POLARIZATION LOSSES IN A DIRECT CARBON FUEL CELL OPERATING ON OIL PALM MESOCARP FIBRE BIOCHAR NIDA JAFRI1*, YOON LI WAN1,WONG WAI YIN2, VEENA DOSHI, CHEAH
KEAN HOW3
1School of Engineering, Faculty of Built Environment, Technology & Design, Taylor’s University, Subang Jaya, Malaysia
2Fuel Cell Institute, University Kebangsaan Malaysia, 43600 UKM Bangi, Selangor DE, Malaysia.
3School of Engineering and Physical Sciences, Heriot-Watt University, Malaysia Campus, Putrajaya, Malaysia
*Corresponding Author: [email protected]
Abstract
The working and performance of a Direct Carbon Fuel Cell (DCFC) was evaluated based on the existing mathematical model of Tafel equation which was used to simulate the performance of the DCFC. The model considers factors such as electrochemical reaction dynamics, mass-transfer and electrode processes. The fuel sources were derived from oil palm mesocarp fibre (OPMF) and commercially available Activated Carbon (AC), using solid oxide based electrolyte. The main polarization losses occurring in the cell components were determined and it was found that the anode activation polarization contributed towards the major potential loss occurring in the circuit. The activation polarization loss was the highest for AC, followed by untreated mesocarp fibre biochar & alkali treated mesocarp fibre biochar. The results showed that the pre-treatment of the biochar significantly reduced the ash content which in turn improved the electrochemical oxidation rate, thereby reducing the activation polarization loss.
Keywords: Direct carbon fuel cell, Oil palm mesocarp fibre, Modelling, Polarization losses.
1. Introduction
DCFC has lately garnered attention as it offers an interesting approach for biomass utilization for the production of clean energy. The DCFC comprises of three principal components: anode, cathode and electrolyte. It offers a relatively
technologies and also the method of fuel preparation is not expensive. This fuel cell is a pragmatic system as it provides the possibility of using the abundant
lignocellulosic biomass waste as a fuel source with only minor pre-treatment.
Nomenclature F’
G J j0
jc jlim
js j0 I n
Faraday’s constant (96,487 Cmol−1) Gibbs-free energy (J)
current density (Am−2)
exchange current density (Am−2)
current density in the electrode phase (Am−2) limiting current density (Am−2)
current density in the electrolyte phase (Am−2) exchange current density (Am−2)
current (A)
numbers of electrical charge transfer in an electrochemical reaction (mole)
PCO2 partial pressure of CO2 in gas phase (Pa) PO2
R R’
Rc Rct Rce Rs
partial pressure of O2 in gas phase (Pa) resistance (Ω m2)
gas constant (8.314 J mol K−1)
resistance in the electrode phase (Ω m2) charge transfer resistance (Ω m2)
resistance of all components except for anode (Ω m2)
resistance in the electrolyte (Ω m2) T
Vcell
VN
V0
temperature (K) cell voltage (V) open circuit voltage (V)
open circuit voltage at standard state (V) Greek Symbols
apparent charge transfer coefficient
act
con
ohm
polarization loss (V) activation polarization (V) concentration polarization (V) ohmic polarization (V) Superscript
act activation
an
cat con ohm
anode cathode concentration ohmic
DCFCs convert the chemical energy of carbon directly without a reforming process. The exhaust gases that are produced at the anode side consists mostly of pure carbon dioxide, making it easy to be captured and sequestrated without any added processing. DCFC systems employs diverse electrolytes such as: molten carbonate [1, 2], molten hydroxide [3] and solid oxide based ceramic such as Yttria- stabilized zirconia (YSZ) [4, 5]. The molten carbonates and molten hydroxide electrolytes are corrosive in nature at the operating temperatures of 400- 800°C. Due to this corrosivity and concerns of stability of the electrolyte at higher temperatures, these electrolytes are limited to operation below 800 °C [6].
Hence, this study utilizes the solid oxide based electrolyte for direct carbon conversion in the DCFC. DCFC is subject to corrosion and degradation during operation which mainly occurs on its membrane-electrodes components [7].
Hence, researchers have tried to utilize coal or carbon black as the substrates as they comprise of comparatively less impurities which affect the operation and lifetime of the DCFC system [8, 9]. Thus, carbon sources which are ash free or have less ash components are required for this technology. Researches have shown that the ash accumulation in the cell system would reduce the cell life and would ultimately affect the performance of the DCFCs [10, 11].
In a DCFC operated on solid oxide based- electrolyte, the major voltage losses occurring are due to (i) activation polarization at the anode (ii) activation polarization at the cathode (iii) ohmic polarization (iv) concentration polarization at the anode (v) concentration polarization at the cathode [12, 13, 14]. The activation polarization and concentration polarization depend largely on the anode interlayer thickness. Using a thick anode interlayer would allow for more active sites of the three phase boundary (TPB), thereby decreasing the voltage loss due to activation polarization at the anode [15, 16]. This also helps in increasing the mechanical strength of the anode interlayer However, the process can result in issues related to transportation limitation where the fuel gas can hardly penetrate the thick anode interlayer.
