Insight into graphite oxidation in a NiO-based hybrid direct carbon fuel cell
Graphical abstract
Graphite carbon (GC) shows excellent durability performance at 0.7 V load in an electrolyte-supported HDCFC with CO as the primary gas in the outlet.
Introduction
A direct carbon fuel cell is a device that converts the chemical energy of solid carbon into electricity with no external gasification or reforming process [1], [2], [3]. In recent years, it receives more attention due to high energy conversion efficiency and its easy availability of the fuel. Solid carbon can be obtained from a variety of natural resources, e. g., forest, agriculture, and food waste [4]. Up to now, so many kinds of carbon fuels have been applied in DCFCs. Researchers have focused on the investigation of the utility of different fuels like pure carbon [5], [6], [7], [8], biochar, coal or even raw biomass in a variety of cell configurations, such as molten hydroxides-based DCFCs, molten carbonates-based DCFCs, and solid oxide conductors DCFCs [9], [10], [11].
In hydroxide-based DCFC, Hackett et al. [12] evaluated various carbonaceous materials, especially graphite (GC). They investigated the effect of GC rods’ fuel structure on the open-circuit voltage and life, and, they also conducted a lifetime comparison.
Weaver et al. [13] presented a molten carbonate electrolyte to be used for direct carbon conversion in 1979. They tested several types of carbon fuels and concluded that devolatilised coal is more reactive than spectroscopic carbon and pyrolytic GC. They linked the high reactivity to the large surface area and the poor crystallisation. The most reactive material, JPL coal, produced 100 mA cm−2 at 0.8 V vs. reference electrode at 700 °C, the potential at 100 mA cm−2 increasing to 0.9 V at 800 °C. Besides, they measured the anode off-gas as 90% CO2 at a high current density and found the CO2/CO ratio decreased at lower current densities. In molten carbonate-based DCFCs, the pore volume and the surface area influence the wettability between the fuel and the molten carbonate, promoting electrochemical reactions in the DCFC system [14]. Chen et al. [15] and Peng et al. [16] have demonstrated that the wetting behaviour of the carbon anode may be as crucial as the molecular mechanism for carbon oxidation in molten carbon DCFCs. Li et al. [17] and co-workers investigated the effects of the chemical and physical properties of fuels such as composition, structure, surface area, and surface functional groups on electrochemical reactions. They found that desirable structure of the carbon fuel for carbonate-based DCFC is to have a high mesoporous surface area and oxygen-rich surface groups [18]. They confirmed that carbon with oxygen-rich surface groups possesses high electrochemical reactivity. They also performed surface modification of activated carbon and carbon particles with acid or air plasma. Similarly, the acid-treated carbon gives the best performance due to the most considerable degree of surface oxygen functional groups [19]. However, Cherepy et al. [20] recently carried out a work using eutectic molten carbonate electrolyte for the direct conversion of solid carbon particulates. They found that the surface area has no substantial effects on the carbon discharge rate.
