The Kinetic and Mechanism of The Oxygen Reduction Reaction on Pt, Au, Cu, PtCu/C and CuAu/C in Alkaline Media THESIS

Transcription

1 The Kinetic and Mechanism of The Oxygen Reduction Reaction on Pt, Au, Cu, PtCu/C and CuAu/C in Alkaline Media THESIS Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of The Ohio State University By Xi Lin Graduate Program in Chemistry The Ohio State University 2016 Master's Examination Committee: Dr. Anne Co, Advisor Dr. Yiying Wu

2 Copyright by Xi Lin 2016

3 Abstract One of the major limitations of fuel cell is its sluggish kinetics for oxygen reduction reaction (ORR) occurring at the cathode and the expensive, but efficient Pt-based catalysts still remains the best catalyst for ORR. The first part of the thesis will discuss the activation energy measurement of ORR on polycrystalline Pt and PtCu/C in basic condition followed by the mechanism study via Tafel slope analysis. The results show that the activation energy of ORR at 0.8 VRHE is 23.2 ± 3.9 kj/mol on polycrystalline Pt and 17.0 ± 3.1 kj/mol on PtCu/C in alkaline media. The Ea obtained are relatively constant under different temperature (27 to 31 C) and potential (0.7 to 0.95 VRHE) ranges. Tafel slope analysis gives ~ 120 mv/dec at high current density region (hcd) and ~ 60 mv/dec at low current density region (lcd) on both polycrystalline Pt and PtCu/C in alkaline media. The results indicate that the rate determining step of ORR on both polycrystalline Pt and PtCu/C is the first electron transfer step at hcd region and either the chemical dissociation step or the proton transfer step at lcd region. Also, the kinetics of Cu, Au and CuAu/C have also been studied and compared to the Pt-based catalysts. Results show that CuAu/C predominantly goes through a four-electron pathway to produce water and only produces ~10% peroxide as side product. When normalized to geometric area, the kinetic current of Au is 4-fold of CuAu/C and 120-fold of Cu. Among all five catalysts discussed in this thesis, PtCu/C has the highest ORR activity, showing that Pt-based catalyst still exhibits highest ORR activity compared to the other catalysts. ii

4 Dedication This thesis is dedicated to my family, friends and myself. iii

5 Acknowledgments I would like to express my greatest appreciation to my advisor Dr. Anne Co and the Co group for their patience, support and guidance throughout this study and thesis work. I would like to thank the Ohio State University Department of Chemistry and Biochemistry for funding this work. iv

6 Vita June Yucai High School B.S. Chemistry, University of Cincinnati Univeristy B.S. Chemistry, South China Normal 2014 to present...graduate Teaching Associate, Department of Chemistry, The Ohio State University Fields of Study Major Field: Chemistry v

7 Table of Contents Abstract... ii Dedication... iii Acknowledgments... iv Vita...v List of Tables... viii List of Figures... ix Chapter 1: Introduction and Background Alkaline Fuel Cells (AFC) Oxygen Reduction Reaction (ORR) Pt-based ORR Catalysts Non-Pt Based ORR Catalysts Cyclic Voltammetry Rotating Disk Electrode (RDE) and Rotating Ring Disk Electrode (RRDE)...14 Chapter 2: Experimental Polycrystalline Pt Preparation PtCu/C Preparation Electrochemical Study...18 vi

8 2.4 Cu, Au, CuAu/C Preparation...21 Chapter 3: Results and Discussion Activation Energy Study of ORR on Polycrystalline Pt in Base Activation Energy Study of ORR on PtCu/C in Base and Comparison to Polycrystalline Pt Kinetic Study of ORR on Cu, Au and CuAu/C Catalysts in Base...61 Chapter 4: Conclusion...81 References...84 vii

9 List of Tables Table 1. Comparison of AFC to PEMFC[4]...7 Table 2. Summary of activation energy study of ORR on single crystalline Pt, Pt(hkl), and polycrystalline Pt at potential at 0.9 VRHE in both acid and base systems...26 Table 3. Data for Electrochemical surface areas (ESCA) and 0.9 VRHE of polycrystalline Pt...32 Table 4. Activation energy of polycrystalline Pt in 0.1 M KOH...35 Table 5. One possible theoretical mechanism for ORR on Pt in base[1]...38 Table 6. Data for Electrochemical surface areas (ESCA) and 0.9 VRHE of PtCu/C..46 Table 7. Activation energy of PtCu/C in 0.1 M KOH...50 Table 5. One possible theoretical mechanism for ORR on Pt in base[1]...53 Table 8. Comparison of Ik on polycrystalline Pt and PtCu/C...57 Table 9. Summary of Ik of ORR on Cu, Au, CuAu/C, poly Pt and 0.8 V in base...80 viii

10 List of Figures Figure 1. A comparison of electrical system efficiencies between a PAFC (PC25, commercial system) and other conventional energy conversion systems. [3]...2 Figure 2. Schematic of an alkaline fuel cell [4]...4 Figure 3. Typical excitation signal for cyclic voltammetry a triangular potential waveform with switching potentials at 0.8 and 0.2 V versus SC[60]...13 Figure 4. Structure of RRDE and flowing profile...15 Figure 5. Experimental set up for ORR...20 Figure 6. CuAu/C (3-minute deposition)...22 Figure 7. Background current of polycrystalline Pt in 0.1 M KOH in at different temperature...28 Figure 8. ORR current of polycrystalline Pt in 0.1 M KOH in at different temperatures.29 Figure 9. Itotal of polycrystalline Pt in 0.1 M KOH in at different temperatures...30 Figure 10. Kinetic current of polycrystalline Pt in 0.1 M KOH in at different temperatures at different temperature in base...31 Figure 11. IR correction comparison of polycrystalline Pt in 0.1 M KOH...33 Figure 12. ln Ik vs. 1000/T of polycrystalline Pt in 0.1 M KOH...34 Figure 13. Activation energy of polycrystalline Pt in 0.1 M KOH...35 ix

