Over the past three decades, lithium-ion batteries have revolutionized the energy industry due to their lighter weight, longer charges and ability to perform better under extreme conditions compared to the nickel-cadmium batteries of the past. A key component of lithium-ion batteries is graphite, the primary material used for one of two electrodes known as the anode.
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When a battery is charged, lithium ions flow from the cathode to the anode through an electrolyte buffer separating these two electrodes. This process is then reversed as the battery discharges energy. While various materials can be used for the cathode, graphite is the go-to material for most anodes, thanks to its abundance, low cost, and long cycle life. Cycle life refers to how long a battery can hold a charge and contributes to technology advancements.
As industries around the globe work to create more powerful lithium-ion batteries to power everything from electric vehicles to grid-scale energy storage stations, graphite plays an increasingly important role. Natural graphite typically contains flakes which need to be converted to a spherical form before they can be used as an anode material. Alternatively, synthetic graphite can be produced in a controlled process to ensure consistent quality. The production of high-quality synthetic graphite requires temperatures as high as 3000°C.
Optimizing the morphology of the graphite allows researchers to create anodes with a higher rate capability and energy density, lower first cycle irreversible capacity loss, longer cycle life and better safety performance. Spherical graphite particles allow for more efficient packing of particles, thereby increasing the overall conductivity. Using a scanning electron microscope (SEM), researchers can visually study the morphology of the particles. Scanning electron microscopes have superior resolution compared to optical microscopes, which would prove difficult to use when looking at a black powder such as graphite.
Often, researchers can’t afford the in-house equipment or expertise and instead send their samples for analysis to testing labs with floor-model SEMs. Not only do these floor models require operation by a trained professional but outsourcing these analyses delays results.
Graphite particle images captured using a Thermo Scientific Phenom XL Desktop SEM. The upper left-hand image is natural graphite. The other images were captured after applying five different treatments to synthetic graphite, with the goal of finding the closest match to the natural material.
By turning to the Thermo Scientific Phenom XL Desktop SEM with a dedicated Auto-Scan script, researchers can dramatically improve turnaround times for these types of battery experiments. The Phenom XL enables researchers to perform their own analyses in-house with very little training.
With the dedicated Auto-Scan script, the Phenom XL can automatically test up to 36 samples at the same time, generating more than 200 images in under 30 minutes. The script is easy to use: the user defines the number of positions that require analysis, and the desired magnification levels at all the positions. Once this is done, image acquisition can be done unattended overnight, dramatically reducing turnaround times while eliminating variations in image quality that are common with different operators.
The Phenom XL can generate images at 10 nanometer resolution, which makes it the ideal instrument to image 20-micron graphite particles. Moreover, the intuitive user interface reduces the need for training, extending these analyses to more users.
As scientists around the globe work to improve graphite for lithium-ion battery anodes, the Phenom XL Desktop SEM with dedicated Auto-Scan script can automate the repetitive testing work required. With the ability to quickly and accurately characterize these samples in-house, users can accelerate the R&D process as they work to design safer, more powerful, and longer lasting lithium-ion batteries.
Willem van Zyl is an application engineer at Thermo Fisher Scientific.
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To learn more about how electron microscopy can enhance battery research, please see our Battery Research website.
Also, watch our on-demand webinar, Advanced Diagnostic Tools for Characterizing Lithium Metal and Solid-State Batteries.
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Choosing an electrode material with good performance and low cost is of great significance for the practical application of the electro-Fenton process. In this study, graphite felt was systematically studied to determine its application performance in an electro-Fenton system. The influence of operating parameters, pH and voltage, on the H 2 O 2 yield and the evolution of iron ions was investigated, which helped to select the optimal parameter values. The removal rate of methylene blue was 97.8% after 20 min electrolysis under the conditions of 7 V voltage and pH 3. Inhibition experiments showed the graphite felt E-Fenton system mainly relied on the indirect oxidation of ·OH and the direct oxidation of the graphite felt anode to degrade the methylene blue. The graphite felt showed good stability as a cathode during repeated use, but the anode conductivity and catalytic performance were decreased, and the adsorption performance was enhanced. Finally, the graphite felt electrode was characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) and X-ray photoelectron spectroscopy (XPS) to preliminarily analyze the reason for the change in anode performance.
