Efficient Hydrogen Production by Direct Electrolysis of Waste Biomass

Subscriber access provided by READING UNIV

Article

Efficient Hydrogen Production by Direct Electrolysis of Waste Biomass at Intermediate Temperatures Takashi Hibino, Kazuyo Kobayashi, Masaya Ito, Qiang Ma, Masahiro Nagao, Mai Fukui, and Shinya Teranishi ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01701 • Publication Date (Web): 24 May 2018 Downloaded from http://pubs.acs.org on May 24, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 29 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

ACS Sustainable Chemistry & Engineering

Efficient Hydrogen Production by Direct Electrolysis of Waste Biomass at Intermediate Temperatures Takashi Hibino *†, Kazuyo Kobayashi †, Masaya Ito †, Qiang Ma †, Masahiro Nagao †, Mai Fukui ‡, Shinya Teranishi ‡ †

Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8601, Japan

‡

Soken Inc., Nisshin, Aichi 470-0111, Japan

*

E-mail: [email protected]

KEYWORDS: Waste biomass; Electrolysis; Hydrogen production; Mesoporous carbon

ABSTRACT

Biomass has been considered as an alternative feedstock for energy and material supply. However, the lack of high-efficiency and low-cost processes for biomass utilization and conversion hinders its large-scale application. This report describes electrochemical hydrogen

ACS Paragon Plus Environment

1

ACS Sustainable Chemistry & Engineering 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

Page 2 of 29

production from waste biomass that does not require large amounts of energy or high production costs. Hydrogen was produced by the electrolysis of bread residue, cypress sawdust, and rice chaff at an onset cell voltage of ca. 0.3 V, with high current efficiencies of approximately 100% for hydrogen production at the cathode and approximately 90% for carbon dioxide production at the anode. The hydrogen yields per 1 mg of the raw material were 0.1–0.2 mg for all tested fuels. Electrolysis proceeded continuously at plateau voltages that were proportional to the current. These characteristics were attributable to the high catalytic activity of the carbonyl-group functionalized mesoporous carbon for the anode reaction, and that the major components of biomass such as cellulose, starch, lignin, protein, and lipid were effectively utilized as fuels for hydrogen production.

Introduction The Paris Agreement was adopted at COP 21 in 2015, so that both developed and developing countries aim to reduce greenhouse gas emissions by tens of percent by 2030.1–3 As part of this effort, the transition from fossil fuels to biofuels has been accelerated, due to their much lower carbon footprint.4–6 However, biofuels are not yet competitive with fossil fuels in terms of cost and performance. To realize the commercialization of biofuels, various problems must be addressed, such as the high total cost for biomass utilization, low efficiency of the conversion process into fuels, and insufficient reactivity of biomass raw materials.7,8 The use of waste biomass such as agricultural and food residues, harvested weeds, and wood chips as feedstock decreases the raw material costs. Furthermore, if hydrogen is directly produced from such waste biomass, then processing costs and fuel availability could be significantly improved.


2

Page 3 of 29 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


A major challenge for direct hydrogen production from biomass is how to gasify carbohydrates, proteins, and fats in the feedstock with high efficiency and low cost. The gasification of biomass to hydrogen is generally conducted using water vapor or air as a gasification agent at temperatures of 800°C or higher, followed by the separation and purification of hydrogen,9,10 which is usually energy expensive. Therefore, it is necessary to develop an alternative approach that leads to high-efficiency and cost-effective conversion of biomass into hydrogen. Furthermore, to obtain a high hydrogen yield, this approach should be fully applied to all biomass components, including starch, cellulose, hemicellulose, lignin, protein, and fat.

Biomass electrolysis enables hydrogen production at onset cell voltages of less than 1 V, depending on the fuel species. Although bioethanol is one of the most promising fuels for electrolysis,11,12 this fuel has the limitations inherent in the hydrolysis and fermentation of biomass, which requires special and expensive procedures for processing. Biomass raw materials, including switchgrass and wood sawdust, can also be used as fuels for electrolysis; however, polyoxometalates or ferric chloride are required to function as redox intermediates.13,14 The raw material previously reacts with the intermediate to form a reduced intermediate in a tank outside the electrolysis cell. The reduced intermediate is subsequently supplied to the anode and releases a proton to the cathode by the application of a voltage. We have recently reported another type of biomass electrolysis that does not require pretreatment of the raw material.15 This method involves the addition (batch operation) or supply (flow operation) of a mixed solution of newspaper and phosphoric acid (H3PO4) to the anode, followed by operation of the electrolysis


3


Page 4 of 29

cell at temperatures between 100 and 150°C. Cellulose16,17 and lignin18 are hydrolyzed to monoand disaccharide derivatives and aliphatic molecules, respectively, by H3PO4 at the anode. These products react with water (H2O) to form protons and carbon dioxide (CO2) at the anode, and hydrogen at the cathode. For example, cellulose electrolysis is expected to proceed as follows. Anode: C6H12O6 + 6H2O → 6CO2 + 24H+ + 24e-

(1)

Cathode: 24H+ + 24e- → 12H2

(2)

In this study, we present hydrogen production by the direct electrolysis of waste biomass. Firstly, the anode material is modified to replace the expensive conventional Pt/C anode with metal-free mesoporous carbon. The electrolysis characteristics of the pure biomass components are then evaluated using the optimized anode. Bread residue, cypress sawdust, and rice chaff are then examined as biomass feedstock in both batch- and flow-type electrolysis cells. The hydrogen yield per weight of biomass is determined in the batch cell. Continuous hydrogen production is demonstrated in the flow cell.

