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X-ray scattering measurements by Rao et al. have recently also identified such species at the related RuO2(110) surface at similar potentials, whereby...
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Ab Initio Thermodynamics Insight into the Structural Evolution of Working IrO Catalysts in Proton-Exchange Membrane Electrolyzers 2

Daniel Opalka, Christoph Scheurer, and Karsten Reuter ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b00796 • Publication Date (Web): 23 Apr 2019 Downloaded from http://pubs.acs.org on April 23, 2019

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Ab Initio Thermodynamics Insight into the Structural Evolution of Working IrO2 Catalysts in Proton-Exchange Membrane Electrolyzers Daniel Opalka, Christoph Scheurer and Karsten Reuter* Chair for Theoretical Chemistry and Catalysis Research Center Technical University of Munich, Lichtenbergstr. 4, D-85747 Garching (Germany) *corresponding author: [email protected]

Abstract At the cell voltages required to reach technologically viable current densities in proton-exchange membrane (PEM) electrolyzers, IrO2 catalysts are suspected to undergo a transformation to an amorphous hydrous form. Here, we present a systematic ab initio thermodynamics study analyzing the shape and stability of IrO2 nanoparticles in this potential range. Our results confirm a thermodynamic instability of the rutile crystal structure induced by the stabilization of highly oxidized O species at the surface already at onset potentials for the oxygen evolution reaction (OER). Intriguingly, this is preceded by a transformation of the equilibrium shape at even lower potentials. Instead of the well-studied IrO2(110) facets, this shape is dominated by IrO2(111) facets that have hitherto barely received attention. Our findings highlight the need to extend detailed characterization studies to this high-potential range, not least to establish more suitable active site models for the OER that may then serve as basis for computational screening studies aimed at reducing the rare-metal content in future PEM OER catalysts. Keywords: Oxygen evolution reaction, PEM electrolysis, ab initio thermodynamics, density-functional theory, iridium oxide 1. Introduction Proton-exchange membrane (PEM) electrolyzers promise to overcome the power-density limitations of established alkaline electrolysis technology and could thereby pave the road towards a scalable energy storage from sustainable power sources [1,2]. A key limiting issue is the very small materials base for oxygen evolution reaction (OER) catalysts that are both active and stable under the harsh acidic PEM operating conditions. Presently, the primary catalyst with a reasonably small overpotential for cost-efficient OER current densities is IrO2, reaching ~2 A/cm2 at cell voltages around 1.7 V vs. the reversible hydrogen electrode (RHE) [3,4]. Notwithstanding, under such conditions even this material seems to undergo a hitherto only incompletely characterized transformation from its rutile crystalline to some amorphous hydrous state with Ir in higher oxidation states [5-11]. The nature of the active sites in this new state is thereby presently as unclear, as is the compatibility of such a strong compositional and morphological transformation with the requirement of a long-term stability for economically viable PEM electrolysis. The latter stability requirement seems even harder to meet, when considering that in emerging times of sustainably-generated intermittent electricity not only long-term steady-state OER operation is of interest. Instead, frequent start-up and shut-down scenarios ramping the voltage up and down have to be addressed [12], in which each time the catalyst would then possibly undergo such a structural transformation. Stability issues at operating voltages around 1.7 V ACS Paragon Plus Environment

