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SOFC and MCFC: Commonalities and opportunities for integrated research
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SOFC and MCFC: Commonalities and opportunities for integrated research Stephen J. McPhail a,*, Anja Aarva b, Hary Devianto a, Roberto Bove c, Angelo Moreno a a
ENEA Energy Department e Hydrogen & Fuel Cells, Rome 00123, Italy TKK Helsinki University of Technology, Espoo, Finland c Alstom Power, Birr, Switzerland b
article info
abstract
Article history:
The present article explores the key issues for research and development that are common
Received 3 February 2010
to current state-of-the-art MCFC and SOFC technologies. By analyzing overlapping aspects
Received in revised form
regarding materials, operating conditions and applications of the two types of high-
26 July 2010
temperature fuel cell (HTFC), the most pressing common challenges are set forth. Simi-
Accepted 22 September 2010
larities between the MCFC and SOFC exist especially on the anode side, given their
Available online 15 October 2010
utilization of nickel as the oxidation catalyst and their suitability for conversion of hydrocarbon fuel. Catalyst deactivation due to contaminant poisoning and carbon depo-
Keywords:
sition thereby emerges as the crucial problem to
Contaminant poisoning
mechanisms is given and of relevant studies in the field of HTFCs. The implications on the
overcome. A brief review of these
Sulphur
HTFC catalyst material given by this review are summarized and put forward as lines of
Carbon deposition
research that can be undertaken jointly by players both in MCFC and SOFC development.
High-temperature fuel cell materials
Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Since there exists a wide variety of fuel cell types, in this study only a certain class will be discussed: the high-temperature fuel cells (HTFC), in particular the Molten Carbonate (MCFC) and Solid Oxide fuel cells (SOFC). The fundamental aspect that differentiates this typology from low-temperature fuel cells, is evidently that e operating typically at temperatures of over 600 C e the heat produced can be utilized in fuel processing and heat generation systems. Generally high-temperature fuel cells have the potential to be independent of a pure hydrogen infrastructure, and can find their application in transitional fuels such as natural gas, syngas and biogas. Thus, as compared to low-temperature fuel cells, they can be more flexible in accepted fuels, which implies the great advantage that they can be implemented more easily into nowadays’ fuel range and infrastructure. Also, where heat
off-take can be guaranteed in addition to power consumption, they can provide unsurpassed efficiencies, approaching 90% total efficiency (electrical plus thermal) [1]. However, this potential still remains unfulfilled since MCFC and SOFC stacks operating in real-world conditions still too easily incur material deactivation or structural change in their active components, leading to unwelcome performance degradation. Success of HTFCs in gaining a solid foothold on the highly competitive power train market, however, depends just as much on the effective integration of many, crucial components that make up the fuel cell system. Apart from the electrochemical heart e the fuel cell stack e its optimal performance depends amongst others on fuel and air conditioning, heat management, process control, load regulation and operating conditions. In this paper, however, we focus on cell active components: anode, cathode and electrolyte. This is done by identifying the commonalities between MCFC and
* Corresponding author. E-mail address: [email protected] (S.J. McPhail). 0360-3199/$ e see front matter Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.09.071
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SOFC operating frames and the implications of these conditions on the current state-of-the-art materials of the two HTFCs. The issues thus evinced are by no means suggested as being the most important in their respective research fields; it is simply stated which areas could benefit from a joint approach and the sharing of knowledge. By recognizing the similar needs of these two technologies, an indication is hoped to be given as regards the focus of research and development efforts in the present and near future on material improvement that would mutually benefit both fuel cell types. Fulfilling these requirements would mean a considerable step forward towards fully fledged maturity of hightemperature fuel cells, and a joint, directed effort by the MCFC and SOFC communities could speed up this process to make them finally meet market requirements.
2.
Cell reactions and fuel conversion
The main features of the MCFC and SOFC [2,3] are tied to their operating temperature. In the former case, a minimum temperature level of 600 C has to be maintained to guarantee the liquid state of the carbonate eutectic and thus the mobility of the carbonate ions. In the latter, oxygen ions are transferred from cathode to anode through oxygen conducting solid electrolyte [2e4]. SOFCs can be operated between 600 and 1000 C [4]. In other words, the charge carriers that are used to close the electrical circuit are different for SOFC and MCFC: ionized oxygen (O¼) in the case of the SOFC, carbonate ions (CO¼ 3 ) in the MCFC. For the latter, this requires an additional feed of CO2 at the cathode side. For the former, this means that if pure hydrogen is introduced at the anode, no carbon is involved in SOFC operation. Electrochemical reactions at the anode and cathode of the MCFC and SOFC are set out schematically in Fig. 1. It should be mentioned that interesting efforts exist to combine the advantages of the MCFC and SOFC by creating a composite electrolyte based on carbonate-impregnated
ceramics (e.g. [5e8]). It is however outside the scope of this article to look into radically new fuel cell concepts, as the purpose is to find accelerated means to render “conventional” state-of-the-art HTFCs a product, fit for market penetration. High-temperature operation offers distinct advantages: (electro) chemical reactions are more rapid resulting in faster reduction and oxidation kinetics, thereby eliminating the necessity for noble metal catalysts. Conventionally nickelbased anode catalysts are used in these HTFCs [2,4]. In addition to cost reduction, this implies that carbon monoxide does not exhibit any poisoning effect on the fuel cell, and on the contrary can be used as an additional fuel [2]. The high temperature is also eminently suitable for light hydrocarbon reforming, which can take place directly inside the cell, by or utilizing a heat-integrated reformer element. The heat released by the electrochemical oxidation of the fuel can be utilized by the endothermic reforming reaction (1) [2]: (1) Cx Hy þ xH2 O/ 1=2y þ x H2 þ xCO The hydrogen thus produced (and to a much lesser degree the carbon monoxide) oxidizes electrochemically at the anode to produce water, which then can be consumed by reaction (1). When utilizing biogenous fuels e i.e. including fossil fuels like natural gas or refined oil e carbon is always part of the reaction. Once reformed to carbon monoxide, this can either react electrochemically with the electrolyte ion to form CO2, or e much more easily e is converted chemically with steam to hydrogen and carbon dioxide, according to the so-called water-gas shift reaction [9]: CO þ H2 O4H2 þ CO2
(2)
As a consequence of these reactions, water and CO2 are formed on the anode side. As H2 is consumed at the anode (electrochemical reactions, Fig. 1) water is formed, and reaction (2) favours the production of more H2 for electrochemical oxidation. For the MCFC, CO2 is needed on the cathode side to generate the charge carrying carbonate ion. Since the amount
Fig. 1 e A schematic overview of a high-temperature fuel cell, with anode and cathode electrochemical reactions in the insets. When hydrocarbons are utilized as fuel, the anode can act as a reforming stage (reaction (1)).
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of CO2 required at the cathode is the same as that formed as a consequence of the anode reaction, spent anodic gas is generally recycled from the anode to the cathode. This suggests the potentially interesting role that MCFCs could have in terms of CO2 separation. As a consequence of the MCFC’s operation, CO2 is continuously transferred from the cathode to the anode. This particular feature could be exploited for separating CO2 originating from a traditional thermal power plant and concentrating it in the anode exit stream where it can be captured and sequestered (Fig. 2).
3.
MCFC and SOFC: common challenges
As was seen from the previous chapter, the chief similarity between MCFCs and SOFC lies on the anode side. Favorable conditions for the reforming of light hydrocarbon fuels combined with the nickel-based catalyst used in both fuel cell types, lead us to focus on anode side issues. In this section the major implications of utilizing carbonaceous fuels in HTFCs will be discussed. In tackling these challenges a common approach and joint effort between MCFC and SOFC can be undertaken, increasing the likelihood and speed of success.
