Elsevier

Surface Science

Volume 681, March 2019, Pages 1-8
Surface Science

Electrochemically active Ir NPs on graphene for OER in acidic aqueous electrolyte investigated by in situ and ex situ spectroscopies

https://doi.org/10.1016/j.susc.2018.10.021Get rights and content

Highlights

Abstract

An electrode for the oxygen evolution reaction based on a conductive bi-layered free standing graphene support functionalized with iridium nanoparticles was fabricated and characterized by means of potentiometric and advanced X-ray spectroscopic techniques. It was found that the electrocatalytic activity of iridium nanoparticles is associated to the formation of Ir 5d electron holes. Strong Ir 5d and O 2p hybridization, however, leads to a concomitant increase O 2p hole character, making oxygen electron deficient and susceptible to nucleophilic attack by water. Consequently, more efficient electrocatalysts can be synthesized by increasing the number of electron-holes shared between the metal d and oxygen 2p.

Introduction

The treatment of greenhouse gases and the mitigation of their effects is one of the most important global challenges humanity is now facing [1]. These challenges include the effects of global warming on the climate and their consequences for humans [2], plants, and animals [3], as well as the negative health effects of toxic gases and suspended nanoparticles [4], [5]. This issue has to be solved while still providing enough energy to feed a growing population (more than 7 billion) and to operate an infrastructure, which requires bigger and more efficient transportation networks [6]. Renewables (including wind, solar and tidal power) enable opportunities for a large, continuous, and clean energy supply [7]. However, one of the biggest challenges for an economically viable implementation of renewable energy technologies in the current energy infrastructure is caused by the intermittent nature of these energetic resources, which does not match the demand. This mismatch requires strategies to store excess energy for later use. Electrochemical energy conversion and storage plays a crucial role in the overall solution of the global energy challenges [8]. In particular, hydrogen obtained via electrochemical oxidation of water has the potential to replace fossil fuels as a storable and clean fuel [9]; however, this technology requires further development before it can be applied on a large scale [7]. In fact, because of the slow kinetics of the anodic oxygen evolution reaction (OER) [10], it is necessary to apply potentials well beyond the thermodynamically required potential of 1.23 V (vs. SHE) to split water molecules into their principal components (hydrogen and oxygen). For example, to achieve current densities of at least 10 mA/cm2 over-potentials of hundreds of millivolts above the OER equilibrium potential have to be applied, with a drastic reduction of the process efficiency [11], [12].

Due to their low overpotential and high stability in acidic media iridium-based oxides have been shown to be good candidates for the electrocatalytic oxidation of water [13], [14]. Unfortunately, iridium is among the scarcest materials in the earth's crust. Thus, optimized strategies are required for its usage; e.g. by increasing the surface to bulk ratio of the iridium particles [15]. An electronic effect exerted by the active Ir metal species causes the enhanced activity observed, however a detailed chemical speciation is still elusive and is even contradictory in literature in many cases, which requires further investigation to optimize synthesis approaches. Well dispersed nanoparticles (NPs) are usually not electrically connected between each other requiring a different strategy to apply the required potential to flow the electrical current necessary to run the reaction. This limitation can be circumvented by using a conductive support material, which drives the current to the catalytic active centers. In this way, the support layer provides electrical contact for the different catalyst NPs yielding high activity and stability under OER conditions, comparable to bulk materials [16]. Graphene was selected as a possible candidate substrate due to its high mechanical strength and flexibility, exceptional thermal and electrical conductivity, and chemical stability under harsh reaction conditions [17], [18], [19], [20]. It is well known that defect-free graphene layer is not electro/chemically active [21] and requires functionalization with heteroatoms to manifest chemical reactivity making it as an ideal substrate for electrocatalytic investigations due to its low contribution to the overall reaction rates, allowing the correlation of the catalytic activity to the supported metal.

Photoelectron spectroscopy (PES) is a powerful surface sensitive technique, which provides information about the electronic structure of the core-levels in a non-destructive way. This technique has been applied to a wide range of systems in order to elucidate their chemical composition. The implementation of this technique under reaction conditions is however complicated by the so called “pressure gap” [22] between the high pressure required for realistic reaction conditions and the ultra-high vacuum (UHV) needed in conventional PES chambers for their operation. Recently, holey grids made of Si nitride were combined with photoelectrons transparent bi-layers graphene (BLG) to form windows, which separate the liquid environment from the evacuated (UHV) PES chamber [23], [24]. With this approach it was possible to investigate electrochemical electrified interfaces in the presence of liquid electrolytes and gases [22] under reaction conditions. By taking advantage of this approach, in situ X-ray photoelectron spectroscopy (XPS) was accomplished, allowing the direct correlation between the electronic structure of IrOx (active sites) and its electrocatalytic activity during OER.

In this work, we investigate the variation in the electronic structure of Ir NPs on conductive bi-layers of graphene (BLG) as consequence of applied potential under OER in acidic conditions. We combine in situ with ex situ X-ray photoelectron spectroscopy (PES) and X-ray absorption near-edge structure (XANES) to unveil still undisclosed electronic structural information on the IrOx active in OER. In particular, valuable information of the occupied (Ir 4f) and partially occupied (Ir 5d, O 2p) orbitals is obtained with these techniques. We show that the formation of electron-holes in the hybridized O 2p/Ir 5d levels is linked to the electrochemical activity shown by iridium oxide. These holes lead to the formation of electron deficient oxygen species, which are the likely centers in the formation of molecular oxygen due to nucleophilic attack by OH or H2O. Furthermore, we report the structural descriptor of the electrocatalytic activity as a consequence of Ir 5d and O 2p hybridization and shared electron-holes and provide guidance for the synthesis of improved electrode materials for OER.

Section snippets

Beamlines

In situ synchrotron radiation based experiments were performed at the ISISS beamline of BESSY II in Berlin (Germany). In this facility, the photons are sourced from a bending magnet (D41) and a plane grating monochromator (PGM) yielding an energy range from 80 eV to 2000 eV (soft X-ray range), a flux of 6  × 1010 photons/s with 0.1 A ring current using a 111 µm slit and a 80 µm × 200 µm beamspot size. The in situ measurements were accomplished in the ambient pressure X-ray photoelectron

Results and discussion

The electrocatalytic performance of the graphene electrode described in the experimental section was investigated by means of linear sweep voltammetry (LSV) and cyclic voltammetry (CV). Fig. 1B shows the cyclic voltammogram (CV) of this assembly performed at room temperature with a scan rate of 20 mV/s in 0.1 M H2SO4 versus a Ag/AgCl reference electrode and a Pt counter electrode. The CV shows two oxidation waves, I and II, and two reduction waves, IV and V, in addition to III, which is

Conclusions

In this work, the electronic structure of a free standing graphene electrode decorated with Ir NPs for OER in acidic environment was conducted. In situ X-ray spectroscopy revealed that the OER on the surface of the IrOx NPs is similar to the oxo-oxyl-bridge formation in the photosystem II. The reaction is driven by the formation of shared electron-holes in the O 2p and Ir 5d which activate oxygen by making it electron-deficient. Thus, the oxidation of water into dioxygen involves the formation

Acknowledgements

The authors acknowledge BESSY II/HZB for allocating beamtime within the project number 16103418CR. This work was further supported by the Ministry of Education and Science of the Russian Federation (RFMEFI61614X0007) and theBundesministerium für Bildung und Forschung (05K14EWA) through the joint Russian-German research project “SYnchrotron and NEutron STudies for Energy Storage” (SYNESTESia).”. We thank DAAD for financial support in the framework of Taiwanese-German collaboration (project ID

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