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 biol

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

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 do 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

CONTENTS

CHAPTER 1: RESEARCH AMBITION AND SIGNIFICANCE 4

I.1 Urea sensor 4

I.2 Direct urea fuel cell 5

I.3 Significance and organization 7

CHAPTER 2: INTRODUCTION 8

II 1 Urea sensors 8

II 1.1 Enzymatic urea sensor 8

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 2 Direct urea fuel cell 14

II 2.1 Anode materials 20

II 2.2 Electrolyte materials 22

II 2.3 Cathode materials 23

II 3 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

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 1 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 2 Electrical system and single stack cell fabrication 56

III 3 Quantitative characterizations 57

CHAPTER 4: RESULTS AND DISCUSSION 57

IV 4.1 NiBTC materials 57

IV 4.1.1Morphological and structural studies 57

IV 4.1.2Electro-catalytic activity of Ni– benzene-1,3,5-tricarboxylic 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 5 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

REFERENCES 139

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) Ni- MOF particle, (c) composite of Ni-MOF/MWCNT, (d) STEM

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

KOH concentration from 0.01 to 1M, inset: current density response of Ni-BTC/MWCNT/ITO non-enzymatic electrode with varying KOH concentration, recorded at a scan rate of 20

mV s-1 67 Figure 4-8 CV curves of MWCNT (A), Ni/MWCNT (B), Ni- MOF (C), and Ni-MOF/MWCNT (D) electrodes in absence and in presence of 10 mM urea at a scan rate of 20 mV s-1 69 Figure 4-9 CV curves of the Ni-BTC/MWCNT catalyst modified in ITO glass at different scan rate form 1-40 mVs-1, inset: plot of oxidation and reduction peak as a function of the square root of scan rate, 5 mM urea 0.1 M KOH solution 72 Figure 4-10 Plot of the anodic peak potential vs logarithm of potential scan rate at 5 mM urea 0.1 M KOH solution 72 Figure 4-11 (a) CA curve to successive additions of urea into the test cell, (b) calibration plots of urea concentration vs current response for different Ni-MOF/MWCNT catalysts

which are different heating times (12, 24, 48 h) in synthesis process 74 Figure 4-12 CA curve of Ni-BTC/MWCNT electrode with respect to the addition of urea and different species of interference: 20 µM creatinine, 1 µM ascorbic acid, 1 µM glucose, and 3 µM uric acid 76 Figure 4-13 XPS spectrum of before cyclic voltammetry testing viz., (A) Ni-MOF/MWCNT, (B) deconvulated Ni 2p, and after cyclic voltammetry testing viz., (C) Ni- MOF/MWCNT, (D) deconvulated Ni 2p 77 Figure 4-14 (A) XRD pattern of LNF-C material and (B) SEM micrographs of LNF-C powder and (C) LNF-C/MWCNT composite 81 Figure 4-15 CV curves of (a) LNF-C, (b) NiO/MWCNT, (c) LNF/MWCNT, and (d) LNF-C/MWCNT material modified in ITO electrodes in 10 mM urea and 0.1 M KOH and (do) LNF- C/MWCNT/ITO electrode in the absence of urea 83

Figure 4-16 Arrangement of generated layer of the prepared electrode for urea detector 84 Figure 4-17 Mechanism of urea electrooxidation based on LNF catalyst 85 Figure 4-18 CV curves of LNF/MWCNT (A,B) and LNF- C/MWCNT (C,D) in the absence (A,C) and presence (B,D) of

20 mM M urea in 0.1 M KOH solution at a scan rate of 20 mV

s-1 88 Figure 4-19 the obtained peak current densities with repeated

CV cycle for LNF/MWCNT and LNF-C/MWCNT modified ITO electrode in 20 mM M urea and 0.1 M KOH solution at a scan rate of 20 mV s-1 89 Figure 4-20(A) CV curves of LNF-C/MWCNT modified on ITO electrode in 10 mM urea and 0.1 M KOH in various of scan rate (1–60 mV s-1), (B) plot of peak current density (Ip)

vs square root of potential scan rate, and (C) plot of Epa vs logarithm of potential scan rate 92