Therefore, this study selected three-dimensional electrode material graphite felt as the cathode and anode to degrade the methylene blue, graphite felt was systematically studied to determine its application performance in the electro-Fenton system. To highlight the influence of operating parameters, the evolution of the iron ions and the effects of voltage and pH on H 2 O 2 production were studied. The experimental data of methylene blue degradation were fitted, and a kinetic model was established. The degradation mechanism for the graphite felt E-Fenton system was studied in detail. Finally, a repeated use experiment was carried out to determine the change of performance of the graphite felt electrode before and after use.
Compared with two-dimensional electrodes, three-dimensional electrodes have the advantages of counteracting the limitations of the low space-time yield and low normalized space velocity from electrochemical processes. 22 Some researchers also used three-dimensional electrode materials as anodes. For example, Yi et al. used activated carbon fiber as the anode to degrade the alizarin red S (ARS) dye, and believed that there is synergy between adsorption and electrochemical oxidation during electrolysis. 23 Since the functions of the anode and the cathode are different in the E-Fenton system, the three-dimensional electrode material as the cathode and the anode to construct the E-Fenton system to degrade the pollutants need to be further studied. However, there are few reports using three-dimensional electrode materials as the cathode and anode in the E-Fenton system to degrade pollutants. Considering that the E-Fenton process is developed for long-term and large-scale applications, the stability of the electrode is important for determining the performance of the EF process. Zhou et al. evaluated the stability of anodized GF electrode for p nitrophenol degradation in 10-times continuous runs, and the TOC removal efficiency decreased within 15%. 24 However, the running time of these tests are not long enough. 13 How to deal with these scrapped electrodes is another issue to consider after the electrodes lose their electrochemical properties, so a lot of repeated experiments are needed to determine the changes in their performance.
In the E-Fenton system, O 2 can react with H + at the cathode to generate H 2 O 2 ( eqn (1) ), and then H 2 O 2 reacts with Fe 2+ to produce the ·OH oxidizing organic pollutants ( eqn (2) and (3) ). 5,6 Compared with traditional Fenton technology, one of the advantages of E-Fenton technology is to eliminate the trouble of adding H 2 O 2 and avoiding the potential danger of H 2 O 2 storage and transportation. 7,8 Another advantage of E-Fenton technology is that Fe 2+ can be regenerated at the cathode, which greatly reduces the production of iron sludge. 9,10 It can be seen that studying the evolution of iron ions and the production of H 2 O 2 are of great significance for further understanding and regulation of E-Fenton technology. pH and voltage are two important factors affecting the evolution of iron ions and the production of H 2 O 2 . However, people are more concerned with the effect of iron ion dosage, pH and voltage on the removal rate of pollutants, no further consideration is given to the effects of iron ions evolution and H 2 O 2 production. 11,12 There are also many scholars who regulate the production of H 2 O 2 , but their purpose is to maximize H 2 O 2 production through technical regulation. 13,14 Therefore, the relationship between H 2 O 2 production, iron ion evolution, and contaminant removal rate needs further study. In addition, choosing an electrode material with good performance is a problem that troubles the practical popularization of E-Fenton technology. 15 Most of the anodes choose Pt, BDD, etc; however, they are rarely used for practical purposes because of their high cost. 1,16 Since both H 2 O 2 production and iron ion regeneration occur at the cathode, most people choose carbon-based three-dimensional electrode materials for the cathode, such as graphite felt, 17 carbon felt, 18 carbon sponges and activated carbon fibres. 19,20 Some researchers have modified three-dimensional electrode materials to improve the application performance. For example, Ganiyu et al. used Fe II Fe III LDH modified carbon felt cathode to achieve efficient metronidazole removal over a wide pH range. 21