11 Figure 14. Six sets of activation energy of polycrystalline Pt in 0.1 M KOH (a, b, c, d, e, f)...37 Figure 15. Tafel plot of polycrystalline Pt in 0.1 M KOH...38 Figure 16. Background current of PtCu/C in 0.1 M KOH in at different temperatures...42 Figure 17. ORR current of PtCu/C in 0.1 M KOH in at different temperatures...43 Figure 18. Itotal of PtCu/C in 0.1 M KOH in at different temperatures...44 Figure 19. Kinetic current of PtCu/C in 0.1 M KOH in at different temperatures...45 Figure 20. IR correction comparison of PtCu/C in 0.1 M KOH...47 Figure 21. ln Ik vs. 1000/T of PtCu/C in 0.1 M KOH...48 Figure 22. Activation energy of PtCu/C in 0.1 M KOH...49 Figure 23. Eight sets of Activation energy of PtCu/C in 0.1 M KOH (a, b, c, d, e, f, g, h)...51 Figure 24. Tafel plot of PtCu/C in 0.1 M KOH at 27 C...53 Figure 25. Background current comparison of poly Pt and PtCu/C in 0.1 M KOH...55 Figure 26. Itotal comparison of poly Pt and PtCu/C in 0.1 M KOH...56 Figure 27. Kinetic current comparison of poly Pt and PtCu/C at 27 C in 0.1 M KOH...57 Figure 28. Activation energy comparison of poly Pt and PtCu/C in 0.1 M KOH...58 Figure 29. Tafel plots comparison of poly Pt and PtCu/C in 0.1 M KOH...59 Figure 30. Background current and ORR current of Cu in 0.1 M KOH at room temperature...62 x

12 Figure 31. Itotal of Cu in 0.1 M KOH at room temperature...63 Figure 32. Kinetic current of Cu in 0.1 M KOH at room temperature...64 Figure 33. Background current and ORR current of Au in 0.1 M KOH at room temperature...65 Figure 34. Itotal of Au in 0.1 M KOH at room temperature...66 Figure 35. Kinetic current of Au in 0.1 M KOH at room temperature...67 Figure 36. Background current and ORR Current of CuAu/C in 0.1 M KOH at room temperature (3 min deposition)...68 Figure 37. Itotal of CuAu/C in 0.1 M KOH at room temperature (3 min deposition)...69 Figure 38. Kinetic current of CuAu/C in 0.1 M KOH at room temperature (3 min deposition)...70 Figure 39. Ik of ORR on 0.8 V at different deposition time in 0.1 M KOH at room temperature...71 Figure 40. H2O2 formation measured from Pt ring hold at 1.2 V during ORR anodic sweep for CuAu/C in 0.1 M KOH at room temperature...73 Figure 41. Background current of Cu, Au and CuAu/C in 0.1 M KOH at room temperature...75 Figure 42. Itotal of Cu, Au and CuAu/C in 0.1 M KOH at room temperature...77 Figure 43. Ik of Cu, Au and CuAu/C in 0.1 M KOH at room temperature...78 Figure 44. Kinetic current of ORR on Cu, Au, CuAu/C, poly Pt and 0.8 V in 0.1 M KOH...79 xi

13 Chapter 1: Introduction and Background A fuel cell is an electrochemical conversion device that directly converts chemical energy to electrical energy. It continuously supply fuel such as hydrogen, natural gas, or methanol and an oxidant such as oxygen, air, or hydrogen peroxide [1]. The invention of fuel cells as energy conversion systems dates back to the middle of the 19 th century. However, their development, lacked the drive during its early day because primary energy sources (coal, oil and gas) were abundant, unrestricted, and inexpensive. One of the major factors that has influenced the development of fuel cells has been the increasing concern about the environmental consequences of fossil fuel use in production of electricity and the propulsion of vehicles. The industrialized countries are highly dependent on fossil fuels. More importantly, the awareness of how human activities are influencing the environment and how sustainable development can be achieved is increasing worldwide. Fuel cells may help reduce our dependence on fossil fuels and harmful emissions into the atmosphere due to the advantage of higher conversion efficiencies compared to heat engines as shown in Figure 1. The efficiency of the phosphoric acid fuel cell (PAFC) is ~ 40% and when combined with heat recovery, up to 80%. However, the conventional heat engines have efficiency limited up to 30%. It should be noted that PAFC even has the lowest efficiencies of all types of fuel cells. Using pure hydrogen, fuel cells only produce 1

14 water, thus eliminate locally all emissions (such as Hg, CO2, SO2) otherwise caused by electricity production. Therefore, fuel cells can provide a new option for future power generation. Figure 1. A comparison of electrical system efficiencies between a PAFC (PC25, commercial system) and other conventional energy conversion systems [2] This chapter will give a brief review on alkaline fuel cell, its Pt-based and non-pt based catalysts as well as techniques used to evaluate catalyst efficiency, such as cyclic voltammetry and RDE/RRDE. This will provide the necessary background to understand the research presented. 2

15 1.1 Alkaline Fuel Cells (AFC) Alkaline fuel cell (AFC) was the first fuel cell technology to be put into practical service and make the generation of electricity form hydrogen feasible. Starting with applications in space the alkaline cell provided high-energy conversion efficiency (~ 70%) with low carbon footprint. AFCs use an aqueous solution of potassium hydroxide as the electrolyte, the overall chemical reactions are given by: Anode reaction: 2H2 + 4OH - 4H2O + 4e - (E = 0 VRHE) Cathode reaction: O2 + 2H2O + 4e - 4OH - (E = 1.23 VRHE) Overall reaction: 2H 2 + O 2 2H 2 O + electric energy + heat (E = 1.23 VRHE) By-product water and heat have to be removed. This is usually achieved by recirculating the electrolyte and using it as the coolant liquid, while water is removed by evaporation. A schematic of the recirculating electrolyte AFC is shown in Figure 2. 3

