ProtonâElectron Conductivity in Thin Films of a CobaltâOxygen

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Proton−Electron Conductivity in Thin Films of a Cobalt−Oxygen Evolving Catalyst Casey N. Brodsky,† D. Kwabena Bediako,† Chenyang Shi,‡ Thomas P. Keane,† Cyrille Costentin,*,†,§ Simon J. L. Billinge,*,‡,∥ and Daniel G. Nocera*,†

ACS Appl. Energy Mater. Downloaded from pubs.acs.org by 5.62.154.38 on 08/24/18. For personal use only.

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Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138, United States ‡ Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York 10027, United States § Laboratoire d’Electrochimie Moléculaire and Unité Mixte de Recherche UniversitéCNRS No. 7591, Université Paris Diderot, Sorbonne Paris Cité, Bâtiment Lavoisier, 15 rue Jean de Baïf, 75205 Cedex 13 Paris, France ∥ Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973, United States S Supporting Information *

ABSTRACT: The conductivities of thin film cobalt oxygen evolving catalysts (Co-OECs) are examined for four variants differing in phosphate and borate anion compositions and pH. The effects of these parameters on the Co-OEC activity is explored by (a) examination of the intermediate range structural order within films through high energy X-ray scattering and pair distribution function analysis, and (b) direct measurement of film conductivity, which has been measured with an interdigitated array microelectrode. We find that the nature of the anion is a critical determinant of the size of cobaltate clusters comprising the Co-OEC thin films which in turn is a critical determinant of film conductivity. Improved conductivity of the Co-OEC films with the size of the metalate clusters is consistent with a model for electron conduction that arises from the hopping of hole equivalents delocalized over metalate clusters. The hole hopping is fast relative to the rate-determining step of OER, which may become limited by proton transport within the films. KEYWORDS: thin films, cobalt oxygen evolving catalysts, conductivity, proton transport

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X-ray structural measurements.12−16 These M-OECs are ubiquitous in electrocatalysis of the OER, as high resolution transmission electron microscopy of conventional metal oxide OER catalysts reveal that the surfaces of the oxides possess an amorphous overlayer comprising the metalate clusters of the M-OECs.17−21 Electrochemical kinetics,22 oxygen isotope labeling,23,24 and spectroscopic measurements25−27 are consistent with the OER catalysis occurring at edge sites of the metalate clusters. This overall catalytic activity may be perturbed by deposition conditions (e.g., electrolyte and thickness), and in these cases, the performance of the M-OEC exhibits charge and mass transfer limitations,13,21,28 the origin of which likely arises from the need to transport electrons, protons, and electrolyte

INTRODUCTION Self-assembling oxidic metalate clusters comprising elements of the first row promote the oxygen evolution half-reaction (OER),1−6 which is the critical determinant of the overall efficiency of solar-to-fuels conversion processes based on the renewable production of hydrogen with water as the hydrogen source. The ability to exploit self-assembling allows OER catalysts to self-heal,7 thus avoiding the harsh conditions of concentrated base that are needed to stabilize typical metal oxide catalysts.8 OER in neutral or near-neutral water greatly simplifies the semiconducting-catalyst materials fabrication in devices such as buried junctions (e.g., the artificial leaf)9,10 and also allows for the more facile implementation of solar energy storage via water splitting, as any water source may be employed. The relaxed requirements on water supply are especially important for implementation of renewables in emerging countries that currently lack access to reliable energy.11 The catalyst films of the self-assembling oxidic metal oxygen evolution catalysts (M-OECs) consist of aggregates of metalate clusters of molecular dimension, as determined from in situ © XXXX American Chemical Society

Special Issue: New Chemistry to Advance the Quest for Sustainable Solar Fuels Received: May 22, 2018 Accepted: July 20, 2018

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DOI: 10.1021/acsaem.8b00785 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials through the porous M-OEC films. The effect of charge transport on catalytic performance has been preliminarily modeled assuming a proton−electron transport process via self-exchange hopping between catalytically active redox sites.29 A large capacitance observed in cyclic voltammograms indicates that electron transport is rather ohmic.30 Experimental measurements, however, of film conductivity have yet to be undertaken, and the origins of transport have yet to be defined in M-OEC films. Because an understanding of the origins of charge transport in M-OEC films is necessary to optimize catalyst performance and also to provide a firm underpinning on which catalysts may be benchmarked,31 we now report conductivity measurements of Co-OEC films. An interdigitated array (IDA) microelectrode is employed to deconvolute current arising from OER catalysis from that of charge transport in Co-OEC films deposited from phosphate (Pi) at pH 7 (designated CoPi7) and borate (Bi) at pH 9 (CoBi9). To isolate the role of the buffering species from that of pH, we also consider Co-OEC deposited from borate electrolyte at pH 7 (“CoBi7”) and from a 1:1 mixture of phosphate and borate electrolyte at pH 7 (“CoPiBi7”). We further correlate the measured conductivity properties of the pair distribution function (PDF) analyzed X-ray structures of the metalate clusters composing the different films. The conductivity of the Co-OEC films increases with the size of the metalate clusters, consistent with a model for electron conduction that arises from the hopping of hole equivalents delocalized over metalate clusters. The hole hopping is fast relative to the ratedetermining step of OER, which may become limited by proton transport within the films.