In recent years, the electrochemical advanced oxidation process (EAOPS) has been developed for the prevention and remediation of environmental pollution, especially for water treatment. 1 The core of EAOPs is a series of physical processes and chemical reactions to generate the hydroxyl radicals (·OH). ·OH has a strong oxidation potential (2.87 V), which is second only to F (2.87 V). 2 ·OH can oxidize the vast majority of organic pollutants in wastewater to CO 2 , H 2 O and inorganic ions. 3 As a kind of environmentally friendly electrochemical advanced oxidation process, electro-Fenton (E-Fenton) technology is highly favoured to treat wastewater because of its advantages, such as high utilization rate of H 2 O 2 and Fe 2+ , process cleaning, low sludge residue and low investment cost. 4
where eqn (5) represents the electric energy required in kW h to remove methyl blue by one order of magnitude in a unit volume (kW h order of methyl blue m −3 ), U is the voltage of DC power (V), P is the rated DC power (W), I is the current of DC power (A), and t is the treatment time (min). V is the volume of water treated at time t (L), C 0 is the initial concentration of methylene blue (mg L −1 ), and C is the final concentration of methylene blue (mg L −1 ).
The surface morphology was characterized by scanning electron microscopy (SEM, Hitachi S-470). The crystal structure of the catalysts was characterized via X-ray diffraction (XRD, Rigaku MiniFlex 600). The surface OGs were analyzed by X-ray photoelectron spectroscopy (XPS) analysis. XPS was obtained with an AXIS-Ultra electron spectrometer (Shimadzu, Japan). The specific surface area and pore volume were analyzed via N 2 adsorption/desorption using the BET and t-plot method with a Novaeseries (Quantachrome Co., USA).
H 2 O 2 and iron ions were sampled from the reactor every 5 minutes during the experiment. Methylene blue was sampled from the reactor every 1 min during the experiment. The concentration of H 2 O 2 was analysed by a UV-vis spectrophotometer at 400 nm using the potassium titanium(iv) oxalate method. 25 The concentrations of total iron and Fe 2+ were determined by the 1,10-phenanthroline spectrophotometric method (λ = 510 nm). 26 The Fe 3+ was determined by subtraction of total iron minus Fe 2+ concentrations. 15 The methylene blue concentration at 664 nm was determined using an ultraviolet spectrophotometer. The ultraviolet spectrophotometer was purchased from HACH Company, Germany. Relative error of all experimental data was less than 1.5%.
Batch studies were performed in a columnar vessel (diameter of 90 mm, height of 160 mm). Then, 500 mL of 20 mg L −1 methylene blue solution was taken for every study. Both the electrodes (anode and cathode) were graphite felt with an effective area of 25 cm 2 . Continuous aeration to the cathode was performed using a fish aerator during the experiment. The graphite felt electrode was kept parallel and the spacing between plates was 4.0 cm. The pH was adjusted using 0.1 M H 2 SO 4 . Na 2 SO 4 was used as the supporting electrolyte. A constant voltage was applied across the electrodes using a DC power supply. If no special instructions were given, all the voltages in the experiment are cell voltages. All experiments were conducted at room temperature (293.15 + 2 K).
Analytical grade methylene blue (7220-79-3, 99%), methanol (67-56-1, 99.5%), FeSO 4 ·7H 2 O (7782-63-0, >99%) and Na 2 SO 4 (7757-82-6, >99%) were purchased from Shengao Chemical Industry, China. These reagents and dyes were directly used without any further purification. All solutions were prepared with ultrapure water (resistivity of 18.20 MΩ). Graphite felt was purchased from Tianjin Carbon Factory, China, which was cut into 12 × 1.5 × 0.5 cm pieces. During the experiment, the graphite felt was immersed in water at a depth of 6 cm. Graphite felt was washed with ultrapure water and then dried at 378.15 K for 24 h.
