preparation and characterization of cu/ysz cathode for high-temperature electrolysis

INTERNATIONAL JOURNAL OF ENERGY RESEARCHInt. J. Energy Res. 2010; 34:438–444Published online 30 November 2009 in Wiley InterScience(www.interscience.wiley.com). DOI: 10.1002/er.1646

Preparation and characterization of Cu/YSZ cathodefor high-temperature electrolysis

Jongmin Kim, Sungkyu Lee, Kyoung-Hoon Kang and Hyun Seon Hong�,y

Center for Plant Engineering, Institute for Advanced Engineering, 633-2 Goan-ri, Baegam-myeon, Cheoin-gu, Yongin-si,Gyeonggi-do 449-863, Republic of Korea

SUMMARY

The Cu/Y2O3-stabilized ZrO2 (YSZ) composite was successfully fabricated by high-energy ball-milling method forpossible use as a cathode material in high-temperature electrolysis (HTE) cell. Thus, the fabricated Cu/YSZ compositewas characterized using various analytical tools such as XRD, SEM, TEM and laser diffraction and scattering method.The Cu/YSZ cermet was observed to consist of crystalline Cu and YSZ in sub-micron scale and evenly distributedwithout forming aggregates. Electrical conductivity of ball-milled and sintered Cu/YSZ cermet pellet was measured by4-probe technique and compared with that of conventional Ni/YSZ cermets. The effect of composites composition onthe electrical conductivity was also investigated and the marked increase in electrical conductivity for Cu/YSZ overNi/YSZ was attributed to more open micro-structural morphology of Cu/YSZ and higher electrical conductivity ofcopper, which explains higher hydrogen production rate for HTE cell with Cu/YSZ cathode compared with HTE cellwith Ni/YSZ cathode during actual HTE at 8001C. Copyright r 2009 John Wiley & Sons, Ltd.

KEY WORDS: Cu/YSZ cathode; high-temperature electrolysis

1. INTRODUCTION

Hydrogen is usually made by steam reforming andpartial oxidation of natural gas or oil fractions ofpetroleum oil. However, these processes emit greenhouse gases into the atmosphere. High-temperatureelectrolysis of steam (HTES) is recognized as anefficient and futuristic technology capable ofproducing hydrogen because HTES do not emit

green house gas. Also, HTES consumes lesselectricity than conventional water electrolysis[1–3]. Ni/Y2O3-stabilized ZrO2 (YSZ, 8mol%)composite is the most common cathode materialfor HTES due to its low cost and chemical stabilityin H2O/H2 mixture prevailing in HTES. On theother hand, Cu has been recently introduced asconstituent of an anode material in SOFC for itslow resistivity, economy and other beneficial

*Correspondence to: Hyun Seon Hong, Center for Plant Engineering, Institute for Advanced Engineering, 633-2 Goan-ri,Baegam-myeon, Cheoin-gu, Yongin-si, Gyeonggi-do 449-863, Republic of Korea.yE-mail: [email protected]

Contract/grant sponsor: Hydrogen Energy R&D Center

Received 1 October 2009

Accepted 1 October 2009Copyright r 2009 John Wiley & Sons, Ltd.

electrical and physical properties [4,5]. As HTES isa reverse operation of SOFC, Cu is used ascathode material for HTES. In cathode cermet,YSZ constitutes a framework for the dispersion of(Ni, Cu) metal particles and acts as an inhibitor forthe sintering and coarsening of metal powders athigh temperature. Besides, it constitutes a signifi-cant portion of ionic conductivity, thus improvingthe triple-phase boundary (TPB) where cathode,electrolyte and pores contributes to overall elec-trochemical cell reaction involving all reactants.The thermal expansion coefficient of cathode canbe managed to be compatible with those of HTEScomponents by mixing YSZ with elemental metalpowders in appropriate ratios [6,7].

Although the density gradient in powder mix-tures of NiO/YSZ and CuO/YSZ is less than inNi/YSZ and Cu/YSZ, it can be minimized orprevented altogether in high-energy ball milling ofblended elemental Ni or Cu and YSZ powders. Inthis study, Ni/YSZ and Cu/YSZ cermets forHTES have been synthesized by the high-energyball milling of powder mixtures of metals andYSZ. The cermets thus fabricated were char-acterized to examine microstructure and electricalconductivity.

