CuZn Alloy- Based Electrocatalyst for CO 2 Reduction. Thesis by. Amira Alazmi

Transcription

1 CuZn Alloy- Based Electrocatalyst for CO 2 Reduction Thesis by Amira Alazmi In Partial Fulfillment of the Requirements For the Degree of Master of Science King Abdullah University of Science and Technology Thuwal, Kingdom of Saudi Arabia June 2014

2 2 EXAMINATION COMMITTEE APPROVALS FORM The thesis of Amira Alazmi is approved by the examination committee. Committee Chairperson Kazuhiro Takanabe Committee Member Niveen Khashab Committee Member Nikolaos Hadjichristidis

3 3 June 2014 Amira Alazmi

4 4 All Rights Reserved ABSTRACT CuZn Alloy- Based Electrocatalyst for CO 2 Reduction Amira Alazmi Carbon dioxide (CO 2 ) is one of the major greenhouse gases and its emission is a significant threat to global economy and sustainability. Efficient CO 2 conversion leads to utilization of CO 2 as a carbon feedstock, but activating the most stable carbon- based molecule, CO 2, is a challenging task. Electrochemical conversion of CO 2 is considered to be the beneficial approach to generate carbon- containing fuels directly from CO 2, especially when the electronic energy is derived from renewable energies, such as solar, wind, geo- thermal and tidal. To achieve this goal, the development of an efficient electrocatalyst for CO 2 reduction is essential. In this thesis, studies on CuZn alloys with heat treatments at different temperatures have been evaluated as electrocatalysts for CO 2 reduction. It was found that the catalytic activity of these electrodes was strongly dependent on the thermal oxidation temperature before their use for electrochemical measurements. The polycrystalline CuZn electrode without thermal treatment shows the Faradaic efficiency for CO formation of only 30% at applied potential ~ 1.0 V vs. RHE with current density of ~ 2.55 ma cm 2. In contrast, the reduction of oxide- based CuZn alloy electrode exhibits 65% Faradaic efficiency for CO at lower applied potential about 1.0 V vs. RHE with current density of 2.55 ma cm 2. Furthermore, stable activity was achieved over several hours of the reduction reaction at the modified electrodes. Based

5 5 on electrokinetic studies, this improvement could be attributed to further stabilization of the CO 2 on the oxide- based Cu- Zn alloy surface.

6 6 ACKNOWLEDGEMENTS In the name of Allah, Most Gracious, Most Merciful I thank all who in one way or another contributed in the completion of this thesis. First, I give thanks to Allah for protection and ability to do work. I am so grateful to King Abdullah University of Science and Technology (KAUST) for making it possible for me to study here. I would like to acknowledge the help and support of a number of people who have helped and supported me during my study. I would like to express my sincere gratitude to my advisor, Professor Kazuhiro Takanabe, for providing invaluable instruction and mentoring in completing this thesis. I am truly thankful for his suggestions, criticisms throughout the courses of my study. The time I spent in his lab provide invaluable development of both my personal and academic skills. I want to extend my thanks to my friends and colleagues for the great time I had in our group. I would also like to thank Dr. Shahid Rasul, for his cooperation, support, guidance and advice during my research, is greatly appreciated. Dr. Weili Yu, I thank you for your help in my first experiments in the lab and for your fruitful suggestions. I would like to thank my thesis committee: Prof. Niveen Khashab, and Prof. Nikolaos Hadjichristidis, for their encouragement, insightful comments. I would also like to acknowledge the University of Dammam for their support. To Mr. Marcos Bracchitta, thank you for modeling great teaching and for furthering my thinking about identity and learning. I would like to thank Nawal Al Hajri, who as a good friend was always willing to help and give her best suggestions. It would have been a lonely lab without her. I think of her as a sister. A

7 7 special acknowledgement goes to my close friend Haneen Omar, thank you for listening, offering me advice, and supporting me throughout my study. Haneen is an amazing person in too many ways. I would also like to express sincere acknowledgements to Mr. Abdulaziz Faden and his lovely family. There are no words to convey how much I thank them. Abdulaziz has been a true and great supporter and has unconditionally encouraged me during my good and bad times. He has been instrumental in instilling confidence on me. I also thank my family especially my mother, Kareema, brother, Sultan, and sisters, Abir and Afkar, who encouraged me and prayed for me throughout the time of my research.

8 8 TABLE OF CONTENTS Page EXAMINATION COMMITTEE APPROVALS FORM... 2 ABSTRACT... 4 ACKNOWLEDGEMENTS... 6 TABLE OF CONTENTS... 8 LIST OF FIGURES LIST OF TABLES CHAPTER 1 INTRDUCTION Introduction Challenges in energy and climate change Electrochemical reduction of CO Fundamental challenges for electrochemical reduction of CO Classification of electrocatalytic metal and reaction selectivity Objectives of the work CHAPTER 2 EXPERIMENTAL Fabrication of electrodes Electrochemical reduction reaction and electrode characterization Problems related with experimental procedures CHAPTER 3 RESULTS AND DISCUSSION Structural and morphology of the CuZn- based electrodes X- Ray photoelectron spectroscopy (XPS) Electrocatalytic performance Effects of formic acid Long- term test for electrocatalytic CO 2 reduction... 38

9 9 3.6 Effects of oxidation temperature Effects of applied potential on Faradaic efficiency Comparison with Zn- based electrode CHAPTER 4 CONCLUSIONS REFERENCES... 50