Figure 4-21 CV curves of of LNF-C/MWCNT modified ITO electrode with various urea concentration (0-30mM) in 0.1 M KOH at scan rate 20 mV s-1 Inset: plot of log (current) vs log (urea concentration) 94 Figure 4-22 CA responses of (A) LNF/MWCNT and (B) LNF-C/MWCNT material modified on ITO electrode corresponding to successive increase of urea concentration in alkaline medium at an applied potential of 0.55 V Inset: a calibation curve of current density vs urea concentration 96

Figure 4-23 CA curve of LNF-C/MWCNT modified in ITO

glass with respect to the addition of 100 μM urea, 20 μM creatinine, 1 μM ascorbic acid, 1 μM glucose, 3 μM uric acid and 100 μM of Na+, Mg2+, Cl-, and SO42- in 0.1M KOH solution 98 Figure 4-24 Stability of LNF-C/MWCNT catalyst composite

as urea sensor activity in 10mM urea and 0.1M KOH 99

Figure 4-25 (A) XRD patterns of metal organic framework – NiBTC, tailored Ni@C sphere and NiO@C core-shell and TGA (black line) and DSC (red line) analysis of NiBTC in air flow (B) and nitrogen flow (C) 105 Figure 4-26 SEM, EDS and FIB-SEM images of (A, B) NiBTC, (C, D) NiO@C core-shell and (E, F) tailored Ni@C sphere, respectively 107 Figure 4-27 TEM and HRTEM of (A and B) NiBTC, (C-E) tailored Ni@C subunit and (F-H) NiO@C core-shell Inset (E) and (H) are the selected area electron diffraction (SAED) of

respectively 109 Figure 4-28 (A) N2 adsorption isotherms and (B) the pore diameter distribution of precursor NiBTC, tailored Ni@C sphere and NiO@C core-shell 111 Figure 4-29 CV curve of precursor NiBTC (A), NiO@C core- shell (B) and tailored Ni@C sphere (C) with and without

100mM Urea in 0.1M KOH at scan rate 10mVs-1 Summary of current densities for presence of 100mM urea and onset potentials (D) The linear relationship between current density peak (I) and urea concentration (C) of precursor NiBTC, NiO@C core-shell and tailored Ni@C sphere (D) CAs of urea electro-oxidation on the precursor NiBTC, NiO@C core-shell and tailored Ni@C sphere at 0.55V in 100mM urea and 0.1M KOH solution (E) Calculated graphs between logarithms of current density peak (I) and urea concentration (C) of typical nickel-based catalysts (F) 115 Figure 4-30 CV curve of precursor NiBTC (A), NiO@C core- shell (B) and tailored Ni@C sphere (C) with various Urea concentration in 0.1M KOH at scan rate 10mVs-1 116 Figure 4-31 Doube layer charging currents recorded with various scan rates and dependence of double layer charging current in double layer region vs scan rate of Ni@C electrode

(A, B) NiBTC electrode (C,D) and NiO@C electrode (E, F) in 20mM urea and 0.1M KOH 118 Figure 4-32 Comparison of cell performances of various anode materials such as commercial Pt/C, precursor NiBTC, tailored Ni@C sphere and NiO@C core-shell; and Pt/C cathode Anolyte: 0.1M urea in 1M KOH at 55 C, catholyte: wet air as oxidant 122 Figure 4-33 Cell performances of NiO@C core-shell anode; and Pt/C cathode Anolyte: vary urea concentration of 0.05- 0.5M in 1M KOH at 55 C, catholyte: wet air as oxidant 123 Figure 4-34 Cell performances of precursor NiBTC anode; and Pt/C cathode Anolyte: vary urea concentration of 0.05- 0.5M in 1M KOH at 55 C, catholyte: wet air as oxidant 123 Figure 4-35 Cell performances of tailored Ni@C sphere anode; and Pt/C cathode Anolyte: vary urea concentration of 0.05-0.5M in 1M KOH at 55 C, catholyte: wet air as oxidant 124

Figure 4-36 Cell performances of tailored Ni@C sphere anode; and Pt/C cathode Anolyte: vary KOH concentrations and in 0.33M urea at 55 C, catholyte: wet air as oxidant Inset: evolution of current density and potential during the diversity

of KOH concentration (0-5M) 124 Figure 4-37 (A) Effect of cell performance on the operation temperature TC (t: 25, 40 and 55) with self-tailored Ni@C sphere anode, Pt/C cathode Anolyte: 1M KOH and 0.33M urea, catholyte: wet air as oxidant, (B) CV curve of assembled hierarchical core-shell Ni@C electrode in 1M KOH and 20M urea at a different temperature of 25 to 80C, (C) linear calculated relation of 1/T and ln current density of CV results followed as Arrhenius equation and (D) stability of urea cell stack with tailored Ni@C sphere anode under current density constant of 20A cm-2, in the solution of 1M KOH and 0.33M urea at 55C 128

