Elsevier

Electrochimica Acta

Volume 398, 1 December 2021, 139338
Electrochimica Acta

Nanostructured Co3O4 electrocatalyst for OER: The role of organic polyelectrolytes as soft templates

https://doi.org/10.1016/j.electacta.2021.139338Get rights and content

Abstract

Designing an efficient electrocatalyst for the oxygen evolution reaction (OER) in alkaline media is highly needed but very challenging task. Herein, we used organic polyelectrolytes such as (carboxymethyl cellulose) CMC and polyacrylamide polymers for the growth of Co3O4 nanostructures by aqueous chemical growth method. The morphology and composition studies were performed on scanning electron microscopy (SEM), energy dispersive X-ray (EDX), powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM) techniques. The structural properties and the surface chemistry of the Co3O4 electrocatalysts were correlated to the OER performance, and the enhancement mechanism with respect to pristine Co3O4 was observed to be specifically related to the polyelectrolyte templating role.

Co3O4@CMC composites displayed reduced crystallite size, producing OER overpotential as low as 290 mV at 10 mAcm−2 in 1.0 KOH and Tafel slope of 71 mVdec−1, suggesting fast transfer of intermediates and electrons during water electrolysis. On the other hand, the use of polyacrylamide and its different templating mechanism resulted in similar crystallite size, but preferential exposed faces and larger surface vacancies content, as demonstrated by HR-TEM and XPS, respectively. Consistently, this material displays cutting-edge OER performance, such as overpotential of 260 mV at 10 mAcm−2 and a low Tafel slope of 63 mVdec−1. The proposed strategy for the preparation of Co3O4 nanostructures in the presence of CMC and polyacrylamide is facile, mass production, thus it could equally contributed towards the realization of hydrogen energy. Therefore, these nanostructures of Co3O4 can be regarded as an alternative and promising materials for the different electrochemical applications including fuel cells, metal air batteries, overall water electrolysis and other energy storage devices.

Introduction

Since several decades, there is a drastic increase in the energy demands as our activities in daily life are mainly supported by the availability of energy. The international energy agency has reported an increase of 2.3% of energy demand in 2018. This rise in energy demand is doubled to that of last decade [1]. Most of the energy (approximately 70%) is coming from the fossil fuels. By the end of 2040, the energy demands will cross by 40% as described in the future- outlook of energy in 2018. The continuous use of fossil fuels is not only increasing their cost, but it also shows adverse effects on our environmental conditions due to the emission of greenhouse gases. Nature is also associated with wide range of alternative energy resources including wind, solar, ocean, water, bioenergy, geothermal, etc [2]. Water splitting is a potential energy resource for reducing the impact of non-renewable energy resources, and it can also reduce the impact of carbon contamination, as this technology produces green fuels with zero carbon emission [3]. Water splitting involves two processes namely oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). The complete dissociation of water produces H2 and O2 via HER at the cathode electrode and OER at the anode electrode, respectively [4]. Electrocatalysis is a promising technology for increasing the energy resources through the consumption of water and electricity [5]. Electrocatalysis shows that two processes (HER/OER) have different kinetics as HER involves a two electrons transfer process [6], whereas OER is four electrons transfer process [7]. Therefore, OER is complicated and kinetically slow, thus requiring efficient catalysts. For this reason, there is a need to design efficient electrocatalysts with high density of catalytic sites to lower the overpotential for the OER process [8,9]. Several catalysts including Ir/Ru based materials are extensively used for the OER, but their large-scale use creates economic burden and rare abundance is another limitation. Furthermore, transition metal oxides are found promising materials for OER applications as they are abundant and low cost. The oxides of Fe, Ni, Co, and Cu are widely studied for OER due to their rich active sites for catalysis [10], [11], [12], [13], [14], [15]. However, these materials have high overpotential, poor stability and durability in alkaline media. To shape the successful overall water splitting for practical applications, efficient OER catalysts must be developed. For this purpose, different earth-abundant cobalt-based catalysts particularly cobalt oxide (Co3O4) materials have been developed [16], [17], [18]. However, the Co3O4 is easy to restack and is associated with poor conductivity, which further limits its OER activity [16].

