Influence of graphene sheet properties as supports of iridium-based N-heterocyclic carbene hybrid materials for water oxidation electrocatalysis

Highlights

• Hybrid materials comprised of graphene modified with Ir-NHC complexes are suitable electrocatalysts for water oxidation.

• EXAFS analysis evidences that the parent graphite influences the iridium local structure in the hybrid materials.

• The iridium local structure determines the electrocatalytic behavior in water oxidation.

Abstract

The effect of the structural properties of graphene materials on the local structure of –OH anchored Ir(I)–NHC complexes is herein investigated. For that, two partially reduced graphene oxides exhibiting different sheet properties due to an adequate selection of the crystalline characteristics of their parent graphite were used. The main differences among them were the size of Csp² domains within their graphenic layers and the distribution of functional groups at the basal planes and edges. Anchoring of N-methylimidazolium moieties through the graphene –OH functional groups and subsequent formation of the Ir(I)–NHC complexes resulted in graphene-based hybrid materials. The structural differences of the support have an influence in the interaction of the supported iridium compounds with the graphene sheet. The oxygenated functional groups in the material with a smaller graphene sheet are closer leaving larger Csp² domains in the graphene layer, favoring their interaction with the supported iridium atoms therefore displacing the chlorido ligand from the first coordination shell. In contrast, the hybrid material in which the distribution of the oxygenated functional groups within the basal planes of the graphenic layer is more homogeneous shows partial chlorido displacement. This fact has an influence on the electrocatalytic performance of the iridium-based hybrid materials as water oxidation catalysts (WOCs), exhibiting improved catalytic activity the catalyst having coordinated chlorido ligands.

Introduction

Nowadays, global warming and depletion of fossil fuels have become a major challenge considering the growing worldwide energy demand [1]. Within this scenario, it became a critical issue to progress towards a more sustainable society moving towards more efficient renewable energies. The use of solar energy in combination with water electrolysis and CO₂ reduction are key processes to produce chemical fuels, being therefore efficient renewable energy storage systems. However, the large over-potential and slow kinetics of the oxygen evolution reaction (OER) has made catalytic water oxidation a major challenge for the later decades [2,3].

Among the most efficient water oxidation catalysts (WOC) are those based on Ru and Ir [4,5,6]. Moreover, homogeneous catalysts based on these metals exhibit a high efficiency in the evolution of oxygen and have more tunable structures when compared to those of heterogeneous systems such as metal oxides, (oxy)sulfides, (oxy)nitrides or metal (oxy)nanoparticles [7,8,9]. However, for a large-scale utilization of these homogeneous catalysts, their immobilization on the surface of heterogeneous electrodes, particularly via covalent attachment, is required since it substantially improves their recyclability, reduces the amount of catalyst, enhances their efficiency and robustness, and prevents deactivation via associative intermolecular pathways [10,11]. Carbon materials are commonly used for developing heterogeneous WOCs [[12], [13], [14], [15]]. Among them, graphene offers additional advantages that could promote a proactive role improving catalytic efficiency, as for example, a unique electronic behavior, high surface area or outstanding chemical stability [16,17]. The graphene materials produced from graphite by the easily scalable chemical route –i.e. via oxidation of graphite to produce graphene oxides (GOs) and/or subsequent reduction to produce partially reduced graphene oxides (RGOs)– are versatile materials which exhibit in their structure a series of oxygenated functional groups from which the organometallic catalyst can be covalently attached. Moreover, this route offers the possibility to selectively control the structural properties of the obtained graphene materials. Thus, it is possible to modulate the type and amount of oxygenated functional groups located at basal planes and edges of GO sheets by selecting the oxidation method [18] or selectively remove certain functional groups during the production of TRGOs [19]. Even further, the type and distribution of oxygenated functional groups in the GO can be also modulated by an adequate selection of the crystallinity of the parent graphite [20,21]. This versatility in processing allows graphene materials with different structure and properties to be obtained, a fact which has been used to improve their field of application (e.g. electrochemical systems, composites, etc.) [22,23]. Catalytic applications are not an exception and there are a great number of studies in which graphene materials act as proactive supports of nanoparticles [24] or even organometallic compounds [25] in different catalytic systems [17,26].

Much more scarce are the studies focused on the graphene properties themselves and the influence of their structural properties on the catalytic performance of the resulting supported hybrid catalytic systems, being most of them centered in studying the effect of graphene sheet reduction. As an example, the positive correlation between hydrogen transfer catalytic activity of iridium N-heterocyclic carbene (NHC) organometallic complexes supported onto GO and TRGO has been recently reported [27].

Bearing this in mind, herein two TRGOs, obtained from two graphites of different crystallinity, have been prepared for the covalent anchorage of an organometallic Ir(I)–NHC complex. This anchorage, the same for the two TRGOs, includes a sequential and specific reaction with the graphene -OH functional groups that gives rise to the supported iridium complexes [28]. The properties of the parent graphenes and hybrid materials have been extensively studied by means of XPS, Raman and EXAFS techniques, and the electrocatalytic water oxidation behavior of the supported iridium catalysts evaluated. Moreover, the objective of this work is double; on one hand it is intended to determine if structural sheet properties lead to some structural changes in the first coordination sphere of the anchored metal compounds and, on the other hand, to determine if these changes lead to different water oxidation catalytic behavior. Interestingly, a correlation has been found between the properties of the parent graphene sheets and the iridium local structure in the supported catalysts which also affect the catalytic performance.