Preparation of controlled one dimensional silver and silver palldium core shell nanostructures and their electrocatalytic performance for ethanol oxidation in alkaline direct alcohol fuel cell

VIETNAM NATIONAL UNIVERSITY HCMC UNIVERSITY OF TECHNOLOGY -o0o - QUANG THE ANH PREPARATION OF CONTROLLED ONE DIMENSIONAL SILVER AND SILVER@PALLADIUM CORE-SHELL NANOSTRUCTURES AND THEIR ELECTROCATALYTIC PERFORMANCE FOR ETHANOL OXIDATION IN ALKALINE DIRECT ALCOHOL FUEL CELLS Speciality: Chemical Engineering Code: 60520301 MASTER’S THESIS HOCHIMINHCITY, 1/2019 ACKNOWLEDGEMENT First, I would like to express my deep sense of gratitude to my supervisor, Dr Nguyen Truong Son, for his encouragement, great support and valuable advice for finalization of my thesis His knowledge about fuel cells help me grow my potential I sincerely would like to thank M.Sc Nguyen Truong Xuan Minh for providing excellent material, experimental techniques and great mentor in the development and conclusion of this thesis I am very grateful to M.Sc Luu Hoang Tam and Ms Nguyen Phuc Thanh Duy for supporting me of equipment as well as knowledge of operation I would like to extend my sincere thanks to Assoc Prof Le Minh Vien and all group members at Inorganic Laboratory for their equipment support and warm hospitality I would like to appreciate Mr Thomas Ng and Bronx Creative & Design Centre for their technical and equipment support I would like to thank Vietnam National Foundation for Science and Technology Development (NAFOSTED) for the financial support under grant number 104.052017.34 Last, but not least, I would like to thank my parents, my sister, my friends, for their unconditional support, encouragement and love, and without which I would not have come this far SUMMARY Fossil fuel reserves are likely depleting and their combustion produces a lot of emission gas Therefore, development of a new energy resource burning a low CO2 gas is an essential mission For decades, studies of alternative energy sources was extending, in which fuel cells have attracted more attention as effective power conversion In fuel cells, chemical reactions convert to electric energy Moreover, the gas emission of devices is low polluting This technology has great potential to become the major power in the future Platinum-based commonly catalyze in fuel cell reaction, but the platinum metal is high cost and low quantity Recently, there have been more and more studies on palladium, which is as active as platinum for electrooxidation reactions [2] Especially, palladium displays as a good catalyst for ethanol electro-oxidation in alkaline media In this work, Pd modified with Ag to form coreshell nanowire structures to improve the stability, the oxidation and the cost The major purpose of thesis is to synthesize the Ag@Pd nanomaterial, investigate the effects of synthesis conditions on the formation of Ag nanowire (the core) and Pd shell and preliminarily examine their catalytic activity for the ethanol oxidation in alkaline media (crucial reaction in alkaline direct ethanol fuel cells) In this study, the Ag nanowire was synthesized by the polyol method Using sodium chloride and sodium bromide formed long and thin Ag nanowires The characterization of Ag nanowires carried out by the transmission electron microscopy(TEM) and X-ray Diffraction (XRD) The results showed that a AgNO3 – PVP molar ratio of 1:1.5, a temperature of 150oC and a reaction time of 2h were the appropriate condition for the formation of Ag nanowires (AgNWs) with the length of to 10 µm, and the diameter of 45 to 55nm Continuously, the AgNW covered by a thin layer of Pd through sequential reduction accompanied by the galvanic displacement reaction The products showed the Ag@Pd core-shell structures, as demonstrated by characterization Highresolution transmission electron microscopy (HRTEM) and