Compared with the traditional Fenton process, the E-Fenton process has the ability to produce H2O2in situ; thus, the study of H2O2 generation is of great significance for understanding and regulation the E-Fenton process. The effect of pH and voltage on the yield of H2O2 was investigated in this study. We performed continuous aeration at the cathode using a fish aerator during the experiment. Since Fe2+ has a catalytic effect on H2O2, Fe2+ was not added. The catalytic effect of iron was lost, and the Fenton reaction stopped during the electrolysis.15 showed that the yield of H2O2 decreased as the pH increased, which is consistent with the study of Lei Zhou et al.27 According to eqn (1), acid environment is a necessary condition for generating H2O2. The higher the pH value, the lower the concentration of H+ in the solution. Therefore, as the pH increased, the yield of H2O2 gradually decreased. showed that as the voltage increased, the yield of H2O2 increased and then reached a maximum when the voltage was 7 V. However, the yield decreased when the voltage was continuously increased to 9 V. According to Faraday's law, the current density increases as the voltage increases, which leads to an increase in H2O2 yield. However, when the current intensity is too high, the oxidation rate of H2O2 will increase. H2O2 can be oxidized to ·HO2 and H+ on the surface of the anode. ·HO2 is unstable and further decomposes into H+ and O2 (eqn (6) and (7)).28,29 Safizadeh et al. suggested that excessive voltage will lead to the aggravation of the hydrogen evolution reaction (eqn (9)), and the hydrogen evolution reaction will reduce the concentration of H+ per unit volume in solution.30 In addition, the reduction of O2 by a carbon electrode mainly occurs via 2 electrons or 4 electrons.31 The 2-electron reaction can produce H2O2 (eqn (1)), and the 4-electron reaction can produce H2O (eqn (8)). The 4-electron side reaction will lead to a decrease in H2O2 yield under certain conditions of the solute.
Open in a separate windowAfter 20 min of reaction, the removal rates of methylene blue at voltages 3, 5, 7 and 9 V were 86.55%, 96.16%, 97.80% and 90.33%, respectively. The highest removal rate of methylene blue was obtained at voltage 7 V. This conclusion was consistent with the highest H2O2 yield at the voltage of 7 V. Therefore, if the production of H2O2 can be further optimized, it is of great significance to improve the degradation rate of organic matter in the E-Fenton system. The removal rates of methylene blue at pH 2.5, 3, 3.5 and 4 were 97.73%, 97.80%, 94.92% and 93.87%, respectively. showed that when the pH was 2.5, the yield of H2O2 was higher than at pH 3. According to the calculation formula for pH (eqn (10)), the acid added at pH 2.5 is times the amount of the acid added at pH 3, which considerably increases the amount of acid. According to Chen et al., the quenching effect of H+ on the ·OH is more obvious under strong acid conditions.32 In addition, the reaction between Fe2+ and H2O2 will be inhibited.33 Therefore, pH 3 was the optimal pH for methylene blue degradation in this experiment.
