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Kun Zheng
supervisor: dr hab. inż. Konrad Świerczek, prof. AGH
Faculty of Energy and Fuels
AGH University of Science and Technology Kraków
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1. Aim of PhD studies
2. Introduction and literature review of SOFCs
3. Experimental
4. Results and discussion
Novel anode materials with perovskite-type structure o Sr2MMoO6-δ (M = Mg, Mn, Fe, Co and Ni) o Sr2-xBaxMMoO6-δ (M = Co, Ni) o Sr2Fe1.5-xMnxMo0.5O6-δ
Novel cathode materials with perovskite-related structure o GdBa0.5Sr0.5Co2-xFexO5+δ o Ln2CuO4+δ (Ln = La, Pr, Nd)
Studies of electrochemical properties of IT-SOFCs, carbon deposition and sulfur poisoning issues
5. Summary, conclusions and recommendations
Outline
The aim was to obtain and study physicochemical properties, as well as optimize novel electrode materials for application in Intermediate-Temperature (600-800 oC) Solid Oxide Fuel Cells (IT-SOFC), which can be fueled by syngas or other non-hydrogen fuels.
Scientific approach:
o Manufacturing of novel single phase anode and cathode materials,
o Comprehensive investigations of physicochemical properties of synthesized compounds,
o Selection of the best candidate materials,
o Construction and studies of electrochemical properties of button-type IT-SOFCs based on selected electrode materials,
o Exploration of carbon deposition and sulfur poisoning issues for selected anode materials.
3 Aim of PhD studies
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Anode: H2 + O
2- → H2O + 2e-
Overall reaction: H2 + 0.5O2 → H2O
Cathode: 0.5O2 + 2e
- → O2-
Introduction and literature review
Anode: H2 + O
2- → H2O + 2e-
CO + O2- → CO2 + 2e-
Overall reaction: H2 + CO + O2 → H2O + CO2
Cell fueled with H2
Cell fueled with syngas (H2 and CO)
Cathode: O2 + 4e
- → 2O2-
Schematics of SOFC cell with O2- conducting solid electrolyte
5 Introduction and literature review
Criteria for possible application of electrode materials: high thermal and chemical stability, chemical compatibility,
high electrical conductivity (> 100 S·cm-1),
high mixed ionic-electronic conductivity with as high as possible ionic component,
high catalytic activity,
adequate thermomechanical properties, thermal expansion coefficient,
preparation of electrode layers with an appropriate porosity, adhesion and
mechanical strength,
long-term stability of all properties,
low cost, environmental friendliness, CO2 tolerance, Cr tolerance,
for cells fueled with non-H2 fuel - carbon deposition tolerance and sulfur
poisoning tolerance.
porous anode
porous cathode
J. Molenda, K. Świerczek, W. Zając, J. Power Sources 173 (2007) 657.
thermomechanical properties
electrocatalytic properties
dense, thin electrolyte, mechanic strength, pure ionic conductivity
matched thermal expansion
coefficients, adhesion between
components
high catalytic activity,
preferably mixed ionic-eletronic conductivity
6 Introduction and literature review
clean, green power significant decrease of gas emissions
higher efficiency, comparing to traditional mechanical energy conversion devices
only SOFCs have such wide fuel flexibility
S.C. Singhal, K. Kendall, editors, High Temperature Solid Oxide Fuel Cells: Fundamentals, Design and Applications, Elsevier, 2003.
low level of noise pollution
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SOFC Technology R&D Needs, Delphi
CHP: combined heat and power generator; CHCP: combined heating cooling and power generator APU: auxiliary power unit; LGP: liquefied petroleum gas; JP-8: jet propellant 8 fuel
Introduction and literature review
8 Introduction and literature review
fueled with syngas from coal
SOFCs fueled by syngas can utilize traditional fuel, coal, in much better, cleaner and efficient way, and may significantly reduce CO2 emissions
SOFC
Development of SOFC for Products, Mitsubishi Heavy Industries, Ltd.
