Water Electrolysis using Flame-Annealed Pencil-Graphite Rods - ACS

Research Article pubs.acs.org/journal/ascecg

Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Water Electrolysis using Flame-Annealed Pencil-Graphite Rods Ryuki Tsuji,† Hideaki Masutani,† Yuichi Haruyama,†,‡ Masahito Niibe,†,‡ Satoru Suzuki,†,‡ Shin-ichi Honda,§ Yoshiaki Matsuo,∥ Akira Heya,† Naoto Matsuo,† and Seigo Ito*,†

Downloaded via AUBURN UNIV on February 27, 2019 at 01:10:29 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

Department of Materials and Synchrotron Radiation Engineering, Graduate School of Engineering, University of Hyogo, 2167 Shosha, Himeji, Hyogo 671-2280, Japan ‡ Laboratory of Advanced Science and Technology for Industry, University of Hyogo, 3-1-2 Kouto, Ako, Hyogo 678-1205, Japan § Department of Electrical Materials and Engineering, Graduate School of Engineering, University of Hyogo, 2167 Shosha, Himeji, Hyogo 671-2280, Japan ∥ Department of Applied Chemistry, Graduate School of Engineering, University of Hyogo, 2167 Shosha, Himeji, Hyogo 671-2280, Japan S Supporting Information *

ABSTRACT: Inexpensive and sensitive graphite electrodes were fabricated by applying flame annealing to pencil-graphite rods (PGRs) as electrodes for water electrolysis cells. The resin (polymer, binder) on the surface of PGR was removed by flame annealing to make it porous, and the graphite electrodes with high activity and low cost were obtained. By flame annealing the PGR, although the PGR electrode became active upon water electrolysis, the PGR electrode became instable for long-time operation. The effects of flame annealing on PGR for water electrolysis were analyzed by SEM, FT-IR spectroscopy, Raman spectroscopy, NEXAFS, and electrochemical impedance spectroscopy (EIS). KEYWORDS: Pencil-graphite electrodes, Graphite, Water electrolysis, Fire-flame annealing, Electrochemical analysis, Resin

■

INTRODUCTION Because of the problems associated with limited fossil fuel resources and global warming due to CO2 emission, power generation from solar and wind can be one of the candidates as the base energy in our society in the future. Actually, renewable energies by solar photovoltaics and wind can be cheaper energy sources compared to those from fossil fuels. In an IRENA report, global weighted average costs over the last 12 months (2017) for onshore wind and solar PV stand at 6 cents USD and 10 cents USD per kWh, respectively. Contrary to this, the cost spectrum for fossil fuel power generation ranges from 5−17 cents USD per kWh.1 But, these natural energies are greatly affected by sun irradiation and seasons and cannot be supplied steadily. In order to accumulate such unstable energy for constant utilization, a water-electrolysis system which converts the natural energies to hydrogen to reserve energy without CO2 emission has attracted attention. Contrary to the present industrial hydrogen production system by cracking of fuel oil with mass CO2 emission, hydrogen production by water electrolysis using solar and wind generation does not emit CO2, which makes it an ideal hydrogen production method for future energy systems.2−4 There are various types of water-electrolysis systems, and graphite electrodes have been one of the standard materials for the study of water electrolysis to produce hydrogen. However, existing graphite electrodes are somewhat expensive (ca. 23 © XXXX American Chemical Society

USD/10 cm rod, ø = 2.0 mm [exchange ratio = 110 JPY/ USD], estimated by a Japanese graphite company). A more inexpensive graphite electrode-based material is necessary for the mass consumption in the study of, research on, and development of water electrolysis electrodes. Pencil graphite rods (PGRs) are an important material for electrodes in electrochemical research, due to their low cost and disposability. Already, three important review papers5−7 and more than 300 papers with the title of “pencil-graphite” have been published (searched by SCOPUS searching system [Elsevier B. V.] checked July 2018). The research topics of PGRs have been developed for sensors,8−10 super capacitors,11−13 water electrolysis,14,15 and so on. Actually, a PGR costs only 27 cents USD/13 cm rod, ø = 2.0 mm (from Mitsubishi Pencil Co. Ltd., Japan). Regarding the PGR, it was noted that resin (binders, polymer) existed in the bulk and on the surface of PGR,16 which can be changed with the hardness of PGR. Since Navratil et al. already showed variation of electrochemical activity using PGRs from different companies,17 the physical and electrochemical characteristics of pencil-graphite rods can also be changed by the hardness. In this paper, in order to remove the coating polymer on the Received: September 14, 2018 Revised: December 31, 2018

