Nickel - based electro - oxidation catalysts for urea sensors and urea fuel cells

A thesis for the Degree of Doctor of Philosophy Nickel – based electro-oxidation catalysts for urea sensors and urea fuel cells By TRAN THAO QUYNH NGAN Department of Chemical and biological engineering Graduate School Gachon University A thesis for the Degree of Doctor of Philosophy Nickel – based electro-oxidation catalysts for urea sensors and urea fuel cells By TRAN THAO QUYNH NGAN Submitted in Fulfillment of the Requirements for the Degree of Doctor of Philosophy July, 2018 Department of Chemical and biological engineering Graduate School Gachon University Thesis for Doctor of Philosophy’s Degree Nickel – based electro-oxidation catalysts for urea sensors and urea fuel cells By TRAN THAO QUYNH NGAN Accepted in Fulfillment of the Requirements for the Degree of Doctor of Philosophy July 2018 Committee Chairman Young Soo Yoon Committee Member Hyon Hee Yoon Committee Member Ho Yu Yong Committee Member Il Tae Kim Committee Member Jae Seung Kim i ACKNOWLEDGEMENTS Firstly, I would like to express my sincere gratitude to my advisor, Prof Hyon Hee Yoon, for his tremendous guidance, support and continuous encouragement During my study for Ph.D course, he not only supported to me the best living and working environment, but also provided his guidance, with complete patience and motivation in the best condition for my research process All my achieved things are direct results of his nourishment of knowledge bestowed upon me during my study Furthermore, his encouragement and advice are the main motivation for me to overcome the difficulties of being a foreign student in Korea Especially, I’d like to give my appreciation to my laboratory members who are friendly beside me and give their hands to support me during this period Sincerely, I would like to thank all the academic and technical staff of department of Chemical and Biological Engineering, Gachon University for their support towards me in one way or another Finally, I would like to express my heartfelt gratitude to my parents who gave me a chance to every incredible thing and always give their continuous love, uninterrupted support and encouragement throughout the studying period here And, many thanks to all my friends who is a part of my life I want to you know I deeply cherish you and always will ii CONTENTS CHAPTER 1: RESEARCH AMBITION AND SIGNIFICANCE I.1 Urea sensor I.2 Direct urea fuel cell .5 I.3 Significance and organization CHAPTER 2: INTRODUCTION II Urea sensors .8 II 1.1 Enzymatic urea sensor