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could thereby be particularly severe for delicate, smaller catalyst particles. Yet, precisely the synthesis of such IrO2 nanoparticles is presently one of the major research strategies to reduce the loading of the catalyst with the noble metal Ir, which after all is among the rarest elements in the earth crust. Latest synthesis routes achieve nanoparticle sizes down to the 1-2 nm range [7,10,13-16]. The significant variation in activity and stability reported for such nanoparticles has been ascribed to different degrees of hydroxylation and particle morphology, only but highlighting our incomplete understanding of the actual state of the working catalyst. In-depth characterization and mechanistic work on model systems, both experimental [17,18] and computational [19-22], has hitherto almost exclusively focused on lower operating potentials. Under such potentials at maximum up to the onset of the OER, the integrity of the IrO2 crystalline form was generally assumed. Even more so, advanced first-principles calculations have unanimously been based on the active site motifs offered by the rutile IrO2(110) facet – in the understanding that this facet is the lowest-energy facet of rutile crystals at 0K and that this facet was indeed repeatedly found as dominant in ex situ characterization of synthesized IrO2 particles. While substantial insight has been built up in these studies, it is unclear how much of this transfers to the above described working catalysts at cell voltages far beyond the OER equilibrium potential at 1.23 V. Recent work by Bernt and Gasteiger suggests that at 1.7 V operating cell voltage, more than 1.5 V potential is actually applied to the catalyst [4]. In this work we therefore conduct ab initio thermodynamic calculations to specifically address this higher potential range. Constructing the surface phase diagrams for all low-index surfaces of rutile IrO2, we find the stabilization of highly oxidized superoxo (OO) species at the surface to induce a general thermodynamic instability of the rutile structure already at potentials not much above the OER onset. While this concurs with the experimentally observed transformation to the amorphous state, we find this instability to be preceded by a general shape transformation already at much lower potentials. Combining the first-principles surface free energy data into a Wulff construction, this equilibrium shape exclusively exhibits hitherto not considered IrO2(111) facets already at OER onset potentials. These first insights into the structural evolution of working IrO2 catalysts highlight the urgent need to extend systematic characterization and mechanistic endeavors to higher potentials to identify the true active sites governing the OER at technologically relevant current densities. 2. Methods The central quantity determining the thermodynamic stability of a specific termination σ of a facet with crystallographic orientation (hkl) in an aqueous environment and under an applied potential U is the surface free energy 𝛾(ℎ𝑘𝑙),𝜎 = ―

1 2𝐴(ℎ𝑘𝑙) 1 2𝐴(ℎ𝑘𝑙)

)𝜇 H O } {𝐺(ℎ𝑘𝑙),𝜎 ― 𝜈(ℎ𝑘𝑙),𝜎 𝐺bulk ― (𝜈(ℎ𝑘𝑙),𝜎 ― 2𝜈(ℎ𝑘𝑙),𝜎 surf Ir O Ir 2

)}(𝜇aq {𝜈(ℎ𝑘𝑙),𝜎 ― 2(𝜈(ℎ𝑘𝑙),𝜎 ― 𝜈(ℎ𝑘𝑙),𝜎 H O Ir H

+

+ 𝜇𝑒 ― ) .

(1)

𝜎 Here, 𝐺(ℎ𝑘𝑙), is the Gibbs free energy of surface termination σ, 𝐺bulk is the Gibbs free energy surf of a bulk unit-cell of rutile IrO2, 𝜇H2O is the chemical potential of water, 𝜇aq H + is the electrochemical potential of a solvated proton, and 𝜇𝑒 ― the electron electrochemical potential in the system. We describe a specific surface termination σ of facet (hkl) with a symmetric slab of surface unit-cell area 𝐴(ℎ𝑘𝑙) and containing 𝜈(ℎ𝑘𝑙),𝜎 , 𝜈(ℎ𝑘𝑙),𝜎 and 𝜈(ℎ𝑘𝑙),𝜎 Ir, O and H atoms, respectively. Ir O H The last term in the first line of eq. (1) accounts thus for any off-stoichiometries in the IrO2 ratio in the considered termination by releasing or taking water molecules from the water environment. As a byproduct, the surface may be (de)protonated, which we assume to occur

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in concert with electron transfer at the metallic IrO2 surface (proton-coupled electron transfer (PCET), the term in the second line of eq. (1)). To avoid the computationally challenging explicit calculation of 𝜇aq H + , we employ the computational hydrogen electrode (CHE) concept of Nørskov and coworkers [23], which relates the sum (𝜇aq H + + 𝜇𝑒 ― ) to the applied potential U (referenced to the RHE, which is reference scale used exclusively in the following). This leaves as central 𝜎 quantities to be determined the solid Gibbs free energies 𝐺(ℎ𝑘𝑙), and 𝐺bulk. For these we follow surf the approach of Reuter and Scheffler [24], and approximate the difference in Gibbs free energies entering eq. (1) with the difference of the corresponding zero-point energy (ZPE)corrected total energy contributions. All ZPE-corrected energy contributions for the molecular or surface systems [25] are then obtained by first-principles density-functional theory (DFT) calculations with the generalizedgradient approximation using the RPBE functional [26]. For the relevant surface terminations determining the particle shape and shown in Fig. 1, solvation effects are approximately taken into account through the SCCS implicit solvation model [27] as detailed in the Supporting Information (SI). Systematic test calculations with increased computational settings as detailed in the SI indicate a convergence of the computed surface free energies within 5 meV/Å2 and no qualitative difference to slab calculations in vacuum. All relevant low-energy terminations have also been calculated with the GGA PBE functional [28] to assess the effect of the approximate DFT functional. Apart from the small relative destabilization of the Obr/OOcus (110)-termination discussed in the text below, all other conclusions regarding the IrO2 instability and shape transformation are equally obtained on the basis of both functionals. 3. Results