energy dispersive spectrometry (EDS) demonstrated core-shell structure clearly Cyclic voltammetry (CV) test used to examine the catalytic activity of the samples and compare their activity to that of Pd nanocatalyst Các nguồn lượng hóa thạch gần bị cạn kiệt việc sử dụng chúng tạo lượng lớn khí thải, phát triển nguồn lượng nhiệm vụ quan trọng Trong năm gần nghiên cứu nguồn lượng thúc đẩy pin nhiên liệu thu hút nhiều quan tâm thiết bị chuyển đổi lượng hiệu Năng lượng hóa học chuyển đổi trực tiếp thành lượng điện thêm vào khí thải thiết bị tạo không đáng kể Kỹ thuật có tiềm to lớn để trở thành nguồn cung cấp lượng tương lai Tuy nhiên thiết bị hoạt động nhiệt độ cao làm hạn chế ứng dụng chúng Khi sử dụng xúc tác nhiệt độ làm việc pin nhiên liệu giảm Các xúc tác tẳng platin thường sử dụng cho phản ứng thiết bị platin có giá cao trữ lượng thấp Có nhiều nghiên cứu chứng minh hoạt tính xúc tác Paladi gần giống với Platin Nội dung luận văn tổng hợp vật liệu có cấu trúc dây nano dạng lõi vỏ, cấu trúc vật liệu tốt có khả ứng dụng để làm xúc tác pin nhiên liệu Mục tiêu luận văn tốt nghiệp tổng hợp vật liệu có cấu trúc lõi vỏ kiểm tra sơ hoạt tính vật liệu phản ứng oxi hóa ethanol mơi trường kiềm.( phản ứng pin nhiên liệu kiềm) Dây nano bạc tổng hợp phương pháp polyol Sử dung natri clorua natri bromua để tạo dây nano dài mỏng Qua trình tổng hợp tìm điều kiện thích hợp để tạo dây nano có đường kính từ 45-55nm, chiều dài từ 7-10µm nhiệt độ 150oC thời gian phản ứng 2h Sau dây nano phủ lớp paladi phương pháp trao đổi ion tạo cấu trúc lỏi võ mong muốn Các phương phân tích đại áp dụng để chứng minh vật liệu đảm bảo đặc tính Mẫu Ag-Pd lõi vỏ tỷ lệ 6:100, 8:100, 10:100, 12:100 14:100 tổng hợp Sử dụng kính hiển vi điện tử truyền qua độ phân giải cao (HRTEM) phổ tán sắc lượng tia X(EDS) để xác định cấu trúc vật liệu Hoạt tính xúc tác cho phản ứng oxy hóa ethanol vật liệu đo điện quét(CV) Bên cạnh so sánh hoạt tính mẫu với paladi dạng nano DECLARATION I declare that “Preparation of controlled one dimensional silver and silver@palladium core-shell nanostructures and their electrocatalytic performance for ethanol oxidation in alkaline direct alcohol fuel cells ” thesis was an original report of my research, which had write by me and submitted for any previous degree The experimental work is entirely my own work; the collaborative contributions indicated and acknowledged, clearly References provided on all supporting literatures and resources TABLE OF CONTENTS ACKNOWLEDGEMENT SUMMARY DECLARATION TABLE OF CONTENTS LIST OF FIGURES LIST OF TABLES ABBREVIATION INTRODUCTION CHAPTER LITERATURE REVIEW 1.1 Overview 1.2 Overview of fuel cells and direct ethanol fuel cells 12 1.2.1 Fuel Cells 12 1.2.2 Direct ethanol fuel cells 13 1.2.3 DEFCs and challenges 15 1.3 Review of core-shell one-dimensional (1-D) nanostructure 17 1.3.1 One-dimensional (1-D) nanostructure 17 1.3.2 Core-shell one-dimensional (1-D) nanostructure 19 1.4 Review of Ag@Pd core-shell nanowires 24 1.4.1 Silver's Properties, Characteristics and Applications 24 1.4.2 Review of silver nanowires 24 1.4.3 Synthesis of silver nanowires 27 1.4.4 Palladium's Properties, Characteristics and Applications 28 1.4.5 Galvanic displacement method 30 1.4.6 Improving catalytic activity of Ag on ethanol oxidation 32 1.5 Applications and related studies 33 1.6 Electrochemical measurement 37 1.6.1 Cyclic voltammetry (CV) test 37 1.6.2 Mechanism of oxidation reaction on palladium in alkaline media 38 1.6.3 Mechanism of ethanol oxidation reaction on palladium in alkaline media 39 1.6.4 Electrochemical active surface area (ECSA) measurements 39 1.7 The scope of thesis 40 CHAPTER 2: EXPERIMENT 41 2.1 Chemicals 41 2.2 Apparatus 41 2.3 Experiment procedure 41 2.3.1 Synthesis of silver nanowires 41 Synthesis of silver nanowires 42 2.3.2 Synthesis of Ag@Pd core-shell nanowire 45 2.4 Characterization