Study of an Electrolyte Free Fuel Cell from Recycled Conventional Solid Oxide Fuel Cell

Fan Liangdong， He Yunjuan， Zhu Bin1，

(1.Department of Energy Technology，Royal Institute of Technology(KTH)，S-100 44 Stockholm，Sweden;2.Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials，F(xiàn)aculty of Physics and Electronic Technology， Hubei University， Wuhan 430062， China)

The ever increased environmental pollution and the energy demand ask for the highly efficient energy conversion and storage technologies.Fuel cells，kinds of electrochemical devices that convert the energy from chemical fuel into the electricity directly，are such a kind of candidates.Since the discovery of the fuel cell at the year of 1830 by Sir Grove[1］， the fuel cell has been subjected to extensive studied and great progress has been made， especially in the last1960.However，all fuel cells are not ready formarket application due to the high cost and insufficient life time.

Among the fuel cells，the solid oxide fuel cell(SOFC)presented the most promising prospective for large scale application，especially for the stationary power plant.With the evolution of the new materials and fabrication technologies，the SOFC hasmade enormous steps towards the industrial application.The recent research tendency is to reduce the operational temperature to improve the fuel cell stability and reduce the system operation cost[2］.High ionic conductive electrolytes and some good electrodematerials have been developed in the past decades，which have largely motivated the fuel cell communities[3-4］.The ceriacarbonate composite material，one of the newly highlighted materials，has attracted wide attentions due to its excellent ionic conductivity and unique hybrid ionic conduction property[5-8］.These exceptional properties have been also demonstrated in the fuel cell measurement and other advanced energy and environmental applications， such as electrolysis[9］， direct carbon fuel cell[10-11］， CO2separation[12］， NH3synthesis[13］and so on.The recent research of the mixed transition metal oxide composite electrode has also demonstrated with outstanding electrode catalytic activity.It shows double catalytic function both for fuel oxidation and oxygen reduction[14-17］.The excellent fuel cell performances，low costmaterial and fabrication technology as well as the operation requirementmake it a prospective system for low temperature and highly efficient energy conversion application.

More recently， a breakthrough research result，the electrolyte free fuel cell(EFFC)[18-24］， has been reported by a Swedish group in Royal Institute of Technology.Instead of the three layer assembly configuration of the conventional SOFCs(anode/electrolyte/cathode)，the electrolyte layer has been physically removed.Such a novel fuel cell only uses one layer compositematerial，containing the ionic conductivematerial and the electron conductive material with semi-conductive property.Although it is structurally simplified，an EFFC could still realize the same，even some cases higher performance as what a conventional SOFC do.EFFC provide simple， harmony， effective and much affordable characteristics over the conventional threelayer fuel cell.

In this work， we will try a new process， a recycle of three-layer fuel cell for fabrication of electrolyte free fuel cell.The electrochemical performances of the conventional three-layer fuel cell and EFFC were investigated and compared.The results will highlight the great prospective of the electrolyte free fuel cell for energy conversion.

1 Experimental

Two kinds ofmaterials are synthesized for fabrication of SOFC and EFFC.One is the ionic conductor，samarium doped ceria-sodium carbonate(SDC/Na2CO3，NSDC)which also serves as the electrolyte for SOFC.It is prepared by a one-step co-precipitation method as described elsewhere[25］.Briefly， Sm(NO3)3·6H2O and Ce(NO3)3·6H2O was homogeneously dissolved in the de-ionized water.The Na2CO3solution was employed as the precipitant.The resulted white precipitant was collected by infiltration， drying， and high temperature sintering before utilization.The LiNiCuZnO oxide composite(LNCZ)，playing as the electro-catalyst for electrode reaction was prepared by a simp le solid state reaction method.All the raw materials for LNCZ were mixed and sintered at 700℃for 3 h.

The conventional three-layer fuel cell with the configuration of LNCZ-NSDC/NSDC/LNCZ-NSDC was prepared by a cold-pressing approach.The electrode of LNCZ/NSDC was prepared by mechanical mixed of LNCZ and NSDC in amass ratio of 55∶45.The thickness of the each layer， i.e.the anode， electrolyte and cathode layer，was varied by the adjusting of the powder mass.Powder weights of 0.26 g，0.16 g and 0.12 g，were respectively used in this study for a anode support SOFC.The single cellwas subjected to 600℃heat-treatment before fuel cell testing.

The electrochemical performances of SOFC were tested in air by electrochemical impedance spectroscopy(EIS)first，then in H2/Air for I-V and I-P characteristics as well as the EIS measurement at 500 and 550℃.During the testing， two silver-paste coated nickel foams were used as the current collector.The hydrogen and air flowing rate are 120 m L/min and 150 m L/min，respectively.The cell was stabilized for 30 min before performing the each measurement.The active area of single cell is 0.64 cm2.

After the electrochemical testing，the discarded three-layer fuel cell， i.e.SOFC pellet， was used for preparation of EFFC.A simple procedure is presented in schematic Fig.1 .The SOFC was crashed in amortar with pestle.The powder was carefully collected and used for EFFC pellet fabrication with similar procedure as the conventional three-layer fuel cell，butmuch easier steps.The EFFC pellet underwent a fuel cell testing immediately right after the relative fabrication without any further treatment.The I-V curve was recorded similar to three-layer fuel cell.A 30 minute shortterm stability testing of the EFFC was also carried out in a fuel cell condition.