Materials and methods Materials A Sn0.9In0.1P2O7-polytetrafluoroethylene (PTFE) membrane was selected as the electrolyte, due to its high proton conductivity (>0.01 S cm-1) at temperatures between 100 and 150°C, and a wide voltage range of 0–2 V.19–21 PTFE powder (0.04 g) was added to 1 g of Sn0.9In0.1P2O7 powder, kneaded using a mortar and pestle, and then cold-rolled to a thickness of 170 µm using a


4

Page 5 of 29 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


laboratory rolling mill. Ketjen black (KB; EC-600JDK) was purchased from Akzo Nobel and modified as follows. The carbon (1 g) was stirred in 50 mL of 24% nitric acid (HNO3) at room temperature for 74 h. After filtering and washing, the residual carbon was dried under vacuum at 120°C for 6 h. The oxygenated carbon was subsequently heated at 600°C for 4 h in a flow of argon (Ar), unless otherwise stated. The heat-treated carbon was dispersed with a small amount of 85% H3PO4 (Wako Chemicals) in a mixer (Thinky AR-100) for 30 min. The slurry obtained was deposited on the surface of carbon fiber paper (Toray TGP-H-090). A commercially available Pt/C (Pt loading: 2 mg cm-2, Electrochem) anode was used as a control anode and as the cathode. Bread, cypress wood, and rice chaff were milled into powder form using a household mixer (Zojirushi BM-RT08-GA). Cellulose (Wako Chemicals), starch (Wako Chemicals), lignin (Tokyo Chemical Industry), protein (Wako Chemicals), and lipid (Wako Chemicals) were used as model biomass components without further purification. Characterization Temperature-programmed desorption (TPD) analysis for the oxygenated carbon sample was performed using on-line mass spectrometry (MS; Pfeiffer Vacuum Thermostar GSD301). 0.5 g of the sample powder was packed in a glass tube and supplied with Ar at a flow rate of 60 mL at 100°C until the signals of carbon monoxide (CO; m/z = 28) and CO2 (m/z = 44) were stable. The sample was subsequently heated to the desired temperature in an Ar flow at a rate of 10°C min-1. The pore characteristics of the carbon samples were determined by nitrogen (N2) adsorption at liquid N2 temperature (Bel Japan BELSORP18PLUS-HT). The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method from adsorption data in the relative pressure range of 0.001 to 0.3. The mesopore volume distributions were determined using the Barrett-Joyner-Halenda (BJH) method. Functional group profiles for the carbon samples were


5


Page 6 of 29

obtained using Fourier-transform infrared spectroscopy (FT-IR; Agilent Excalibur FTS-4000). The chemical charge states of C 1s for the carbon samples were analyzed using X-ray photoelectron spectroscopy (XPS; VG Escallab220i-XL). Morphological changes before and after heating the biomass raw materials were observed using scanning electron microscopy (SEM; Keyence VE-8800). The quantities of hydrogen and CO2 evolved from the cathode and anode, respectively, were monitored using mass spectrometry (MS). Electrochemical measurements Electrolysis tests of batch and flow cells were performed as follows (Figure S1). The anode (area: 1.1 cm2) was used for both cells. The cathode area was adjusted to 0.5 cm2 for the batch cell and 0.2 cm2 for the flow cell, due to the difference in size between the cells. Details of the procedures have been described previously.15 The current density was normalized with respect to the area of the cathode. In the batch cell, fuels (7.5–60.0 mg) were mixed with appropriate quantities of 85% H3PO4 in a weight ratio of fuel to 85% H3PO4 of 1:15. After deposition of the sample mixtures onto the surface of the anode, the electrolyte membrane was sandwiched between the anode and cathode. The anode was attached to a stainless steel current collector (SUS316, 16 mm diameter; Hohsen) and then sealed using a PTFE gasket (Nitto Denko) with PTFE tape (Nitto Denko). In the flow cell, a liquid mixture of fuel and 85% H3PO4 (fuel concentration: 0.79 wt%) was pumped to the anode chamber with flow channels at an injection rate of 16 mL h-1 using a syringe feeder (YMC YPS-301). The anode chamber was sealed using the PTFE gasket with an elastomeric O-ring. The cathode was supplied with Ar at a flow rate of 100 mL min-1 in both cells. All electrochemical measurements were conducted using a potentiostat-galvanostat (Solartron 1287) and an impedance/gain-phase analyzer (Solartron 1260). Current-voltage (I-V) curves were recorded potentiostatically at a scan rate of 20 mV s-1.


6

Page 7 of 29 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


Voltage-time curves were acquired galvanostatically in the current range of 0.05–0.20 A cm-2. Impedance spectra were obtained at a bias voltage of 0.4 V in the frequency range of 0.1–106 Hz.