of Ag@Pd core-shell nanowires 47 CHAPTER 3: RESULTS AND DISCUSSION 48 3.1 The result of silver nanowires 48 3.1.1 Effect of PVP concentration on silver nanowire formation 48 3.1.2 Effect of AgNO3 concentration on silver nanowire formation 49 3.1.3 Effects of reaction temperature on silver nanowire formation 50 3.1.4 Effects of reaction time on silver nanowire formation 51 3.1.5 XRD diagram of the silver nanowire 51 3.1.6 The optimal conditions synthesize the Ag nanowires 52 3.2 The result of Ag@Pd core - shell nanowire 53 3.2.1 Preparing Pd(NO3)2 and Ag nanowire in deionized water to synthesize the core-shell structure 53 3.2.2 Preparing Pd(NO3)2 in ethylene glycol on the formation of Ag@Pd core shell nanomaterial 54 3.2.3 Preparing Pd(NO3)2 in deionized water to synthesize Ag@Pd core-shell nanowires 54 3.2.4 XRD pattern of Ag@Pd core- shell nanostructures 56 3.2.5 EDS analysis of Ag@Pd core-shell nanowire 57 3.2.6 HRTEM image of Ag@Pd core-shell nanowires 58 3.2.7 Distribution of material on Vulcan XC-72R 59 3.3 Performance of the synthesized Ag@Pd for ethanol electro-oxidation in alkaline media 59 3.3.1 Preparation for cyclic voltammetry(CV) test 59 3.3.2 Cyclic voltammetry(CV) result of silver nanowire 60 3.3.3 The catalytic activity of samples in 1M KOH + 1M C2H5OH 61 3.3.4 Comparison to CV plot of Ag@Pd nanowires to Pd/C nanoparticle in 1M KOH 63 3.3.5 Comparison to CV plot of Ag@Pd nanowires to Pd nanocatalyst in 1M KOH and 1M C2H5OH 63 CHAPTER 4: CONCLUSION AND RECOMMENDATION 65 4.1 Conclusion 65 4.2 Recommendation 66 LIST OFPUBLICATIONS 67 REFERENCES 68 LIST OF FIGURES Fig 1.1 The image shows a reaction flowchart for direct ethanol fuel cells 16 Fig 1.2 The complete electro-oxidation of ethanol in ADEFC produce [3] 17 Fig 0.1 Schematic representation of 1D nanostructures From top to bottom: nanowire, nanorod, nanotube, and nanobelt nanoribbon……………………………………………….…19 Fig 1.4 Concentric spherical core-shell nanoparticles [10] 20 Fig 1.5 Scanning electron microscopy (SEM) images of ZnO/CdSe nanowire arrays after different annealing treatments: h at 250°C: (a) plan view and (b) cross section; h at 400 °C : (c) plan view and (d) cross section; h at 350°C plus h at 400 °C: (e) plan view and (f) cross section In Figure 1b the circled area shows the local thickness of CdSe nanowire shell 21 Fig 1.6 Typical SEM images of Sn/Pt nanotube arrays 22 Fig 1.7 Schematic view of Ge-Si core-shell nanowires (a) Side view of Ge-Si core- shell nanowire; (b) top view of Ge-Si core-shell nanowire[14] 23 Fig 1.9 a) Schematic of the displacement process Transmission electron microscopy images of material, b) Ag nanowire, c) corresponding Pt nanotube following galvanic displacement[19] 32 Fig 1.10 Typical cyclic voltammogram where ipc and ipa show the peak cathodic and anodic current respectively for a reversible reaction 37 Fig 1.11 The cyclic voltammogram of the Pd/C in 1M KOH solution (scan rate: 20 mV s−1) [3] 38 Fig 1.12 The cyclic voltammogram of the Pd/C in 1M KOH + 1M EtOH solution 39 Fig 2.1 The synthsis of Ag nanowire 43 Fig 2.3 The synthesis of Ag@Pd core-shell nanowires 46 Fig 3.1 The TEM images of as-synthesized products with different PVP amounts of 0.3(a), 0.4(b), 0.5(c), 0.6(d), 0.65(e) and 0.7(f)M 48 Fig 3.2 The TEM images of the as-synthesized silver nanowires with AgNO3 concentrations of 0.1(a), 0.2(b), 0.3(c), 0.4(d), 0.5(e), 0.6M(f), respectively 49 Fig 3.3 (a–e) depict the TEM images of the products obtained at 140(a), 150(b), 160(c), 170(d) and 180oC(e), respectively 50 Fig 0.2 The TEM images of the silver nanowires synthesized with reaction times of 60(a), 90(b), 120(c), 150(d) and 180(e) minutes……………………………………………………….51 Fig 3.5 XRD diagram of the silver nanowires 52 Fig 3.6 The result of synthesis of optimal condition 53 Fig 3.7 Image TEM of the Pd weren't successfully used to coat the