10 10 LIST OF FIGURES Figure 1 Pourbaix diagram for carbon dioxide reduction reaction at 25 C Figure 2 Product distribution as a function of potential obtained by Hori and co- workers Figure 3 Scheme of electrode preparation (a), Scheme of the electrochemical cell used for electrochemical characterization of the polycrystalline CuZn alloy and several thermally oxidized electrodes (b) Figure 4 X- ray diffraction (XRD) patterns of polycrystalline CuZn alloy (a), thermally oxidized electrode at 800 C for 2h as prepared (b), after the reduction (b) Figure 5 SEM images of polycrystalline CuZn alloy before the reduction (a) after the reduction (b) Figure 6 SEM images of (a) and (b), polycrystalline CuZn;(c) and (d) CuZn oxide/ 800 C before the reduction ;(e) and (f) CuZn oxide/800 C after reduction Figure 7 (SEM/EDS) images of (a), CuZn oxide/800 C before reduction; (b), CuZn oxide/800 C after reduction Figure 8 XPS spectra of the three electrodes (polycrystalline CuZn alloy, thermally oxidized electrode at 800 C for 2h as prepared and after the reduction): (a) Cu (2p) peak, (b) Zn (2p) peak (c) O (1s) signal Figure 9 CPE and Faradaic efficiency profiles of CuZn alloy (a), CuZn oxide/500 C (b), CuZn oxide/600 C (c), CuZn oxide/700 C (d), CuZn oxide/800 C (e) and CuZn oxide/900 C (f) electrodes in 0.1 M KHCO 3, ph 6.8 for 1h Figure 10 CPE and Faradaic efficiency profiles of CuZn alloy (a) and CuZn alloy/hcooh (b) electrodes in 0.1 M KHCO 3, ph 6.8 for 1h Figure 11 Total current density vs time of CuZn alloy (a), CuZn oxide/500 C (b), CuZn oxide/600 C (c), CuZn oxide/700 C (d), CuZn oxide/800 C (e) and CuZn oxide/900 C (f) electrodes at various potentials between 0.6 and 1.0 V vs RHE in 0.1 M KHCO 3, ph Figure 12 Faradaic efficiency profiles vs potential of polycrystalline CuZn alloy (a), Oxidized CuZn / 500 C (b), Oxidized CuZn / 600 C (c), Oxidized CuZn / 700 C (d), Oxidized CuZn / 800 C (e) and Oxidized CuZn / 900 C (f) electrodes for one hour reduction in 0.1 M KHCO 3, ph

11 11 Figure 13 Faradaic efficiency profiles vs different air oxidation temperature of CuZn alloy electrodes at various potentials between 0.6 and 1.0 V vs RHE (a), 0.6 V vs RHE; (b), 0.7 V vs RHE; (c), 0.8 V vs RHE; (d), 0.9 V vs RHE; (e), 1.0 V vs RHE in 0.1 M KHCO 3, ph Figure 14 CO 2 reduction electrolysis data at V vs RHE for polycrystalline CuZn and several annealed electrodes. (a f) Total current density vs time, Faradaic efficiency for CO vs time and total FE for HCO 2 H for electrodes Figure 15 Comparison of the electrocatalytic activities of polycrystalline CuZn and CuZn annealed at 800 C for 2 h Figure 16 CPE and Faradaic efficiency profiles of Zn (a) and Zn oxide/400 C (b) electrodes in 0.1 M KHCO 3, ph 6.8 for 1h

12 12 LIST OF TABLES Table 1 Product distribution for the CO 2 electrochemical reduction on different metal Table 2 Faradaic efficiency (%) and total current density for the CO 2 reduction products

13 13 CHAPTER 1: INTRODUCTION 1.1 Introduction Challenges in energy and climate change Concerns about global warming suggest moving from fossil fuel to environmentally friendly sources of energy. 1 As a result, a large number of researches have focused on the development of energy sources in current years. 2 Human activities (e.g. combustion of fossil fuels, biomass burning) have increased carbon dioxide emissions into the atmosphere. Carbon dioxide (CO 2 ) is one of the major contributors to the greenhouse gas emissions which can be utilized as a free source of carbon and energy. Moreover, the significant accumulation of CO 2 in the atmosphere has linked to the large increase in emissions over the past couple centuries. Allowed further and continued rise of greenhouse gases will lead to an increase in global temperatures and considerable changes in global climate. 3-6 Therefore, there are many problems that will be raised in different areas for example; human health, agriculture, natural ecosystems, and coastal areas. Hence, the pressure on countries and scientists have been increased to reduce CO 2 emissions and improve CO 2 utilization and capture systems. 7 The reduction of carbon dioxide emissions is a long- term and complex mission. It is known that there are three potential strategies to address this issue; first, storage of CO 2 ; second, usage of CO 2 ; and third, conversion of CO 2 produced To apply the first strategy, a move from fossil fuels toward less carbon intensive energy sources such as hydrogen and renewable energy needs to be developed. 9, 11 A well- established process

14 14 as the storage of CO 2 demands the development of new technologies for the capture and sequestration of CO 2. 8, 9, 11 In fact, carbon dioxide conversion and utilization appears to be the finest approach to solve all of these problems. Carbon dioxide conversion has many potential approaches to be reached like chemical approaches 12-21, photocatalytic 19, and electrocatalytic reduction Electrochemical conversion of CO 2 The process of nature is used by plants converting carbon dioxide (CO 2 ) and H 2 O by solar energy as a driving force to store chemical energy in alcohols and hydrocarbons. Electrochemically generated electricity from renewable sources is used for electro- reduction of CO 2 which can be considered as artificial photosynthesis. 31 Carbon dioxide can be electrochemically converted into beneficial products under minor conditions Electrolyzes allows electro- reduction of CO 2 by applying a potential difference between two electrodes. At the anode, protons and electrons released when the water oxidation occurs at enough high potential difference. At the cathode, CO 2 and protons are reduced into hydrogen and hydrocarbons. Carbone dioxide conversion field have been extensively studied from the early 1900 s. Metal electrodes have received attention for example amalgamated copper, amalgamated zinc, mercury and the main product was formic acid. 30, In 1985, Frese et al. showed CO 2 reduction at ruthenium electrode led to interesting and promising results in this field. 40 Metallic copper was discovered as a capable electrocatalyst to produce hydrocarbons from reducing carbon dioxide. 22 Figure 1 shows the equilibrium reduction potentials with respect to ph showing on the Pourbaix diagram. Excellent

15 15 1, 11, 41, 42 reviews are available and provided better understanding for CO 2 reduction. Several organic products can be obtained by reducing carbon dioxide electrochemically with high Faradaic efficiency such as formic acid, carbon monoxide, 46 47, 48 methane, and ethylene. 39 Recently, there are a great number of publications which focused on studying the efficiency and the selectivity by different catalysts either in aqueous 49, 50 or 51, 52 non- aqueous media. Figure 1: Pourbaix diagram for carbon dioxide reduction reaction at 25 C Fundamental challenges for electrochemical reduction of CO 2 Carbon dioxide has become an attractive C 1 building block for making organic chemicals, materials, and carbohydrates (e.g., foods) as an economical, safe and renewable carbon source. 2 Production of chemicals by utilizing CO 2 as a feedstock provides a huge challenge in new concepts and opportunities for catalytic and industrial development. 54 Indeed, the extreme thermodynamic stability of carbon dioxide exhibits difficulty in the chemical activation of carbon dioxide. 11, 55 Electrical energy input is

