Single chamber Solid Oxide Fuel Cells selective electrodes: A real chance with brownmillerite-based nanocomposites
Introduction
Solid Oxide Fuel Cell technology is in a period of great development and since now, has been reserved to stationary applications, due to their size and their thermomechanical fragility.
A particular type of SOFC has been proposed and studied: Single Chamber SOFC (SC–SOFC). A SC-SOFC [1,2] resolves most of the flaws of a traditional SOFC and in principle would have several advantages [3,4], especially for small and cheap devices. In the SC-SOFCs both the electrodes are immersed in the same atmosphere. The obtainment of an effective sealing at high temperature is complicated, and tight connections between the cell and the other sealing elements can be the source of tensions leading to failures. For Single Chamber devices, cell design becomes more compact, and in general fabrication is easier. Moreover, specific applications can be devised; an example is the use of SC-SOFCs operating with exhaust gases in thermal engines [5]. In this application the cell might be embedded at the outlet of engine and could turn unburned hydrocarbons present in the exhaust gas into electricity allowing, for instance the energy supplying of electronic devices in vehicles; such application can also consider all cases in which hydrocarbons are burnt, as in stationary power plants.
After start-up, SC-SOFCs can operate without external heating, because the direct oxidation of fuel by oxygen is exothermal and can sustain autonomously the cell, at expense of a fraction of efficiency. The main drawbacks are the low efficiency due to parasitic reactions, and the risk of explosion related to the presence of fuel and oxygen in the same mixture. For safety reasons, SC-SOFCs are fed only with methane and rarely with hydrogen; methane at the anode is subjected to partial oxidation and the syngas produced is the real fuel [6].
In SC-SOFCs the selectivity of electrode materials for the respective electrochemical reaction is critical, and has been the main restrain to a large diffusion of these devices. No material turned out to have the selectivity required, and only a maximum 8% efficiency has been reached [4]. Instead, great advancements were accomplished in cell design, with indisputably good results in terms of cell power [7].
Coplanar cells are electrolyte plates in which both the electrodes are printed on the same side [8,9]. Flow-through cells (also named Mixed Reactant Cell) in which the gas mixture is forced through repeated porous layers of cathode, electrolyte and anode are another possibility [10].
Riess [6,[11], [12], [13]], stressed the importance of the selectivity of the electrodes on SC-SOFCs performance. A SC-SOFC is operated in an anomalous way compared to other cells. A strong imbalance in fuel (methane)/oxygen ratio, which ranges between 1 and 2 [14,15] and a high gas flow, allow a large part of the fuel to pass through the cell unreacted.
The oxygen poor mixture is necessary to promote selective methane partial oxidation to syngas at the anode [6]. The cathode should not be able to catalyse methane oxidation, and hardly methane can be used directly for electrochemical oxidation. Syngas is the real fuel so feeding this mixture of methane and oxygen is a way to feed fuel only at the anode. Feeding directly with syngas would lead to loss of performances, because the cathode is able to electrocatalyse its oxidation. A fake but effective selectivity of the anode can be created with a fuel rich mixture, but the cathode must be selective by itself. A single chamber cell with an efficient cathode could operate with low flows and reach high fuel utilization efficiencies, up to 25% each cell and to 1-(3/4)n for a stack of n cells [6].
Electrochemical reactions require limited residence times, while chemical reactions, that should be suppressed in a SC-SOFC, take longer times; as a reference, 10 ms is a suitable time for electrocatalysis reactions while being low enough to avoid catalytic effect [12]. Thus, high flow is a mean to decrease residence times and to avoid the catalytic effect of the used materials, but that entails a very low fuel utilization; residence time is also influenced by materials porosity, cell geometry and gas flow. Anode activity is stressed by the mixed atmosphere (stability, deactivation due to the reduction/oxidation cycles) [[16], [17], [18]].
Aim of the present work is to developed a perovskite/brownmillerite nanocomposite in which the perovskite is selected for catalytic selectivity toward oxygen reduction and the brownmillerite for ionic and electronic conductivity: this is expected to help obtaining a selective cathode for SC-SOFC.
Several materials, mainly perovskites, have been considered interesting to improve selectivity and stability of cathode: ferrites, cobaltates, manganates [[19], [20], [21], [22], [23], [24], [25], [26], [27], [28]]: among these we focused on iron containing ones.
All materials used as SOFC cathodes are Mixed Ionic Electronic Conductive (MIEC) oxides or composites of MIEC materials [29]. Several perovskite based oxides show good oxygen ion mobility and conductivity [[30], [31], [32]] as well as electronic conductivity [33] but are characterized by high catalytic activity in oxidation. Because of this reason, among MIECs the choice has been restricted to a brownmillerite (A2B2O5) of the type Ca2FeAlO5 (CFA) [34]. In principle, CFA-based oxides are less reactive than perovskites toward oxidation and are characterized by a good chemical stability. The brownmillerite structure differs from the perovskite one for the presence of ordered oxygen vacancies in the (010) plane. Depending on the B cation and on the synthesis procedure, fully (Ibm2 and Pnma) or not fully (Icmm) ordered structures can be obtained. The presence of cations that prefer tetrahedral coordination (as Al) contribute to increase the stability. The distribution of Al and Fe in Ca2FeAlO5 follows the “homogeneous layer principle” [35]: Al and Fe tend to form homogeneous octahedral layers or tetrahedral chains rather than mixed layers or chains of two elements along the b axis. To improve conductivity, a small amount of magnesium was inserted, so the actual formula of CFA is Ca2FeAl0.95Mg0.05O5 [36]. Also Co-doping was considered in literature for improving conductivity [[37], [38], [39]] in our work we decided to avoid the use of Critical Raw Materials and Mg was preferred. The substitution of Al(III) with Mg(II) should help increasing Fe(IV) amount for charge compensation. Mg also affects the structure having a high octahedral site preference (higher than Fe(III)) and enhances the number of iron cations in tetrahedral environment; its solubility in CFA is low (<0.1) [36]. After its synthesis by citric acid route, CFA has been characterized by means of XRD, TPR, SEM, XPS, EDX and N2 adsorption isotherms; its catalytic activity towards methane oxidation has been measured; once verified the low activity toward methane oxidation, CFA was selected as the MIEC support for the catalytically active phase.