The research in this paper centers on the assessment of the voltage losses occurring for the DCFC single cell. The aim is to evaluate and simulate the various polarizations and V–j relation in connection to the experimentally obtained power densities. The commercial software MATLABTM R2017b version is used for this work.
2. Materials and Sample Preparation
The Oil Palm Mesocarp Fibre (OPMF) samples used in this study were collected from an oil palm plant in Sri Ulu Langat in Dengkil, Selangor, Malaysia. The Activated Carbon (AC) samples were procured commercially from Sigma Aldrich (M) Sdn Bhd. OPMF samples were washed with distilled water to remove the naked impurities. The samples were then dried in an oven (UN75, Memmert, USA) at 105 ºC for 24 h for moisture removal (ASTM D2867-09). It was then grinded in a grinder (JK-SG-160, KGC Scientific) and separated into particle sizes of 0.5-2.0 mm using a sieve shaker (RX-812-1, Tyler). The OPMF biomass was stored in a desiccator prior to usage to avoid moisture loss. Alkali pre-
treatment with 2.0M NaOH (99.9 %) was then carried out on 20 g of oven dried OPMF samples. Pre-treated OPMF samples were oven dried again (UN75, Memmert, USA) following the same procedure of ASTM D2867-09 at 105 ºC for 24 h for moisture elimination. The OPMF sample was then subjected to pyrolysis at 500 °C to be converted to fine biochar. The research involved using two types of fuel derived from OPMF biomass and one commercially available AC fuel.
The nomenclature of the biochar fuels are summarized in Table 1.
Table 1: Nomenclature of different fuels used in the study Fuel Description
AC Activated Carbon, commercially supplied by Sigma Aldrich used without any pre- treatment or pyrolysis.
UMF Untreated OPMF pyrolyzed at 500 °C
MF_2.0 Alkali treated OPMF pre-treated with 2.0M NaOH and pyrolyzed at 500 °C
3. Design and Principle of a Direct Carbon Fuel Cell (DCFC)
The solid oxide based ceramic electrolyte employed for the electrochemical oxidation of carbon is seen to be a promising option [17, 18, 19]. Yttria-stabilized zirconia (YSZ) was employed as the electrolyte. OPMF biochar and AC are used as the anode fuels. The elemental carbon in the fuel contains high energy density and is oxidized electrochemically with the following reactions occurring at the electrodes [20, 21]:
Anode: C + 2O2- → CO2+ 4e- (1) Cathode: O2 + 4e- → 2O2- (2) Overall: C + O2 → CO2 Eo = 1.02 V (3) The solid carbon fuel is placed in the anode compartment where it is
electrochemically oxidized to CO2. Thereby, electrical power is generated at a high temperature and the overall cell reaction is depicted in Eq. (3).
3.1. System configuration
An optimal design of the DCFC set-up was developed to assess the activity of the biochars, as shown in Fig. 1. The DCFC setup was supplied with commercial button cell from Ningbo SOFCMAN Energy, Technology Ltd., Co. China. The cathode of the DCFC was made up of Lanthanum strontium manganite (LSM) which is an oxide ceramic having a thickness of 25 µm. The anode material was made up of Nickel-Yttria stabilized zirconia (Ni-YSZ) having a thickness of 400 µm which is the conductive surface for the transportation of oxygen anions. The electrolyte was made up of ceramic material, Yttria-stabilized zirconia (YSZ)
with a thickness of 15 µm. Silver wires were used as electrical contacts on the electrodes. The button cell was positioned between the two vertical ceramic tubes.
Fig. 1: Setup of Direct Carbon Fuel Cell. 1: flange, 2: ceramic tube, 3:
furnace, 4:
anode current collector, 5: biochar placed on top of button cell, 6: cathode current collector.
Fig. 1 shows the schematic design and components of the DCFC. 0.1 g of OPMF biochar was placed on the cathode side of the button cell (cell surface area of 1.743 cm2). The final operating temperature of the vertical furnace was set to 800 °C with a ramping rate of 10 °C min-1 and was held at 800 °C for 2 hrs.
Nitrogen gas was supplied into the system through the anode and cathode side at 200 and 600 ml min-1 respectively. The gas stream at the cathode end was changed to oxygen gas with a flow rate of 600 ml min-1about 10 mins before the system reached the final temperature of 800 °C. The electrochemical testing was performed at a temperature of 800 °C after the Open Circuit Potential (OCP)
reached a steady state. The anode side was purged with N2 gas (constant flow rate of 200 ml min-1, STP) and the cathode side was purged with O2 gas (constant flow rate of 600 ml min-1, STP). The OCP was recorded during the experimental run using a Potentiostat (model Gamry Interface 1000). The electrochemical impedance spectroscopy (EIS) of the DCFC was calculated at a temperature of 800°C with a ramping 10 °C min-1 in the frequency range of 100 KHz to 0.1 Hz.
The current-voltage (I-V) curve of the run was evaluated through an electrochemical terminal with a scan rate of 10 mV s-1 [22, 23].
