⁻⁷S/cm and a good photoconductivity of 1.3×10⁻⁴S/cm.     

Oxidative reforming of n-heptane in gliding arc plasma reformer for hydrogen production

The exploration of novel technologies to reduce the air pollution and greenhouse gas emissions has been of great interest. Gliding arc plasma reformer at atmospheric pressure has been developed for converting n-heptane to hydrogen. The system has been evaluated by H₂ yield and energy yield via continuous n-heptane oxidative reforming at room temperature. Effects of some process parameters (discharge gap, input power, residence time, and O/C) have been studied on the reaction performance. The maximum H₂ yield and energy yield are 50.1% and 94.5 L (kW h)⁻¹. To investigate the role of inert gas (N₂, Ar) in the plasma oxidative reforming system, the performance of C₇H₁₆/air, C₇H₁₆/N₂/O₂/Ar and C₇H₁₆/O₂/Ar have been investigated. The results show that N₂ (B³Π_g) and Ar¹ can accelerate the formation of active oxygen species (such as O⁺, O (¹D) and O). The presence of active oxygen species promotes the progress of the oxidative reforming reaction. What's more, N₂ (B³Π_g) is also conducive to the direct conversion of n-heptane. The reaction mechanism of hydrogen production from gliding arc plasma oxidative reforming of n-heptane was proposed based on the analysis of the OES and GC–MS.