Fig. 1: Computed surface free energies 𝛾(ℎ𝑘𝑙),𝜎 of energetically most relevant surface terminations σ at the (110) (left panel) and (111) (right panel) facets of rutile IrO2. The potential range where a termination is most stable is drawn with bold lines. The complete bold line therefore constitutes the surface phase diagram of the corresponding facet in the potential range from open-circuit (U = 0 V vs. RHE) to PEM operating conditions (shaded gray area, taken to be U > 1.3 V, see text). See text for an explanation of the nomenclature used to describe the various terminations.

We have computed the surface free energies of >70 surface terminations according to eq. (1), systematically considering the different possibilities to adsorb O, H, OH, OH2, OOH and OO species at the five symmetry-inequivalent low-index facets of rutile IrO2 in (1 x 1) surface unitcells. Attractive lateral interactions between different adsorbed species and repulsive lateral interactions between the same adsorbed species could stabilize mixed and lower coverage ACS Paragon Plus Environment

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configurations over finite potential ranges. For the (110) facet with the highest surface density of active sites, we have tested this explicitly by additionally considering corresponding configurations in (1 x 2) cells with (co-)adsorbed O, OOH and OO species (that are most relevant for the high-potential range focused on in this work). In agreement with similar findings in preceding computational work on this and other rutile facets [20,21,25,29], this does not show any significant lateral interactions that would render any such configuration most stable in the potential range above 1 V. For the remainder of this work we can therefore restrict the discussion to (1 x 1) terminations. Also stating the symmetry-equivalent facets, the five low-index facets of rutile are the (110), the (011)/(101), the (010)/(100), the (111) and the (001) facets, here ordered in ascending surface energy at 0 K in vacuum. While this ordering is often still used to rationalize the traditional focus on the (110) facet, the true surface free energies in the electrochemical environment depend on pH and the applied potential U, cf. eq. (1). Figure 1 compiles computed surface free energies 𝛾(ℎ𝑘𝑙),𝜎 for energetically most relevant terminations σ at the IrO2(110) and IrO2(111) facet. Analogous plots for the other three facets are provided in the SI. For each facet separately, the lowest-energy terminations at different potentials correspond to the stable phases. In the present implicit pH-dependence at the RHE scale, the resulting surface phase diagrams thus constitute the one-dimensional analog to a Pourbaix diagram. For all five facets, these diagrams show the intuitively expected sequence, starting with fully hydrated or hydroxylated surfaces at open-circuit conditions that are gradually deprotonated at increasing potentials until pure O-terminations and eventually terminations with higher oxidized superoxo species become most stable at OER-relevant potentials U > 1.3 V. As shown in Fig. 1, for the hitherto most studied (110) facet with its two undercoordinated adsorption sites, br (bridge) and cus (coordinatively unsaturated site), the higher-potential transitions in this sequence are for instance at 0.9 V (OHbr/OHcus → Obr/OHcus) and 1.4 V (Obr/OHcus → Obr/OOcus). In previous such ab initio thermodynamics phase diagrams for IrO2(110) OOH and OO terminations were not explicitly considered [19-21]. In the lower potential range, where we can compare to this preceding work, we obtain excellent agreement, despite partly different solvation treatments ranging from pure vacuum in the earliest studies to explicit ice layers and implicit solvation models similar to the one employed here. In contrast to IrO2(110), the IrO2(111) facet features two br and one cus site [29]. Here, the transitions from hydrated to hydroxylated, oxygen-terminated and superoxo-containing phases occur at 0.8 V (Obr1/H2Ocus/Obr2 → Obr1/OHcus/Obr2), 1.3 V (Obr1/OHcus/Obr2 → Obr1/Ocus/Obr2) and 1.4 V (Obr1/Ocus/Obr2 → Obr1/OOcus/Obr2). Even though the transition to the superoxo-containing phase takes thus place at the same potential as on the IrO2(110) facet, the absolute surface free energy of the IrO2(111) facet at this transition potential is much lower. We will see in the following that this has important implications for the shape of rutile IrO2 nanoparticles in this higher potential range. As a consequence, it is also the surface free energy of this Obr1/OOcus/Obr2 phase at IrO2(111) that first becomes negative at further increasing potentials. As apparent from Fig. 1, this zero-transition happens at U = 1.3 V, whereas it happens at 1.4 V and higher for the other facets. As we will argue in the discussion below, this early zero transition raises profound doubts as to the long-term stability of rutile IrO2 catalysts at potentials required for technologically relevant current densities.