2. EXPERIMENT

Figure 1 shows a flowchart for fabrication of high-temperature electrolysis (HTE) cathode compo-site. The Ni/YSZ composite was fabricated bymechanical alloying of Ni and YSZ powders.Nickel (Johnson Mathey, Alfa Aesar,o3 mm) and8mol% YSZ (Tosoh Co., TZ-8Y,o0.3 mm) pow-ders were used as starting materials with appro-priate volume ratios of Ni powder for finalcomposition of 60 vol.% Ni-balance YSZ. Me-chanical alloying was carried out in planetary ballmill (Fritsch Pulverisette 6) under ambient atmo-sphere. The ball-milling media consisted of ZrO2

balls (2mm in dia) and a ZrO2 bowl (250mL). Theball to powder weight ratio was 15 to 1 and therotating speed was 400 rpm. On the other hand,Cu/YSZ cermet was fabricated from Cu (SigmaAldrich, dendritic type,o3 mm) and YSZ (TosohCo., TZ-8Y,o0.3) powders with appropriate

volume ratios of Cu to YSZ powders for finalcompositions of 40 and 60 vol.% Cu-balance YSZ.The Cu and YSZ powders were ball milled ina-Terpineol (90%, Aldrich), which reduced YSZagglomerates to approximately the same dimen-sion as those of nickel and copper particles [8]. Theball-milled powder mixture was subsequentlybaked at 1101C in air for 10 h. The Ni/YSZ andCu/YSZ powder composites thus prepared werepressed into pellet shape (Ø 10mm� 2mm) andsubsequently sintered at 9001C under flowingAr-5% H2 reducing gas mixture for 2 h in ahorizontal quartz tube furnace of conventionaltype. The morphology and structure of thecomposite particles were examined by scanningelectron microscopy (SEM, Model LEO SUPRA55, Carl Zeiss, Germany) and transmission elec-tron microscopy (TEM, Model JEM 3010, Japan)with energy dispersive analysis (EDS) of X-raysperformed in each step of the cermet fabricationprocess. High-energy ball-milled particle size dis-tribution was quantitatively characterized usinglaser diffraction and scattering method. Thecrystal structure of the fabricated composite wasdetermined by X-ray diffraction (XRD, ModelM18XHF-SRA, Mac Science, Japan). Electricalconductivity of sintered Ni/YSZ and Cu/YSZcermet pellets was measured using 4-probe techni-que at room temperature and the effect ofcomposite compositions on the electrical conduc-tivity investigated.

Figure 1. Schematic diagram of for fabrication ofNi/YSZ and Cu/YSZ composites.

PREPARATION AND CHARACTERIZATION OF CU/YSZ CATHODE 439

Copyright r 2009 John Wiley & Sons, Ltd. Int. J. Energy Res. 2010; 34:438–444

DOI: 10.1002/er

3. RESULTS AND DISCUSSION

The XRD patterns of the Ni/YSZ and Cu/YSZcomposites are shown in Figure 2. The XRDpatterns show that the ball-milled Ni/YSZ andCu/YSZ composites consisted of crystalline Ni, Cuand YSZ. XRD peak of CuO is attributed to theoxidation of copper powders during the planetaryball mill, whereas the high temperature caused bymechanical impulse during high-energy ballmilling most probably caused the CuO to form.Formation of CuO has some adverse effect onelectrical conductivity.

Subsequent reduction process under flowing Ar-5% H2 gas mixture enhances the electrical

conductivity of the composite. The observed in-crease in electrical conductivity after reductionsintering is possibly explained by densification ofparticles and consequent improvement of electronmigration paths as depicted in Figure 6. Figure 3shows the SEM micrographs of Ni/YSZ andCu/YSZ composite particles prepared by themechanical alloying method. It was also obviousfrom Figure 3 that Ni and Cu powders were finelyground to about 200nm size particles. SEM-EDAX analysis also revealed that the ball-milledNi and Cu particles were partially covered with fineparticles YSZ.

Average particles sizes of the ball-milled Ni andYSZ were about 0.2 and 7 mm, respectively, inFigure 4, which is qualitatively measured usinglaser diffraction and scattering method. The high-energy ball milling of Ni/YSZ composite powdersat 400 rpm for 24 h yielded very fine particle sizedistribution while high-energy ball-milled Cu/YSZpowders mainly consisted of 5 mm average particlesize. Therefore, composite particles of less than10 mm size were successfully fabricated by usinghigh-energy ball milling. Increased particle size, asobserved by SEM images of Ni, Cu and YSZparticles, is attributed to repetitive breaking andsubsequent pressure welding of powders duringhigh-energy ball milling. The morphology of theparticles also changed from initial spherical formto angular or acicular form below 1 mm thicknessduring high-energy ball-milling process (Figure 3).

Figure 5 shows TEM images of Ni/YSZ andCu/YSZ composites, where YSZ particles are

Figure 3. SEM images of the Ni/YSZ (a) and Cu/YSZ (b) composites powder prepared by high-energy ball milling.

