Reticulated porous carbon foam with cobalt oxide nanoparticles for excellent oxygen evolution reaction

Highlights

• Developed novel 3D macro porous graphitized carbon foams (GCF) using coal tar pitch containing polyaromatic hydrocarbons.

• GCFs modified with multiwalled carbon nanotubes and cobalt oxide nanoparticles by direct annealing method.

• Modified GCFs exhibited electrical conductivity of 555 S/cm and surface area of 18.36 m²/g.

• Current density of 10 mA/cm² was achieved for modified GCFs at over potential of 440 mV and found to be stable upto 5 h.

Abstract

Carbon (graphitic) foam is a 3D porous, rigid and permeable material used for many applications due to its excellent electrical and thermal properties. In this study, coal tar pitch containing polyaromatic hydrocarbons has been used as precursor for preparation of carbon foam by sacrificial template method and graphitized carbon foam (GCF) was obtained by heat treatment at 2350 °C. The GCF was modified by growing multiwalled carbon nanotubes (MWCNTs) using chemical vapour deposition method and loaded with cobalt oxide nanoparticles (CoO) using simple chemical method. Different GCF based modified samples were tested for electrochemical performance as electrocatalyst for splitting of water. The best performing electrocatalyst was GCF decorated with MWCNTs and CoO nanoparticles (GCF-CNT-Co) which showed the current density of ∼10 mA/cm² at overpotential of ∼0.44 V/RHE which is comparable with the overpotential (0.33 V/RHE) for commercial benchmark Pt/C electrocatalyst. All coal tar pitch based GCF samples with different modifications showed excellent electrolytic stability for 5 h in 0.1 M KOH solution.

Introduction

Water electrolysis is considered as an efficient technique for generation of hydrogen (H₂) and oxygen (O₂) using electrocatalyst. In this technique, the required electric potential is applied between two electrodes and an electric current pass-through water, thus oxidation and reduction reactions take place and hydrogen evolution reaction (HER) occurs at cathode and oxygen evolution reaction (OER) at anode. Theoretical minimum value of 1.23 V is needed for splitting of water but due to sluggish reaction, this value increases drastically depending on properties of electrocatalyst used. Hydrogen (H₂) evolution by electrocatalysis is considered as a clean, renewable, efficient and sustainable energy resource for mankind. Hydrogen has many advantages over conventional energy sources such as high heat of combustion and high energy storage capacity [1]. Whereas, OER is a limiting reaction in electrochemical process for generation of molecular oxygen due to sluggish kinetics at anode which requires large amount of input energy to drive the reaction [2]. The reaction rate and efficiency of water splitting mainly depend upon the material, termed as electrocatalyst, as well as the pH of electrolyte or water. Therefore, to achieve high efficiency water splitting through electrocatalysis, an efficient electrocatalyst is required in order to increase the current density, lower the overpotential and accelerate the reaction rate. The main requirements for efficient and well performing catalyst include its easy availability, economic viability, high electrical conductivity, low intrinsic overpotential and chemical as well as electrochemical stability in acids and strong base [3]. Based on the requirement, an intensive research is going on in search of a low cost, abundantly available and highly efficient electrocatalyst for splitting of water for OER and HER.

To date, only expensive noble metal-based catalysts, such as platinum (Pt), iridium (Ir), ruthenium (Ru) and their corresponding metal oxides are known and well-studied electrocatalyst because they can deliver the desired efficiency due to their favorable properties [[4], [5], [6], [7]]. Currently, the platinum based catalysts are found to be most suitable and efficient electrocatalysts. However, high cost and limited natural abundance of noble metals hinder their commercial applications. Therefore, the current research in this area is mainly focused in two directions: to reduce the content of noble metals in an electrocatalyst and explore other metals having low production cost and high natural abundance without compromising the electrocatalytic performance [[8], [9], [10]]. In recent exploration, the cost effective, efficient catalyst have been synthesized based on various class of earth abundant transition metals such as cobalt, molybdenum, nickel, tungsten, etc. and their chalcogenides, sulphides, oxides, carbides, hydroxides and reported to be promising candidates as electrocatalyst for HER and OER [[11], [12], [13], [14], [15], [16], [17]]. Out of various transition metals, cobalt and molybdenum and their oxides are well studied electrocatalyst and showing good performance. But the designing of cost effective, efficient and stable electrocatalyst for water splitting based on these metals is a challenging task.

Recently, various mesoporous carbon materials loaded with/without various metal/metal oxides and some elements like Nitrogen, Sulphur, and phosphorous doped carbon are being used for electrocatalysis of water because of their unique and tunable structures, stability in wide pH range of electrolyte and electrical conductivity [8,13,18,19]. Tavakkoli et al. reported the deposition of maghemite nanoparticles and pseudo-atomic-scale Pt on carbon nanotubes and studied performance of the material for water splitting [20]. In further studies, the Pt and cobalt oxide nanoparticles were deposited on light weight, porous, inexpensive carbon foam (CF) and used for electrocatalytic water splitting [21,22]. Shen et al. fabricated the nanostructured cobalt–iron double sulfides covalently entrapped in nitrogen-doped mesoporous graphitic carbon (Co_0.5Fe_0.5S@N-MC) and reported the resultant material as an efficient oxygen evolution reaction (OER) catalyst under alkaline conditions [23]. In another study, a metal free 3D porous carbon electrode has been reported as a water splitting electrocatalyst. In that study, the porous carbon, called as carbon cloth, was doped with nitrogen, phosphorus and oxygen and exhibited a high performance with the current density of 10 mA/cm² at the applied voltage of 1.66 V and offered excellent catalytic performance in wide pH range [24,25]. Therefore, in the recent years, porous carbon materials having micropores and loaded with various nanomaterials have attracted the great attention of scientific community towards electrocatalytic water splitting due to excellent favorable properties of carbon as well as permeable porous structures.

In the present work, we are reporting, for the first time, the high softening point coal tar pitch (HSCTP) based light weight, porous and conducting 3D carbon foam for oxygen evolution reaction (OER) during water splitting and study the stability. Coal tar pitch is a black solid material at room temperature and complex mixture of polycyclic aromatic hydrocarbons, hetero cyclic compounds and phenols and their compositions vary with the source of coal tar and the processing conditions. It is a great graphitizable material used for making variety of carbon products with excellent electrical and thermal conductivity. HSCTP is derived from coal tar pitch (CTP) and contains an appreciable amount of anisotropic phase or liquid crystalline phase and an isotropic phase. Here, we develop HSCTP based CF by using template method and polyurethane (PU) foam as template [26]. To increase the electrical conductivity and other properties, the CFs were carbonized and subsequently graphitized at 2350 °C. After graphitization, the CFs were decorated with CoO nanoparticles on as such GCF and MWCNT decorated CF to improve electrocatalytic kinetics. These samples were tested for electrochemical studies as electrocatalyst in 0.1 M KOH solution.