Fig.1 Schematic representation of the fabrication of an EFFC from a conventional SOFC

2 Results and discussion

The electro-catalytic activity of the transition metal oxide composite for oxygen reduction reaction is presented in the Fig.2 a.Generally，in the impedance spectroscopy，the high frequency intercept is defined as the ohmic resistance，the intermediate frequency semi-arc after the high frequency is set as the charge transfer resistance，and the tail after the semi-arc is thought as the gas diffusion resistance[26］.The ohmic resistance is around 0.8Ω，while the charge transfer resistance，about5.4Ω(3.456Ω·cm2in area specific resistance)，is much higher than the ohmic resistance，and the required values for high effective electrode[27］.The apparent gas diffusion resistance is also several times larger than the ohmic resistance.It is well known that the oxygen reduction process takes up major voltage loss of the whole fuel cell，especially at the reduced temperature range.In addition，the gas diffusion resistance is a little higher than we could expected，though the composite electrode is subjected to low temperature heat-treatment of 600℃in this study.Thus the bulk density of the composite electrode should be reduced.Some pore forming agent，such as activation carbon and organic material，could be added to facilitate the gas diffusion in the composite electrode.

Fig.2 EIS of 3-layer SOFC at 550℃，a)in air and b)in H 2/air

The fuel cell impedance in H2/air condition is presented in Fig.2 b.It can be seen that the impedance spectroscopy shape changes much compared with that in the air.In detail，both the ohmic resistance and the electrode polarization are much reduced in the fuel cell condition.One side of pellets is subjected to the hydrogen oxidation reaction whose kinetics ismuch favorable compared with that for oxygen reduction reaction at the same temperature.In addition，as can be seen from Fig.2 ，the ohmic resistance is much reduced in fuel cell atmosphere vs.in air，0.6Ωvs.0.8Ω.The proton conduction in the composite electrolyte could explain the difference.It iswell recognized that the composite electrolyte is good at both oxygen ion conduction and proton conduction[28-29］.The co-transportation of O2_and H+gives the high ionic conductivity of the composite，subsequently the reduced ohmic resistance in the real fuel cell atmosphere compared that in the air only.On the other hand，the existence of the proton conduction in the composite is expected to improve the cathode reaction kinetic[30］， leading to lower electrode polarization resistance，as shown in Fig.2 b.

The electrochemical performances of the three-layer fuel cells at500 and 550℃are shown in the Fig.3 .The open circuit voltage is 0.98 V for the 550 ℃ test，indicating the electrolyte has acceptable densification and negligible electronic conduction.However，the high electrode polarization resistance as demonstrated in the Fig.2 b leads to a low short circuit current and a low peak power density.The peak power output is only 95 mW/cm2and 160 mW/cm2at500 and 550℃，respectively.Furthermore，an obvious concentration polarization is observed in this case，the fuel cell voltage decreases significantly at the high current density range.Insufficient porosity as indicated in the EIS study has limited the quick gas diffusion indispensable for an electrode reaction.

Fig.3 I-V and I-P characteristics of three-layer fuel cell in H 2/air at 500℃and 550℃

The I-V and I-P characteristics of one layer fuel cell recycled and re-fabricated from the three-layer fuel cell in H2/air are displayed in the Fig.4 .The open circuit voltage(OCV)of the one layer fuel cell is 0.74 V， much lower than its counterpart， the threelayer fuel cell.There is also one clear activation polarization at the low current density range.The relative low OCV is consisted with the high porosity of the constructed fuel cell pellet due to high temperature heattreatment free process and the large constructed particle size as shown in the cross-section SEM image of the single pellet(Fig.5 ).The particle size is in micrometer level and highly aggregated after the high temperature treatment and fuel cell operation.The enough porosity can be indirectly reflected by the disappearance the concentration polarization in the I-V curve.Moreover，it is surprising to see that the maximum power density of the single layer fuel cell is 130 mW/cm2at 500℃，almost1.5 times higher than that of the threelayer fuel cell，and close to its performance at550 ℃.The excellent fuel cell performance identifies the potential prospective of EFFC for energy conversion.

Fig.4 I-V and I-P characteristics of one layer fuel cell in H 2/air at 500℃

Fig.5 SEM image of cross-section of the one layer fuel cell

The single layer fuel cell stability testing was carried out under a constant voltage of 0.2 V.The current values were recorded with the time as shown in the Fig.6 .The current gradually increases with the time during the 30 minutes testing.The performance is even higher when the undergoing testing of the single layer fuel cell is switched from a stability testing to a I-V testing.Of course，a long term operation is still needed for a further confirmation of a stability issue for EFFCs.

Fig.6 A short-term performance of a single layer fuel cell

3 Conclusions

In summary，an electrolyte free fuel cell(EFFC)was fabricated using the materials recycled from an operated conventional three-layer solid oxide fuel cell(SOFC).Its electrochemical performance was investigated and compared with the SOFC.The high electrode polarization resistance of the SOFC leads to a low fuel cell performance，while the EFFC gives a lower OCV but a higher peak power output(130 vs.95 mW/cm2at 500℃)compared with the SOFC.The EFFC also presents a reasonable stability during a short term testing of 30 minutes.It is expected that the EFFC gives an alternative approach for recycling and reuse the materials in the operated conventional SOFC for future energy conversion.

Acknow ledgment

This research is supported by the Chinese Scholarship Council(2010625060)，Swedish Research Council(VR，Contract No.621-2011-4983)，the Swedish Agency for Innovation Systems(VINNOVA，Contract No.P36545-1)，and the EC FP7 TriSOFC project(Contract No.303454)and the Hubei provincial 100-talent distinguished professor program.

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