Fe Oxyhydroxides supported on Perovskite Oxides as Oxygen

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Co/Fe Oxyhydroxides supported on Perovskite Oxides as Oxygen Evolution Reaction Catalyst Systems Xi Cheng, Bae-Jung Kim, Emiliana Fabbri, and Thomas J. Schmidt ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b04456 • Publication Date (Web): 30 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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ACS Applied Materials & Interfaces

Co/Fe Oxyhydroxides supported on Perovskite Oxides as Oxygen Evolution Reaction Catalyst Systems Xi Cheng,1 Bae-Jung Kim, 1 Emiliana Fabbri,1* and Thomas J. Schmidt 1,2 1

Electrochemistry Laboratory, Paul Scherrer Institut, Forschungsstrasse 111, 5232 Villigen PSI,

Switzerland 2Laboratory

of Physical Chemistry, ETH Zürich, 8093 Zürich, Switzerland

KEYWORDS Electrolysis, Water splitting, CoOOH, catalyst/support interaction, operando XAS

ABSTRACT Co/Fe oxyhydroxide catalysts have been deposited onto the surface of amorphous carbon or different perovskite oxides. By performing electrochemical characterizations and operando Xray absorption spectroscopy measurements, novel insights into Co-Fe oxyhydroxide catalysts and their interactions with perovskite oxides have been revealed. The addition of Fe into Co oxyhydroxide catalysts greatly enhances the OER activity by stabilizing the Co cations into a lower oxidation state under operative conditions compared to the case of undoped Co oxyhydroxide. A beneficial Co-Fe electronic interaction for OER can be also achieved by

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depositing Co oxyhydroxide on Fe containing oxide supports, such as the LaFeO3 perovskite. Finally, it was found that despite the lower Fe content in the Ba0.5Sr0.5Co0.8Fe0.2O3- (BSCF) perovskite structure, Co oxyhydroxides supported on this perovskite exhibit the highest OER activity. Therefore, our findings suggest that perovskite structures presenting a large content of oxygen vacancies and undergoing surface reconstruction, such as BSCF, offer the best interface for Co oxyhydroxides. Finally, profiting from the beneficial Co-Fe electronic interaction and the perovskite interface interaction, the highest OER activity has been achieved by depositing Co-Fe oxyhydroxide on the surface of BSCF perovskite.

INTRODUCTION In the last decade, the growth and diffusion of renewable energy is clearly remarkable. However, while primary renewable energies present the advantages of being sustainable with minor negative impacts on the environment and the human health, the majorities are intermittent with considerable variability in the supply. In this scenario, electrolyzers are vital for the development of a renewable energy-based economy. Indeed, the excess of secondary energy produced by primary renewable sources can be converted by electrolyzers into H2, which can be easily stored even for a long term and reused where and when it is required. The electrochemical reaction mostly hampering the performance of water electrolyzers is the anodic water splitting reaction, i.e., the oxygen evolution reaction (OER). Therefore, the search for highly active anodic catalysts is currently extremely lively. 2 ACS Paragon Plus Environment

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Perovskite oxides show potentials of being candidates as electrode catalysts for the OER in alkaline electrolytes.1-7 By partial substituting in the ABO3- perovskite structure the A- and/or B-site bwith other elements, some complex perovskite compositions have shown very high OER activity.6, 8-14 Moreover, it is generally acknowledged that the B site of the perovskite structure is the active site,15 which is, for most of the OER active perovskites, occupied by cobalt and iron in different ratio.8, 13-14, 16-18 Our previous study have demonstrated that Ba0.5Sr0.5Co0.8Fe0.2O3- (BSCF) nanopowder exhibits an outstanding performance both at laboratory and industrial scale.16 Operando X-ray absorption spectroscopy (XAS) measurements (i.e., direct observation of changes of the catalyst’s electronic and local structures during the water splitting reaction under operative conditions) have shown that the unique features of BSCF allow during the OER a dynamic self-reconstruction of the material’s surface. We have further suggested that the flexible perovskite structures presenting a large content of oxygen vacancy, such as BSCF,16 favor the occurrence of the lattice oxygen evolution reaction (LOER)14-15, 19 and the operando formation of a Co/Fe oxyhydroxide surface layer16 which might actually contribute to the excellent OER activity of BSCF. Therefore, it appears crucial to understand if the key for a high OER activity is only the self-assembled oxyhydroxide layer or the interaction between the oxyhydroxide layer and the underneath perovskite structure. In addition to this open research question, a comprehensive literature regarding the reason behind the improvement in the OER activity of Fe-substituted Co oxide-based catalysts (both as binary CoOx or as perovskite structure, i.e. ACoO3) is currently not available.16, 20-23 In order to obtain more fundamental understandings about the operando self-assembled Co/Fe oxyhydroxide on the surface of perovskite catalysts, herein, we propose a systematic study where Co/Fe oxyhydroxides are intentionally deposited on different substrates. In particular, using a 3 ACS Paragon Plus Environment


