Heteroatoms doped graphene, non-precious and precious metal catalyst, oxygen and hydrogen evolution reaction, alkaline water electrolyzer
With the ever-growing energy demand, fossil fuel consumption, and their adverse impact on the environment over the world, the advanced research complying to develop clean energy resources as an ideal alternative to fossil fuels. Electrochemical water (H2O) conversion into hydrogen and oxygen gas (2H2O 2H2 + O2), a nonfossil fuel-based technology has gained great attention around the world scientific community due to its potentials replacement of traditional fossil fuel. The water electrolysis involving two half cell reactions in the electrolyzer, one is a cathodic reaction known as hydrogen evolution reaction (HER) and another one is an anodic reaction, which known as oxygen evolution reaction (OER). However, their commercialization has been hindered by high cost and low durability of best noble metal-based catalysts such as Pt/C for HER and IrO2/RuO2 for OER in the water electrolyzer. The major drawbacks of these catalysts are rear availability, the tendency of particle agglomeration and oxidization in the oxidizing environment leads to poor activity and stability. Therefore, significant ongoing research concentrated on the developing of non-noble metal-based catalysts or reducing the noble metal usage with non-noble metal/non-metal catalysts by improving intrinsic activity and stability. To address these issues, we have carried out the research to develop new kinds of cata-lyst supports and nonprecious metal/minimal amount of precious metal-based catalysts for improving the performance and durability of HER and OER catalysts in alkaline water electrolyzer. Our advanced research acknowledges that there is still plenty of scope for the development and improvement of HER and OER elec-trocatalysts.
The work detailed in this dissertation is divided into five parts, in which the first chapter aims at providing the general introduction of electrochemical water splitting as well as the motivation, which focused on our interest in the perspective of scientific challenge. In addition, we elaborate on the reaction pathway of HER and OER in the different electrolytes with a general overview of catalyst development. The second chapter deals with the physical and electrochemical characterization techniques.
In chapter three, we present the first report on the synthesis of a new kind of conductive, robust honeycomb structured few-layer S, N-doped crystalline graphene from S-doped carbon nitride (SCN) to support the mini-mal amount of Rh clusters and its HER electrocatalytic properties in alkaline media. The introduction of ul-tra-small Rh clusters increases the active sites and the synergistic effects between Rh and S, N elements on the graphene framework. The interaction between heteroatoms (S, N) and Rh can be modulated the electron-ic structure of Rh, which increases the active sites for H adsorption and avoid the aggregation of catalyst re-sulting in favor of outstanding HER performance.
In Chapter four, ultra-small NiMo nanoparticles anchored N-doped graphene electrocatalyst is prepared for HER in alkaline media, where N-doped graphene is synthesized from nonconductive graphitic carbon nitride (CN). The optimized catalyst shows excellent HER activity and stability in alkaline media. A variety of char-acterization techniques suggest that the catalytically active sites of this unique catalyst are associated with the particle size, metal centers, and the metal coordinated to the nitrogen within the graphene framework.
Lastly, in Chapter five, we have rationally designed new kinds of cheap transitional metal (M = Co, Ni) single atom coordinated N doped graphene as electrocatalyst for OER in alkaline medium. Metal single atom coor-dinated N-doped graphene is prepared by the pyrolysis of [M(EDTA)]2- complex. It is observed from ultra-high-resolution transmission electron microscopy (UHR-TEM) that single metal atom distributed over N-doped graphene. The metal single-atom coordinated with N over graphene and act as the active sites for OER. The metal single-atom coordinated N on graphene electrocatalysts showed excellent electrocatalytic OER activity, which is better than of commercial OER catalyst. Excellent electrocatalytic OER activity and stability is mainly originated from the active sites of single atom coordinated N within graphene framework.