Results and discussion Anode design For oxygen-functionalized carbon anodes, the redox pair between carbonyl (C=O) and phenol (C–OH) groups is one of the most important hydrogen carrier sites.22,23 The origin of this redox reaction is ascribed to the following equilibrium reaction: C=O + H+ + e- ⇌ C–OH

(3)

In contrast, carboxyl groups (COOH) have little reversible redox ability and are also one of the least conductive functional groups.24,25 Accordingly, the overpotential and resistive loss of the anode would be decreased by enhancement of the density of carbonyl groups relative to carboxyl groups on the carbon surface. Figure 1(a) shows TPD profiles for CO (m/z = 28) and CO2 (m/z = 44) in a flow of Ar with the HNO3-treated KB. CO2 attributed to carboxyl group desorption26,27 was observed in the temperature range of 150 to 550°C, while CO originating from carbonyl group desorption26,27 became more intensive at 550°C or higher, which reflects the difference in thermal stability between the two functional groups. This also implies that carboxyl groups are more preferentially eliminated from the carbon surface than carbonyl groups by heat treatment of the oxygenated KB at temperatures between 500 and 600°C in an Ar flow. Therefore, the oxygenated KB was heat-treated at 600 and 1,000°C for test and control experiments, respectively.


7


(b)

6.1E-10 m/z = 28 5.1E-10 4.1E-10 3.1E-10 2.1E-10 1.1E-10

dVp / dlogdp / cm3 g-1

(a)

Intensity / -

14 12 8 6 4 2 0

m/z = 44

0.1

0

(c)

unheated o 600 C o 1000 C

10

1.0E-11 200

400 600 800 Temperature / oC

1

10 100 Pore Size dp / nm

1000 1200

(d)

0.14

1000

3000

0.12

2500

0.10

ν C─O

0.08

ν C=C ν C=O

0.06

unheated 600 oC o 1000 C

0.04

Intensity / -

Absorbance

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

Page 8 of 29

2000 1500 C=O

1000 500

0.02 0.00

COO

0

2500

2000 1500 1000 Wavenumber / cm-1

500

289 287 Binding Energy / eV

285

Figure 1. Characterization of the unheated KB, and KB heated at 600 and 1,000°C: (a) TPD-MS spectra of CO and CO2 for the unheated KB. (b) BJH pore size distributions. (c) FT-IR spectra. (d) XPS spectra.

Figure 1(b) shows pore size distributions for the unheated KB, and the KB heated at 600 and 1,000°C. Similar characteristics were found for the three samples; the average mesopore diameter and volume were 6.2, 7.1, and 6.8 nm and 2.6, 2.6, and 2.7 cm3 g-1 for the unheated KB, and the KB heated at 600 and 1000°C, respectively. The BET specific surface areas for the samples were also almost unchanged among the samples, and were 1,864, 1,772, and 1,828 m2 g-


8

Page 9 of 29 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


1

, respectively. These results demonstrate that even heat treatment at 1000°C does not cause

significant structural deformation of the KB. As shown in Figure 1(c), the FT-IR spectrum for the HNO3-treated KB indicated the presence of carbonyl groups (absorption at 1,720 cm-1) and phenol groups (absorption at 1,190 cm-1).28,29 The intensities of these absorption bands decreased with an increase of the heating temperature. To obtain additional information regarding the effect of the heat treatment on the functional groups, XPS measurements were conducted for the KB before and after the heat treatment. Figure 1(d) revealed less intensive carbonylic C=O (287.4 eV) and carboxylic COO (288.5 eV) peaks30 upon an increase of the heating temperature, the extent of which was much greater for the COO peak (ca. 44% down) than for the C=O peak (ca. 10% down). It was thus confirmed that the relatively carbonyl group-rich carbon sample was successfully obtained by heat treatment at 600°C, as expected from the TPD profile.

Electrolysis of pure cellulose (15 mg) was conducted for the unheated KB anode, and the KB anodes heated at 600 and 1,000°C. Figure 2(a) shows I-V curves for the electrolysis cells at a temperature of 150°C. The onset cell voltage of electrolysis was ca. 0.3 V for all the tested anodes, whereas the current density at each cell voltage increased in the order: 600°C-heated anode > 1000°C-heated anode > unheated anode. Impedance spectra for the electrolysis cells were measured at a bias voltage of 0.4 V [Figure 2(b)], and curve fitting was performed based on an equivalent circuit model (Figure S2). The cell with the unheated anode presented a typical Nyquist plot, the intersection of which with the abscissa at a high frequency of 40,000 Hz was assigned to the ohmic resistance of the cell; the semicircular arcs at medium (3–40,000 Hz) and low (0.1–3 Hz) frequencies correspond to the polarization and mass-transfer resistances of the electrodes, respectively. This cell also possessed the largest ohmic and polarization resistances


9


Page 10 of 29

among the tested cells, which can be explained by the presence of large amounts of carboxyl groups introduced onto the surface through the HNO3 treatment. It is most likely that these functional groups lead to poor electrical contact between the carbon particles. In contrast, the heat treatment of such an oxygenated anode, especially at 600°C, significantly decreased both the ohmic and polarization resistances due to the removal of carboxyl groups without significant loss of carbonyl groups from the carbon surface. Subsequent experimental trials were assessed using the 600°C-heated anode because it was reasonable to conclude that there was no optimal condition for the carbonyl group at temperatures above 600°C.


10

Page 11 of 29 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


Figure 2. Electrolysis characteristics of the batch cells using the unheated KB anode, and the KB anodes heated at 600 and 1,000°C: (a) I-V curves and (b) impedance spectra of the cells at 150°C using the unheated KB anode, and the KB anodes heated at 600 and 1,000°C. (c) I-V curves and (d) impedance spectra of the cell using the 600°C-heated KB anode between 100 and 150°C. Data for the batch cell using the Pt/C anode are also included.

