Carbon Surface Characteristics after Electrochemical Oxidation in a

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Carbon surface characteristics after electrochemical oxidation in a direct carbon fuel cell using a single carbon pellet and molten carbonates Hirotatsu Watanabe, Tomoaki Furuyama, and Ken Okazaki Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b00978 • Publication Date (Web): 02 Jul 2015 Downloaded from http://pubs.acs.org on July 5, 2015

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Carbon surface characteristics after electrochemical oxidation in a direct carbon fuel cell using a single carbon pellet and molten carbonates Hirotatsu Watanabe1*, Tomoaki Furuyama1 and Ken Okazaki1 1

Department of Mechanical and Control Engineering, Graduate School of Science and

Engineering, Tokyo Institute of Technology, 2-12-1-I6-7, Ookayama, Meguro-ku, Tokyo 1528552 Japan, Tel: +81-3-5734-2179, Fax: +81-3-5734-2179, Email: [email protected] KEYWORDS: direct carbon fuel cell, anode reaction, single carbon, observation after discharge ABSTRACT. This study investigates carbon surface characteristics after discharge in a direct carbon fuel cell using molten carbonates. In the experiment, the top surface of a single carbon pellet is pressed on the Au electrode submerged in molten carbonates, and discharge is started at a constant current. First, the effect of the press force of the carbon pellet on voltage was investigated. With decreasing press force, voltage reduction was observed at high current, whereas the voltage remained constant at low current. Then, after the pellet was washed, its top and side surfaces after discharge were observed and analyzed. In Auger electron spectroscopy spectra, Na peaks were not detected on the carbon surface after discharge, indicating that the washing of the pellet was sufficient to remove the carbonates. On the top surface that was in

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contact with the electrode, surface roughness parameters decreased after discharge, indicating that the surface became smooth after discharge. In addition, changes in surface morphologies were observed after discharge; smooth hemispherical particles were observed after discharge, whereas angular particles and edges were observed before discharge. Moreover, cleavages were observed after discharge. Meanwhile, on the side surface that was not in contact with the electrode, the only observed changes were thermal cracks caused by exposure to high temperature molten carbonates. The results suggest that hemispherical particles were formed after discharge because of the combination of press force and electrochemical oxidation on the flat Au electrode, which caused edges to be smoothed during discharge. Observations indicated that the triple phase boundary (carbon, electrolyte, and anode) played an important role in changing carbon surface morphologies during discharge.

1. Introduction New high-efficiency technologies for the conversion of coal to electricity are required for clean energy production and sustainable energy supply. Direct carbon fuel cells (DCFCs) have attracted much attention because of their 100 % theoretical efficiency. Many researchers have developed DCFCs with a carbon/electrolyte slurry that improves to form a triple phase boundary (carbon, electrolyte, and anode) when pulverized carbon particles are used [1-8]. Molten carbonates have often been used as an electrolyte because of numerous advantages such as longterm stability in CO2 and high ionic conductivity [1]. Although a wide range of designs and concepts have been tested, overall effort to develop DCFC technology has been relatively small in comparison with other major fuel cell technologies [1].


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In particular, further investigation is required to understand the electrochemical oxidation of carbon. Supported by some indirect evidence, a mechanism for the anode oxidation of carbon has been proposed and summarized in recent studies [9]. Electrochemical oxidation, which releases four electrons per carbon (as given by R1), and cathode (R2) and overall reaction (R3) are expressed as follows: C + 2 CO32- → 3 CO2 + 4e-

(R1)

O2 + 2 CO2 + 4e- → 2 CO32-

(R2)

C + O2 → CO2

(R3)

(E0 = 1.03 V at 1073 K)

In addition to reaction mechanisms, molten carbonate transfer to the reaction site is important in improving the performance of a DCFC using the carbon/carbonate slurry. Observation and analysis of carbon fuel after discharge help to understand the electrochemical oxidation of carbon in a DCFC. Recently, some studies have observed carbon particles after discharge in a DCFC using the carbon/electrolyte slurry. Guo et al. showed that carbon particle size decreased, and cleavages between layered structures became larger with carbon consumption in a DCFC using molten NaOH [10]. Li et al. used carbon nanofibers (CNFs) in a DCFC using molten carbonates, and observed the morphology of CNFs after discharge [11]. In their studies, the oxidation of CNFs resulted in short and sintered filamentous fragments. However, finding the carbon reaction site is difficult in a DCFC using the carbon/electrolyte slurry because a large number of small carbon particles exist in the slurry. Thus, another approach is required for detailed observation and analysis of the carbon fuel after discharge. In this study, the electrochemical oxidation of carbon in molten carbonates is studied using a single carbon pellet that presses on the Au electrode in the carbonates during discharge.