16 16 required for the carbon dioxide reduction reaction. This is due to the positive Gibbs free energy of CO 2 reduction in medium and high ph range and the theoretical potentials are negative. 56 References to kinetic, electrochemical reduction of carbon dioxide always occurs at overpotential greater than 1.0 V to obtain reasonable amounts of fuels, such as methane, ethylene, etc. 56 On carbon dioxide reduction reaction in an aqueous electrolyte, the major by- product will be hydrogen. 56 It has been reported that one possible way to repress hydrogen evolution is by using high hydrogen overvoltage metals such as Hg. Metals like Hg produce formate ions (HCOO ) at very high overpotentials. 56 Thus, the development of electrocatalysts for the fixation of CO 2 to fuel is highly required finding solutions for this scientific problem requires a good understanding of the chemistry of CO 2 activation This is so because breaking the C- O bond and forming C- H and C- C bonds is needed such as multifunctional catalysts by 56, 60 using electricity. Depending on reaction conditions and the catalyst different reactions occur at the cathode. Carbon dioxide reduction reactions with standard potentials E 0 are listed below. 30 2CO H e à C 2 H 4 + 4H 2 O CO 2 + 8H + + 8e à CH 4 + 2H 2 O CO 2 + 6H + + 6e à CH 3 OH + 2H 2 O CO 2 + 2H + + 2e à CO + H 2 O CO 2 + H + + 2e à HCOO 2H + + 2e à H 2 E 0 = V vs. RHE E 0 = V vs. RHE E 0 = V vs. RHE E 0 = V vs. RHE E 0 = V vs. RHE E 0 = 0.0 V vs. RHE

17 17 At the anode oxygen produced by oxidized the water: 2H 2 O à O 2 +4H + + 4e Assembly of the nuclei and formation of chemical bonds is the fundamental problems of the conversion of CO 2 into fuels. 56 Converting comparatively simple molecule as carbon dioxide into further energetic and complex molecules is complicated. CO 2 conversion is already a difficult process even for the single electron reduction of CO 2 to CO 2 as a first step to activate CO 2 for subsequent reduction steps. 56 Thermodynamically hydrogen and CO 2 products are formed at similar potentials. In practice, however, reduction of CO 2 occurs at potentials below 0.0 V vs RHE and it is accompanied by the formation of hydrogen. Large overpotential is required for hydrocarbon formation as it s a complex process which includes several electron transfers and intermediates. One of the fundamental challenges of CO 2 reduction is the products selectivity. 61,62 There are many influences, such as temperature, electrocatalyst 23, 63 material, concentration of the reactants, electrode potential, and electrolyte solution. 63 When electroreduction conditions are comparable, the electrocatalyst materials rule the selectivity of CO 2 reduction. Achieving and understanding the high selectivity of CO 2 conversion at low overpotentials is the goal of CO 2 electrochemical reduction study. Homogenous catalyst have used by dissolving a transition metals complex in the electrolyte or reaction electrode made of conductive material can be used as a heterogeneous catalyst. 10, 66, 67 Therefore, in aqueous electrolytes hydrogen evolution reaction (HER) takes place by cathodic polarization. In comparison, carbon

18 18 dioxide concentration drastically decreases in a basic solution, while HER is prevalent in acidic solutions. Consequently, neutral electrolyte solutions (i.e M NaHCO 3 ) has been used for the most of CO 2 reduction study Classification of electrocatalytic metals and reaction selectivity A set of excellent research on CO 2 reduction by Hori s group found that it is possible to divide the electrocatalytic metals based on product selectivity into four groups, as shown in Table 1. 22, The classification was found by using several metals for CO 2 electro- reduction in 0.5 M KHCO 3 aqueous solution at a fixed current of 5 ma / cm 2. Different potentials were required, depends on catalyst activity, to obtain 5 ma/cm Table 1: Product distribution for the CO 2 electrochemical reduction on different metal (Adapted from ref. 30).

19 19 In the first group the main product is formate, because of high hydrogen overpotential and the negligible CO adsorption properties. Thus make it not active towards the CO 2 electroreduction. In the second group (Au, Ag, Zn, Pd, and Ga), CO is formed as major product, as they have medium hydrogen overvoltages and weak CO adsorption properties. Hence, ability to catalyze the breakage of the C- O bond in CO 2 but no forward reduction due to it is allowed the CO desorption. Third group include high active catalysts for hydrogen evolution reaction (HER). Since, they have low hydrogen overvoltages the main reaction is reduction of water to H 2. In addition, they have strong CO adsorption properties. Fourth group, copper metal is the only metallic catalyst for CO 2 reduction. The ability of the further reduction of CO 2 makes copper unique catalyst for this reaction. 30 By using copper metal catalyst for CO 2 conversion the products are mixture of several products and occurred at a high overpotential. Indeed, the understanding of how to control the selectivity of CO 2 reduction to produce useful 53, fuels like methanol and ethylene is not completed yet. Polycrystalline metals electrodes have been tested in aqueous solutions for CO 2 electroreduction over many years. 30 Hori and co- workers have studied the product distribution as a function of potential on copper electrocatalyst for the CO 2 conversion reaction for the first time. 77 From Figure 2 the CO 2 starting to reduce at higher over potentials than 0.5 V vs. RHE and only hydrogen was detected at lower potentials. At potential ~ 0.6 V vs. RHE CO and HCOO started to be formed and at around 0.8 V vs. RHE The highest efficiency was achieved, which corresponds to an overpotential of about 0.9 V. Ethylene and methane, hydrocarbons formation,starts form at 0.7 V. The