Figure 2(c) shows I-V curves for the electrolysis cell using cellulose as the fuel at temperatures between 100 and 150°C. The I-V curve was dependent on the operation temperature, which resulted in I-V characteristics recorded at 150°C that were comparable to the characteristics observed for the Pt/C anode under the same conditions. This temperature dependency was attributable mainly to the polarization and mass-transfer resistances of the anode, rather than the ohmic resistance of the cell [upper panel of Figure 2(d)]; the ohmic resistance remained almost unchanged and the polarization resistance decreased from ca. 1.5 to ca. 0.3 Ω cm2 with an increase in the temperature from 100 to 150°C. The mass-transfer resistance obtained at 100°C was too large for quantification. A similar temperature dependency of the impedance was observed for the Pt/C anode; however, at all temperatures, the semicircular arcs related to the mass-transfer resistance were larger than those recorded for the optimized mesoporous carbon anode [lower of Figure 2(d)]. This is probably due to the difference in the mesopore volume between the carbon species used. Vulcan XC-72R used as the carbon support of the Pt/C anode has a total pore volume of 0.32 cm3 g-1,31 which is approximately one eighth of that (2.6 cm3 g-1) observed for the KB anode. Therefore, the decomposition products of cellulose may not diffuse easily in such a small space.


11


Page 12 of 29

Electrolysis of pure biomass components The electrolysis of expected biomass components other than cellulose was also investigated at a temperature of 150°C. Figure 3(a) shows I-V curves using various pure components (15 mg of each) as fuels. The current densities recorded with the components at each voltage were one and two orders of magnitude greater than those obtained without fuel, which indicates that these components are viable electrolysis fuels. The electrolysis onset cell voltage was ca. 0.3 V for cellulose, starch, and protein and ca. 0.4 V for lignin and lipid. We cannot calculate the theoretical onset cell voltage for the electrolysis reactions, because these fuels are partially decomposed to various products at the anode. However, as shown in Figure 2(c), the onset cell voltage using the 600°C-heated KB and Pt/C anodes was independent of the anode species, but dependent on the temperature. Therefore, it is most likely that the onset cell voltage is determined by a thermodynamic factor rather than a kinetic factor. Another important result was the dependency of the I-V characteristics on the component species: cellulose = starch > protein > lignin = lipid. The internal resistances of the cells with the component fuels were compared using impedance measurements to better understand this dependency. As shown in Figure 3(b), the impedance spectra revealed three distinctive features. Firstly, similar polarization and masstransfer resistances were observed for cellulose (1.9 Ω cm2) and starch (1.3 Ω cm2), which suggests a similarity of the decomposition products (mono- and disaccharides) for these components. Secondly, protein showed somewhat larger polarization and mass-transfer resistances (6.2 Ω cm2) than those acquired for cellulose and starch, which is attributable to the difference in the reducibility of the functional group in the molecule between the carbonyl group in glucose and the carboxyl group in amino acid. Thirdly, the polarization and mass-transfer


12

Page 13 of 29 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


resistances obtained for lignin (92.7 Ω cm2) and lipid (157.6 Ω cm2) were much larger than those recorded for the other components. These results reflect that the two components are more acidinsoluble than the other components, which leads to a reduction of available small molecule fuels at the anode. Figure 3(b) also showed that the ohmic resistance of the cell was especially large with the use of lignin as a fuel. Approximately 60 wt% solid residue was observed with the lignin fuel, which was identified as unreacted lignin, as previously reported.18 Therefore, the large ohmic resistance observed for lignin is caused by poor electrical contact at the interface between the anode and electrolyte, due to the presence of the solid residue.


13


Page 14 of 29

Figure 3. Electrolysis characteristics of the batch cell using various model biomass components as fuels at 150°C: (a) I-V curves and (b) impedance spectra. (c) Formation rates of hydrogen at the cathode, and CO2 and NO2 at the anode. Theoretical values are included for comparison. (d) Formation rates of hydrogen, CO2, and NO2 during electrolysis of protein for 600 s at 0.25 A cm2

.

Figure 3(c) shows hydrogen formation rates from the cathode at a temperature of 150°C, including the theoretical values (black dashed line) calculated according to Faraday’s law, based on the two-electron reaction represented in Reaction (2). The observed rates agreed approximately with the theoretical values for all tested components, which demonstrates that the current efficiency for hydrogen production was 95–106% (see Supporting Information). Figure 3(c) also shows CO2 and NO2 formation rates from the anode. The theoretical rates shown by the red dashed line in Figure 3(c) were calculated under the assumption that a carbon or nitrogen atom in the fuel is oxidized to a CO2 or nitrogen dioxide (NO2) molecule, and four protons with two oxygen atoms in the fuel and H2O molecules through the following four-electron reaction. C or N in fuel + 2O in fuel and H2O → CO2 or NO2 + 4H+ + 4e-

(4)

No products, except CO2 (and only NO2 for protein), were detected using MS. The current efficiency for CO2 formation was estimated to be 90% for cellulose, 85% for starch, 74% for lignin, 70% for lipid, and 67% for protein (see Supporting Information). It is likely that oxide compounds not detected by MS are also produced, especially from the lignin and lipid fuels. However, the current efficiency for NO2 formation from protein was 29%; therefore, almost all currents were considered to be consumed for CO2 and NO2 formation at the anode. [The


14

Page 15 of 29 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


formation rates of these gases in addition to hydrogen during a short electrolysis period at 0.25 A cm-2 are shown in Figure 3(d).] Based on these observations, it is concluded that hydrogen is produced by the electrolysis of most of the biomass components at onset cell voltages less than 0.5 V, which motivates the development of waste biomass electrolysis because these feedstock materials are composed mainly of the biomass components tested.