H2O2 → ·HO2 + H+ + e−
6
·HO2 → O2 + H+ + e−
7
O2 + 4H+ + 4e− → 2H2O
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8
O2 + 4H+ + 4e− → 2H2↑
9
pH = −lg C(H+)
10
In the E-Fenton process, Fe3+ can be electronically reduced to Fe2+ at the cathode to achieve the cycle of Fe2+ and Fe3+, which considerably reduces the dosage of Fe2+ and the production of iron sludge. Therefore, the evolution of iron ions in this experiment was investigated. When the water contains dissolved oxygen, the H2O2 produced by the cathode will react with Fe2+. Thus, we adopted the method of continuous bubbling N2 by Aboudalle et al. during the experiment to remove the dissolved oxygen in the water.34 showed the total iron concentration did not remain constant when the Fe3+ was added to the solution. The total iron ions in the solution gradually decreased, and the Fe2+ concentration gradually increased as the reaction proceeded ( ). When electrolysed for 20 min at voltages of 3, 5, 7 and 9 V, the total iron ions decreased by 0.91%, 8.55%, 12.43%, and 19.33%, respectively. Therefore, the higher the voltage, the more the total iron ion concentration decreases. The effect of voltage on the iron ions may be related to the variation in pH. According to previous studies, when adding 1 mmol L−1 Fe3+ to solution, [Fe(H2O)6]3+ or simply Fe3+ as the sole species at pH 0, Fe3+ practically disappears at pH 4. At the optimum pH of 2.8, only approximately one-half of the free Fe3+ is always present in the bulk.22 In this experiment, with an increase in voltage, the cathode localized alkalization affected the pH near the cathode,22,35 which caused the total iron ion concentration to gradually decrease. showed that the rate of regeneration of Fe2+ first increased and then decreased with an increase in voltage. Although the total iron ion concentration was the highest at a voltage of 5 V, the rate of Fe3+ reduction was faster because of the higher current density at the voltage of 7 V. Thus, there was little difference in the Fe2+ regeneration rate at voltage 5 V and 7 V (76.75% and 73.33%, respectively). When the voltage was 3 V, the cathode localized alkalization was not significant; thus, the total iron ion concentration was almost constant in solution. However, the regeneration rate of Fe2+ was only 60.19% due to a lower current density. When the voltage was 9 V, the current density was large. Thus, the regeneration rate of Fe2+ at the early stage was higher than 3 V. However, the total iron ion concentration sharply decreased with the reaction, resulting in a regeneration rate of Fe2+ of only 61.33%.
Open in a separate windowshowed the ability of Fe3+ to regenerate Fe2+ under anaerobic conditions, whereas according to eqn (1) and (4), the regeneration and consumption of Fe2+ are simultaneous during the actual reaction. Therefore, the evolution of iron ions was studied during the degradation process of methylene blue in the graphite felt E-Fenton system, and the reaction time was extended to 90 min; the results are shown in . As the reaction proceeded, the total iron ion concentration continued to decreased in the solution, which was consistent with the conclusions shown in . However, when electrolysis was conducted for 20 min, the total iron ion concentration dropped to 25.33%, which was twice as much as under anaerobic conditions (12.43%). The electrode localized alkalization was very significant in the actual treatment process. During the actual treatment process, O2 continuously consumed H+ at the cathode to produce H2O2, and Fe2+ reacted with H2O2 to generate OH− (eqn (1) and (2)). The local alkalization of the electrodes caused by these reactions was more pronounced, resulting in a final decrease in total iron concentration. In addition, Fe2+ can not maintain a stable concentration during the entire reaction process. The Fe2+ concentration gradually decreased with the electrolysis time, and the Fe3+ concentration gradually increased. This phenomenon was consistent with the research of Ramirez-Pereda et al., which showed that the regeneration rate of Fe2+ was lower than its consumption rate during the actual electrolysis process.15
Methylene blue (20 mg L−1) was degraded at a voltage of 7 V with Fe2+ at 0.2 mmol L−1, the supporting electrolyte, Na2SO4, at 10 mmol L−1 and at pH 3. The removal rate of methylene blue was 97.80% after 20 min of electrolysis. The E-Fenton oxidation of methylene blue can be represented by the following nth-order reaction kinetics.
11
where C represents the dye concentration, t represents the time, k represents the reaction rate coefficient and n represents the order of the reaction.
For the first order reaction, eqn (11) was integrated and converted:
Ct = C0 e−kt
12
in which C0 is the initial dye concentration. For the second order reaction, eqn (11) was integrated and converted:
13
The degradation data of methylene blue in the graphite felt E-Fenton system was plotted via first-order reaction kinetics and second-order reaction kinetics. showed a comparison of the two kinetic models for the experimental results, and shows the parameters obtained from the curve fitting. The second-order kinetics does not fit the data very well for the entire E-Fenton reaction due to the lower regression coefficients (<0.96). The first-order kinetics fits the results best as evidenced by the high regression coefficient (>0.99). Therefore, the kinetics process of degradation of methylene blue by the graphite felt E-Fenton method was determined to be first-order reaction kinetics.