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chemical precipitation/co-precipitation method we have investigated the electrochemical properties of single metal oxyhydroxides (single Co or Fe oxyhydroxides) and mixed Co-Fe oxyhydroxides deposited directly on to the surface of different perovskites and carbon supports (the latter being used only as model substrate to study underlying mechanisms rather than being proposed as technical suitable support). Furthermore, standard ex situ characterizations might not fully reveal the structural and the electronic properties of the catalyst system under OER conditions, where the applied potential can induce substantial modifications to the catalyst chemical state and structure.16, 24 In this regard, XAS measurements carried out at operating conditions and at different applied potentials (operando mode) have been used as a valuable tool to investigate the electronic structure of the catalysts of interest during the water splitting reaction.

Regarding single and mixed oxyhydroxides deposited on a carbon support, our study highlights that although the Fe oxyhydroxide is a poor OER catalyst, a relatively small incorporation of Fe into Co oxyhydroxide structure can significantly enhance the OER activity of the Co oxyhydroxide catalysts (which also largely surpasses the OER activity of the Fe oxyhydroxide). Operando XAS measurements suggest that the improvement in the OER activity of the Co oxyhydroxide by Fe doping is due to the suppression of typical 2+/3+ Co redox couple between 1 and 1.3 VRHE, stabilizing the overall Co oxidation state to a value below 3+ even during the OER process. A similar effect as recently proposed for Fe incorporation into NiO.25 Furthermore, in order to confirm if an interaction does actually exist between a superficial oxyhydroxide catalyst and an underneath perovskite, Co oxyhydroxide has been also deposited


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onto the surface of different perovskites. The OER results suggest that the unique features of BSCF perovskite are an indispensable to achieve high OER activity for Co oxyhydroxide, significantly exceeding the OER activity of an identical Co oxyhydroxide structure supported on “inert” substrates such as carbon or different perovskite structures, e.g., LaCoO3. Among all the investigated catalysts, the highest OER activity can be achieved by depositing a mixed Co-Fe oxyhydroxide onto the surface of BSCF. Thus, the present work highlights for the first time (i) the effect of Fe substitution into Co oxyhydroxide catalysts by operando characterizations and (ii) the unique features of BSCF perovskite as “active” oxyhydroxide support to achieve high OER activity.

RESULTS AND DISCUSSION Different samples made of Co-Fe oxyhydroxides deposited by a chemical precipitation/coprecipitation method on the surface of different substrates (carbon and perovskite oxides with different composition) have been prepared as illustrated in Figure 1. Scanning electron microscopy (SEM) investigations have revealed that the oxyhydroxide are dispersed in form of agglomerated particles on the carbon and perovskite support (see Figure 1). Interestingly, even though the particle size of the carbon support is much smaller than that of the perovskites, being the first below 200 nm and the latter above 1 mm, a better oxyhydroxide dispersion seems to be achieved using the perovskite support. Energy-dispersive X-ray spectroscopy (EDX) elemental analysis has been also performed for the Co-Feoxh/C electrode revealing a Co to Fe atomic ratio of 5 to 1.



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Figure 1. On top: Sketch of the investigated samples. Random grey spheres (first line from the top) represent the carbon support, while black and grey spheres resemble a perovskite structure (second line from the top). In particular, LaCoO3, La0.2Sr0.8CoO3, LaFeO3 and Ba0.5Sr0.5Co0.8Fe0.2O3 (BSCF) were used as support for the Co oxyhydroxide phase, while only BSCF was investigated as support for the Co-Fe oxyhydroxide. Red, green, and violet spheres represent Co, Fe, and Co-Fe oxyhydroxide particles, respectively. On the bottom: SEM micrographs of Co-Fe oxyhydroxide deposited on carbon support (left side) and on BSCF perovskite (right side).