effectively working IT-SOFC
fueled by syngas
1. carbon deposition resistance
2. sulfur tolerance
3. catalytic activity in
600-800 °C range
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Issues 1 and 2 concern anode materials Currently used in H2-fueled cells Ni-YSZ cermet cannot be used due to insufficient stability in C- and S-containing fuels
Issue 3 concerns mostly cathode materials Currently used La1-xSrxMnO3 cannot be used due to low electrical conductivity and insufficient catalytic activity, La1-xSrxCo1-yFeyO3-δ perform better, but still unsatisfactorily
Introduction and literature review
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Carbon deposition may occur as an effect of:
2CO ↔ C + CO2 - Boudouard reaction
CO + H2O ↔ H2 + CO2 - water gas shift reaction
Carbon deposition causes decrease of performance due to:
blocking of catalytic reaction sites,
formation of cracks,
mechanical damage and sealing failure.
Apart from thermodynamic considerations, also:
gas flow,
choice of anode materials,
microstructure of the anode,
current density,
play important role.
Introduction and literature review
Thermodynamic considerations of carbon deposition: chemical equilibrium diagram of C-H-O system at 750 °C
X.F. Ye et al., Journal of Power Sources 195 (2010) 7264 and Journal of Power Sources 196 (2011) 5499
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Sulfur poisoning may occur due to impurity sulfur present in syngas.
Sulfur poisoning causes decrease of performance due to:
physical adsorption (reaction of H2S with Ni-YSZ anode) H2S(g) ↔ HS(ads) + H(g/ads) ↔ S(ads) + H2(g/ads)
chemisorption and sulfidation Ni + H2S(g) → NiS + H2(g/ads) 3Ni + xH2S(g) → Ni3Sx + xH2(g/ads)
Initial and reversible cell voltage drop
within a short time is followed by fatal
larger cell voltage drop.
Introduction and literature review
sulfur poisoning sulfur tolerance
M. Gong, X. Liu, J. Trembly, C. Johnson, Journal of Power Sources 168 (2007) 289
Choice of materials resistant to sulfur poisoning can be made considering sulfur and hydrogen adsorption energies on metals.
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Weaker adsorption of sulfur on Mo suggests that Mo-containing materials may show sulfur tolerance.
M.Liu.et.al., Development of sulfur-tolerant anodes for SOFCs, DOE 7thAnnual SECA workshop, 2006
Introduction and literature review
Novel, carbon deposition tolerant and sulfur poisoning resistant anode materials are essential for development of SOFC technology.
Choice of chemical composition of anode materials studied in this thesis:
single phase oxide materials stable in reducing atmospheres,
materials contain molybdenum,
high oxygen nonstoichiometry for ionic conduction,
presence of Mo5+/Mo6+ redox couple for electrical conductivity,
materials contain 3d metals for possible enhancement of conductivity.
13 Introduction and literature review
A2MMoO6-δ (A = Sr, Ba, M = Mn, Fe, Co, Ni, Mg) oxides with perovskite-type crystal structure
14 Introduction and literature review
Conventional SOFCs work typically in 900-1000 oC range, which causes problems with long-term performance, due to corrosion and thermal degradation.
Advantages of IT-SOFCs (operation in 600-800 oC range):
cheaper construction materials (steels) can be used,
alleviation of corrosion problems,
improvement of long-time stability.
However, there are problems related to catalytic activity and electrical conductivity, especially in relation to the cathode materials.
15 Introduction and literature review
Novel cathode materials, which exhibit high, mixed ionic-electronic conductivity and high catalytic activity are also essential for development of SOFC technology.
Choice of chemical composition of cathode materials studied in this thesis:
single phase oxide materials stable in oxidizing atmospheres,
high oxygen nonstoichiometry for ionic conduction,
high values of oxygen diffusion coefficient D and surface exchange coefficient K
presence of 3d metal redox couples (e.g. Co3+/Co4+) for high electrical conductivity.