Figure 1. CVs of (a) CoPi7, (b) CoPiBi7, (c) CoBi7, and (d) CoBi9 films in KPi solutions of varying pH, from 7.0 to 12.0. Each CV is initiated at open circuit potential and scanned anodically and then cathodically at 50 mV s−1. Films were deposited by the passage of 1 mC cm−2 charge.

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RESULTS Electrochemistry of CoPi/Bi Films. The four film types CoPi7, CoPiBi7, CoBi7, and CoBi9 were deposited by applying an anodic potential to fluorinated tin oxide (FTO) electrodes immersed in phosphate or borate solutions containing Co2+. Detailed deposition procedures for each of the films are provided in the Supporting Information (SI). The cobalt content in the films was controlled with the amount of charge passed and quantitated by inductively coupled plasma mass spectrometry (ICP-MS). The cyclic voltammograms (CVs) of the four CoPi/ Bi films exhibit well-defined electrochemistry. The effect of the nature of the electrolyte was sought from a comparison of CoBi7 and CoPi7. A similar Bi/Pi comparison could not be performed at pH 9 because at this pH, enough PO43− is present to lead to rapid precipitation of Co3(PO4)2 in the deposition bath. Thus, only CoBi9 was studied at pH 9. Studies of CoPiBi7 were undertaken to understand which anion dominates as the structuredirecting agent of the metalate clusters composing the Co-OEC film. Figure 1 shows the CVs for films (charge passed = 1 mC cm−2) in 0.1 M KPi solutions of pH varying from 7 to 12; overlays of representative CVs of each film at pH 7 and 9 are shown in Supporting Information Figure S1. Each of the four films exhibits a reversible Co(III/II) couple with the Co(IV/III) couple buried within the catalytic wave.28,30,33 The CV waves shift cathodically with increasing pH, by approximately 80 mV per pH unit. CoBi7 also exhibits a small prefeature before the main Co(III/II) couple. The catalytic performance of the four film types was evaluated by collecting overpotential, η, vs current density, j, data in a 1 M KBi solution buffered at pH 9 between η = 250 mV and η = 450 mV. Films of varying thickness were prepared by passing a deposition charge of 2, 25, 100, 200, and 400 mC cm−2 (Figure 2)

Figure 2. Tafel plots (η = E − E0 − iR) of (a) CoPi7, (b) CoPiBi7, (c) CoBi7, and (d) CoBi9 operated in 1 M KBi, pH 9. Films were prepared by passage of 2 (magenta squares), 25 (purple squares), 100 (pink squares), 200 (red squares), and 400 mC cm−2 (maroon squares) of the appropriate buffer solution. The linear regions of each plot were fitted to a line with a slope of (RT ln10)/F (59 mV).

(∼20−3000 nm). The plots are described by the linear Tafel equation:32 RT ln 10 [log j − log j 0 ] (1) αF where the slope b = (RT ln 10)/αF is governed by transfer coefficient (α), which is determined by the catalytic mechanism. A mechanism governed by a reversible one electron preequilibrium preceding a chemical rate-determining step gives rise to an α of 1 and a slope of b = 59 mV, as has been shown to be operative for Co-OECs.33 All four films studied here exhibit a slope of approximately 59 mV at low catalyst loadings (2 and 25 mC cm−2 films, Figure 2). At higher loadings the Tafel plots exhibit two slopes: approximately 59 mV at low overpotentials and approximately 100 mV at high overpotentials (100 mC cm−2 films and greater, Figure 2). Artificially high Tafel slopes with η=

B


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ACS Applied Energy Materials increasing film thickness have been ascribed to limitations in charge and/or mass transport.34−38 For each Tafel plot, the linear region at low overpotential was fit to a line with a slope of 59 mV in order to extract the nominal exchange current density (current density at zero overpotential). Figure 3 shows a plot of exchange current density vs catalyst

Figure 4. Intermediate range structure of Co-OECs. (a) X-ray pair distribution functions for CoPiBi7 (purple line) and CoBi7 (blue line) with fits (yellow line) using a 4 × 4 and a 7 × 7 model, respectively. The difference spectra are shown in gray. Spectra are offset vertically for clarity. (b) Goodness of fit given by Rw for model fits of different size clusters for CoPiBi7 (purple triangles) and CoBi7 (blue squares). (c) Best model for a single cluster of CoPiBi7, using a 4 × 4 cluster. (d) Best model for a single cluster of CoBi7, using a 7 × 7 cluster.

Figure 3. Exchange current density vs catalyst film loading. The linear fits in Figure 2 were extrapolated to η = 0 V and plotted against catalyst loading, determined for each film by ICP-MS, for CoPi7 (red triangles), CoPiBi7 (purple triangles), CoBi7 (blue triangles), and CoBi9 (green triangles). The data were fitted to hyperbolic tangent eq 9.

