Starbon with Zn-N and Zn-O active sites: An efficient electrocatalyst for oxygen reduction reaction in energy conversion devices
Graphical abstract
The schematic representation of ORR favourable active sites on NS/ZnO electrocatalyst.
Introduction
High consumption of non-renewable energy sources and extended environmental pollution has triggered universal challenges to explore renewable energy sources. Thus, researchers all over the world are putting immense efforts to design highly efficient, durable electrocatalyst for ORR which is rate determining step in energy conversion and storage systems such as metal air batteries and proton-exchange membrane fuel cells [1]. In the present scenario, platinum supported on high surface area carbon (Pt/C) catalyst is considered as state-of-art catalyst for ORR in fuel cells. However, the major issues with Pt/C catalyst such as high cost, scarcity of Pt and inadequate stability during functioning of fuel cells retarded the commercialization of fuel cells [2]. Hence, designing the low cost, eco-friendly, noble-metal free electrocatalysts and understanding the mechanism of ORR are two main challenges towards the commercialization of fuel cells.
Recently, carbon based materials including carbon nanotubes, graphene, fullerene, graphite oxide, carbon black and vulcan carbon have been reported for versatile applications in energy conversion and storage electrochemical devices [[3], [4], [5], [6]]. These carbon nanomaterials are usually obtained from fossil sources using hazardous chemicals or treatment of organic precursor compounds at high temperature. Usually, the methods that are used to prepare the carbon nanostructures are laser ablation, chemical vapor deposition etc. [7]. The expensive techniques used for synthesis hinder the use of carbon nanocomposites in large scale. Hence, utilization of renewable sources, specifically biomass derived precursors for synthesis of carbonaceous materials is an emerging area for versatile applications including water purification, separation media, heterogeneous catalysis, acid-catalyzed reactions [8], recovery of critical metals [9] etc.; which can also be used in energy conversion as well as energy storage devices [10,11]. Basically, biomass derived precursors demonstrated sustainable approach for production of porous carbon materials because of the escape of moisture during heating process. Biomass derived porous carbonaceous materials exhibit variable properties such as high surface area, high pore volume, hydrophobicity and are simple to activate with various functional groups [8]. The additional advantages of biomass derived precursors are that they are renewable sources, abundant in nature, available at low cost [12,8], non-pollutant and thus can be considered as green catalysts for variety of applications. Specifically, ‘starbon’ a name derived from “starch was born” has attracted the attention of researchers as a porous carbon material. This type of carbon with tunable surface functionalities and porosity is produced by varying carbonization temperature of the polysaccharides [10]. Alginic acid is a well-known acidic polysaccharide, with polyuronide block copolymer structure obtained from algal biomass as well as seaweed [10]. The presence of carboxylic as well as hydroxyl groups in alginic acid help to create porosity by their escape at high temperature and also could aid metal particles to embed on it [13]. However, in order to be used as an ORR catalyst starbon needs to be activated by heteroatom doping. Besides, zinc oxide, a traditional sunscreen as called by ancient people has been found to be interesting for ORR owing to its oxygen vacancies within its lattice structure [14]. ZnO has also been widely used as a wastewater treatment catalyst by the photocatalysis process. The properties of ZnO such as controllable morphology, absorption range, photo stability, high chemical stability and high electrochemical coupling coefficient make it promising candidate in variable applications [15]. The properties of ZnO depend on the band gap of ZnO and the treatment with porous carbon may reduce its band gap [16]. So far, ZnO has been studied as a catalyst for ORR in corrosion studies [[17], [18], [19]] but the use of ZnO/starbon composite for ORR in fuel cell is in its infancy. In case of Zn based carbonaceous materials, very high temperature and prolonged heating involved in the synthesis usually leads to evaporation of Zn providing porosity in carbon framework [20]. On the other hand, Zn single atom sites without formation of nanoparticles/clusters of ZnO have also been reported; however Zn single atom loading is critical and challenging in terms of synthesis owing to high volatility of Zn precursor during annealing at higher temperature [[21], [22], [23], [24]]. In this study, employing appropriate synthetic parameters we design a catalyst with suitable porosity in starbon and uniform distribution of ZnO nanostructures to generate sufficient Zn-N functionalities on its surface in order to study catalytic activity of ZnO/N-doped starbon composite as against previously reported single atom Zn sites in the porous carbon [24].
Therefore, here our focus is to design a cost effective catalyst derived from renewable sources. We prepared a composite using alginic acid, urea and zinc chloride which act as source of starbon, nitrogen and zinc oxide respectively. Zinc chloride also introduces the porosity in the starbon structure with concomitant formation of active Zn-Nx linkages, which could also render a better surface anchoring of ZnO. The evolution of chlorine at high temperature resulting in the porous structure with conversion of Zn to ZnO has also been studied previously [25]. In the present case, the crystal structure study using XRD reveals the formation of zinc oxide on N-doped starbon, whereas the surface analysis by XPS confirms the formation of active Zn-Nx linkages. From electrochemical assessment, the activity of the catalyst towards ORR in basic media has been found to be superior as compared to benchmark Pt/C catalyst. The E1/2 and onset potential of optimized composite are observed around 1.02 V and 1.11 V vs RHE which are more positive than commercial Pt/C. Furthermore, the prepared catalyst showed current density of 5.5 mA cm−2 at 1600 rpm and followed four electron transfer mechanism with less than 2 % peroxide yield. It also showed significant stability up to 15,000 potential cycles (loss in E1/2 is ∼27 mV only) and high selectivity towards ORR.
Section snippets
Experimental
Zinc chloride (ZnCl2), urea (CH4N2O) were procured from Ranchem chemical (India) and alginic acid (C6H8O6)n was purchased from Loba Chem. All the chemicals and solvents were used without any further purification. De-ionized water was used for electrochemical measurements.
Results and discussion
Synthesis of N-doped starbon/zinc oxide (NS/ZnO) composite from zinc chloride, urea and alginic acid is shown in Scheme 1. The physical characterization of the prepared composites was carried out using different techniques for understanding the nature of active site and catalytic mechanism. Here, alginic acid acts as a source of starbon, urea as a source of nitrogen, whereas zinc chloride acts as a source of ZnO as well as activating agent for creating pores in the starbon framework [25]. It is
Conclusion
We have demonstrated one step, in situ synthesis of composite of ZnO on N-doped starbon as a noble metal free electrocatalyst for ORR. Here activation of starbon, doping of nitrogen in the starbon skeleton followed by formation of ZnO on the surface of starbon took place at high temperature in one process. The electrocatalytic study revealed that NS/ZnO-2 composition demonstrates excellent catalytic activity for ORR with Eonset 1.11 V vs RHE, JL 5.5 mA cm−2 following four electron transfer
CRediT authorship contribution statement
Sagar Ingavale: Methodology, Investigation, Writing - original draft. Phiralang Marbaniang: Methodology, Investigation. Bhalchandra Kakade: Resources, Writing - review & editing. Anita Swami: Writing - review & editing, Funding acquisition, Supervision.
Declaration of Competing Interest
The authors declare no conflict of interest.
Acknowledgment
The financial support from Department of Science and Technology - Science and Engineering Research Board, India (DST-SERB; No. ECR/2016/000680) and DST-FIST, India (fund for improvement of S&T infrastructure) No. SR/FST/CST-266/2015(c) are greatly acknowledged. Authors also acknowledge the Raman and HRTEM facility at SRM IST.
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