II 1.2 Non-enzymatic urea sensor 10 II 1.3 Metal-organic frameworks (MOFs) 11 II 1.4 CeO2-modified perovskite oxide (LaNi0.6Fe0.4O3-CeO2) 12 II 1.5 The role of MWCNT in fabrication of urea sensor .14 II Direct urea fuel cell .14 II 2.1 Anode materials 20 II 2.2 Electrolyte materials 22 II 2.3 Cathode materials 23 II Modelling of DUFC 26 II 3.1 I-V behavior 26 II 3.2 Basic assumption and model structure 27 II 3.3 Mathematical model 32 II.3.3.1 Anode side 33 iii 3.3.1.1.Urea transport to gas diffusion layer (GDL): 33 3.3.1.2.Diffusion layer 33 3.3.1.3.Catalyst layer 35 II.3.3.2 Anion exchange membrane (AEM) 38 II.3.3.3 Ohmic overpotential 40 II.3.3.4 At cathode 43 3.3.4.1.Diffusion layer: .43 3.3.4.2.Catalyst layer: 44 II.3.3.5Solution procedure 45 3.3.5.1.Anode 46 3.3.5.2.Cathode 48 CHAPTER 3: EXPERIMENTAL SECTION 51 III Materials processing 51 III.1.1 LNF-C synthesized and electrode fabrication 51 III.1.2 Synthesis of Ni– benzene-1,3,5-tricarboxylic acid metal– organic framework 53 III.1.3 Ni@C, NiO@C synthesis 56 III Electrical system and single stack cell fabrication 56 III Quantitative characterizations .57 CHAPTER 4: RESULTS AND DISCUSSION 57 IV Urea sensor 57 iv IV 4.1 NiBTC materials .57 IV 4.1.1Morphological and structural studies .57 IV 4.1.2Electro-catalytic activity of Ni– benzene-1,3,5tricarboxylic acid metal–organic framework for urea electro-oxidation 65 IV 4.1.3Electrochemical performance of urea sensor 70 IV 4.1.4Interference, reproducibility and shelf life 75 IV 4.1.5Analysis of urea in urine sample 78 IV 4.2 LNF-CeO material .80 IV 4.2.1Structural and morphological characterization 80 IV 4.2.2Electrochemical analysis .82 IV 4.2.3Interference, stability, and real sample analysis 97 IV 4.3 Summary 100 IV Ni@C, NiO@C and NiBTC for direct urea fuel cell .104 IV 5.1 Characterization and optimization 104 IV 5.2 Catalytic performance of urea electro-oxidation on various forms of nickel supported-carbon 112 IV 5.3 Performances of DUFC 120 CHAPTER 5: SUMMARY AND CONCLUSIONS 133 CHAPTER 6: FUTURE WORK 135 APPENDICES 138 v REFERENCES 139 vi LIST OF FIGURES Figure 2-1 Comparison of energy density of different energy resources [89] 17 Figure 2-2 Schematic diagram of urea fuel cell system 19 Figure 2-3 Pictorial summary of major factors that contribute to fuel cell performance 27 Figure 2-4 The general model structure 31 Figure 2-5 Schematic of anode and cathode bonded to AEM in DUFC 32 Figure 3-1 The synthesis schematic of LNF-C material 52 Figure 3-2 Schematic diagram of Ni-BTC/MWCNT composite preparation and its application