A

DOI: 10.1021/acssuschemeng.8b04688 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. SEM images of pencil graphite 4H (a, b) and 4B (c, d) before (a, c) and after (b, d) flame annealing. For the annealing of 4H hardness PGR, the applied flame for the carbon should be increased gradually. If the instant annealing with strong fire flame was applied on the 4H PGR, the rod would brake due to the large amount of resin (binder, polymer) inside of the 4H PGR (Figure S1c). On the other hand, due to the small amount of polymer in the 4B PGR, the possibility of cracking during the instant annealing by fire flame was quite negligible. Characterization Methods. The structure of the PGR electrode surface was observed by scanning electron microscopy (SEM, JSM6510, JEOL). The porous texture of the electrodes was investigated by a nitrogen-desorption isotherm measured at 77 K using specific surface area and a pore distribution measuring instrument (BELSORP-max, MicrotracBEL). In this experiment for the N2 adsorption technique, the PGRs were heated at 120 °C for 120 min to obtain the sample because high temperature annealing at higher than 120 °C would change the condition of binder in PGR, which cannot project the results of water electrolysis. The specific surface area was calculated from the Brunauer−Emmett−Teller (BET) plot, and the pore size distribution of the electrodes was calculated from N2-adsorption branch isotherms. Raman and Fourier transform infrared (FT-IR) spectroscopies were performed using NRS-2100 (JASCO, Japan) and Lumos (Bulker) instrumentation, respectively. After water electrolysis, each electrode was analyzed by Raman and FTIR spectroscopies, once again. The chemical bonding states between graphite and oxygen in these electrodes were characterized by total electron yield the near edge X-ray absorption fine structure (TEY-NEXAFS) with soft X-rays from a synchrotron radiation beamline (BL-09A) in the NewSUBARU SR facility (University of Hyogo, Japan). In order to cancel the carbon orientation,18 the incident angle of X-rays for TEY-NEXAFS measurement was 35° from the normal direction on the sample surface (magic angle). The reference samples for the CK absorption edge was a highly oriented pyrolytic graphite (HOPG, ZYA grade, mozaic spread = 0.4° ± 0.1°, NT-MDT) and a standard graphite (High Purity Chemicals Co., Japan). Electrochemical Measurements. The electrochemical performances of the PGR electrodes were characterized by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and water electrolysis experiments using a Hoffman electrolysis apparatus (Hshaped test tubes). The CV and EIS analyses were performed by a three-electrode system using a platinum foil with a large surface area as the counter electrode and an Ag/AgCl electrode as a reference

surface and to activate the graphite surface, the PGRs with different hardnesses were annealed in gas flame. Using poresize distribution, Raman spectroscopy, FT-IR, NEXAFS, and electrochemical impedance spectroscopy (EIS) analyses, we have analyzed the PGR for the utilization at water electrolysis to produce hydrogen and oxygen and considered the effect of the gas-flame annealing on PGR.

■

EXPERIMENTAL SECTION

Pencil Graphite as an Electrode Material. The expressions of PGR hardness are different across countries (informed by Wikipedia). In the United States (USA), the hardness (color density) was ranked from #1 (softest, darkest) to #4 (hardest, lightest). In the European Union (E.U.) and Japan, expressions of H (hardness), B (blackness), and F (firmness/fineness) have been utilized due to definition by a pencil lead company (Brookman, in London). In the E.U., the hardness (color density) was ranked as 10B (softest, darkest), 9H, 8H, 7H, 6H, 5H, 4H, 3H, 2H, H, F, HB, B, 2B, 3B, 4B, 5B, 6B, 7B, 8B, 9B, and 10B (hardest, lightest). The corresponding numbers are #1 = B, #2 = HB, #3 = H, and #4 = 2H. In Japan, the expression was similar to that in the E.U., but the exact hardness was different from that in the E.U. The PGR hardness (color density) in Japan was ranked 9H (softest, darkest), 8H, 7H, 6H, 5H, 4H, 3H, 2H, H, F, HB, B, 2B, 3B, 4B, 5B, and 6B (hardest, lightest). In this work, the PGRs (hardness = 4B and 4H, Uni exchangeable pencil-graphite rod, ø = 2.0 mm × 130 mm, Mitsubishi Pencil Co., Ltd., Japan) were used for both the anode and the cathode as water electrolysis electrodes. Although the experiments had been performed on various PGRs with 10 types of hardness (4B, 3B, 2B, B, HB, F, 2H, 3H, and 4H), this paper shows only the data of 4B and 4H, as the representative samples. 4B and 4H PGRs are the softest and hardest ones from Mitsubishi Pencil Co., Ltd., respectively. PGRs are generally a mixture of graphite, resin, and clay; 4B PGR has the largest graphite content, and 4H PGR has the least. First, the cornshaped tip at the top edge of the PGR was cut off, and then, the cross section was scraped by a file to be a flat-shaped edge. In order to remove the coating polymer on the surface and to activate the surface graphite, the whole of the PGR was annealed for 1 min in flame using liquefied petroleum gas (LPG) (Figure S1a) until the PGR emitted red light by the heat (Figure S1b). After it was air-cooled until it reached room temperature, the targeting PGR electrode was obtained. B


Research Article

ACS Sustainable Chemistry & Engineering electrode (Figure S2) in a Na2SO4 (1 M) aqueous electrolyte. In the CV test, the applied voltage was set in the range from −2.0 to 2.0 V (vs. Ag/AgCl) with a scan speed of 25 mV s−1, and the results of the third cycle are shown. EIS measurements were performed using a three electrode system (working electrode = PRG; counter electrode = Pt foil; and reference electrode = Ag/AgCl) (Figure S2) by applying various-frequency voltage waves with an amplitude of 15 mV at the bias voltage of +1.7 V against the Ag/AgCl reference electrode. The frequency range of the applied AC voltage was set from 10 mHz to 1 kHz. For the water electrolysis experiment, the applied voltage was set at 5 V and/or 10 V, which is due to the significant resistance by the electrode distance (60 mm) in the Hoffman cell apparatus (Figure S3), and measurement of hydrogen generation was continued until the hydrogen amount reached 50 mL.