Table Of Contents
List of Contents
Abstract i List of contents iii List of Figures v
I. Chapter 1. Introduction 8 1.1 Motivation 8 1.2 Hydrogen evolution reaction 9 1.3 Oxygen evolution reaction 11 1.4 Scope 13 1.5 References 14 II. Chapter 2. Characterization technique 18 2.1 Materials characterization 18 2.2 Electrochemical characterization 18 2.3 References 19 III. Chapter 3. S-doped carbon nitride driven honeycomb structure S, N-doped crystalline graphene framework to support Rh clusters: a new benchmark electrocatalyst for hydrogen evolution reaction 21 3.1 Introduction 21 3.2 Experimental 23 3.3 Results and discussion 25 3.4 Conclusion 41 3.5 References 42 IV. Chapter 4. Graphitic carbon nitride driven N-doped graphene supported NiMo nanoparticles electrocatalyst for hydrogen evolution reaction 47 4.1 Introduction 47 4.2 Experimental 49 4.3 Results and discussion 51 4.4 Conclusion 63 4.5 References 63 V. Chapter 5. Transitional metal (M=Co, Ni) single atom coordinated N-doped graphene electrocatalyst for oxygen evolution reaction in alkaline water electrolysis 67 5.1 Introduction 67 5.2 Experimental 68 5.3 Results and discussion 69 5.4 Conclusion 80 5.5 References 80
List of Figures Figure 3-1 (a) Schematic illustration of the synthesis process of Rh anchored S, N doped graphene. (b) XRD patterns of SCN, SCNMg-650, SCNMg-750, SCNMg-850, and SCNMg-950. (c) XRD patterns of SCNMg-850-ASP. (d) Raman spectrum, (e) N2 adsorption-desorption isotherm, and (f) conductivity plot of SCN, SCNMg-650, SCNMg-750, SCNMg-850, and SCNMg-950 26 Figure 3-2 SEM and TEM image of (a, f) SCN, (b, g) SCNMg-650, (c, h) SCNMg-750, (d, i) SCNMg-850, and (e, j) SCNMg-950 29 Figure 3-3 XPS spectrum of (a) C 1s, (b) N 1s, (c) S 2p, (d) O1s of SCN and SCNMg-850. Proposed model of (e) SCN, and (f) S, N doped graphene. (g) HER polarization curve, and (h) Nyquist plots of SCN, SCNMg-650, SCNMg-750, SCNMg-850, and SCNMg-950 31 Figure 3-4 (a) XRD pattern of SCNMg-850-73Rh-500, SCNMg-850-36Rh-500, SCNMg-850-18Rh-500. (b) TEM of SCNMg-850-36Rh-500. (c) HAADF-STEM image of SCNMg-850-36Rh-500. (d-f) HR-TEM image of SCNMg-850-36Rh-500. (g) EDS mapping of SCNMg-850-36Rh-500. TEM image of (h) SCNMg-850-18Rh-500, (i) SCNMg-850-36Rh-500 33 Figure 3-5 (a) LSV polarization curves of SCNMg-850-18Rh-500, SCNMg-850-36Rh-500, SCNMg-850-73Rh-500, 20% and 46% commercial Pt/C. (b) TGA plot of SCNMg-850-36Rh-500, VC-36Rh-500, rGO-36Rh-500, and SCN-36Rh-500. (c) EIS Nyquist plots of SCNMg-850-18Rh-500, SCNMg-850-36Rh-500, SCNMg-850-73Rh-500, 20% and 46% commercial Pt/C. (d) LSV polarization curves of SCNMg-850-36Rh-500, SCNMg-850-36Rh-700, SCNMg-850-36Rh-500, 20% and 46% commercial Pt/C. TEM image of (e) SCNMg-850-36Rh-500, (f) SCNMg-850-36Rh-700, and (g) SCNMg-850-36Rh-900 35 Figure 3-6 (a) LSV polarization curves of SCNMg-850-36Rh-500, rGO-36Rh-500, VC-36Rh-500, SCN-36Rh-500, 20% and 46% commercial Pt/C. TEM image of (b) VC-36Rh-500, (c) rGO-36Rh-500, and (d) SCN-36Rh-500. (e) EIS Nyquist plots of SCNMg-850-36Rh-500, rGO-36Rh-500, VC-36Rh-500, SCN-36Rh-500, 20% and 46% commercial Pt/C. (f) Tafel’s plots of SCNMg-850-36Rh-500, rGO-36Rh-500, VC-36Rh-500, SCN-36Rh-500, and 46% commercial Pt/C. (g) CV curve of SCNMg-850-36Rh-500 and 46% Pt/C at the potential window of 0-1.2V (vs RHE) 37 Figure 3-7 (a) Chronoamperometric response at a constant potential for SCNMg-850-36Rh-500. (b) HAADF-STEM image of SCNMg-850-36Rh-500 after 500 hours stability test. (c) HR-TEM image of