for electrochemical detection of urea 55 Figure 4-1 SEM images of (a) Ni-MOF-24 powder, (b) NiMOF particle, (c) composite of Ni-MOF/MWCNT, (d) STEM vii image of single Ni-MOF particle; and distribution of the nickel, carbon, and oxygen elements 59 Figure 4-2 SEM images (a) and particle-size distribution of Ni-MOF powder 60 Figure 4-3 Barrett-Joynet-Halenda (BJH) pore size distribution curves of Ni-MOF-24 60 Figure 4-4 (a) XRD patterns of MWCNT, Ni-MOF in 24h of hydrothermal reaction and Ni-MOF/MWCNT composite, (b) FT-IR spectra of H3BTC and Ni-MOFs, (c) XPS spectrum of Ni-MOF/MWCNT and (d) Ni 2p 63 Figure 4-5 Schematic representation of synthesis process of Ni-(BTC)MOF 63 Figure 4-6 Possible structure of Ni-(BTC)MOF 64 Figure 4-7 (a) Electrochemical response of Ni- BTC/MWCNT electrodes in a range concentration of 0-20 mM urea in 0.1M KOH solution, and (b) repetitive CV of the Ni-BTC/MWCNT/ITO non-enzymatic electrode in different viii Due to the generated water during urea oxidation, according to material balance for water, water flux through catalyst layer (𝑁𝐻2𝑂 ) is given as: 𝑵𝑯𝟐𝑶 = 𝒋𝒂𝒏 𝟐𝑭 + 𝑵𝒅𝑯𝟐𝑶 Equation 23 And material balance for urea, urea through catalyst layer (𝑁𝑢𝑟𝑒𝑎 ) is given as: 𝒅𝑵𝒖𝒓𝒆𝒂 𝒅𝒛 𝟏 𝒅𝒋𝒂𝒏 𝒅𝒛 = − 𝟐𝑭 Equation 24 The variation of anodic overpotential within the catalyst layer is defined as: ηan(z) = Φs (z) + Φm (z) Equation 25 with:  The potential of the electronic phase of the catalyst layer is followed Ohm’s law equation as: 𝒅Φs 𝒅𝒛 =− 𝟏 𝒆𝒇𝒇 𝒌𝒔 ( j – jan) Equation 26  The potential of the ionomer phase of the catalyst layer is followed Ohm’s law equation as:  37 𝒅Φm 𝒅𝒛 =− 𝟏 j 𝒆𝒇𝒇 an 𝒌𝒎 Equation 27 Modifying Equation 25 by using Equation 26 and Equation 27, one can be written as: 𝒅ηan 𝒅𝒛 =( 𝟏 𝒆𝒇𝒇 𝒌𝒔 + 𝟏 𝒆𝒇𝒇 𝒌𝒎 ) jan - 𝟏 𝒆𝒇𝒇 𝒌𝒔 j Equation 28 With:  j: the cell current density 𝑒𝑓𝑓  𝑘𝑠 𝑒𝑓𝑓 and 𝑘𝑚 : effective conductivities of the solid and ionomer phase, respectively, which can be calculated by using Bruggeman’s correction for conductivity II.3.3.2 Anion exchange membrane (AEM) 𝑚 Water flux transport through membrane (𝑁𝐻2𝑂 ) is caused by effect of 𝑚 𝑚 electro-osmotic drag (𝑁𝑑𝑟𝑎𝑔 ) and diffusion (𝑁𝑑𝑖𝑓𝑓 ): 𝒎 𝒎 𝑵𝒎 𝑯𝟐𝑶 = 𝑵𝒅𝒓𝒂𝒈 + 𝑵𝒅𝒊𝒇𝒇 Equation 29 With:  Water flux through the membrane due to electro-osmotic drag by solvation effect In the AEM fuel cell, water transports from cathode to anode side, through AEM due 38 to electro-osmotic drag, as hydroxyl ion generated at the cathode moves to anode The water flux is constant at cell temperature as: 𝒋 𝑵𝒎 𝒅𝒓𝒂𝒈 = -nH2O 𝑭 Equation 30  Water flux due to diffusion of different water