loading, as determined by ICP-MS. These plots exhibit curvature with increasing catalyst loading, and for the case of CoPi7, a clear limiting plateau is attained. We note that the presence of Pi limits the extent of catalyst loading as compared to Bi as clearly shown from plots of charge passed vs catalyst loading (Figure S2). Intermediate Range Structure. The size of the metalate clusters has been shown to vary with the nature of the electrolyte. X-ray scattering and PDF analysis of CoPi7 and CoBi913,15 have revealed that Bi furnishes a more ordered mesostructure of larger-sized metalate clusters than that obtained with Pi as the electrolyte (150 Å2 and 960 Å2 for CoPi7 and CoBi9, respectively). Here, this technique has been extended to CoBi7 and CoPiBi7 films. Synchrotron X-ray total scattering experiments were conducted at beamline X17A at the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory, and spectra were analyzed and modeled using FIT2D, PDFgetX3, and Diffpy-CMI (full details of experimental setup and fitting in the SI).39−45 The PDF plots of the CoBi7 and CoPiBi7 films are shown in Figure 4a and exhibit interatomic correlations up to r = 15 Å. As previously observed for CoPi7 and CoBi9,13 the data could once again be modeled well by clusters of edge-sharing CoO6 octahedra. Clusters of varying sizes were modeled, and the fit error RW was plotted against cluster size, given by the number of CoO6 units per edge (Figure 4b). Within the context of an approximate rhombohedral model, the best fits were determined to be domains consisting of 4 and 7 CoO6 units per edge, with corresponding areas of 100 and 330 Å2 for CoPiBi7 and CoBi7, respectively; the model clusters are shown in Figure 4c,d. The simulated PDFs of the model clusters are shown against the experimental PDFs with difference plots in Figure 4a. The difference plots show some error between 3 and 7 Å, likely due to minor interlayer correlations that were not modeled by the single-slab fits. Extending the PDFs to layering models of the metalate clusters was unsuccessful suggesting that any stacking is turbostratic. Indications of such disordered stacking correlations can be seen in the plots of raw synchrotron X-ray intensity. These I(Q) plots are shown in Figure S3, plotted against that of the layered bulk material CoOOH. The presence of a peak at ∼1.4 Å−1 indicates stacking, although it does not distinguish between ordered and turbostratically disordered stacking.

CoOOH exhibits a sharp peak in this region, CoBi7 and CoBi9 exhibit a broad peak, and CoPi7 and CoPiBi7 have no peak in this region, indicating that borate-only clusters allow layering to some extent within the films. Importantly, the presence of phosphate precludes layering even in borate-containing deposition media. Owing to the disordered stacking in these amorphous films, as shown in Tables S1 and S2, the best fits were achieved with a single slab. Electronic Conductivity. The conductivity of the films was measured with interdigitated microelectrode array (IDA) chips, as has been implemented for measurements of conductivity in polymer films46,47 and thin film metal oxides.4,48,49 The IDA chip (Scheme 1) was fabricated to have two sets of working Scheme 1. Schematic Representation of the Interdigitated Microelectrode Array Chip and Experiment: Top View (Left) and Cross-Sectional View (Right)

electrodes separated by 5 μm (details of fabrication presented in the SI). Co-OEC catalyst films were deposited directly onto the chip and bridged the gaps between electrode sets. Current was passed between the working “source” and “drain” electrodes, which were maintained at a 5 mV differential over a 0.7−1.3 V window. Figure 5 shows the conductivity of the four film types measured in 0.1 M KPi pH 7 and 0.1 M KBi pH 9 buffer solutions. Each conductivity scan has two regions: (i) a rising conductivity in a near-exponential fashion (i.e., near-linear on the semilogarithmic C


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Figure 5. Direct conductivity scans for the four film types CoPi7 (red trace), CoPiBi7 (purple trace), CoBi7 (blue trace), and CoBi9 (green trace), operated in (a) 0.1 M KPi, pH 7 and (b) 0.1 M KBi, pH 9. Each scan starts at high potential and scans toward more negative potentials, and then the sweep direction is reversed. Scan rate = 1 mV s−1.

Figure 7. Role of protons and buffer in conductivity. (a) Conductivity scans of CoBi9 operated in 0.1 M KBi pH or pD 8.8 in H2O (teal trace) and D2O (pink trace). (b) Conductivity scans of CoBi9 operated in 0.001 (black trace), 0.01 (navy trace), 0.1 (blue trace), and 0.5 M KBi at pH 9 (purple trace). Each scan starts at high potential and scans toward more negative potentials, and then the sweep direction is reversed. Scan rate = 1 mV s−1.

scale in Figure 5) from low to high potential, and (ii) a plateau at high potentials where a maximum conductivity is achieved. The maximal conductivity of the films increases in the order CoPi7 < CoPiBi7 < CoBi7 < CoBi9. The CoBi7 and CoBi9 films exhibit a sharper transition between these two regions. All scans exhibit hysteresis, and this is more pronounced in films where Pi is present and when the scan rate of the working electrodes is increased (Figure S4). Therefore, a slow scan rate of 1 mV s−1 was used for all conductivity experiments. The effects of film deposition conditions, Bi vs Pi, and pH on conductivity were explored. For films deposited at pH 7, both the plateau conductivity and the lateral position of the rising region (measured at the midpoint of the rising region) correlate with buffer identity (Figure S5): CoBi7 onsets the earliest (at the least positive potential) and exhibits the highest plateau conductivity, followed by CoPiBi7 and CoPi7 with the latest onset and lowest plateau. The effect of deposition pH can be seen in the comparison between CoBi7 and CoBi9 (Figure S6): CoBi9 plateaus slightly higher than CoBi7, but the lateral position of the rising region of the CoBi7 trace is shifted by approximately 100 mV cathodically from that of CoBi9. The effect of protons on electron conductivity was probed for CoPi7 and CoBi9 in solutions of varying pH buffered by KPi and KBi, respectively (Figure 6). The lateral position of the rising

