Preparation of mesoporous carbons from amphiphilic carbonaceous material for high-performance electric double-layer capacitors

Amphiphilic carbonaceous material (ACM), with nanoscale dispersion in alkaline aqueous solutions, is synthesized from green needle coke. As a special precursor with small particle size, plenty of functional groups and widened d₀₀₂ simultaneously, ACM guarantees subsequent ACM-based activated carbons (AACs) with high specific surface area over 3000 m² g⁻¹ as well as well-developed mesoporous structure after KOH activation. Such pore properties enable AACs’ high performances as electrode materials for electric double-layer capacitors (EDLCs). In particular, surface area up to 3347 m² g⁻¹ together with notable mesopore proportion (26.9%) gives sample AAC814 outstanding EDLC behaviors during a series of electrochemical tests including galvanostatic charge/discharge, CV and electrochemical impedance spectroscopy. The electrode gets satisfactory gravimetric and volumetric specific capacitance at the current density of 50 mA g⁻¹, up to 348 F g⁻¹ and 162 F cm⁻³, respectively. Furthermore, for the mesoporosity, there is only a slight capacitance reduction for AAC814 as the current density reaches 1000 mA g⁻¹, indicating its good rate performance. It is all the ACM's unique characteristics that make AACs a sort of competitive EDLC electrode materials, both in terms of specific capacitance and rate capability.     

Influence of physical properties of activated carbons on characteristics of electric double-layer capacitors

Electrochemical characterization has been carried out for several activated carbons used as polarizable electrodes of electric double-layer capacitors in an aqueous electrolytic solution. The rest potential of the activated carbon was proportional to the logarithm of the oxygen content or to the concentration of the acidic surface functional groups of the activated carbon. The result of triangular voltage-sweep cyclic voltammetry was the same as that of the residual current measurement. The oxygen content and concentration of the acidic surface groups of activated carbon influenced the electrochemical characteristics of the activated carbon. Under anodic polarization, gas evolution was observed at the electrode surface of activated carbon with high oxygen content at 0.8 V versus saturated calomel electrode ( SCE). Gas evolution was not observed at the electrode surface of activated carbon with low oxygen content even to 1.0 V versus SCE. Under cathodic polarization of activated carbon with high oxygen content, the peak was observed at approximately −0.2 V versus SCE, but there was no gas evolution at the electrode surface of the activated carbon. Bubbles were not observed at the electrode surface of activated carbon with low oxygen content at −0.5 V versus SCE. Electric double-layer capacitors were made from activated carbons used for electrochemical measurements; load-life tests have been carried out. Thickness and internal resistance of the capacitor composed of activated carbon with high oxygen content increased. The changes in thickness and internal resistance of the capacitor composed of activated carbon with low oxygen content were small.     

Effect of pore size and surface area of carbide derived carbons on specific capacitance

This work presents a systematic study on how pore size and specific surface area (SSA) of carbon effect specific capacitance and frequency response behavior. Carbide derived carbons (CDC) produced by leaching metals from TiC and ZrC at temperatures from 600 to 1200 °C have highly tailorable microstructure and porosity, allowing them to serve as excellent model systems for porous carbons in general. BET SSA and average pore size increased with synthesis temperature and was 600–2000 m² g⁻¹ and 0.7–1.85 nm, respectively. Maximum specific capacitance in 1 M H₂SO₄ was found to occur at an intermediate synthesis temperature, 800 °C, for both ZrC and TiC derived carbons and was 190 and 150 F g⁻¹, respectively. Volumetric capacitance for TiC and ZrC derived carbons was maximum at 140 and 110 F cm⁻³. These results contradict an oft-reported axiom that increasing pore size and SSA, all other things being held constant, increases specific capacitance. A correlation between specific capacitance and SSA of micropores (less than 2 nm in diameter) has been shown. As expected, increasing pore size was found to improve the frequency response. However, CDCs with similar pore size distributions but obtained from different starting materials showed noticeable differences in impedance behavior. This highlights the importance of not only the pore size and specific surface area measured using gas sorption techniques, but also the pore shape or tortuousity, which is non-trivial to characterize, on energy storage.     