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Experiments using a single carbon pellet yield detailed observation and analysis. The effect of the press force of the pellet on the electrode is investigated, and the carbon surface is observed before and after electrochemical oxidation.

2. Experimental section Figure 1 shows a schematic of a DCFC using a single carbon pellet in molten carbonates. In this study, the working electrode (WE), counter electrode (CE), and reference electrode (RE) were made from gold sheet. RE was used in this study. The area in contact with molten carbonate was 1.0 cm2 for CE and RE. Details of WE will be shown later. Each gold sheet was spotwelded to a gold wire and extended to connect to the Potentiostat/Galvanostat (HAL-3001, Hokuto Denko). The gas feed tubes for the CE and RE were high purity alumina (outer diameter: 8 mm, inner diameter: 5 mm), and these were supported by silicone plugs attached to the top of a quartz reactor (outer diameter: 100 mm, inner diameter: 95 mm). A high purity alumina cruicible (outer diameter: 87 mm, inner diameter 82 mm) was used as a holder for carbonate. CO32produced by the cathode reaction transports to the anode side, and it is consumed by the anode reaction. The anode compartment was separated from the electrolyte by a porous alumina tube with an average pore diameter of 95.6 nm. In this experiment, a single carbon pellet was used as fuel in DCFC. The single carbon pellet with a diameter of 4.5 mm was used as fuel in the DCFC. Figure 2 shows the pellet attached to the tip of a high purity alumina tube (a) and the magnified image of the top surface of the pellet before discharge (b). The alumina tube with the pellet was attached to a force gauge mounted on a traverse system that moves vertically. This system can press carbon on the WE at a specific force. The carbon surface before and after electrochemical


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oxidation can be easily analyzed in this system, contrary to the DCFC cell using the carbon/carbonate slurry. Commercial activated carbon pellets (charcoal activated, 01084-12, Kanto Chemical Co. Inc.) were used in this study. Table 1 shows the elemental analysis (dry-base) and ash content of samples. The top surface of the pellet was roughly shaved to creat a flat surface. Table 2 shows the apparent density (ρa), true density (ρt), and porosity (ε) of activated carbon pellets. Alumina crucible and porous alumina tube contained 112.5 g and 4.95 g of dry ternary carbonate powder (Li2CO3:Na2CO3:K2CO3 = 1:4:4 (weight ratio)), respectively. Before melting the carbonate powder, the WE, CE, and RE compartments were not inserted into the porous alumina tube and alumina crucible. After the fuel cell was assembled, the reactor was heated by an electric furnace to 1103 K. During heating, Ar (200 ml/min) was introduced into the reactor. After melting the carbonates, the CE and RE compartments were inserted into the molten carbonates in the alumina crucible, and O2 (50 ml/min) and CO2 (100 ml/min) gases were introduced. Then, the single carbon pellet attached to the alumina tube was pressed against the Au electrode in the molten carbonates. Press force (F) was held constant during discharge at uniform current flow (I). Several studies have reported that solid oxide based DCFCs, in which the carbon was in a direct contact with the anode surface [6,12,13]. The DCFC cell developed in this study was categorized to physical contact type DCFC in the molten carbonate based cells. To study the carbon surface after electrochemical oxidation, the single carbon pellet was pressed against the WE at 1.0 N, and the current was set to 10 mA. After a 12 min discharge, the carbon pellet was washed thoroughly by distillated hot water to remove carbonates and then dried. Thereafter, the top surface (contact surface) and side surface were analyzed using Auger electron spectroscopy (AES, JAMP-9500F, JEOL). AES analysis was performed with an


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accelerating voltage of 10.0 keV. Carbon surfaces before inserting carbonates were also observed to ascertain changes before and after discharge. Moreover, the top surface of the pellet was examined with laser scanning confocal microscopy (VK-X250, KEYENCE) to obtain 3D images and surface roughness parameters. Observation area was 1.9 mm x 1.4 mm around the center of the top surface of the pellet before and after discharge. This color laser microscope, which used violet laser with the wavelength of 408 nm, achieved a plane coordinates resolution of 1 nm and a height resolution of 0.5 nm. Figure 3 shows the schematic of the carbon pellet and Au electrode for WE in this experimental system. Changes to the top surface after discharge were caused not only by the electrochemical oxidation but also through exposure to high temperature molten carbonates. Meanwhile, changes to the side surface after discharge were caused only by exposure to high temperature molten carbonates. Therefore, changes due to the electrochemical oxidation were studied by comparing the top and side surfaces before and after discharge. When the side surface of the carbon pellet was observed, it was at point 2 mm from the top surface.