Table 1 e Contaminants and their tolerance limits for MCFCs [11]. Contaminant
Tolerance
Sulphides e.g. H2S, COS, CS2 Halides e.g. HCl, HF Siloxanes e.g. HDMS, D5 Particulates Tars Heavy metals e.g. As, Pb, Zn, Cd, Hg
0.5e1 ppm
Fuel impurities
Though high-temperature fuel cells have the advantage of not requiring noble metal catalysts for the electrochemical reactions, some species have a poisoning effect on the catalytic properties of the electrodes. Giving accurate tolerance limits for all the possible fuel impurities and their effects on HTFC materials can be quite difficult. Especially for SOFCs, that are operated in a wide range of temperatures (600e1000 C) with a wide range of catalysts (this will be discussed more in chapter 4), strict categorization of certain contaminant effects and comparing of results is complicated. MCFC operating conditions and catalyst materials are more established than for SOFC, and an overview of contaminant effects can be attempted [11] (Table 1). Even for the MCFC, however, the tolerance levels are indicative (a margin of safety is included in the values of Table 1) and the extent of their harmful effect may depend on the partial pressure of other species in the gas (e.g. hydrogen, water), the current density at which the fuel cell is operated, temperature and the fuel utilization factor. Exposure time to the various impurities is also determinant as regards the extent of the
10e100 ppm 10e100 ppm 2000 ppm 1e20 ppm
Electrode deactivation Usurpation of electrolyte Corrosion Usurpation of electrolyte Silicate deposits Deposition, plugging Carbon deposition Deposition Usurpation of electrolyte
damage caused and the potential for its reversal. Elaborate investigations into the endurance to contaminants are few, since experimentation of these effects is necessarily destructive and of long duration, but an accurate knowledge of the conditions which are deleterious is highly desired. Identification of the limits of safe operation would greatly enhance fuel cell durability as well as optimise the integration of the fuel clean-up stage in terms of requirements and cost.
3.1.1. 3.1.
0.1e1 ppm
Effects
Special case: hydrogen sulphide poisoning
Many of the possible fuels for HTFCs, for instance biogas derived from sewage sludge, can contain significant amounts of sulphur compounds [11,12]. Sulphur-containing odorant is also often added in natural gas for safety reasons [2,13]. Traces of sulphur in the fuel can lead to a severe decrease in the cell performance due to catalyst deactivation [3,14]. In past research, above all the effects of hydrogen sulphide (H2S) have been studied, as this is the most abundant contaminant present in gases from organic origin. However also organic sulphur compounds can be present and are equally bad for the nickel-based catalyst (see Fig. 3). The effects of sulphides are dependent on many factors such as the bulk concentration, the concentration relative to hydrogen in the fuel, humidity, electrical load and temperature. As temperature decreases, the propensity of Ni to react with sulphur tends to increase [15]. Though experiments have shown that the drastic effect of sulphur-containing compounds on electrode activity seems irreversible upon enduring exposure to concentrations of more than 10 ppm or even 5 ppm [9,12], thermodynamic equilibrium calculations show that no permanent, bulk nickel-sulphide phases should be formed until concentrations of over 100 ppm [15].
Fig. 2 e The MCFC as a CO2 separator [10].
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Fig. 3 e (a) open-cell reticulated Ni foam pre-test and (b) open-cell reticulated Ni foam post-test with H2S exposure [16].
In MCFCs hydrogen sulphide not only reacts with the anode material, but it also interacts with the electrolyte [17]. Reaction of hydrogen sulphide with the nickel on the anode leads to blocking and deactivating the electrochemically active sites for hydrogen oxidation [18e20]. The affected sites give rise to morphological changes in the anode structure, and can thereby cause further deterioration of cell performance through secondary effects like impeded gas diffusion, volume change or reduced wetting by the electrolyte. At the electrolyte, hydrogen sulphide can react chemically with carbonates to form either sulphide or sulphate ions [18], thereby uses electrochemically active charge carriers which would otherwise be available for the hydrogen oxidation. This translates in reduced cell performance. However, hydrogen sulphide can also react electrochemically with carbonates [21], releasing electrons, but yielding harmful, ionized sulphate compounds. There has also been evidence that suggest that the electrolyte material has an effect on the sulphur tolerance in SOFCs [12]. The complex interaction of chemical, electrochemical, equilibrium and kinetic mechanisms governing the fate of H2S in HTFC operating conditions make it exceedingly difficult to separate and quantify the several poisoning effects of hydrogen sulphide. A preliminary study by Hansen [22] has investigated the applicability of so-called Temkin kinetics to unify the effects of different parameters in a unique correlation. Here, the quantity of sulphur coverage is identified as the governing variable that determines the loss of performance of a nickel-based anode at given current density. The correlation is tested on published experimental results obtained with three different anodes and a wide range of temperatures and sulphur concentrations. The data are consistently approximated by the correlation with a coefficient of determination (R2) of 0.9 or higher. Sulphur poisoning can be reversible at low concentrations [23,24], thus it can be possible e within limits not yet welldefined e to recuperate the fuel cell’s original performance. These reactions are under investigation and could provide a crucial, added controlling parameter to guarantee long-term reliability of a nickel-based HTFC stack. To find the governing conditions that establish the limits of safe operation and poisoning reversibility is of exceeding importance, especially
when fuels are considered with intrinsically fluctuating compositions and impurity levels, like biogas from anaerobic digestion or syngas from biomass gasification.
3.2.
Carbon deposition
In addition to fuel impurities, also fuel components can cause problems. When hydrocarbons are used as a fuel there is always a risk of carbon formation. Unfortunately nickel is not only a good electrocatalyst and reforming catalyst, it also has high activity towards carbon deposition [2e4]. The use of carbon-based fuels leads to a well-known problem in HTFCs, namely carbon deposition on the anode. The main consequence is the loss of electrochemically active sites, and ultimately, reduced performance. In addition, it has been reported that the growth of filaments attached to the anode crystallites can modify anode micro structure [25], drastically reducing its durability [26]. Plugging of the gas passages by the accumulation of carbon on the anode is another plain consequence. When methane is considered as fuel, among other possible reactions, Boudouard (3) and methane cracking (4) are considered the most probable responsibles for carbon deposition, together with carbon gasification (5) [9,27,28]: 2CO4C þ CO2
(3)
CH4 4C þ 2H2
(4)
CO þ H2 4C þ H2 O
(5)
A first thermodynamic consideration is that the increase of the operating pressure favours C formation, according to equations (3) and (4), while water addition reduces formation of C (see Fig. 4). In fact, gasifying agents, such as water, can remove carbon from the catalyst surface (reverse reaction (5)) [29,30]. When methane steam reforming is considered, at 600 C, the steam-to-carbon value (mole-based) below which carbon formation begins is 1.5, and reduces to 1 at 800 C and higher [9]. Higher hydrocarbons show a higher tendency towards carbon formation than methane. It is a common
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Fig. 4 e SEM mapping of carbon deposition on Ni-10 wt%Cr MCFC anode at cell temperature 650 C, pressure 1 atm, exposure time 1000 h: (a) at S/C ratio 3.5 (b) at S/C ratio 2.5.
practice to use a high steam-to-carbon ratio of the fuel feed in order to suppress carbon formation [29,31]. However high water content of the fuel feed is unwanted because it reduces the equilibrium potential and thus the power output of the cell [3]. In addition thermodynamic calculations can be an unreliable way to predict carbon formation [32,33], because the rates of carbon removal and carbon deposition are not always fast enough to establish equilibrium, as shown in [34]. Carbon deposition is well-known from the steam-reforming industry. However, when reforming reactions occur at the anode, additional phenomena can drive carbon deposition since electro catalysis can favour coke formation kinetics. Similarly, direct reaction of a carbon-containing fuel at the HTFC anode can give rise to a number of factors leading to carbon deposition. Catalyst activity is influenced by current density, and it is expected that carbon deposition is also dependent on the current density at which the cell is operated, in addition to fuel chemical composition and temperature [35]. Furthermore, the current density influences the variation of the chemical composition of the gas within the anode. Singh et al. [27] performed a thermodynamic evaluation of carbon formation in a SOFC operated on syngas from a biomass gasifier. For different tar content, a so-called threshold current density is calculated. This quantity is defined as the current above which no carbon deposition is expected. Recently, Mermelstein et al. [36] reported experimental results of carbon deposition due to tars in a Ni-YSZ-based SOFC. The results, as expected, show that carbon formation is relevant when no water is added to the mixture. However, contrary to thermodynamic calculations, carbon deposition is not completely suppressed at steam-to-carbon values from 2 to 3. Experimental results on subscale MCFCs operated at 10 bar on simulated gasified coal humidified at 165 C, showed no carbon deposition [9]. While carbon deposition can be avoided through proper design and operation of the system, there is also an interest in defining novel materials that prevent it. In particular, additives can be used in the anode for reducing carbon deposition. This will be discussed in Section 4.