8.8 is shown in Figure 7a, with analogous experiments at pH/pD 8.5 and 9.5 in Figure S7. The dependence of catalyst film conductivity on borate buffer concentration was explored by varying [KBi] from 1 to 500 mM (Figure 7b). Borate buffer at pH 9 was preferred for this study because, for Co-OEC films, activity is most affected by Bi as opposed to Pi.23 For CoBi9, the change in average lateral position of the rising region is minimal, with no change in the height of the conductivity plateau. At very low buffer concentration, the main effect observed is a larger hysteresis presumably due to insufficient buffer capacity to maintain pH. The conductivity behavior of CoPi/Bi films sharply contrasts that of films of Co4O4 cubane, which constitutes the basic structural element of CoPi/Bi catalysts. The cubanes have been used as a molecular model to inform on the nature of Co(IV)51−53 in CoPi/Bi catalyst films and on the Co(III)−OH|Co(IV)−O redox couple of CoPi/Bi catalysts.54 Cubane films for conductivity measurements were prepared by substituting the pyridines of the Co4O4(py)4(OAc)4 cubane with those of poly(4vinylpyridine) (PVP). The PVP-Co4O4 cubane film was dropcast onto electrodes from an aqueous suspension.55 CVs of the Co4O4−PVP adduct on FTO show a quasi-reversible Co(IV/III) couple at 1.16 V (Figure S8), which shows a diffusion-controlled waveform dictated by diffusion of charge carriers through the PVP film. The conductivity trace of the PVP-Co4O4 cubane film is shown in Figure 8. A noisy signal is due to low conductivity for

Figure 6. Effect of solution pH on film conductivity. (a) Conductivity scans of CoPi7 film operated in KPi solutions of varying pH, from 6.0 to 8.0. (b) Conductivity scans of CoBi9 film operated in KBi solutions of varying pH, from 8.0 to 12.0. Each scan starts at high potential and scans toward more negative potentials, and then the sweep direction is reversed. Scan rate = 1 mV s−1.

Figure 8. Direct conductivity scan for a Co4O4−PVP adduct operated in 0.1 M aqueous KPF6. The scan starts at high potential and scans toward more negative potentials. Scan rate = 1 mV s−1.

region of the scans shifts by approximately 100 mV per pH unit for CoPi7 and 80 mV per pH unit for CoBi9, but the plateau heights are unaffected by pH. Additionally, the lateral position of the rising region of the conductivity trace shifts cathodically by about 30 mV for H2O operation solution as compared to D2O, but the plateau height is unaffected; a representative H/D experiment on CoBi9 operated in KBi conducted at pH or pD50

charge transport among the Co4O4 cubanes in PVP. Notwithstanding, a clear maximum near 1.16 V, the standard potential for the cubane Co(IV/III) couple, is observed in the conductivity trace. Unlike CoPi/Bi films, the conductivity does not exhibit a plateau-limiting current as observed for CoPi/Bi films. D


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Figure 9. Overlay of a direct conductivity scan (blue trace) and a CV (red trace) of catalyst films operated in KPi solutions buffered at pH 7 for (a) CoPi7 and (b) CoBi9.

Table 1. Parameters of Intermediate Range Structure, Conductivity, and Catalytic Performance in Co-OEC Films film type

av cluster size/Å2

CoPi7 CoPiBi7 CoBi7 CoBi9

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150 100 330 960

stacked layers? no no yes yes

plateau onset in Bi9/V vs NHE

−5

5.55 × 10 9.60 × 10−5 1.13 × 10−4 1.48 × 10−4

0.88 0.88 0.71 0.78

DISCUSSION The OER activity of the M-OECs is derived from metal−oxyl edge sites of the metalate clusters. The generation of the metal− oxyl of the precatalytic state occurs by proton-coupled electron transfer.6,23 Accordingly, the activity of OER by M-OECs depends intimately on the charge transport of electrons and protons through the film. The efficacy of charge transport is reflected in the conductivity of the M-OEC film. Charge transport through the film may be isolated in an IDA by measuring the conductivity between source and drain electrodes when they are maintained at a small differential voltage bias of 5 mV over a voltage window of 0.7− 1.3 V vs NHE. Using the linear Tafel eq S4, the difference in current due to catalysis at the two working electrodes set at a 5 mV differential is negligible, even when the average voltage is within the OER potential window. Taking a difference between i1 and i2 eliminates the current due to OER, and therefore the current due solely to film conductivity, iσ, is given by iσ = (i2 − i1)/2

iσ may be converted to direct conductivity, σ, by i σ= σ UA

σ at 1.1 V vs NHE in Bi9/(S cm−1)

−2 nopt dep/(μmol of Co cm )