Co/Fe oxyhydroxides deposited on carbon support


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In order to gain a fundamental understanding of the electrochemical properties of Co/Fe oxyhydroxides, cyclic voltammetry (CV) and chronoamperometric measurements have been performed, sequentially, on single metal oxyhydroxides (single Co or Fe oxyhydroxides) and on mixed Co-Fe oxyhydroxides deposited on functionalized acetylene black carbon substrate (C).26 Furthermore, ex situ and operando X-ray absorption spectroscopy (XAS) was used to follow potential-induced changes of the prepared catalysts in their local electronic and geometric structure. Figure 2 shows CVs recorded between 1 and 1.7 VRHE at 10 mV s-1 for the three different oxyhydroxide catalysts named Cooxh/C, Feoxh/C, and Co-Feoxh/C, respectively. After the first CV (for more details, refer to Supporting Information Fig.S1) the Cooxh/C electrode presents a stable, quasi-reversible Co(III)/Co(II) redox couple (oxidation peak at ~1.15 VRHE, reduction peak at ~1.08 VRHE).21, 23 Differently, Feoxh/C electrodes do not show any redox features in the investigated potential range (Fig. 2b).21, 23 Finally, the mixed Co-Feoxh/C electrodes present a barely visible Co(III)/Co(II) redox couple (oxidation peak at ~1.24 VRHE, reduction peak at ~1.16 VRHE) in the investigated potential range (Fig. 2c) indicating that the presence of Fe significantly suppresses or shifts the Co(III)/Co(II) redox couple to higher potentials (i.e. in the OER regime where is no more detectable).

Figure 2. Cyclic voltammograms (30th cycles at 10 mV s-1) of (a) Cooxh/C, (b) Feoxh/C, and (c) Co-Feoxh/C electrodes in 0.1 M KOH electrolyte saturated with synthetic air at 1600 rpm.



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Overall, the cyclic voltammetry results indicate that a strong electronic interaction must occur between the Co and Fe cations in the Co-Feoxh/C electrodes leading to a suppression or a shift to higher potential of the Co(III)/Co(II) redox couple and, thus, most likely to a modification of the Co electronic structure. This point is further verified by ex situ and operando X-ray absorption near edge structure (XANES) measurements. XANES spectra at the Co K-edge of the Cooxh/C and Co-Feoxh/C precursors show that Co cations have initially the same oxidation state close to 2+, consistent with the fact that the same Co(NO3)2 precursor has been used for the two electrodes (Figure 3a). When in contact with KOH electrolyte under open circuit (OC), the Co oxidation state of Cooxh/C sample increases significantly (close to that of Co3O4), while no significant changes were observed for the Co K-edge of the Co-Feoxh/C electrode (Figure 3b). This can be presumably ascribed to an electronic interaction between Co and Fe cations, the latter stabilizing the former in a ~2+ oxidation state.

Figure 3. XANES spectra recorded at the Co K-edge of (a) the Cooxh/C and Co-Feoxh/C precursors and, (b) after the Cooxh/C and Co-Feoxh/C electrodes immersed in the KOH electrolyte


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under open circuit (OC). Additionally the Co K-edge XANES spectra of commercial CoO and Co3O4 (Aldrich) are taken as reference

Figure 4 shows the XANES spectra at Co K-edge of Cooxh/C and Co-Feoxh/C electrodes under the selected applied potentials (operando conditions). By anodically polarizing the electrode up to 1.45 VRHE, the Co K-edge position of Cooxh/C sample shifts towards a higher energy revealing an increase in the average Co oxidation state. When the potential is cathodically reversed to 1.00 VRHE, the Co K-edge of Cooxh/C shifts back to a lower energy overlapping in a fair extend with the initial Co K-edge at 1.00 VRHE during the anodic scan, which indicates a quasi-reversible change in the Co oxidation state. This result is consistent with the quasi-reversible Co(III)/Co(II) redox couple observed in the CVs of Cooxh/C. Regarding the Co K-edge of Co-Feoxh/C electrodes, a quasi-irreversible shift toward a higher energy is observed during the polarization up to 1.45 VRHE, but still an average Co oxidation state lower than ~2.67 (formal Co oxidation state for the reference sample Co3O4) is maintained, indicating that in the mixed Co-Feoxh catalyst the presence of Fe enables the stabilization of Co cations in a lower oxidation state (below 3+) than compared to that of Co cations in Cooxh. This can explain the suppression of the quasi-reversible Co(III)/Co(II) redox couple in the cyclic voltammetry of Co-Feoxh/C electrodes. The operando XANES spectra for Feoxh/C indicates that the oxidation state of Fe does not change under the applied potentials (See supporting information Fig.S2).