3. Results and discussion Figure 4 shows a photograph of the anode compartment taken from the top of the reactor during discharge. In this observation, the carbon pellet attached to the tip of the alumina tube presses on the Au electrode at 1.3 N. When the current is set to 50 mA (Fig. 4a) or 80 mA (Fig. 4b), the voltage indicates 0.22 V or 0.07 V, respectively. Although the carbon pellet is not observed because of its position in the alumina tube, bubble generation is continuously observed near the tip of the tube. Because bubble generation is a result of the electrochemical oxidation of carbon, the amount of bubbles increases with increasing current.


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Figure 5 shows the effect of press force on the voltage under different currents. The potential difference between RE and WE is defined as voltage. Voltage variation at the same press force is within 5 % at 40 mA. When the current is 20 mA, the voltage remains constant at various forces. This implies that the effect of press force on voltage does not appear in this force range. However, when the current is 40 mA, voltage reduction appears below 0.5 N. In this experiment, carbon had a roughness surface. Thus, voltage reduction observed at low press force comes from increasing contact resistance between carbon and the electrode, or decreasing contact area between the carbon pellet and electrode. The voltage reduction from contact resistence becomes insignificant at lower current. A key finding from this experiment is that voltage is almost constant over 1.0 N even at high current. Therefore, the press force is set to 1.0 N in subsequent tests. Figure 6 shows a constant current discharge curve at I = 10 mA and F = 1.0 N. After the voltage drop due to an overpotential, voltage remains constant at 0.53 V. After discharge for 12 min and washing the carbon pellet, the top and side surfaces are investigated, as shown in Figs. 7, 9 and 10. Figure 7 shows the AES spectra of the top surface of the carbon pellet after discharge. We can detect surface elements from the AES spectra, and the measuring area of the spectra is 1 µm × 1 µm. Only two peaks are observed in the AES spectra; these peaks indicate C and O. Finding here is that Na peaks that appear at 990 eV [14] are not detected on the carbon surface after discharge. In addition to AES analysis shown in Fig. 7, 12 positions on the carbon surface are randomly analyzed using AES, and peaks other than C and O are not detected on the carbon surface after discharge. This indicates that the washing of carbon after discharge by distillated hot water is sufficient to remove the carbonates.


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Figures 8 and 9 show the 3D surface measurement of the top surface of the pellet (a) and its height displacement from averaged value along vertical direction (b) before and after discharge, respectively. The color scale indicates the height displacements of the top surface. It is shown that the top surface becomes flat after discharge. Considering the 3D surface, it is difficult to estimate the exact contact area in this study; however, the contact area is supposed to be changed by the pressure force as described above. Surface roughness parameters of the top surface of the pellet are shown in Table 3. Table 3 shows the cross section area (Ac), the surface area (As), the ratio of surface area to cross-sectional area (As/Ac), and the 3D surface roughness (Sa), which is defined as the average roughness evaluated over 3D measured surface. As/Ac and Sa decrease after discharge, indicating that the surface roughness becomes smooth after discharge. Table 4 shows surface roughness parameters of Au electrode used in this study. Compared with the carbon pellet, the surface of Au electrode is quite smooth and flat. Figure 10 shows secondary electron images of the contact surface of the carbon pellet before (a) and after (b) discharge for 12 min at I = 10 mA and F = 1.0 N. Two major differences are observed in surface morphologies before and after discharge. One difference is that smooth hemispherical particles are found on the surface after discharge, whereas angular particles and edges are found before discharge. The electrochemical oxidation by R1 progressed on the flat Au electrode, resulting in smooth edges after discharge. These results suggested that smooth hemispherical particles form during discharge because of the combination of the press force and reaction on the Au electrode. This corresponds to the change of parameters of the top surface of the pellet before and after discharge as shown in Table 3. Another difference is that cleavage is observed after discharge, whereas it is not observed before discharge. Carbon appears to be consumed from the border in this cleavage.