the removal of binders by annealing. In addition, since the 4H PGR was much porous than 4B PGR (Figure 2b), it was found that 4H PGR has a specific surface area (1.13 m2 g−1) that is larger than that of 4B PGR (0.305 m2 g−1). These pores were formed by fire-flame annealing the PGR, because the resin contained in PGR was removed by flame annealing from the inside of the PGR, keeping the porous structure; the specific surface area was increased by the flame annealing. Hence, this phenomenon is due to the amount variation of resin contained in the PGR, and it is considered that 4H PGR contains a large amount of resin to make it harder than 4B PGR. The PGRs are composed of graphite and resin, and the effects of flame annealing on the graphite condition was analyzed using Raman spectroscopy with 514.5 nm excitation (Figure 3). The Raman signal of the surface was larger than

■

RESULTS AND DISCUSSION The surface structure of PGR before and after flame annealing were confirmed by SEM images (Figure 1). The surfaces of both 4B and 4H PGR became rough after the fire-flame annealing. For the further investigation of the porosity of the PGR electrode before and after annealing, N2-adsorption isothermal analysis was performed (Figure 2a). With the fireflame annealing, both 4B and 4H PGR showed a type-I adsorption−desorption diagram, indicating the presence of micropores.19,20

Figure 3. Raman spectra of pencil graphite (4B and 4H) with/ without flame annealing of a cross-section (a) and at the surface (b).

that of the cross section, because the PGR surface was smoother than that of the cross section, and the smoother surface shows a higher Raman signal. The Raman spectra of the PGR electrode surface show bands at about 1352, 1580, and 2728 cm−1, which correspond to the D, G, and 2D (G′) bands of graphite, respectively.21−23 Highly oriented pyrolytic graphite (HOPG, close-to-perfect-structured graphite material, for STM, mosaic spread = 0.8 ± 0.2°, Techno Chemics, Japan) has an intense band observed at 1580 cm−1 (Figure 3b). When the graphite structure is perturbed, additional bands at around 1350 cm−1 (D) and 1620 cm−1 (D′) appear.24 The ID/IG ratio could exactly reflect the disorder of the graphite in the materials. The ID/IG ratio of original 4B PGR (without annealing) was 0.65, and that of the annealed 4B one was decreased to 0.48. Hence, it was conceivable that the crystalline property of graphite increased by flame annealing. Each peak was not observed from the surface of 4H PGR without annealing, but each peak was observed from the cross section (Figure 3b). Hence, it was confirmed that the resin

Figure 2. (a) N2-adsorption isotherms of pencil graphite (4B and 4H) with flame annealing and (b) pore-size distributions (analyzed by adsorption-branch isotherm) of pencil graphite (4B and 4H) with flame annealing.

Figure 2b shows the pore size distribution of annealed 4H and 4B obtained from the adsorption branch isotherm. Since the adsorption isotherm of original PGR (without annealing) showed a negative value for both 4B and 4H due to the very small specific surface area, the specific surface area analysis could be impossible. After annealing, however, it was found that there were micropores that ranged from 0.5 to 1.5 nm in both hardnesses. The pores in PGR were formed because of C


Research Article

ACS Sustainable Chemistry & Engineering covered the electrode surface on 4H PGR. Comparing the Raman results of 4B and 4H PGR, 4B PGR has higher graphite crystallinity than 4H PGR, due to the greater amount of resin in 4H PGR. The surface of flame annealed PGR was analyzed using FTIR (Figure 4). Since the FT-IR spectra of 4B and 4H PGR

Figure 4. FT-IR spectra of pencil graphite (4B and 4H) with/without flame annealing.

surfaces without annealing show peaks at about 1240, 1480, 1710, and 2920 cm−1, which correspond to the CO stretch, CH bending, CO stretch, and CH stretch, respectively, the resin present on the surface of PGR can be close to acrylic resin (PMMA).25−28 However, in the annealed 4B and 4H PGRs the PMMA patterns were not observed. Therefore, the FT-IR spectrum shows the removal of surface capping resin by flame annealing, which is important for the application of the electrodes in electrochemistry. The bonding condition of graphite and oxygen before and after flame annealing was analyzed using TEY-NEXAFS. Figure 5a shows the TEY-NEXAFS at the CK-edge of the 4B PGR with/without annealing, the reference standard graphite, and the second reference standard graphite (HOPG). In all materials, the π* and σ* peaks were observed at around 285.5 and 293 eV, respectively, showing existence of sp2 graphite.29,30 From the PGR electrodes and standard graphite, a peak at 288 eV between π* and σ* peaks was observed. This peak can appear when a carbon atom is with other elements, due to some bonding with neighboring atoms (for example, CO, CH, etc.), indicating that the PGR is low crystallinity graphite.31−33 The large peak at 288 eV of the standard graphite would be due to the natural oxidation, which is related to the freshness of the sample after opening the package. The 288 eV peak of a fresh sample should be much smaller. Also, since HOPG is a highly pure graphite, this peak can be small. Compared to original PGR (without annealing), the π* and σ* peaks became larger after annealing, which are very close to the spectra of standard graphite. From the above results, it was confirmed that by applying flame annealing, the resin on the surface was removed and the graphite layer appeared on the outermost surface, thereby increasing the proportion of standard graphite peak. Figure 5b shows TEY-NEXAFS at the OK-edge. The structures at 532 and 538 eV were attributed to the π* peak of CO bonds and the σ* peak of the OH bond, respectively.34 The spectrum of original PGR is similar in shape to that of standard graphite. Compared with the original PGR (without annealing), although the π* peak (CO bond) did not changed, the proportion of σ*