concentration in anode and cathode was calculated as following Equation 14 𝒎 𝑵𝒎 𝒅𝒊𝒇𝒇 = 𝑫𝑯𝟐𝑶 𝒄𝒂𝒕𝒉 𝒄𝒂𝒏 𝑯𝟐𝑶 − 𝒄𝑯𝟐𝑶 𝒍𝒎 𝒋 = 𝑵𝒅𝑯𝟐𝑶 + 𝟐𝑭 Equation 31 𝑚 Modifying Equation 29 with Equation 30 and Equation 31, 𝑁𝐻2𝑂 is expressed as: 𝒋 𝒋 𝒅 𝑵𝒎 𝑯𝟐𝑶 = 𝑵𝑯𝟐𝑶 + 𝟐𝑭 -nH2O 𝑭 Equation 32 Urea transport through AEM follows the sample way as transport through diffusion layer, according to Equation 19 and Equation 20, it is given by: 𝑵𝒎 𝒖𝒓𝒆𝒂 𝒗𝒎 /𝒌𝒎 −𝒄𝒄𝒂𝒕𝒉 𝒄𝒂𝒏 𝒖𝒓𝒆𝒂 𝒇𝒖𝒓𝒆𝒂 𝒆 = 𝒎 𝒎 𝒆𝒗 /𝒌 −𝟏 𝒗𝒎 Equation 33 In the case of AEM: 39 𝑚,𝑒𝑓𝑓  Mass transfer coefficient in membrane layer: km = 𝐷𝑢𝑟𝑒𝑎 /lm  Superficial velocity of water in membrane layer: vm = 𝑚 𝑀𝐻2𝑂 𝑁𝐻2𝑂 𝜌𝐻2𝑂 With 𝒄𝒄𝒂𝒕𝒉 𝒖𝒓𝒆𝒂 = 𝟎 => 𝑵𝒎 𝒖𝒓𝒆𝒂 = 𝒗𝒎 /𝒌𝒎 𝒄𝒂𝒏 𝒖𝒓𝒆𝒂 𝒆 𝒎 𝒎 𝒆𝒗 /𝒌 −𝟏 𝒗𝒎 Equation 34 II.3.3.3 Ohmic overpotential Occurrence of ohmic overpotential is caused of the resistive losses in three regions consisting of electrolyte, electrodes and electrolyteelectrode interface Thus, the ohmic overpotential in DUFC is expressed as [118]: ηohmic = j*a*Rin = j*a*(RAEM + Rinterfaces) Equation 35 With:  a: geometric area (cm2)  Rin: the internal resistance of the cell (Ω), which can be measured by experiment with impedance spectroscopy, is comprised of resistance of membrane (RAEM) and resistance of interfaces (Rinterfaces) and electrodes (Relectrode) According to Grew and Chiu [124], conductivity of alkaline anion exchange membrane is expressed as: 40 𝑭𝟐 Km = 𝑹𝑻 ( 𝟏 𝒆𝒇𝒇 𝑫𝑶𝑯,𝑯𝟐𝑶 + 𝟏 𝒆𝒇𝒇 𝑫𝑶𝑯,𝒎 )-1 𝒛𝟐𝑶𝑯 cOH,m Equation 36 With:  zOH: the charge of OH-  cOH,m: concentration of OH- in membrane 𝑒𝑓𝑓 𝑒𝑓𝑓  𝐷𝑂𝐻,𝐻2𝑂 and 𝐷𝑂𝐻,𝑚 are diffusion coefficients of OH- in water and membrane, respectively, which is derived by Bruggeman’s correction A fitting parameter is used to treat the diffusion coefficient between mobile species and membrane structure: χ= 𝑨𝒐 𝑫𝑶𝑯,𝑯𝟐𝑶 𝑨𝟏 𝑫𝑶𝑯,𝒎 Equation 37 With correction parameter: Ao = (ε – εo)1.5 ε: volume fraction of water in the membrane εo: the volume fraction of water in the membrane at percolation limit Thus, Equation 36 can be rewritten as: 𝑭𝟐 Km = (ε – εo)1.5 𝑫𝑶𝑯,𝑯𝟐𝑶 𝑹𝑻(𝟏+𝝌) cOH,m Equation 38 The pore volume fraction of water is given by 41 ̅ 𝑯𝟐𝑶 𝝀Ѵ ̅ 𝑯𝟐𝑶 𝒎 +𝝀 Ѵ ε = 𝑽̅ Equation 39 With: ̅ 𝐻2𝑂 : partial molar volume of membrane and water  𝑉̅𝑚 and Ѵ  𝜆: water uptake coefficient which can be calculated by Brunauer– Emmett–Teller (BET) equation [125]: 𝝀 𝒃𝒂 𝝀𝒎𝒐𝒏𝒐 = (𝟏− 𝒂𝑯𝟐𝑶 ) 𝑯𝟐𝑶 𝟏−(𝒏+𝟏)𝒂𝑯𝟐𝑶 +𝒏 𝒂𝒏+𝟏 𝑯𝟐𝑶 𝟏+(𝒃+𝟏)𝒂𝑯𝟐𝑶 −𝒃 𝒂𝒏+𝟏 𝑯𝟐𝑶 Equation 40 With:  𝜆𝑚𝑜𝑛𝑜 : water uptake coefficient at monolayer coverage  𝑎𝐻2𝑂 : water vapor activity  b and n: adsorption isotherm constant determined by fitting experimental