Review of Cathodes for Use in Proton-conducting Solid Oxide Fuel Cells

In this literature review, research of cathode material for solid oxide fuel cell/electrolysers shall be discussed, specifically investigating novel material research into proton-conducting solid oxide fuel cells. Research in this topic is being divided into two main field; materials that block or conduct protons. The review covers previous cathode materials used within generic oxide- conducting fuel cell and modern research articles relevant to the development of Proton-conducting solid oxide fuel cell cathodes and their key properties between the two categories.  

1. Introduction

First invented by Sir William Grove in 1839 in Swansea, Fuel Cells (FC) are electrochemical devices that convert chemical energy into electrical energy and sometimes heat energy. Fuel cell technology has been more prominent in recent years due to being; more efficient than traditional chemical combustion devices, being silent and potential of being more reliable due to have no moving parts, producing minimal to zero harmful emissions including NOx and SOx and due to their modular nature can be scalable in terms of power and capacity unlike batteries. Fuel Cells main components are an electrolyte usually in the centre of the cell, which allows ions to move from each Electrode; an anode and a cathode which are where the half reaction (fuel oxidation reaction or HOR and oxygen reduction reaction or ORR respectively) take place. There’s also an external circuit which connects the anode to the cathode which conducts free electrons. The biggest challenges facing Fuel cells the large expense of the materials and techniques in cell production, the need for high purity within its fuel and durability over a long period, especially within stop-start cycle conditions.

There are many types of fuel cells, usually named after the material of the electrolyte. One of the most promising types is Solid Oxide Fuel Cells (SOFC’s). SOFC’s are made from metal oxides with the electrolyte commonly a ceramic material. They conventionally work at an operating of 500-1000 ^oC, rather high compared to other FC’s but allows the energy output to both electrical and heat. Because of these dual energy outputs SOFC’s are a possible combined heat and power alternative for stationary purposes in permanent installations such a homes and large buildings. Another advantage of SOFC’s is there fuel flexibility running on essentially any pure form of proton-containing fuel (pure hydrogen, hydrocarbons, ammonia etc). This allows SOFC’s to be potentially implemented regardless of the fuel infrastructure of a given country or region.

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The largest barriers for SOFC’s to overcome before being commercially viable is the reduction usage of expensive materials, increasing in durability (Especially hydrocarbon-based fuels which have impurities) and the reduction in operating temperature to >500 ^oC. A novel attempt at solving these barriers is the use of proton-conducting oxides to form a proton conducting solid oxide fuel cell (H-SOFC’s). Conventionally, SOFC has a carrier ion of oxide ions which are formed by the ORR within the cathode and electrolyte conducts the ions to the anode where the ions oxidise the chosen fuel. H-SOFC’s work by changing the carrier ion to the protons being conducted to the cathode. Theoretically the use of proton-conducting oxides would allow for low-cost material use, lower operating temperature with a maximum of around 600 ^oC, and many of the issue around durability and voltage losses being reduced.

One of the biggest challenges facing this new technology is the losses that where present in the anode material (usually a nickel-based material that had issues with carbon deposition and sulphur poisoning) are reduced but issue is faced at the cathode. Cathodes are now required to be have good conductivity of proton, oxygen ions and electrons, be highly stable at the operating temperature and oxidisation, to have a similar thermal expansion coefficient (TEC) as the electrolyte (Usually a form of barium cerium yttrium zirconate or BZCY and to provide good catalytic ability to the ORR.

2. Conventional SOFC’s Cathode Materials

This section shall discuss the basis of cathode material research and specifically developments of cathodes in SOFC’s more generally.