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Fig. 2: (Top panels) Potential-dependent equilibrium nanoparticle shape as derived by a Wulff construction based on the most stable surface free energies of each of the five lowindex facets. (Bottom panels) Facet distributions, with color coding as used in the top panels and Fig. 1 for the corresponding terminations. Left: Rod-like shape at open-circuit conditions (U = 0); right: lenticular shape at the OER onset (U = 1.3 V).

When we combine the surface free energies of the lowest-energy termination of each facet at any given potential within a Wulff construction [30], we can predict the equilibrium shape of a nanoparticle at the corresponding potential. Starting with the shape obtained for open-circuit conditions (U = 0 V), Fig. 2 (left) shows a rod-like crystal form. This form is dominated by fully hydroxylated (110) facets (OHbr/OHcus), reflecting the very low surface free energy of this termination in the low potential range, cf. Fig. 1. The O-H bonds of both OH-groups align approximately parallel to the surface and form a network of strong hydrogen bonds, which substantially contributes to the stabilization of these lateral (110) facets [21]. In addition, the Wulff shape also exhibits two apical facets, namely (001) and (011) facets, which together have a joint contribution of about 25% to the total nanoparticle surface. Also the (001) facets are hydrated at U = 0 V. However, overall the effect of the electrochemical environment on the particle shape is negligible in this low potential range and we obtain the typical shape expected for rutile crystals at low temperature in gas phase. This situation changes dramatically at higher potentials, notably due to the strong lowering of the surface free energy of the highly O-rich (111)-(Obr1/OOcus/Obr2)-termination. Starting at U = 0.9 V, we therefore see the emergence of these (111) apical facets in the predicted Wulff shape. Their share of the total nanoparticle surface increases quickly with further increasing potential, until the entire nanoparticle is only formed of these (111) facets for U > 1.3 V. As shown in Fig. 2, this goes hand in hand with a complete change of the nanoparticle shape, which eventually becomes lenticular in the OER regime. 4. Discussion Superoxo species at rutile IrO2: Our results point first of all at the significant presence of highly oxidized O species at the surface of rutile IrO2 at potentials of interest for the OER with technologically relevant current densities. As described initially we assume such potentials to ACS Paragon Plus Environment