16 Figure 2. Schematic of an alkaline fuel cell [3] 4

17 The AFC has the advantage of exhibiting the highest electrical efficiencies of all fuel cells [4]. However, despite its early success and leadership role in fuel cell technology, AFCs have fallen out of favour with the research community and have been eclipsed by the rapid development of the proton exchange membrane fuel cell (PEMFC) as the technology of choice for vehicular applications. Typical PEMFC performance describes a system in which current densities are greater than 1 A/cm 2 at 0.6 V or higher, volumetric densities exceed 1 kw/l and gravimetric densities exceed 1 kw/kg. Table 1 shows the advantages and disadvantages of the AFC and the PEMFC for comparison. The main reason for the AFC s early decline is because it only works properly with very pure gas which is a major restraint in most applications. The formation of carbonate decreases the ionic conductivity of the electrolyte solution and could potentially lead to blockage of the electrolyte pathways and/or electrode pores in the following reaction: CO2 + 2OH - (CO3) 2- + H2O However, a number of methods have been developed to deal with this problem. Kordesch and Simader [5] mentioned the removal of the 0.03% carbon dioxide from the air through chemical absorption in a tower filled with, e.g., soda lime. Experimental numbers have been given by Appleby and Foulkes [6] indicate kw h per kilogram of limestone at 20 30% oxygen utilization. Ahuja et. al. [7] uses molecular sieves to reduce atmospheric carbon dioxide to acceptable levels and it can be used for both reformed gases on the anode side and for air on the cathode side. Electrochemical removal of the carbonates from the electrolyte has [6], condensing carbon dioxide out of 5

18 the air by using liquid hydrogen [7, 8], using solid ionomer alkaline membrane [9], operating at higher temperature to increase the solubility of the K2CO3 [10], and circulating electrolyte to significantly improve the AFC tolerance to CO2 [11] have also been proposed. Also, in recent research, alkaline anion exchange membrane fuel cell (AAEMFC) has been proposed which employs a solid polymer electrolyte instead of aqueous potassium hydroxide as the electrolyte. A typical anion exchange membrane (AEM) is composed of a polymer backbone with tethered cationic ion-exchange groups to facilitate the movement of free OH - ions. This AEM can conduct hydroxide ions without having electrolyte leakage and carbonate precipitation while maintaining the advantage of alkaline fuel cells [12, 13]. Therefore, with the development in the CO2 removal methods, it appears there is still a great potential for alkaline fuel cells to be more wildly commercialized in the fuel cell market. 6

19 Table 1: Comparison of AFC to PEMFC[3] Advantages Alkaline Fuel Cell (AFC) mechanically rechargeable low-cost KOH electrolyte Polymer Electrolyte Membrane Fuel Cell (PEMFC) nonvolatile electrolyte few materials problems CO2 rejecting electrolyte pressure differential between anode and cathode polymer electrolyte Disadvantages limited activated life intolerant of impurities in gas streams CO2 and CO pure H2 only suitable fuel expensive catalysts required CO a strong poison H2O management essential high-cost electrolyte pure H2 only suitable fuel oxygen kinetics are slow intolerant of impurities limited life water management essential Comments pure H2 only suitable fuel Apollo fuel cell operates at room temp to 80 C demo in vehicles in the 1970s operates best at C originally developed for space by GE hydrogen fuel (re-formed hydrocarbons, pure H2, MH storage) main development efforts for automotive and stationary applications 7

20 1.2 Oxygen Reduction Reaction (ORR) Despite the advantage of high conversion, high energy density and low carbon footprint, fuel cells are not wildly available commercialy primarily due to the slow kinetics of the oxygen reduction reaction (ORR) occurring at the cathode [14]. ORR is a multielectron reaction that may include a number of elementary steps involving different reaction intermediates. Of various reaction schemes proposed for the ORR on Pt [10], a modified Wroblowa et al.,[15] scheme provides a wildly accepted schematic to describe the complicated reaction pathway by which O2 is reduced at metal surfaces: Based on this reaction scheme, in basic solutions, O2 can be electrochemically reduced either directly to OH - with the rate constant k1 without formation of HO2 -, ad intermediate (so-called direct 4e - reduction) or to HO2 -,ad with the rate constant k2 ( series 2e - reduction). The HO2 -,ad can be electrochemically reduced to OH - with the rate constant k3 ( series 4e - pathway), catalytically decomposed on the electrode surface (k4), or desorbed into the bulk of the solution (k5). Although a number of important problems pertaining to the interpretation of the reaction pathway for the ORR on single crystalline Pt, Pt(hkl), such as Pt(111), Pt(100) and Pt(110) have yet to be resolved, recent studies [16-18] suggest that a series pathway via an HO2 -,ad intermediate may be operative on 8

21 Pt and Pt bimetallic catalysts. This can be considered as a special case of the general mechanism where k1 is essentially zero, i.e., there is no splitting of the O-O bond before a peroxide species is formed. HO2 -, ad, on the other hand may (k5=0) or may not (k5 0) be further reduced to HO2 -. In either case, the rate determining step appears to be the addition of the first electron to O2,ad. The rate expression is then [18] i = nfk[o2](1-θad) x exp(-βfe/rt)exp(-γrθad//rt) where n is the number of electrons, K is the chemical rate constant, [O2] is the concentration of O2 in the solution, θad is the total surface coverage by all adsorbed species, x is either 1 or 2 depending on the site requirements of the adsorbates, i is the observed current, E is the applied potential, b and c are the symmetry factors (assumed to be 1/2), and rθad is parameter characterizing the rate of change of the apparent standard free energy of adsorption with the surface coverage by adsorbing species. Assuming that the coverage of ORR intermediates is small under reaction conditions[19], for most cases, only two adsorbed species need to be considered, OHad and specifically adsorbing anions like (bi)sulfate or halides [4]. 1.3 Pt-based ORR Catalysts In spite of many attempts in the last decade by researchers and fuel cell developers to create a non-pt catalyst for low temperature (< 200 C) air cathodes, Pt remains the catalyst of choice. However, it still not fully compensate for their expense with their performances [20, 21]. Even with the use of high loadings of Pt in the electrode, the activation overpotential for the ORR is on the order of 500 mv at acceptable current 9