2.1 Traditional perovskite’s

Cathodes tend to need to be mixed ionic electronic conductors (MIEC) and traditionally, the choice of material has predominately been perovskite structures (ABO₃). The current state of the art material is lanthanum strontium manganite (LSM or La_xSr_1-xMnO₃). Doping the material with strontium was found to enhance the mixed variance of the manganese (Mn3+/Mn4+) forming a p-type semiconductor material. The changing in variance means LSM is high catalytic to the ORR, however has poor oxygen ion conductivity. Efforts have been used to combined with yttria-stabilised zirconia, a very porous and well-used electrolyte material to combat poor ion conductivity. However, issues remain with the use of YSZ due to deceasing the performance of the cell.

A similar cathode material is lanthanum strontium cobalt (LSC or La_1-xSr_xCoO₃). LSC has been found to have improved cell performance, especially when used with cerium gadolinium oxide (CGO) electrolytes. The main research of the material is looking how to reduce the material high TEC in comparison to other material within the cell. The main interest seems to be looking at either doping with Fe (becoming LSCF) which improves high temperature stability and reduced TEC and forming other cobalt composites for low temperature uses i.e.  gadolinium strontium cobalt (GSC or Gd_1-xSr_xCoO₃) and samarium strontium cobalt (SSC or Sm_1-xSr_xCoO₃).

3. H-SOFC’s cathode research

Due to the difference in carrier ion, H-SOFC’s have other requirements to consider to more general SOFC’s. Herein the current research of H-SOFC cathodes is broken into two section based on the two mains types of material and its effect on ion; being proton-blocking or proton-conducting.

3.1 Proton-blocking cathodes (PBC)

3.1.1 Single Electronic Conductor (SEC)

SEC is materials which conduct electrons usually in a single-phase material. Oxygen gas usually interacts with the electrons on the surface of the cathode, but the ion needs to move around the cathode and interact with proton at the three-phase boundary site, forming water within the electrolyte.

SEC’s tend not to be the best form of cathode within H-SOFC. Although they have usually excellent electron conduction, there poor oxygen ion mobility is a real issue because it impedes the ORR and causing slower kinetics within the cell.

Examples of materials tested that are electronic conductor include Pt and (La_0.6Ba_0.4)MnO₃

• Xie et al
  •  
• Hou et al

3.1.2 Mixed Ionic and Electronic Conductors (MIEC) 

The traditional perovskite such as LSM are within the category of MIEC. With the ability to conduct of electrons and ions they allow ORR to within the interface electrolyte and the cathode, allowing for easier mass transport of water. The majority of formed from metal oxides and normal are structured perovskites, double layered perovskites and ruddleston-popper phase materials.

The main characteristics of a MIEC material is that they have high electrochemical catalytic ability towards ORR but tend to have lower oxygen ion transport and has slower reaction kinetics due to the slow mobility, but less of an issue than an oxygen-ion carrier electrolyte. The research has prioritised this effect due to the cathode material being an integral part of the ORR, therefore reactivity of the material is prioritised over the ion conduction.

Research has also used multi-layered to help improve ion conduction with MIEC materials. These usually involves mixing with the electrolyte usually a more porous material chosen for its ion conduction and an added effect is improved matching of thermal expansion coefficient to electrolyte material. However, optimising the concertation of mixture is critical so that electron conduction and catalytic effectiveness is not affected.

An alternative, in terms of material structure too perovskite is the Ruddleston-Popper phases (A_n+1B_nO_3n+1). These are in fact, two perovskites adjacent to another and interleaved via the cations in the form of a rock salt layer or layers (see Figure).  The materials give very strong electronic conductivity in comparison to perovskite due to its structural characterises. The most researched of these structures is the La₂NiO_4+δ. There has also been testing of the material which have doped or substituted Ni on the B sites. Materials chosen to have included Co, Fe and Cu and have been shown to perform well with lanthanum strontium gallium manganate electrolytes (LSGM). However, all types of above Ruddleston-Popper structures have reactivity with numerous electrolytes including CGO and YSZ, reducing effectiveness of the cell.