20 20 predominant hydrocarbon such as ethylene form between 0.7 and 0.9 V. Indeed, high yields of hydrocarbons requird overpotential ~ 1.0 V,which constitute an important energy loss. Figure 2: Product distribution as a function of potential obtained by Hori and co- workers (Figure adapted from 81 data from 77). Copper based alloys (Cu- Ni, Cu- Sn, Cu- Pb, Cu- Zn and Cu- Cd) have been studied by Watanabe et al. and they found that the major products are the CO and HCO 2 H on CuZn alloy electrodes. 78 In general, CuZn alloy surface is catalytically active for CO 2 reduction. 78 The large overpotential for CO 2 reduction at Cu- Zu alloy surface is may be due to the poorly stabilization of the CO 2 at Cu- Zu alloy surface. It is known that many 30, 53, 79, 80 metal electrodes have the same mechanistic scenario. As discussed in detail in this chapter, many of the interwoven problems in energy can be addressed by developing efficient electrochemical materials for conversion of CO 2 to useful products. A key technological fact of the electrochemical conversion of

21 21 CO 2 is to use renewable electricity to convert CO 2 into products such as formic acid, methanol and CO (or syngas). In general, the electrochemical conversion of CO 2 presents a number of challenges for catalyst development. Poor selectivity, high overpotential requirement, and quick loss of activity in favor of H 2 O reduction are the three potential problems polycrystalline metal electrodes usually suffer in aqueous solutions. 27, Investigators have identified several potential catalysts that are able to reduce CO 2 electrochemically in aqueous electrolytes. 25, 27, In fact, methanol production at oxidized Cu electrodes 87, 88 containing a Cu 2 O surface layer displayed a greater activity than native Cu electrodes, which mostly yielded CO 2 to CH 4 and C 2 H 4. 23, 71, 84, 89 Some catalysts including Ag, Au, and Zn show high selectivity towards CO. 30 It is known that electrodes derived from metal oxides such as copper and gold prepared by reducing a micron thick Cu 2 O and Au oxide films, respectively, have shown high improvement over activity and stability, with lower overpotentials. 27, 84 Nevertheless, high cost and low abundance of gold led to make it inappropriate for large- scale applications. To commercialize CO 2 reduction processes, reduction of the costs associated with catalyst composition which possesses high selectivity is demanded. The CO product can be used as feedstock in the Fischer Tropsch process, a well- known and well- characterized process that has been used in industry to produce chemicals and synthetic fuels from syngas (CO+H 2 ) for many decades. 90 By coupling the catalytic reduction of CO 2 to CO with the Fischer Tropsch process to produce synthetic

22 22 fuels and industrial chemicals, the estimated maximum reduction of atmospheric CO 2 emissions is 40%. 91 In this study, the catalytic results are given as total current densities and Faradaic efficiencies. Faradaic efficiency: It is a measure of how selective an electrocatalyst is towards a particular product and it is given by the percentage of the total charge that is used in the formation of each product. A highly selective electrocatalyst leads to high Faradaic efficiency towards a given product (close to 100%), however, it does not relate to its activity. Indeed, Faradaic efficiency is governed by the amount of electrons needed to reduce CO 2 into that specific product as well as the amount of product formed. Moreover, the Faradaic efficiency towards methane is 4 times larger than CO, as there are 8 electrons involved in the formation of methane and only 2 in the formation of CO. The observations on different thermally oxidized electrodes in this study are in agreement with Kanan's investigations CO 2 electroreduction on metal oxide electrodes in aqueous solution of. 27, 80 These investigations include testing the improvement of the electrocatalytic activity of metal oxides electrodes that can be compared with the thermally oxidized metal alloy electrodes in the current study. Kanan et al. observed that the reduction of oxide metal electrodes exhibits high selectivity and that the reactions occur at lower overpotentials for a specific current density compared to the metal electrode. These researchers also concluded that the selectivity have increased due to further stabilization of the CO 2 on oxide metal electrodes. 27, 80 In this study, the polycrystalline CuZn alloy electrode was oxidatively treated prior to CO 2 reduction in attempt to lower the overpotential and improve the selectivity. This catalytic study

23 23 shows significant effects of the oxidation treatment on the Faradaic efficiency and selectivity. Characterization of the electrode suggests the presence of the oxide species at the surface of the electrode, which in turn improved CO 2 reduction towards carbon monoxide production over hydrogen evolution. 1.2 Objectives of the work This work involves systematic studies concerning the modification and characterization of CuZn- based electrocatalyst for CO and HCO 2 H formation by CO 2 reduction. CuZn alloy has been chosen as the active electrocatalyst targeting to enhance the CO 2 reduction performance of the Cu- based material with atomically dispersed Zn in the metal. Recently, Kanan et al. has proved that the reduction of metal oxide (Au, Cu 27, 80, 84 and Sn) is beneficial route to improve CO 2 reduction activity. The thermal treatment of polycrystalline CuZn alloy electrode is thus elucidated whether further stabilization of the CO 2 can be achieved at oxide- based Cu- Zu alloy electrode. 27, 80 The effect of the presence of oxide species on the surface of the CuZn alloy during the reduction reaction on its catalytic activity is studied, by comparing five different annealing temperatures of the polycrystalline CuZn alloy. Reduction of the thermally- oxidized surface may improve the Faradaic catalytic activity of metal alloy electrodes.

24 24 CHAPTER 2: EXPERIMENTAL 2.1 Fabrication of electrodes Commercial brass sheet (70 wt% Cu - 30 wt% Zn) was cleaned with HCl 0.1 M for 20 s, ethanol, and deionized water sequentially in order to remove any impurities at the surface. The Cu- Zn alloy sheet was loaded into a ceramic substrate located in the center of a box furnace then heating temperature of (500, 600, 700, 800 and 900 C) for 2 h in ambient atmosphere (Figure 3a). 2.2 Electrochemical reduction reaction and electrode characterization A typical three electrode cell (25 ml volume) was used for electrochemical reduction of CO 2 at different potentials for one hour. An Ag/AgCl electrode saturated with KCl was selected as the reference electrode along with a Pt wire as the counter electrode. The cathode area ranges typically 2 cm 2. An aqueous electrolyte (0.1 M KHCO 3 ) is saturated with CO 2 by bubbling CO 2 gas for 1 h (298 K, ph 6.8). The CO 2 flow` rate to the cathode was set to 10 ml min 1 by a mass- flow controller which was initially calibrated for CO 2 gas. Liquid and gas phase samples are taken from the sealed reactor via a syringe septum for analyzing the reaction products. The electrocatalytic activity was examined by using Chrono- amperometric analysis. Gas chromatography (GC) detects and analyzes the gas phase products (Figure 3b). To categorize the different components of the gaseous mixture analytical technique which allows separating the gas phase products. 92 The liquid phase products were high performance liquid chromatography (HPLC). The efficiency and selectivity quantification of the catalyst

