Scholarly article on topic 'Investigation of Ni-YSZ SOFC Anode Fabricated Via Electroless Nickel Co-Deposition'

Investigation of Ni-YSZ SOFC Anode Fabricated Via Electroless Nickel Co-Deposition Academic research paper on "Materials engineering"

CC BY-NC-ND
0
0
Share paper
Academic journal
Procedia Engineering
OECD Field of science
Keywords
{SOFC / "electroless nickel co-deposition" / cermet / conductivity}

Abstract of research paper on Materials engineering, author of scientific article — Nor Bahiyah Baba, Alan Davidson

Abstract The paper discusses investigation of nickel – yttria-stabilised zirconia (Ni-YSZ) solid oxide fuel cell (SOFC) anode fabricated via electroless nickel co-deposition. Electroless nickel co-deposition is an in-situ method in fabricating Ni- YSZ anode cermet in one single process without high temperature sintering unlike the common fabrication techniques. The Ni-YSZ anode was analysed in term of its microstructure and electronic conductivity which is the main criteria in determining the overall performance of the anode in SOFC application. The microstructure of Ni- YSZ anode was analysed using scanning electron microscope (SEM) coupled with energy dispersive x-ray analysis (EDXA). It was found that the main elements of the anode is uniformly distributed which ensures good electronic networks and balance ceramic networks ensure minimal internal crack or delamination between the cermet anode and the ceramic electrolyte. The electronic conductivity of Ni-YSZ SOFC anode was measured up to 800 oC in air and nitrogen behaving as a biphasic system. Both the microstructure and electronic conductivity performance of Ni-YSZ cermet anode fabricated by electroless nickel co-deposition was found to be consistence and comparable to the other studies.

Academic research paper on topic "Investigation of Ni-YSZ SOFC Anode Fabricated Via Electroless Nickel Co-Deposition"

Available online at www.sciencedirect.com

Procedía Engineering

ELSEVIER

Procedía Engineering23 (2011)474 - 478

www.elsevier.com/locate/procedia

Power Elactronics and Engineering Application

Investigation of Ni-YSZ SOFC anode fabricated via alactrolass nickel co-deposition

Nor Bahiyah Babaa*, Alan Davidsonb

aTATI Universtiy College (TATIUC), 24000 Kemaman, Terengganu, MALAYSIA bEdinburgh Napier University, Edinburgh EH10 5DT, Scotland, U.K.

Abstract

The paper discusses investigation of nickel - yttria-stabilised zirconia (Ni-YSZ) solid oxide fuel cell (SOFC) anode fabricated via electroless nickel co-deposition. Electroless nickel co-deposition is an in-situ method in fabricating Ni-YSZ anode cermet in one single process without high temperature sintering unlike the common fabrication techniques. The Ni-YSZ anode was analysed in term of its microstructure and electronic conductivity which is the main criteria in determining the overall performance of the anode in SOFC application. The microstructure of Ni-YSZ anode was analysed using scanning electron microscope (SEM) coupled with energy dispersive x-ray analysis (EDXA). It was found that the main elements of the anode is uniformly distributed which ensures good electronic networks and balance ceramic networks ensure minimal internal crack or delamination between the cermet anode and the ceramic electrolyte. The electronic conductivity of Ni-YSZ SOFC anode was measured up to 800oC in air and nitrogen behaving as a biphasic system. Both the microstructure and electronic conductivity performance of Ni-YSZ cermet anode fabricated by electroless nickel co-deposition was found to be consistence and comparable to the other studies.

© 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of [name organizer]

Keywords: SOFC; electroless nickel co-deposition; cermet; conductivity

1. Introduction

Solid oxide fuel cells (SOFCs) are of major interest in fuel cell development due to their high energy conversion efficiency, wide range of fuels and environmental friendliness. One important obstacle for

* Corresponding author. Tel.: +649-8641193; fax: +649-8632453. E-mail address: bahiyah@tatiuc.edu.my.

1877-7058 © 2011 Published by Elsevier Ltd. doi:10.1016/j.proeng.2011.11.2533

their industrial development is their processing difficulties. The common fabrication techniques of Ni-YSZ as an anode material for SOFCs including conventional ceramic process [1,2], thermal spraying [3], vapour deposition [4] and laser deposition [5] technologies. The traditional and most common techniques for producing anodes for SOFCs involve producing cermets through silk screening and subsequent sintering [6,7] followed by consolidation by sintering.

Three recent techniques for producing anodes for SOFCs are (i) applying an electroless coating on the ceramic powders [8,9], (ii) chemical precipitation coating [10] and (iii) chemical co-precipitation [11] -all followed by solid state sintering. Problems arise in the sintering process where the reduction of nickel oxide produces larger nickel grains which reduces the cell's performance [12] and adds processing steps and costs.