GdBa0.5Sr0.5Co2-xFexO5+δ perovskite-type oxides Ln2CuO4+δ (Ln - La, Pr, Nd) perovskite-related oxides
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1. Preparation techniques of materials: High temperature solid state reaction
Soft chemistry method
2. Characterization techniques: Crystal structure characterization (XRD) with Rietveld analysis
Scanning Electron Microscopy (SEM) analysis
Thermogravimetric (TG) studies
Differential Scanning Calorimetry (DSC) analysis
Mass relaxation experiments:
Development and programing of mathematical solution, modeling and data analyzing
Determination of oxygen diffusion coefficient (D) and surface exchange coefficient (K)
Electrical conductivity (σ) and Seebeck coefficient (α) measurements
Calculations of ionic conductivity (σ𝑖𝑜𝑛) Thermal and chemical stability studies
3. Characterization of button-type IT-SOFCs Assembling of cells
Studies of electrochemical properties (open circuit voltage, current density-voltage characteristics, impedance measurements)
SEM studies after tests
Experimental
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Studies of anode materials
Sr2MMoO6-δ (M = Mg, Mn, Fe, Co and Ni)
Sr2-xBaxMMoO6-δ (M = Co, Ni; x = 0-2)
Sr2Fe1.5-xMnxMo0.5O6-δ (x = 0, 0.5)
Published in: 1. K. Zheng, K. Świerczek, J. Eur. Ceram. Soc. 34 (2014) 4273, IF 2.307 2. K. Zheng, K. Świerczek, W. Zając, A. Klimkowicz, Solid State Ionics 257 (2014)
9, IF 2.112 3. K. Zheng, K. Świerczek, J. Bratek, A. Klimkowicz, Solid State Ionics 262 (2014)
354, IF 2.112 4. K. Zheng, K. Świerczek, N.M. Carcases, T. Norby, ECS Transactions 64 (2)
(2014) 103
Results and discussion
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Crystal structure
Results and discussion: novel anode materials
Single phase anode materials were synthesized in whole series Perovskite-type B-site rock-salt ordered (double perovskite) structure was confirmed
A2BB’O6 double perovskite ABO3 simple perovskite
XRD
1:1 ordered perovskites
G. King, P.M. Woodward, J. Mater. Chem. 20 (2010) 5785.
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Typical SEM micrograph of anode material Sr2FeMoO6-δ
Results and discussion: novel anode materials
Lack of linear dependence between unit cell parameters and volume as a function of ionic radius of M2+, due to overlapping of M2+/M3+ and Mo6+/Mo5+ states and partial mixing between B-site cations.
Crystal structure Microstructure of powder
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P21/n → (P21/n, I4/m) → I4/m → Fm-3m series of phase transitions were observed for the first time for Sr2MnMoO6-δ material
Crystal structure at high temperatures - phase transitions
First order-type transition was confirmed by DSC study for Sr2MnMoO6-δ
Continuous I4/m → Fm-3m transition was observed for other Sr2MMoO6-δ compounds
Results and discussion: novel anode materials
XRD
DSC
21 Results and discussion: novel anode materials
anode materials Sr2NiMoO6-δ Sr2MgMoO6-δ Sr2CoMoO6-δ Sr2FeMoO6-δ Sr2MnMoO6-δ
TEC [10-6 K-1] 12.8 13.8-18.2 13.4 13.8 11.5-14.8
TECs of Sr2MMoO6-δ are similar to TECs of typical electrolyte materials, which is attractive from the point of view of application
typical electrolyte materials Ce0.8Ge0.2O1.9 Zr0.85Y0.15O1.93 La0.9Sr0.1Ga0.8Mg0.2O3
TEC [10-6 K-1] 12.5 10.8 12.2
Thermal expansion coefficients (TEC)
Oxygen nonstoichiometry and transport properties
Oxygen nonstoichiometry for Sr2MMoO6-δ strongly depends on M cation
High conductivity of Sr2FeMoO6-δ (1000 S·cm-1)
overlapping of Fe2+/Fe3+ and Mo6+/Mo5+ states
TG σ
22 Results and discussion: novel anode materials
conditions Sr2NiMoO6-δ Sr2MgMoO6-δ Sr2CoMoO6-δ Sr2FeMoO6-δ Sr2MnMoO6-δ
air at 800 °C (oxidizing)
stable stable stable unstable unstable
5 vol.% H2/Ar at 800 °C (reducing)
unstable stable unstable stable stable
Re-synthesis effect for Sr2FeMoO6-δ material
Sr2NiMoO6-δ and Sr2CoMoO6-δ are unstable in reducing conditions (precipitation of Ni/Co).