25 25 determine by the information that result from Gas chromatography and HPLC techniques. Indeed, they allowed calculating the concentration of compounds formed during the reaction. Nevertheless, it is not enough information to compare the difference of electrocatalysts performance because the amount of products also depends on the current passing through the specific area of the catalyst. The structural phase evolution of the based CuZn electrodes (untreated, thermal treated at 800 C for 2 h, and reduced electrodes) were investigated by X- ray diffraction (XRD). Scanning electron microscope with energy dispersive spectroscopy (SEM/EDS) was used to obtain backscatter images and semi- quantitative mineral chemical compositions of the electrode. Chrono- potentiometric analysis was used as an initial evaluation of the electrolytic activity and stability and X- ray photoelectron spectroscopy (XPS) determined the chemical stat of the electrodes. (a) (b) Figure 3: Scheme of electrode preparation (a), Scheme of the electrochemical cell used for electrochemical characterization of the polycrystalline CuZn alloy and several thermally oxidized electrodes (b).

26 Problems related with experimental procedures There are several technical problems that must be overcome during the reduction reaction on the course of this study. In this electrochemical system, the anodic and cathodic reactions have to be separated by using a porous frit to prevent the effect of the anodic reaction at the counter electrode. The frit could be blocked by the generated gas bubble during the measurement which exhibits an error on the analysis. To solve this problem the pores have to be cleaned with vacuum after each analysis. In addition, the electrochemical system must be completely air- tight, which is crucial to measure reactions successfully. During the long- time measurement, water oxidation on the anodic side will lead to decrease the internal electrolyte. To overcome this problem an additional electrolyte need to be added to the anodic side. The additional value should be added to the volume of the electrolyte as a total volume to calculate the Faradaic efficiency for each product. Moreover, the electrode preparation has to be controlled, by keeping the electrodes cleaning time constant. Any changes will enhance inhomogeneous electrode. Another issue is associated with the convection of the electrolyte and removal of the gas formed on the surface of the electrode. This can be avoided by controlling the rotation speed of the magnetic stirrer bar.

27 27 CHAPTER 3: RESULTS AND DISCUSSIONS 3.1 Structural and morphology of the CuZn- based electrodes XRD analysis has been used to evaluate the structural phase of the polycrystalline CuZn alloy and the thermally oxidized electrode at 800 C for 2 h before and after the reduction reaction (Figure 4). The diffraction data on polycrystalline CuZn alloy shows three diffraction peaks (111), (200), and (220) (Figure 4a). In case of oxidized CuZn alloy the results suggest the presence of a ZnO (100), (002), (101), (102), (110), (103), (200), (112), and (201) orientations for the thermally oxidized electrode at 800 C for 2 h as prepared. In addition to these peaks, the peak at ~40 was assigned to CuO (Figure 4b). In order to evaluate electrode structure after the reduction reaction at various potentials (Figure 4c) illustrates that ZnO has shown (100), (002), (101), (102), (110), (103) and (112) orientations, but with lower intensity. In addition, small (200) diffraction peak of CuO was observed after the reduction reaction. Moreover, only (111) diffraction peak for cooper metal and two diffraction peaks for zinc metal with (100) and (101) orientations are observed. Thus, results indicate the presence of oxide species during and after the reduction reaction.

28 28 (c) (b) (a) Figure 4: X- ray diffraction (XRD) patterns of polycrystalline CuZn alloy (a), thermally oxidized electrode at 800 C for 2h as prepared (b), after the reduction (b). The SEM image of the polycrystalline CuZn alloy shows a smooth and flat surface (Figure 5a). The results display a negligible difference in the smoothness of the electrode after carried out CO 2 reduction reaction (Figure 5b). Hence, the electrode surface has not influenced by the reduction reaction. Figure 6 shows SEM images of polycrystalline CuZn alloy and thermally oxidized electrode at 800 C for 2 h as prepared and after the reduction. The chemical composition of the electrodes is identified by Energy- dispersive X- ray spectroscopy (EDS) (Figure 7). The results show a clear difference in the morphology of the electrodes (Figure 6). In the SEM images (Figure 6) from the low (Figure 6b) and high (Figure 6c) magnification SEM image a dense array of nanowires were stacked disorderly at the surface of the electrode. These nanowires are

29 29 mixed oxide of ZnO and CuO at the electrode surface, as evidenced by XRD pattern. Furthermore, the EDX analysis confirmed that the percentage of oxygen was 26%, Cu was 37% and Zn was 29% in the nanowires were formed by oxidizing polycrystalline CuZn alloy at 800 C for 2 h (Figure 7a). Following CO 2 reduction electrolysis, SEM indicated that the nanowires morphology have changed to ZnO flowerlike structures. Clearly, from the low (Figure 6d) and high (Figure 6f) magnification SEM image, the ZnO crystals show flowerlike structures built up by many sheets which are smooth. The sizes of these sheets are not uniform, and their lengths are several microns. As evidenced via EDS analysis on selected flowerlike structure, no Cu is detected (Figure 7b). The present of oxygen was detected by EDS after the reduction as well as XRD. The results confirmed that the electrode has a ZnO and CuO species on the surface even after the reduction over several hours which may lead to the high stability of the oxide- derived CuZn alloy at 800 C electrode. To obtain more information about the electrodes and evaluate the surface chemical state for each electrode, X- Ray Photoelectron Spectroscopy (XPS) analysis was used.

30 30 Figure 5: SEM images of polycrystalline CuZn alloy before the reduction (a) after the reduction (b).

31 31 Figure 6: SEM images of (a) and (b), polycrystalline CuZn alloy; (c) and (d), CuZn oxide/800 C before reaction; (e) and (f), CuZn oxide/800 C after reduction.

