SOFC - An Introduction
Solid oxide fuel cells (SOFC) convert chemical to electrical energy
directly, much like the common battery, except that they can be
constantly supplied with fuel for a continuous source of power. SOFCs
are very attractive as a source of power as they can convert a wide
variety of fuels with high efficiency (40-60% unassisted, up to 70% in
pressurized hybrid system) compared to engines and modern thermal power
plants (30-40% efficient) [1].
In addition to high efficiency and fuel adaptability SOFCs are
clean, reliable, and almost entirely nonpolluting. They are
non-vibrating due to the absence of moving parts and the noise
pollution associated with power generation is also eliminated.
Figure 1
A SOFC essentially comprises two porous electrodes which sandwich an
electrolyte. Air flows along the cathode (the "air electrode"). At the
cathode/electrolyte interface, oxygen catalytically acquires four
electrons and gets converted to two oxygen ions. They migrate through
the electrolyte to the positively charged anode (the "fuel electrode")
where they oxidize the fuel resulting in the formation of water, carbon
dioxide, heat and electrons which complete the external circuit. The
charge flow in the external circuit is balanced by ionic current flow
within the electrolyte. The ideal voltage (Eo) from a single cell under
open circuit conditions is close to 1.0V dc as calculated from the
Nernst equation. However, the useful voltage output (V) under load
conditions, that is, when a current passes through the cell, is given
by [2]
where I is the current passing through the cell, R the electrical resistance of the cell, and ηc and ηa the polarization losses associated with the cathode and anode, respectively.
Although the operating concept of SOFCs is simple, the selection of
materials for the individual components presents enormous challenges.
Traditional solid-oxide fuel cells operate at high temperatures,
generally in the range 800-1000 °C, oxidizing hydrocarbon fuels to
carbon dioxide and water. These high temperatures place great demands
on the fuel cell components such as the anodes, cathodes, electrodes
and all supporting structures. Due to the high internal temperatures
involved, SOFCs have been primarily developed for stationary power
generation in which cell life and overall safety can be better ensured.
Lowering the SOFC working temperature would extend durability of
SOFC cells, expand applicable kinds of materials and elevate SOFC cost
performance [3 ]. Materials research focused on solid oxide fuel cells
(SOFC) is driven by the recognition that processing and operating at
lower temperatures would not only directly address many reliability
problems, but also have the potential of leading to lower cost
manufacturing and lowering the cost of energy produced from the
resulting power sources. In addition, for potential applications in
transportation that involve frequent thermal cycling and rapid
start-up, the operating temperature is expected to be around 500 ºC [4].
The porous anode of a SOFC serves to provide electrochemical
reaction sites for oxidation of the fuel, allow the fuel and byproducts
to be delivered and removed from the surface sites, and to provide a
path for electrons to be transported from the electrolyte/anode
reaction sites to the interconnect in SOFC stacks. Ni-YSZ is commonly
used as the anode that works at around 1000 ºC. There has been
extensive studies to develop low temperature anodes for SOFC.
Nickel alloy - YSZ composites have been reported to be a good anode
material for low and intermediate temperature (~600 ºC) SOFC [5, 6, 7].
While earlier studies have focused on bimetal alloys, comprising mainly
of Ni and Co, a recent report by Xie and coworkers have established
that better cell performance can be obtained with a trimetal alloy
comprising Fe, Co and Ni in SDC (Samarium doped ceria) support [8].
Figure. 2 Impedance spectra measured at 450 degree C in hydrogen atmosphere for symmetrical cells with Fe0.25Co0.25Ni0.5-SDC and Ni-SDC electrodes.
The electrical property or electrochemical activity of the Ni
alloy-SDC cermet as an SOFC anode is affected by the electrical
conductivity of each constituent component as well as the
microstructural parameters like size, portion, spatial distribution and
contiguity of each constituent phase [9 ]. Thus, it is necessary to
characterize the correlation between electrical property and
microstructural parameters of the composite for the proper utilization
of Ni-ceria cermet with optimum condition. So far, there have only been
studies to establish low-temperature anode activity. There have
however, been no optimization of these anodes for improved performance.