Co3O4 growth can be templated by various small molecules, producing a wide set of different morphologies, and their electrochemical activities have been studied [19], [20], [21], [22]. The same approach has not been experienced with polymeric sacrificial templates, providing both many functional groups and mechanical constraints resulting from the backbone chain. In this study, we investigated the role of two different polymers, namely polyacrylamide and sodium carboxymethyl cellulose as soft surface template for the hydrothermal growth of Co3O4 nanostructures.

These polymers are both displaying nucleophilic functional groups and, in the reaction conditions, a large number of negative charges due to the presence of carboxylate groups, expected to complex the cobalt ions during the cobalt hydroxide precipitation [23], as well as to stabilize the growing nanocrystals.

Carboxymethyl cellulose (CMC) is a common cellulose derivative obtained by cellulose reaction with chloroacetic acid. The resulting carboxylic acid derivatives are deprotonated due to the basic environment and the obtained material is generally distributed as sodium salt. CMC is widely employed as a thickener or viscosity modifier [24] due to the possibility to tune the hydrophilic character and the resulting rheological properties by controlling the degree of substitution of pristine cellulose. The tunable hydrophilicity of CMC is also employed to stabilize multiple suspensions [25] in industrial applications. Additionally, CMC is inexpensive, chemically stable and highly suitable for mass production of composite materials. There are several examples of CMC based hydrogels integrating metal oxides [26] and few examples of metal oxides growth templated by CMC microspheres [27] but the role of the polyelectrolyte during hydrothermal synthesis process was not investigated in the existing literature. CMC is also known to spontaneously produce nanofibers that might influence the metal oxide growth process and the resulting porosity, enhancing the performance of electrochemical reactions [28].

Polyacrylamide (PA) is a hydrophilic polymer resulting from the homo-polymerization of acrylamide. The hydrophilicity is driven by the large number of amide functional groups, whose slightly nucleophilic character is known to stabilize electrophilic metal derivatives [29]. Polyacrylamide is also know to spontaneously decompose at elevated temperature or pH [30] due to the hydrolysis of the amide group and the resulting formation of a carboxylic acid and ammonia. The hydrolyzed polymer becomes a temperature-induced polyelectrolyte with either anionic or cationic character [31] depending on the reaction conditions. PA, in its anionic form, is widely employed as stationary phase in gel electrophoresis. In addition, thanks to the large number of negative charges, it is widely employed in wastewater treatment to flocculate insoluble particles and metal precipitation [32]. A recent study shows that polyacrylamide microspheres exhibit metallophylic properties which significantly carried precursor salt with the homogenous distribution [33].

In this study, we used a polyacrylamide/CMC as soft surface template for the deposition of cobalt oxide (Co3O4) nanostructures. Unlike previous reports of hydrothermal growth methods for Co3O4 nanostructures employing small molecular templates, the use of polymeric templates with multiple functional charged groups is expected to direct the metal oxide growth and efficiently stabilize the growing structure. The use of these organic polyelectrolytes (PA is acting as one in the hydrothermal conditions employed) allows for a cheap and robust preparation process of a functional materials with robust OER activity, which fosters the OER kinetics. The unique and attractive nature of polyelectrolytes as a sacrificing templates for controlling the growth kinetics, morphology and improving the functionalities of Co3O4 nanostructures have not been reported in the available literature. Also, the simultaneous OER performance evaluation of soft templated Co3O3 nanostructures with two different polyelectrolyte polymers has not been studied in the past.