Electrolysis of waste biomass Bread, cypress, and rice chaff were tested as fuels in a similar manner to that for cellulose. The raw materials were used after being milled without purification; therefore, the species, concentration, and fragment length of the biomass components varied according to the biomass source and the manufacturing process. This was confirmed by room temperature SEM observations, as shown in Figures 4(b), (e), and (h), where the sample powders were impregnated with H3PO4. The three raw materials presented intrinsic structures and sizes in the acid solvent. These samples were gradually liquefied when heated to 125°C, but not completely dissolved at this temperature [Figures 4(c), (f), and (i)]. In particular, rice chaff usually contains ca. 15% silicon dioxide,32 which further interferes with dissolution of the fragments into the acid solvent.


15


Page 16 of 29

Figure 4. Photographs and SEM micrographs of (a), (b), and (c) bread, (d), (e), and (f) cypress, and (g), (h), and (i) rice chaff impregnated with H3PO4, followed by heat treatment at 125°C.

I-V curves using the biomass fuels were measured for batch operation at a temperature of 150°C [Figure S3(a)]. Electrolysis of the three fuels commonly began at cell voltages greater than ca. 0.3 V and provided current densities of ca. 0.3 A cm-2 at a cell voltage of 1 V, which are characteristics similar to those recorded for cellulose and starch. This is supported by the high content of starch or cellulose contained in these feedstock materials: 70% starch in bread32 and 35% and 38% cellulose in cypress33 and rice chaff34, respectively. Furthermore, the magnitudes of the total impedance measured for the three fuels [Figure S3(b)] were closer to those obtained


16

Page 17 of 29 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


for cellulose and starch than those observed for protein, lignin, and lipid [Figure 3(b)]. Nevertheless, there were some differences in the ohmic, polarization and mass-transfer resistances among the cells with the three fuels, which is related to the differences in the component species and their proportions in the fuels.32–34

The amount of available fuel at the anode decreased with time for galvanostatic batch-mode electrolysis, which leads to the depletion of fuel at the anode at some point in time. Therefore, the quantity of produced hydrogen per unit weight of fuel can be estimated from the depletion time according to Faraday’s law. Figures 5(a) and (b) show voltage-time curves for the cells and formation rates for hydrogen, CO2, and NO2, respectively, during continuous electrolysis of the bread, cypress, and rice chaff (15 mg of each) fuels with a current density of 0.1 A cm-2 at 150°C. The voltage-time curves were characterized by plateau-like behavior in voltage and by a subsequent rapid increase in voltage. These characteristics were associated with the consumption and depletion of the fuel because hydrogen and CO2 were constantly produced until the plateaulike behavior of the voltage was terminated. (The current efficiencies for hydrogen, CO2, and NO2 were 98–101%, 83–99%, and 0–4%, respectively, during this period.) The depletion time varied from 4,000 to 6,000 s, depending on the fuel. Voltage-time curves obtained for rice chaff fuel with various weights are displayed in Figure 5(c). The cell voltage recorded at each time before depletion of the fuel became gradually decreased as the fuel weight increased, which is reflected by an increase in the amount of the available fuel captured by the anode, as will be discussed later. More importantly, the depletion time lengthened as the fuel weight increased. As a result, the weight of the hydrogen estimated from the depletion time increased with the weight of the fuel used, as shown in Figure 5(d). Similar results were also obtained for the bread and


17


Page 18 of 29

cypress fuels. Based on these results, 0.12–0.22 mg, 0.14–0.24 mg, and 0.13–0.27 mg of hydrogen can be produced from 1 mg of rice chaff, cypress, and bread, respectively. It is also noted that the hydrogen yield per unit weight of fuel tends to decrease with the weight of the fuel, due to the deterioration of the utilization efficiency of the fuel at the anode. This issue would be avoided by stirring the fuel in a batch cell.

Figure 5. Electrolysis characteristics of the batch cell using the bread, cypress, and rice chaff fuels at 150°C: (a) voltage-time curves and (b) formation rates of H2 at the cathode, and CO2 and NO2 at the anode during continuous electrolysis of the bread, cypress, and rice chaff (15 mg of each) at 0.1 A cm-2. (c) Voltage-time curves during continuous electrolysis of rice chaff with


18

Page 19 of 29 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


various weights at 0.1 A cm-2. (d) Weight of produced hydrogen as a function of the rice chaff fuel weight. Data for the bread and cypress fuels are also included.

Figure 6. Electrolysis characteristics of the flow cell using the bread, cypress, and rice chaff fuels at 150°C: (a) voltage-time curves, (b) impedance spectra, and (c) formation rate of hydrogen at the cathode during continuous electrolysis of the bread, cypress, and rice chaff at 0.1


19


Page 20 of 29

A cm-2. (d) Voltage-time curves and formation rate of hydrogen during continuous electrolysis of the bread at various current densities.