Open in a separate windowReactionFirst-orderSecond-order K (min−1) R 2 K (L mg−1 min) R 2 E-Fenton0.260.9950.030.911Open in a separate windowIts kinetic equation is:
Ct = 20 e−0.26
14
When methylene blue was degraded, its degradation rate decreased significantly after 15 minutes. In order to reduce the experimental error and to more clearly see the experimental differences under each reaction condition, the electrolysis time was set to 15 min during the investigation of degradation mechanism. Each set of experiments was repeated 3 times, and the average value was obtained.
H showed the individual adsorption effect of graphite felt on the removal rate of methylene blue, which was 3.26% after 15 min of adsorption. In addition, when H2O2 was added alone, methylene blue exhibited almost no degradation; thus, H2O2 cannot oxidize methylene blue alone without an electric field. showed the degradation rate of methylene under the conditions of Fe2+ at 0.2 mmol L−1 and 0 mmol L−1, respectively. When Fe2+ was added to the solution, the removal rate of methylene blue increased by 41.37%; thus, the Fenton reaction to produce ·OH and ·HO2 was the main factor for the degradation of methylene blue. Methanol is recognized as ·OH scavenger. The reaction rate of methanol with ·OH is 1.2 × 109 to 2.8 × 109 M−1 s−1.36 In the presence of Fe2+, 5 mL of methanol was added to the solution. The removal rate of methylene blue decreased by 52.33%, which confirmed that ·OH was the main active substance in the E-Fenton system ( ). When 5 mL of methanol was added without the Fe2+-forming Fenton reagent in the solution, the removal rate of methylene blue did not significantly change ( ). This showed that the methylene blue is directly oxidized by the graphite felt anode. According to , the removal rate of methylene blue decreased when 10 mL of H2O2 was added to the solution; thus, H2O2 cannot produce ·OH under the action of an electric field without Fe2+. In contrast, the competition between H2O2 anodic oxidation and methylene blue anodic oxidation resulted in a decrease in the removal rate of methylene blue.
Open in a separate windowIn addition, aeration had a significant effect on the degradation of methylene blue in the graphite felt E-Fenton system. When the solution did not contain Fe2+, the removal rate was 3.13% and 55.07% under conditions of no aeration and aeration, respectively ( ). When the solution contained Fe2+, the removal rate was 77.09% and 96.44% under conditions of no aeration and aeration, respectively ( ). Tammeveski et al. thought that because an activated carbon fibre surface contains a large number of functional groups, O2 will be reduced to ·O2− at the cathode.37 Based on this, we speculate that aeration may play three roles in the graphite felt E-Fenton system: (i) as active substance, where ·O2− was produced under the action of an electric field; (ii) to accelerate the mass transfer of the solutions; (iii) and to produce the Fenton reagent, H2O2, at the cathode. According to , when N2 or O2 was exposed to the solution without Fe2+, the removal rate of methylene blue did not significantly change; thus, it can be concluded that no ·O2− was produced. When Fe2+ was not present in this system, the main role of O2 was to accelerate the mass transfer. When Fe2+ was present, the role of O2 was to accelerate the solution mass transfer and the generation of the Fenton reagent, H2O2. When Fe2+ was present and the solution was not aerated, the potential difference between the anode and the cathode was the migration power of the ions. Fe2+ and H+ migrated to the cathode and generated ·OH viaeqn (1) and (2). In addition, methylene blue formed a highly chromatic monovalent cationic quaternary ammonium salt ionic group in aqueous solution, and the quaternary ammonium salt ionic group migrated to the cathode under the action of an electric field, which accelerated the reaction rate of methylene blue with ·OH. Therefore, there was a high degradation rate of methylene blue even under the condition of no aeration. When no Fe2+ was in the solution, the degradation of methylene blue depended on the direct oxidation of the anode; however, the anode will repel methylene blue because of the charged nature of methylene blue. Therefore, the degradation of methylene blue was very slow under unaerated conditions. When continuous aeration was applied to the solution, the disturbance of oxygen on water overcomes the repulsive force of the anode on methylene blue and enabled methylene to be oxidized directly at the anode.