The application of electroless nickel co-deposition in producing composites by in-situ incorporation of inert particles in the conventional Ni-P matrix has been extensively carried out. These include the incorporation of diamond, silicon carbide, silicon nitride, silicon oxide, boron carbide, alumina, ceria, yttria and zirconia particles. The application of ceramic YSZ in electroless nickel composite coating is still new [13]. The approach of the work is focusing on fabricating Ni-YSZ anode via electroless nickel co-deposition. This approach also applicable to many fields including thermal barrier coatings (TBCs) and composite surface coatings.

2. Experimental work

2.1. Electaaless ca-depasitiac

A ceramic substrate of alumina tile (Coors Ceramics, U.K.) as the base for the composite deposition was used. The substrate has a manufacturer standard dimension of 50 x 50 x 1 mm. The ceramic alumina substrate requires sensitising to activate the surface. This was followed by electroless nickel composite deposition of Ni-YSZ after the four pre-treatment processes consecutively.

A ceramic powder of 50g/l was added into the bath along with the substrate. An appropriate bath agitation methods required to keep the ceramic powders suspended in the bath. The coating time was kept constant at 60 minutes. The bath temperature was kept constant at 89 ± 20C. The Ni composition was measured using Cambridge Stereoscan 90 Scanning Electron Microscope (SEM) coupled with Energy Dispersive X-ray Analysis (EDXA) and porosity by Archimedes.

2.2. Faunpaictpnabe

The anode fabricated via eletroless nickel co-deposition was tested in term of its electrical resistivity and conductivity performance measured using 4-point probe. The conductivity performance of the Ni-YSZ anode fabricated by electroless nickel co-deposition was tested in simulated SOFC condition. In real practice SOFC is operated at 800oC where it is heated up slowly to avoid drastic temperature change to the ceramic electrolyte.

SOFC used hydrogen gas as a fuel and nitrogen is used when it is not operating or during heating or cooling. The simulated SOFC condition was created in a furnace, heating the anode in air environment up to 800oC. Then another set of test where the nitrogen gas was purged into the furnace up to 600oC. The anode sample was heated at 20oC/min. A 4-point probe was used to measure the sheet resistance and thus the resistivity and conductivity of the anode at every 5-15oC increment under 50 mA current.

3. Results and discussion

3.1 Surface characterisations

Surface of Ni-YSZ deposition was analysed using SEM/EDXA. The microstructure of Ni-YSZ at 6k magnification is shown in Fig. 1(a). The figure shows the white particles represent the YSZ ceramic and the grey area is the metallic nickel matrix. There are few micro pores visible on the anode surface. Fig. 1(b) shows the nickel elemental mapping of the Ni-YSZ anode surface. The mapping indicates uniform distribution of metallic nickel element within the anode. This balance that the coefficient of thermal expansion within the Ni-YSZ cermet. This reduces the possibility of cracking of the anode.

Fig.1: Surface characterization (a) SEM micrograph (b) EDX nickel elemental mapping

2.4 Anode performance

The Ni-YSZ co-deposition was deposited onto a ceramic substrate representing the SOFC anode. The initial sample with the thickness of 13 ^m Ni-YSZ co-deposition contains 50 vol.% Ni. The initial electronic conductivity tests were carried out at room temperature (25oC) and involved measurements at two different points on the surface of the anode sample. The tests were carried out at three different currents- 1mA, 50mA and 100mA. The resistance, resistivity and conductivity of the sample at the three different currents are given in Table 1.

Tablel: Initial 4-point probe test at various current

Current 1 mA 50 mA 100 mA

1st Point 2nd Point 1st Point 2nd Point 1st Point 2nd Point

Resistance/ Q 0.117 0.210 0.353 0.425 0.317 0.261

Resistivity/ 10-4Qcm 1.52 2.73 4.59 5.52 4.12 3.4

Conductivity/104Scm-1 0.66 0.36 0.22 0.18 0.24 0.29

The observed values were very encouraging. The initial test was carried out at room temperature using 4-point probe measurement. Since YSZ is non-metallic and Ni is metallic, the Ni-YSZ cermet behaves as biphasic composite system - having a conductivity percolation threshold at an adequate amount of Ni. The conductivity values of 50 vol.% Ni-YSZ is a factor of ten less than half the value of pure metallic Ni at room temperature (11.8 x104 (Qcm)-1). Thus the obtained values for this initial test are still comparable.

Two series of conductivity performance tests were done on 50%Ni-YSZ anode fabricated via electroless nickel co-deposition. These were carried out in two different environments - in air varying temperatures from 25oC to 800oC and in nitrogen varying temperatures from 25oC to 600°C. Both anode samples in air and nitrogen had coating deposition thickness of 10 microns. The conductivity plots for both series are given in Fig. 2. The conductivity trend decreases with temperature, an indication that it has a metallic conductivity.

Temperature/'

Fig. 2: Scatter plot of conductivity against temperature in both nitrogen and air

The conductivity values are similar in air and nitrogen environment although the former is slightly higher. This observation might be due to the high moisture content in air compared to nitrogen. A review by Zhu and Deevi [14] found that the Ni-YSZ anode overpotential is significantly reduced in the presence of moisture or steam. Lowering anodic overpotential enhanced the electronic conductivity.