4. Material implications for active components As has been discussed, most similarity between the two technologies exists at the anode, where conventionally nickel
provides the active sites for the electrochemical oxidation. This is why, when utilizing hydrocarbon fuels rather than pure hydrogen, the problems arise that are common to both MCFC and SOFC and that are the chief obstacles to carefree operation. In this chapter we present a short introduction to current stateof-the-art materials used for the fuel cell active components, i.e. anode, cathode and electrolyte, combined with a discussion of how the challenges described above are tackled in component development. Again, no pretence is made to list all the important material issues that are still prevalent in both MCFC and SOFC (TEC mismatch, acidic corrosion, electrolyte evaporation, redox, chromium poisoning, particle agglomeration, phase instability etc.). Being specific to one of the two technologies, many are neglected on purpose. For these, the reader is referred to reference texts such as [2] and [3].
4.1.
Anode materials
4.1.1.
MCFC anode
Since the early seventies, porous nickel is the material used for the anode, strengthened against creep and fracture by the addition of chromium and/or aluminium. The chromium, like the aluminium, is effective against loss of surface area, pore growth and sintering. However chromium has the tendency to consume carbonate through lithiation by the electrolyte, which decreases performance and jeopardises the integrity of the NieCr alloy. Aluminium appears to be less prone to lithiation with the carbonates in the long run. Adding alloying agents can compromise porosity and conductivity requirements. The current level of Cr or Al addition is between 6 and 10%, which seems to have resolved most problems relating to structural strength, performance and cell life time [37]. The porous anode structure is impregnated with an excess of the liquid electrolyte in order to prevent depletion of the electrolyte through leakage, metal reaction and evaporation during long-term operation. However the impregnation of the anode by the electrolyte has to be weighed against the diminution of active surface area and conductivity, and the increase of the gas phase mass transport resistance, leading to additional polarization losses. The optimal electrolyte filling degree, furthermore, depends on pore size distribution, electrode thickness, electrolyte composition and even on fuel gas composition [38]. The current additives to the anode base material, added to the right degree and following careful procedures for their
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dispersion, guarantee sufficient mechanical strength and have in a large part solved the problem of long-term creep. Main issues left open are therefore resistance to carbon deposition and especially contaminant poisoning e in particular by sulphur and halogens. A significant development in this respect, is the intermixing of ceramic oxides with the nickel. Being less susceptible to carbon deposition, ceramic oxides (like lanthanum, cerium, zirconium and samarium, with additives for the improvement of electronic conductivity), offer the prospects of being fuelled by dry methane, as is the case with SOFCs [39]. Moreover, ceramic-based or ceramic-coated anodes improve sulphur tolerance notably, possibly by an order of magnitude, in addition to being morphologically stable during cell operation and increasing electrolyte wetting [40,41]. Furthermore, they are potentially low-cost. Continued studies indicate the validity of the concept [42]: the challenge is to optimise the procedures for regeneration of the contaminated anode, since the harmful species trapped on the ceramic oxides can saturate their selectivity. Regeneration can take place passing a clean gas over the anode that reabsorbs the pollutant species adhered to the catalytic sites, either by reducing the catalyst or by oxidizing and releasing the pollutants. This procedure is not ideal from a steady state process point of view and research is directed towards minimizing or even bypassing this step. A solution might be to find a material that selectively repels the actual adhesion of contaminants to the reactive sites, possibly through pre-saturation, but the viability of this idea has not yet been proved. In all cases it is necessary to find a coating or protective material that is morphologically and structurally stable and a fabrication process that maintains cost-effectiveness.
4.1.2.
SOFC anode
The SOFC anode is typically a cermet structure i.e. a mixture of nickel and the ceramic electrolyte, usually YSZ [3]. Nickel has been the main anode catalyst material used in SOFCs since 1964 [4]. Nickel is an active electrocatalyst, especially when hydrogen is used as a fuel, and it is chemically stable with electrolyte material [3]. Unlike cathode and electrolyte materials, nickel shows good performance at all temperature ranges [43]. Also the price of nickel is competitive [3,43]. As discussed before in Section 3, the Ni/YSZ anode has a good performance when hydrogen is used as a fuel, but problems occur when hydrocarbons are fed to the cell. Although nickel catalyses internal reforming of hydrocarbons, nickel also has high activity for carbon formation [3,4,32]. As an alternative to Ni/YSZ other cermet structures have been investigated. Ni/SSZ has shown lower overpotential and better stability at 800 C than Ni/YSZ when hydrogen was used as a fuel [43]. It has also been reported by Sasaki et al. that Ni/ SSZ anode has increased tolerance against sulphur poisoning [8]. Ni combined with ceria-based ion conductors has also given promising results [3,43]. For instance Ni/GDC is reported to improve ionic conductivity, catalytic activity and long-term stability [43]. In addition, tendency to form carbon is suppressed [3,43]. In order to reduce carbon deposits there has been growing interest in replacing nickel with copper [43] and zirconia with ceria [32]. Copper cermets can be less expensive than
nickel anodes and they have demonstrated to have better resistance against carbon formation. Also alloying copper with nickel reduces carbon formation [43]. Other possible elements that have been examined are titanium, cobalt, iron, silver, gold, platinum, ruthenium and rhodium [4,43,44]. Unfortunately platinum does not last long and peels off after few hours in SOFC operating conditions [4]. Mixing some gold (1%) to Ni/YSZ anode has shown to decrease the anode’s tendency to form carbon [44]. Although gold, silver and copper are very good electric conductors they are not highly active oxidation catalysts [32]. Silver (mp 962 C) and copper (mp 1083 C) have melting points that are too low for conventional cermet sintering [4,32]. Iron and cobalt are not resistant enough to the corrosive atmosphere [4]. In addition cobalt, iron, platinum, ruthenium and rhodium are said to be too expensive or not to offer any advantages compared to nickel [43,44]. One possibility is to use mixed ionic electronic conductive ceramics [3,32]. It has to be noted, that the status of research for conventional cermet anodes is to optimise their performance through controlling the micro structure and adjusting minor additives, whereas the situation with MIECs is still at the stage of identifying suitable candidates [32]. In general the ceramics show a very low activity for carbon formation and also provide stability against redox cycles [3]. The materials can be zirconia, ceria or lanthanum-based ceramics [3,43]. Lanthanum chromate (La1xSrxCrO3), which is also widely studied as interconnect material, could be a good candidate due to its stability. Addition of dopant (La1xSrxCr1yMyO3, M ¼ Mn, Fe, Co, Ni) has shown to improve the catalytic activity. Especially lanthanum chromium manganite has given promising results [32]. Good results have also been achieved with ceria-based materials, with GDC (Ce0.6Gd0.4O1.8) particularly [3,32]. Ceria (CeO2) is a well-known oxidation catalyst [32,45] and can also be used to increase the sulphur tolerance of the anode [46]. Another approach is to use relatively inert metal such as copper to provide electric conductivity and combine it with metal oxides for catalytic activity and ionic conductivity. Cu/CeO2/YSZ, which has given promising results, is an example of this type [32,32,46].
4.2.
Cathode materials
4.2.1.
