Insight into graphite oxidation in a NiO-based hybrid direct carbon fuel cell

https://doi.org/10.1016/j.ijhydene.2019.08.208Get rights and content

Highlights

  • Graphite oxidation is investigated in a NiO-based HDCFC.

  • Graphite shows the best cell performance on the electrolyte-supported HDCFC.

  • Cell performance is dependent on both of the fuel property and the cell configuration.

  • In-situ gas analysis is an effective way of identifying possible electrochemical reactions.

  • CO is the main gas for graphite oxidation.

Abstract

A direct carbon fuel cell is an electricity generation device using solid carbon as a fuel directly with no reforming process. In this study, three-carbon fuels, graphitic carbon (GC), carbon black (CB), and biomass carbon (BC) are tested as the fuel to investigate the influence of carbon fuel properties on the cell performance in HDCFC with a traditional nickel oxide as the anode. Either an electrolyte-supported cell with a thin nickel oxide anode or an anode-supported cell with a thick nickel oxide anode is used to evaluate the electrochemical reactivity of carbon samples. These three-carbon fuels are characterised on the crystal structure, particle size, composition, and surface property. It is found that GC shows excellent cell performance on thin nickel oxide anode. However, it displays relatively slow electrochemical reactivity on the thick anode due to its great extent of carbon oxidation. BC shows good initial cell performance but fast degradation of the cell performance, as much more hydrogen is released at the beginning of the cell test. The anode reactions of HDCFCs are explored by the in-situ gas analysis in open circuits and under current load conditions. It is observed that GC produces the highest amount of CO among these three fuels, suggesting that carbon oxidation is the dominant electrochemical process in HDCFCs after a certain time when most of the hydrogen is released from the pyrolysis process.

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.

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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)

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