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Other than electrochemical oxidation (R1), carbonate decomposition (R4) and carbothermic reaction (R5) are considered to be occur in the anode compartment. M2CO3 → M2O + CO2

(R4)

C + M2CO3 → M2O + CO2

(R5)

where M is Li, Na, and K. Both reactions produce CO2. Decomposition mechanism of Na2CO3 and Li2CO3 has been studied using thermogravimetric analysis [15], and it was shown that Li2CO3 was slowly decomposed at 1073 K. In this study, the bubble generation on the Au was rarely observed at OCV condition, whereas it was continuously observed at current load condition as shown in Fig. 4. These indicate that the carbon pellet is mainly consumed by not the carbonate reactions (R4 and R5) but the electrochemical reaction (R1) in this study. If the carbon pellet is consumed by the anode reaction which releases four electrons (R1), the carbon consumption rate (Qc) is given as:

Qc =

I 4F

(1)

where F is the Faraday constant (96,450 C mol-1) and I is the constant current flow (C s-1). When a pellet diameter is 4.5 mm and oxidation time is 12 min at 10 mA, carbon pellet length is decreased by 21 µm after discharge. The decrease by 21 µm is sufficient to change carbon surface morphologies; however, it is difficult to compare this estimation (21 µm) with the experiment shown in Figs 8 and 9 because the apparent density varies in such a short length. Figure 11 shows the mechanism of how carbon surface morphologies change during discharge. Because the carbon pellet surface is not smooth, the molten carbonates easily


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penetrate into the clearance between the pellet and Au electrode. The reaction process of electrochemical carbon consumption is summarized in previous papers [2,3,7,8]. This is briefly summarized here. First, the reaction is initiated by the dissociation of carbonate to form O2- (for example, R6). Then, O2- adsorbs on the reactive carbon surface (CRS) and forms a strongly bound CO functional group (CRSO) (R7). Then, O2- absorbs at CRSO (R8), and CO2 is produced electrochemically (R9). CO32- → CO2 + O2-

(R6)

CRS + O2- → CRSO + 2e-

(R7)

CRSO + O2- → CRSO22-

(R8)

CRSO22- → CO2 + 2e-

(R9)

Carbon is mainly consumed through above reactions (overall reaction is given as R1) at the triple phase boundary (carbon, electrolyte, and anode) that is supposed to form at the carbon edges in contact with the Au electrode. This forms a smooth carbon surface. When carbon is consumed near the hole, cleavage forms, as shown in Fig. 9b. This also indicates that the cleavage becomes larger with increasing carbon consumption at the cleavage border. Previously, mesopores of carbon have been reported to enhance electrolyte transfer, resulting in a high power density in the DCFC [4]. Holes observed in Fig. 8a are quite larger than mesopores; however, the molten carbonates are expected to easily penetrate into the holes, thereby enhancing triple phase boundary formation on the border if the electrode is in contact with the holes. This leads to advancing carbon consumption by electrochemical oxidation. The triple phase boundary is shown to play an important role in changing carbon surface morphologies during discharge.


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Figure 12 shows secondary electron images of the side surface of the carbon pellet. The carbon pellet shown here is the same as that shown in Fig. 10. Contrary to the top surface (Fig. 10), differences in the side view before (Fig. 12a) and after discharge (Fig. 12b) are caused only by exposure to the high temperature molten carbonates. In fact, the electrochemical oxidation is not advanced on the side surface. A thermal crack appears on the side surface, and the carbon layer appears to be removed because the carbon pellet is inserted into high temperature molten carbonates. If the same heat transfer was given to the pellet, similar thermal crack is supposed to occur without molten carbonates. However, cleavages and smooth surfaces observed on the top surface (Fig. 10b) are not found on the side surface. This indicates that changes observed in Fig. 10 are not caused by exposure to the high temperature molten carbonates but by the electrochemical oxidation of carbon. In the DCFCs using carbon/carbonate slurry [e.g. 4,5,8], the carbon is also contact with the flat electrode; therefore, similar changes of surface morphologies are expected to occur during discharge in the DCFC.