surface of the Ag nanowire 53 Fig 3.8 Presented representative TEM images of the as-synthesized Ag@Pd nanomaterial 54 Fig 3.9 Ag nanowires and the TEM images of the Ag@Pd core-shell nanowires synthesized with the ratio of 6:100, 8:100, 10:100, 12:100, 14:100, respectively 55 Fig 3.10 Distribution of Ag@Pd core-shell nanowires in 5µm scale 56 Fig 3.11 XRD pattern of Ag@Pd core- shell nanostructures 57 Fig 3.13 The HRTEM image of Ag@Pd core – shell nanowire 58 Fig 3.14 Distribution of Ag@Pd material in Vulcan XC-72R 59 Fig 3.15 Detector, sample, working electrode of CV test 60 Fig 3.16 CV plots of Ag in 1M KOH and 1M KOH + 1M C2H5OH with a 61 Fig 3.17 Linear sweep voltammograms for ethanol oxidation on in 1M KOH + 1M C2H5OH with 50 mV/s scan rate with the ratio of 6:100(CS1), 8:100(CS2), 10:100(CS3), 12: 100(CS4), 14:100(CS5) and Pd nanocatalyst 62 Fig 3.18 CV plot of Ag@Pd nanowire(the ratio of 10:100) and Pd nanocatalyst 63 Fig 3.19 CV plot of Ag@Pd nanowires to Pd nanoparticle in 1M KOH and 1M C2H5OH 64 LIST OF TABLES Table Table Table Table Table 1.1 Performance of various types of power generation[8] 12 1.2 Fuel cells classification 13 1.3 The nobility of metals follows their standard redox potential 31 2.1 Detailed conditions of experiments in polyol method 44 2.2 Detailed conditions of experiments Pd@Ag 45 Coating Ag nanowires in solution open up new possibilities to deposit assynthesized core-shell nanowires onto the desired substrates at large scales The fine structures of the Ag@Pd core-shell nanowires were further characterized using HRTEM Figures 3.13 (a, b, c and d ) showed the HRTEM images of one nanotube at different positions The Ag@Pd nanomaterial indeed had a nanowire structure with a thin layer (approx 5–10 nm) of Pd 3.2.7 Distribution of material on Vulcan XC-72R Ag@Pd/C core-shell nanowires dispersed in 1ml ethanol by sonication for hours to obtain a suspension The nano-materials are loaded on the conductive carbon material which not only maximizes the availability of nano-sized electro-catalyst surface area for electron transfer but also provides better mass transport of reactants to the electro-catalyst Fig 3.14 Distribution of Ag@Pd material in Vulcan XC-72R 3.3 Performance of the synthesized Ag@Pd for ethanol electro-oxidation in alkaline media 3.3.1 Preparation for cyclic voltammetry(CV) test The sample preparation procedure for the electrochemical analysis: 3.3mg of each catalyst was dispersed in ml of ethanol by sonication for 2h Then 5µl of this suspension was dropped onto a glassy carbon electrode (GCE, mm in diameter) and dried at room temperature 5µl of 0.5% Nafion in ethanol was added onto GCE to fix the catalyst Electrochemical tests were performed on an auto machine potentiostat 59 with a three electrode cell using a Pt grid and saturated calomel electrode (SCE) (+0.244 V vs SHE), as the counter and reference electrode, respectively Blank voltammograms were done in 1M KOH solution (blank solutions) and other tests were done in 1M KOH + 1M C2H5OH solution[9] Fig 3.15 Detector, sample, working electrode of CV test 3.3.2 Cyclic voltammetry(CV) result of silver nanowire The experiment defined the activity of Ag nanowires catalysts for ethanol oxidation in alkaline media that perform in 1M KOH solution (blank solution) and (1M KOH + 1M C2H5OH) solution Figure 3.16 show the result of CV plots, the range of 0.0 – 0.2V have a pair of anodic and cathodic peaks This can be explained that oxidize Ag to Ag2O From -0.4 to -0.3V, there is a small speak because of the oxygen reduction The peak of ethanol oxidation don’t appear in the diagram, it can assert that Ag nanowire doesn’t catalyze the ethanol oxidation in alkaline media 60 Fig 3.16 CV plots of Ag in 1M KOH and 1M KOH + 1M C2H5OH with a scan rate of 50 mV/s 3.3.3 The catalytic activity of samples in 1M KOH + 1M C2H5OH