32 32 Figure 7: (SEM/EDS) images of (a), CuZn oxide/800 C before reduction; (b), CuZn oxide/800 C after reduction. 3.2 X- ray photoelectron spectroscopy (XPS) The surface chemical state of the polycrystalline CuZn alloy and thermally oxidized electrode at 800 C for 2h, and after the reduction electrodes was analyzed by XPS (Figure 8). The polycrystalline CuZn alloy surface exhibits the following two main peaks signals for copper: Cu 2p 3/2 at ev and Cu 2p 1/2 at ev, corresponding to metallic copper. 93 (Figure 8 a). These peaks were accompanied by weak shake- up satellite peaks at approximately 943 ev, which is indicative of the presence of the paramagnetic chemical state of Cu 2+ (Figure 8 a). 94 In addition, this electrode exhibits the following signals for zinc: Zn 2p 3/2 at ev and Zn 2p 1/2 at ev, corresponding to metallic zinc (Figure 5b). 95 On the other hand, thermally oxidized electrode at 800 C for 2h as prepared exhibits the following signals for copper: Cu 2p 3/2 at ev and Cu 2p 1/2 at ev, corresponding to Cu 2 O (Figure 8 a) It is known that for Cu- containing compounds, the existence of the characteristic satellite peak in the Cu- 2p spectra is indicative of Cu 2+ presence, since the d shells of Cu 0 and Cu + are filled,

33 33 therefore, cannot give rise to satellite peaks. The satellite peaks characteristic of CuO 94, 99 do not appear in its spectrum and the Cu2p 3/2 would be ev if CuO was formed. 96, 99, 100 In addition, this electrode exhibits the following signals for zinc: Zn 2p 3/2 at ev and Zn 2p 1/2 at ev, corresponding to zinc oxide, 101 with practically no metallic specie is present. Following CO 2 reduction electrolysis, XPS data shows the following signals for copper: Cu 2p 3/2 at ev and Cu 2p 1/2 at ev, corresponding to CuO (Figure 8a). 102 Moreover, a broad shoulder at ev and ev appeared in thermally oxidized electrode after the reduction, which was assigned to CuO and Cu 2 O, respectively. 97,100 In additions, this electrode exhibits the following signals for zinc: Zn 2p 3/2 at ev and Zn 2p 1/2 at ev, corresponding to zinc oxide(figure 8b). 101 The O 1 S signal detected from the surface of the polycrystalline CuZn alloy shows only one peak at ev. This implies that the surface of the polycrystalline CuZn alloy is covered with the hydroxide groups, apparently formed due to the exposure of electrodes to humidity or the presence of CuO as the satellite peaks has observed on copper spectrum in (Figure 8b) These hydroxide groups can be removed by the heat treatment of samples as the results of thermal desorption or decomposition. The O 1 S spectrum in Figure 8 shows two peaks. The peak at ev represents the O- Zn bond in ZnO The O 1s peak at ev in XPS analysis is associated with oxygen deficient, revealing that the ZnO contain many oxygen vacancies. 107 Figure 8 c shows the O 1s spectrum for the thermally oxidized electrode at 800 C after the reduction reaction. The oxygen peak, at ev, is assigned to the O 105, 108 Zn bond in the ZnO lattice (Figure 8c).

34 34 According to XRD patterns, EDS analysis, SEM images and XPS analysis, the formation of ZnO on the surface of thermally oxidized CuZn alloy at 800 C was confirmed. The surface of polycrystalline CuZn alloy has covered by ZnO, since the oxides and the diffusivity of Zn is two to five folds higher and the former is more thermodynamically stable than of Cu. Consequently, Cu- oxides forming get to be restrained. 109 Based on previous study which includes annealing of different CuZn alloy (brass) contents various concentrations of Zn. By annealing the brass at 500 C in air for 48 h they found that with increase the Zn concentrations, the CuO/Cu 2 O peaks gradually become weaker and then disappeared for the Zn 40 wt% brass. The ZnO structure began to appear from the Zn 10 wt% and dominated from Zn 40 wt% brass. Obviously, the formation of ZnO is dominant due to the lower melting point and higher vapor pressure of Zn than those of Cu under the same conditions. In this study, the 70wt% Cu- 30wt% Zn were thermally oxidized at 800 C, the ZnO layer forming were dominated whether has optimum temperature let Zn diffusion from bulk to surface. The increase of the oxidation temperature favors the Zn segregation from the bulk to surface and thus the formation of ZnO on the surface. 110 This could explain the absent of CuO on the surface of the electrode according to XPS analysis. After the reduction reaction thermally oxidize electrode surface has been shown.

35 35 (a) (b) (c) Figure 8: XPS spectra of the three electrodes (polycrystalline CuZn alloy, thermally oxidized electrode at 800 C for 2h as prepared and after the reduction): (a) Cu (2p) peak, (b) Zn (2p) peak (c) O (1s) signal. 3.3 Electrocatalytic performance To test the behavior of the electrodes, chronopotentiometric analysis was carried out at constant current ( 1.67 ma cm 2 ) in a three- electrode electrolysis cell. Figure 9 illustrates the chronopotentiograms for polycrystalline CuZn alloy and several thermally oxidized electrodes. The initial reduction potential for polycrystalline CuZn alloy electrode was 808 mv, gradually higher potential (1000 mv) was reduced after 12 min (Figure 9 a). These results suggest that it may possibly be related to a presence of

36 36 small quantities of oxide species on the electrode surface apparently formed due to the exposure of electrode to humidity or reduction of CuO, which is consistent with the XPS data (Figure 8). Initially at low overpotential the major product was H 2 at this electrode. As the overpotential increase the CO 2 conversion start to arise. On the other hand, there was no formic acid formation at this electrode. This could be due to the high overpotential required for HCOOH formation at this electrode. It is worth to mention that steady- state conditions have not reached at this course of time. Clearly, at thermally oxidized electrodes almost the same stable potential (~1000 mv) was produced. Thermally oxidized electrode at 500 C electrode for 2 h and after reached steady- state conditions Faradaic efficiency for CO was significantly increased approaching to 77%. This increasing could be attributed to the reduction of the oxide species presence on the surface of the electrode as observed from XPS data (Figure 8 c). In addition, the formation of H 2 has remarkably decreased. This result suggests that the electrons selectively reduced the CO 2 rather than H 2 formation or the mixed oxide reduction. Comparable results were obtained at the thermally oxidized electrode at 600 C and 700 C electrode (Figure 9 (b) and (c)). At thermally oxidized electrode at 800 C and 900 C the Faradaic efficiency for CO was negligibly decreased (Figure 9 (d) and (f). Overall these results have showed high Faradaic efficiency towards CO formation. Further investigation of the electrodes stability and activity chronoamperometry measurements were carried out at various potentials between 0.6 and 1.0 V vs RHE in 0.1 M KHCO 3, ph 6.8 for 1 h. Moreover, the effect of formic acid has been studied as will discuss later.