4. Conclusion In this study, the electrochemical oxidation of carbon in molten carbonates was studied using a single carbon pellet. First, the effect of press force on voltage was investigated. With decreasing press force, voltage reduction was observed at high current, whereas voltage remained constant at low current. Then, the top and side surfaces of the carbon pellet after discharge were observed and analyzed. On the top surface, surface roughness parameters decreased after discharge, indicating that the surface became smooth after discharge. Smooth hemispherical particles were observed on the top surface after discharge, whereas angular particles and edges were observed


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before discharge. In addition, cleavages were formed on the top surface after discharge. A comparison of the top and side surfaces of the carbon pellet before and after discharge showed that these changes were caused by the electrochemical oxidation of carbon. Hemispherical particles were suggested to be formed because of the combination of the press force and electrochemical oxidation on the flat Au electrode, which created smooth edges after discharge. Observations indicate that the triple phase boundary (carbon, electrolyte, and anode) played an important role in changing the carbon surface during discharge.

ACKNOWLEDGMENT This study was partly supported by a JSPS Grant-in-Aid for challenging Exploratory Research (No. 25630064), and J-Power. Elemental analyses and AES analysis were performed at the Center for Advanced Material Analysis at Tokyo Institute of Technology.

Reference [1] Giddey S, Badwal SPS, Kulkarni A, Munnings C. A comprehensive review of direct carbon fuel cell technology. Progress Energy and Combustion Science 2012;38:360-399. [2] Rady AC, Giddey S, Badwal SPS, Ladewig BP, Bhattacharya S. Review of fuels for direct carbon fuel cells. Energy and Fuels 2012;26,1471-1488.


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[3] Cherepy NJ, Krueger R, Fiet KJ, Jankowski AK, Cooper JF, Direct conversion of carbon fuels in a molten carbonate fuel cell. J. Electrochem. Soc. 2005;152,A80-A87. [4] Li X, Zhu ZH, Marco RD, Dicks A, Bradley J, Liu S, Lu GQ. Factors that determine the performance of carbon fuels in the direct carbon fuel cell. Ind. Eng. Chem. Res. 2008;47:9670-9677. [5] Li X, Zhonghua Z, Chen J, Marco RD, Dicks A, Bradley J, Lu G. Surface modification of carbon fuels for direct carbon fuel cells, J. Power Sources 2009;186:1-9. [6] Li C, Shi Y, Cai N. Mechanism for carbon direct electrochemical reactions in a solid oxide electrolyte direct carbon fuel cell. J. Power Sources 2011;196:754-763. [7] Wang CQ, Liu J, Zeng J, Yin JL, Wang GL, Cao DX, Significant improvement of electrooxidation performance of carbon in molten carbonates by the introduction of transition metal oxides. J. Power Sources, 2013;233:244-251. [8] Watanabe H, Furuyama T, Okazaki K. Enhancing the efficiency of direct carbon fuel cells by bubbling Ar gas in carbon/carbonate slurry, J. Power Sources 2015;273:340-350. [9] Cao D, Sun Y, Wang G. Direct carbon fuel cell: Fundamentals and recent developments, J. Power Source 2007;167:250-257. [10] Gür TM. Mechanistic Modes for Solid Carbon Conversion in High Temperature Fuel Cells. J. Electrochem. Soc. 2010;157:B751-B759. [11] Guo L, Calo JM, DiCocco E, Bain EJ. Development of a low temperature, molten hydroxide direct carbon fuel cell, Energy & Fuels 2013;27:1712-1719. [12] Li X, Zhonghua Z, Marco RD, Bradley J, Dicks A. Carbon nanofibers synthesized by catalytic decomposition of methane and their electrochemical performance in a direct carbon fuel cell, Energy & Fuels 2009;23:3721-3731.


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[13] Guzman F, Singh R, Chuang SC. Direct use of sulfur-containing coke on a Ni-Yttriastabilized zirconia anode solid oxide fuel cell, Energy & Fuels 2011;25:2179-2186. [14] Desclaux P, Nürnberger S, Rzepka M, Stimming U. Investigation of direct carbon conversion at the surface of a YSZ electrolyte in a SOFC, Int. J. Hydrogen Energy 2011;36:10278-10281.