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Figure 4. XANES spectra recorded at the Co K-edge of (a) the Cooxh/C and, (b) Co-Feoxh/C electrodes during different anodic and cathodic potential holding in 0.1 M KOH and compared with the XANES spectra of CoO and Co3O4 commercial metal oxide (Aldrich).

The Tafel plots and the OER activity (in terms of A gmetals-1 at 1.55 VRHE) of the Cooxh/C, Feoxh/C and, Co-Feoxh/C electrodes are summarized in Figure 5. The Fe oxyhydroxide deposited on carbon exhibits an extremely low activity towards the OER, while considerably higher current density is observed for the Cooxh/C sample. Nonetheless, the Co-Feoxh/C sample shows almost 4 times higher OER activity than the single Co oxyhydroxide catalyst. Differently, the Tafel slopes were very similar among the three samples, being 51, 56, and 55 mV/dec for Cooxh/C, Feoxh/C and, Co-Feoxh/C, respectively. The OER activity results indicate that the presence of Fe, with the effect of stabilizing Co cations in a lower oxidation state, is extremely beneficial to boost the OER activity of Co oxyhydroxide catalysts. Several reports have demonstrated that the presence of Fe could enhance the OER performance of Co-based oxide/oxyhydroxide catalysts.20-23, 27 However, to the best of our knowledge, this study reports first time that the operando behavior of


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Co-based oxyhydroxide catalysts, in terms of Co electronic state changes during polarization into the oxygen evolution regime.

Figure 5. (a) Tafel plots and (b) OER activity expressed as mass normalized current at 1.55 VRHE for the Cooxh/C, Feoxh/C and, Co-Feoxh/C electrodes.

Co/Fe oxyhydroxides deposited on perovskite oxides Figure 6 shows CVs recorded between 1 and 1.7 VRHE at 10 mV s-1 of Co oxyhydroxide deposited on different perovskite oxides; namely, Cooxh/LaCoO3, Cooxh/La0.2Sr0.8CoO3, Cooxh/LaFeO3, and Cooxh/BSCF. The electrodes made of Cooxh/LaCoO3 and Cooxh/La0.2Sr0.8CoO3 exhibit a quasi-reversible Co(III)/Co(II) redox couple (oxidation peak at ~1.16 VRHE, reduction peak at +1.10 VRHE). Differently for the Cooxh/LaFeO3 sample, the quasi-reversible Co(III)/Co(II) redox couple is barely visible and shifted to higher potentials. According to the operando XAS results for Co-Feoxh/C (Figure 2), the suppression of the Co(III)/Co(II) redox couple can be



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ascribed to an electronic interaction of Co cations in the Co oxyhydroxide structure with the Fe cations from the perovskite support, which enables the stabilization of Co cations in a lower oxidation state. For Cooxh/BSCF the pseudo-capacitive current changes upon cycling. The initial CV shows a redox couple at about 1.20/1.08 VRHE, which could be ascribed to a Co(III)/Co(II) redox couple but shifted to higher potentials compared to that observed for a Co oxyhydroxide supported on carbon. A strong suppression of the Co(III)/Co(II) redox couple is not observed for the Cooxh/BSCF compared to the case of Cooxh/LaFeO3 probably because the amount of Fe in BSCF is much lower than that in LaFeO3 to significantly interact with the Co cations in the superficial oxyhydroxide structures (for BSCF, only 20% of the B-sites are occupied by Fe, in the case of LaFeO3, this percentage is 100%). Therefore, presumably only a very limited fraction of the Co cations in the superficial oxyhydroxide structures can be stabilized in a lower oxidation level by the Fe in BSCF. The significant change in the CV of Cooxh/BSCF upon potential cycling is most likely due to BSCF surface reconstruction, as shown previously in ref 16. Indeed, both the CVs of BSCF (only the bare perovskite, without the addition of Co oxyhydroxide) and Cooxh/BSCF shows drastic changes in the CV upon potential cycling in the OER range (See Supporting Information Fig.S3).