Figure 5. TEY-NEXAFS of pencil graphite (4B) with/without flame annealing (CK-edge (a) and OK-edge (b)).

peak (OH bond) increased with annealing. It was thought that this was due to oxidation of graphite surface during flame annealing and increase of hydroxyl groups on the surface. The hydrogen of this hydroxyl group would be derived from the resin on the graphite surface and the polymer component inside. The schematic and photograph of the CV test system are shown in Figure S2a and S2b, respectively. Figure 6 shows the CV curves of the PGR (4B and 4H) with/without flame annealing at a scan rate of 25 mV s−1 from −2.0 to 2.0 V (vs. Ag/AgCl). The C−V results using three PGRs for each condition are shown in Figure S4, which show the

Figure 6. Cyclic voltammetry of pencil-graphite electrodes (4B and 4H) with/without flame annealing at a scan rate of 25 mV s−1 and current densities within a potential range of −2.0 to 2.0 V (vs. Ag/ AgCl) in Na2SO4 (1 M) aqueous electrolyte. D


Research Article

ACS Sustainable Chemistry & Engineering reproducibility of the flame annealing process. It was confirmed that the flame annealing process on PGR can give quite reliable results in electrochemical measurement. In the case of 4B PGR, the current value after the flame annealing was higher than that before the flame annealing. It can be noticed that the onset position of the current in the oxidation and reduction is shifted to 0 V by the flame annealing. Although it may look like similar performances between the original and annealed 4H PGR in Figure 6, the oxidation current using annealed 4B PGR at 2 V was 3 times higher than that of the original one. The rising voltage of the reduction current using annealed 4B PGR was apparently lower than that of the original one. Hence, it was confirmed that the annealing effect on PGR was important to improve the electrochemical activity. This means that the flame annealing improved electrochemical activity in water electrolysis. In addition, it was confirmed that the hysteresis becomes larger with annealing, which was thought to be an influence by making the porous surface by flame annealing. In order to compare the C−V results with those of other types of electrodes, an electrode of Pt rod, which has the same size with PGR, has been measured (Figure 6). The current densities of +50 and −50 mA/cm2 were obtained at 1.9 V for OER and −1.3 V for HER. Contrarily, the average voltage for +50 and −50 mA/cm2 using 4B annealed PGR was 1.9 and −1.8 V, respectively. Hence, the 4B-annealed PGR and Pt rod have close overpotentials for OER, but the overpotentials of Pt for HER was lower than that of PGR. According to physisorption results, BET of the 4B rod is 4 times smaller than that of the 4H rod, which would imply the opposite trend. The higher graphitic quality of the 4B PGR product is the reason for the higher current at the water electrolysis compared to the 4H PGR. The large amount of binder in 4H PGR deteriorated the graphitic quality and made it very porous. Kayan et al. reported a value of −250 mA cm−2 (at −1.5 V vs. Ag/AgCl) using PGR, which is 10 times larger than that of the value in this report.14 The difference is due to the variation of electrolyte; Kayan et al. used an electrolyte of 0.5 M H2SO4, and we used an electrolyte of 1.0 M Na2SO4. The schematic and photograph of the water electrolysis experiment system are shown in Figure S3a and S3b, respectively. The experiment was conducted with PGR electrodes placed on the anode and the cathode and by filling the test tube with Na2SO4 (1 M) as an aqueous electrolyte. The photographs of the 4B PGR electrode after water electrolysis for 50 min are shown in Figure 7. No significant variation of electrode shape was confirmed for all cathodes (hydrogen generation electrode) (Figure 7e−h). On the contrary, for the anode (oxygen generating electrode), although the PGR at 5 V without annealing did not show the variation, collapse of the electrode was confirmed at 10 V without annealing and at 5 V and at 10 V with annealing (Figure 7b−d). The PGR at 10 V without annealing (Figure 7b) gradually disintegrated as carbon particles from the surface. The PGR at 5 and 10 V with annealing (Figure 7c, d) split into branches. Of note, the 10 V voltage condition was significantly tough compared to the 5 V one. The reason the PGR electrode collapses (Figure 7c, d) is thought to be due to the bonding strength between carbon particles becoming weakened by removing the binding adhesive by flame annealing.