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fall around and above 1.5 V (at cell voltages above 1.7 V [4]). Especially for the most investigated IrO2(110) facet our model study finds an OOcus terminated surface to be the most stable in this potential range. X-ray scattering measurements by Rao et al. have recently also identified such species at the related RuO2(110) surface at similar potentials, whereby the insensitivity of the measurements to hydrogen did not allow to differentiate between OOH or OO [31]. Similarly, experiments by Pfeifer et al. at IrO2 particles had also pointed at the presence of highly oxidized O species, there directly correlating these electrophilic species with the OER activity [32]. While early computational studies had dismissed such oxidized terminations at IrO2(110) for this potential range [19,20], in particular the significant stabilization of tilted OOHcus configurations due to hydrogen-bonding with nearby bridging oxygen atoms (or in lower-coverage configurations with nearby Ocus atoms) has been highlighted in a number of mechanistic studies at IrO2(110) and RuO2(110) [21,22,31,33]. With the exception of the work by Rao et al., the latter works have, however, only considered OOHcus and OOcus as fleeting reaction intermediates and not as dominant surface species that define the resting state of the catalyst surface. In this respect though, we note that the strong stability of OOcus that we also confirm in our calculations had been questioned in several of these works. First of all, this is on mechanistic grounds. Starting from an OOcus terminated surface as the resting state, the first and ratedetermining step in the catalytic OER cycle would be the O2 evolution leading to an empty Ircus site at the surface. This is inconsistent with measured Tafel slopes of ~100-120 mV/dec (transfer coefficient ~0.5) in this potential range [17], unless one invokes a potential dependence of this chemical step as has indeed been found for the chemical water dissociation step at IrO2(110) and RuO2(110) [21,33]. The second reason to distrust the stability of OOcus [21,22] is the known overbinding of gas-phase O2 in current generalized-gradient approximation (GGA) functionals [34]. For the PBE functional [28] that was found to yield an optimum description of the bulk IrO2 electronic structure [35] this overbinding is almost 1 eV per O2 molecule. For the here employed RPBE functional that was partly especially designed to mitigate this problem, this overbinding is still 0.7 eV per O2 molecule [26]. With a surface unit-cell area of IrO2(110) of ~20 Å2, this could translate to an underestimation of the surface free energy of the Obr/OOcus termination by ~0.7/20 = 35 meV/Å2 in the current RPBE-based computational setup. However, already an upshift by 17 meV/Å2 would be sufficient to never have this termination correspond to the most stable phase up until a potential of 1.5 V (at the IrO2(111) surface only 9 meV/Å2 would even be required). Instead, we would then obtain the Obr/Ocus termination as most stable. Correspondingly, it would also be the latter configuration that represents the resting state in this potential range close to 1.5 V and that should be chosen as basis for mechanistic studies. For a further mechanistic understanding, also considering other mechanisms circumventing the hyperoxo-formation step [36], a further analysis of the true stability of OOcus at the surface is thus pivotal. Likely, future computational studies will have to address improved functionals beyond present-day GGAs for metallic oxides, as well as a better description of (dynamical and potential-dependent) solvation effects beyond the explicit ice or implicit solvation models considered in this and previous work [37]. In contrast, for the catalyst stability discussion we want to lead here, the uncertainty in the OOcus termination is of lesser importance. The close energetics of this and the competing, second most stable O-rich terminations in the relevant potential range around 1.5 V, cf. Fig. 1, renders it irrelevant, which of the two terminations enters the following stability considerations. If OOcus is eventually indeed confirmed to be overbound with presently tractable computational setups, all arguments and conclusions put forward below will still hold – then being based on the currently second most stable Obr/Ocus or Obr1/Ocus/Obr2 termination at (110) and (111), respectively.