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Figure 6. Cyclic voltammograms (30th cycle) recorded at at 10 mV s-1 for (a) Cooxh/LaCoO3, (b) Cooxh/La0.2Sr0.8CoO3, (c) Cooxh/LaFeO3 and, (d) Cooxh/BSCF electrodes in 0.1 M KOH electrolyte saturated with synthetic air at 1600 rpm.

To verify that in Cooxh/BSCF electrodes not a significant fraction of Co cations in the superficial oxyhydroxide structures can be stabilized in a lower oxidation, XANES spectra at Co K-edge of Cooxh/BSCF electrode under OC and selected applied potentials were recorded and they are shown in Figure 7. Figure 7a demonstrates that the initial Co K-edge of Cooxh/BSCF electrode is at a similar energy position than that of Co3O4 reference sample (as shown previously for



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Cooxh/C electrode) indicating that a very limited fraction Co cations in the Co oxyhydroxide supported by BSCF are stabilized in a lower oxidation state. By anodically polarizing the electrode up to 1.45 VRHE (Figure 7b), the Co K-edge position of Cooxh/BSCF electrode shifts towards a higher energy, revealing an increase in the Co oxidation state. When the potential is cathodically reversed to 1.00 VRHE, the Co K-edge of Cooxh/BSCF slightly shifts back towards a lower energy, but without fully recovering to its initial position. Therefore, the Co cation in Cooxh/BSCF under operative conditions present an intermediate behavior between those of Co and Co-Fe oxyhydroxide catalysts supported on carbon. Indeed, even though in the Co K-edge of Cooxh/BSCF at 1.45 VRHE shifts to higher energies than that of Co3O4 reference sample, as in the case of Cooxh/C electrode, cathodically reversing the potential to 1.00 VRHE leads to an irreversible change in the Co oxidation state as in the case of Co-Feoxh/C electrode. It must be acknowledged, though, that also the Co cations of the BSCF perovskite contribute to the Co Kedge spectra. However, all the perovskite oxides produced by sol gel in the present study possess a low surface area as reported in ref. 14, and thus, a low surface to bulk ratio. In other words, only a small fraction of the perovskite Co cations can take place in the OER possibly modifying their oxidation state.


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Figure 7. XANES spectra recorded at the Co K-edge (a) of the Cooxh/C, Co-Feoxh/C and Cooxh/BSCF electrode after immersion in the KOH electrolyte under open circuit (OC) and (b) of Cooxh/BSCF electrode during different anodic and cathodic potential holding in 0.1 M KOH. The XANES spectra of CoO and Co3O4 commercial metal oxide (Aldrich) are also included.

OER activity comparison The OER activity in terms of mass current densities normalized by the precipitated Co oxyhydroxide on the surface of different perovskites (A gCo-1 at 1.55 VRHE) is summarized in Figure 8. These results are also compared to the OER activity of the Co oxyhydroxide supported on carbon to comprehend if an interaction does exist between the oxyhydroxide structure and the substrate underneath. Furthermore, for comparing only the OER activity of the pure precipitated oxyhydroxides, the OER current of the mixed oxyhydroxide/support electrodes is background corrected by the current measured from the support (carbon or perovskite support) under identical conditions. The OER activity of the Co oxyhydroxide on the LaCoO3 and La0.2Sr0.8CoO3 surface are quite similar to that Cooxh/C, indicating that these two perovskites serves as a pure support and do not have any interaction with the Co oxyhydroxide structure. Secondly, the OER activity of the Co oxyhydroxide supported on the LaFeO3 is higher than that of Cooxh/LaCoO3, Cooxh/La0.2Sr0.8CoO3 and, Cooxh/C. Given the enhanced OER activity of CoFeoxh/C electrode compared to that of Cooxh/C, we can deduce that the higher OER activity of the Cooxh/LaFeO3 is the results of the electronic interaction between the Fe cations of the perovskite structure and the Co cations in the oxyhydroxide. Finally, among the series of Co oxyhydroxides deposited on different perovskite oxides, Cooxh/BSCF shows the highest OER activity. However,



