Thesis entitled "Carbon electrode of high activity for SO2 electro-oxidation" will be held in hybrid mode on December 16, 2024 as per the following schedule
Date: December 16, 2024
Time: 10:00 AM
Venue - Seminar Room, Department of Chemical Engineering
Abstract:
Hydrogen, with its high energy density and minimal environmental impact, stands as a promising substitute for fossil fuels in various energy applications. Presently, hydrogen production primarily relies on fossil fuel-based methods, such as steam methane reforming, oil, and coal gasification, which contribute to CO2, SO2, and other harmful emissions. Alternatively, water electrolysis, powered by renewable sources like solar, wind, or nuclear energy, offers a greener pathway for hydro-gen production. However, water electrolysis faces challenges due to the energy-intensive and sluggish oxygen evolution reaction (OER) at the anode. One efficient alternative is replacing OER with the electrochemical oxidation of sulfur dioxide (SO2) and water into sulfuric acid (H2SO4). This approach finds relevance in processes like the sulfur-iodine (S – I) and hybrid sulfur (HyS) cycles for indirect water-splitting to produce hydrogen. However, the SO2 electro-oxidation is also
not a fast reaction and requires higher potentials for appreciable reaction rate, causing the use of excess energy in electric form. A fast and efficient SO2 electro-oxidation requires use of noble metal electro-catalysts namely Pt, Au and Pd. However, noble metals are prone to catalyst poisoning and their utility is limited by large cost, especially for very large scale production. Therefore, it is very attractive to develop a low cost electrode material ix with high activity for SO2 electro-oxidation, which could also be used in other industrially important applications like flue gas desulfurization and SO2 sensing.
Carbon is a widely used electrode material of low cost, wide availability, good chemical as well as electrochemical stability, and is also available in different forms like graphite, carbon black, pyrographite, activated carbon, glassy carbon, etc. Though carbon is commonly used as support or current collector for electrodes having noble metal-electrocatalyst as active material, it is one of the least active materials for SO2 electro-oxidation, precluding its use as a standalone electrode for the reaction. In the present study, one of the focuses was on simple electrochem-ical synthesis of high activity, self-standing carbon fiber paper (CFP) and highly oriented pyrolytic graphite (HOPG) electrodes for SO2 oxidation. The other focus was to reduce the peak potential, onset potential and charge transfer resistance of the CFP electrodes by optimizing their surface oxygen content via anodization in sulfuric acid and annealing in a nitrogen atmosphere and to optimize the activity
of the CFP electrodes to the level of the platinum electrodes.
In the first experimental study, our objective was to improve the activity of a commercially available HOPG sheet by a simple, two-step, electrochemical process. The first step involved the partial exfoliation of HOPG sheet in H2SO4 solution for the generation of oxygen functional groups at the electrode surface. This process not only induced structural defects but also resulted in the creation of vertically oriented graphene flakes on the surface, significantly increasing the surface area of the electrode. The second step of the electrode synthesis involved electrochemical reduction of aqueous SO2 over the partially exfoliated HOPG surface. It resulted in the formation of a sulfur layer over the electrode surface, thereby increasing the activity of the electrode towards SO2 electro-oxidation. Different process pa- x rameters, such as concentration of the electrolyte (in the first step) and reduction potential (in the second step), were optimized, and the best electrode showed an oxidation peak at a significantly lower potential (∼1.0 V) than the pristine HOPG (>1.2 V).
In the second work, our primary focus was the synthesis of a carbon-based electrode of high activity for SO2 electro-oxidation. Significant improvements in the activity of the electrode were obtained by introducing surface oxygen func-tionalities through the anodization of carbon fiber paper in sulfuric acid solution. The mechanical strength of the CFP allows it to be a self-standing electrode of high conductivity. We show that anodization of CFP in sulfuric acid leads to changes on the surface that make it highly active for SO2 electro-oxidation. Elec-trodes of varying surface oxygen content (2.90 atomic% to 25.60 atomic%) and relative proportion of different types of oxygen functional groups were obtained by changing the anodization potential and the concentration of sulfuric acid. The activity of the optimal electrode was quantified by exchange current density and charge transfer resistance, respectively. The exchange current density was about 52-times the pristine CFP electrode (2.39 μA cm– 2 compared to 0.046 μA cm– 2). The morphological and structural changes in the electrode on anodization were determined using field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD) and Raman analysis. The chemical analysis of the electrode surface, done using X-ray photoelectron spectroscopy (XPS), showed a strong cor-relation between the electrode’s activity and the ratio of the percentage of hydroxyl groups to the sum of percentages of epoxy and carbonyl groups. The electrode performed well in an accelerated stability test based on cyclic voltammetry for over 1000 cycles.
In the last study, we report a significant increase in the activity of a carbon elec-trode for SO2 electro-oxidation by treating its surface to obtain partially reduced graphite oxide (RGO). The RGO surface was formed on a self-standing CFP by a two-step process wherein the CFP was first anodized in sulfuric acid to generate surface oxygen functionalities. The anodized CFP was then annealed in the nitro- gen atmosphere to obtain the RGO surface. The structural and chemical features of the electrode surface were varied by an independent variation of anodization po-
tential and annealing temperature. The XPS analysis showed that annealing not only reduced the surface oxygen content of the electrode from ∼21 – 26 atomic% to ∼3 – 5 atomic% but also allowed tailoring of the oxygen functionalities by pref-erentially increasing the percentage of hydroxyl groups. The cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to quantify the activity of the electrode for SO2 electro-oxidation. The electrode’s charge trans-fer resistance for SO2 electro-oxidation, determined from impedance spectroscopy, showed a linear correlation with the ratio of the percentage of surface hydroxyl groups to the sum of the percentages of epoxy and carbonyl groups. The effect of the activation process on structural characteristics like crystal domain size and defect density was studied using XRD and Raman spectroscopy. For SO2 electro-oxidation, the activated carbon electrode showed superior performance than the
Pt electrode both in terms of substantially lower onset potential (0.26 V versus Ag/AgCl reference electrode compared to 0.35 V for Pt) and 36-times higher ex-change current density (6.16 μA cm−2 compared to 0.17 μA cm−2 for Pt).
Keywords: SO2 electro-oxidation, CFP, oxygen functionalization, partially reduced graphite oxide, anodization, HOPG, electrochemical surface modifica-
tions