data on conductivity Partial molar volume of membrane can be calculated as: ̅ 𝒎 = 𝑬𝑾 𝑽 𝝆 𝒎,𝒅𝒓𝒚 Equation 41 With:  EW : equivalent weight of membrane  𝜌𝑚,𝑑𝑟𝑦 : density of dry membrane Hydroxyl ions concentration in the membrane can be expressed: 42 ̅ 𝑯𝟐𝑶 Ѵ ̅ 𝑯𝟐𝑶 𝒎+Ѵ COH,m = 𝑽̅ Equation 42 Using Equation 38-Equation 42, hydroxyl ion conductivity in membrane is calculated and by subtracting it from the internal cell resistance (estimated from impedance spectroscopy), one can obtained the electrolyte–electrode interface resistance in cell II.3.3.4 At cathode In the cathode side, the modeling is followed the work of You and Liu [126] The governing equations was represented in the individual sections 3.3.4.1 Diffusion layer: There is no chemical reaction in this region, Fick’s law is followed by oxygen flux [127] 𝒅,𝒆𝒇𝒇 𝒅𝒄𝑶𝟐 𝒅𝒛 𝑵𝒅𝑶𝟐 = -𝑫𝑶𝟐 Equation 43 With:  Effective diffusion coefficient of oxygen in diffusion layer: 𝑑,𝑒𝑓𝑓 𝐷𝑂2 which can calculated by using Bruggeman’s correction (Equation 16)  Local concentration of oxygen: cO2 43 3.3.4.2 Catalyst layer: In the cathode side, the electrochemical reaction in the same way of anode side is described by using Tafel type Butler-Volmer equation [123] with respect to oxygen as following: 𝒅𝒋𝒂𝒎 𝒅𝒛 = -Ac 𝒋𝒄𝟎,𝒓𝒆𝒇 ( 𝒄 𝒄𝑶𝟐 𝑶𝟐,𝒓𝒆𝒇 𝜹 −𝜶𝒄 𝑭𝜼𝒄𝒂𝒕𝒉 ) [exp ( 𝑹𝑻 ) – exp ( 𝜶𝒂 𝑭𝜼𝒄𝒂𝒕𝒉 𝑹𝑻 ) Equation 44 With:  Ac : specific area of the reaction surface at cathode 𝑐  𝑗0,𝑟𝑒𝑓 : Reference cathode exchange current density  𝛿: order of the reaction with respect to urea concentration 𝑟𝑒𝑓  𝑐𝑂2 : Reference concentration of oxygen which is associated with 𝑐 𝑗0,𝑟𝑒𝑓  𝛼𝑎 and 𝛼𝑐 : anodic and cathodic transfer coefficient for oxidation reduction reaction (ORR)  𝜂𝑐𝑎𝑡ℎ : cathodic over potential According to mass balance for oxygen in cathode side, it can be expressed as: 𝒅𝑵𝑶𝟐 𝒅𝒛 𝟏 𝒅𝒋𝒂𝒏 𝒅𝒛 = − 𝟒𝑭 Equation 45 44 Following the Fick’s law, the oxygen flux in catalyst layer can be described as: 𝒄,𝒆𝒇𝒇 𝒅𝒄𝑶𝟐 𝒅𝒛 NO2 = -𝑫𝑶𝟐 Equation 46 𝑐,𝑒𝑓𝑓 With, effective diffusion coefficient of oxygen in catalyst layer: 𝐷𝑂2 which can calculated by using Bruggeman’s correction (Equation 16) Cathode overpotential in the catalyst layer can be determined as: ηcath(z) = Φs (z) + Φm (z) Equation 47 According to Ohm law for ionomer phase, the ionomer phase potential of cathode catalyst layer can be defined as: 𝒅Φm 𝒅𝒛 = 𝟏 j 𝒆𝒇𝒇 an 𝒌𝒎 Equation 48 Because the electron conductivity of carbon matrix is substantially greater than ionic conductivity of ionomer, one can assume that the potential drop through the solid carbon matrix carbon is negligible compared to