0.77 1.62 2.18 2.98

of a Co(IV) has been shown to facilely delocalize over neighboring Co centers in Co4O4 cubanes, which are the basic structural element of Co-OEC.56 Analysis of the intervalence charge transfer (IVCT) band of mixed-valence Co4O4 cubanes indicates a charge transfer rate constant within the cubane to be 3 × 1012 s−1,53 engendering Robin−Day Class II mixedvalence character.57 Indeed, Co(IV) centers in the cubane are completely delocalized over the time scale of EPR measurements, and X-ray crystallography shows a symmetric cubane core.53,54 In the case of Co-OECs, PDF analysis reveals the average metalate cluster sizes listed Table 1, which approximately consist of 15−50 Co centers. Considering the large IVCT rate constant, hole delocalization within the cluster is considered to be fast relative to the slower electron transport rate through the film, as indicated by the low conductivities of the Co-OEC films (Table 1). We therefore consider electronic conduction to arise from the hopping between metalate clusters of the delocalized hole equivalents. In the context of the conductivity traces of Figures 5−7, at low anodic potentials, hole equivalents are introduced into the metalate clusters as charge carriers. In this regime, transport is driven by a gradient in hole concentration, and a rising increase in conductivity is observed with increased potential. At a high population of hole equivalents introduced into the film, charge carrier hopping between metalate clusters is characterized by a high density of states within the metalate clusters and further increases in potential do not lead to an increase in conductivity, which is governed by the rate of hopping between metalate clusters. The threshold for charge transport in this plateau regime is likely governed by a percolative process.58−62 In the plateau region, the intercluster transfer is driven by the potential gradient between the two electrodes of the IDA (i.e., Galvani potential), consistent with the observation that the plot of i1 − i2 vs U (Figure S9) is linear. Several observations are consistent with delocalized hole hopping among metalate clusters. First, the conductivity seems to slightly increase for larger metalate clusters (Table 1). Second, the onset of the plateau changes at potentials positive of the Co(III/II) couple but prior to the potential for OER and does not appear at a discrete anodic potential in the different films (Figures S10 and S11). For this reason, introduction of hole equivalents at the applied potentials of the conductivity measurements is not well-described as generating localized

(2)

(3)

where U is the 5 mV voltage offset divided by the distance between electrode arrays and A is the total effective catalyst area. To confirm that iσ is indeed directly proportional to U, a plot of i1 − i2 vs U was constructed in 1 and 100 mM KBi, pH 9 (Figure S9), and a linear relationship was observed. Mechanism of Proton and Electron Conduction. The overlay of the conductivity traces with CVs for CoPi7 and CoBi9 films (Figure 9) highlights the relationship between film conductivity and oxidation of the Co-OEC. Similar overlays for the other films in KPi at pH 7 and KBi at pH 9 are shown in Figures S10 and S11. The conductivity of the film is greatly increased as the film is oxidized, though the multinuclearity of the large clusters prevents unequivocal assignment of cobalt oxidation numbers related to free charge carriers responsible for conductivity. Notwithstanding, the plateau onset in conductivity occurs anodically near or beyond the localized formal Co(III/II) couple (responsible for the CV peak), indicating that the presence of formal Co(IV) plays a role in film conduction. The hole E


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PCET is involved for generating hole equivalents, but once a sufficient concentration of hole equivalents is achieved, electron conduction becomes independent of protons, as charge balance in the plateau region is accomplished by the movement of electrons across the gradient of the Galvani potential. Effects of Deposition Buffer and pH. Different deposition conditions give rise to different cluster sizes, and hence differences in conductivity, as shown by the correlation of conductivity vs cluster size in Table 1. The buffer anion present in the deposition solution mediates cluster size with Bi giving rise to larger clusters. Nucleation and growth of Co-OECs exhibit an inverse dependence on buffer concentration, as we have shown that the buffering anion must dissociate from edges to allow cluster growth.67 Pi exchanges slowly at dicobalt edge sites (on the order of hours) by a dissociative substitution mechanism. Conversely, Bi can exchange at edge sites on the order of seconds via Lewis acid−base mechanism.23 Thus, Pi leads to fewer edge sites available for cluster growth as opposed to Bi, and consequently the sizes of metalate clusters in Pi buffer are smaller than that of Bi, as reflected in the PDF data of Table 1. That CoPiBi7 clusters are closer in size to those of CoPi7 than those of CoBi7, indicates that the more kinetically inert Pi acts as the dominant structure-directing agent. Consistent with the trend in cluster sizes for films deposited at pH = 7, the CoBi7 film exhibits an earlier rising portion and onset of the plateau conductivity, and a larger plateau value, as compared to CoPi7, with the mixed film CoPiBi7 exhibiting intermediate behavior. The lateral shift of the rising conductivity toward cathodic potentials with increasing cluster size (Table 1) results from redox leveling, offered by delocalizing hole equivalents across a larger cluster. In line with this contention, XAS spectra of CoOEC films exhibit valency edge shifts sensitive to cluster size.68 We note that CoBi7 and CoBi9 do not show this potential−size trend, indicating that other factors may also be at play. In Bi buffer, the active binding species that interacts with dicobalt edge sites is boric acid (B(OH)3; eq 5) rather than borate (B(OH)4−). The pKa of Bi is 9.2, and therefore a solution of Bi at pH 7 has a much larger concentration of B(OH)3 than that of a solution buffered at pH 9. The greater concentration of binding species leads to smaller cluster sizes in CoBi7 as compared to CoBi9, reflected in the PDF data and in the maximal plateau conductivity of the two films. In addition, the conductivity trace of CoBi7 is shifted cathodically by about 70 mV relative to that of CoBi9. This may be explained by the different local pH environments of the two film types. CoBi7 has a larger number of surface associated B(OH)3 species, and therefore a lower local surface pH, than CoBi9. The edge sites of CoBi7 are easier to deprotonate, and therefore easier to oxidize, resulting in an overall cathodic shift of the conductivity trace. This is seen clearly in the film CVs, where CoBi7 has a Co(III/II) prefeature that CoBi9 lacks (Figure S1). Larger cluster sizes will also be beneficial to conductivity via better stacking (as shown by I(Q) PDF plots in Figure S3), higher charge carrier concentration at a given potential, and possibly lower edge-site pKas, thus permitting a more facile PCET associated with hole addition to the metalate cluster. Relation of Film Conductivity with OER. The Tafel slopes for OER catalysis by Co-OEC films are well-behaved at low catalyst loadings showing a 59 mV per decade slope (Figure 2), which is typical for this OER catalyst.6 However, an increase in Tafel slope is observed (∼100 mV per decade) at high loadings and high overpotentials, as well as a nonlinear relation between catalyst loading and exchange current density (Figure 3). In this