In recent years, significant efforts have been made to study the relationship between the characteristics of the carbon fuel and the performance of the DCFC device [21], [22], [23]. Carbon fuel properties are vitally important to the output performance of direct carbon fuel cells (DCFCs). Features such as surface functional groups, surface area, pore structure, the degree of crystallinity have been investigated, and the effects of these parameters on the cell performance have been evaluated [6]. Although the utilisation of carbon in fuel cells has been widely studied, the oxidation mechanism of carbon at the anode is not fully understood. GC is a typical fuel for DCFCs as its graphitic structure. A literature survey demonstrates that GC has been extensively studied and experimentally evaluated as a fuel in a variety of DCFC configurations. In different settings, the carbon oxidation mechanism might change remarkably. Thus, different conclusions were reported. It is said that carbon fuels with a high surface area (e.g., devolatilised coal) are more accessible to the anode reaction than GC [13]. However, Cherepy et al. [20] experimented with various carbon samples in a molten carbonate electrolyte and obtained the best performance of 50 mA cm−2 at 0.8 V on GC carbon, which might be due to its excellent conductivity. Vutetakis et al. [24] reported GC-fuelled molten carbonate-based DCFCs shows better performance than that of an anthracite-fuelled DCFC, but poorer performance than that of a diamond-fuelled DCFC. Li et al. [18] carried out a comparative experiment on GC, carbon black, and active carbon in molten carbonate-DCFCs. In their research, GC showed the lowest electrochemical reactivity at different potentials and temperatures, and the peak power density delivered was around 17 mW cm−2 at 800 °C, implying low electrochemical activity of the highly ordered GC. Recently, Chen et al. [25] studied the performance of GC as a fuel in DCFC based molten carbonate Li2CO3/K2CO3/Al2O3. They concluded that GC alone could not perform well as the anode fuel in DCFC although it has a regular structure, a high electrical conductivity, and a low resistance. Meanwhile, the influence of GC fuel on the performance and lifetime of the DCFC are still up for debate. From the above results, we can see that most research is focused on the molten carbonate DCFCs. The reactivities of GC in hybrid direct carbon fuel cells with a molten carbonate fuel cell and a solid oxide fuel cell is not extensively studied.
We have demonstrated the carbon oxidation process in a hybrid direct carbon fuel cell using carbon black as the fuel in our previous research [26]. Our initial investigation on the application of different carbon fuels in HDCFCs showed that GC in an anode-supported cell exhibited poor cell performance of 75 mW cm−2, which is much lower than 100 and 210 mW cm−2 at 750 °C with carbon black and activated carbon as the fuel, respectively [27]. Among all the fuels, biomass carbon (BC, pyrolysed medium density fibreboard) generated the best cell performance with a maximum power density of 878 mW cm−2 at 750 °C [28].
In this study, we will carry out further investigation on the key factors affecting the electrochemical reactivity of carbon fuels in the HDCFC system. We will focus on the GC fuel on a thin NiO-electrode and a thick NiO-anode. A standard fuel of carbon black (CB) widely used in other literature and a biomass carbon (BC) [29] are used for comparison regarding the difference in chemical and surface properties. In this paper, for the first time, we investigated the electrochemical oxidation of fuel in the DCFC with in-situ gas analysis under electrochemical test.
Section snippets
Pre-treatment of carbon fuels
The carbon fuels for experiments are graphite carbon (GC, Cabot), carbon black (CB, XC-72R, Cabot), and biomass carbon (BC). The biomass carbon was pyrolysed at 400 °C in nitrogen. A mixture of 62 mol% lithium carbonate (Aldrich Chemical Co., WI, USA) and 38 mol% potassium carbonate (Fisher, UK) was pre-mixed by ball milling in acetone for 24 h before mixed with different carbons. The weight ratio of carbon to Li2CO3– K2CO3 is 4:1. The same process was used for the carbon-carbonate mixture,
Electrochemical performance of electrolyte-supported cells
The electrochemical reactivity of different carbon fuels was evaluated on HDCFC with a thin NiO anode. Fig. 1 shows electrochemical performance of HDCFCs with GC, CB, and BC fuels. The polarisation curves of these three carbon fuels are similar in shape with linear curves presented. It demonstrates that BC produces the highest cell performance (Fig. 1a). GC-fuelled cell gives slightly lower performance than the BC-fuelled cell. The HDCFC operated with CB fuel offeres the lowest performance. The
Conclusion
The electrochemical reactivity of carbon fuels on nickel oxide-based hybrid direct carbon fuel cells (HDCFCs) has been investigated. Graphite (GC), carbon black (CB), and biomass carbon (BC) fuels were chosen as the fuel. GC generated good initial cell performance and long-term stability on the electrolyte-supported HDCFC. It suggested that the HDCFC performance is affected by composition and surface property, and is less likely to be dependent on the particle size and the crystal structure
Acknowledgement
This work was supported by Sichuan Science and Technology Program (2019YFH0177), the talent introduction plan of Sichuan University of Science and Engineering (2016RCL36, 2016RCL37), the enterprise cooperation project of Sichuan Oil Technology Co. Ltd. (HX2017087); and the opening project of Material Corrosion and Protection Key Laboratory of Sichuan Province (2017CL11 and 2017CL13), China. CJ acknowledges the Royal Society of Edinburgh for an RSE BP Hutton Prize in Energy Innovation.