Influence of the mesoporous structure on capacitance of the RuO2 electrode

The difference in capacitive performance between high and low surface area RuO₂ electrodes, synthesized with and without a mesoporous silica template, respectively, was investigated in aqueous solutions of sulfuric acid and sulfates by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). RuO₂ synthesized with the template was crystalline and the formation of the mesoporous structure with a 6.5 nm diameter was confirmed using a transmission electron microscope and the nitrogen adsorption and desorption isotherm. From the CV at the scan rate of 1 mV s⁻¹, the specific capacitance of the high surface area electrode in H₂SO₄(aq) was determined to be 200 F g⁻¹. The high surface area RuO₂ has a three times higher BET specific surface area (140 m² g⁻¹) than the low surface area sample (39 m² g⁻¹). Introducing the mesoporous structure was proved effective for increasing the capacitance per mass of the RuO₂, though not all the surface functions as a capacitor. Both the CV and EIS suggest that by increasing the charging rate or frequency, the mesoporous structure of the electrode leads to a lower capacitance decrease (higher capacitance retention) than the low surface area electrode. The EIS also indicates that the response time of the capacitor is hardly influenced by the presence of the mesoporous structure.     

Structures and electrochemical performances of pyrolized carbons from graphite oxides for electric double-layer capacitor

The structural features and the electrochemical performances of pyrolized needle cokes from oxidized cokes are examined and compared with those of KOH-activated needle coke. The structure of needle coke is changed to a single phase of graphite oxide after oxidation treatment with an acidic solution having an NaClO₃/needle coke composition ratio of above 7.5, and the inter-layer distance of the oxidized needle coke is expanded to 6.9 Å with increasing oxygen content. After heating at 200 °C, the oxidized needle coke is reduced to a graphite structure with an inter-layer distance of 3.6 Å. By contrast, a change in the inter-layer distance in KOH-activated needle coke is not observed.     

Influence of mesophase activation conditions on the specific capacitance of the resulting carbons

Mesophase pitch AR24 was directly activated with KOH using different proportions of the activating agent and activation temperatures, to study the effect on the textural characteristics of the resultant activated carbons and how these characteristics influence their behaviour as electrodes in supercapacitors. The textural properties of the activated carbons were studied by gas adsorption and immersion calorimetry. The results indicate that all the carbons produced were mainly microporous, with pore size around 1 nm. The behaviour of these carbons as electrodes in supercapacitors was studied from galvanostatic charge–discharge cycles. The specific capacitance values obtained were very high, reaching 400 and 200 F g⁻¹ at low and high current densities respectively, for the sample activated with (5:1) KOH to mesophase ratio. Nevertheless, the reasons for this high capacitance values cannot be explained only on the basis of the textural characteristics of the activated carbons, as the results indicated that other factors might be also playing a significant role in their electrochemical behaviour.     

Studies of the activated carbons used in double-layer supercapacitors

The specific capacitance of activated carbons is determined by both the ratio of edge/basal orientation and the nature of functional group on the surface. The difference between the edge and the basal layers results from the semiconductive properties of basal layer. The ratio of edge/basal orientation can be estimated by X-ray diffraction (XRD). The wettability of activated carbons is determined by the nature of functional groups on the surface. Most of the surface groups are electrochemically active. The impact of the surface groups on electrochemical performance of the activated carbon electrodes was investigated by means of various surfactant treatments. The ac impedance and constant current discharge techniques were used. Two types of surface groups which had “capacitor-like” or “battery-like” behaviors, respectively, were revealed and discussed in detail. The surface groups with “battery-like” behavior should be avoided. “Non-symmetric” electrode arrangement should be considered for a double-layer supercapacitor in order to take the advantages of pseudo-capacitance of the surface groups with “capacitor-like” behaviors.     

