Structure and Reactivity of IrO_x Nanoparticles for the Oxygen Evolution Reaction in Electrocatalysis: An Electronic Structure Theory Study

Abstract

In this work, we employ electronic structure methods to investigate the structure and reactivity of IrO x nanoparticle models as catalysts for the oxygen evolution reaction (OER). Based on the explicit inclusion of the potential and pH in a constant potential framework, a computational approach is applied to investigate the thermodynamics of the proton and electron transfer process of IrO x cluster models. We address structural changes of the clusters under electrochemical conditions by constructing potential−pH diagrams from our computational results. Comparison of two IrO x cluster structures suggests that the charge transport to the clusters strongly depends on the pH. As a result, structures with a maximum number of on-top hydroxyl (OH μ1 ) species are stable at low potentials and deprotonation becomes favorable with increasing potential. An assessment of the Ir oxidation states in our models shows that mixed oxidation states, i.e., Ir IV and Ir V , occur around the OER onset potential and increase to higher oxidation states (Ir VI ) in the high potential regime. Furthermore, an investigation of the water adsorption mechanism is carried out at different potentials.The results suggest that the potential controls the energetics of intermediates as well as transition states during the OER.

The Sabatier Principle in Electrocatalysis: Basics, Limitations, and Extensions

“…Recent progress in this field put forth grand canonical models beyond the CHE method (Hansen and Rossmeisl, 2016;Hörmann et al, 2019Hörmann et al, , 2020Abidi et al, 2020), allowing the electrode potential to be explicitly included into the DFT calculations (Govind Rajan et al, 2020;Groß, 2020). Yet, a limitation of this approach is that the outcome may severely depend on the chosen value for the proton/hydrogen work function, for which values between 3.9 eV and 4.7 eV have been reported in the literature (Busch et al, 2020;Bhattacharyya et al, 2021).…”

Metal Coordination Determines the Catalytic Activity of IrO2 Nanoparticles for the Oxygen Evolution Reaction

“…On the other hand, regarding the OER on cluster models, Auer and co-workers analyzed the effect of the applied potential and pH on the structure of two models containing either 3 or 13 iridium centers. [58] Results for the larger cluster show that at acidic reaction conditions the surface presents mainly Ir-O and Ir-OH groups, the number of Ir-OH decreasing when increasing the applied potential and the pH. Moreover, the O-O bond formation through the WNA mechanism is predicted to present a non-negligible energy barrier that decreases while increasing the applied potential.…”

“…All calculations were performed with the VASP code, [62,63] using the spin polarized formalism, the PBE density functional [64] and the Grimme's D2 empirical correction to account for dispersion forces. [65] PBE functional has been widely used in modeling the OER with iridium based materials including finite clusters [39,40,49,50,52,56,58] and according to Goddard and co-workers represents the metallic nonmagnetic electronic structure of the bulk in better agreement with experiments than hybrid functionals or the PBE-U approach. [66] PAW pseudopotentials were used to describe the ionic cores.…”

“…From a computational point of view, the catalytic activity of iridium-based materials for the OER has been addressed either with slab models of the (110) surface [39,45,[54][55][56][46][47][48][49][50][51][52][53] or small clusters up to 13 Ir centers. [57,58] Regarding the studies with slab models, most contributions limit the study to the thermodynamic cost of each PCET step. Only in a few cases, the energy barriers associated with some of the elementary steps have been computed (mostly the chemical ones) and the associated energy barriers tend to be low, [49,50,53,55,56] thus suggesting that the thermodynamics of the PCET is sufficient to get overpotential trends.…”

Electrocatalysis for the Oxygen Evolution Reaction in Acidic Media: Progress and Challenges

“…Proton exchange membrane water electrolysis (PEMWE) operating in acidic environments has offered an effective way to produce sustainable, high-purity hydrogen through targeted electrochemical reactions since the 1960s [16] (Figure 1). PEMWE has the advantages of a faster dynamic response, a higher current density, and lower crossover of gases and is considered to be the basis of a hydrogen society in the future [17][18][19]. Electrochemical water splitting involves two heterogeneous multi-step half-reactions, which are referred to as the cathodic hydrogen evolution reaction (HER) and the anodic oxygen evolution reaction (OER) [20,21].…”