Figure 7. Photographs of pencil-graphite electrodes (4B) after water electrolysis [(a−d) anode (O2 generating electrode), (e−h) cathode (H2 generating electrode); (a, c, e, g) 5 V, (b, d, f, h) 10 V; (a, b, e, f) without anneal, (c, d, g, h) with anneal].

The hardness variation (4B or 4H) of PGR gave the similar phenomena about the electrode stability during water electrolysis (Figures 7 and 8). The different points were about the 4H anode at 5 V without annealing (Figure 8a) and the 4H cathode at 10 V without annealing (Figure 8f), which gradually disintegrated as carbon particles from the surface at the 4B anode at 10 V without annealing (Figure 8b). It was reported that graphite electrodes can be decomposed easily with SO42− anions.35,36 We are working to explain the reason for disintegration of flame-annealed graphite rod samples. We are working to solve the reason for graphite disintegration, which is due to pH, SO42−, and surface catalytic effects for oxygen evolution reaction, and our results will be published soon. Figure 9a and b show data of current variation during water electrolysis experiments at applied voltages of 10 and 5 V, respectively. At 10 V voltage, 4B PGR performed with a higher current than 4H PGR at any time point. Initially, the annealed 4B PGR performed with a higher current than that without annealing, reflecting the CV results (Figure 6). For the longtime water electrolysis, however, the current of annealed 4B PGR deteriorated due to the anode decomposition, which is thought to be due to the collapse of the PGR of the cathode, as shown in Figures 7d and 8d. The situation was the same to 4H PGR. Using the PGR without annealing at the applied voltage of 10 V, the current value gradually increased with the lapse of time, which would be due to the removal of surface coating E


Research Article


and to the refreshment of carbon surface during the water electrolysis. At an applied voltage of 5 V, the current value was relatively stable (Figure 9b). This is because the applied voltage was so low and the electrodes for O2 generation did not collapse significantly, as shown in Figures 7a, 7c, 8a, and 8c. At 5 V voltage, only annealed 4B PGR performed a higher current than the others. However, the current at 5 V was quite lower than that at 10 V (Figure 9a). Therefore, the speed of H2 and O2 gas production was quite slow at 5 V using the H-type Hoffman cell, which is not suitable for the research on water electrolysis. Afterward, the experiments on water electrolysis had been performed with 10 V applied voltage. Figure 10 shows variation of Faraday efficiency during the water electrolysis experiment with the applied voltage at 10 V. The Faraday efficiencies of cathodes (hydrogen generating electrode) were initially close to 100% and decreased to around 90% at the end in all of the PGR experiments. Contrarily, those of the anodes (oxygen generating electrode) were as low as 45% to 60%. Basically, such low Faraday efficiency by the anodes is due to the oxidation of anode carbon electrodes, resulting in the decomposition of electrodes (Figures 7 and 8). At the same time, some kind of carbonsourced molecule can be emitted as a redox couple from the surface of PGR by the oxidation, resulting in decreasing the Faraday efficiency at the cathode reaction due to the crossover of the reaction between the electrodes. Specifically, the 4B PGR showed increments of anodic reaction (O2 generation) (Figure 10a, b), and the annealed one improved the anodic current significantly (Figure 10b). On the other hand, the anodic current by 4H PGR was relatively constant (Figure 10c, d). This phenomena may be due to the amount of resin (binder, polymer) in and on PGR; the 4B PGR with annealing had the highest improvement due to the smallest amount of binder, and that without annealing, the improvement was decreased due to the remaining binder. Basically, since the 4H PGR has a greater amount of binder than the 4B one, the 4H PGR did not increase the amount of O2. However, the exact cause to explain the phenomena is still ambiguous. In order to understand the phenomena about the decomposition of PGR electrode during water electrolysis, we analyzed the anode PGR electrode before and after water electrolysis (applied voltage at 10 and 5 V) by Raman and FTIR spectroscopies (Figures S5 and S6). In Figure S5a, S5b, and S5d, the peaks of the 2D band were noticeable before water electrolysis, whereas the 2D band disappeared after water electrolysis. The G band after water electrolysis became broad. Hence, the water electrolysis can decompose the carbon crystal on the PGR electrode surface. Regarding the 4H PGR without annealing (Figure S5c), the peaks of D and G bands were not observed before water electrolysis, but appeared after that, because the surface resin was removed by water electrolysis. Figure S6a−d shows the FT-IR spectrum of the surface of the PGR electrode before and after water electrolysis. After water electrolysis (10 and 5 V), spectrum of acrylic resin coated on 4B and 4H PGR before annealing was not observed (Figure S6a and S6c). Hence, it was confirmed that acrylic resin on the surface was removed by water electrolysis. After annealing (Figure S6b and S6d), no significant change was observed by FT-IR. To understand the electrochemical phenomena, electrochemical impedance spectroscopy (EIS) was performed on the

Figure 8. Photographs of pencil-graphite electrodes (4H) after water electrolysis [(a−d) anode (O2 generating electrode), (e−h) cathode (H2 generating electrode); (a, c, e, g) 5 V, (b, d, f, h) 10 V; (a, b, e, f) without anneal, (c, d, g, h) with anneal].