Co(IV) centers and is better considered as an increase of the average oxidation state of the metalate clusters beyond Co(III). Indeed, electronically conductive behavior assessed by a capacitive double layer measurement was observed at a low buffer concentration in the absence of a Co(IV/III) redox feature.30 Third, a recent study on redox state quantification by X-ray absorption spectroscopy indicated that Co-OEC redox changes are not modeled well by the standard Nernst equation,63 as the multinuclearity of the large clusters results in significant metal− metal interactions. Accordingly, the conductivity of the films should not abide by a Nernstian model for charge transport between isolated redox centers at a well-defined redox couple. For the case of redox self-exchange between localized sites, as has been observed for redox-active polymers,64−66 conductivity obeys a Nernstian rate law, rate = k[red][ox], which results in a parabolic profile for the variation of conductivity with potential; a maximum in conductivity is observed at the potential at which a 50/50 mixture of redox states is reached. This is indeed the case for the conductivity associated with hole transport for films of the Co4O4−PVP adduct, as shown in Figure 8. No such maximum is observed in the conductivity traces of the four CoPi/Bi films, demonstrating that a transport model based on redox self-exchange between localized sites is inadequate for the Co-OEC films. The introduction of holes into the Co-OEC films is protoncoupled whereas the propagation of holes through the film is proton independent. This contention is indicated by the dependence of the horizontal shift of the rising conductivity of Co-OEC films on pH (Figure 6), buffer concentration (Figure 7), and H/D isotope (Figures 7 and S8), in contradistinction to the invariance of the plateau region on each of these conditions. We note that the pH dependence of hole generation is reflected in the pH dependence of the CV peaks (Figure 1) of Co-OEC films, which has previously been observed for CoPi.28 The oxidation of metalate clusters has been shown electrochemically to occur by proton-coupled electron transfer (PCET); Co oxidation is accompanied by edge-site proton loss thus accounting for the H/D isotope and pH dependence of the rising edge of the conductivity traces. Edge-site reactivity also explains the general shapes of the rising edge of Co-OEC films in Pi and Bi buffer, as substitution at edge sites for the latter is more labile (eqs 4 and 5), thus exposing more OHx edge sites with facility for PCET and accounting for a sharper conductivity trace for Co-OEC in Bi.

In addition, there is an issue of buffering capacity that is reflected in Figure 7b by the hysteresis observed at lower buffer concentrations. With the application of cathodic potential, protons are added to the film and the buffering capacity is overwhelmed, leading to an increase of the effective pH and an attendant cathodic shift of the effective potential. Interestingly, even at the low buffer concentration, the same value of conductivity in the plateau region is achieved. This further supports the notion that F


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ACS Applied Energy Materials regime, film conductivity may be expected to limit overall catalytic performance either due to electron or proton transport or both. Assuming that film conductivity is much smaller than solution conductivity (which is reasonable based on measured film conductivities), the effect of bulk film electronic conductivity on Tafel plots depends on a single dimensionless paramete λ, j 0 df rB RT /F

λ=

capacitance and cobalt loading30), and thus a linear increase of exchange current density with cobalt loading is observed. For thick films, an approximate constant Sa with decreasing porosity will result in an increase of the apparent thickness of the bulk material and the formation of a diffusion−reaction layer in the film resulting from mutual compensation of proton-coupled redox-active sites propagation and catalytic reaction. Intersection between both limiting behaviors allows definition of the −2 optimal catalyst loading (nopt dep) in μmol of Co cm , directly opt proportional to df , which describes optimal catalyst loading in terms of film thickness in nm. A clear plateau is discerned for −2 CoPi7 in Figure 3, permitting nopt dep = 0.76 μmol of Co cm to be ascertained. In view of the absence of a plateau region for the other three films, the limiting behaviors can be combined and simplified by assuming that Sa/S is approximately constant for all films, yielding (details in SI),

(6)

where j is the exchange current density, rB is bulk film resistivity, and df is the film thickness. Simulations38 show that interference of electronic conductivity on Tafel plot slopes begins at an overpotential corresponding to 0

log λ = −

Fη RT ln 10

ij ndep yz 0 tanhjjjj opt zzzz j 0 = jmax j ndep z k {

(7)