Linear sweep voltammograms for ethanol oxidation on in 1M KOH + 1M C2H5OH The rate-determining is proposed to be the formula: Pd  (CH 3CO)ads + Pd  OH ads  Pd  CH 3COOH + Pd : slow   In which the adsorbed ethoxi intermediate is removed by adsorbed hydroxyl ions to form acetate as the main product and release free active catalytic sites The effect of Ag composition examine by CV test in 1M KOH + 1M C2H5OH with a scan rate of 50 mV/s Fig 3.18 image display the result of linear sweep voltammograms for ethanol oxidation which determine by active surface area(Act) According to literature , a value reduction charge of PdO monolayer is 405µC/cm2 or 4.05C/m2 61 Fig 3.17 Linear sweep voltammograms for ethanol oxidation on in 1M KOH + 1M C2H5OH with 50 mV/s scan rate with the ratio of 6:100(CS1), 8:100(CS2), 10:100(CS3), 12: 100(CS4), 14:100(CS5) and Pd nanocatalyst The chart shows information about linear sweep voltammograms for ethanol oxidation on in 1M KOH + 1M C2H5OH with 50 mV/s scan rate with the ratio of 6:100(CS1), 8:100(CS2), 10:100(CS3), 12: 100(CS4), 14:100(CS5) and Pd nanocatalyst A considerable increase of intensity in ratio of 6:100(CS1), 8:100(CS2), 10:100(CS3) The ratio of 10:100(CS3) has the highest activity toward ethanol oxidation and peak current while the ratio of 6:100(CS1) has the lowest one In other hand, the intensity of 12:100(CS4), 14:100(CS5) were decreased Thus, this may be the high Pd content, it cause a suspension of Pd The nanoparticle is easily conglomerating The active surface area (ASA) of CS3 sample was higher than Pd nanocatalyst 62 3.3.4 Comparison to CV plot of Ag@Pd nanowires to Pd/C nanoparticle in 1M KOH Fig 3.18 shows the cyclic voltammogram of the Ag@Pd and Pd catalysts in 1M KOH solution The range of -0.1 – 0.1V, the plot of Ag@Pd catalyst can attribute to the formation of silver oxide and silver oxide reduction The Pd nanocatalyst is not this peak The range of -0.4 – -0.3V, the reduction peak is observed on the Ag@Pd and Pd catalyst The peaks can attribute to the reduction of the Pd(II) oxide In addition, the peak of Ag@Pd is larger than the Pd catalyst The result is demonstrated that Ag@Pd nanowires has good catalytic activity in 1M KOH[34] Fig 3.18 CV plot of Ag@Pd nanowire(the ratio of 10:100) and Pd nanocatalyst 3.3.5 Comparison to CV plot of Ag@Pd nanowires to Pd nanocatalyst in 1M KOH and 1M C2H5OH As a comparison, CVs of Ag@Pd nanowires and Pd nanocatalyst in Fig 3.19 The ethanol oxidation reaction starts at −0.8 V and a current peak centered at −0.25V is observed during the forward scan The addition of Ag to Pd causes a tensile strain in the structure of surface Pd and shifts up the d-band center of Pd The peak of Ag 63 oxidation observe at 0.00V In the forward scan, the oxidation of ethanol form carbonaceous species In the reserve sweep, this species is oxidized[38] Fig 3.19 CV plot of Ag@Pd nanowires to Pd nanoparticle in 1M KOH and 1M C2H5OH If the peak of forward scan is higher than the peak of backward scan, the oxidation of ethanol is completed Additionally, The ratio of the forward sweep peak current if to the reverse scan peak current ib represents the tolerance of catalyst poisoning, and a higher If/Ib indicates more complete oxidation of ethanol and less accumulation of carbonaceous chemicals According to Fig 3.19, The If/Ib ratio of Ag@Pd reaches 1.28 while while the value of Pd/C is only 0.53 Catalytic measurements demonstrated that the CS3 sample has a better activity in ethanol oxidation than Pd/C It is evident that Ag can promote oxidation behavior for this reaction in alkaline media 64 CHAPTER 4: CONCLUSION AND RECOMMENDATION 4.1 Conclusion The major objective of graduated thesis is to investigate the Ag@Pd core-shell nanowire structures This material is catalyzed for oxidation ethanol in alkaline media The extension apply in fuel