Figure 9. Water electrolysis variation with time using PGR electrodes (4B and 4H) with/without flame annealing, performed in Na2SO4 (1 M) aqueous electrolyte at 10 V (a) and 5 V (b).

F


Research Article


Figure 10. Variations of Faraday efficiency during water electrolysis using PGR electrodes (4B and 4H) with/without flame annealing in Na2SO4 (1 M) aqueous electrolyte [(a) original 4B PGR, (b) annealed 4B PGR, (c) original 4H PGR, (d) annealed 4H PGR].

PGR electrodes before and after water electrolysis with/ without fire-flame annealing. For the preparation of electrodes, the water electrolysis was performed for 30 min with a 2electrode system (before the structure decomposition of PGR, as shown in Figures 7 and 8), and then, each anode PGR was transferred to a 3-electrode cell with a reference electrode (Figure S2) for the impedance measurement. Figure 11a shows the equivalent circuit for EIS analysis. Figure 11b and 11c shows typical Cole−Cole plots obtained by EIS and the fitting results for the representing examples (original and annealed 4B PGR). Table 1 summarized the measured results of each component obtained by EIS. Before the water electrolysis, the flame annealing reduced the surface interface component (Rp) and increased the capacitance (CPE-T), due to removal of resin on the surface and to make the surface porous, respectively. However, the annealing effect on the CPE-T and Rp after water electrolysis became obscure, because of the removal of surface resin by the water electrolysis procedure.

■

CONCLUSIONS The image of fire-flame annealing on PGR is summarized at Figure 12. Through the application of flame annealing to the pencil-graphite electrode, the surface-coating resin (binder, polymer) was removed, making it porous, and a high-activity and low cost graphite electrode was obtained. For the longtime water electrolysis (50 min), however, the PGR decomposed to a tree-like structure. When PGR without flame annealing was used for a water electrolysis electrode, the resin on the surface of the PGR peeled off during the water electrolysis, and then, the electrode became stable for the water electrolysis with emitting particulate graphite from its surface. Among the samples used in this study, it was revealed that the

Figure 11. (a) An equivalent circuit for the analysis of pencil-graphite electrodes before/after the water electrolysis. Representative impedance data and fitting results of original (b) and annealed (c) pencil-graphite electrodes.

4B PGR electrode was much more suitable compared to the 4H PGR for use as an electrode for a water electrolysis cell. In this paper, we suggested very inexpensive pencil-graphite electrodes for water electrolysis. However, since certain phenomena can be changed with the manufactured products, each researcher should check the characterization of a pencil G


Research Article


Table 1. Impedance Data of Pencil-Graphite Working Electrodes with a Pt Counter Electrode and a Ag/AgCl Counter Electrode as a Function of Potential in 15 mV Steps from 0 to +1.7 V (vs. Ag/AgCl) in Na2SO4 (1 M) Aqueous Solution hardness 4B

water electrolysis before at 5 V at 10 V

4H

before at 5 V at 10 V

annealing

Rs [Ω]

before after before after before after before after before after before after

CPE-T [F] −5

4.9 × 10 0.077 0.0058 0.055 0.0034 0.0052 5.7 × 10−5 0.019 0.0046 0.0061 0.0030 0.0034

10.18 2.42 1.07 0.97 1.12 1.14 5.57 7.58 1.14 1.37 1.44 0.99

■

CPE-P

Rp [Ω]

0.73 0.57 0.87 0.89 0.93 0.87 0.62 0.54 0.80 0.83 0.93 0.85

144.5 4.56 0.54 1.04 0.51 0.89 428.9 7.81 1.40 1.00 0.81 1.28

picture and a photograph (Figure S3); cyclic voltammetry curves using three pencil-graphite electrodes (Figure S4); Raman spectra of surface of pencil-graphite electrodes (Figure S5); and FT-IR spectra of pencilgraphite electrodes (Figure S6) (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +81-79-267-4908. Fax: +81-79-267-4885. ORCID

Seigo Ito: 0000-0002-8582-5268 Notes

The authors declare no competing financial interest.

■

Figure 12. Effect of flame annealing for water electrolysis on pencil graphite: (a) the original material as a PGR with resin; (b) initial condition of the water electrolysis without annealing (not so active, but the surface polymer was removed); (c) after 50 min water electrolysis (the surface was decomposed to particulates, but the rod bulk was stable due to the polymer inside); (d) annealing of PGR by fire-flame annealing; (e) PGR after annealing without resin; (f) initial condition of the water electrolysis after annealing (very active); and (g) after 50 min water electrolysis using annealed PGR (the PGR decomposed to a tree-like structure).

graphite in order to use them in experiments. By using the fireflame annealed pencil-graphite electrode, it is possible to supply very inexpensive graphite electrodes for the study of catalyst addition on the graphite electrode. This report could help electrochemical research works in the future.