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we have previously shown that the small amount of Fe in BSCF could not stabilize all the Co cations of the oxyhydroxide into a lower oxidation state (i.e. below the formal Co oxidation state of Co3O4) as it happens in the case of Co-Fe oxyhydroxide. Therefore, only a limited Fe-Co electronic interaction beneficial for the OER activity is expected in the Cooxh/BSCF electrodes. The activities of the Cooxh/perovskite systems are also not a function of the oxide electronic conductivity as the ex situ conductivities of LaCoO3, La0.2Sr0.8CoO3, LaFeO3 and, BSCF are 3.25 10-5 Scm-1, 0.23 Scm-1, 8.9 10-9 Scm-1 and, 0.0032 Scm-1, respectively (i.e. in decreasing order La0.2Sr0.8CoO3>BSCF> LaCoO3> LaFeO3). Therefore, other interactions besides the Co-Fe interaction or the electronic conductivity of the support must contribute to the highest activity of Cooxh/BSCF electrodes. We have previously reported that BSCF presents a flexible structure with a large content of oxygen vacancies (see supporting information Table S1), which can favor the operando formation of a highly OER-active Co/Fe oxyhydroxide surface phase.16 The present results seem to indicate that such a flexible structure with a large oxygen vacancy content (=0.66 in the as prepared Ba0.5Sr0.5Co0.8Fe0.2O3-) might also play a role in boosting the OER activity of a deposited Co oxyhydroxide structure. Therefore, it is then reasonable to assume that an interaction between the oxyhydroxide and the BSCF does exist and this interaction is responsible for the enhanced OER activity.


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Figure 8. (a) Tafel plot and (b) OER activity expressed in terms of A gCooxh-1 at 1.55 VRHE of the additional Co oxyhydroxide on different supports, i.e. carbon (C), LaCoO3, La0.2Sr0.8CoO3, LaFeO3 and, BSCF. From the Tafel plot in Figure 8a, Tafel slopes of 52, 57, 59, 50 and, 62 mV/dec have been calculated for Cooxh/C, Cooxh/LaCoO3, Cooxh/La0.2Sr0.8CoO3, Cooxh/LaFeO3 and, Cooxh/BSCF, respectively.

Based on these results, the two distinct interactions revealed above, i.e. the Fe-Co electronic interaction and the perovskite/oxyhydroxide interaction should be combined to maximize the OER activity by depositing directly the Co-Fe oxyhydroxide catalyst on the surface of BSCF. Figure 9 shows that the OER activity of the Co-Feoxh/BSCF electrode is almost 5 times higher than that of Co-Feoxh/C. Since these two electrodes are prepared by the same condition (i.e. direct deposition of additional Co-Fe oxyhydroxide) the difference between their OER activities can be attributed to pure support effect, that is, the interaction between BSCF and the oxyhydroxide. The improved OER activity of Co-Feoxh/BSCF electrode compared that of the other catalyst systems clearly indicates that perovskite oxides able to dynamically self-assemble a superficial



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Co-Fe oxyhydroxide phase, as BSCF,13, 16 do not merely play the role of precursors of the active superficial phase, but are essential to support high OER activity. .

Figure 9. Tafel plots of for Cooxh/C, Co-Feoxh/C and, Co-Feoxh/BSCF electrode measured in 0.1 M KOH electrolyte.

CONCLUSIONS A chemical precipitation/co-precipitation method has been used to directly deposit Co/Fe oxyhydroxide catalysts on the surface of amorphous carbon and perovskite oxides. Combining electrochemical characterizations and operando X-ray absorption spectroscopy measurements, we could reveal that the presence of Fe in mixed Co-Fe oxyhydroxide catalysts stabilizes the Co cations in a lower oxidation state, boosting the OER activity. Furthermore, the comparison of the OER activity of Co/Fe oxyhydroxides on different supports suggests that also interplay between the oxyhydroxide and the support can take place. One interplay can be attributed to the Fe-Co electronic interaction as observed in mixed Co-Fe oxyhydroxides supported on carbon, when the 18 ACS Paragon Plus Environment

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support structure contains a large number of Fe cations as in the case of Cooxh/LaFeO3 electrodes. Furthermore, it has been also revealed that flexible perovskite support structures with a large content of oxygen vacancies, as BSCF, also leads to an enhanced Co-Fe oxyhydroxide OER activity, being Co-Feoxh/BSCF the most OER active sample among the whole Co/Fe oxyhydroxides supported on carbon and perovskite supports. This result indicates that to achieve the highest activity, the perovskite that eventually build up a self-assembled, superficial oxyhydroxide phase is not only a bare precursor of the active phase but plays an essential role in sustaining high OER activity.