cell, the products replacing for Pt-based catalysis is low price The Ag@Pd bimetallic core-shell structures were successful by galvanic exchange method The achievements of this work are described below: The silver nanowire was formed successfully The diameter is about 40 60nm, the length is about -10µm The high specific surface area enhance electrochemical activity The synthesis of AgNWs material via the polyol process was investigated based on the change of factors: reaction time, PVP concentration, NaCl concentration and temperature reaction The result show by TEM image reflecting good material The high crystalline structure were exhibited by XRD pattern The Ag@Pd core-shell nanowire structure was obtained by using galvanic exchange method The structures has been the dependency of concentration The TEM image explain the difference of material XRD diagram and EDS analysis demonstrate the observation of Ag and Pd The HRTEM show the diameter of Pd coat According to result of cyclic voltammetry, the sample have active catalysts for oxidation ethanol in alkaline media The results show that the ratio of 10:100 has the highest activity toward ethanol oxidation The addition of Ag to Pd causes a tensile strain in the structure of surface Pd and shifts up the d-band center of Pd The peak of Ag oxidation observed The recuperation of Pd prove the effective of Ag It can be maintaining a long-term work 65 4.2 Recommendation In the first stage of project, Although the Ag@Pd core – shell nanomaterial was successful But, It is not complete The recommendation can enhance electrochemical of samples: A suggestion for future research would be to investigate salt concentration (KBr) Other concentration could be used for synthesizing silver nanowire to improve the silver wire The effect of the reaction time, PVP concentration, and temperature reaction is expected to be made This work could improve the Ag@Pd core-shell nanowire structure and enhance the catalytic activity of material Besides other new bimetallic nanomaterial would be synthesized and characterized for using as catalyst support in fuel cell 66 LIST OF PUBLICATIONS MTX Nguyen1, Anh The Quang1, TTM Bui1, Dung P L1, DA Tran1, TH Luu2, AT Nguyen1, H Huynh1, ST Nguyen1,*, Effects of synthesis conditions on the formation and morphology of silver nanowires, Viet Nam journal of science and technology, 2018 Vol 56(2A), p 112-118, 67 REFERENCES Antolini, E., Catalysts for direct ethanol fuel cells Journal of Power Sources, 2007 170(1): p 1-12 Xu, C., P kang Shen, and Y Liu, Ethanol electrooxidation on Pt/C and Pd/C catalysts promoted with oxide Journal of Power Sources, 2007 164(2): p 527-531 Nguyen, S.T., et al., Enhancement effect of Ag for Pd/C towards the ethanol electro-oxidation in alkaline media Applied Catalysis B: Environmental, 2009 91(1): p 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34618-34623, 2017 56 Z Liang, T Zhao, J Xu, and L Zhu, "Mechanism study of the ethanol oxidation reaction on palladium in alkaline media," Electrochimica Acta, vol 54, no 8, pp 2203-2208, 2009 57 P Zhang et al., "Silver nanowires: Synthesis technologies, growth mechanism and multifunctional applications," Materials Science and Engineering: B, vol 223, pp 1-23, 2017 73 ... core- shell nanostructures and their electrocatalytic performance for ethanol oxidation in alkaline direct alcohol fuel cells ” thesis was an original report of my research, which had write by me and. .. core) and Pd shell and preliminarily examine their catalytic activity for the ethanol oxidation in alkaline media (crucial reaction in alkaline direct ethanol fuel cells) In this study, the Ag... stability and poison tolerance[3] In this research, 1D -dimensional material examined as the catalyst for the oxidation reaction of ethanol in alkaline direct ethanol fuel cells (ADEFCs) Ag@Pd core- shell