Another type of perovskite which has been heavily researched is the layered double perovskite. There structure, AA’B₂O_6-δ usually is a mixture of a Ba and a chosen Lanthanide and the B-site consisting of a front row transition metal. The most studied version of double perovskites is LnBaCo₂O_5+δ where Ln can be variety of different lanthanides i.e Pr, Nd, Sm and Gd. The structure works by barium and the lanthanide formed in alternate layers.

With these types of materials, the larger the ionic radius of the material, the more likely they too have improved oxide ion mobility. An example of this would be PrBaCo2O_5+δ, which has been shown to have excellent ion conduction and has been testing within a cell. However, the largest disadvantage is the high activation energy required by the material.

GdBaCo₂O_5+δ is another candidate having showed good ASR value on a CGO electrolyte at the operational temperature of 625 ⁰C. GdBaCo₂O_5+δ has been tested on other electrolytes at various operating temperatures. The material was shown to be highly reactive with a YSZ based cell at a higher temperature of 700 ⁰C

Example of new materials researched with the last decade of Proton-Blocking blockers include within a BZCY electrolyte Ba_0.5Sr_0.5FeO_3-δ-SDC¹, Sm_0.5Sr_0.5Fe_0.8Cu_0.2O_3-δ-SDC², La_0.7Sr_0.3FeO_3-δ-SDC^3,4, Ba_0.5Sr_0.5Fe_0.9Ni_0.1O_3-δ-SDC⁵, La0.6Pr0.2Sr0.2FeO_3-δ-SDC⁶, La₂NiO_4+δ-LaNi_0.6Fe_0.4O_3-δ73⁷, Pr_0.6Sr_0.4Cu_0.2Fe_0.8O_3-δ-SDC⁸, La₂NiO_4+δ-LaNi_0.6Fe_0.4O_3-δ⁹. Pr₂NiO_4+d¹⁰, PrBaCuFeo_5+x¹¹, SmBaCuCoO_5+X¹², SmBaCuFeO_5+X¹²

• Hou et al

• Hou et al

3.2 Proton-conducting cathode (PCC)

3.2.1 Mixed Protonic and Electronic conductors (MPEC)

MPEC’s are a mixture of good mobility of protons and electrons within a material. MPEC are usually material which has been doped by a transition and post transition elements e.g Fe, Co and Bi. Cobalt is especially good when it comes to a doping metal within MPEC’s due to its ability to improve the protonic ability of the material.

Example of new materials researched with the last decade of Proton-conducting blockers include within a BZCY electrolyte include; BaCe_0.5B_0.5O₃¹, BaCe_0.5Fe_0.5O₃², Ba(Pr_0.8Gd_0.2)O_2.9³.

• Tao et al Bi doping
• Tao et al Fe doping
• Has strong performance with Co
• Duan et al
• Relatively poor electro-catalytic abilities
• Zhu et al with BZCY electrolytes

3.2.2 Mixed Proton, Ion and Electronic conductors (MPIEC or MPOEC)

• Combinations of oxygen ion, proton and electron transport
• Tends mixture of MIEC and/with a proton conductor
• Most research looks at multi-phase materials
• Very likely to contain cobalt
• Tends to be costly and to have high thermal expansion coefficients
• Also issues with water incorporation.

Example of new materials researched with the last decade of MIEC’s with a proton conducting include Sm_0.5Sr_0.5CoO_3-δ-BaCe_0.8Sm_0.2O_3-δ ¹, PrBaCo2O_5+δ-BZCY², GaBaCoFe_5+δ-BZCY³, La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ-BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3-δ⁴, Ba_0.5Sr_0.5Co_0.8Fe_0.2O₃-BaCe_0.8Sm_0.2O_2.9⁵

