Graphene featuring imidazolium rings and electrostatically immobilized polyacrylate chains as metal-free electrocatalyst for selective oxygen reduction to hydrogen peroxide

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Abstract

While graphene-based materials hold a strong potential as electrocatalysts, owing to their astonishing properties and functionalization capability, their intrinsic catalytic activity is more often masked under the umbrella of the precious and industrial metals that are used as counterparts for boosting the electrocatalytic performance. In the present work, we envisioned and realized a simple, stepwise surface engineering path for the modification of graphene oxide, aiming at elevating its intrinsic electrocatalytic activity, excluding any metals, towards the oxygen reduction reaction (ORR). More specifically, oxidized graphene was initially covalently decorated with amphoteric imidazole rings, which were then converted to the corresponding imidazolium counterparts. At the last step, anion exchange of the grafted imidazolium species was performed to immobilize via electrostatic interactions polyacrylate chains. The electrocatalytic performance of the so-formed graphene-based ensembles towards the ORR in alkaline media was evaluated, unveiling the 2e reduction of oxygen, selectively producing hydrogen peroxide. The results indicate that both covalent and non-covalent surface engineering of graphene with organic chains matters towards electrocatalysis and should not be considered negligible. Our findings and insights provided are a stepping stone on the quest of graphene-based ensembles as metal-free electrocatalysts.

Introduction

Graphene oxide (GO) is the cheapest and most readily available graphene derivative, easily prepared in large scale by several procedures based on the archetypal method of Hummers and Offeman [1]. Although GO nanosheets dispossess the exceptional electronic properties of pristine mechanically exfoliated graphene [2], due to the disruption of the graphitic 2D network by oxygen functionalities, GO can be easily chemically modified thanks to them [3]. Graphene derivatives’ chemical functionalization lights new ways for its processing and utilization in diverse technological fields, one of them being electrocatalysis [4]. Indeed, graphene holds the potential to contribute significantly in this field as a substituent to the expensive and scarce dominant metal catalysts. For example, in energy conversion and storage applications, rechargeable metal-air batteries and polymer electrolyte fuel-cells, where the electrochemical oxygen reduction reaction (ORR) plays a pivotal role, graphene derivatives have been extensively studied as efficient catalysts [5]. Markedly, the electrocatalytic properties of graphene derivatives are usually underestimated or masked by the high catalytic activity of the loaded metal-based catalysts [6]. However, this approach exemplifies the use of graphene derivatives mostly as a catalytic substrate, setting aside their intrinsic electrocatalytic characteristics. Furthermore, metal nanoparticles, transition metal carbides, transition metal dichalcogenides and transition metal oxides that are utilized as counterparts to graphene-based hybrid ORR electrocatalysts, are strategically selected to promote the 4e ORR pathway towards the generation of clean electricity in a fuel cell [6].

Whilst the 4e ORR pathway is direct, clean and has been traditionally preferred, the 2e pathway is less studied and leads to H2O2 formation. Hydrogen peroxide is considered one of the most important chemicals in the world due to its applicability in the chemical [7], cosmetic, textiles industry [8], environmental protection [9], medicine and so on, while also being cost effective. In fact, owing to its oxidative power and strong bleaching activity, H2O2 is an excellent example of chemical ubiquity, while its production is a billion dollar industry [10]. However, hydrogen peroxide’s conventional syntheses include elaborate equipment, expensive Pd catalysts, dangerous storage and high-pressure mixing conditions, inefficient yield and waste by-products [7]. Electrochemical oxygen reduction is an efficient alternative to these methods that avoids the aforementioned undesired conditions, while at the same time produces electricity in the concept of a fuel cell [11]. In this context, specifically designed graphene nanosheets could be promising electrocatalysts for the production of this valuable chemical.

In this work, we investigate the impact of surface engineering of modified graphene nanosheets carrying organic moieties, but not any metal species, with respect to the electrocatalytic activity for ORR. In parallel, the selectivity towards H2O2 is evaluated. In more detail, we firstly introduced chemically anchored amphoteric imidazole rings onto oxidized graphene, then converted them to the corresponding imidazolium ones and subsequently a supramolecular immobilization of negatively charged polyacrylate chains took place. Notably, imidazole-based compounds have been explored as agents to tune the electrochemical interface of ORR [12]. Herein, we used imidazole as a chemically simple, cheap and readily available amphoteric organic unit to alter the charge of graphene nanosheets on demand. On the other hand, polyacrylic acid is widely employed as alkaline gel electrolyte for energy conversion in batteries [13] and it can be easily converted to its anionic form. Following this simple modification approach, we evaluated the impact of surface modification, focusing in the selectivity towards electrochemical hydrogen peroxide production in alkaline ORR conditions, while avoiding any metals in the catalytic process.

Section snippets

Synthesis of poly(acrylic acid), PAA, via reversible addition−fragmentation chain-transfer (RAFT) polymerization

In a 25 mL round-bottom flask, the monomer acrylic acid (1.5 g), the chain-transfer agent 2-(dodecylthiocarbonothioylthio)− 2-methylpropionic acid (0.11 g, 0.3 mmol), the initiator 2,2′-azobis(2-methylpropionitrile) (0.005 g, 0.03 mmol) and the solvent 1,4-dioxane (6 mL) were added. Then, constant bubbling of high purity N2 gas was performed in the reaction mixture, which was placed in a pre-heated oil bath at 70 °C. The reaction mixture was left in the oil-bath under stirring for 15 h, then

Results and discussion

Polyacrylic acid (PAA) chains were immobilized on oxidized graphene (GO) carrying positively charged imidazole rings, as illustrated in Scheme 1.

More specifically, GO was initially treated with thionyl chloride to promote the conversion of the carboxylic acid functionalities to the corresponding reactive acyl chloride ones. A simple condensation reaction with 1-(3-aminopropyl)imidazole (APIm) furnished modified graphene nanosheets decorated with imidazole groups covalently grafted via amide

Conclusion

In conclusion, a simple stepwise surface modification protocol of graphene was applied and alongside, the electrochemical properties of all functionalized graphene-based materials for the electrocatalytic ORR in alkaline media were also investigated. Starting off with oxidized graphene, amphoteric imidazole rings were covalently grafted, which were then converted to the corresponding imidazolium counterparts. Finally, anion exchange of the grafted imidazolium species took place to immobilize

CRediT authorship contribution statement

Maria-Lydia Vorvila: Formal analysis, Investigation, Data curation, Visualization. Ioanna K. Sideri: Formal analysis, Investigation, Data curation, Writing – original draft, Writing – review & editing, Visualization. Anastasios Stergiou: Investigation, Data curation, Writing – original draft, Visualization. Martha Kafetzi: Investigation, Data curation, Visualization. Stergios Pispas: Methodology, Resources, Writing – review & editing, Visualization, Supervision. Raul Arenal: Methodology,

Declaration of Competing Interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Raul Arenal reports financial support was provided by EC H2020. Raul Arenal reports financial support was provided by Spanish MICINN.

Acknowledgments

The TEM studies were performed in the Laboratorio de Microscopias Avanzadas (LMA), Universidad de Zaragoza (Spain). R.A. acknowledges support from Spanish MICINN (PID2019-104739GB-100/AEI/10.13039/501100011033), Government of Aragon (projects DGA E13-20R (FEDER, EU)) and from EU H2020 “ESTEEM3″ (Grant number 823717) and Graphene Flagship (Grant number 881603).

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