MCFC cathode
Before 1970 the MCFC cathode was made of silver or copper. Today, a porous nickel electrode is utilized in the cell. During the first hours of operation it is in-situ oxidized and lithiated by the LieK carbonates melt to lithiated-NiO (i.e. LixNi2þ12xNi3þxO). This process gives it its characteristic dual pore size distribution: larger pores for the migration of the reactants from the oxidant gas, and smaller pores for the capillary absorption of the molten electrolyte from the matrix. Thus the three-phase interface (between the solid catalyst, the liquid ion carrier and the reactant gas), required for proper electrochemical operation, is created [47,48]. The utilization of fuel derived from alternative sources implies some difficulties regarding contaminants contained therein, also for the cathode. Though the fuel gas is fed initially to the anode, often, to provide the CO2 that is required at the cathode, the reacted exhaust gas from the anode is
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recirculated to the cathode inlet. Thus, any fuel contaminants like sulphur, halogen compounds and nitrides will also be present at the cathode inlet, though usually in some oxidized form. The currently accepted tolerance levels of state-of-theart anodes to these species seem to guarantee safe operation of the cathode in the event of their recirculation, but this needs to be specifically determined [49].
4.2.2.
SOFC cathode
At the moment strontium-doped lanthanum manganite (LSM, LaxSr(1x)MnO3) is the most commonly used cathode material in high temperatures [2,43]. LSM is often mixed with electrolyte material (YSZ) in order to extend the TPB [3,43]. Introducing an oxide ion path by YSZ into the LSM cathode enlarges the electrochemical reaction sites throughout the cathode body, although the main sites are at the electrolyteecathode interface [4]. Thus a composite of an electronic conducting perovskite and ionic conducting oxide is formed [3,4,50]. In the case of the SOFC, exhaust recirculation similar to that of the MCFC is not needed, but there are other driving forces in cathode material development. Lately, a common trend in SOFC development has been reducing the operation temperature [4,43]. That is because lower temperature allows the use of less expensive, metallic interconnect materials. Unfortunately the use of metallic interconnects introduces a new problem: chromium poisoning of the cathode [4]. Cathode performance is very dependent on temperature (but less dependent than the performance of electrolyte materials) [43], thus different temperatures require different cathode materials (as well as different electrolyte materials). LSF (LaxSr(1x)FeO3) is one of the best candidates to replace LSM at lower temperatures (600e800 C). Delphi has used LSF in their power units breaking the trend of LSM being the only one used in commercial products [43]. Several other cathode materials (for instance LSC (LaxSr(1x)CoO3) [33,43] and gadolina-based GSC (GdxSr(1x)CoO3) [43]) have shown varying success in conductivity and stability improvements over LSM and LSF. Using other cathode materials, improvements have been demonstrated but not always without introducing other issues.
4.3.
Electrolyte materials
4.3.1.
MCFC electrolyte
As the name suggests MCFC has a liquid electrolyte. For the requirement of good ionic conductivity, lithium is the best material for use in the carbonate melt due to its low atomic weight. In the beginning, eutectic mixtures of LieNa carbonate (Li2CO3eNa2CO3) were used; then attention shifted to LieK (Li2CO3eK2CO3), especially for atmospheric applications due to its increased reactivity; ternary mixtures of LieKeNa have been investigated, but the present tendency is to go back to mixtures of LieNa, due to their lower vapour pressure, which reduces electrolyte loss, their lower acidity and their increased activity especially at high operating pressure. The proportion of Li, Na and/or K in the eutectic mixture is determined by the resulting properties such as melting point, surface tension, viscosity, vapour pressure, solubility. Approximately 50 mol% Li2CO3 is necessary to meet the melting point and performance
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requirements of MCFCs, though it makes up for almost all of the electrolyte cost [51]. The liquid electrolyte is contained by a porous support structure called the matrix. The matrix must be porous to the right degree to be impregnated by the liquid electrolyte, while maintaining fuel and oxidant separation and providing mechanical stability; it must be resistant against the corrosive properties of the electrolyte both in the reducing conditions of the anode as in the oxidizing conditions of the cathode, being able to maintain its pore structure and stability for the projected life time of the fuel cell. This pore structure must be narrowly tailored to fit the pore structures of the adjacent electrodes, so that the electrolyte distribution at the three-phase interface is optimal for the electrochemical reactions. The matrix material most commonly used is high surface area submicron g-LiAlO2 powder, with Al2O3, ZrO2 fibre (or derivatives) or coarse particulates incorporated for strengthening [5,52]. The matrix, being a ceramic (and thus relatively inert) and not electrocatalytically active, is not particularly affected by carbon deposition or contaminant poisoning. The electrolyte however, being a molten salt, has large affinity for reacting with contaminant species like sulphur compounds [53]. This leads to the occupation by the poisoning specie of potential charge carriers in the electrolyte, reducing their availability for ion transport [17,24]. Molten carbonate is actually highly effective in the capture of contaminants [53], thereby potentially protecting the nickel from being attacked. However, unless a way is found to replace the carbonates during operation, the sulphur species simply accumulate in the electrolyte impregnating the anode, so that eventually the catalyst is affected as well.
4.3.2.
SOFC electrolyte
In contrast to MCFCs, the SOFC has a solid electrolyte. Yttria stabilized zirconia (YSZ, (ZrO2)1x(Y2O3)x) is the most commonly used electrolyte material in SOFCs [43]. YSZ is not the most effective ionic conductor there is and other oxides, such as bismuth oxide compositions, can have higher conductivity, especially at lower temperatures [4,50]. However these other oxides often have disadvantages like non-negligible electronic conductivity, high cost, or difficulties in processing [4]. However the lowest operation temperature limit for YSZ is estimated to be around 700 C [4]. As in cathode development the aim to decrease the operation temperature guides the electrolyte material research. Promising alternatives for low-temperature operation are scandia stabilized zirconia (SSZ (ZrO2)x(Sc2O3)1x), where scandia is used as a dopant instead of yttria [4,43], ceria-based oxide ion conductors such as gadolina doped ceria (GDC, CexGd(1x)Oy) and samaria doped ceria (SDC, CexSm1xOy) and finally lanthanum gallate perovskite electrolytes, particularly LSGM (LaxSr(1x)GayMg(1y)O3) [4,43]. However, in addition to the problems created by the low operation temperature, electrolyte material can have an effect on the usual problems at the anodeeelectrolyte interface: carbon formation and sulphur poisoning. The catalyst’s tendency for carbon formation depends also on the interaction between the Ni and the supporting oxide [30,54] and results of Sasaki et al. [12] suggest that the choice of the electrolyte material has an effect on the sulphur tolerance of the anode.
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5.
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Conclusions
The strong potential of high-temperature fuel cells (HTFC) is best brought to the fore by operating them on hydrocarbon fuels like natural gas, biogas and syngas. Unsurpassed efficiencies and ready implementation into today’s fuel and energy supply infrastructure are the main drivers for this configuration. An overview has been presented of the MCFC’s and SOFC’s current best-practice materials to discover similarities between the two technologies. Also the main difficulties tied to common operating conditions have been identified, so that the combination of these areas provide the basis for a joint approach in overcoming the problems that are proving persistent in both fuel cell types. The objective hereby is to bring together the communities in sharing knowledge that is relevant for both MCFC and SOFC in their upgrade to commercialization. Carbon deposition and contaminant poisoning (in particular by sulphurous compounds) are such issues, tied mainly to the presence of an unprotected nickel catalyst. Although these phenomena have been studied extensively in the past in other fields of catalyst development, the particular conditions in high-temperature fuel cells (electrocatalysis, high temperature, reducing environment, interacting compounds, etc.) overlap several mechanisms, defeating theoretical and equilibrium analysis. Prediction and control are thus more of an iterative process, and a suitable correlation of conditions and effects is still under investigation. Research is exploring the possibilities of adding or substituting ceramic oxides in the basic anode make-up, since they provide resistance both against carbon deposition as against sulphur poisoning. Among these, especially ceria-based oxides are currently researched both in MCFCs and SOFCs, thanks to their high capacity for sulphide take-up and ease of regeneration. Investigations are required to determine the optimum compound for both electrocatalytic performance and resistance. One major challenge is also the fabrication procedure for their optimal incorporation in, or onto, the state of the art anode catalysts. A joint effort on the issues which are common to both the MCFC and SOFC communities has been shown to be possible, and could speed up the development to make HTFCs finally meet market requirements.
