The direct carbon solid oxide fuel cell with H2 and H2O feeds
Graphical abstract
Introduction
The direct carbon fuel cell (DCFC) is a promising approach for converting the chemical energy of solid carbon to electricity without combustion or upstream gasification and reforming reactions. DCFCs offer a number of potential advantages: (i) high overall efficiency because of the direct conversion of solid carbon to CO2 [1]; (ii) ultra low NOx emission due to the low reaction temperature of carbon [2], [3]; (iii) simple and low cost operation [4], [5]; and (iv) convenient storage and transportation of carbon with low fire/explosion hazards [6].
DCFC has been studied in molten salt electrolyte fuel cells at 400–750 °C and solid oxide fuel cells (SOFC) at 600–1000 °C [7]. The molten salt electrolyte fuel cells produced peak power densities ranging from 10 to 112 mW/cm2, listed in Table 1. This type of cell suffered from difficulties in maintaining a stable operation due to the potential leakage of liquid electrolytes and their interactions with fuel contaminants.
Carbon-based solid oxide fuel cell (C-SOFC) avoids the technical issues resulting from molten electrolyte. C-SOFCs have been studied with various carbon fuels including coal, petroleum carbon, graphitic carbon, and biomass carbon, as shown in Table 2.
One key advantage of the DCFC is its high theoretical efficiency due to the solid carbon oxidation, reaction (1). The production of carbon dioxide leads to a positive entropy change (ΔS0 = 1.6 J mol− 1 K− 1 at 600 °C) that yields higher than 100% thermodynamic efficiency (η = ΔGT/ΔH298) for carbon-based fuel [14].
This overall oxidation of carbon to CO2 occurs in the SOFC through a number of steps, illustrated in the inset of Fig. 1, (i) reduction of O2 on the cathode to produce oxygen anion, O2 −, (ii) diffusion of oxygen anions across the solid electrolyte to the anode side, and (iii) electrochemical oxidation of carbon fuel to CO2 and CO on the anode catalyst according to reactions (2), (3).
Incomplete oxidation of carbon leading to the formation CO (reaction (3)) is an undesirable reaction because of the low thermodynamic efficiency and the poisonous nature of CO. CO could also decompose to form carbon on the Ni surface, leading to coke formation and cracking of the Ni anode. One objective of this study is to determine the extent of oxidation of carbon by measuring the concentration of CO2 and CO produced on the anode.
CO produced could be further electrochemically oxidized to CO2, reaction (4), and the CO2 product can initiate the reverse Boudouard reaction, reaction (5) [16], [17].
Under high temperature of SOFC operating condition, the addition of hydrogen to carbon fuel can produce methane, reaction (6), which can be further oxidized to CO2 and CO. Hydrogen can be electrochemically oxidized to H2O, which may initiate a number of secondary reactions, reaction (7), (8), in the anode chamber [18], [20].
The CO and H2, the secondary products, could further diffuse into the anode interlayer, contributing to electricity generation; H2O may decrease the coke formation from CO (Boudouard reaction) by consuming carbon monoxide through reaction (8), slowing down the anode degradation. The rate of each above step is controlled by its reaction kinetics governed by the concentration of the reactants and rate constants. Table 3 lists the rate of these possible reactions which are in the same order of magnitude as the rate of electrochemical oxidation for producing CO and CO2 determined from our previous study [17]. The comparable rates of chemical and electrochemical reactions indicated that the concentration of CO, CO2, H2 and H2O can play a significant role in affecting the overall performance of the fuel cell since the gaseous species such as CO or H2 can further produce electricity through electrochemical oxidation. Although adding catalysts onto carbon could further enhance its reactions with H2, H2O, and CO, the residual catalyst can deposit on the anode surface, changing the catalytic properties of the anode surface.
Electrochemical oxidation of solid carbon particles occurs only on the external surface of the anode. In contrast, the electrochemical oxidation of gaseous hydrogen and methane fuels primarily take place on the active sites, which are located inside of the porous anode structure and are inaccessible to solid carbon [16]. The key challenge of developing carbon fuel cell is the delivery of solid fuel to the electrode/electrolyte interface, in which majority of fuel electrochemical oxidation occurs [24]. In our previous work a solid fuel injection system has been developed that allowed bringing solid fuels to the inside of the anode chamber without exposure to air [16]. In this study, we further added H2 and H2O to gasify the solid carbon in the anode chamber. The added H2 and H2O may react with the carbon fuel to produce a sufficient quantity of CO to access the interior TPB, which is inaccessible to carbon. Thus, another objective of this study was to evaluate the promoting effect of low hydrogen content feed (3 mol% H2 in He) and steam (7 mol% H2O in helium) on the C-SOFC performance using coconut carbon as fuel. Coconut fuel was used because it has been shown to contain low ash that minimizes the fouling of anode [25], [26].
Section snippets
Fuel cell fabrication
The anode-supported cell comprised of a NiO/YSZ (Yttria stabilized Zirconia) anode support (850 μm, 65 wt.% NiO), NiO/YSZ anode interlayer (40 μm, 60 wt.% NiO), 30 μm YSZ electrolyte, 25 μm LSM (lanthanum strontium manganite)/YSZ (70 wt.% LSM) cathode interlayer, and 35 μm LSM cathode current collection layer. The half cell (anode support, anode interlayer, and electrolyte) was fabricated by co-tape casting technique using viscous slips containing NiO (AEE Co.), YSZ (TZ-8Y, Tosoh), binder,
Fuel cell testing
C-SOFC was operated by introducing solid carbon and a pure helium flow to the anode chamber. CO and CO2 were the major products of carbon oxidation via reactions (2) and (3). Voltage–current curve of C-SOFC along with the molar concentration of CO and CO2 at the exhaust of the anode chamber is shown in Fig. 2. Increasing the current density caused a significantly larger increase in the formation of CO than that of CO2. The low concentration of CO2 has been observed in our previous studies [16],
Conclusions
One of the key technical challenges for DCFCs is the delivery of solid fuel to the electrode/electrolyte interface, in which majority of fuel electrochemical oxidation occurs. Molten salt and molten metal fuel cells or fuel processing can be used to surmount this problem. In this work, the addition of H2 and H2O to gasify the solid carbon was used as a fuel processing technique. This study shows that C-SOFC produced CO as a major product. CO was produced from electrochemical oxidation of carbon
Acknowledgments
This work received financial support from Department of Energy (DE-FC 3606G086055), Ohio Coal Development Office (DE-FC36-08GO0881114), and FirstEnergy Corporation (DE-FE-0000528).
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