the potential drop in the electrolyte phase: 𝒅(−𝜼𝒄𝒂𝒕𝒉) 𝒅𝒛 = 𝟏 j 𝒆𝒇𝒇 an 𝒌𝒎 Equation 49 II.3.3.5 Solution procedure 45 3.3.5.1 Anode Combining the material balance toward urea and water in cathode side Equation 23 and Equation 24 with differentiating Equation 22 respect to z axis can obtain as: 𝒄,𝒆𝒇𝒇 𝒅𝟐 𝒄𝒖𝒓𝒆𝒂 𝒅𝒛𝟐 𝑫𝒖𝒓𝒆𝒂 = (1+ 𝑴𝑯𝟐𝑶 𝒄𝒖𝒓𝒆𝒂 𝝆𝑯𝟐𝑶 ) 𝟏 𝒅𝒋𝒂𝒏 𝟐𝑭 𝒅𝒛 + 𝑴𝑯𝟐𝑶 𝒋𝒂𝒏 𝝆𝑯𝟐𝑶 ( 𝟐𝑭 + 𝑵𝒅𝑯𝟐𝑶 ) 𝒅𝒄𝒖𝒓𝒆𝒂 𝒅𝒛 Equation 50 Again, combining overpotential equation Equation 28 and material balance for urea Equation 24 with differentiating Tafel type ButlerVolmer equation Equation 21 respect to z axis obtains as: 𝒅𝟐 𝒋𝒂𝒏 𝒅𝟐 𝒛 ={𝒄 𝜸 𝒅𝒄𝒖𝒓𝒆𝒂 𝒖𝒓𝒆𝒂 𝒅𝒛 + 𝟐𝜶𝒂 𝑭 𝑹𝑻 [( 𝟏 𝒆𝒇𝒇 𝒌𝒔 + 𝟏 𝒆𝒇𝒇 𝒌𝒎 ) jan - 𝟏 𝒆𝒇𝒇 𝒌𝒔 j ]} 𝒅𝒋𝒂𝒏 𝒅𝒛 Equation 51 There are two dependent variables, 𝑗𝑎𝑛 and 𝑐𝑢𝑟𝑒𝑎 , which involved in two second-order differential equations, Equation 50 and Equation 51 These two equations can be transformed into four first-order differential equation as: 𝒅𝒋𝒂𝒏 𝒅𝒛 =𝒋̃ Equation 52 𝒅𝒄𝒖𝒓𝒆𝒂 𝒅𝒛 = 𝒄̃𝒖𝒓𝒆𝒂 Equation 53 Equation 50 => 46 𝒅𝒄̃𝒖𝒓𝒆𝒂 𝒅𝒛 𝑴 = (1+ 𝝆 𝑯𝟐𝑶 𝒄𝒖𝒓𝒆𝒂 ) 𝑯𝟐𝑶 𝟏 𝒄,𝒆𝒇𝒇 𝑫𝒖𝒓𝒆𝒂 𝟐𝑭 𝒋̃ + 𝑴𝑯𝟐𝑶 𝒋 ( 𝒂𝒏 + 𝑵𝒅𝑯𝟐𝑶 ) 𝒄̃𝒖𝒓𝒆𝒂 𝒄,𝒆𝒇𝒇 𝑫𝒖𝒓𝒆𝒂 𝝆𝑯𝟐𝑶 𝟐𝑭 Equation 54 Equation 51=> 𝒅𝒋̃ 𝒅𝒛 = {𝒄 𝜸 𝒖𝒓𝒆𝒂 𝒄̃𝒖𝒓𝒆𝒂 + 𝟐𝜶𝒂 𝑭 𝑹𝑻 ( 𝟏 + 𝒆𝒇𝒇 𝒌𝒔 𝟏 𝒆𝒇𝒇 𝒌𝒎 ) jan - 𝟐𝜶𝒂 𝑭 𝒆𝒇𝒇 𝑹𝑻𝒌𝒔 j } 𝒋̃ Equation 55 Using Runge Kutta method, at z = 0, the four ordinary differential equations with four dependent variables are solved At z=0, the anionic current density must be zero: 𝒋𝒂𝒏|𝐳=𝟎 = Equation 56 At z = 0, urea diffusion flux: 𝒄,𝒆𝒇𝒇 𝑵𝒅𝒖𝒓𝒆𝒂 = -𝑫𝒖𝒓𝒆𝒂 𝒄̃𝒖𝒓𝒆𝒂|𝒛=𝟎 + 𝑴𝑯𝟐𝑶 𝝆𝑯𝟐𝑶 𝒄𝒖𝒓𝒆𝒂|𝒛=𝟎 𝑵𝒅𝑯𝟐𝑶 Equation 57 By equating Equation 20 and Equation 57, 𝑐𝑢𝑟𝑒𝑎|𝑧=0 is related to 𝑐̃𝑢𝑟𝑒𝑎|𝑧=0 , 𝑗̃z=0 and 𝑐𝑢𝑟𝑒𝑎|𝑧=0 are determined And at the catalyst layerAEM interface, z = ξ, anionic current density is cell current density The urea flux through the catalyst layer is partly consumed due to urea oxidation reaction and remaining passes through the AEM known as urea crossover, therefore: 47 𝒋𝒂𝒏|𝐳=𝛏 = 𝒋 Equation 58 𝐣 𝐦 ̃𝐮𝐫𝐞𝐚|𝐳=𝟎 + 𝐍𝐮𝐫𝐞𝐚 + 𝟐𝐅 = -𝐃𝐜,𝐞𝐟𝐟 𝐮𝐫𝐞𝐚 𝐜 𝐌𝐇𝟐𝐎 𝛒𝐇𝟐𝐎 𝐝 𝐜𝐮𝐫𝐞𝐚|𝐳=𝟎 𝐍𝐇𝟐𝐎 Equation 59 𝑚 𝑁𝑢𝑟𝑒𝑎 can be calculated from Equation 34, and hence two initial valve, 𝑗̃z=0 and 𝑐𝑢𝑟𝑒𝑎|𝑧=0 are still unknown 3.3.5.2 Cathode Similar to anode layer, there are four first-order differential equations with four different variables:𝑗𝑎𝑛 , NO2, cO2, ηcath The appropriate boundary conditions will be considered in the following section At gas diffuser-catalyst layer interface, z = 0, 𝑗𝑎𝑛 equals to zero, and NO2 at this interface must be equal to oxygen reduction rate at cell current density gets: 𝒋𝒂𝒏|𝐳=𝟎 = Equation 60 From Equation 45=> 𝑁𝑂2|𝑧=0 = 4𝐹 Equation 61 