Only Sr2MgMoO6-δ shows redox stability, but has very low conductivity.
Sr2FeMoO6-δ is unstable in oxidizing conditions, but high conductivity makes it very interesting as novel anode material.
Thermal and chemical stability
XRD
23 Results and discussion: novel anode materials
Instability of SrBaCoMoO6-δ in reducing conditions
Improved electrical conductivity of SrBaCoMoO6-δ in reducing conditions due to
precipitation of metallic Co
Systematic research on Ba-doped Sr2-xBaxCoMoO6-δ and Sr2-xBaxNiMoO6-δ series was also performed, in search for materials with better chemical stability.
Only small modification of physicochemical properties (crystal structure, oxygen nonstoichiometry, electrical conductivity) was observed. No significant improvement of stability was achieved.
Shown instability of Co- and Ni-containing oxides in reducing conditions limits their application and puts doubt the interpretation of all electrochemical data recorded in such conditions in the literature.
σ
XRD
Further studies were directed towards B-site modification of Sr2FeMoO6-δ oxide.
Systematical research on physicochemical properties of Sr2Fe1.5-xMnxMo0.5O6-δ materials was performed.
Sr2Fe1.5-xMnxMo0.5O6-δ were found to be redox stable, which property allows to use them as both, anode and cathode materials for symmetrical IT-SOFCs.
In reducing conditions the oxides show n-type conductivity, in oxidizing, p-type.
Decreased Mo content is favorable, due to decreased costs.
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Relatively good conductivity of 50 S·cm-1 at 850 °C was recorded for SrFe0.75Mo0.25O3-δ
Results and discussion: novel anode materials
B-site modification changes crystal structure (Pm-3m simple perovskite) and improves stability
σ
XRD
Studies of cathode materials
GdBa0.5Sr0.5Co2-xFexO5+δ (x = 0-2) Ln2CuO4+δ (Ln = La, Pr, Nd)
Published in: 5. K. Zheng, A. Gorzkowska-Sobaś, K. Świerczek, Mater. Res. Bull. 47(12) (2012)
4089, IF 1.968. 6. C. Kuroda, K. Zheng, K. Świerczek, Int. J. Hydrogen Energy 38(2) (2013) 1027,
IF 2.930. 7. K. Świerczek, N. Yoshikura, K. Zheng, A. Klimkowicz, Solid State Ionics 262
(2014) 645, IF 2.112. 8. K. Świerczek, K. Zheng, A. Klimkowicz, ECS Transactions 57(1) (2013) 1993. Determination of oxygen transport coefficients: 9. K. Zheng, A. Klimkowicz, K. Świerczek, A. Malik, Y. Ariga, T. Tominaga, A.
Takasaki, J. Alloys Compd. (2014, accepted), IF 2.726. 10. D. Baster, K. Zheng, W. Zając, K. Świerczek, J. Molenda, Electrochim. Acta 92
(2013) 79, IF 4.086. 11. W. Zając, D. Rusinek, K. Zheng, J. Molenda, Cent. Eur. J. Chem. 11(4) (2013)
471, IF 1.329.
25 Results and discussion
26 Results and discussion: novel cathode materials
AA’B2O6 double (layered) perovskite
1:1 ordered perovskites
G. King, P.M. Woodward, J. Mater. Chem. 20 (2010) 5785.
Perovskite-type A-site layer ordered (double perovskite) structure was confirmed Increasing Fe content decreases tendency for formation of ordered structure
Crystal structure
ABO3 simple perovskite
double simple
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Co-rich oxides possess very high electrical conductivity (> 100 S·cm-1)
High Co-containing oxides show large oxygen nonstoichiometry
GdBa0.5Sr0.5Co1.5Fe0.5O5+δ shows relatively high TEC. However, composite-type (cathode material and electrolyte powder mixture) can be used for manufacturing of electrode for IT-SOFCs.
