Influence of graphene sheet properties as supports of iridium-based N-heterocyclic carbene hybrid materials for water oxidation electrocatalysis
Graphical abstract
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.
Section snippets
Materials
Two graphites were used in this work as graphene oxide precursors. These graphites were obtained from coal-derived samples, coal tar for G-1 and anthracene oil for G-2, by successive thermal polymerization, carbonization and graphitization to 2700 °C.
The imidazolium salt [MeImH(CH2)3OH]Cl and the starting organometallic compound [Ir(μ-OMe)(cod)]2 were prepared according to standard literature procedures [29,30]. All other chemicals were purchased from Aldrich. HPLC grade reagents were employed
Properties of parent thermally reduced graphene oxides (TRGO)
With the aim of studying the influence of the graphene structure on the catalytic behavior of supported Ir–NHC hybrid catalysts, two thermally reduced graphene oxides (TRGO-X, X = 1 and 2) were selected as catalyst support. They were prepared from two graphites (G-1 and G-2) with different crystalline structure. G-1 exhibits a more compact graphitic structure than G-2, as observed by the higher Lc value (51 vs 15 nm) and slightly shorter interlayer distance (d002, 0.336 vs 0.337 nm), measured
Conclusions
We have confirmed that the covalent anchorage of Ir(I)–NHC complexes through carbonate functions to thermally reduced graphene oxides of different properties lead to hybrid materials suitable as water oxidation electrocatalysts. This work also demonstrates that the structural properties of the graphene layers play a crucial role not only in the overall structure of the supported iridium compounds but also in their subsequent electrocatalytic behavior towards water oxidation. In this sense, it
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
Financial support from the Ministerio de Ciencia e Innovación (MICINN/FEDER) under the projects CTQ2016-75884-P and RTI2018-098537-B-C22, Gobierno de Aragón/FEDER 2014–2020 “Building Europe from Aragón” (groups E42_17R and E12_20R) and Principado de Asturias/FEDER (IDI/2018/000121) are gratefully acknowledged. The authors also acknowledge ALBA synchrotron for granting beamtime and the collaboration of the CLAESS beamline staff.
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