LIST OF TABLES

Table 1 Description of the urea fuel cell model 30 Table 2 Comparison of current response of five different NiBTC/MWCNT electrodes and one electrode for five successive measurements in KOH 0.1M solution 79 Table 3 Determination of urea in diluted real urea samples 79 Table 4 Comparison of electrochemically surface area of different electrode materials 84 Table 5 Examination of real urine sample by using CV and colorimetric method 99 Table 6 Comparison of the analytical performance of amperometric urea biosensors 101 Table 7 Pore volume and surface area of precursor NiBTC, tailored Ni@C sphere, NiO@C core-shell 111

Table 8 Comparison of electrochemically surface area of precursor NiBTC, NiO@C core-shell and tailored Ni@C sphere 119 Table 9: Competition of performance of direct urea fuel cell 130

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

TRAN THAO QUYNH NGAN Supervisor: Prof HYON HEE YOON Department of Chemical and biological engineering Graduate School of Gachon University

Recently, nickel-based electrocatalysts for urea oxidation have gained tremendous attention owing to their exceptional catalytic performance and economic viability Thus, Ni-based catalysts have been increasingly used in sensors and fuel cells applications In a pursuit to develop a state

of the art Ni-based catalytic system with efficiency as par to platinum group metals, diverse strategies have been adopted such as novel composition, different morphology, and structures In this regard, in the current study Ni-based catalysts with unique morphology, structure, and compositions have been developed as novel electrocatalysts for urea oxidation and nonenzymatic urea sensor applications The developed Ni-based catalysts demonstrated outstanding performances in urea sensor and direct urea fuel cell which were better than some of the reported literature values

In case of the non-enzymatic sensor, perovskite-type oxide (LaNi0.6Fe0.4O3-CeO2, LNF-C) synthesized by the sol-gel approach which was then amalgamated with multiwalled carbon nanotube (MWCNT) to device a novel amperometric urea sensor The urea electrooxidation mechanism studied by cyclic voltammetry revealed a 6-electrons pathway, and a half-order reaction in an alkaline medium The LNF-C/MWCNT modified electrodes displayed a sensitivity of 195.6 μAmM-1cm-2 in the linear range from 25 to 670 μM of urea A low detection limit of 1 μM with a fast response time (5 s) and good self-life stability were some of the fascinating attributes of this novel sensor In addition, analysis of real urine samples using this sensor showed excellent feasibility

To further enhance the urea sensing performance a porous Ni-organic framework (Ni3(benzene-1,3,5-tricarboxylic)2.12H2O) (Ni-MOF) supported on MWCNTs was also synthesized The structure elucidation

of Ni-MOF revealed a large porous spherical conglomerate consisting of nanometer-sized units coalesced together The Ni-MOF/MWCNT showed a higher sensitivity of 685 µA mM-1cm-2, in the linear range of 0.1 to 1.12mM urea concentration with lower detection limit of 3 µmol and a response time of 10 s The sensor showed no change in activity

even after 30 days of storage at room temperature Thus, the Ni-MOF showed the potentiality to fabricate urea sensor for practical applications Urea electrocatalysts such as Ni@C and NiO@C nano-structured materials were also synthesized for application in direct urea fuel cell Ni-based metal organic framework (Ni-MOF) precursors were converted

to highly porous Ni@C and NiO@C by a thermal conversion process The strategy adopted here resulted in a tailored hierarchical structure integrated structure of metal and carbon The urea fuel cell operated using the Ni@C as anode material resulted in an open circuit potential of 0.93V with a power density of 13.82 mWcm-2 at 50C with 0.33 M urea solution in 1 M KOH as the fuel source The cell periodically operated after an interval of 3 days exhibited outstanding stability with no variation on the performance Thus, in the current study, it was demonstrated that a superior catalyst derived from a heterogenous structure greatly enhanced the electrochemical performance of urea fuel cell, thus realizing the utilization of urea as a renewable fuel