3.2. Electrochemical thermodynamic equations
The open circuit voltage VN is determined by Nernst equation and given by- (4)
(5)
3.3. Polarization losses
As current flows through a cell circuit, there is a drop in voltage. This drop in voltage is called as the polarization loss ƞ, and occurs due to charge transfer ƞact, ion concentration gradient ƞcon, and circuit Ohmic resistance ƞohm. Hence, the cell voltage ƞcell is calculated by deducting these polarization losses from the open circuit voltage.
(6)
3.3.1. Activation polarization
To calculate the activation polarizations for anode and cathode, respectively, the general form of Bulter–Volmer equation was used-
(7)
The apparent charge transfer-coefficient β value is typically given a value of 0.5 for fuel cell systems [24]. The anode exchange current density j0,an is correlated to the charge transfer resistance Rct and is expressed as-
(8) and
(9) (10)
3.3.2. Ohmic polarization
The current flowing through the electrolyte is expressed as- (11) and the current flowing through the electrodes is-
(12) The resistances occurring in the other parts is expressed as- (13)
where pO2 is the oxygen partial pressure at the cathode surface, CR is a constant referring to the Ohmic resistance of the connections, D is a kinetic factor and is proportional to the amount of electrolyte. The expression for the complete Ohmic polarization is given as-
(14)
(15)
4. Results and discussion
4.1 Proximate and Ultimate Analysis of Fuels
The values for proximate and ultimate analysis for all the fuels used in the study are given in Table 2.
Table 2. Proximate and ultimate analysis of the fuels used in the study.
MF_2.0 UMF AC Elemental analysis
C (wt %) 73.07 77.40 96.42
H (wt %) 1.36 1.97 0.61
N (wt %) 6.77 2.96 0.00
S (wt %) 0.05 0.24 0.61
O (wt %) 18.73 17.43 0.20
Proximate analysis
Ash (wt %) 0.35 0.42 2.16
Moisture (wt %) 7.85 4.69 5.29
4.2. Electrochemical dynamic characteristics 4.2.1. V–j relations and power density
Table 3 gives the comparison of DCFC results for all the fuels used in the study.
The MF_2.0 fuel gave the OCP value of 0.82 V with the highest current density among all the three fuels. This result can be linked to the reduced ash content. The ash content dropped from 8.5% (raw OPMF biomass) to 0.35 % for the pre- treated MF_2.0 fuel.
Table. 3. Obtained experimental values in DCFC for the fuels & their respective ash content
Samples Op.
Temp.
(°C)
OCP
(V) Current Density
(Am-2)
Power Density (Wcm-2)
Ash (%)
Raw_OPMF - - 8.58
AC 800 0.82 3.44 2.11 2.16
UMF 800 0.81 2.87 2.32 0.42
MF2.0 800 0.82 6 2.82 0.35
4.2.2. Activation polarization
The activation polarization occurring in the cell circuit accounts for the most serious loss. The loss is the highest in the case of the AC fuel, followed by UMF fuel and MF_2.0 fuel as seen in Fig. 2 and Fig. 3. This phenomenon probably could be the result of the low electrochemical oxidation rate of the AC fuel mainly because of the high content of ash present in it. In contrast, the pre-treated MF_2.0 fuel performs well with the lowest loss which is a result of its low ash content. These results also indicate that the reaction rate at the anode surface is a crucial factor which determines the voltage losses.
To minimize this activation loss, selection is done for carbon fuels having a high electrochemical oxidation reaction rate [8]. This could be achieved by pre- treating the carbon fuel with alkali or mild acids to wash off the unwanted ash components from the raw fuel source.
Fig. 2. Anode activation polarization loss vs. current density at 800 oC
Fig. 3. Cathode activation polarization loss vs. current density at 800 oC
4.2.3. Ohmic polarization
The voltage losses due to ohmic polarizations for fuel cells operating on low current densities is usually small as compared to the activation loss. At low current densities, the activation loss is dominated. As seen in Fig. 4, the ohmic polarization loss is the highest for AC fuel, followed by UMF and MF_2.0 fuels.
The results show that the functioning of the DCFC is majorly influenced by activation polarization and Ohmic polarization.
Fig. 4. Ohmic polarization loss vs. current density at 800 oC
4. Conclusions
The performance of a DCFC single cell was studied. Some concluding observations from the investigation are given below.
The major voltage loss in the DCFC occurs due to the activation polarization for all the three carbon fuel samples. However, the highest loss was recorded for AC fuel which is a result of the high ash content present in it. The ash components make the electrochemical reaction rate sluggish.
The results also strongly suggest that the anode surface reaction kinetics play an essential role in determining the electrochemical reaction rate. This also means that the contact area of the biochar with anode, carbon particle size and the transport mechanism are other important factors to be considered for a better operation.
The alkali pre-treatment of the OPMF biomass was able to reduce the ash content which in turn reduced the voltage losses occurring in the circuit. Thus, the pre-treated biochar samples were able to perform decently well in a DCFC.
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