22 densities. Using such high Pt loadings (0.4 mg/cm 2 ) lead to high fuel cell costs by at least an order of magnitude (two orders of magnitude for transportation applications) to permit commercialization [22]. Therefore, efforts must be taken to decrease Pt content while maintaining a high level of ORR activity. In recent years, much research and theory has been dedicated to develope bimetallic catalyst systems where Pt is alloyed with 3d block metals, usually Fe, Ni, Co or Cu[4, 23-34]. These catalysts have lower Pt loadings and increased activity relative to bulk Pt, mainly due to geometric and electronic effects [23, 35]. There are many approaches to preparing these catalysts, but most involve a.) preparing Pt-X (X = Fe, Ni, Co, or Cu) catalyst nanoparticle [36-39] or b.) preparing core-shell (core = Ni, Co, Cu or PtX; shell = Pt) catalysts [23, 26, 32, 36, 40-49]. While these bimetallic methods have all proven to significantly enhance the ORR, they possess certain intrinsic shortcomings. First catalyst designs relying upon nanoparticles are susceptible to activity losses due to particle growth. Second, core-shell and nanoparticle catalyst designs usually require very intensive synthesis processes and/or temperatures exceeding 800 C [23, 43, 44, 46] neither of which are ideal for industrial scale catalyst production. Recently, galvanic displacement has emerged as a powerful yet facile route to the development of highly active Pt-based bimetallic ORR catalyst [48-51]. Our previous group member Eric Coleman employed galvanic displacement to coat thick layer of Pt on three dimensional nanoporous Cu foams to produce highly active and stable core-shell ORR catalysts. There are several advantages to this method: 1) catalysts remain electrically connected because they are supported on a continuous metal substrate, 2) 10

23 catalysts maintain a high surface area the porous nanostructure offers a similar catalytic enhancement to nanoparticles, 3) oxygen is able to freely diffuse into pores, increasing overall catalyst utilization, 4) particle growth from Ostwald ripening is eliminated, and 5) catalyst synthesis is facile and can easily be brought to industrial scale [52]. However, as good ORR catalyst, the mechanism of the ORR on PtCu/C is still unknown. 1.4 Non-Pt Based ORR Catalysts Even though Pt has shown the best catalytic performance, its high cost has limited a large-scale application. Therefore, the search for inexpensive, non-noble metal catalysts to substitute Pt-based catalyst has become a critical issue in the ORR research field. With the need of looking for comparatively cheap materials in basic media, a number of substitutes for Pt have been considered, including Pd-based catalysts [53] and non-noble metals and functionalized carbon materials [54-56]. Among the non-noble metal catalysts, Fe [55], Co [57], and Mn [58] are the most studied material. As an earthabundant element, the use of Cu to catalyze the ORR has been explored with the ultimate target of finding a replacement for Pt-based catalysts in fuel cells. Au has also gain interests as a fuel cell catalyst in basic media. It has been suggested that the first electron transfer step in alkaline media takes place by an outer sphere mechanism, where both reactants and products are close to the electrode but still solvated. Study has shown that on several metals, in particular on gold, the reaction depends on the surface plane, and it is faster on Au (100) than on the other principle planes [59]. 11

24 1.4 Cyclic Voltammetry Cyclic voltammetry (CV) is perhaps the most versatile electroanalytical technique for the study of electroactive species. Its versatility combined with ease of measurement has resulted in extensive use of CV in the fields of electrochemistry, inorganic chemistry, organic chemistry, and biochemistry. Cyclic voltammetry is often the first experiment performed in an electrochemical study of a compound, a biological material, or an electrode surface. The effectiveness of CV results from its capability for rapidly observing the redox behavior over a wide potential range. The resulting voltammogram is analogous to a conventional spectrum in that it conveys information as a function of an energy scan. CV consists of cycling the potential of an electrode, which is immersed in an unstirred solution, and measuring the resulting current. The controlling potential, which is applied across a working electrode and a reference electrode, can be considered as an excitation signal. The excitation signal for CV is a linear potential scan with a triangular waveform. This triangular potential excitation signal sweeps the potential of the electrode between two values, as shown in Figure 2 below. A cyclic voltammogram is obtained by measuring the current at the working electrode during the potential scan. As shown in Figure 3, this technique involves sweeping between two potential endpoints (b and d) at a constant potential scan rate for a predetermined number of cycles. For each cycle, there is a forward (a anodic) and a reverse (c cathodic) scan. Data are recorded in the form of an I-E curve, which can yield information about the onset potential and the kinetics of an electrochemical process. 12

25 Figure 3. Typical excitation signal for cyclic voltammetry a triangular potential waveform with switching potentials at 0.8 and 0.2 V versus SC[60] 13

26 1.5 Rotating Disk Electrode (RDE) and Rotating Ring Disk Electrode (RRDE) A rotating disk electrode (RDE) is a small metal disk inlaid into an insulting cylinder having a large base. A disk electrode is set in an insulting rod, which is rotated at a constant frequency in a solution. The solution near its base moves angularly, both radially and axially. Because of friction a thin layer of solution in contact with the base move flows the rotation of the cylinder. Due to the solution s viscosity this angular movement extends into a thicker layer of the solution, but its rate decreases exponentially with the distance from the cylinder s surface. A rotating ring disk electrode (RRDE) is a double-working electrode (WE) used in hydrodynamic voltammetry, very similar to an RDE. The electrode actually rotates during experiments, including a flux of the analyte to the electrode The difference between an RRDE and and RDE is the addition of a second WE in the form ring around the central disk of the first WE. The two electrode are separated by a nonconductive barrier and connected to the potentiostat through different leads. To operate such an electrode, it is necessary to use a bipotentiostat or a potentiostat capable of controlling a four-electrodes system. Figure 4 shows a typically structure of an RRDE, consisting of a thin metallic ring inlaid around the metallic disk situated in the center of the base the insulting cylinder. Because of the radial component of the solution s movement, which caused by the cylinder s rotation, the products of the electrode reaction formed on the disk electrode are carried over the insulating gap toward the ring electrode, where they can be detected and 14

27 analyzed. The electrode rotates around its own axis, the solution rotates as it does in an RDE, and the ring and the disk are related via electrolyte transmission [61]. Figure 4. Structure of RRDE and flowing profile 15