MATERIALS AND METHODS Sample preparation Perovskites in forms of powders were synthesized using a modified sol-gel process. Stoichiometric quantities of La2O3 ( from Aldrich, 99.9%), Sr(NO3)2 ( from Aldrich, 99%), Ba(NO3)2 (from Aldrich, 99%), Co(NO3)2 · 6H2O (from Aldrich, 98%) and, Fe(NO3)3 · 9H2O (from Aldrich, 99.5%) were initially dissolved in a 0.2 M nitric acid aqueous solution. Citric acid was added as a chelating agent using a 2:1 ratio with respect to the total content of metal cations. After a transparent solution was obtained, the pH value was adjusted between 9 -10 by adding NH4OH. Subsequently, the prepared solution was heated under stirring until a viscous gel was obtained. Finally, the gel was ignited to flame, resulting in ash. In order to obtain single phase perovskite catalysts, the powders were calcined in air at 1050°C for 2 h. Before use the acetylene black carbon (Alfa Aesar) was treated as reported in reference 26.



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Physico-chemical characterizations Phase identification of the synthesized perovskite powders was performed by X-ray diffraction (XRD, Bruker D8 system) with Cu K polychromatic radiation ( = 0.15418 nm) in a Bragg Brentano geometry (See supporting information Fig.S4). The catalyst microstructure was investigated by scanning electron microscope (SEM, Zeiss ULTRA 55) while elemental analysis was investigated by energy-dispersive X-ray spectroscopy (EDX, APOLLO XV).

Electrode preparation and electrochemical characterization For the electrochemical characterization, thin-film rotating disk electrode (RDE) measurements were carried out as reported in reference.30-31 Thin film electrodes32 of the bare supports were deposited from ink suspension made of 7.5 mg of the perovskite powder or 2.5 mg of acetylene black carbon, 2500 L of isopropanol and 10 µL of Na+ exchanged Nafion binder. All the inks have been initially sonicated for 30 mins and, successively, rotating glassy carbon electrodes (mirror polished with a total surface area of0.196 cm2) were drop-coated with 10 µL of the sonicated. The OER activity of the supports was measured using a home-made Teflon cell connected to potentiostat (Biologic VMP-300). The electrochemical measurements were carried out at room temperature in synthetic air-saturated 0.1 M KOH electrolyte (prepared from Milli-Q water and KOH from Sigma Aldrich, 99.99%), using a hydrogen reference electrode (RHE) separated by a salt bridge and a gold counter electrode. Initially, 30 cyclic voltammograms between 1-1.7 VRHE were recorded at 10 mV s-1 and 1600 rpm in electrolyte. Subsequently, chronoamperometry measurements were performed by holding each selected potential in the1-


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1.7 VRHE range for 30 sec, achieving a steady-state current with no capacitive contribution. All potentials were corrected for the ohmic-drop determined by impedance spectroscopy. To deposit the metal oxyhydroxide onto the surface of acetylene black carbon or perovskite support, 10 uL of the ink made of acetylene black carbon or perovskite support (prepared as reported before) were firstly dropped on the rotating mirror polished glassy carbon electrode. Secondly, before the ink becomes totally dry, 5 uL of known concentration of Co(NO3)2 or Fe(NO3)3 or mixed Co-Fe nitrate solution (Co to Fe ratio is 4:1) were added on the same glassy carbon electrode and then dried. Finally, the glassy carbon electrode was immerged into the 0.1 M KOH electrolyte. The nitrate salt is then precipitated as metal oxyhydroxide onto the surface of the support. The electrochemical measurements were performed directly on the as prepared electrodes without any further material treatment. According to the solubility product calculation (Ksp), the Co and Fe cation are fully precipitated in 0.1 M KOH electrolyte. (See supporting information: solubility calculation). The loading of the metal oxyhydroxide is controlled by the nitrate salt concentration. The relationship between the metal oxyhydroxide loading and the OER activity was firstly studied to find the optimized metal oxyhydroxide loading for the OER measurements (See supporting information Fig. S5). The selected metal loading was 316 gcm-2 for the Cooxh/C, Feoxh/C and, Co-Feoxh/C electrodes and 180 gcm-2 for the Cooxh/perovskite, Feoxh/perovskite and, Co-Feoxh/perovskite electrodes. To obtain the OER activity of the pure precipitated oxyhydroxide catalysts, firstly, the OER current of the mixed oxyhydroxide/support electrodes is measured using the same configuration and the same protocol as described for the supports above. Then the oxyhydroxide/support OER current is background corrected by the current measured from the support under identical conditions. Finally, the background corrected current has been normalized by the metal mass 21 ACS Paragon Plus Environment