37 37 Figure 9: CPE and Faradaic efficiency profiles of CuZn alloy (a), CuZn oxide/500 C (b), CuZn oxide/600 C (c), CuZn oxide/700 C (d), CuZn oxide/800 C (e) and CuZn oxide/900 C (f) electrodes in 0.1 M KHCO 3, ph 6.8 for 1 h. 3.4 Effects of formic acid To study the effect of Formic acid during the reduction reaction 25 mm of Formic acid was added to the electrolyte. After running the CPE analysis for 1 h only 6 % decrease in Formic acid concentration was observed, which may due to oxidation of Formic acid. The Faradaic efficiency for CO formation has negligible changed. This result indicates that the presence of Formic acid will not have a significant effect if we do continually use the same electrode under different potential (under the same condition) (Figure 10).

38 38 Figure 10: CPE and Faradaic efficiency profiles of CuZn alloy (a) and CuZn alloy/hcooh (b) electrodes in 0.1 M KHCO 3, ph 6.8 for 1h. 3.5 Long- term test for electrocatalytic CO 2 reduction Further investigation of the CO 2 conversion activity and the potentials dependence of the electrodes was by applying varies potentials at polycrystalline CuZn electrode and several thermally oxidized (at different temperature from 500 C to 800 C) electrodes. Figure 11 shows the total geometric current density (j tot ) versus time at various overpotentials from 0.6 to 1.0 V vs RHE for all electrodes. In general, at low overpotentials H 2 is the main product; however, at high overpotential the conversion of CO 2 is initiated. Figure 11 shows a direct correlation between the applied potentials and the total geometric current density (j tot ). In addition, the maximum Faradaic efficiencies for formic acid were very low 11% at polycrystalline CuZn electrode and 8% for the thermally oxidized electrodes. In all chronoamperometry measurements, an initial high current density could be observed and is associated mainly with the double- layer charging. This was followed by a quick drop of the current, then the current becoming relatively stable. It could be

39 39 rationalized that at the beginning of the analysis, the active sites of the electrocatalyst electrodes were free of adsorbed CO 2 molecules, thus explaining the high current density. However, the adsorption of new CO 2 molecules is dependent on the release of the active sites from the intermediate species (Figure 10). (Figure 10 a- f) show the performance of the polycrystalline CuZn alloy and several thermally oxidized electrodes with various thermal oxidation treatment temperatures. These thermally oxidized electrodes exhibit a similar current densities with respect to each overpotential applied; however, the selectivity towards CO and HCOOH displays a dependence on overpotential. The current density showed a negligible dependence on the thermal oxidation treatment temperature of the electrodes, while the selectivity towards the formation of CO and formate exhibited a strong relationship with applied potentials on each electrode.

40 40 Figure 11: Total current density vs time of CuZn alloy (a), CuZn oxide/500 C (b), CuZn oxide/600 C (c), CuZn oxide/700 C (d), CuZn oxide/800 C (e) and CuZn oxide/900 C (f) electrodes at various potentials between 0.6 and 1.0 V vs RHE in 0.1 M KHCO 3, ph Effect of oxidation temperature Figure 12 illustrates the Faradaic efficiency for CO 2 conversion products with respect to potential for the electrodes. Clearly, the catalytic activity of these electrodes shows a strong dependence on the increase of thermal oxidation treatment temperature. At low thermal oxidation treatment temperatures the electrodes exhibit high H 2 formation (Figure 12 a, b and c). At high thermal oxidation treatment temperatures the Faradaic efficiency for CO increases (Figure 12 d, e and f). Maximum Faradaic efficiency for CO was obtained at thermally oxidized electrode at 800 C approaching to 65 % at 1.0 V vs. RHE (Figure 12 e). Notably, the Faradaic efficiency for CO has decreased at thermally oxidized electrode at 900 C approaching to 46 % at 1.0

41 41 V vs. RHE (Figure 12 f). This could be due to additional ZnO presence on the surface of the electrode. As described on the XPS analysis above the oxidation of Zn is favorable than Cu oxidation. As the temperature increase the amount of ZnO is increase. According to XPS data Cu 2 O and ZnO are present on the surface of thermally oxidized electrode at 800 C that with absent of CuO. It is worth to mention that this direct correlation between the Faradaic efficiency and thermal oxidation treatment temperature is also governed by the applied potentials. The reduction of Cu 2 O could be responsible of the improvement on electrocatalytic activity, as reported by Kanan et al. 84 Furthermore, the electrocatalytic activity has the possibility to attribute to reduction of both Cu 2 O and ZnO. Since the reduction of zinc oxide has shown electrocatalytic activity for CO 2 reduction Figure 12. Moreover, From XPS and XRD the thermal oxidized electrode at 800 C after reduction reaction has shown metallic species of copper and zinc. These results suggested that CO 2 reduction may be occur at copper or zinc metallic or both of metallic species are responsible for the catalytic activity on this study. The Faradaic efficiency towards CO formation increases slightly with increasing thermal oxidation treatment temperature, achieving the maximum of 65% for the thermally oxidized electrode at 800 C. With increasing the thermal oxidation treatment temperature, the Faradaic efficiency towards CO formation decreases to 45% in the thermally oxidized electrode at 900 C. In contrast, the Faradaic efficiency towards formate formation stabilizes around 1-3% in all electrodes.

42 42 Figure 12: Faradaic efficiency profiles vs potential of polycrystalline CuZn alloy (a), Oxidized CuZn / 500 C (b), Oxidized CuZn / 600 C (c), Oxidized CuZn / 700 C (d), Oxidized CuZn / 800 C (e) and Oxidized CuZn / 900 C (f) electrodes for one hour reduction in 0.1 M KHCO 3, ph Effects of applied potential on Faradaic efficiency It has proposed by researchers that high overpotentials are needed to convert CO 2. 79, 111 As discussed in details on the first chapter that the first step in CO 2 conversion is the formation of a CO 2 (it is whatever species forms when an electron is added to CO 2 ). Thermodynamically, The equilibrium potential for CO 2 formation is very negative in water and in most common solvents. 79, 111 Thus, it is essential to run the cathode at very high overpotential for the reaction to occur. In fact, a highly negative overpotential is required to inject an electron into CO 2 which is considered as intermediate step on