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Stability of rutile IrO2 at OER conditions: With regards to the overall stability of the IrO2 catalyst, a central result of our study are the relatively low potentials at which the surface free energy of different facets become zero. As apparent from Fig. 1, at IrO2(110) this happens at 1.4 V (1.5 V, if OOcus was artificially overbound as discussed above). At IrO2(111) this happens already at 1.3 V (1.3 V, if OOcus was overbound), i.e. for both facets right at or even below the potential range required for technologically viable current densities. A surface free energy below zero indicates a general thermodynamic instability of the corresponding system. The bulk system can gain energy by creating an increasing amount of such surfaces. The identification of this early zero transition is thus a severe finding regarding the stability of IrO2 catalysts for acidic OER in general. Previous computational studies determining ab initio thermodynamics phase diagrams for IrO2 did not pay attention to this aspect as they typically only evaluated relative Gibbs free energy differences [19-21]. A proper referencing to the bulk Gibbs free energy as done in eq. (1) of this work instead allows to explicitly compute the surface free energy and therewith to also evaluate this important stability criterion. An important factor that arises when using this proper bulk reference is thereby the consideration of solvation effects not only through the thermodynamic reservoirs in eq. (1), but also at the electronic structure level. Solvation will generally stabilize a surface compared to 𝜎 vacuum. Correspondingly, the DFT total energy entering 𝐺(ℎ𝑘𝑙), will become lower upon surf consideration of a solvation model and thus also the surface free energy will become lower, thereby shifting any of the above zero transitions to lower potentials. In the present case, without considering any solvation effects (as had been common practice up until recently) we would obtain the zero transition for IrO2(110) and IrO2(111) at 0.2 V and 0.15 V higher potentials, respectively, as compared to the stated results using an implicit solvation model. In contrast, when evaluating only relative Gibbs free energy differences to determine the most stable among different terminations, much of this solvation effect may cancel out (i.e. solvation leads to a similar stabilization of the various terminations). For example, we calculate the potential for the transition from the Obr/Ocus to the Obr/OOcus termination at IrO2(110) as essentially identical with or without solvation model. However, this is not a general result and we do find other transitions to be sensitive to the inclusion of solvation effects. The Obr1/Ocus/Obr2 to Obr1/OOcus/Obr2 transition potential at IrO2(111) shifts for instance by 0.15 V downwards when including implicit solvation in our calculations. Now, an implicit solvation model does certainly not yet capture the full effects of an aqueous environment [37]. Notably, it lacks explicit hydrogen bonding as has recently been highlighted for IrO2(110) by Gauthier et al. [22]. However, for the present stability discussion we would expect such additional solvation effects to generally only lead to an even stronger stabilization of the various surface terminations. This means their surface free energies would become even smaller and possible zero transitions would occur at even lower potentials as determined in this work. Correspondingly, the here identified overall instability of IrO2 at potentials for technologically relevant OER would be even aggravated. Overall, we therefore conclude that the present results raise substantial doubts as to a long-term stability of operating rutile IrO2 catalysts. This is fully consistent with a previous assessment on oxide corrosion on the basis of bulk thermodynamics by Binninger et al. [38] and concurs with the operando studies favoring an amorphous hydrous state as the truly active phase of working IrO2 catalysts [5-10,31,39]. Insight into the structural evolution: Of course, assessments of thermodynamic stability disregard kinetic effects that may stabilize rutile IrO2 particles for any amounts of time. Generally, one could imagine such stabilization to be stronger for larger particles (with a smaller surface to volume ratio) and one could imagine such stabilization to be stronger for lower overpotentials (with lower current densities and a smaller thermodynamic driving force). In this respect, detailed studies in particular on rutile IrO2 single crystals and only probing the ACS Paragon Plus Environment

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OER onset are less likely to contribute to this stability discussion. On the other hand, smallest IrO2 nanocatalysts in the 1-2 nm size range [7,10,13-16] are more prone to a structural transformation. However, corresponding studies, in particular when operating at technologically relevant potentials, have a much lower characterization depth and are often restricted to e.g. an analysis of oxidation states as derived from X-ray photoelectron spectroscopy. A high coverage of superoxo species at the surface of 1-2 nm nanoparticles as suggested by this work may then well be interpreted in terms of a bulk structural transformation. Intriguingly, structural information on the basis of electron microscopy images is furthermore often only provided after synthesis and not operando or after extended hours on stream. Correspondingly, there is presently a pronounced lack of insight into the detailed structural and compositional evolution of working IrO2 catalysts. Also to this end our present study contributes important first leads. First of all, our first-principles Wulff construction predicts a change of the equilibrium particle shape and this at potentials that are much lower than even the OER equilibrium potential. Already at 0.9 V we see the appearance of IrO2(111) facets, which finally make up for the entire particle surface at 1.3 V. This heavily questions the exclusive use of IrO2(110) cus and br sites as relevant active site models for this material. Even if the nanoparticles remained in the rutile crystal structure during long-term OER, then mechanistic studies should at least include the IrO2(111) facet; a facet that has hitherto essentially not received attention at all. However, the shape change is also closely tight to the general rutile instability issue. The zero transition of the surface free energy at the (111) facet occurs already at lower potential than on the other facets, and it is entirely robust to a possible uncertainty in the description of the OOcus species. The zero-transition of the second most stable Obr1/Ocus/Obr2 termination occurs at essentially the same potential, namely already at 1.3 V. In consequence, the present ab initio thermodynamics results suggest the following picture for the onset of the structural evolution of a working IrO2 catalyst: Once reaching a critical potential of 0.9 V, the particle shape will change from rod-like to lenticular, increasingly exhibiting (111) facets. The latter facets are particularly unstable and are (possibly together with undercoordinated atoms at facet edges not yet considered in our Wulff model) the likely attack points for further morphological and compositional transformations at potentials above 1.3 V. The kinetics of both types of transformations (shape and morphology/composition) will be different depending on the particle size and on the applied overpotentials. The large variation with respect to both parameters in existing studies may therefore rationalize the often widely differing reports on structure and activity in terms of varying degrees of completion of the one or the other transformation (with an unknown degree of polycrystallinity of larger nanoparticles likely adding further variation). Both transformations are also particularly consequential for start-up and shut-down scenarios. It is clear that a compositional and morphological transformation (oxidizing and reducing the particle) induced by such ramps is likely to lead to degradation; in the PEM setup with fast exchanging electrolyte prominently through washing out of dissolved Ir or irreversible Ir deposition on the membrane. However, the same would also hold for the here identified shape transformation which equally requires mass transport through dissolved Ir ions – only that the latter transformation begins already at much lower potentials way below the OER onset. 5. Conclusions In summary, we feel there is an urgent need for further in-depth investigations of the structural evolution of rutile IrO2 catalysts to whatever is the active phase under OER conditions up until potentials yielding technologically viable current densities. Not least, this is to establish better models for the active sites that can then serve as basis for future computational studies. Up to ACS Paragon Plus Environment