28 Chapter 2: Experimental 2.1 Polycrystalline Pt Preparation A polycrystalline Pt rotating disk electrode (Pine, 5 mm diameter) was mirror polished and repolished with a 0.05 micron MasterPolish (Buehler) slurry for 1 min at rotating rate of 700 rpm. The slurry was then washed away by sonicating the electrode in ultrapure water (Milli-Q 18.2 M ) for 1 min. The washing procedure was repeated twice. 2.2 PtCu/C Preparation Nanoporous Cu (npcu) synthesis An AlCu alloy (83 at% Al) was prepared from bulk high-purity (99.9%) metals at The Ohio State University Solidification and Metal Casting Laboratories (OSU foundry). The alloy was cut into coins (24 mm diameter 2 mm thick). The nanoporous Cu (npcu) structure was created by etching CuAl alloys in NaOH (6 M, at 80 C for 16 h), followed by continuous rinsing in ultrapure water (>2 h) to remove residual solvent and byproducts. The npcu was dried and stored in a desiccator soon after preparation. Prior to its use as an ORR catalyst support, npcu was heated in an H 2 atmosphere (2 h at 450 C), to ensure the reduction of any formed Cu oxides (during the etching and storage process) to Cu metal. 16

29 2.2.2 Thin-layer-npCu electrode preparation Following heat treatment of the npcu in H 2, the reduced copper coin was ground into a fine powder using a mortar and pestle. A 7.5 mg portion of npcu powder was added to 7.5 mg Vulcan XC- 72 carbon, and the mixture was dispersed ultrasonically in 10 ml ultrapure H 2 O for 10 min. A diluted Nafion solution (5 wt%, Alfa Aesar, 40 L) was added to the npcu dispersion. The resulting solution was sonicated for another 10 min. Immediately following sonication, 20 L of the suspension was drop-casted onto a mirror polished glassy carbon (GC) disk electrode (Pine, 5 mm diameter). The prepared electrode was dried under vacuum (55 C for 1.5 h). After drying, the coated electrode was allowed to cool down to room temperature Deposition of Pt A 120 ml portion of a 1.2 mm K 2 PtCl 4 (Sigma Aldrich) solution (ultrapure H 2 O from Milli-Q 18.2 M ) was added to a standard three-electrode cell with heating jacket. The cell was thermostatically controlled at 50 C. The npcu-coated GC electrode was attached to a Pine electrode rotator and immersed in the K 2 PtCl 4 solution for 2.5 min. To ensure uniform deposition of platinum, the electrode was rotated at 500 rpm during deposition. Immediately after the timed deposition, the electrode was removed from the platinum solution, and rinsed in ultrapure water for 1 min at 500 rpm, twice, to quench the reaction. 17

30 2.3 Electrochemical Study Electrochemical measurements were performed in a jacketed standard three-electrode cell using a disk electrode (Pine, 5 mm diameter) equipped with a CH Instruments bipotentiostat (CH 760D) capable of concurrent rotation control. A reversible hydrogen electrode (RHE) with a Luggin capillary was used as the reference electrode for all electrochemical measurements. The counter electrode was a Pt mesh. The electrolyte used was 0.1 M KOH was prepared by dissolving KOH pellets (Sigma-Aldrich, ACS grade) in ultrapure deionized H 2 O. All cyclic voltammograms were recorded at K with a scan rate of mv/s and a rotation rate of 1600 rpm. The prepared electrodes were transferred to the electrochemical cell and immersed in N2-saturated electrolyte, deaerated for at least 45 min. The electrodes were electrochemically washed by cycling potential between 0.5 V and 1.2 V at 1 V/s 50 times in 0.1 M HClO4 (Fisher) solution to dealloy and stabilize the catalyst. Different temperatures have been applied to the cell by using Isotemp (Fisher Scientific, Inc) with a range of K respectively. All CV scans were measured until a steady state voltammogram was attained. The Pt electrochemical surface area (Pt ECSA) was determined from a N 2 -saturated voltammogram via the average integrated charge of the underpotentially deposited hydrogen (H UPD ) region ( V) after double-layer correction. The widely accepted conversion of 210 C/cm -2 Pt for polycrystalline Pt was assumed. ORR CV measurements were performed in an O2-saturated electrolyte (0.1 M KOH) with the potential cycled between 0.03 V and 1.2 V at 20 mv/s. Polarization curves were obtained by subtracting the N 2 -saturated voltammogram from the O 2 -saturated voltammogram to remove any 18

31 background contributions. Kinetic ORR activity (I k ) was calculated for the anodic sweep curve via the following relationship: Ilim Itotal Ik = Ilim Itotal where Ilim is the diffusion-limited current and Ii total is the total current. Pt-specific activities were determined from Pt-ECSA data as follows: specific activity = Ik Pt ESCA Electrochemical impedance spectroscopy (EIS) was used to determine the solution resistance for the electrochemical system to check if it is necessary to correct for IR loss (E IR ). A CH instruments bipotentiostat (CH 760 D) was used to measure impedance at OCP at 0.9V (amplitude of 5 mv from 10,000 Hz to 0.01 Hz). During the IR correction check, the Ereal has been calculated as follows: Ereal = Emeasured- EIR 19

32 Figure 5. Experimental set up for ORR measurement 20

33 2.4 Cu, Au, CuAu/C Preparation Cu Disk Preparation A polycrystalline Cu rotating disk electrode (Pine, 5mm diameter) was mirror polished and being re-polished by 0.05 micron MasterPolish (Buehler) slurry for 1 min at rotating rate of 700 rpm each time before experiment. The slurry was then washed away by sonicating the electrode in ultrapure water (Milli-Q 18.2 M ) for 1 min. The washing procedure was repeated twice Au Disk Preparation A mirror polished glassy carbon (GC) disk electrode (Pine, 5 mm diameter) was repolished by 0.05 micron MasterPolish (Buehler) slurry for 1 min at rotating rate of 700 rpm. The slurry was then washed away by sonicating the electrode in ultrapure water (Milli-Q 18.2 M ) for 1 min. The washing procedure was repeated twice. After dried the electrode under vacuum (55 C for 0.5 h), the electrode was put into Au sputter coater (Deton Vacuum Desk II) with current of 45 ma for 100 s to sputter a thin layer of Au on GC CuAu/C Preparation Nanoporous Cu was synthesized in the same way as described in section and dropcasted onto a mirror polished glassy carbon (GC) disk electrode (Pine, 5 mm diameter) as described in After obtained the thin-layer Cu GC electrode, Au was galvanically deposited onto the electrode by immerging the electrode into 100 ml portion of a

