6deposition of YSZ on Ni/YSZ composite film by rf sputtering  technique;

7deposition of Ag and YSZ on YSZ film by dc and rf sputtering  technique, respectively;

8deposition of Ag on Ag/YSZ composite film by dc sputtering technique; and

9removal of silicon nitrate from backside of structure by reactive  ion etching。

Prepared in this way mSOFC-MEMS provides 0。15 W at 3168C (Morse and Jankowski, 1999)。 The film can be deposited by other techniques, e。g。 pulse laser deposition (Rupp and Gauckler, 2006), but the sputtering methods are the most popular (La O et al。, 2004; Hertz and Tuller, 2004; Bieberle-Hutter and Tuller,  2006)。

Currently the ETH, Zurich is developing the mSOFC system called ONETBAT。 It is fabricated on Foturanw which is a glass ceramic 300 mm thick substrate, by spray pyrolysis, pulse laser deposition or sputtering (Bieberle-Hu¨ tter et al。, 2005, 2008)。 To prepare the mSOFC, first the platinum current collector-anode (,50 nm thick) is deposited and then on top subsequently the ,550 nm thick YSZ electrolyte and

,200 nm thick LSCF perovskite cathode。 Next, the structure is annealed at 6008C to crystallize the ceramic layers and then

Figure 7 Schematic view of mSOFC prepared by MEMS technology

Cathode Electrolyte Anode

O2 (0。21 atm) e–

Silicon support

Source: Srikar et al。 (2004) reproduced with permission of Elsevier

the selected parts of the Foturanw substrate are UV-exposed and  back  etched  to  release  the   ceramic  membrane。 The mSOFC at 5508C provided the open circuit voltage    of

1。06 V and the power of 150 mW/cm2。

It is important to note that the ceramics films are deposited at low temperature and therefore most likely they are going to have nanocrystalline structure。 Therefore, one might  expect that the grain boundary conductivity might be a significant portion of the electrolyte total conductivity, which will influence the performance of the mSOFC。 As an example, the temperature dependence of nanocrystalline conductivity of samarium doped  ceria electrolyte is shown in Figure    8。

3。Single chamber solid oxide fuel  cells

The working mechanism of SC-SOFC operation is based on the difference in electrocatalytic activity of the  electrodes to air and hydrocarbon。 If anode is catalytically active to hydrocarbon reforming, then a following chemical reaction is expected  to  occur (1):

O2 þ 4e2 ! 2O22 ð2Þ

H2 þ O22 ! H2O þ 2e2 ð3Þ

CO þ O22 ! CO2 þ 2e2 ð4Þ

Reaction (1) is exothermic and causes increase of the fuel cell temperature。 This is advantageous because the higher temperature lowers resistances of electrolyte and    electrodes。

The major advantage of SC-SOFCs is a possibility of direct use of hydrocarbon without loss of fuel cell  performance。 While in case of the SOFC direct use of hydrocarbon fuel usually leads to carbon deposition on anode side and  as  a result the fuel cell performance decrease。 Another advantage is related to the possibility of application of porous electrolyte (Suzuki et al。, 2005), because mechanism of  operation  is based on difference of catalytic activities of electrodes。 This significantly simplifies technological requirements and cost effective  technologies  can  be  used  for  fuel  cell  fabrication,

e。g。 screen printing (Suzuki et al。, 2005)。 A major drawback of the  SC-SOFCs  is  lower  fuel  utilization  than     conventional