Following literature, that underline the capability of iron containing oxides to dissociatively chemisorb oxygen [[40], [41], [42], [43]] and our previous experience, the supporting CFA powder has been impregnated with iron and thermally treated with the aim of inducing the formation of a catalytically active iron based active phase enclosed in a brownmillerite. We refer to this second material as CFA + FeOx. Catalysis and electrocatalysis are diffuse applications of these nanocomposition techniques [[44], [45], [46]]. To the best of our knowledge brownmillerite-based nanocomposites are not so diffused and some few example concerns composites of the type perovskite/brownmillerites [47,48].
Symmetrical cells with Cerium Gadolinium Oxide (CGO) electrolyte have been prepared using both CFA and CFA + FeOx for electrochemical impedance spectroscopy (EIS) tests. The electrode firing temperature has been optimized; complete cells with a Ni/CGO anode have been fabricated and their polarization curves have been measured. The same treatments necessary to assure electrode/electrolyte adhesion have been carried out to the CFA + FeOx powders to better investigate the evolution of nanocomposite during the device optimization.
Section snippets
Synthesis and characterization
CFA has been synthesized with a solid combustion synthesis. Precursors CaCO3, Fe, Al(NO3)3 and Mg(OH)2 were dissolved in water with the help of HNO3 when needed. Citric acid in amount 1.9 times the cations in solution was then added. The solution was neutralized adding drop-by-drop NH4OH; water was eliminated by mildly heating on hot plate, gel formation happens at the end of this step. Increasing of hot-plate heating over 250 °C triggers gel self-combustion. The obtained powder was grinded and
XRD
CFA pattern (Fig. 1Sa -Supporting Information) matches with a brownmillerite phase Ca2FeAlO5: the magnesium concentration is only minimal and insufficient to produce significant deviations from parent Ca2FeAlO5 structure but probably un-favours crystalline order. Brownmillerite's crystal structure is similar to the perovskite's one, from which it can be obtained with the ordered removal of one oxygen atom every six [54]. With this transformation, each layer every two of the perovskite becomes a
Conclusions
A specific material for cathodes of SC-SOFCs, Ca2FeAl0.95Mg0.05O5, has been designed aiming to obtain a material with specific properties in terms of process efficiency. During the material design, the requirement of a high final power output was deliberately dropped in order to maximize the results. CFA and its derivative CFA + FeOx have been successfully synthesized and completely characterized. Symmetrical cells with CFA and CFA + FeOx have been fabricated and tested by means of EIS, the
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
The authors want to express their gratitude to Dr. Clematis from University of Genova, for his great help in the interpretation of EIS spectra.
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Elaboration and characterization of the brownmillerite Ca<inf>2</inf>Fe<inf>2</inf>O<inf>5</inf> synthesized by citrate sol-gel method. Application for photocatalytic H<inf>2</inf>-Production under visible light over Ca<inf>2</inf>Fe<inf>2</inf>O<inf>5</inf>/ZnO hetero-system
2022, International Journal of Hydrogen EnergyCitation Excerpt :They can be also used in photocatalysis under visible light owing to their narrow band gap energy activity [23,24]. In this category, the brownmillerites A2B2O5 (A = alkaline earth and B = Fe, Co), are used in solid oxide fuel cells [25], Li-ion batteries [26] and photoelectrochemical conversion [27] because of their oxygen ionic conduction and moderate, electronic conductivity [28]. Due to oxygen vacancies, they are formed with alternate sheets of octahedra and tetrahedra [29], thus permitting the accommodation of 3d cations with mixed valences [30].
Ca<inf>2</inf>Fe<inf>1.95</inf>Mg<inf>0.05</inf>O<inf>5</inf>: Innovative low cost cathode material for intermediate temperature solid oxide fuel cell
2021, International Journal of Hydrogen EnergyCitation Excerpt :Furthermore, it presents a TEC of 11.3 × 10−6 C−1 between 700 and 1000 °C, a value compatible with that one for Gadolinium Doped Ceria (GDC), the most used electrolyte for SOFC operating at intermediate temperatures (IT-SOFCs) [27,28]. The presence of Fe (IV) cations in the structure, confirmed in literature by Mössbauer spectroscopy [29], in addition with Fe (III) ions, is considered responsible for electronic conductivity, based on polaron hopping through the cations of the lattice [30]. To further increase the electronic conductivity of the material an aliovalent doping on the B site with Mg was chosen in this work to promote the presence of Fe (IV) and, consequently, to improve the electronic conduction.
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