According Henry’s Law, oxygen concentration at the interface is therefore calculated as: 48 cO2|z=0 = 𝑷𝑶𝟐|𝒛=𝟎 𝑯𝑶𝟐 Equation 62 By using Equation 43 and Equation 62, at the gas diffusion layercatalyst layer interface, oxygen pressure is determined in terms of pressure as: 𝑃𝑜𝑢𝑡|𝑧=0 = 𝑃𝑖𝑛 - 𝑙𝑑 𝑅𝑇𝑗 𝑑,𝑒𝑓𝑓 4𝐹𝐷𝑂2 Equation 63 With:  𝑃𝑖𝑛 : inlet pressure of oxygen At the catalyst layer-AEM interface, z = ξ, the anionic current density is equal to the cell current density (Equation 58) There are the first order nonlinear differential equations, which are solved numerically by 4th order Runge Kutta method, on the each electrodes:  On the anode side: 𝑐𝑢𝑟𝑒𝑎|𝑧=0 ,𝑐̃𝑢𝑟𝑒𝑎|𝑧=0 , 𝑗̃z=0 and 𝑗𝑎𝑛 with the first order nonlinear differential equations of Equation 52 - Equation 55  On the cathode side: NO2, cO2, ηcath with the first order nonlinear differential equations of Equation 44 - Equation 46 and Equation 49 49 Finally, DUFC potential at a given current density may expressed as: V = E – ηan|z=lc − ηcath|z=lc − ηohmic Equation 64 50 CHAPTER 3: EXPERIMENTAL SECTION III Materials processing III.1.1 LNF-C synthesized and electrode fabrication Perovskite (LaNi0.6Fe0.4O3) doped cerium oxide structure was prepared by a citrate assisted sol–gel method as described elsewhere [128– 130](Figure 2-1) Briefly, 0.01mol La(NO3)3 6H2O and 0.06 mol citric acid was dissolved in 100 mL deionized water under vigorous stirring until a transparent solution was obtained Then, to above the solution 50 mL of 0.01 mol Ce(NO3)3 6H2O, 0.006 mol Ni(NO3)2 6H2O, 0.004 mol ferric citrate aqueous solution was added to obtain a sol The preparedsol solution was maintained at pH 7-8 with ammonia solution and was concentrated by heating at 80-90 0C for 3-4 h The hydrothermal treatment of the gel was performed at 200 0C for h in a furnace The resultant powder was then calcined at 950 0C for h to get the perovskite structure Finally, the catalyst powder was obtained by ball-milling and dry with ethanol 51 ... 119 Table 9: Competition of performance of direct urea fuel cell 130 xviii Nickel – based electro- oxidation catalysts for urea sensors and urea fuel cells TRAN THAO QUYNH NGAN Supervisor:... Chemical and biological engineering Graduate School Gachon University Thesis for Doctor of Philosophy’s Degree Nickel – based electro- oxidation catalysts for urea sensors and urea fuel cells By... thesis for the Degree of Doctor of Philosophy Nickel – based electro- oxidation catalysts for urea sensors and urea fuel cells By TRAN THAO QUYNH NGAN Submitted in Fulfillment of the Requirements for