In the Co-OEC films, a deviation from the Tafel slope of 59 mV appears (for thicker films) at η ∼ 0.32 V which gives log λ ∼ −5.4. Letting j0 ∼ 10−10 A cm−2 (average value in Figure 3) and rB = 1/σ ∼ 104 Ω μm, we find that the film must be 103 μm thick for transport to perturb the Tafel slope. Using the approximation for film thickness (df) as related to deposition charge passed (Cdep),30 df /nm = 7.8(Cdep/(mC cm−2))

(9)

Although eq 9 is an approximation of the complex behavior seen in Figure 3, it is sufficient in providing a model with which to extract values of nopt dep for each film type in order to make comparisons among the different films (Table 1). The robustness of eq 9 can be independently verified with CoPi7 as eq 9 furnishes −2 nopt dep = 0.77 μmol of Co cm , which compares well to the value obtained from the intersection of the two linear regions (as described above) for CoPi7 in Figure 3. In this case of limiting proton transport, dopt f is related to the apparent diffusion coefficient of protons in the bulk material, DH, by

(8) −2

the thickest film, deposited at 400 mC cm , is 3.1 μm, which is 2 orders of magnitude less than the requisite 103 μm; for films deposited at 40 mC/cm2, the film would be expected to be 1/10 as thick, as confirmed by SEM measurements (Figure S12) showing CoPi films to be ca. 280 nm. Therefore, film conductivity limited by electron transport is not expected to limit catalytic performance, and thus any limiting behavior to OER as film thickness is increased is ascribed to a proton diffusion/ hopping process (Scheme 2). We note that whereas proton transfer is not needed for charge balance in the two-electrode IDA setup−as charge is kept constant by a continual hole flux

dfopt =

DH kcat

(10)

A limiting OER behavior defined by proton transport augments our previous model of limiting proton−electron transport,29 which results if there is localized self-exchange between discrete redox centers, as is the case for Co4O4-PVP, as indicated by the maximum in the conductivity trace shown in Figure 8. In contrast, the plateau behavior observed in the conductivity studies described here for Co-OEC films establish that electron transport is not limiting and that any limiting behavior in OER, when the film becomes thick, as described by eq 9, arises from proton mass transport. It is noteworthy that proton extraction from the bulk into the Co-OEC film bears some similarities to the kinetically limiting lithium cation intercalation in lithium-ion battery cathode materials.69 In the present case, kinetic limitation by proton extraction is revealed in thick films by the highly demanding OER turnover.

Scheme 2. Proton−Electron Transport through Co-OECs

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CONCLUSION The model for charge conductivity shown in Scheme 2 is consistent with the data summarized in Table 1, which reveal that the redox properties, conductivity, intermediate range structure, and catalytic performance are intimately correlated. IDA conductivity measurements reveal that Co-OEC film conductivity depends on three factors: (i) how easily a film is oxidized, (ii) how fast holes can hop between clusters to propagate charge through the film, and (iii) the maximal charge carrier concentration. The films conduct via facile hole mobility as compared to catalytic rate, with concomitant uncoupled proton hopping. X-ray PDF analysis reveals that the presence of phosphate anions in the deposition solution limits the size and

under an electric field gradient−it is required during OER catalysis in the single working electrode experiment. The plots of exchange current density in Figure 3 may be best modeled by two limiting behaviors (full details of derivation in SI). For thin films, an increase of cobalt loading simply results in an increase of the accessible surface area, Sa (as attested by capacitance measurements showing proportionality between G