CHAPTER 1: RESEARCH AMBITION AND SIGNIFICANCE

It is well known to be a wide distribution of urea in nature and industry as: (i) 15 to 40mg/dl (2.5–7.5 mM/l) of normal level of urea in serum [1]; (ii)urea appearance in many ointments of pharmaceutical industry [2]; (iii) in the food, for example in milk; and (iv) annual exceeding 100 million metric tonnes of urea production almost for fertilizer and so on

In addition, the nitrate contamination in urea-rich wastewater from both animal urine and industry often place dangers to the environmental issues toward drinking water and the ground Currently, urea sensor has been

of significant interest in a wide range of clinical as well as in food and environmental monitoring Therefore, reliable and selective analytical detection of urea has been an attractive research topicin fabrication of point-of-care diagnostic devices [1,3] Recently, the amalgamation of nanodimensional materials for sensing application has generated a wide interest in biomedical and environmental monitoring, as they combine the merits of each individual component [1,4–6] The unique features of these nanodimensional materials such as high surface area, excellent electrochemical properties and mechanical stability make them suitable for the development of a reliable and efficient sensor system This

of biomolecules toward diversity of temperature, pH and ionic strength and the limitations of traditional sensor system comprising only of native enzymes [1] Therefore, based on the aforementioned discussions, we developed non-enzymatic sensors with these main points:

 To develop and evaluate the detection property of 1,3,5-tricarboxylic)2.12H2O) (Ni-BTC) and perovskite-type oxide (LaNi0.6Fe0.4O3-CeO2, LNF-C) toward urea

Ni3(benzene- To investigate the sensitivity, detecting range of urea concentration, detection limit of urea, response duration, and stability of material in the testing medium

 Furthermore, analyze the urea component in real urine samples

to demonstrate the feasibility study

I.2 Direct urea fuel cell

Under growing concerns of energy demand and also depletion of fossil fuel, renewable energy resources such as gas/liquid hydrogen carriers have been investigated [7,8] Thus, electric energy from pollutants consisting urea, a non-toxic chemical, wide distribution in industry and natural systems as well as mammal waste, has been offering as a great replacing fuel because of its electricity efficiency and relieving contamination issue [9,10] Furthermore, its natural stability together with easy and safe transport and storage also make an advantaged

property to develop DUFC Direct urea fuel cell (DUFC) is known as a key point but challenging renewable energy production technology However, reducing the over potential to drive the sluggish kinetics of urea oxidation reaction due to a 6 e- transfer process, which was demonstrated from previous report, has been investigated [11] In addition, insufficient stability, conductivity reduction due to “poisoning”

of gas products are great challenges which need to overcome[12–14] Therefore, an open question of how to gain an optimizing electric energy efficiency from urea and use their practice application is a great theme

of model scientific technology In this study, we focused on the main goals as:

 To develop and evaluate the electro-oxidation property of urea of different nickel-based catalysts: NiMOF, Ni@C and NiO@C

 To apply these catalysts as anode materials in a fuel cell stack for gaining the power performance of direct urea fuel cell

 To investigate the power output of DUFC in different operating conditions (25-55C of operating temperature, 0.1-5M of OH-concentration, 0.05-1M of urea concentration)

 Comparison the power output of DUFC with different anode materials of NiMOF, Ni@C, NiO@C and commercial Pt/C

 To investigate the stability characteristics of a unit cell under 20 hours of the operating duration

I.3 Significance and organization

Herein, six chapters in this thesis have represented the generated results The second chapter is a general view and background of literature toward utilized materials and advancement The third chapter shows nickel-nanostructure on fabrication techniques and characterization The fourth chapter reveals the obtained results in the present research, which is analyzed descriptively The fifth chapter is an overall view and conclusion of the study The last chapter provides the insight for the future perspectives that could improve the output performance of direct urea fuel cell

CHAPTER 2: INTRODUCTION

II 1 Urea sensors

Urea is a colorless and odorless nitrogen containing organic molecule, which has been of paramount importance in evaluating kidney and liver function by examining human excreta or blood samples [1,15,16] Moreover, it has also been one of contributors in industrial polluted waste water/ground and aquatic system [17,18] Therefore, it plays a pivotal role to monitor urea in food and drinking water for the preservation of the ecosystem and human health [19–21] Electrochemical analysis methods such as amperometry [22,23], potentiometry [24–27], and impedimetry [28] are preferred over routinely used methods such as chromatography [20], colorimeter [19,29], and the likes owing to their robustness, simplicity, cost effectiveness and relatively high sensitivity The simplest urea sensors utilize urease enzyme as the sensing element, which is urea biosensor or enzymatic urea sensor However, urea biosensor has faced a numerous

of challenges in storage and limitation on detection which non-enzymatic urea sensor can propose more advantages