For batch-mode electrolysis, the voltage-time characteristics could be improved by an increase in the weight of the fuel; however, this effect decreased with time [Figure 5(c)], due to an increase in the internal resistance of the cell. Accordingly, continuous electrolysis characteristics of the present approach were reassessed in flow mode. Figure 6(a) shows voltage-time curves for the flow cells with the bread, cypress, and rice chaff fuels with a current density of 0.1 A cm-2 at 150°C. Data recorded for the batch cell with the rice chaff fuel (60 mg) is also included for comparison. The flow cells exhibited more plateau-like voltages and lower cell voltages compared with the results obtained for the batch cells. The sudden small increase in voltage observed for the cypress fuel at ca. 2,000 s is probably due to unknown disturbance of the flow in the fuel nozzle. The voltage plateau value at 3,000 s was decreased in the order: rice chaff > bread > cypress. Figure 6(b) shows impedance spectra for the flow cells at a bias voltage of 0.4 V, including data obtained for the batch cell with the rice chaff fuel. The difference in impedance among the flow cells was too small to explain the difference in the plateau voltage among them. However, note that the impedances of the flow cells were much smaller than that of the batch cell, which is ascribable to the constant supply of fresh fuel to the anode in the flow process.

Figure 6(c) shows the rates of hydrogen formation during continuous flow electrolysis under the same conditions as those shown in Figure 6(a). The formation rate for hydrogen was almost the same in the three cells, which is in agreement with the theoretical rates calculated using


20

Page 21 of 29 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


Faraday’s law. The electrical energy (kWh) consumed for electrolysis of each fuel was calculated by integrating the area under the voltage-time curve shown in Figure 6(a) and multiplying by the current. These energies were then normalized according to the volumes of experimentally produced hydrogen that were computed by integration of the data shown in Figure 6(c). The estimated energies were 0.75, 0.50, and 0.83 kWh (Nm3)-1 for bread, cypress, and rice chaff, respectively, which are much less than those reported for water electrolysis [ca. 5 kWh (Nm3)-1] and ethanol electrolysis [ca. 2 kWh (Nm3)-1].35 It is evident that these significant energy savings could be achieved by suppression of the cell voltage to ca. 0.4 V at 0.1 A cm-2. To confirm such energy savings over a wide range of current densities, electrolysis tests were performed using bread as the fuel at various current densities. Figure 6(d) shows that the flow cell provided voltages of 0.19, 0.35, and 0.53 V at 0.05, 0.10, and 0.15 A cm-2, respectively. There is a proportional relationship between the voltage and current density, where the voltage increases by ca. 0.18 V per increment of 0.05 A cm-2. However, at a current density of 0.20 A cm-2, the cell voltage increased gradually from 0.64 to 0.72 V with time. The initial voltage of 0.64 V is likely counted lower, compared with the other data sets, while the terminal voltage of 0.72 V is in the proportional region. One possible explanation for this behavior is the temperature increase of the cell due to the heat generated by the internal resistance of the cell, and subsequent recovery of the temperature by an external heater. This also suggests the possibility of the thermal sustainability of the cell that would allow maintenance of the operation temperature in practical applications.36 Figure 6(d) also shows that hydrogen was produced in proportion to the current density and that the rate recorded at each current density was approximately 100% of that calculated using Faraday’s law. These results, together with the results obtained for the batch cell, demonstrate that the biomass feedstocks examined could be


21


Page 22 of 29

electrolyzed efficiently and continuously in one step to hydrogen. It is also concluded that any of these feedstock materials could be utilized as fuels without significant differences in performance. These characteristics are attributed to the effective production of hydrogen from most of the raw material components.

In conclusion, hydrogen could be successfully produced directly from biomass waste such as bread residue, cypress sawdust, and rice chaff by electrolysis of a mixture of the raw material and an 85% H3PO4 solvent at a temperature of 150°C. A carbonyl-group functionalized mesoporous carbon exhibited electrochemically catalytic activity for the anode reaction that was comparable to the activity observed for a Pt/C anode. The current efficiency for hydrogen production reached approximately 100% for all the tested fuels in both the batch and flow operations. Note that the present approach provided more significant savings in terms of the energy required for hydrogen production compared to that reported for the electrolysis of water and bioethanol. ASSOCIATED CONTENT The following files are available free of charge. Supporting Information (3 figures) AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Author Contributions


22

Page 23 of 29 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


All authors contributed to the manuscript and approved the final version. Funding Sources Nagoya University received grants from the Japan Society for the Promotion of Science (JSPS): Nos. 26000008, 17H01895, and 17K19087. ACKNOWLEDGMENT The authors would like to thank Dr. Miki Niwa (Professor Emeritus, Tottori University) for numerous useful discussions. This work was supported by Kakenhi Grants-in-Aid (Nos. 26000008, 17H01895, and 17K19087) from JSPS. REFERENCES 1. Anderson, K.; Peters, G. The trouble with negative emissions. Science 2016, 354, 182–183. DOI: 10.1126/science.aah4567 2. Schellnhuber, H. J.; Rahmstorf, S.; Winkelmann, R. Why the right climate target was agreed in Paris. Nat. Clim. Change 2016, 6, 649–653. DOI:10.1038/nclimate3013 3. Aldy, J.; Pizer, W.; Tavoni, M.; Reis, L. A.; Akimoto, K.; Blanford, G.; Carraro, C.; Clarke, L. E.; Edmonds, J.; Iyer, G. C.; McJeon, H. C.; Richels, R.; Rose, S.; F. Sano, Economic tools to promote transparency and comparability in the Paris agreement. Nat. Clim. Change 2016, 6, 1000–1004. DOI:10.1038/nclimate3106 4. Kumar, G.; Shobana, S.; Chen, W. H.; Bach, Q. V.; Kim, S. H.; Atabani, A. E.; Chang, J. S. A review of thermochemical conversion of microalgal biomass for biofuels: chemistry and processes. Green Chem. 2017, 19, 44–67. DOI: 10.1039/C6GC01937D