• Wang et al

3.3.1 Comparing both categories

Sun et al has started comparing the two categories by using mixed phases with the same cathode-base. As in example, he evaluated the different composites, one a proton-blocking material, La_0.7Sr_0.3FeO_3-δ-Ce_0.8Sm_0.2O_2-δ or LSF-SDC and the other a proton-conducting, La_0.7Sr_0.3FeO_3-δ-BaZr_0.1Ce_0.7Y_0.2O_3-δ or LSF-BZCY. Both materials where characterised via XRD, Impedance and then cell tested with a BZCY electrolyte-based H-SOFC cell. The results demonstrated that as a composite cathode, LSF-BZCY showed lower area specific polarization resistances than the LSF-SDC, whilst LSF-SDC demonstrated higher performance than its proton-conducting counterpart. However, whilst measuring as a single cell the polarization resistances were very similar (LSF-SFC 0.11 Ω cm^-2 and LSF-BZCY value of 0.13 0.11 Ω cm^-2) and power densities showed that the proton-blocking conductor material was higher and more powerful, 449 mW cm^-2, 405 mW cm^-2 at 700 ⁰C, LSF-SDC and LSF-BZCY respectively. Sun concluded that LSF-SDC was “superior” than LSF-BZCY as H-SOFC cathode material and suggested than in the case of BZCY electrolytes that proton-blocking cathodes are preferable than proton-conducting materials.

Sun et al is one of a very limited number of papers that tend to suggest that proton-blocking conductor materials are the preferred category material for cathodes. However, the limitation of the paper available suggest that more specific work should be conducted over several types of cathode and electrolyte materials.

4. Conclusions

• PBC is weaker oxygen ion mobility than PCC but has issues with electronic conduction
• Single layers seem not to be the answer and multi-layers are being heavily researched

References

• J Hou, L Miao, J Hui, L Bi, W Liu, J Irvine, J. Mater. Chem. A, 2018, 6, 10411
• J Hou, J Qian, L Bi, Z Gong, R Peng, W Liu, J. Mater. Chem. A, 2015, 3, 2207-2215
• Z Wang, W Yang, S Shafi, L Bi, Z Wang, C Xia, W Liu, Y Liu, J. Mater. Chem A, 2015, 3, 8405-8412
• General Fuel Cell Books
• R O’Hayre, S-W Cha, W Colella, F Prinz, Fuel Cell Fundamentals, John Wiley and Sons Inc, Hoboken, 10^th edn., 2006
• A Appleby, F Foulkes, Fuel Cell Handbook, Van Nostrand Reinhold, New York, 1th Edn, 1989
• A Dicks, D Rand, Fuel Cell Systems Explained, John Wiley and Sons Inc, 3^rd edn, 2018

Electronic Conductor

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2. H Iwahara, T Yajima, H Uchida and K Morimoto, Proceddings of the Second Internatinal Symposium on Solid Oxide Fuel Cells, held from 2 to 5 July 1991 in Athens, Greece, CEC, Brussels, 1991

Mixed Ionic and Electronic Conductors (MIEC) 

1. W. Sun, Z. Shi, S Fang, L. Yan, Z. Zhu and W.Liu, Int J. Hydrogen Energy, 2010, 35, 7925-7929
2. Y. Ling, J. Yu, B. Lin, X. Zhang, L. Zhao and Z. Liu, J .Power Sources, 2011, 196, 2631-2634
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Mixed Protonic and Electronic conductors (MPEC)

1. Z Tao, L. Bi, L Yan, W. Sun, Z. Zhu, R Peng and W.Liu, Electrochem. Commun, 2009, 11
2. Z Tao, L Bi, Z Zhu and W Liu, J. Power Sources, 2009, 194, 801-804
3. Same as reference 1 in Electronic Conductor

Mixed Proton, Ion and Electronic conductors (MPIEC or MPOEC)

1. F. He, T. Wu, R. Peng and C. Xia, J. Power Sources, 2009, 194, 263-268
2. C. Yang and Q. Xu, J. Power Sources, 2012, 212, 186-191
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Comparing both categories

1. W. Sun, S. Fang, L.Yan and W. Liu, J.Electrochem. Soc., 2011, 158, B1432