■

REFERENCES

(1) IRENA. Onshore Wind Power Now as Affordable as Any Other Source, Solar to Halve by 2020. IRENA Assembly Press Release; International Renewable Energy Agency (IRENA): 2018. (2) Saba, S. M.; Müller, M.; Robinius, M.; Stolten, D. The investment costs of electrolysis e A comparison of cost studies from the past 30 years. Int. J. Hydrogen Energy 2018, 43, 1209−1223. (3) Schmidt, O.; Gambhir, A.; Staffell, I.; Hawkes, A.; Nelson, J.; Few, S. Future cost and performance of water electrolysis: An expert elicitation study. Int. J. Hydrogen Energy 2017, 42, 30470−30492. (4) Vincent, I.; Bessarabov, D. Low cost hydrogen production by anion exchange membrane electrolysis: A review. Renewable Sustainable Energy Rev. 2018, 81, 1690−1704. (5) Akanda, M. R.; Sohail, M.; Aziz, M. A.; Kawde, A.-N. Recent Advances in Nanomaterial-Modified Pencil Graphite Electrodes for Electroanalysis. Electroanalysis 2016, 28, 408−424. (6) Kawde, A.-N.; Baig, N.; Sajid, M. Graphite pencil electrodes as electrochemical sensors for environmental analysis: a review of features, developments, and applications. RSC Adv. 2016, 6, 91325− 91340. (7) David, I. G.; Popa, D.-E.; Buleandra, M. Pencil Graphite Electrodes: A Versatile Tool in Electroanalysis. J. Anal. Methods Chem. 2017, 2017, 1. (8) Justice Babu, K.; Sheet, S.; Lee, Y. S.; Gnana Kumar, G. ThreeDimensional Dendrite Cu−Co/Reduced Graphene Oxide Architectures on a Disposable Pencil Graphite Electrode as an Electrochemical Sensor for Nonenzymatic Glucose Detection. ACS Sustainable Chem. Eng. 2018, 6, 1909−1918. (9) Pala, B. B.; Vural, T.; Kuralay, F.; Cırak, T.; Bolat, G.; Abacı, S.; Denkbas, E. B. Disposable pencil graphite electrode modified with peptidenanotubes for Vitamin B12 analysis. Appl. Surf. Sci. 2014, 303, 37−45.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b04688. Pictures at flame annealing of graphite carbon rods (Figure S1); schematic figure and photograph of cyclic voltammetry of PGR electrodes with/without flame annealing at a scan rate of 25 mV s−1 and current densities within a potential range of −2.0 to 2.0 V (vs. Ag/AgCl) in Na2SO4 (1 M) aqueous electrolyte (Figure S2); water electrolysis using a Hoffman electrolysis apparatus (H-shaped test tubes) with a schematic H