Fabrication of high-power electric double-layer capacitors

The electrochemical behavior of activated carbon/carbon (AC/C) composite electrodes was investigated for high-power electric doublelayer capacitors (EDLCs). It was found that high-rate charge/discharge characteristics are affected by the resistance of the electrolyte phase in the pores of the electrode. The charge/discharge characteristics were improved by optimizing the pore-size distribution of the electrodes. The size and total volume of the macro-pores in the electrodes were controlled by mixing and burning out polymer spheres. A high-power EDLC (15V, 470 F), which can discharge as much as 500 A, was fabricated by using improved AC/C composite electrodes.     

The capacitive characteristics of activated carbons—comparisons of the activation methods on the pore structure and effects of the pore structure and electrolyte on the capacitive performance

Fir wood-derived carbons activated with steam, KOH, and KOH + CO₂ were found to exhibit the high-power, low ESR, and highly reversible characteristics between −0.1 and 0.9 V in aqueous electrolytes, which were demonstrated to be promising electrode materials for supercapacitors. The pore structure of these activated carbons was systematically characterized by the t-plot method based on N₂ adsorption isotherms. Activated carbons prepared through the above three activation methods under different conditions (i.e., the gasification time of CO₂, KOH/char ratio, and activation time of steam) generally showed excellent capacitive performance in aqueous media, mainly attributed to the development of both micropores and mesopores (with the meso-pore volume ratio, V_meso/V_pore, ranging from 0.18 to 0.52). Scanning electron microscopic (SEM) photographs showed that the surface morphologies of honeycombed holes were found to depend on the activation methods. The average specific capacitance of the activated carbon with a combination of KOH etching and CO₂ gasification (with gasification time of 15 min) reached 197 F g⁻¹ between −0.1 and 0.9 V in H₂SO₄. The capacitive characteristics of steam- and KOH-activated carbons in NaNO₃ and H₂SO₄ could be roughly estimated from the pore structure and BET surface area although the correlation may be only applicable for the fir wood-derived activated carbons.     

Effects of pore structure and electrolyte on the capacitive characteristics of steam- and KOH-activated carbons for supercapacitors

Four kinds of activated carbons (denoted as ACs) with specific surface area of ca. 1050 m² g⁻¹ were fabricated from fir wood and pistachio shell by means of steam activation or chemical activation with KOH. Pore structures of ACs were characterized by a t-plot method based on N₂ adsorption isotherms. The amount of mesopores within KOH-activated carbons ranged from 9.2 to 15.3% while 33.3–49.5% of mesopores were obtained for the steam-activated carbons. The pore structure, surface functional groups, and raw materials of ACs, as well as pH and the supporting electrolyte were also found to be significant factors determining the capacitive characteristics of ACs. The excellent capacitive characteristics in both acidic and neutral media and the weak dependence of the specific capacitance on the scan rate of cyclic voltammetry (CV) for the ACs derived from the pistachio shell with steam activation (denoted as P-H₂O-AC) revealed their promising potential in the application of supercapacitors. The ACs derived from fir wood with KOH activation (denoted as F-KOH-AC), on the other hand, showed the best capacitive performance in H₂SO₄ due to excellent reversibility and high specific capacitance (180 F g⁻¹ measured at 10 mV s⁻¹), which is obviously larger than 100 F g⁻¹ (a typical value of activated carbons with specific surface areas equal to/above 1000 m² g⁻¹).     

Supported PtRu on mesoporous carbons for direct methanol fuel cells

We prepared and characterized several cryogel mesoporous carbons of different pore size distribution and report the catalytic activity of PtRu supported on mesoporous carbons of pore size >15 nm in passive and in active direct methanol fuel cells (DMFCs). At room temperature (RT), the specific maximum power of the passive DMFCs with mesoporous carbon/PtRu systems as anode was in the range 3–5 W g⁻¹. Passive DMFC assembly and RT tests limit the performance of the electrocatalytic systems and the anodes were thus tested in active DMFCs at 30, 60 and 80 °C. Their responses were also compared to those of commercial Vulcan carbon/PtRu. At 80 °C, the specific maximum power of the active DMFC with C656/PtRu was 37 W g⁻¹ and the required amount of Pt per kW estimated at 0.4 V cell voltage was 31 g kW⁻¹, a value less than half that of Vulcan carbon/PtRu.     