SCNMg-850-36Rh-500 after 500 hours stability test. (i) Energy dispersion spectroscopy mapping of SCNMg-850-36Rh-500-after stability of carbon (green), sulfur (yellow), nitrogen (blue), rhodium (red), and oxygen (orange), scale bar, 10nm 38 Figure 3-8 XPS spectrum of (a) N 1s, (b) S 2p (c) C 1s, (d) O 1s 0f SCNMg-850-36Rh-500 and SCNMg-850. (e) XPS spectra Rh 3d of SCNMg-850-36Rh-500 40 Figure 4-1 (a) Schematic diagram of the synthesis method. (b) XRD pattern of CN, CN-650Ar/H2, CNMg-650-B, and CNMg-650. (c) XRD pattern of CN, CNMg-650, CNMg-750, CNMg-850, and CNMg-950. (d) Raman spectrum of CN, CN-650Ar/H2, CNMg-650, CNMg-750, CNMg-850, and CNMg-950 52 Figure 4-2 SEM and TEM image of (a,f) CN, (b,g) CNMg-650, (c,h) CNMg-750, (d,i) CNMg-850, and (e,j) CNMg-950 53 Figure 4-3 (a) N2 adsorption-desorption isotherm, (b) pores size distribution, (c) electrical conductivity against pressure plot of CN, CNMg-650Ar/H2, CNMg-650, CNMg-750, CNMg-850, CNMg-950. (d) FTIR spectrum of CN, CNMg-650Ar/H2, CNMg-650 54 Figure 4-4 XPS spectrum of (a) C 1s, (b) N1s, (c) O 1s of CN and CNMg-650 56 Figure 4-5 (a) TEM image of 1Ni2Mo nanoparticles. (b) EDS mapping of 1Ni2Mo nanoparticles. (c) EDS spectrum of 1Ni2Mo nanoparticles. (d) XRD pattern of CNMg-650-1Ni2Mo. (e) TEM image of CNMg-650-1Ni2Mo. (f) TGA curve of CNMg-650-1Ni2Mo. (g) TEM image of CNMg-650-1Ni1Mo. (h) TEM image of CNMg-650-2Ni1Mo 58 Figure 4-6 (a) LSV polarization curve, (b) Tafel’s plot of CNMg-650-1Ni2Mo, CNMg-650-1Ni1Mo, CNMg-650-2Ni1Mo, 20% Pt/C. (c) Nyquist plot of CNMg-650-1Ni2Mo, CNMg-650-1Ni1Mo, CNMg-650-2Ni1Mo, 20% Pt/C, and GC. (d) LSV polarization curve, (e) Tafels plot, (f) Nyquist plot of CN-1Ni2Mo, CN-650Ar/H2-1Ni2Mo, C-1Ni2Mo, rGO-1Ni2Mo, and CNMg-650-1Ni2Mo. (g) Time vs current density plot of CNMg-650-1Ni2Mo 60 Figure 4-7 XPS spectrum of (a) Ni2p, (b) Mo 3d of CNMg-650-1Ni2Mo, (c) N 1s, (d) C1s, and (e) O1s of CNMg-650 and CNMg-650-1Ni2Mo 62 Figure 5-1 (a) Schematic diagram for the synthesis of EDMX-750-Ar/H2. (b) XRD pattern of EDCo0.5-750-B, EDCo0.5-750, EDNi0.5-750-B, EDNi0.5-750. TEM image of (c) EDCo0.5-750-B, (d) EDNi0.5-750-B. XRD pattern of (e) EDCo0.5-750, EDCo1-750, EDCo2-750, EDCo3-750. (f) EDNi0.5-750, EDNi1-750, EDNi2-750, EDNi3-750 70 Figure 5-2 SEM and TEM image of (a, e) EDCo0.5-750, (b, f) EDCo1-750, (c, g) EDCo2-750, (d, h) EDCo3-750, (i, m) EDNi0.5-750, (j, n) EDNi1-750, (k, o) EDNi2-750, (l, p) EDNi3-750 71 Figure 5-3 Raman spectrum of (a) EDCoX-750 (b) EDNiX-750. N2 adsorption-desorption isotherm of (c) EDCoX-750, (d) EDNiX-750. Pore size distribution plot of (e) EDCoX-750, (f) EDNiX-750 72 Figure 5-4 (a) OER polarization curve of EDCoX-750, RuO2 (b) OER polarization curve of EDNiX-750, RuO2 74 Figure 5-5 (a) TEM image of EDCo2-750-Ar/H2. (b) HADDF-STEM image of EDCo2-750-Ar/H2. (c) EDS mapping of EDCo2-750Ar/H2. (d) TEM image of EDNi1-750-Ar/H2. (e) HADDF-STEM image of EDNi1-750-Ar/H2. (c) EDS mapping of EDNi1-750Ar/H2 75 Figure 5-6 (a) N2 adsorption-desorption isotherm, (b) pore size distribution of EDCo2-750-Ar/H2, (c) N2 adsorption-desorption isotherm, (b) pore size distribution of EDNi1-750-Ar/H2 76 Figure 5-7 (a) OER polarization curve of EDCo2-750, EDCo2-750-Ar/H2, RuO2. (b) OER polarization curve of EDNi1-750, EDNi1-750-Ar/H2, RuO2. (c) TGA plot of EDCo2-750-Ar/H2 and EDNi1-750-Ar/H2 77 Figure 5-8 XPS spectrum of (a) C1s, (b) N 1s, (c) O 1s, (d) Co 2p of EDCo2-750-Ar/H2. XPS spectrum of (e) C1s, (f) N 1s, (g) O 1s, (h) Ni 2p of EDNi1-750-Ar/H2 78