34 mm AuCl 3 (Acros Organics) solution (ultrapure H 2 O from Milli-Q 18.2 M ) at room temperature (24 C) for various of time (1 min to 5 min). The npcu-coated GC electrode was attached to a Pine electrode rotator and in order to ensure uniform deposition, the rotating rate was controlled at 500 rpm during deposition. Immediately after the timed deposition, the electrode was removed from the AuCl 3 solution, and rinsed in ultrapure water for 2 min at 500 rpm, twice, to quench the reaction. Figure 6. CuAu/C (3 minutes deposition) 22

35 2.4.4 Electrochemical Study ORR Kinetic Study The electrochemical measurement system and 0.1 M KOH electrolyte were prepared the same way as described in section 2.3. All cyclic voltammograms were recorded at 298 K with a scan rates of mv/s and a rotation rate of 1600 rpm. The electrodes were first washed in 0.1 M HClO4 (Fisher) electrochemically to stabilize the catalyst. The potential was cycled 50 times between 0.5 V and 1.2 V at 1 V/s in 0.1 M HClO4 (Fisher) solution. Then the prepared electrodes were transferred to the N2-saturated 0.1 M KOH electrochemical cell, deaerated for at least 45 min. For polycrystalline Cu and CuAu/C, the CV scans between 0.03 V and 1.2 V at 20 mv/s and 100 mv/s until steady state voltammograms were attained. For Au, the CV scans between 0.03 V and 1.6 V at 20 mv/s and 100 mv/s until a steady state voltammogram was attained. ORR CV measurements were performed in an O2-saturated electrolyte (0.1 M KOH) with the potential cycled between 0.03 V and 1.2 V at 20 mv/s for polycrystalline Cu and CuAu/C, 0.03 V and 1.6 V for Au. Itotal was obtained by subtracting the N 2 -saturated voltammogram from the O 2 -saturated voltammogram to remove any background contributions. Kinetic ORR activity (I k ) was calculated for the anodic sweep curve via the following relationship: Ilim Itotal Ik = Ilim Itotal where Ilim is the diffusion-limited current and I tot is the total current. All data was 23

36 normalized to geometric area with diameter of 5mm Hydrogen Peroxide Production Study of CuAu/C Hydrogen peroxide production during ORR was measured via a rotating ring-disk electrode (RRDE, Pine) with a Pt ring electrode held at 1.2 V vs RHE during the measurement of the ORR on the CuAu/C disk. The RRDE experiments were conducted in O2-saturated 0.1 M KOH at room temperature. Before the ORR RRDE scan, a potential hold was measured at the ring in the absence of an applied potential at the disk to establish a background current. The ring background current was subtracted from the peroxide oxidation ring current. The collection efficiency, N, for the RRDE was All electrochemistry experiments were repeated to ensure reproducibility. 24

37 Chapter 3: Results and Discussion 3.1 Activation Energy Study of ORR on Polycrystalline Pt in Base Fuel cells are not wildly available commercially used mainly due to the sluggish oxygen reduction reaction (ORR) activity on the cathode. Therefore, researchers have been studying the mechanism of ORR in order to gain fundamental knowledge of the ORR to develop more efficient catalyst. Pt has been well studied among many research groups. Of various reaction schemes proposed for the ORR on Pt [10], a modified Wroblowa et al. scheme [15] provides a wildly accepted schematic that describes the complicated reaction pathway by which O2 is reduced on metal surfaces: Most of detailed mechanism study of the ORR on polycrystalline Pt was based on acidic systems. Most recent activation studies of ORR on polycrystalline Pt in polymer electrolyte membrane fuel cell (PEMFC) report an activation energy ~ kj/mol in 25

38 0.1 M HClO4 by Markovic et al. [4, 62] Stamenkovic et al. has reported the activation erergy as 21 kj/mol [63] and Paulus et al., reported 22 kj/mol [64] in 0.1 M H2SO4. Song et al. reported 28.3 kj/mol at low current density region (lcd) and 57.3 kj/mol at high current density region (hcd) [65]. The activation energy of the ORR on single crystalline Pt such as Pt(111), Pt(100) and Pt(110) has also been reported. as ~ 42 kj/mol by Markovic et al.[4] However, studies in basic system are more limited. Activation energy of the ORR on single crystalline Pt, has been reported by both Markovic et al. and Schmidt et al. as 40 kj/mol in 0.1 M KOH [4, 66], but no value on polycrystalline Pt has ever been reported. Table 2. Summary of activation energy study of ORR on single crystalline Pt, Pt(hkl), and polycrystalline Pt at potential at 0.9 VRHE in both acidic and basic systems Pt(hkl) Poly Pt Acidic 42 kj/mol, 0.1 M HClO4 [4] 21 kj/mol, 0.1 M H2SO4 [63] 22 kj/mol, 0.1 M H2SO4 [64] kj/mol, 0.1 M HClO4 [4] kj/mol, 0.1 M HClO4 [62] 28.3 kj/mol (lcd), 57.3 kj/mol (hcd) [65] Basic 40 kj/mol, 0.1 M KOH [66] None 40 kj/mol, 0.1 M KOH [4] Therefore, this chapter will focus on the measurement of the activation energy of ORR on 26

39 polycrystalline Pt and its possible mechanism. The activation energy of ORR on polycrystalline Pt in basic systems will be reported for the first time. 27

40 3.1.1 Background current and ORR current Figure 7. Background current of polycrystalline Pt in 0.1 M KOH in at different temperatures The background current of polycrystalline Pt has been obtained by running cyclic voltammetry from 0.02 V to 1.20 V (vs. RHE) in N2-saturated 0.1 M KOH at room temperature (27 C). After measuring the background current at 27 C, temperature has been increased to 31 C, 36 C, 41 C, 46 C. Figure 7 shows the change of background current under different temperatures. As temperature increases Pt oxidation and reduction peaks at 0.8 V remains unchanged (with difference within 1%). This shows that at this temperature range, increasing temperature does not have any significant impact on polycrystalline Pt. Therefore, the surface at each temperature are pretty reproducible. 28