Research Article

ACS Sustainable Chemistry & Engineering (10) Eksin, E.; Zor, E.; Erdem, A.; Bingol, H. Electrochemical monitoring of biointeraction by graphene-based material modified pencil graphite electrode. Biosens. Bioelectron. 2017, 92, 207−214. (11) Vishnu, N.; Gopalakrishnan, A.; Badhulika, S. Impact of intrinsic iron on electrochemical oxidation of pencil graphite and its application as supercapacitors. Electrochim. Acta 2018, 269, 274−281. (12) Hür, E.; Arslan, A. New electrode active materials for supercapacitors: Pencil graphite electrode coated with cobalt ion doped poly(3-methylthiophene) and poly(3,4-ethylenedioxythiophene). Synth. Met. 2014, 193, 81−88. (13) Arslan, A.; Hür, E. Supercapacitor Applications of Polyaniline and Poly(N-methylaniline) Coated Pencil Graphite Electrode. Int. J. Electrochem. Sci. 2012, 7, 12558−12572. (14) Kayan, D. B.; Koçak, D.; Ilhan, M.; Koca, A. Electrocatalytic hydrogen production on a modified pencil graphite electrode. Int. J. Hydrogen Energy 2017, 42, 2457−2463. (15) Vural, T.; Kuralay, F.; Bayram, C.; Abaci, S.; Denkbas, E. B. Preparation and physical/electrochemical characterization of carbon nanotube−chitosan modified pencil graphite electrode. Appl. Surf. Sci. 2010, 257, 622−627. (16) Arai, H.; Banzai, S.; Mitsubishi Pencil Co., Ltd. U.S. Patent 2017/0174923 A1, June 22, 2017. (17) Navratil, R.; Kotzianova, A.; Halouzka, V.; Opletal, T.; Triskova, I.; Trnkova, L.; Hrbac, J. Polymer lead pencil graphite as electrode material: Voltammetric, XPS and Raman study. J. Electroanal. Chem. 2016, 783, 152−160. (18) Niibe, M.; Muaki, M.; Miyamoto, S.; Shoji, Y.; Hashimoto, S.; Ando, A.; Tanaka, T.; Miyai, M.; Kitamura, H. Characterization of Light Radiated from 11m Long Undulator. AIP Conf. Proc. 2003, 705, 576−579. (19) Gregg, S. J.; Sing, K. S. W. Adsorption Surface Area and Porosity, 2nd ed.; Academic Press INC.: London, 1982. (20) Toriya, S.; Takei, T.; Fuji, M.; Chikazawa, M. Characterization of silica-pillared derivatives from aluminum-containing kanemite. J. Colloid Interface Sci. 2003, 268, 435−440. (21) Ferrari, A. C.; Robertson, J. Interpretation of Raman spectra of disordered and amorphous graphite. Phys. Rev. B: Condens. Matter Mater. Phys. 2000, 61, 14095−14107. (22) Ferrari, A. C.; Meyer, J. C.; Scardaci, V.; Casiraghi, C.; Lazzeri, M.; Mauri, F.; Piscanec, S.; Jiang, D.; Novoselov, K. S.; Roth, S.; Geim, A. K. Raman Spectrum of Graphene and Graphene Layers. Phys. Rev. Lett. 2006, 97, 187401−1−4. (23) McCreery, R. L. Advanced Carbon Electrode Materials for Molecular Electrochemistry. Chem. Rev. 2008, 108, 2646−2687. (24) Pimenta, M. A.; Dresselhaus, G.; Dresselhaus, M. S.; Cançado, L. G.; Jorio, A.; Saito, R. Studying disorder in graphite-based systems by Raman spectroscopy. Phys. Chem. Chem. Phys. 2007, 9, 1276− 1290. (25) Stephan, A. M.; Kumar, T. P.; Renganathan, N. G.; Pitchumani, S.; Thirunakaran, R.; Muniyandi, N. Ionic conductivity and FT-IR studies on plasticized PVC/PMMA blend polymer electrolytes. J. Power Sources 2000, 89, 80−87. (26) Alkan, C.; Sarı, A.; Uzun, O.; Karaipekli, A. Preparation, characterization, and thermal properties of microencapsulated phase change material for thermal energy storage. Sol. Energy Mater. Sol. Cells 2009, 93, 143−147. (27) Melo, M. J.; Bracci, S.; Camaiti, M.; Chiantore, O.; Piacenti, F. Photodegradation of acrylic resins used in the conservation of stone. Polym. Degrad. Stab. 1999, 66, 23−30. (28) Duan, G.; Zhang, C.; Li, A.; Yang, Y.; Lu, L.; Wang, X. Preparation and Characterization of Mesoporous Zirconia Made by Using a Poly (methyl methacrylate) Template. Nanoscale Res. Lett. 2008, 3, 118−122. (29) Muramatsu, Y.; Gullikson, E. M. Peak Intensity Ratios Between the π* and σ* Peaks in the Total-Electron-Yield CK-XANES of Graphite Materials; Mixed Particle Systems and Molecular Systems Composed of the sp2 and sp3 Graphite Atoms. Adv. X Ray Anal, Japan 2012, 43, 425−36. Article in Japanese.

(30) Brandes, J. A.; Cody, G. D.; Rumble, D.; Haberstroh, P.; Wirick, S.; Gelinas, Y. Carbon K-edge XANES spectromicroscopy of natural graphite. Carbon 2008, 46, 1424−1434. (31) Abbas, M.; Wu, Z. Y.; Zhong, J.; Ibrahim, K.; Fiori, A.; Orlanducci, S.; Sessa, V.; Terranova, M. L.; Davoli, I. X-ray absorption and photoelectron spectroscopy studies on graphite and single walled carbon nanotubes: Oxygen effect. Appl. Phys. Lett. 2005, 87, 051923. (32) Fedoseeva, Y. V.; Pozdnyakov, G. A.; Okotrub, A. V.; Kanygin, M. A.; Nastaushev, Y. V.; Vilkov, O. Y.; Bulusheva, L. G. Effect of substrate temperature on the structure of amorphous oxygenated hydrocarbon films grown with a pulsed supersonic methane plasma flow. Appl. Surf. Sci. 2016, 385, 464−471. (33) Wu, Y.; Fu, H.; Roy, A.; Song, P.; Lin, Y.; Kizilkaya, O.; Xu, J. Facile one-pot synthesis of 3D graphite−SiO2 composite foam for negative resistance devices. RSC Adv. 2017, 7, 41812−41818. (34) Wang, Y. F.; Singh, S. B.; Limaye, M. V.; Shao, Y. C.; Hsieh, S. H.; Chen, L. Y.; Hsueh, H. C.; Wang, H. T.; Chiou, J. W.; Yeh, Y. C.; Chen, C. W.; Chen, C. H.; Ray, S. C.; Wang, J.; Pong, W. F.; Takagi, Y.; Ohigashi, T.; Yokoyama, T.; Kosugi, N. Visualizing chemical states and defects induced magnetism of graphene oxide by spatiallyresolved-X-ray microscopy and spectroscopy. Sci. Rep. 2015, 5, 15439. (35) Rabah, M. A.; Abdul Azim, A. A.; Ismail, A. Wear of graphite anodes during electrolysis of acid sulphate solutions. J. Appl. Electrochem. 1981, 11, 41−47. (36) Pei, S.; Wei, Q.; Huang, K.; Cheng, H.-M.; Ren, W. Green synthesis of graphene oxide by seconds timescale water electrolytic oxidation. Nat. Commun. 2018, 9, 145.

I


Water Electrolysis using Flame-Annealed Pencil-Graphite Rods - ACS

Recommend Documents