Herein, the role of these two polymers, CMC and polyacrylamide, is investigated with respect to their ability to act as soft templates for Co3O4 growth. The resulting morphology and structure are thoroughly investigated, and the material is eventually evaluated as electrocatalyst for OER. The Co3O4 nanostructures grown in the presence of CMC display a low overpotential of 290 mV at 10 mAcm−2 and a small Tafel slope of 71 mVdec−1, whereas the use of polyacrylamide resulted even lower Tafel slope and overpotential of 63 mVdec−1 and 260 mV, respectively. The improved electrochemical performance was correlated with the modified nanoscale morphology, higher amount of oxygen vacancies, electrochemical active surface area and the turnover frequency (TOF).

Section snippets

Synthesis of Co3O4 nanostructures

Polyacrylamide, (carboxymethyl cellulose) CMC, urea, cobalt chloride hexahydrate, potassium hydroxide, 20% RuO2/C and absolute ethanol were obtained from Sigma Aldrich Karachi, Pakistan. All the solutions were prepared in the deionized water. For the synthesis of cobalt oxide nanostructures using nucleophilic CMC/polyacrylamide polymers, the following methodology was used: An equimolar solution (0.1M) of cobalt chloride hexahydrate and urea was prepared in two separate beakers with a volume of

The physical characterization of nanostructured Co3O4 material

Fig. 1 is describing the chemical structure of polyacrylamide and carboxymethyl cellulose. SEM micrographs of the samples are displayed Fig. 2. All samples are characterized by complex nanostructures. Pristine Co3O4 exhibit quasi-spherical nanoparticles aggregated into platelets-like structures. The platelets exhibit a length of few microns and diameter of 200–500 nm Fig. 2a. The Co3O4@CMC-1 exhibit more irregular character, with elongated nanowire-like structures composed by nanoparticles, as

Conclusions

In summary, we have produced Co3O4 nanostructures in the presence of polyacrylamide and CMC as nucleophilic polymers by wet chemical method. The Co3O4 nanostructures were found efficient for OER in 1.0M KOH basic conditions. The prepared materials are physically characterized by XRD, SEM, EDS and HRTEM techniques, highlighting a different role of the templating polyelectrolyte on the nanocrystals morphology. These studies confirmed the short-range nanowire morphology consisting chain of

Credit author statement

Adeel Liaquat Bhattia, carried out partial material synthesis and electrochemical measurement

Aneela Tahirab, measured the XRD of prepared materials and analyzed it

Alessandro Gradoned,j, did the TEM measurement

Raffaello Mazzarod,k*, did the TEM analysis

Vittorio Morandid, Supervised the TEM measurement and edited the TEM draft

Umair aftabc, did the electrochemical impedance analysis

Muhammad Ishaq Abroc, partially involved in the electrochemical measurements

Ayman Nafadyh, did the SEM analysis

Kezhen

Declaration of Competing Interest

Authors declare that the presented work is original and only considered for this journal.

Acknowledgment

Authors acknowledge the Higher Education of Pakistan for the partial financial support of this research work. A.N thanks Researchers Supporting project (RSP-2021/79) at King Saud University, Riyadh, Saudi Arabia. Authors acknowledge the XPS support from Dr. Kezhen Qi though his financial support under the project “Liaoning Revitalization Talents Program (XLYC1807238).” China. A.I.M. acknowledges RTI2018-099668-BC22, RyC-2015-17870 and UMA18-FEDERJA-126 projects (Spain). A.G., R.M. and V.M.

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      Co, Ni, and Fe are used as a single atom or doped with other transition metals to perform better electrochemical activities, these transition metals have reported near metal conductivity values and are easily approachable and cheap, making them suitable for OER applications. For example, Bhatti et al. described Co3O4 nanoparticles grown on polyelectrolyte-like carboxymethyl cellulose (Co3O4@CMC), which showed OER activity at a much-decreased overpotential of 0.29 V [13]. Likewise, Ni3(OH)2 microspheres were incorporated with g-C3N4, serving as substrate layer by single-step solvothermal method functioned as an electrochemical catalyst depicting excellent OER performance at 0.24 V [14].

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