References (37)
- et al.
A high-performing direct carbon fuel cell with a 3D architectured anode operated below 600 degrees C
Adv Mater
(2018) - et al.
Direct carbon fuel cell operation on brown coal
Appl Energy
(2014) - et al.
Surface modification of carbon fuels for direct carbon fuel cells
J Power Sources
(2009) - et al.
Solid state electrochemistry of direct carbon/air fuel cells
Solid State Ion
(2008) - et al.
A novel direct carbon fuel cell by approach of tubular solid oxide fuel cells
J Power Sources
(2010) - et al.
Performance enhancement of molten carbonate-based direct carbon fuel cell (MC-DCFC) via adding mixed ionic-electronic conductors into Ni anode catalyst layer
J Power Sources
(2018) - et al.
Evaluation of Sc2O3-CeO2-ZrO2 electrolyte-based tubular fuel cells using activated charcoal and hydrogen fuels
Electrochim Acta
(2018) - et al.
Evaluation of carbon materials for use in a direct carbon fuel cell
J Power Sources
(2007) - et al.
Application of refuse fuels in a direct carbon fuel cell system
Energy
(2013) - et al.
Direct Carbon Fuel Cells - wetting behavior of graphitic carbon in molten carbonate
Int J Hydrogen Energy
(2016)
Evaluation of raw coals as fuels for direct carbon fuel cells
J Power Sources
Utilization of wood biomass char in a direct carbon fuel cell (DCFC) system
Appl Energy
Use of ash-free "Hyper-coal" as a fuel for a direct carbon fuel cell with solid oxide electrolyte
Int J Hydrogen Energy
Experimental investigation of Direct Carbon Fuel Cell fueled by almond shell biochar: Part II. Improvement of cell stability and performance by a three-layer planar configuration
Int J Hydrogen Energy
Lignite as a fuel for direct carbon fuel cell system
Int J Hydrogen Energy
Carbon anode in direct carbon fuel cell
Int J Hydrogen Energy
Catalysis and oxidation of carbon in a hybrid direct carbon fuel cell
J Power Sources
Electrochemical performance of different carbon fuels on a hybrid direct carbon fuel cell
Int J Hydrogen Energy
Cited by (7)
Impacts of reactant flow nonuniformity on fuel cell performance and scaling-up: Comprehensive review, critical analysis and potential recommendations
2021, International Journal of Hydrogen EnergyCitation Excerpt :The anode and the cathode are composite materials of ceramics and metals (cermet) of porous nature [41–44], they allow the flow of gases between the channels and the electrolyte, and they serve as catalysts for the electrochemical reactions and transmit the current flow [45]. Fuel oxidation occurs in the anode while oxidant reduction takes place in the cathode, the electrolyte conducts the ions (H+ and O2−) while the electrons, after being generated due to the anode semi reaction, travel through an output circuit into the cathode to take part in the cathode semi reaction [46–49]. There are several types of fuel cells, among them there are, proton exchange membrane fuel cells (PEMFC) [50], direct methanol fuel cells (DMFC) [51], alkaline fuel cells (AFC) [52], phosphoric acid fuel cells (PAFC) [53], solid oxide fuel cells (SOFC) [54], molten carbonate fuel cells (MCFC) [55], microfluidic fuel cells [56], and biofuel cells [57,58].
Carbon derived from treated rice husk as fuel for direct carbon fuel cells
2022, International Journal of Energy ResearchResearch Progress of Fuels for Direct Carbon Fuel Cells
2022, Rengong Jingti Xuebao/Journal of Synthetic CrystalsReview of molten carbonate-based direct carbon fuel cells
2021, Materials for Renewable and Sustainable Energy