Electric double layer capacitance on hierarchical porous carbons in an organic electrolyte

Nanoporous carbons were prepared by using colloidal crystal as a template. Nitrogen adsorption/desorption isotherms and transmission electron microscope images revealed that the porous carbons exhibit hierarchical porous structures with meso/macropores and micropores. Electric double layer capacitor performance of the porous carbons was investigated in an organic electrolyte of 1 M LiClO₄ in propylene carbonate and dimethoxy ethane. The hierarchical porous carbons exhibited large specific double layer capacitance of ca. 120 F g⁻¹ due to their large surface areas. In addition, the large capacitance was still obtained at a large current density up to 10 A g⁻¹, which satisfies demands from the high power application such as hybrid electric vehicles. Capacitance analysis of the hierarchical porous structures revealed the contribution of meso/macropores and micropore to the electric double layer capacitance to be 8.4 and 8.1 μF cm⁻², respectively. The results indicated electric double layer is formed even when solvated ions are larger than pore diameters.     

Influence of pore structure on the gravity separation performance of fine lignite

Influence of lignite porosity on the density fluctuation and subsequent beneficiation efficiency was studied. Using copper as tracer element, SEM+EDS was used to study the wetting rate of medium to particles. Furthermore, the particle quality changes before and after the wetting was analyzed. Separation experiments were carried out to verify the hypothesis. Results show that the wetting process is less than three seconds and there is no significant relationship between the quality growth rate and the particle density, which leads to the irregular change of particle density. Separation experiment proved the low separation efficiency and high content of mismatch particles.     

Electrochemical capacitor performance of N-doped mesoporous carbons prepared by ammoxidation

The electrochemical double-layer capacitive properties of mesoporous carbon (MC) materials with a moderate amount of nitrogen functionality are reported. Ordered mesoporous carbon is prepared using mesoporous silica (MS) as a template and sucrose as a carbon source. Two types of N-doped MCs are prepared by ammoxidation performed at different stages of the MC preparation process—ammoxidation before (NC) and after (CN) carbonization. Irrespective of the ammoxidation sequence, N-doped MCs maintain mesoporous properties such as a high surface area with narrow pore-size distribution. However, the amounts and chemical states of incorporated nitrogen are highly dependent on the sequence of ammoxidation. In a cyclic voltammetry test, N-doped MCs, compared with MC, exhibit higher capacitance in addition to fast charge/discharge characteristics, which results from their mesoporosity and the pseudo-capacitive effect of incorporated nitrogen. In particular, the NC-type MCs show the best capacitive properties among the materials studied due to the large amount of pyridinic species that modifies the electron donor/acceptor properties of the surface and thereby results in an enhanced, fast and reversible faradaic redox reaction.     

Highly ordered mesoporous carbons as the support for Pt catalysts towards alcohol electrooxidation: The combined effect of pore size and electrical conductivity

Pt nanoparticles were successfully deposited on ordered mesoporous carbons (CMK-3) using a pulse microwave-assisted polyol method. CMK-3 with three different pore sizes, obtained by using boric acid as the pore-expanding agent, were adopted. The pore size was controlled to be 4.4, 6.1, and 6.7 nm while the highly ordered structure was still maintained. With these different CMK-3 samples as the support, the particle size of Pt was identical, about 2 nm. It was found that the electrochemical surface area was almost the same in these three cases and the alcohol electrooxidation activity was not increased but decreased a little along with the pore size increment. The increased pore size of CMK-3 had no obviously positive effect on easier mass transfer and then a better electrocatalytic activity. This could be attributed to the very good 3-D interconnection of the nanospacings of CMK-3 per se. Moreover, combined with the theoretical calculation and confirmed by alternative current impedance results, the electrical conductivity of CMK-3 was dramatically decreased along with the pore size increasing, which counteracted the beneficial effect from easier mass transfer in the case of bigger pore size. Based on the present and reported results, it hints that when the carbon skeleton of carbon materials is different, the modification skills are changed for improving their structure in order to make them suitable for electrocatalysts supports.     