In summary, a probable mechanism of the graphite felt E-Fenton system to remove the methylene blue is shown in . During the electrolysis process, the graphite felt anode can directly oxidize the methylene blue. H2O2 was electro-generated from the reduction of O2 on the graphite felt cathode under the condition of aeration. H2O2 then migrated into the liquid phase to react with Fe2+ and Fe3+. The ·OH and ·HO2 were generated by the Fenton and Fenton-like reaction. Then, the methylene blue was degraded by ·OH and ·HO2 indirectly.
Open in a separate windowIn order to study the stability of graphite felt electrode, the experiment of reusing was carried out. showed the change in methylene blue removal rate and system current with the number of uses under the optimal conditions for 15 min of electrolysis. The graphite felt electrode was not regenerated during the process of repeated use. According to , the methylene blue removal rate and current showed a downward trend with increasing usage counts. After 90 times of repeated use, the removal rate decreased by 26.02%, and the current decreased by 76.56%. Linear fitting was performed on the change of methylene blue removal rate, and the slope of the fitting line was −0.20. The removal rate of graphite felt only decreased by 0.20% for each use, which reflected the good reusable performance of graphite felt. In addition, the current drop was much larger than the removal rate during the reusing process. According to , the adsorption performance of graphite felt was greatly improved after repeated use. The adsorption removal rate of unused graphite felt to the methylene blue was only 3.26%. After 90 repeated times of use, the adsorption removal rate of graphite felt to the methylene blue was 38.91%, and the adsorption effect was considerably improved. Although the current density of the E-Fenton system decreased during repeated use, the increase in the adsorption capacity was helpful to shorten the reaction path between methylene blue and active substances, which made up for repeated use current greatly reduced defects to some extent.
Open in a separate windowThis study initially investigated the reasons for the decrease of current and removal rate during the repeated use of graphite felt E-Fenton system. According to eqn (15), under the condition of constant voltage and constant solution composition, the reason for the decrease in current was the increase in the internal resistance of the graphite felt electrode.
15
where I represents the current (mA), U represents the voltage (V), Ri represents the internal resistance of the graphite felt electrode, and Re represents the external resistance of the solution.
We used the original graphite felt A, the repeated use graphite felt B (used as the anode) and the repeated use graphite felt C (used as the cathode) and completed a total of 7 permutation combinations to study the reasons for the decrease in current and removal rate ( ). The experimental results were shown in .
ElectrodeDifferent electrode combinationsAnodeAAABBCCCathodeABCACABOpen in a separate windowOpen in a separate windowAccording to , the current of the A + A system was 64 mA; however, the currents of the A + B, B + A, C + B, and B + C systems were 25, 18, 13, and 15 mA, respectively. These results indicated that in the presence of the B electrode, the current value of the entire E-Fenton system is low regardless of whether the B electrode continues to function as the anode or the cathode, or whether the counter electrode or the new electrode is used. The currents of the A + C and C + A system were 55 mA and 67 mA, respectively, indicating that the repeated use cathode C has little influence on the current drop. Thus, the decrease in current was because of the increase in anode resistance.
showed the first-order linear fitting results for methylene blue removal. The K values (the reaction rate coefficients) of the A + A, C + A, A + C, B + A, B + C, A + B, and C + B systems were 0.22, 0.19, 0.14, 0.11, 0.08, 0.07, and 0.06 min−1, respectively. Upon comparing the K value of B + C (0.08) with the B + A (0.11), A + B (0.07), and C + B (0.06) systems, the removal rate of methylene blue was very low regardless of how the electrode was replaced in the presence of electrode B. Upon comparing B + C (0.08) with C + A (0.19) and A + C (0.14), the K value increased as long as electrode B was replaced. Thus, the decrease in catalytic activity of electrode B was the main reason for the decrease in the methylene blue removal rate.