In general, the conductivity at 600-800oC of Ni-YSZ fabricated via electroless nickel co-deposition ranges between 700-1000 Scm-1 in both air and nitrogen environments. These values are comparable with the published data from several studies as tabulated in Table 2. The role of phosphorus might be important. Parkinson stated that the electrical resistivity of EN deposited Ni increases with phosphorus content [15].

Table 2: Electronic conductivity of Ni-YSZ anode by other authors

Anode T/oC Fabrication Environment a/ Scm 1 Ref.

40vol.% Ni-YSZ 600 Solid state coating H2 1500 [10]

Solid state mixing 900

800 Solid state coating H2 1400

Solid state mixing 800

45wt.% Ni-YSZ 600-800 Solid state with calcinations H2/Ar 500 [16]

Conventional solid state 430

The anode fabricated via electroless nickel co-deposition is a possibility for in-situ fabrication of SOFC anode onto electrolyte. It is proven the anode of 50 vol.% Ni has a metallic conductivity with highest conductivity of 700 Scm-1 in N2; 1000 Scm-1 in air at 600oC and 750 Scm-1 at 800oC in air.

3. Conclusions

In terms of the electrical conductivity performance, the initial electrical conductivity test carried out at room temperature showed an encouraging outcome in that the value for a 50 vol.% Ni-YSZ anode was only a factor of ten less than the equivalent loading of pure Ni. The electrical conductivity of this anode at temperatures up to 800oC, the electrical conductivity tests were carried out in two different environments - air and nitrogen - and results were comparable to those in the public domain.

References

[1] Marinsek M and K. Z. J. Macek. Preparation of Ni-YSZ composite materials for solid oxide fuel cell anodes by the gel-precipitation method. Journal of Power Sources 2000; 86: 383-9.

[2] Simwonis D, Thulen H, Dias HJ, Naoumidis A, Stover D. Properties of Ni/YSZ porous cermets for SOFC anode substrates prepared by tape casting and coat-mix process. Journal of Materials Processing Technology 1999; 92-93: 107-111.

[3] Stover D, Hathiramani D, VaBen R, Damani RJ. Plasma-sprayed components for SOFC applications. Surface & Coatings Technology 2006; 201: 2002-5.

[4] Haldane MA and Etsell TH. Fabrication of composite SOFC anodes. Materials Science anddnoineerino 2005; B121: 120-5.

[5] Yang D, Zhang X, Nikumb S, Deces-Petit C, Hui R, Maric R, Ghosh D. Low temperature solid oxide fuel cells with pulsed laser deposited bi-layer electrolyte. Journal of Power Sources 2006, 164: 182-8.

[6] Wincewicz KC and Cooper JS. Taxonomies of SOFC material and manufacturing alternatives. Journal of Power Sources 2005;140: 280-96.

[7] Abe H, Murata K, Fukui T, Moon WJ, Kaneko K, Naito M. Microstructural control of Ni-YSZ cermet anode for planer thin-film solid oxide fuel cells. Thin Solid Films 2006; 496: 49 - 52.

[8] Pratihar SK, Sharma AD, Basu RN, Maiti HS. Preparation of nickel coated YSZ powder for application as an anode for solid oxide fuel cells. Journal of Power Sources 2004; 129: 138-42.

[9] Wen G, Guo ZX, Davies CKL. Microstructural Characteristion of Electroless-Nickel Coatings on Zirconia Powder. Scripta materialia 2000: 43: 307-11.

[10] Kim SD, Moon H, Hyun S-H, Moon J, Kim J, Lee H-W. Performance and durability of Ni-coated YSZ anodes for intermediate temperature solid oxide fuel cells. Solid State Ionics 2006; 177: 931-8.

[11] Mosch S, Trofimenko N, Kusnezoff M, Betz T, Kellner M. Performance and stability of SOFC anode prepared by co-precipitation. Solid State Ionics 2008; 179: 1606-10

[12] Simwonis D, Tietz F, Stover D. Nickel coarsening in annealed Ni/8YSZ anode substrates for solid oxide fuel cells. Solid State Ionics 2000; 132: 241-51.

[13] Baba, N.B., W. Waugh, and A.M. Davidson. Manufacture of Electroless Nickel/YSZ Composite Coatings. AroceeDinos of World Academy of Science, dnoineerino and Technology (WASdT), Dubai, UAE 2009; 37: 715-20.

[14] Zhu, WZ and Deevi SC. A review on the status of anode materials for solid oxide fuel cells. Materials Science and

dngineering 2003; A362: 228-39.

[15] Parkinson R. Properties and Application of Electroless Nickel. Nickel Development Institute 1997; 33.

[16] Han KR, Jeong Y, Lee H, Kim C-S. Fabrication of NiO/YSZ anode material for SOFC via mixed NiO precursors, Materials Letters 2006; 61: 1242-45.