The solid oxide electrolyte conducts oxygen ions from cathode to
anode. The electrolyte has to be chemically, morphologically, and
dimensionally stable in both oxidizing and reducing conditions. The
electrolyte should also be thermally compatible with other cell
components. High ionic conductivity of the electrolyte is required for
the SOFC operated at low temperatures. The electrolyte also has to be
made fully dense to prevent gas leakage.
The following materials are most commonly used as SOFC electrolytes at present.
- Yittrium Stabilized Zirconia (YSZ)
- Samarium Doped Ceria (SDC)
- Gadolinium Doped Ceria (GDC)
The poor activity of cathode materials to oxygen reduction is one of
the key obstacles for the development of Low temperature-SOFCs. It is
known that La1-xSrxMnO3 exhibits high
performance as the cathode catalyst for SOFCs operated at temperature
above 800 °C. However, as the temperature decreases, its electric
conductivity decreases dramatically and the polarization resistance
increases [10].
Thermodynamic and experimental studies have show that mixed ionic
electronic conduction (MIEC) materials could be an efficient
alternative to enhance the performance of the cathode at low
temperatures due to the fact that they can simultaneously give high
electric and ionic conductivity and high oxygen electrochemical
reduction activity [11,12,13].
Recently, Shao and Haile reported a new cathode material, Ba0.5Sr0.5Co0.8Fe0.2O3
(BSCF) for low-temperature SOFCs, which has been extensively
investigated as an oxygen permeation membrane material for oxygen
generation [14]. With this BSCF cathode, good performance has been
obtained at low temperatures from a single cell based on a 20 micron
thick Sm0.15Ce0.85O (SDC) electrolyte film. Maximum power densities of 402 mWcm-2 were produced at 500 oC, which is the highest performance in reported literature until now [15].
References
- Singhal, S.C; "Science and Technology of Solid-Oxide Fuel Cells," MRS Bull.25,.3, 16-21 [2000]
- Hirschenhofer, J.H; Sauffer, O.B; Engleman, R. R; Klett, M.G
(Eds.), Fuel Cell Handbook, US Department of Energy, Morgantown, WV, 3
[1998]
- Suda, S; Itagaki, M; Node, E; Takahashi, S; Kawanob, M; Yoshida, H;
Inagaki, T; Journal of the European Ceramic Society; 26, 4-5; 593-597
[2006]
- Doshi, R; Richards, V.L; Carter, J.D; Wang, X; Krumpelt, M; J. Electrochem. Soc. 146; 1273 [1999]
- Ringuede, A; Labrincha, J.A; Frade, J.R: Solid State Ionics 141-142; 549 [2001]
- Sato, K; Ohmine, Y; Ogasa, K; Tsuji, S; in: S.C. Singhal, M. Dokiya
(Eds.), SOFC-VIII, vol. 2003-07, Electrochem. Soc., Pennington, NJ,
2003, p. 695.
- Ringued´e, A; Fagg, D.P; Frade, J. R; J. Eur. Ceram. Soc. 24; 1355 [2004]
- Xie, Z; Zhu,W; Zhu, B; Xia, C; Electrochimica Acta 51; 3052-3057 [2006]
- Lee, J. -H; Moon, H. Lee, H, -W; Kima, J; Kima, J. -D; Yoon, K.H; Solid State Ionics; 148, 1-2 , 15-26 [2002]
- Minh, N.Q; Takahashi, T; Science and Technology of Ceramic Fuel cells, Elsevier, Amsterdam 117 [1995]
- Stevenson, J.W; Armstrong, T.R: Carneim, R.D; Pederson, L.R: Weber, W.J; J. Electrochem. Soc. 143 2722 [1996]
- Sahibzada, M; Benson, S.J; Rudkin, R.A: Kilner, J.A; Solid State Ionics 113-115; 285 [1998]
- Skinner, S.J; Fuel Cells Bull. 4. 6 [2001]
- Shao, Z.P; Haile, S.M; Nature (London) 431; 170 [2004]
- Yan, A: Cheng, M; Donga, Y; Yang, W; Maragou, V; Song, S: Tsiakaras, P; Applied Catalysis B: Environmental; 66, 1-2 64 [2006]
Comments @ nOnoscience
October 1st, 2007 at 8:57 am
[…] micro-pores) and the grass would give it the special flavor that can only be experienced.Read more here. Post a […]
October 1st, 2007 at 1:12 pm
It is really suprising that Vettiver has so much use. In tamil, as you know vetti means jobless. i had a classmate in my undergraduate days who used to be called vetti (Not that he was stupid). Now he is a pulmonolgist in your town. So i can understand from your research, vetti can evolve to have lot of uses.