One-step dual template synthesis of platinum on mesoporous carbon nanowires for electrocatalysts

One-step methodology has been utilized to fabricate platinum catalysts on well-aligned mesoporous carbon nanowires (Pt/MCNWs) in dual templates which are the combination of porous anodic alumina (PAA) hard template and triblock polymer ethanol solution as soft template with evaporation-induced self-assembly technique. The mesoporous nanowires structure is confirmed by transmission electron microscopy (TEM). The Pt/MCNWs has a higher electrochemical active surface area and better catalytic performance than a commercial Pt/Vulcan XC-72. It could attribute to the good dispersion of Pt and a special morphology of MCNWs. Hence, the Pt/MCNWs synthesized by the dual template method might be a promising candidate to be utilized as electrocatalyst.     

Wormholelike mesoporous carbons as the support for Pt2Sn1 towards ethanol electrooxidation: Effect of pore diameter

Pt₂Sn₁ nanoparticles supported on wormholelike mesoporous carbons (WMCs) with three different pore diameters, namely WMC-F7, WMC-F30, and WMC-F0 have been synthesized by adopting a modified pulse microwave-assisted polyol method. The pore diameters (D_p) of WMC-F7, WMC-F30, and WMC-F0 are 8.5 nm, 4.4 nm, and 3.1 nm respectively, while the particle size of Pt₂Sn₁ (D_Pt) on each support is identical (∼3 nm). Based on the experimental results, it has been found that the pore diameter plays an important role in the electrochemical activity of Pt₂Sn₁ catalysts towards ethanol electrooxidation reaction (EOR). Pt₂Sn₁/WMC-F7 catalyst exhibits the highest electrochemical surface area (ESA) and activity towards EOR. Moreover, Pt₂Sn₁/WMC-F7 gives the comparable activity to the Pt₂Sn₁ supported on the commercial XC-72 carbon. However, in the cases of WMC-F30 (D_Pt < Dp < 2 D_Pt) and WMC-F0 (Dp = D_Pt) as the supports, the corresponding catalysts obtain much lower ESA and EOR activity with respect to WMC-F7 (Dp > 2 D_Pt). This could be attributed to the easy accessibility of Pt₂Sn₁ nanoparticles in the case of WMC-F7 with Dp > 2 D_Pt for the easy fuel transportation, and consequently the EOR activity has been greatly improved.     

动力型双电层电容器过载使用的性能

采用过流、过压、短路放电、高低温的过载方法,研究了过载对有轨电车用动力型双电层电容器的性能影响.结果表明:双电层电容器单体在充放电的平均电流相同条件下循环测试,单体的电化学性能保持一致.平均电流大于额定电流时,单体产热量大,容易造成漏液.单体可进行短路放电,瞬时短路放电电流为6200 A,且单体不会损坏.在额定电流持续...     

Importance of pore size in high-pressure hydrogen storage by porous carbons

Development of high-capacity hydrogen-storage systems utilizing physisorption at high pressure and low temperature is hindered by poor understanding of the pore size/shape requirements for achieving the maximum hydrogen uptake. Tuning the carbon structure and pore size of carbide-derived carbons (CDCs) with high accuracy by using different starting carbides, chlorination temperatures and activation temperatures allows rational design of carbon materials with increased hydrogen-storage capacity. Systematic experimental investigation of a large number of CDCs with controlled pore size distributions and specific surface area (SSA) shows that pores larger than ∼1.5 nm contribute little to hydrogen storage. It has been experimentally demonstrated that, just as at ambient pressure, pores of 0.6–0.7 nm in diameter provide the largest H₂ uptake per unit SSA at elevated pressures and liquid nitrogen temperatures. The effect of pore size was stronger than the effect of surface chemistry on the hydrogen uptake.