references
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Available at www.sciencedirect.com
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SOFC and MCFC: Commonalities and opportunities for integrated research Stephen J. McPhail a,*, Anja Aarva b, Hary Devianto a, Roberto Bove c, Angelo Moreno a a
ENEA Energy Department e Hydrogen & Fuel Cells, Rome 00123, Italy TKK Helsinki University of Technology, Espoo, Finland c Alstom Power, Birr, Switzerland b
article info
abstract
Article history:
The present article explores the key issues for research and development that are common
Received 3 February 2010
to current state-of-the-art MCFC and SOFC technologies. By analyzing overlapping aspects
Received in revised form
regarding materials, operating conditions and applications of the two types of high-
26 July 2010
temperature fuel cell (HTFC), the most pressing common challenges are set forth. Simi-
Accepted 22 September 2010
larities between the MCFC and SOFC exist especially on the anode side, given their
Available online 15 October 2010
utilization of nickel as the oxidation catalyst and their suitability for conversion of hydrocarbon fuel. Catalyst deactivation due to contaminant poisoning and carbon depo-
Keywords:
sition thereby emerges as the crucial problem to
Contaminant poisoning
mechanisms is given and of relevant studies in the field of HTFCs. The implications on the
overcome. A brief review of these
Sulphur
HTFC catalyst material given by this review are summarized and put forward as lines of
Carbon deposition
research that can be undertaken jointly by players both in MCFC and SOFC development.
High-temperature fuel cell materials
Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Since there exists a wide variety of fuel cell types, in this study only a certain class will be discussed: the high-temperature fuel cells (HTFC), in particular the Molten Carbonate (MCFC) and Solid Oxide fuel cells (SOFC). The fundamental aspect that differentiates this typology from low-temperature fuel cells, is evidently that e operating typically at temperatures of over 600 C e the heat produced can be utilized in fuel processing and heat generation systems. Generally high-temperature fuel cells have the potential to be independent of a pure hydrogen infrastructure, and can find their application in transitional fuels such as natural gas, syngas and biogas. Thus, as compared to low-temperature fuel cells, they can be more flexible in accepted fuels, which implies the great advantage that they can be implemented more easily into nowadays’ fuel range and infrastructure. Also, where heat
off-take can be guaranteed in addition to power consumption, they can provide unsurpassed efficiencies, approaching 90% total efficiency (electrical plus thermal) [1]. However, this potential still remains unfulfilled since MCFC and SOFC stacks operating in real-world conditions still too easily incur material deactivation or structural change in their active components, leading to unwelcome performance degradation. Success of HTFCs in gaining a solid foothold on the highly competitive power train market, however, depends just as much on the effective integration of many, crucial components that make up the fuel cell system. Apart from the electrochemical heart e the fuel cell stack e its optimal performance depends amongst others on fuel and air conditioning, heat management, process control, load regulation and operating conditions. In this paper, however, we focus on cell active components: anode, cathode and electrolyte. This is done by identifying the commonalities between MCFC and
* Corresponding author. E-mail address: [email protected] (S.J. McPhail). 0360-3199/$ e see front matter Copyright ª 2010, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.09.071
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SOFC operating frames and the implications of these conditions on the current state-of-the-art materials of the two HTFCs. The issues thus evinced are by no means suggested as being the most important in their respective research fields; it is simply stated which areas could benefit from a joint approach and the sharing of knowledge. By recognizing the similar needs of these two technologies, an indication is hoped to be given as regards the focus of research and development efforts in the present and near future on material improvement that would mutually benefit both fuel cell types. Fulfilling these requirements would mean a considerable step forward towards fully fledged maturity of hightemperature fuel cells, and a joint, directed effort by the MCFC and SOFC communities could speed up this process to make them finally meet market requirements.
2.
Cell reactions and fuel conversion
The main features of the MCFC and SOFC [2,3] are tied to their operating temperature. In the former case, a minimum temperature level of 600 C has to be maintained to guarantee the liquid state of the carbonate eutectic and thus the mobility of the carbonate ions. In the latter, oxygen ions are transferred from cathode to anode through oxygen conducting solid electrolyte [2e4]. SOFCs can be operated between 600 and 1000 C [4]. In other words, the charge carriers that are used to close the electrical circuit are different for SOFC and MCFC: ionized oxygen (O¼) in the case of the SOFC, carbonate ions (CO¼ 3 ) in the MCFC. For the latter, this requires an additional feed of CO2 at the cathode side. For the former, this means that if pure hydrogen is introduced at the anode, no carbon is involved in SOFC operation. Electrochemical reactions at the anode and cathode of the MCFC and SOFC are set out schematically in Fig. 1. It should be mentioned that interesting efforts exist to combine the advantages of the MCFC and SOFC by creating a composite electrolyte based on carbonate-impregnated
ceramics (e.g. [5e8]). It is however outside the scope of this article to look into radically new fuel cell concepts, as the purpose is to find accelerated means to render “conventional” state-of-the-art HTFCs a product, fit for market penetration. High-temperature operation offers distinct advantages: (electro) chemical reactions are more rapid resulting in faster reduction and oxidation kinetics, thereby eliminating the necessity for noble metal catalysts. Conventionally nickelbased anode catalysts are used in these HTFCs [2,4]. In addition to cost reduction, this implies that carbon monoxide does not exhibit any poisoning effect on the fuel cell, and on the contrary can be used as an additional fuel [2]. The high temperature is also eminently suitable for light hydrocarbon reforming, which can take place directly inside the cell, by or utilizing a heat-integrated reformer element. The heat released by the electrochemical oxidation of the fuel can be utilized by the endothermic reforming reaction (1) [2]: (1) Cx Hy þ xH2 O/ 1=2y þ x H2 þ xCO The hydrogen thus produced (and to a much lesser degree the carbon monoxide) oxidizes electrochemically at the anode to produce water, which then can be consumed by reaction (1). When utilizing biogenous fuels e i.e. including fossil fuels like natural gas or refined oil e carbon is always part of the reaction. Once reformed to carbon monoxide, this can either react electrochemically with the electrolyte ion to form CO2, or e much more easily e is converted chemically with steam to hydrogen and carbon dioxide, according to the so-called water-gas shift reaction [9]: CO þ H2 O4H2 þ CO2
(2)
As a consequence of these reactions, water and CO2 are formed on the anode side. As H2 is consumed at the anode (electrochemical reactions, Fig. 1) water is formed, and reaction (2) favours the production of more H2 for electrochemical oxidation. For the MCFC, CO2 is needed on the cathode side to generate the charge carrying carbonate ion. Since the amount
Fig. 1 e A schematic overview of a high-temperature fuel cell, with anode and cathode electrochemical reactions in the insets. When hydrocarbons are utilized as fuel, the anode can act as a reforming stage (reaction (1)).
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of CO2 required at the cathode is the same as that formed as a consequence of the anode reaction, spent anodic gas is generally recycled from the anode to the cathode. This suggests the potentially interesting role that MCFCs could have in terms of CO2 separation. As a consequence of the MCFC’s operation, CO2 is continuously transferred from the cathode to the anode. This particular feature could be exploited for separating CO2 originating from a traditional thermal power plant and concentrating it in the anode exit stream where it can be captured and sequestered (Fig. 2).
3.
MCFC and SOFC: common challenges
As was seen from the previous chapter, the chief similarity between MCFCs and SOFC lies on the anode side. Favorable conditions for the reforming of light hydrocarbon fuels combined with the nickel-based catalyst used in both fuel cell types, lead us to focus on anode side issues. In this section the major implications of utilizing carbonaceous fuels in HTFCs will be discussed. In tackling these challenges a common approach and joint effort between MCFC and SOFC can be undertaken, increasing the likelihood and speed of success.