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Containing Iron, Cobalt, and Nickel. J. Am. Chem. Soc. 2013, 135, 11580. (6) Bediako, D. K.; Ullman, A. M.; Nocera, D. G. Catalytic Oxygen Evolution by Cobalt Oxido Thin Films. Top. Curr. Chem. 2015, 371, 173. (7) Costentin, C.; Nocera, D. G. Self-Healing Catalysis in Water. Proc. Natl. Acad. Sci. U. S. A. 2017, 114, 13380. (8) Surendranath, Y.; Nocera, D. G. Oxygen Evolution Reaction Chemistry of Oxide-Based Electrodes. Prog. Inorg. Chem. 2011, 57, 505. (9) Reece, S. Y.; Hamel, J. A.; Sung, K.; Jarvi, T. D.; Esswein, A. J.; Pijpers, J. J. H.; Nocera, D. G. Wireless Solar Water Splitting Using Silicon-Based Semiconductors and Earth-Abundant Catalysts. Science 2011, 334, 645. (10) Nocera, D. G. The Artificial Leaf. Acc. Chem. Res. 2012, 45, 767. (11) Nocera, D. G. On the Future of Global Energy. Daedalus 2006, 135, 112. (12) Bediako, D. K.; Lassalle-Kaiser, B.; Surendranath, Y.; Yano, J.; Yachandra, V. K.; Nocera, D. G. Structure-Activity Correlations in a Nickel-Borate Oxygen Evolution Catalyst. J. Am. Chem. Soc. 2012, 134, 6801. (13) Farrow, C. L.; Bediako, D. K.; Surendranath, Y.; Nocera, D. G.; Billinge, S. J. L. Intermediate-Range Structure of Self-Assembled Cobalt-Based Oxygen-Evolving Catalyst. J. Am. Chem. Soc. 2013, 135, 6403. (14) Liu, Y.; Nocera, D. G. Spectroscopic Studies of Nanoparticulate Thin Films of a Cobalt-Based Oxygen Evolution Catalyst. J. Phys. Chem. C 2014, 118, 17060. (15) Du, P.; Kokhan, O.; Chapman, K.; Chupas, P.; Tiede, D. Elucidating the Domain Structure of the Cobalt Oxide Water Splitting Catalyst by X-ray Pair Distribution Function Analysis. J. Am. Chem. Soc. 2012, 134, 11096. (16) Huynh, M.; Shi, C.; Billinge, S. J. L.; Nocera, D. G. Nature of Activated Manganese Oxide for Oxygen Evolution. J. Am. Chem. Soc. 2015, 137, 14887. (17) Lee, S. W.; Carlton, C.; Risch, M.; Surendranath, Y.; Chen, S.; Furutsuki, S.; Yamada, A.; Nocera, D. G.; Shao-Horn, Y. The Nature of Lithium Battery Materials under Oxygen Evolution Reaction Conditions. J. Am. Chem. Soc. 2012, 134, 16959. (18) Friebel, D.; Bajdich, M.; Yeo, B. S.; Louie, M. W.; Miller, D. J.; Sanchez-Casalongue, H.; Mbuga, F.; Weng, T.-C.; Nordlund, D.; Sokaras, D.; Alonso-Mori, R.; Bell, A. T.; Nilsson, A. On the Chemical State of Co Oxide Electrocatalysts During Alkaline Water Splitting. Phys. Chem. Chem. Phys. 2013, 15, 17460. (19) Bergmann, A.; Martinez-Moreno, E.; Teschner, D.; Chernev, P.; Gliech, M.; de Araujo, J. F.; Reier, T.; Dau, H.; Strasser, P. Reversible Amorphization and the Catalytically Active State of Crystalline Co3O4 during Oxygen Evolution. Nat. Commun. 2015, 6, 8625. (20) May, K. J.; Carlton, C. E.; Stoerzinger, K. A.; Risch, M.; Suntivich, J.; Lee, Y.-L.; Grimaud, A.; Shao-Horn, Y. Influence of Oxygen Evolution during Water Oxidation on the Surface of Perovskite Oxide Catalysts. J. Phys. Chem. Lett. 2012, 3, 3264. (21) Gonzalez-Flores, D.; Sanchez, I.; Zaharieva, I.; Klingan, K.; Heidkamp, J.; Chernev, P.; Menezes, P. W.; Driess, M.; Dau, H.; Montero, M. L. Heterogeneous Water Oxidation: Surface Activity versus Amorphization Activation in Cobalt Phosphate Catalysts. Angew. Chem., Int. Ed. 2015, 54, 2472. (22) Surendranath, Y.; Kanan, M. W.; Nocera, D. G. Mechanistic Studies of the Oxygen Evolution Reaction by a Cobalt-Phosphate Catalyst at Neutral pH. J. Am. Chem. Soc. 2010, 132, 16501. (23) Ullman, A. M.; Brodsky, C. N.; Li, N.; Zheng, S.-L.; Nocera, D. G. Probing Edge Site Reactivity of Oxidic Cobalt Water Oxidation Catalysts. J. Am. Chem. Soc. 2016, 138, 4229. (24) Koroidov, S.; Anderlund, M. F.; Styring, S.; Thapper, A.; Messinger, J. First Turnover Analysis of Water-Oxidation Catalyzed by Co-Oxide Nanoparticles. Energy Environ. Sci. 2015, 8, 2492. (25) Zhang, M.; Frei, H. Towards a Molecular Level Understanding of the Multi-Electron Catalysis of Water Oxidation on Metal Oxide Surfaces. Catal. Lett. 2015, 145, 420.

stacking ability of growing cobaltate clusters, while borate buffer allows for the growth of large clusters with some stacking coherency. These film microstructure properties are manifested in the overall OER catalytic activity, which plateaus at an optimal film thickness, nopt dep, beyond which catalytic turnover is limited by proton hopping through the film. These studies highlight that benchmarking heterogeneous catalysts31 is a tenuous exercise without knowledge of the precise nature of the catalyst and of the factors governing charge transport.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.8b00785.

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Full experimental details, PDF modeling, further direct conductivity experiments, CVs, and mathematical modeling of conductivity behavior (PDF)

AUTHOR INFORMATION

Corresponding Authors

*(C.C.) E-mail: [email protected]. *(S.J.L.B.) E-mail: [email protected]. *(D.G.N.) E-mail: [email protected]. ORCID

Cyrille Costentin: 0000-0002-7098-3132 Daniel G. Nocera: 0000-0001-5055-320X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported by the U.S. Department of Energy Office of Science, Office of Basic Energy Sciences under Award No. DE-SC0017619. C.N.B. and T.P.K. acknowledge the National Science Foundation’s Graduate Research Fellowship Program. Work in the Billinge group was funded by NSF through Grant DMR-1534910. Use of the National Synchrotron Light Source, Brookhaven National Laboratory, was supported by DOE-BES under Contract No. DE-AC02-98CH10886. C.C. acknowledges Agence National de la Recherche (Grant ANR CATMEC 14-CE05-0014-01) for partial support. We thank Nancy Li for help with ICP-MS measurements, Dr. Thomas Porter for recording the SEM image of the 40 mC/cm2 CoPi films, and Dr. Michael Huynh and Dr. Guillaume Passard for helpful discussions.

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REFERENCES

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