Chapter 3: Biochar-Assisted Water Electrolysis
3.3 Evaluation of continuous electrolysis
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where nH2, θc, I and F are the rate of H2 evolution (mol/s), current efficiency, current (A) and Faraday’s constant (96485 C/mol), respectively. Though not shown data here, θc is almost 100% due to the pure water electrolysis result at the same experimental conditions (3 mol/L H2SO4 aq.). The time-integrated nH2 agreed well with that calculated within an error of ±3.5%. Thus, the H2 evolution in the continuous electrolysis followed the stoichiometry (anode reaction) and Faraday’s law. Due to calculation of efficiency η by Equation 2-1, theoretical E of water electrolysis at 25 °C is 1.23 V, and it corresponds to η of 1.02. The value of η in BAWE at E = 1 V is 1.18. This value of η proves that the integration of the chemical energy of biochar into that of H2 together with the electricity.
2. The trend of the CO2 formation is much different from that of the H2 formation. The rate of CO2 formation is much lower than that expected from the stoichiometry. Against the CO2/H2 ratio of 1/2 in the stoichiometry, that was only 1/25 in the continuous electrolysis. This does not mean the occurrence of normal electrolysis of water (2H2O → 2H2 + O2) because no O2 was evolved at the anode. It was hence suggested that the water-derived oxygen was chemically incorporated into the biochar.
3. The current decreases monotonously with time. The current is initially 0.11 A, and it decreases to 0.06 A at the end of the run (time; 100 min, total current; ca., 473 C). This shows that the electrochemical reactivity of the biochar was lost during BAWE.
Above results allow us to propose the mechanism of the reaction on anode as shown in Figure 3-10. At relatively low interelectrode potential (E = 1 V) of BAWE, it is believed that biochar was mainly electrochemically oxidized to oxygen-functional groups. Though those oxidations partially evolved as CO2 during continuous electrolysis, large portion still remained at the surface and prevented the further oxidation of biochar.
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Figure 3-10. Proposed mechanism for BAWE
Assuming that the deactivation of the biochar was due to the accumulation of oxygen-functional groups and decrease in the amount of ‘active’ carbon, the elemental compositions of the fresh and spent biochars were measured and compared. The data shown in Table 3-4 prove the accumulation of oxygen. By measuring the mass before and after continuous electrolysis, in the electrolysis with 5000 C, the amount of carbon evolved as CO2 was 5.1% (mol-C/100-mol-C-initial-char). The spent char (5000 C) had an O/C atomic ratio of 0.17. This corresponded to the accumulation of 8.5 mol-O per 100-mol-C of the biochar subjected to the BAWE. It was also confirmed that the heat treatment successfully removed such oxygen probably as CO and CO2.
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Table 3-4. Elemental compositions of biochar with TC = 850 °C and those after the continuous BAWE and subsequent heat treatment (HT)
Elemental composition (wt%-daf)
Atomic ratio (–)
Biochar C H Oa H/C O/C
Fresh (TC = 850 °C) 90.2 0.67 8.7 0.089 0.073
After 1000 C 86.7 0.66 12.2 0.091 0.11
After 5000 C 80.9 0.82 17.9 0.12 0.17
After 5000 C and HT at 250 °C 85.3 0.79 13.5 0.11 0.12
After 5000 C and HT at 500 °C 90.6 0.50 8.6 0.066 0.071
After 5000 C and HT at 850 °C 93.1 0.38 5.9 0.048 0.049
a by difference
The electrochemical reactivity of spent biochars with and without heat treatment were evaluated by LSV, of which profiles are shown in Figure 3-11. As expected from a trend seen in Figure 3-7, i.e., the continuous decrease in the current, the reactivity of the spent biochars is clearly lower than that of the fresh one. Eonset seems to become greater as the total current in the BAWE increases. It is also noted that the heat treatment recuperates the biochar by removing oxygen functional groups. The heat treatment at higher temperature is more effective, and that at 850 °C fully reproduces the fresh char. It also seems that the biochar recuperated by 850 °C heat treatment undergoes the oxidation (current occurrence) even at E = 0.2–0.5 V. The reheat treatment of biochar thus recuperated it regenerating active aromatic carbon.
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Figure 3-11. Change in LSV profiles for five types of biochars with different electrochemical and/or thermal histories
The spent biochars from the continuous electrolysis runs were analyzed by NMR, FT-IR and TGA. It was found that detection of O-containing groups as well as their identification was difficult by NMR. FT-IR result is shown in Figure 3-12. After 1000 and 5000 C continuous electrolysis, spectra shows slightly difference at the range from 1800 cm-1 to 1000 cm-1, but not significant. Difference in this range is generally considered to the growth of O-containing groups such as carboxylic compounds3. Peak observed at 1710 cm-1 is reasonably ascribed to the C=O, while peaks from range of 1600 cm-1 to 1500 cm-1 is considered to be the C=C bond. Moreover, TGA successfully detected those groups that were released upon the biochar heating. Figure 3-13 exhibits DTG profiles for the fresh and spent biochars. The rate of mass release from the spent biochars is maximized around 250 °C and 750 °C. The mass release around 250 °C is
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attributed to decomposition of carboxylic groups and their conversion into CO24, 5. On the other hand, that around 750 °C is reasonably ascribed to CO formation from decomposition of ether and carbonyl groups bonded to aromatic carbon6, 7.
Figure 3-12. FT-IR for fresh biochar (TC = 850 °C) and spent chars from continuous electrolysis run with 1000 C and 5000 C
Figure 3-13. DTG profiles for heating fresh biochar (TC = 850 °C) and spent chars from continuous electrolysis with 1000 C and 5000 C. Heating rate; 5 °C/min
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It is believed that most of oxygen introduction, as much as 8.5 mol-O/100-mol-C, occurred at surfaces of meso-/micro-pores of the biochar, in other words, intra-particle surfaces. The outer surface area, which is as small as the particle’s geometric surface area of particle, is negligibly small compared with that arisen from meso-/micro-pores. It is thus confirmed that the accumulation of oxygen occurred in the pore system of the biochar.
The intra-particle oxidation is a type of the indirect oxidation, which needs the presence or abundance of the reactive mediator that can diffuse into pores and react with active carbon sites of the biochar. The oxidation was not enough to form CO2 but leaving oxygen functional groups, further oxidation of which was difficult under the present experimental conditions. It is known that oxidative degradation of aromatic rings in liquid phase forms not only CO2 but also carboxylic and other O-containing groups directly bonded to aromatic carbon, and their further oxidation into CO2 is difficult8. Such functional groups are, however, labile at elevated temperature, and easily decomposed to CO2 and CO. The heat treatment was therefore effective for the recuperation of the biochar.