Table 1 e Contaminants and their tolerance limits for MCFCs [11]. Contaminant
Tolerance
Sulphides e.g. H2S, COS, CS2 Halides e.g. HCl, HF Siloxanes e.g. HDMS, D5 Particulates Tars Heavy metals e.g. As, Pb, Zn, Cd, Hg
0.5e1 ppm
Fuel impurities
Though high-temperature fuel cells have the advantage of not requiring noble metal catalysts for the electrochemical reactions, some species have a poisoning effect on the catalytic properties of the electrodes. Giving accurate tolerance limits for all the possible fuel impurities and their effects on HTFC materials can be quite difficult. Especially for SOFCs, that are operated in a wide range of temperatures (600e1000 C) with a wide range of catalysts (this will be discussed more in chapter 4), strict categorization of certain contaminant effects and comparing of results is complicated. MCFC operating conditions and catalyst materials are more established than for SOFC, and an overview of contaminant effects can be attempted [11] (Table 1). Even for the MCFC, however, the tolerance levels are indicative (a margin of safety is included in the values of Table 1) and the extent of their harmful effect may depend on the partial pressure of other species in the gas (e.g. hydrogen, water), the current density at which the fuel cell is operated, temperature and the fuel utilization factor. Exposure time to the various impurities is also determinant as regards the extent of the
10e100 ppm 10e100 ppm 2000 ppm 1e20 ppm
Electrode deactivation Usurpation of electrolyte Corrosion Usurpation of electrolyte Silicate deposits Deposition, plugging Carbon deposition Deposition Usurpation of electrolyte
damage caused and the potential for its reversal. Elaborate investigations into the endurance to contaminants are few, since experimentation of these effects is necessarily destructive and of long duration, but an accurate knowledge of the conditions which are deleterious is highly desired. Identification of the limits of safe operation would greatly enhance fuel cell durability as well as optimise the integration of the fuel clean-up stage in terms of requirements and cost.
3.1.1. 3.1.
0.1e1 ppm
Effects
Special case: hydrogen sulphide poisoning
Many of the possible fuels for HTFCs, for instance biogas derived from sewage sludge, can contain significant amounts of sulphur compounds [11,12]. Sulphur-containing odorant is also often added in natural gas for safety reasons [2,13]. Traces of sulphur in the fuel can lead to a severe decrease in the cell performance due to catalyst deactivation [3,14]. In past research, above all the effects of hydrogen sulphide (H2S) have been studied, as this is the most abundant contaminant present in gases from organic origin. However also organic sulphur compounds can be present and are equally bad for the nickel-based catalyst (see Fig. 3). The effects of sulphides are dependent on many factors such as the bulk concentration, the concentration relative to hydrogen in the fuel, humidity, electrical load and temperature. As temperature decreases, the propensity of Ni to react with sulphur tends to increase [15]. Though experiments have shown that the drastic effect of sulphur-containing compounds on electrode activity seems irreversible upon enduring exposure to concentrations of more than 10 ppm or even 5 ppm [9,12], thermodynamic equilibrium calculations show that no permanent, bulk nickel-sulphide phases should be formed until concentrations of over 100 ppm [15].
Fig. 2 e The MCFC as a CO2 separator [10].
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Fig. 3 e (a) open-cell reticulated Ni foam pre-test and (b) open-cell reticulated Ni foam post-test with H2S exposure [16].
In MCFCs hydrogen sulphide not only reacts with the anode material, but it also interacts with the electrolyte [17]. Reaction of hydrogen sulphide with the nickel on the anode leads to blocking and deactivating the electrochemically active sites for hydrogen oxidation [18e20]. The affected sites give rise to morphological changes in the anode structure, and can thereby cause further deterioration of cell performance through secondary effects like impeded gas diffusion, volume change or reduced wetting by the electrolyte. At the electrolyte, hydrogen sulphide can react chemically with carbonates to form either sulphide or sulphate ions [18], thereby uses electrochemically active charge carriers which would otherwise be available for the hydrogen oxidation. This translates in reduced cell performance. However, hydrogen sulphide can also react electrochemically with carbonates [21], releasing electrons, but yielding harmful, ionized sulphate compounds. There has also been evidence that suggest that the electrolyte material has an effect on the sulphur tolerance in SOFCs [12]. The complex interaction of chemical, electrochemical, equilibrium and kinetic mechanisms governing the fate of H2S in HTFC operating conditions make it exceedingly difficult to separate and quantify the several poisoning effects of hydrogen sulphide. A preliminary study by Hansen [22] has investigated the applicability of so-called Temkin kinetics to unify the effects of different parameters in a unique correlation. Here, the quantity of sulphur coverage is identified as the governing variable that determines the loss of performance of a nickel-based anode at given current density. The correlation is tested on published experimental results obtained with three different anodes and a wide range of temperatures and sulphur concentrations. The data are consistently approximated by the correlation with a coefficient of determination (R2) of 0.9 or higher. Sulphur poisoning can be reversible at low concentrations [23,24], thus it can be possible e within limits not yet welldefined e to recuperate the fuel cell’s original performance. These reactions are under investigation and could provide a crucial, added controlling parameter to guarantee long-term reliability of a nickel-based HTFC stack. To find the governing conditions that establish the limits of safe operation and poisoning reversibility is of exceeding importance, especially
when fuels are considered with intrinsically fluctuating compositions and impurity levels, like biogas from anaerobic digestion or syngas from biomass gasification.
3.2.
Carbon deposition
In addition to fuel impurities, also fuel components can cause problems. When hydrocarbons are used as a fuel there is always a risk of carbon formation. Unfortunately nickel is not only a good electrocatalyst and reforming catalyst, it also has high activity towards carbon deposition [2e4]. The use of carbon-based fuels leads to a well-known problem in HTFCs, namely carbon deposition on the anode. The main consequence is the loss of electrochemically active sites, and ultimately, reduced performance. In addition, it has been reported that the growth of filaments attached to the anode crystallites can modify anode micro structure [25], drastically reducing its durability [26]. Plugging of the gas passages by the accumulation of carbon on the anode is another plain consequence. When methane is considered as fuel, among other possible reactions, Boudouard (3) and methane cracking (4) are considered the most probable responsibles for carbon deposition, together with carbon gasification (5) [9,27,28]: 2CO4C þ CO2
(3)
CH4 4C þ 2H2
(4)
CO þ H2 4C þ H2 O
(5)
A first thermodynamic consideration is that the increase of the operating pressure favours C formation, according to equations (3) and (4), while water addition reduces formation of C (see Fig. 4). In fact, gasifying agents, such as water, can remove carbon from the catalyst surface (reverse reaction (5)) [29,30]. When methane steam reforming is considered, at 600 C, the steam-to-carbon value (mole-based) below which carbon formation begins is 1.5, and reduces to 1 at 800 C and higher [9]. Higher hydrocarbons show a higher tendency towards carbon formation than methane. It is a common
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Fig. 4 e SEM mapping of carbon deposition on Ni-10 wt%Cr MCFC anode at cell temperature 650 C, pressure 1 atm, exposure time 1000 h: (a) at S/C ratio 3.5 (b) at S/C ratio 2.5.
practice to use a high steam-to-carbon ratio of the fuel feed in order to suppress carbon formation [29,31]. However high water content of the fuel feed is unwanted because it reduces the equilibrium potential and thus the power output of the cell [3]. In addition thermodynamic calculations can be an unreliable way to predict carbon formation [32,33], because the rates of carbon removal and carbon deposition are not always fast enough to establish equilibrium, as shown in [34]. Carbon deposition is well-known from the steam-reforming industry. However, when reforming reactions occur at the anode, additional phenomena can drive carbon deposition since electro catalysis can favour coke formation kinetics. Similarly, direct reaction of a carbon-containing fuel at the HTFC anode can give rise to a number of factors leading to carbon deposition. Catalyst activity is influenced by current density, and it is expected that carbon deposition is also dependent on the current density at which the cell is operated, in addition to fuel chemical composition and temperature [35]. Furthermore, the current density influences the variation of the chemical composition of the gas within the anode. Singh et al. [27] performed a thermodynamic evaluation of carbon formation in a SOFC operated on syngas from a biomass gasifier. For different tar content, a so-called threshold current density is calculated. This quantity is defined as the current above which no carbon deposition is expected. Recently, Mermelstein et al. [36] reported experimental results of carbon deposition due to tars in a Ni-YSZ-based SOFC. The results, as expected, show that carbon formation is relevant when no water is added to the mixture. However, contrary to thermodynamic calculations, carbon deposition is not completely suppressed at steam-to-carbon values from 2 to 3. Experimental results on subscale MCFCs operated at 10 bar on simulated gasified coal humidified at 165 C, showed no carbon deposition [9]. While carbon deposition can be avoided through proper design and operation of the system, there is also an interest in defining novel materials that prevent it. In particular, additives can be used in the anode for reducing carbon deposition. This will be discussed in Section 4.