[15] Pantano CG, Dove DB, Onoda GY. AES analysis of sodium in a corroded bioglass using a low temperature technique, Applied physics letters 1975;26:601-602. Table 1 Ultimate analysis of activated carbon pellet [wt%] Activated Carbon (AC) C H N S Ash

95.4 0.25 0.24 0.15 0.8

Table 2 Apparent density, true density, and porocity of activated carbon pellet apparent density (ρa) [kgm-3]

673

true density (ρt) [kgm-3]

1670

porosity (ε) [-]

0.597

Table 3 Surface roughness parameters of the top surface of the pellet

Before After

Ac [mm2] (Cross-sectional area)

As [mm2] (Surface area)

As/Ac [-] (Surface area / Cross-sectional area)

Sa [µm]

2.71 2.76

6.95 5.15

2.56 1.87

39 12


14

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Table 4 Surface roughness parameters of Au electrode

Before

As/Ac [-] (Surface area / Cross-sectional area)

Sa [µm]

1.109

0.661


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Potentio/Galvanostat Cathode

Force gauge O2/CO2 Alumina tube for gas seal Press

Ar (out)

Anode Ar (in)

Quartz reactor Alumina crucible

CO2

Porous alumina tube

Electric furnace CO32Molten carbonate

Cathode (Au) (O2 + 2 CO2 + 4e→ 2 CO32-)

Anode compartment

Anode (Au) (C + 2 CO32- → 3 CO2 + 4e-) Single carbon pellet

Fig. 1 Experimental setup


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(a)

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Carbon pellet

Alumina tube

(b)

1 mm

mm Fig. 2 Carbon pellet attached to the alumina tube (a) and magnified image of the top surface of the pellet before discharge (b)

Carbon pellet

Press

Side surface (at 2 mm from the top surface) 2 mm

Top surface (contact surface)

Au

10 mm

5 mm Fig. 3 Schematic of the carbon pellet and Au electrode in this system


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Alumina tube holding carbon particle at the tip

Porous alumina tube Au surface Bubble generation

(a) 50 mA, E = 0.22 V

Molten carbonate Bubble generation

(b) 80 mA, E = 0.07 V Fig. 4 Photograph of the anode compartment taken from the top of the reactor


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0.6

20 mA 40 mA

20 mA

0.55 0.5 Voltage [V]

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0.45

40 mA

0.4 0.35 0.3 0

0.5

1 Force [N]

1.5

2

Fig. 5 The effect of press force on voltage under different currents


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1

0.8

Voltage [V]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.6

0.4

0.2

0 0

5 10 Elapsed time [min]

15

Fig. 6 Constant current discharge curve at I = 10 mA and F = 1.0 N


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200

C

100 Auger Intensity [a.u.]

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O

0 -100 -200 -300 -400 0

200

400

600

800

1000 1200

Electron energy [eV]

Fig. 7 AES spectra of the top surface of the carbon pellet after discharge


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The top side 1.4

Vertical distance [mm]

1.2

y

z

1 0.8 0.6 0.4 0.2 0 -0.1

x (a) 3D representation of surface roughness

-0.05 0 0.05 0.1 Displacement from averaged height [mm]

(b) Height displacement from averaged value along y direction at x = 1 [mm]

Fig. 8 Surface roughness of the top surface of the pellet before discharge

The top side 1.4 1.2 Vertical distance [mm]

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y

z

1 0.8 0.6 0.4 0.2

x

0 -0.1

(a) 3D representation of surface roughness

-0.05 0 0.05 0.1 Displacement from averaged height [mm]

(b) Height displacement from averaged value along y direction at x = 1 [mm]

Fig. 9 Surface roughness of the top surface of the pellet after discharge ACS Paragon Plus Environment

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Energy & Fuels

Hole

4 m

4 m

Angular particles

Angular particles

Angular particles

2 m

2 m

Fig. 10 (a) Secondary electron images of the contact surface of the carbon pellet before discharge


Energy & Fuels

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4 m

Hemispherical particle

2 m

Border of cleavage

Cleavage

2 m

Fig. 10 (b) Secondary electron images of the contact surface of the carbon pellet after discharge at I = 10 mA and F = 1.0 N


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Energy & Fuels

Press

Molten carbonates

Press

Molten carbonates

Hole Carbon pellet

Carbon pellet

Au

Au Angular particles (triple phase boundaries are on edges)

Cleavage (carbon is consumed from the border)

(a) Before discharge

Hemispherical particles (Edges become smooth)

(b) After discharge

Fig. 11 Mechanism of how carbon surface morphologies change during discharge


Energy & Fuels

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(a) Before discharge

10 m

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(b) After discharge

10 m

Thermal crack

Fig. 12 Secondary electron images of the side surface of the carbon pellet before (a) and after (b) discharge


Carbon Surface Characteristics after Electrochemical Oxidation in a

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