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contained in the oxyhydroxide catalyst. This specific current is taken as a parameter for the OER activity of the precipitated metal oxyhydroxide catalysts. The stability protocol consists of switching the potential between 1 and 1.6 VRHE in 0.1 M KOH electrolyte, with a holding time at each potential for 10 sec and a rotation rate of 1600 rpm to avoid bubble formation. Each potential switch between 1 and 1.6 VRHE has been repeated 500 times (number of cycles). Every 25 cycles chronoamperometric measurements between 1.0 and 1.7 VRHE as described above has been recorded and the electrode activity is evaluated at 1.6 VRHE. To identify the change in OER activity over the stability test (i.e., number of cycles) the percentage of the current compared to the initial value at 1.6 VRHE is plotted vs the number of cycles (See supporting information Fig. S6). For a measurement of the ex situ conductivity of the supporting perovskites, impedance spectroscopy measurements were performed as reported in details in reference 14. In brief, keeping the oxide powders under a constant 0.6 MPa pressure for 5 min, by 4-wire impedance spectroscopy measurements the electrical resistivity was evaluated. The measurements were carried out applying a bias of 100 mV within a frequency range of 1 MHz and 1 Hz and at room temperature. After the Ohmic resistance (R) of the electrode was extrapolated from the obtained Nyquist plots, the total conductivity (σ) was calculated using the formula: σ = L/RA where A and L represent the area and the thickness of the pressed powder, respectively.

Operando X-ray absorption spectroscopy (XAS) For X-ray absorption spectroscopy (XAS) measurements, acetylene black carbon or perovskite powders with nitrate salt were dispersed in H2O/isopropanol/Nafion (4:3:2) solution and then 22 ACS Paragon Plus Environment

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sonicated for 30 min. To obtain relatively tick catalyst film for the XAS operando measurements, the inks were spray coated on a custom-made Kapton film coated with gold (for more details see references 10 and 13 . Operando XAS experiments were also performed using the same protocol reported in reference 10 and 13. For more details see the Supporting information.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge. Figure S1. Initial 30 cyclic voltammograms at 10 mV s-1 in air-saturated 0.1 M KOH electrolyte at 1600 rpm for the Cooxh/C electrode. Figure S2. XANES spectra recorded at the Fe K-edge of the Feoxh/C electrode during different anodic and cathodic potential holding in 0.1 M KOH. Figure S3. Cyclic voltammograms (30 cycles at 10 mV s-1) of a) BSCF and b) Cooxh/BSCF electrodes in air-saturated 0.1 M KOH electrolyte at 1600 rpm. Table S1. Perovskite oxygen vacancy content (). Figure S4. X-ray diffraction pattern of LaCoO3, La0.2Sr0.8CoO3, LaFeO3 and, BSCF perovskites. Solubility calculation Figure S5. Measured current vs. additional Co loading for different support materials. Figure S6. Electrochemical stability tests for Co-Feoxh/BSCF and BSCF electrodes perfomed by a series of chronoamperometric potential-step experiments in which the potential was stepped from 1.0 V to 1.6 VRHE. Operando X-ray absorption spectroscopy methods

AUTHOR INFORMATION 23 ACS Paragon Plus Environment


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Corresponding Author *[email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT The authors gratefully acknowledge the Swiss National Science Foundation through its Ambizione Program, Innosuisse and, the Swiss Competence Center for Energy Research (SCCER) eat & Electricity Storage for financial contributions to this work. The authors thank the Swiss Light Source for providing beamtime at the SuperXAS beamline.

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Fe Oxyhydroxides supported on Perovskite Oxides as Oxygen

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