4. Material implications for active components As has been discussed, most similarity between the two technologies exists at the anode, where conventionally nickel
provides the active sites for the electrochemical oxidation. This is why, when utilizing hydrocarbon fuels rather than pure hydrogen, the problems arise that are common to both MCFC and SOFC and that are the chief obstacles to carefree operation. In this chapter we present a short introduction to current stateof-the-art materials used for the fuel cell active components, i.e. anode, cathode and electrolyte, combined with a discussion of how the challenges described above are tackled in component development. Again, no pretence is made to list all the important material issues that are still prevalent in both MCFC and SOFC (TEC mismatch, acidic corrosion, electrolyte evaporation, redox, chromium poisoning, particle agglomeration, phase instability etc.). Being specific to one of the two technologies, many are neglected on purpose. For these, the reader is referred to reference texts such as [2] and [3].
4.1.
Anode materials
4.1.1.
MCFC anode
Since the early seventies, porous nickel is the material used for the anode, strengthened against creep and fracture by the addition of chromium and/or aluminium. The chromium, like the aluminium, is effective against loss of surface area, pore growth and sintering. However chromium has the tendency to consume carbonate through lithiation by the electrolyte, which decreases performance and jeopardises the integrity of the NieCr alloy. Aluminium appears to be less prone to lithiation with the carbonates in the long run. Adding alloying agents can compromise porosity and conductivity requirements. The current level of Cr or Al addition is between 6 and 10%, which seems to have resolved most problems relating to structural strength, performance and cell life time [37]. The porous anode structure is impregnated with an excess of the liquid electrolyte in order to prevent depletion of the electrolyte through leakage, metal reaction and evaporation during long-term operation. However the impregnation of the anode by the electrolyte has to be weighed against the diminution of active surface area and conductivity, and the increase of the gas phase mass transport resistance, leading to additional polarization losses. The optimal electrolyte filling degree, furthermore, depends on pore size distribution, electrode thickness, electrolyte composition and even on fuel gas composition [38]. The current additives to the anode base material, added to the right degree and following careful procedures for their
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dispersion, guarantee sufficient mechanical strength and have in a large part solved the problem of long-term creep. Main issues left open are therefore resistance to carbon deposition and especially contaminant poisoning e in particular by sulphur and halogens. A significant development in this respect, is the intermixing of ceramic oxides with the nickel. Being less susceptible to carbon deposition, ceramic oxides (like lanthanum, cerium, zirconium and samarium, with additives for the improvement of electronic conductivity), offer the prospects of being fuelled by dry methane, as is the case with SOFCs [39]. Moreover, ceramic-based or ceramic-coated anodes improve sulphur tolerance notably, possibly by an order of magnitude, in addition to being morphologically stable during cell operation and increasing electrolyte wetting [40,41]. Furthermore, they are potentially low-cost. Continued studies indicate the validity of the concept [42]: the challenge is to optimise the procedures for regeneration of the contaminated anode, since the harmful species trapped on the ceramic oxides can saturate their selectivity. Regeneration can take place passing a clean gas over the anode that reabsorbs the pollutant species adhered to the catalytic sites, either by reducing the catalyst or by oxidizing and releasing the pollutants. This procedure is not ideal from a steady state process point of view and research is directed towards minimizing or even bypassing this step. A solution might be to find a material that selectively repels the actual adhesion of contaminants to the reactive sites, possibly through pre-saturation, but the viability of this idea has not yet been proved. In all cases it is necessary to find a coating or protective material that is morphologically and structurally stable and a fabrication process that maintains cost-effectiveness.
4.1.2.
SOFC anode
The SOFC anode is typically a cermet structure i.e. a mixture of nickel and the ceramic electrolyte, usually YSZ [3]. Nickel has been the main anode catalyst material used in SOFCs since 1964 [4]. Nickel is an active electrocatalyst, especially when hydrogen is used as a fuel, and it is chemically stable with electrolyte material [3]. Unlike cathode and electrolyte materials, nickel shows good performance at all temperature ranges [43]. Also the price of nickel is competitive [3,43]. As discussed before in Section 3, the Ni/YSZ anode has a good performance when hydrogen is used as a fuel, but problems occur when hydrocarbons are fed to the cell. Although nickel catalyses internal reforming of hydrocarbons, nickel also has high activity for carbon formation [3,4,32]. As an alternative to Ni/YSZ other cermet structures have been investigated. Ni/SSZ has shown lower overpotential and better stability at 800 C than Ni/YSZ when hydrogen was used as a fuel [43]. It has also been reported by Sasaki et al. that Ni/ SSZ anode has increased tolerance against sulphur poisoning [8]. Ni combined with ceria-based ion conductors has also given promising results [3,43]. For instance Ni/GDC is reported to improve ionic conductivity, catalytic activity and long-term stability [43]. In addition, tendency to form carbon is suppressed [3,43]. In order to reduce carbon deposits there has been growing interest in replacing nickel with copper [43] and zirconia with ceria [32]. Copper cermets can be less expensive than
nickel anodes and they have demonstrated to have better resistance against carbon formation. Also alloying copper with nickel reduces carbon formation [43]. Other possible elements that have been examined are titanium, cobalt, iron, silver, gold, platinum, ruthenium and rhodium [4,43,44]. Unfortunately platinum does not last long and peels off after few hours in SOFC operating conditions [4]. Mixing some gold (1%) to Ni/YSZ anode has shown to decrease the anode’s tendency to form carbon [44]. Although gold, silver and copper are very good electric conductors they are not highly active oxidation catalysts [32]. Silver (mp 962 C) and copper (mp 1083 C) have melting points that are too low for conventional cermet sintering [4,32]. Iron and cobalt are not resistant enough to the corrosive atmosphere [4]. In addition cobalt, iron, platinum, ruthenium and rhodium are said to be too expensive or not to offer any advantages compared to nickel [43,44]. One possibility is to use mixed ionic electronic conductive ceramics [3,32]. It has to be noted, that the status of research for conventional cermet anodes is to optimise their performance through controlling the micro structure and adjusting minor additives, whereas the situation with MIECs is still at the stage of identifying suitable candidates [32]. In general the ceramics show a very low activity for carbon formation and also provide stability against redox cycles [3]. The materials can be zirconia, ceria or lanthanum-based ceramics [3,43]. Lanthanum chromate (La1xSrxCrO3), which is also widely studied as interconnect material, could be a good candidate due to its stability. Addition of dopant (La1xSrxCr1yMyO3, M ¼ Mn, Fe, Co, Ni) has shown to improve the catalytic activity. Especially lanthanum chromium manganite has given promising results [32]. Good results have also been achieved with ceria-based materials, with GDC (Ce0.6Gd0.4O1.8) particularly [3,32]. Ceria (CeO2) is a well-known oxidation catalyst [32,45] and can also be used to increase the sulphur tolerance of the anode [46]. Another approach is to use relatively inert metal such as copper to provide electric conductivity and combine it with metal oxides for catalytic activity and ionic conductivity. Cu/CeO2/YSZ, which has given promising results, is an example of this type [32,32,46].
4.2.
Cathode materials
4.2.1.