Figure 2. XRD patterns of Ni/YSZ and Cu/YSZcomposites powder.

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evenly distributed to Ni and Cu particles. EDSanalysis of Figure 6 also corroborates this argu-ment. Thus, it is strongly inferred that uniformdistribution of constituent particles improveselectrical and ionic conductivities of the compo-sites, which in turn enhances cathode performanceduring HTE. It is highly probable that improvedelectrical conductivities derived from (nano) crys-tallinity of composites afforded by high-energyball milling. Figures 7 and 8 show schematic

diagram of electronic and ionic migration pathsalong with SEM images of sintered cermets pellets.

The electrical conductivities of the Ni/YSZ andCu/YSZ composites were measured at roomtemperature. The electrical conductivities of ball-milled and sintered Ni/YSZ composite was about0.5� 103 S cm�1 compared with the 1.8� 103 S cm�1

of Cu/YSZ composite. A possible explanation forthe large difference may be due to higher electricalconductivity of copper particles for the homo-geneous distribution of constituent particles asdepicted in Figure 6.

Figure 5 also shows that Cu, Ni and YSZparticles were randomly and homogeneously dis-tributed in the Cu/YSZ and Ni/YSZ composites,rendering electrical conductivities proportional toNi and Cu contents as well as connectivity amongelectrically conducting particles. Although thispartly justifies superior cathode performance ofCu/YSZ composite at high temperature of HTEoperation, it has to be cautioned that electricalconductivity decreases with temperature.

Therefore, hydrogen production efficiency wasmeasured at 8001C using Cu/YSZ and Ni/YSZ ascathode component of HTE cells. The HTE resultsof Figure 9 are qualitatively explained by Figures8(a), (b) in terms of Ni/YSZ and Cu/YSZ com-posite morphologies. More open pore structure ofCu/YSZ resulted from slightly coarser particlesof Cu/YSZ composite. Thus, finer pore structures

Figure 5. TEM images of the Ni/YSZ (a) and Cu/YSZ (b) composites powder prepared by high-energy ball milling.

Figure 4. Average particle size distribution of ball-milled Ni/YSZ and Cu/YSZ composites as analyzed by

laser diffraction and scattering method.



DOI: 10.1002/er

of the Ni/YSZ electrode were not as efficient for achannel of water vapor and hydrogen passage.Therefore, TPB of Ni/YSZ cathode is less thanthat of Cu/YSZ, resulting in lower production rateof hydrogen as shown in Figure 9.

It is again strongly inferred that hydrogenproduction performance is significantly dependent

on the characteristics of TPB pore morphology ofCu/YSZ and Ni/YSZ composites.

4. CONCLUSIONS

Cu/YSZ composite was fabricated by high-energyball-milling method for possible use in HTE as a

Figure 6. Distribution of elements Cu/YSZ and Ni/YSZ cermets after ball milling.

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cathode and its electrical conductivity was mea-sured by 4-probe technique for comparison withthat of Ni/YSZ. The Cu/YSZ cathode performedbetter than Ni/YSZ as a cathode component ofHTE cell and this is attributed to more open poremorphology, which facilitated easy material trans-port. Therefore, Cu/YSZ has proved to be a highlypromising cathode material for HTE at 8001C. Inadditon, high-temperature stability and durabilityof Cu add to better performance of Cu/YSZcomposite as a HTE cathode material.

ACKNOWLEDGEMENTS

The present research was supported by the HydrogenEnergy R&D Center under one of the 21st CenturyFrontier R&D Programs, Ministry of Eucation, Scienceand Technology, Republic of Korea.

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Figure 9. Comparison of hydrogen production rate forCu/YSZ and Ni/YSZ cathodes during high-temperature

electrolysis.

Figure 8. SEM images of 9001C sintered Ni/YSZ (a) and Cu/YSZ (b) composites.

Figure 7. Electronic and ionic migration paths forcomposites along the conducting metal particles and

YSZ particles.



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6. Hong HS, Chae U-S, Choo S-T, Lee KS. Microstructure andelectrical conductivity of Ni/YSZ and NiO/YSZ compositesfor high-temperature electrolysis prepared by mechanicalalloying. Journal of Power Sources 2005; 149:84–89.

7. Hong HS, Chae U-S, Choo S-T. The effect of ball millingparameters and Ni concentration on a YSZ-coated Nicomposite for a high temperature electrolysis cathode.Journal of Alloys and Compounds 2005; 449:331–334.

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preparation and characterization of cu/ysz cathode for high-temperature electrolysis

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