23


Page 24 of 29

5. Gaurav, N.; Sivasankari, S.; Kiran, G.; Ninawe, A.; Selvin, J. Utilization of bioresources for sustainable biofuels: a review. Renewable Sustainable Energy Rev. 2017, 73, 205–214. DOI: org/10.1016/j.rser.2017.01.070 6. Voloshin, R. A.; Rodionova, M. V.; Zharmukhamedov, S. K.; Veziroglu, T. N.; Allakhverdiev, S. I. Review. Biofuel production from plant and algal biomass. Int. J. Hydrogen Energy 2016, 41, 17257–17273. DOI: org/10.1016/j.ijhydene.2016.07.084 7. Wainaina, S.; Horvath, I. S.; Taherzadeh, M. J. Biochemicals from food waste and recalcitrant biomass via syngas fermentation: a review. Bioresour. Technol. 2018, 248, 113–121. DOI: org/10.1016/j.biortech.2017.06.075 8. Arevalo-Gallegos, A.; Ahmad, Z.; Asgher, M.; Parra-Saldivar, R.; Iqbal, H. M. N. Lignocellulose: a sustainable material to produce value-added products with a zero waste approach–a review. Int. J. Biol. Macromol. 2017, 99, 308–318. DOI: org/10.1016/j.ijbiomac.2017.02.097 9. Sikarwar, V. S.; Zhao, M.; Fennell, P. S.; Shah, N.; Anthony, E. J. Progress in biofuel production from gasification. Prog. Energy Combust. Sci. 2017, 61, 189–248. DOI: org/10.1016/j.pecs.2017.04.001 10. Sikarwar, V. S.; Zhao, M.; Clough, P.; Yao, J.; Zhong, X.; Memon, M. Z.; Shah, N.; Anthony, E. J.; Fennell, P. S. An overview of advances in biomass gasification, Energy Environ. Sci. 2016, 9, 2939–2977. DOI: 10.1039/C6EE00935B 11. Caravaca, A.; Sapountzi, F. M.; Lucas-Consuegra, A.; Molina-Mora, C.; Dorado, F.; Valverde, J. L. Electrochemical reforming of ethanol–water solutions for pure H2 production in a


24

Page 25 of 29 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


PEM electrolysis cell. Int. J. Hydrogen Energy 2012, 37, 9504–9513. DOI: org/10.1016/j.ijhydene.2012.03.062. 12. Silveira, J. L.; Braga, L. B.; de Souza, A. C. C.; Antunes, J. S.; Zanzi, R. The benefits of ethanol use for hydrogen production in urban transportation. Renew. Sustain. Energy Rev. 2009, 13, 2525–2534. DOI: org/10.1016/j.rser.2009.06.032 13. Du, X.; Liu, W.; Zhang, Mulyadi, A.; Brittain, A.; Gong, J.; Deng, Y. Low-energy catalytic electrolysis for simultaneous hydrogen evolution and lignin depolymerization. ChemSusChem 2017, 10, 847–854. DOI: org/10.1002/cssc.201601685 14. Xu, F.; Li, H.; Liu, Y.; Jing, Q. Advanced redox flow fuel cell using ferric chloride as main catalyst for complete conversion from carbohydrates to electricity. Sci. Rep. 2017, 7, 5142. DOI: 10.1038/s41598-017-05535-2 15. Hibino, T.; Kobayashi, K.; Ito, M.; Nagao, M.; Fukui, M.; Teranishi, S. Direct electrolysis of waste newspaper for sustainable hydrogen production: an oxygen-functionalized porous carbon anode. Appl. Catal. B 2018, 231, 191–199. DOI: org/10.1016/j.apcatb.2018.03.021 16. Hibino, T.; Kobayashi, K.; Lv, P.; Nagao, M.; Teranishi, S.; Mori, T. An intermediatetemperature biomass fuel cell using wood sawdust and pulp directly as fuel. J. Electrochem. Soc. 2017, 164, F557–F563. DOI: 10.1149/2.0511706jes 17. Hibino, T.; Kobayashi, K.; Lv, P.; Nagao, M.; Teranishi, S. High performance anode for direct cellulosic biomass fuel cells operating at intermediate temperatures. Bull. Chem. Soc. Jpn. 2017, 90, 1017–1026. DOI: org/10.1246/bcsj.20170163