43 43 the CO 2 reaction pathways, however, these reaction pathways are quite unreasonable in terms of energy and activation. 112 Figure 13 shows the dependence of the electrodes activity on applied potentials. At low overpotentials the major product is H 2 at polycrystalline CuZn alloy electrode and the Faradaic efficiency balance is almost 100%. As the thermal oxidation treatment temperature increase, H 2 formation decrease and the Faradaic efficiency balance is very low. This could be attributed to reduction of the oxide surface of the electrodes at these potentials (Figure 13 a, b and c). On the other hand, at high overpotentials CO formation is increase and H 2 formation is decrease. The increase of Faradaic efficiency for CO indicates that high overpotentials were required for CO 2 conversion at these electrodes (Figure 13 d, and e). Table 2 shows Faradaic efficiency (%) and total current density for the CO 2 reduction products at various potentials. The best potential ( 1.0 V vs RHE) applied in term of current and Faradaic efficiency for CO was selected for further discussion. Figure 14 shows the total geometric current density (j tot ) and the Faradaic efficiency for CO as a function of time profile and the overall Faradaic efficiency for HCO 2 H for the polycrystalline CuZn alloy electrode and several thermally oxidized electrodes at 1.0 V vs RHE. The polycrystalline CuZn alloy electrode exhibits a j tot of ~ 2.55 ma/cm 2, a Faradaic efficiency for CO that increase from ~4% after 12 min from starting the electrolysis to 30% over the course of 1 h. HPLC analysis of the electrolyte at the conclusion of the experiment indicated a Faradaic efficiency of 11% for HCO 2 H; H 2

44 44 formation accounted for the remainder of the current (Figure 14a). These values are even better than the previously measured activity for CuZn alloy electrode in KHCO 3 electrolytes. 78 Thermally oxidized CuZn alloy electrodes are tested for the first time in this study for electrolysis under these conditions. The electrode thermally oxidized at 500 C for 2 h exhibits a j tot of 2.10 ma/cm 2. Subsequently; the j tot and Faradaic efficiencies were a bit smaller than those of the polycrystalline electrode (Figure 14b). At thermally oxidized CuZn at 600 C for 2 h electrode the j tot and Faradaic efficiencies were very similar to those of the polycrystalline electrode (Figure 14c). Figure 13 :Faradaic efficiency profiles vs different air oxidation temperature of CuZn alloy electrodes at various potentials between 0.6 and 1.0 V vs RHE (a), 0.6 V vs RHE; (b), 0.7 V vs RHE; (c), 0.8 V vs RHE; (d), 0.9 V vs RHE; (e), 1.0 V vs RHE in 0.1 M KHCO 3, ph 6.8. In compare to these results, the electrodes thermally oxidized CuZn at higher temperatures exhibited larger j tot values and improved CO 2 reduction Faradaic

45 45 efficiencies for CO. The electrode thermally oxidized CuZn at 700 C for 2 h exhibits an initial j tot of ~ 4.5 ma/cm 2 for 75 s as the mixed oxide surface was reduced and subsequently a stable j tot of 1.9 ma/cm 2. The Faradaic efficiency for CO was 17% during the first 20 min of electrolysis before reach to 52% over 1 h; the Faradaic efficiency for HCO 2 H on the reduced electrode was 3%. Further improvements were obtained by increasing the thermal oxidation treatment temperature. After mixed oxide surface reduction of the electrode thermally oxidized CuZn at 800 C for 2 h, j tot reached a stable value of 2.5 ma/cm 2, the Faradaic efficiency for CO reached 65% and the Faradaic efficiency for HCO 2 H was 3% (Figure 14d). Thermally oxidized CuZn at 900 C for 2 h exhibits a stable j tot of 2 ma/cm 2. This electrode produced CO with 45% Faradaic efficiency and HCO 2 H with 3% Faradaic efficiency. This decrease in Faradaic efficiency for CO could be due to additional ZnO presence on the surface of this electrode. Notably, the Faradaic efficiency for CO was maintained at 65% throughout the electrolysis, indicating not only efficient, but also stable activity for CO 2 reduction on electrode thermally oxidized at 800 C for 2 h surface. The oxide- derived CuZn at 800 C exhibits a high Faradaic efficiency 65% for CO at relatively negative potential 1.0 V vs RHE. By comparison, the polycrystalline CuZn alloy electrode Faradaic efficiency for CO was 30% at the same potential (Figure 15). On the other hand, the Faradaic efficiency for formic acid was very low on both electrodes (Figure 15).

46 46 Figure 14: CO 2 reduction electrolysis data at 1.0 V vs RHE for polycrystalline CuZn and several annealed electrodes. (a f) Total current density vs time, Faradaic efficiency for CO vs time and total FE for HCO 2 H for electrodes. Figure 15: Comparison of the electrocatalytic activities of polycrystalline CuZn and CuZn annealed at 800 C for 2 h.

47 47 Table 2 Faradaic efficiency (%) and total current density for the CO 2 reduction products 3.8 Comparison with Zn- based electrode To test the effect of the Zn and ZnO on the electrodes chronopotentiometric analysis was carried out at constant current ( 1.67 ma cm 2 ). Figure 16 shows the chronopotentiograms for polycrystalline Zn and thermally oxidized at 400 C for 2 h electrodes. Faradaic efficiencies for CO at untreated polycrystalline Zn electrode and at thermally oxidized Zn electrode at 400 C for 2 h were 40% and 46 %, respectively. The

48 48 thermally oxidized Zn / 400 C and thermally oxidized at CuZn at 900 C for 2 h electrodes exhibits identical Faradaic efficiency for CO 46 %. This may help to explain the decrease on Faradaic efficiency at the thermally oxidized CuZn at 900 C electrode which, could be attributed to further ZnO on the surface of this electrode.on the other hand, the thermally oxidized Zn electrode at 400 C for 2 h exhibits higher Faradaic efficiency 35 % for formic acid compared to 2% with thermally oxidized CuZn / 900 C.The lower formation of formic acid can be explained by the lower over potential recorded at thermally oxidized CuZn / 900 C for 2 h 1.1 V vs RHE than 1.3 V vs RHE with thermally oxidized Zn electrode at 400 C for 2 h. Figure 16: CPE and Faradaic efficiency profiles of Zn (a) and Zn oxide/400 C (b) electrodes in 0.1 M KHCO 3, ph 6.8 for 1 h.