TG σ
Thermal expansion coefficient (TEC)
Oxygen nonstoichiometry and transport properties
Results and discussion: novel cathode materials
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𝝈𝒊𝒐𝒏 =𝒄𝐎 · 𝒒
𝟐𝑫𝐬𝑹𝑻
=𝟐𝒄𝐎 · 𝒒
𝟐𝑫 · 𝒅𝒍𝒏𝒄𝐎𝑹𝑻 · 𝒅𝒍𝒏𝑷𝑶𝟐
Nernst-Einstein equation
where: cO - concentration of oxygen, q - charge of mole of oxygen anions, Ds - self-diffusion coefficient, R - gas constant, T - temperature.
Results and discussion: novel cathode materials
For effective operation in lowered temperatures, cathode materials must possess high values of oxygen transport coefficients D and K, as well as high ionic conductivity 𝝈𝒊𝒐𝒏.
Values of D and K were determined
by mass relaxation studies, which allowed also to calculate 𝝈𝒊𝒐𝒏 for selected GdBa0.5Sr0.5Co1.5Fe0.5O5+δ oxide in 600-800 °C range.
In GdBa0.5Sr0.5Co1.5Fe0.5O5+δ ionic conductivity exceeds 0.01 S·cm-1 at 600 °C and 0.04 S·cm-1 at 800 °C.
This cathode material seems to be a good choice for application in IT-SOFCs.
Transport coefficients D and K were determined by fitting measured data with
developed Matlab code
Mass relaxation profile
D and K
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Only Pr2CuO4+δ oxide shows suitable electrical conductivity at high temperatures
TG measurements in air and reducing conditions allow to determine oxygen content
Results and discussion: novel cathode materials
Systematical research on Ln2CuO4+δ (Ln = La, Pr, Nd) cathode materials with possible interstitial oxygen was also performed.
Unfortunately, contrary to previously studied by the author nickelates Ln2NiO4+δ (Ln = La, Pr, Nd), cuprates exhibit worse physicochemical properties.
Low interstitial oxygen content (0.03-0.05) present in these materials may limit their ionic conductivity and catalytic ability.
Only Pr2CuO4+δ oxide shows good total electrical conductivity (> 100 S·cm-1). This material has acceptable chemical stability.
TG
σ
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Ce0.8Gd0.2O1.9 electrolyte
Selected anode materials: Sr2FeMoO6-δ
SrFe0.75Mo0.25O3-δ
Selected cathode materials: GdBa0.5Sr0.5Co1.5Fe0.5O5+δ
SrFe0.75Mo0.25O3-δ
porous anode
porous cathode
Manufactured button-type electrolyte-supported IT-SOFCs For better match of TECs, composite-type electrodes were also prepared
Results and discussion
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IT-SOFCs with GdBa0.5Sr0.5Co1.5Fe0.5O5+δ cathode and reference anode, fueled with H2
Results and discussion: IT-SOFCs
GdBa0.5Sr0.5Co1.5Fe0.5O5+δ-based and Sr2FeMoO6-δ-based IT-SOFCs showed good electrochemical performance when fueled by hydrogen. Predominant ohmic resistance at higher temperature ranges indicates that IT-SOFC performance can be improved by reducing thickness of the electrolyte.
IT-SOFCs with Sr2FeMoO6-δ anode and reference cathode, fueled with H2
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IT-SOFCs with selected SrFe0.75Mo0.25O3-δ anode and GdBa0.5Sr0.5Co1.5Fe0.5O5+δ cathode showed acceptable performance (in low CO content fuel), but experienced power degradation during long time annealing.
Results and discussion: IT-SOFCs
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Symmetrical IT-SOFCs with SrFe0.75Mo0.25O3-δ-based electrodes in 10 vol.% CO/CO2 showed good electrochemical performance, no degradation with time and no carbon deposition on the anode was observed.