Biosensors are analytical devices that based on biological materials such

as enzymes, DNA and RNA, living-organisms, antibodies, and cell

analyte and a physico-chemical transducer [1,30] Physico-chemical transducers are components that are likely to convert a biochemical signal into a response signal [30] Enzymatic biosensors utilize high selectivity of bio-specificity of enzymatic reactions In the case of urea biosensor, urease is commonly utilized as the bio-detecting element

which biochemical reaction is carried out as

Equation 1 Product (ammonium ion) of this decomposition reaction of

urea in present of urease can be detected and quantified by using a

 To provide support material

 To improve the stability of biomolecules in various effects of temperature, pH and ionic strength;

 To increase shelf-life;

 To reduce cost

Therefore, these biosensors are limited by their inability to perform in harsh environment, impaired by complicated immobilization procedures and devoid of long term stability17–19

II 1.2 Non-enzymatic urea sensor

Recently, non-enzymatic sensors have gained wide interests in urea sensing applications, this new age sensors alleviate the problems associated with the traditional enzymatic sensors [39–41] Moreover, these materials are fascinating as they are not limited to sensor applications and can be extended for using as catalysts in urea based fuel cell Metal and metal oxides exhibits good catalytic properties towards electro-oxidation of urea in buffer or alkaline medium In contrast to novel metal catalyst (Pt [6], PtPd [42], IrO2 [43], etc), nickel and its oxides are cheaper and demonstrate excellent activity towards urea oxidation in alkaline media [33,40,44–47] Such behavior with the high isoelectric point (IEP=10.7) of NiO reveals an advantage point for binding biomolecules via strong electrostatic interaction [33] In practically, there are experiments carried on the probability of a diffusion–controlled process of Nickel-based catalytic performance for urea oxidation in the excellent electron transfer assistance of supper-porous structure [11,40] These properties have been extensively exploited in urea oxidation to achieve high catalytic performance Various Ni based structures such as nanoparticles, nanorods, nanowires and nanoflower have been employed in urea oxidation, such structures

with high surface area enhance the catalytic performance and are economically favorable [33,48]

Metal-organic frameworks (MOFs) have generated considerable amount

of interest in the field of catalysis [49–51], separation [52,53], gas storage [54,55], and biomedical imaging [56–58] owing to their large surface area and porosity, high thermal stability, and open metal sites MOFs are formed by assembling the diverse building units in different manners of numerous metal ions or clusters with bridging organic linker groups A wide range of MOF’s can be obtained with varying structures such as one-, two-, or three-dimensions due to the flexibility in the synthesis process [59] MOFs were primarily sought for gas storage applications; however the metallic components as active centers were observed to undergo wide range of oxidation-reduction reactions adding its suitability in catalysis [60,61] Zhang et al [62] reported the electro-oxidation activity towards hydrogen peroxide in alkaline solution with Cu-MOF modified electrode based on a reversible pair of redox reaction

CuIII(OH)-MOF/CuII(H2O)-MOF Liu et al [63] devised a enzymatic glucose sensor and studied the effect of various shapes of [Cu3(BTC)2] MOF (BTC = 1,3,5-benzene-tricarboxylate) nanocrystals

non-on the electro-oxidatinon-on of glucose However, MOFs were marred by low

electrical conductivity, weak dimensional stability and inferior catalytic properties which in sensor applications were a prerequisite for good sensitivity [64] Thus, to overcome these potential drawbacks, Wang et

al [65] synthesized Cu(tpa) (copper terephthalate metal−organic framework) modified graphene oxide composite for electrochemical sensing of dopamine (DA) and acetaminophen (ACOP) Yang et al [64] adopted a novel approach, wherein copper-metal–organic framework [Cu3(BTC)2] was anchored onto graphene surface by an one-step synthesis method, the composite electrode exhibited good sensing ability for H2O2 and ascorbic acid with high electrochemical stability