41 Figure 8. ORR current of polycrystalline Pt in 0.1 M KOH in at different temperatures Right after the background current was collected, the ORR current was obtained at each temperature in O2-satruated solution, keeping all other conditions the same. The most recent convention is to compare the ORR activity of Pt-based catalyst via the anodic sweep, assuming the oxide-free surface. Unless otherwise stated, oxygen reduction curves presented in this thesis was anodic sweeps only. Figure 8 shows the change of ORR current under different temperatures (27 C, 31 C, 36 C, 41 C, 46 C). Three regions, the kinetic controlled region (> 0.85 V), combined kinetic-diffusion control (0.6 V < E <0.85 V) and mass transport region (< 0.6 V) have been shown. As shown in the figure, with the increase of temperature, the ORR current gradually increases as well. For instance, the ORR current at 0.9 V increases by 0.1 ma/cm 2 Pt area (~ 16% increase). 29

42 3.1.2 Derivation of kinetic current of ORR Figure 9. Itotal of polycrystalline Pt in 0.1 M KOH in at different temperatures After background current and ORR current have been obtained at each temperature, Itotal has been calculated by subtracting background current from ORR current to remove background contribution. Figure 9 shows Itotal of polycrystalline Pt at different temperatures. As temperature increases, Itotal also increases indicating a higher ORR activity. 30

43 Figure 10. Kinetic current of polycrystalline Pt in 0.1 M KOH in at different temperatures at different temperature in base Kinetic ORR activity (Ik) was calculated for the anodic sweep polarization curve via the equation, Ik = Iilim Itotal. Pt specific activities were determined from Pt-ECSA data as Ilim Itotal specific activity = Ik Pt ESCA. Figure 10 shows kinetic current (Ik) of polycrystalline Pt at different temperatures. As temperature increases, Ik also increases indicating a higher ORR activity. 31

44 Table 3. Data for Electrochemical surface areas (ESCA) and 0.9 VRHE of polycrystalline Pt Temperature ( C) ECSA (cm 2 ) Ik (ma/cm 2 Pt)

45 Figure 11. IR correction comparison of polycrystalline Pt in 0.1 M KOH Electrochemical impedance spectroscopy (EIS) was measured to determine the solution resistance for the electrochemical system at all five different temperatures (27 C, 31 C, 36 C, 41 C, 46 C) in order to know the effect of IR loss. The impedance was measured at OCP and at 0.9 V giving R= 2. Figure 11 shows the kinetic current before and after applying correction and due to the small impedance, no significant difference has been shown. Therefore, no IR correction has been applied to all figure presented in this thesis. 33

46 3.1.3 Derivation of activation energy of ORR Figure 12. ln Ik vs. 1000/T of polycrystalline Pt in 0.1 M KOH Six different potentials (0.75 V, 0.8 V, 0.85 V, 0.9 V, 0.95 V) have been selected to examine their Pt specific activity. At each potential, a set of six Ik can be obtained from six different temperatures. According to Arrhenius equation k = A*exp(- Ea ), rate equation Rate=k[O2], and Ik = nf(rate), ln(ik) = (- Ea R )(1 ) + ln(nfa[o2]) has been derived, T meaning that ln(ik) is proportional to 1. Figure 12 shows the plot of ln T Ik verus 1, giving T Ea = - slope R. Therefore, the activation energy at different potentials has be obtained. Also, the R 2 of the linear regression reach 0.999, indicates a good linearity and the effectiveness of the analysis method of deriving activation energy. RT 34

47 Figure 13. Activation energy of polycrystalline Pt in 0.1 M KOH Figure 13 shows the activation energy at different potential of polycrystalline Pt in 0.1 M KOH. In this figure, each Ea has been derived from the average Ik from six sets of independent experiments showing in Figure 14, giving the error bars shown in table 4. At 0.9 V, the activation energy of ORR on polycrystalline Pt in base is 23.3 ± 3.9 kj/mol. Also, with the potential increasing from 0.7 to 0.9 V, the activation energy remains relatively constant under the influence of error bar. This number is in the same range of the activation energy of ORR in acid (20~25 kj/mol) [4, 62-65]. This indicates that the rate determining step in acidic and basic system may or may not be the same, but their activation energy are relatively close to each other. Table 4. Activation energy of polycrystalline Pt in 0.1 M KOH 35

48 Potential (V) Ea (kj/mol) ± ± ± ± ± ± 4.3 With a closer look into Figure 14 shown below, the activation energy in each single set of data derived from different days remains relatively the same as potential increases. This indicates that in Figure 6, it is not the existence of error bars from multiple experiments that makes the activation remains constant. It is also worth mentioning that the reproducibility of the experimental data being used in this thesis has been all examined by overlapping the background current before use. 36

49 Figure 14. Six sets of activation energy of polycrystalline Pt in 0.1 M KOH (a, b, c, d, e, f) 37

50 3.1.4 Derivation of Tafel slope of ORR Figure 15. Tafel plot of polycrystalline Pt in 0.1 M KOH Table 5. One possible theoretical mechanism for ORR on Pt in base[67] Step Reaction Tafel slope 1 O 2 (ads) + e - à O - 2 (ads) 118 mv/dec 2 O - 2 (ads)à O(ads) + O - (ads) 60 mv/dec 3 H 2 O(ads)+ O - (ads) à OH (ads)+ OH - (aq) 60 mv/dec 4 O(ads) + H 2 O(aq) + e - à OH(ads) + OH - (aq) 40 mv/dec 5 2 [OH(ads) + H 2 O(aq) + e - à H 2 O(ads) + OH - (aq)] 24 mv/dec 38