MCFC cathode
Before 1970 the MCFC cathode was made of silver or copper. Today, a porous nickel electrode is utilized in the cell. During the first hours of operation it is in-situ oxidized and lithiated by the LieK carbonates melt to lithiated-NiO (i.e. LixNi2þ12xNi3þxO). This process gives it its characteristic dual pore size distribution: larger pores for the migration of the reactants from the oxidant gas, and smaller pores for the capillary absorption of the molten electrolyte from the matrix. Thus the three-phase interface (between the solid catalyst, the liquid ion carrier and the reactant gas), required for proper electrochemical operation, is created [47,48]. The utilization of fuel derived from alternative sources implies some difficulties regarding contaminants contained therein, also for the cathode. Though the fuel gas is fed initially to the anode, often, to provide the CO2 that is required at the cathode, the reacted exhaust gas from the anode is
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recirculated to the cathode inlet. Thus, any fuel contaminants like sulphur, halogen compounds and nitrides will also be present at the cathode inlet, though usually in some oxidized form. The currently accepted tolerance levels of state-of-theart anodes to these species seem to guarantee safe operation of the cathode in the event of their recirculation, but this needs to be specifically determined [49].
4.2.2.
SOFC cathode
At the moment strontium-doped lanthanum manganite (LSM, LaxSr(1x)MnO3) is the most commonly used cathode material in high temperatures [2,43]. LSM is often mixed with electrolyte material (YSZ) in order to extend the TPB [3,43]. Introducing an oxide ion path by YSZ into the LSM cathode enlarges the electrochemical reaction sites throughout the cathode body, although the main sites are at the electrolyteecathode interface [4]. Thus a composite of an electronic conducting perovskite and ionic conducting oxide is formed [3,4,50]. In the case of the SOFC, exhaust recirculation similar to that of the MCFC is not needed, but there are other driving forces in cathode material development. Lately, a common trend in SOFC development has been reducing the operation temperature [4,43]. That is because lower temperature allows the use of less expensive, metallic interconnect materials. Unfortunately the use of metallic interconnects introduces a new problem: chromium poisoning of the cathode [4]. Cathode performance is very dependent on temperature (but less dependent than the performance of electrolyte materials) [43], thus different temperatures require different cathode materials (as well as different electrolyte materials). LSF (LaxSr(1x)FeO3) is one of the best candidates to replace LSM at lower temperatures (600e800 C). Delphi has used LSF in their power units breaking the trend of LSM being the only one used in commercial products [43]. Several other cathode materials (for instance LSC (LaxSr(1x)CoO3) [33,43] and gadolina-based GSC (GdxSr(1x)CoO3) [43]) have shown varying success in conductivity and stability improvements over LSM and LSF. Using other cathode materials, improvements have been demonstrated but not always without introducing other issues.
4.3.
Electrolyte materials
4.3.1.
MCFC electrolyte
As the name suggests MCFC has a liquid electrolyte. For the requirement of good ionic conductivity, lithium is the best material for use in the carbonate melt due to its low atomic weight. In the beginning, eutectic mixtures of LieNa carbonate (Li2CO3eNa2CO3) were used; then attention shifted to LieK (Li2CO3eK2CO3), especially for atmospheric applications due to its increased reactivity; ternary mixtures of LieKeNa have been investigated, but the present tendency is to go back to mixtures of LieNa, due to their lower vapour pressure, which reduces electrolyte loss, their lower acidity and their increased activity especially at high operating pressure. The proportion of Li, Na and/or K in the eutectic mixture is determined by the resulting properties such as melting point, surface tension, viscosity, vapour pressure, solubility. Approximately 50 mol% Li2CO3 is necessary to meet the melting point and performance
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requirements of MCFCs, though it makes up for almost all of the electrolyte cost [51]. The liquid electrolyte is contained by a porous support structure called the matrix. The matrix must be porous to the right degree to be impregnated by the liquid electrolyte, while maintaining fuel and oxidant separation and providing mechanical stability; it must be resistant against the corrosive properties of the electrolyte both in the reducing conditions of the anode as in the oxidizing conditions of the cathode, being able to maintain its pore structure and stability for the projected life time of the fuel cell. This pore structure must be narrowly tailored to fit the pore structures of the adjacent electrodes, so that the electrolyte distribution at the three-phase interface is optimal for the electrochemical reactions. The matrix material most commonly used is high surface area submicron g-LiAlO2 powder, with Al2O3, ZrO2 fibre (or derivatives) or coarse particulates incorporated for strengthening [5,52]. The matrix, being a ceramic (and thus relatively inert) and not electrocatalytically active, is not particularly affected by carbon deposition or contaminant poisoning. The electrolyte however, being a molten salt, has large affinity for reacting with contaminant species like sulphur compounds [53]. This leads to the occupation by the poisoning specie of potential charge carriers in the electrolyte, reducing their availability for ion transport [17,24]. Molten carbonate is actually highly effective in the capture of contaminants [53], thereby potentially protecting the nickel from being attacked. However, unless a way is found to replace the carbonates during operation, the sulphur species simply accumulate in the electrolyte impregnating the anode, so that eventually the catalyst is affected as well.
4.3.2.
SOFC electrolyte
In contrast to MCFCs, the SOFC has a solid electrolyte. Yttria stabilized zirconia (YSZ, (ZrO2)1x(Y2O3)x) is the most commonly used electrolyte material in SOFCs [43]. YSZ is not the most effective ionic conductor there is and other oxides, such as bismuth oxide compositions, can have higher conductivity, especially at lower temperatures [4,50]. However these other oxides often have disadvantages like non-negligible electronic conductivity, high cost, or difficulties in processing [4]. However the lowest operation temperature limit for YSZ is estimated to be around 700 C [4]. As in cathode development the aim to decrease the operation temperature guides the electrolyte material research. Promising alternatives for low-temperature operation are scandia stabilized zirconia (SSZ (ZrO2)x(Sc2O3)1x), where scandia is used as a dopant instead of yttria [4,43], ceria-based oxide ion conductors such as gadolina doped ceria (GDC, CexGd(1x)Oy) and samaria doped ceria (SDC, CexSm1xOy) and finally lanthanum gallate perovskite electrolytes, particularly LSGM (LaxSr(1x)GayMg(1y)O3) [4,43]. However, in addition to the problems created by the low operation temperature, electrolyte material can have an effect on the usual problems at the anodeeelectrolyte interface: carbon formation and sulphur poisoning. The catalyst’s tendency for carbon formation depends also on the interaction between the Ni and the supporting oxide [30,54] and results of Sasaki et al. [12] suggest that the choice of the electrolyte material has an effect on the sulphur tolerance of the anode.
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Conclusions
The strong potential of high-temperature fuel cells (HTFC) is best brought to the fore by operating them on hydrocarbon fuels like natural gas, biogas and syngas. Unsurpassed efficiencies and ready implementation into today’s fuel and energy supply infrastructure are the main drivers for this configuration. An overview has been presented of the MCFC’s and SOFC’s current best-practice materials to discover similarities between the two technologies. Also the main difficulties tied to common operating conditions have been identified, so that the combination of these areas provide the basis for a joint approach in overcoming the problems that are proving persistent in both fuel cell types. The objective hereby is to bring together the communities in sharing knowledge that is relevant for both MCFC and SOFC in their upgrade to commercialization. Carbon deposition and contaminant poisoning (in particular by sulphurous compounds) are such issues, tied mainly to the presence of an unprotected nickel catalyst. Although these phenomena have been studied extensively in the past in other fields of catalyst development, the particular conditions in high-temperature fuel cells (electrocatalysis, high temperature, reducing environment, interacting compounds, etc.) overlap several mechanisms, defeating theoretical and equilibrium analysis. Prediction and control are thus more of an iterative process, and a suitable correlation of conditions and effects is still under investigation. Research is exploring the possibilities of adding or substituting ceramic oxides in the basic anode make-up, since they provide resistance both against carbon deposition as against sulphur poisoning. Among these, especially ceria-based oxides are currently researched both in MCFCs and SOFCs, thanks to their high capacity for sulphide take-up and ease of regeneration. Investigations are required to determine the optimum compound for both electrocatalytic performance and resistance. One major challenge is also the fabrication procedure for their optimal incorporation in, or onto, the state of the art anode catalysts. A joint effort on the issues which are common to both the MCFC and SOFC communities has been shown to be possible, and could speed up the development to make HTFCs finally meet market requirements.
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