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now, mechanistic analyses have unanimously focused on the IrO2(110) facet and its undercoordinated Ircus sites. Our present work shows already that this is unlikely a useful model for working IrO2 particles, even if the catalyst remains in its rutile crystal structure. Already at potentials much lower than the OER onset, the nanoparticles start to reshape to eventually only exhibit (111) facets. Correspondingly, computational screening studies that are based on an (110) active site model and employ descriptors extracted from the mechanistic understanding gained are certainly misled. This is important as there are currently strong research efforts towards such materials exploration, be that to thrift the rare Ir component in core-shell structures, in doped Ir oxides or in rutiles that are completely based on more abundant metals. Preliminary calculations predict the same shape transformation to (111)facetted particles for rutile RuO2 catalysts at potentials below the OER equilibrium potential. The intrinsically higher activity of this oxide may not require as high overpotentials as for IrO2 to reach comparable current densities [17,40]. Correspondingly, the here identified shape transformation might be even more relevant for this material, as it may be possible to stay below potentials where a consecutive morphological/compositional transformation as seen for IrO2 would set in.

Supporting Information The Supporting Information is available free of charge on the ACS publications website at DOI: 10.XXXX/XXXXXX. See Supporting Information for additional detailed information on the ab initio thermodynamics approach, the numerical convergence of the underlying DFT calculations, as well as for a detailed presentation of the surface phase diagrams analogous to Fig. 1 for the (011)/(101), the (010)/(100), and the (001) facet. Acknowledgements This research was supported by the Kopernikus/P2X programme (Cluster FC-A1) of the German Federal Ministry of Education and Research. We acknowledge PRACE for awarding us access to Curie at GENCI@CEA, France. References (1) Carmo, M.; Fritz, D. L.; Mergel, J.; Stolten, D. A Comprehensive Review on PEM Water Electrolysis. Int. J. Hydrogen Energy 2013, 38, 4901–4934. (2) Buttler, A.; Spliethoff, H. Current Status of Water Electrolysis for Energy Storage, Grid Balancing and Sector Coupling via Power-to-Gas and Power-to-Liquids: A Review. Renewable Sustainable Energy Rev 2018, 82, 2440–2454. (3) Cherevko, S.; Geiger, S.; Kasian, O.; Kulyk, N.; Grote, J.-P.; Savan, A.; Shrestha, B. R.; Merzlikin, S.; Breitbach, B.; Ludwig, A.; Mayrhofer, K. J. J. Oxygen and Hydrogen Evolution Reactions on Ru, RuO2, Ir, and IrO2 Thin Film Electrodes in Acidic and Alkaline Electrolytes: A Comparative Study on Activity and Stability. Catal. Today 2016, 262, 170–180. (4) Bernt, M.; Gasteiger, H. A. Influence of Ionomer Content in IrO2 /TiO2 Electrodes on PEM Water Electrolyzer Performance. J. Electrochem. Soc. 2016, 163, F3179-F3189.

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