II 1.4 CeO 2 -modified perovskite oxide (LaNi 0.6 Fe 0.4 O 3 -CeO 2 )

Perovskite oxides have the empirical formula of ABO3 In an ABX3 (or ABO3)-type perovskite structure, both A- and B-sites can be flexibly designed by using metal cations with a difference of valences or ionic radii for improving chemical, electronic, and physical properties In the recently decades, they have been considered wide interest in sensors [66] and fuel cells [67] with porous structures, effective conductivity, and flexible modification in structure Thus, nickel-based catalyst with perovskite structure such as LaNiO3 [68], LaNi0.5Ti0.5O3-CoFe2O4 composite [69], and LaNi0.6Co0.4O3 [70] have been employed as

Robin P Forslund et al used LaNiO3 pervoskite as an excellent catalyst for the electro-oxidation of urea [16] Nickel in the 3+ oxidation state can react with urea molecule, in the case of LaNiO3 pervoskite, in which Ni3+

is the inherent oxidation state, can be more active than NiO (2+ oxidation state) However, it was observed that the LaNiO3 catalyst showed an unstable structure at high pH because of its restructure and CO2-poisoning for long operating duration [13]

With the oxidation reaction occuring in Ni-based catalyst surface in the alkaline media, adsorption behavior of OH- plays a crucial role toward catalytic activity It is caused by forming Ni3+ (NiOOH), a reactive species for the urea electrooxidation, from reaction of OH- adsorption

on Ni2+ (Ni(OH)2) surface Great attempts have recently been reached on enhancing OH- adsorption capability by using CeO2, which was modified

Pt catalyst with more electrochemical activity for CO, methanol, and ethanol oxidation [71] In addition, “poisoning” CO groups on NiOOH [18,72], formed during the urea oxidation process, may be decreased by the presence of CeO2 It was demontrated that the adsorbed OH- species

on CeO2 can promote the oxidative removal of CO on Ni sites [73] Moreover, CeO2 was also reported as a key effect to increase the stability

of Ni(OH) catalyst for the oxidation reaction of H2O2 [74] and prednisone analysis [75]

II 1.5 The role of MWCNT in fabrication of urea sensor

Recently, availability of nanostructured conducting materials like carbon nanotubes (CNTs) [76], graphene [77] and conducting polymer has contributed [1] to crucial break-throughs in the field of urea sensor CNTs with diameters between 1 nm (SWCNTs) and 10 nm (MWCNTs) can get placed in close proximity of the active site of nickel-based catalyst, thus, playing an important role for interfacing active site with electronic circuitry MWCNTs as an electrode material and conductive support for sensor applications have been appreciated to improve the performance of electrodes in terms of electroactive surface, porosity and electron transfer rate [78,79] These features could guarantee its application in electrochemical sensor application since it can significantly attenuate the sensor performance [80,81] In the present investigation, a novel electrode employing MWCNT composite with Ni- BTC and CeO2-modified perovskite oxide (LaNi0.6Fe0.4O3-CeO2) was

prepared for the non-enzymatic urea detection

II 2 Direct urea fuel cell

Overall of fuel cell

The global energy demand has annually been increased exponentially with the depletion of natural resources such as coal, petroleum, fossil fuel

solution for these issues, and are a promising way of the alternative energy such with a numerous of advantages of higher efficiency and environment impact In the nineteenth century (1839), an invention of William Grove is known as the first fuel cell which have recently been approaching in the expectation as an alternative energy technology [84,85] Fuel cells are known as electrical power devices which can convert chemical energy into electrical energy [86–88] These electrical energy devices are able to work during fuel supplying process; once the supply is cut off, it automatically halts power output Although, fuel cells have recently come in many varieties, their theoretical operating principle all works in the same general manner A typical fuel cell consists of three adjacent segments: anode, electrolyte, and cathode The chemical reactions occur at the interfaces of the three different segments Nowadays, they are categorized into five different types of fuel cells which are based on electrolyte membrane and mode of operation [84,88]:

1 Polymer electrolyte membrane fuel cell (PEMFC)

2 Alkaline fuel cell (AFC)

3 Phosphoric acid fuel cell (PAFC)

4 Molten carbonate fuel cell (MCFC)

5 Solid oxide fuel cell (SOFC)

Direct urea fuel cell (DUFC)