51 To provide further insight into the mechanistic pathway of the ORR, the elementary steps of one of the possible mechanism for the ORR Pt in base are presented in Table 5. For this particular mechanism, step one involves O2 (ads) to be reduced to adsorbed to adsorbed superoxide, O2 - (ads), which dissociates to O(ads) + O - (ads). Then a proton transfer form H2O(ads) to O - (ads), forms OH(ads) + OH - (aq). The second electron transfer reacts O(ads) with H2O(ads) to form a second OH(ads) + OH - (aq). The third and fourth electron transfers react the two OH(ads) with two H2O(ads) + two OH - (aq).[67] Their theoretical Tafel slope of each elementary step has also been calculated and shown in Table 5. In Figure 15, the experimental Tafel plot for ORR on polycrystalline Pt in base is presented. At low current density region (lcd), defined as spanning the range of potentials where the kinetic current is between 0.25 % of the limiting current and where the Tafel line begins to deviate from linearity, it gives ~ 60 mv/dec and at high density region (hcd), defined as the linear Tafel range between the lcd Tafel region and the diffusionlimited current region, it gives ~120mV/dec of ORR on polycrystalline Pt in base. The Tafel slope at hcd, ~120 mv/dec, corresponds well to the theoretical Tafel slope of step one in Table 5, indicating that the first electron transfer step is the rate determining step at hcd region. The Tafel slope at lcd region, on the other hand, correspond to the theoretical Tafel slop of step two and step three in Table 5, indicating that the rate determining step at lcd region is either the chemical dissociation step and proton transfer step. It is also worth mentioning that all Tafel slope for ORR on polycrystalline Pt in base 39

52 remains constant under all five different temperatures. This means at this temperature range (27 46 C), the mechanism of ORR on polycrystalline Pt in base does not change. Therefore, it is clear that at different current density region, the rate determining step for ORR on polycrystalline Pt in base is different. At hcd region, the rate determining step is the first electron transfer step and at lcd region, the rate determining step is either the dissociation step and proton transfer step. Looking back at Table 4, it can be concluded that at hcd region, the activation energy measured is measured for the first electron transfer step and at lcd region, the activation energy measured is either for the dissociation step and proton transfer step. In terms of future study, in-situ spectroscopy techniques are needed to detect the intermediates during ORR in order to fully understand the ORR mechanism. 40

53 3.2 Activation Energy Study of ORR on PtCu/C in base and comparison to polycrystalline Pt Due to the high cost and limited resource of Pt, researchers have been dedicated to decrease Pt content of ORR catalyst by making bimetallic catalysts where Pt is alloyed with 3d block metals, usually Fe, Ni, Co or Cu [4, 23-34]. There are many approaches to preparing these catalysts, but most involve a) preparing Pt-X (X = Fe, Ni, Co, or Cu) catalyst nanoparticle [36-39] or b) preparing core-shell (core = Ni, Co, Cu or PtX; shell = Pt) catalysts [23, 26, 32, 36, 40-49]. While these bimetallic methods have all proven to significantly enhance the ORR, they possess certain intrinsic shortcomings. First catalyst designs relying upon nanoparticles are susceptible to activity losses due to particle growth. Second, core-shell and nanoparticle catalyst designs usually require very intensive synthesis processes and/or temperatures exceeding 800 C [23, 43, 44, 46], neither of which are ideal for industrial scale catalyst production. Recently, galvanic displacement has emerged as a powerful yet facile route to the development of highly active Pt-based bimetallic ORR catalyst [48-51]. Our previous group member Eric Coleman employed galvanic displacement to coat thick layer of Pt on three dimensional nanoporous Cu foams to produce highly active and stable core-shell ORR catalysts. This PtCu/C catalyst exhibits good ORR activity in both acidic and basic systems [68], but the ORR mechanism is still unclear. Therefore, this chapter will focus on the derivation of activation energy of ORR on PtCu/C and its possible mechanism. The number of the activation of ORR on PtCu/C in basic system will be reported for the first time. 41

54 3.2.1 Background current and ORR current Figure 16. Background current of PtCu/C in 0.1 M KOH in at different temperatures The background current of PtCu/C has been obtained by running cyclic voltammetry from 0.02 V to 1.20 V (vs. RHE) in N2-saturated 0.1 M KOH at five different temperatures, 27 C, 30 C, 35 C, 40 C, 45 C. Figure 16 shows the change of background current under different temperatures. As temperature increases Pt oxidation and reduction peaks at 0.8 V slightly decreases but the classical Pt features still remains. Also, as shown in Table 6, there is a decrease in ECSA as temperature increases, which indicates a decrease of Pt active area. This could result from the impact of heat that changes the surface of the catalyst. However, the ECSA still stay within the reasonable range, meaning even with the slight change, the catalyst still function the same. 42

55 Figure 17. ORR current of PtCu/C in 0.1 M KOH in at different temperatures Right after the background current was collected, the ORR current was obtained at each temperature in O2-satruated solution, keeping all other conditions the same. Figure 17 shows the change of ORR current under different temperatures (27 C, 30 C, 35 C, 40 C, 45 C). Three regions, the kinetic controlled region (> 0.85 V), combined kineticdiffusion control (0.6 V < E <0.85 V) and mass transport region (< 0.6 V) have been shown. As shown in the figure, with the increase of temperature, the ORR current gradually increases as well. For instance, the ORR current at 0.9 V increases by 0.05 ma/cm 2 Pt area (~ 9% increase). 43

56 Derivation of kinetic current of ORR Figure 18. Itotal of PtCu/C in 0.1 M KOH in at different temperatures After background current and ORR current have been obtained at each temperature, Itotal has been calculated by subtracting background current from ORR current to remove background contribution. Figure 18 shows Itotal of PtCu/C at different temperatures. As temperature increases, Itotal also increases, indicating a higher ORR activity. 44

57 Figure 19. Kinetic current of PtCu/C in 0.1 M KOH in at different temperatures Kinetic ORR activity (Ik) was calculated for the anodic sweep polarization curve via the equation, Ik = specific activity = Ilim Itotal Ilim Itotal Ik Pt ESCA. Pt specific activities were determined from Pt-ECSA data as. Figure 19 shows kinetic current (Ik) of PtCu/C at different temperatures. As temperature increases, Ik also increases, indicating a higher ORR activity. 45