October 1st, 2007 at 7:11 pm
Lakshmi,
I recently came across Honey Bee Network (started by Anil Gupta); lot of stuff about it on the internet. May be this can fit into one of their programmes.
October 1st, 2007 at 9:16 pm
hey do you know what ‘alum’ is called in tamizh. this vettiver, reminded me of that and have been raking my brains to come up with the tamizh name for alum…help, pls.
October 1st, 2007 at 9:47 pm e
Sowmya: good to see you here. If I remember right, isn’t it padikaram படிகாரம் ?
October 1st, 2007 at 9:59 pm
smalltowndoc, thank you for the anecdote. As they say in Tamil, “Siru Thurumbum Pal Kutha Udhavum” (Even a small splinter can be used to floss your teeth
Swarup, thank you for the tip. The Honey Bee Network does seem like a good place to sound this post.
Sowmya, just curious. Why did vettiver remind you of padikaaram? As far as I can see, one is organic and the other inorganic
October 2nd, 2007 at 1:59 am
Don’t they use padikaram n water and vettiver also with water. I guess that is the link in my head.
October 2nd, 2007 at 9:57 am
Grandmother used to hang up curtains made of dried vettiver, and pour water over it, so that as the hot wind blew through the porous curtain, they would instantly cool down before entering the house…
Hey, cool! I’m a sucker for passive heating and cooling techniques, and I’ll have to give this one a try someday.
October 2nd, 2007 at 4:13 pm e
[…] When I was a little girl, summer would mean fun - non-stop games with cousins and friends, riding bicycle in the sweltering heat, […]
October 3rd, 2007 at 9:23 pm
[…] When I was young, I used to go out with my grandfather and scout around Oddakkadu fields (our farm’s name) for a certain root used as perfume and in preparations in south India, called Vettiver in tamil. I loved pulling it up and taking in the aroma along with the smell of moist dirt. Wonderfully refreshing. Lakshmi at nonscience led me down memory lane with a post on vettiver. […]
October 4th, 2007 at 10:23 pm e
very nice article Lakshmi….it brought back fond memories. The only times I’d have “vettiveer” flavored anything, or see it often, would be during my numerous visits to tirunelvelli to visit cousins, and to take a break from my cityslicker lifestyle. And that smell was always incredible.
October 15th, 2007 at 6:25 pm
Lakshmi writes aboutVettiverand apparently it can combat termites and floods! This post on ‘ Oxcytocin and Childbirth’ in A Blog around the Clock was very interesting to read. I will not pretend I understand all that stuff, but what little I did was worth the read.
October 17th, 2007 at 9:07 am
[…] Lakshmi wrote an interesting post today on V for VettiverHere’s a quick excerptIn the east, especially in its original home, India, and in other South East Asian countries, vettiver has been used in an variety of bioengineering applications including water conservation, slope stabilization, pollution control, … […]
October 22nd, 2007 at 7:58 pm
Late replies, still…
Sowmya, Ah ! And thanks for the trackback.
Aaron, Living in a tropical country has its benefits, you see ! Necessity becomes the mother of some really useful inventions
Sunil, at least in big cities, old stuff is becoming fashionable again. What grandmom did for a few annas then for practical solutions, interior designers do to add the “oomph” to the modern house, with a major dent in your bank balance