25


Page 26 of 29

18. Hibino, T.; Kobayashi, K.; Nagao, M.; Teranishi, S. Hydrogen production by direct lignin electrolysis at intermediate temperatures. ChemElectroChem 2017, 4, 3032–3036. DOI: org/10.1002/celc.201700917 19. Lee, K. S.; Maurya, S.; Kim, Y. S.; Kreller, C. R.; Wilson, M. S.; Larsen, D.; Elangovanb, E.; Mukundan, R. Intermediate temperature fuel cells via an ion-pair coordinated polymer electrolyte. Energy Environ. Sci. 2018, 11, 979–987. DOI: 10.1039/C7EE03595K 20. Hibino, T.; Kobayashi, K.; Nagao, M.; Kawasaki, S. High-temperature supercapacitor with a proton-conducting metal pyrophosphate electrolyte, Sci. Rep. 2015, 5, 7903. DOI: 10.1038/srep07903 21. Scott, K.; Xu, C.; Wu, X. Intermediate temperature proton-conducting membrane electrolytes for fuel cells, Wiley Interdiscip. Rev. Energy Environ. 2014, 3, 24–41. DOI: org/10.1002/wene.64 22. Brunmark, A.; Cadenas, E. Redox and addition chemistry of quinoid compounds and its biological implications. Free Radical Biol. Med. 1989, 7, 435–477. DOI: org/10.1016/08915849(89)90126-3 23. Revenga, J.; Rodriguez, F.; Tijero, J. Study of the redox behavior of anthraquinone in aqueous medium. J. Electrochem. Soc. 1994, 141, 330–333. DOI: 10.1149/1.2054725 24. Shen, J.; Liu, A.; Tu, Y.; Foo, G.; Yeo, C.; Chan-Park, M. B.; Jiang, R.; Chen, Y. How carboxylic groups improve the performance of single-walled carbon nanotube electrochemical capacitors? Energy Environ. Sci. 2011, 4, 4220–4229. DOI: 10.1039/C1EE01479J


26

Page 27 of 29 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


25. Pantea, D.; Darmstadt, H.; Kaliaguine, S.; Sümmchen, L. Electrical conductivity of thermal carbon blacks: Influence of surface chemistry. Carbon 2001, 39, 1147–1158. DOI: org/10.1016/S0008-6223(00)00239-6 26. Sebastián, D.; Suelves, I.; Moliner, R.; Lázaro, M. J. The effect of the functionalization of carbon nanofibers on their electronic conductivity. Carbon 2010, 48, 4421–4431. DOI: org/10.1016/j.carbon.2010.07.059 27. Figueiredo, J. L.; Pereira, M. F. R.; Freitas, M. M. A.; Órfão, J. J. M. Modification of the surface chemistry of activated carbons. Carbon 1999, 37, 1379–1389. DOI: org/10.1016/S00086223(98)00333-9 28. Takaoka, M.; Yokokawa, H.; Takeda, N. The effect of treatment of activated carbon by H2O2 or HNO3 on the decomposition of pentachlorobenzene. Appl. Catal. B. 2007, 74, 179–186. DOI: org/10.1016/j.apcatb.2007.02.009 29. Dandekar, A.; Baker, R. T. K.; Vannice, M. A. Characterization of activated carbon, graphitized carbon fibers and synthetic diamond powder using TPD and DRIFTS. Carbon 1998 36, 1821–1831. DOI: org/10.1016/S0008-6223(98)00154-7 30. Terzyk, A. P. The influence of activated carbon surface chemical composition on the adsorption of acetaminophen (paracetamol) in vitro: Part II. TG, FTIR, and XPS analysis of carbons and the temperature dependence of adsorption kinetics at the neutral pH. Colloids Surf., A: , 2001, 177, 23–45. DOI: org/10.1016/S0927-7757(00)00594-X 31. Chai, G. S.; Shin, I. S.; Yu, J. S. Synthesis of ordered, uniform, macroporous carbons with mesoporous walls templated by aggregates of polystyrene spheres and silica particles for use as


27


Page 28 of 29

catalyst supports in direct methanol fuel cells. Adv. Mater. 2004, 16, 2057–2061. DOI: org/10.1002/adma.200400283 32. Nakamura, M.; Yuyama, Y. Development of a composition database for various types of biomass. Tech. Rep. Natl. Res. Inst. Agric. Eng. Japan 2005, 203, 57–80. 33. Kawashima, A.; Akihiro, H.; Morita, H.; Fukuoka, M.; Honda, K.; Morita, M. Dioxin-like polychlorinated biphenyl adsorbent obtained from enzymatic saccharification residue of lignocellulose.

Bioresour.

Technol.

2011,

102,

4682–4687.

DOI:

org/10.1016/j.biortech.2011.01.033 34. Johar, N.; Ahmad I.; Dufresne, A. Extraction, preparation and characterization of cellulose fibers and nanocrystals from rice husk. Ind. Crops Prod. 2012, 37, 93–99. DOI: org/10.1016/j.indcrop.2011.12.016 35. Lamy, C.; Jaubert, T.; Baranton, S.; Coutanceau, C. Clean hydrogen generation through the electrocatalytic oxidation of ethanol in a proton exchange membrane electrolysis cell (PEMEC): Effect of the nature and structure of the catalytic anode. J. Power Sources 2014, 245, 927–936. DOI: org/10.1016/j.jpowsour.2013.07.028 36. Lotrič, A.; Sekavčnik, M.; Pohar, A.; Likozar, B.; Hočevar, S. Conceptual design of an integrated thermally self-sustained methanol steam reformer – High-temperature PEM fuel cell stack manportable power generator. Int. J. Hydrogen Energy 2017, 42, 16700–16713. DOI: org/10.1016/j.ijhydene.2017.05.057


28

Page 29 of 29 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


Table of contents

Direct biomass waste electrolysis produces hydrogen with significant energy and cost savings.


29

Efficient Hydrogen Production by Direct Electrolysis of Waste Biomass

Recommend Documents