Although the catalytic activity of electrode C is lowered, the magnitude of the decrease was much smaller than the decrease of the electrode B. Therefore, the method of replacing the anode B can be used to restore the degradation efficiency of the E-Fenton system in practical application. However, it did not work well to only use the new electrode A to replace the used anode B (the K value of A + C was 0.14). According to C + A (0.19) and A + C (0.14), the new electrode A was used to replace electrode C as a cathode, and electrode C was no longer used as a cathode but was used as an anode. This electrode replacement method can restore the maximum removal efficiency of the E-Fenton system, and the K value is only 0.03 different from that of the new electrode system A + A.
It can be determined graphite felt will cause three changes after being used as anode: the conductivity and catalytic performance will be decreased, and the adsorption performance will be increased. According to the characterization results of , the reasons for the above performance changes can be preliminarily inferred. The pore width distribution showed the number of micropores increased. BET test showed the specific surface area of anode graphite felt increased by 3.69 m2 g−1 compared with the raw graphite felt. In addition, performance changes may be related to changes in crystal structure.38 The XRD results showed that the diffraction peak at 2θ = 23° was stronger, corresponding to the (002) crystal plane of the graphite crystallite; the diffraction peak at 2θ = 44° was weaker, corresponding to the (010) crystal plane of the graphite crystallite, and the peak shape of the two diffraction peaks was wider, which proves that the graphite felt was a graphite microcrystalline layered structure. And the characteristic diffraction peak position of the anode did not change substantially, indicating that the graphite felt electrode did not change the graphite fiber structure before and after use. However, the intensity of the anode peak was weakened, the peak height was shortened, and the width was increased, indicating that the anode graphite felt grain becomes smaller, the delocalized electrons decreased, the grain boundary hindered the electron conduction, and the resistance increased. SEM image showed that the anode attachment increases, which may affect the conductivity. Moreover, cracks appeared in the structure of anode fiber, which indicated that the physical strength and mechanical strength of graphite felt were damaged greatly and the electrode life was reduced. The XPS results showed that the O 1s peak was greatly enhanced, while the C element peak intensity was slightly lowered. The content of C Created by potrace 1.16, written by Peter Selinger 2001-2019 O decreased from 47.12% to 40.31% and the content of H–O–H increased from 19.87% to 27.42%. The decrease in electrical conductivity and catalytic performance may be related to changes in oxygen-containing functional groups.13 Malitesta et al. indicated the oxygen can be located not only at the beta carbon but also on the alpha carbon, leading to the breakage of polymer chains, resulting in a decrease in conductivity.39
Open in a separate windowIn this study, the removal rate of methylene blue was 97.80% after electrolysis for 20 min in the graphite felt E-Fenton system, and these results were very competitive if compared with the traditional graphite electrode system.16 Electric energy consumption is a key index to evaluate the scale-up and industrial applications of the graphite felt E-Fenton system. In this study, the voltage and current of the DC power were 7 V and 64 mA, respectively. The rated power of the fish aerator was 1 W. C0 and C were 20 mg L−1 and 0.44 mg L−1, respectively. The volume of water treated was 0.5 L, and the electrolysis time was 20 min. Taking the above data into eqn (5), the EE/O of the graphite felt system was calculated, and the results were 0.55 kW h m−3, which revealed a significant reduction in energy consumption.15 The graphite felt also showed a good price advantage. The price of graphite felt purchased by our laboratory was 35 yuan (20 × 123 × 0.5 cm pieces). After cutting it into electrodes (12 × 1.5 × 0.5 cm pieces), each graphite felt electrode was only approximately 0.3 Yuan. Moreover, the adsorption performance of the graphite felt used in the E-Fenton system was considerably improved; therefore, the anode graphite felt can be used alone as an adsorbent. In general, the graphite felt E-Fenton system has a good practical significance.
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