Results and discussion: IT-SOFCs
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before annealing composite with CGO after annealing at 800 °C
after annealing at 600 °C
SEM micrographs for SrFe0.75Mo0.25O3-δ annealing in CH4 at 600 °C or 800 °C for 16 h
after annealing at 800 °C
Symmetrical SOFCs with SrFe0.75Mo0.25O3-δ based composite electrodes in CH4
No carbon deposition was observed on SrFe0.75Mo0.25O3-δ anode after annealing in CH4 at 600 °C, and presence of Ce0.8Gd0.2O1.9 electrolyte can significantly suppress the carbon deposition
Carbon deposition free zone can be achieved when IT-SOFC is operating at temperatures ≤ 700 °C.
Results and discussion: carbon deposition studies
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TG measurements, annealing for 12 h
SrFe0.75Mo0.25O3-δ before H2S study
SrFe0.75-xMnxMo0.25O3-δ and Sr2FeMoO6-δ are not resistant to sulfur poisoning during annealing in 800 ppm H2S-containing gas. It seems that there are no oxide-based materials developed, which can work in such conditions. This important problem needs further studies.
SrFe0.75Mo0.25O3-δ after H2S study
TG measurements, annealing for 12 h
Results and discussion: sulfur poisoning studies
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Novel anode and cathode materials for IT-SOFCs, belonging to series of: o Sr2MMoO6-δ (M = Mg, Mn, Fe, Co and Ni) o Sr2-xBaxMMoO6-δ (M = Co, Ni) o Sr2Fe1.5-xMnxMo0.5O6-δ o GdBa0.5Sr0.5Co2-xFexO5+δ o Ln2CuO4+δ (Ln = La, Pr, Nd)
oxides with perovskite-type structure were successfully obtained and systematically studied in terms of their physicochemical properties.
Crystal structure at room and high temperatures, presence of phase transitions, oxygen nonstoichiometry, transport properties, including ionic conductivity, thermal expansion and chemical stability, as well as electrochemical properties of the obtained materials were investigated. Studies allowed to select the best candidate anode and cathode materials.
Summary, conclusions and recommendations
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Laboratory-scale button-type electrolyte-supported IT-SOFCs with selected electrode materials were manufactured and studied in terms of their electrochemical properties in hydrogen and CO fuels. Good electrochemical properties were recorded.
Redox stable SrFe0.75Mo0.25O3-δ material was successfully applied
as anode and cathode material in symmetric SOFCs, and it showed carbon deposition tolerance in 10 vol.% CO/CO2 and CH4 at temperatures ≤ 700 °C. This approach seems especially interesting and important for future development of IT-SOFCs.
Studied anode materials are not resistant to sulfur poisoning in 800 ppm H2S-containing atmospheres. Sulfur poisoning conditions should be thoroughly investigated in the further studies, in order to determine the safe working zone for anode materials (i.e. temperature, H2S level).
Summary, conclusions and recommendations
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Other publications concerning physicochemical properties of rare earth and transition metal oxides: 12. A. Klimkowicz, K. Świerczek, K. Zheng, M. Baranowska, A. Takasaki, B. Dabrowski,
Solid State Ionics 262 (2014) 659, IF 2.112. 13. Z. Du, H. Zhao, Y. Shen, L. Wang, M. Fang, K. Świerczek, K. Zheng, J. Mater. Chem. A
2(26) (2014) 10290. 14. Y. Shen, H. Zhao, J. Xu, X. Zhang, K. Zheng, K. Świerczek, Int. J. Hydrogen Energy
39(2) (2014) 1023, IF 2.930. 15. K. Świerczek, A. Klimkowicz, K. Zheng, B. Dabrowski, J. Solid State Chem. 203 (2013)
68, IF 2.200. 16. J. Han, K. Zheng, K. Świerczek, Func. Mater. Lett. 4(2) (2011) 151, IF 0.724. 17. A. Klimkowicz, K. Zheng, G. Fiołka, K. Świerczek, Chemik 67(12) (2013) 11. 18. A. Klimkowicz, K. Zheng, G. Fiołka, K. Świerczek, Materiały ceramiczne / Ceramic
Materials 65(1) (2013) 92.
39 Obrona rozprawy doktorskiej – Kun Zheng