Cocatalyst in Porous Photoanodes for Water Splitting - ACS Publications

Jun 5, 2017 - 49a, 01601 Kyiv, Ukraine. ⊥. Matter and Materials Group, College of Science, Technology and Engineering, James Cook University, ...
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Ultrasmall CoO(OH) nanoparticles as a highly efficient “true” cocatalyst in porous photoanodes for water splitting Lidong Wang, Dariusz Mitoraj, S. Turner, Oleksiy V. Khavryuchenko, Timo Jacob, Rosalie K. Hocking, and Radim Beranek ACS Catal., Just Accepted Manuscript • Publication Date (Web): 05 Jun 2017 Downloaded from http://pubs.acs.org on June 5, 2017

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Ultrasmall CoO(OH)x nanoparticles as a highly efficient “true” cocatalyst in porous photoanodes for water splitting Lidong Wang,a Dariusz Mitoraj,a,b Stuart Turner,c Oleksiy V. Khavryuchenko,d Timo Jacob,b Rosalie K. Hocking,e and Radim Beranek a,b*

a

b

Faculty of Chemistry and Biochemistry, Ruhr University Bochum, 44780 Bochum, Germany

Institute of Electrochemistry, Ulm University, Albert-Einstein-Allee 47, 89081 Ulm, Germany c

d

EMAT, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium

TMM LLC, Research and Development Department, Volodymyrska str. 49a, 01601 Kyiv, Ukraine e

Matter and Materials Group, College of Science, Technology and Engineering, James Cook University, Townsville, 4811, Australia

KEYWORDS Solar fuels, Artificial photosynthesis, Photoelectrochemistry, Oxygen evolution, Electrolyte effects 1 Environment ACS Paragon Plus

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ABSTRACT The coupling of light absorbers to cocatalysts with well-designed optical and catalytic properties is of fundamental importance for the development of efficient photoelectrocatalytic devices for solar-driven water splitting. We achieved an effective loading of visible light-active porous hybrid photoanodes for water photooxidation with ultrasmall (~1-2 nm), highly disordered CoO(OH)x nanoparticles using a two-step impregnation method. Under visible light (λ > 420 nm) irradiation, the resulting photoanodes significantly outperformed photoanodes loaded with conventional cobalt-based cocatalyst (Co-Pi) comprising larger nanoparticles (~5 nm) both in terms of Faradaic efficiency of oxygen evolution (by the factor of 2) and performance stability under long-term irradiation. A combination of STEM, XAS, cyclic voltammetry, and photoelectrochemical techniques was used to elucidate the advantages of using ultrasmall CoO(OH)x nanoparticles as cocatalysts. Specifically, due to the high transparency of ultrasmall CoO(OH)x nanoparticles in visible range, higher loading of porous photoanodes with cobalt catalytic sites can be achieved, while the photocurrent losses due to parasitic light absorption by the cocatalyst are minimized. Notably, a significant enhancement in stability of ultrasmall CoO(OH)x nanoparticles in borate electrolytes as compared to phosphate electrolytes has been observed. EXAFS data recorded before and after photoelectrocatalysis indicated that the effect of the electrolyte on the stability can be explained by the difference in structural ordering dictated by different interaction of the electrolyte anions with cobalt ions, as corroborated by DFT calculations. This study highlights the strong impact of structural and optical properties of cocatalysts as well as the strong influence of the electrolyte composition on the activity and stability of photoelectrocatalytic systems comprising transition metal oxide electrocatalysts.

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INTRODUCTION From the energetic point of view, the development of human society is currently mainly powered by conversion of chemical energy stored in carbon-based fossil fuels (coal, oil, gas) which had been produced by natural photosynthesis over millions of years and are not renewable on a practical timescale. The development of new carbon-neutral energy sources is therefore a key scientific challenge for the 21st century.1 One of the most attractive strategies to tackle this challenge is to mimic the natural photosynthesis by developing artificial photosynthetic devices allowing the solar energy to be captured, converted and directly stored in high-energy chemical bonds of hydrogen molecules produced by water splitting.2-3 The hydrogen produced could be utilized either directly in fuel cells or used for reduction of low-energy feedstock like CO2 to produce high energy fuels like hydrocarbons or alcohols. From the technological point of view, the most mature systems for solar-driven water splitting are based on a combination of conventional solar cells with electrolyzers in which the solar-toelectricity conversion and the electricity-to-hydrogen conversion are decoupled. Currently achievable overall solar-to-hydrogen conversion efficiencies are around 17%.4 Much less developed alternative approaches are represented by integrated photoelectrochemical systems, which typically employ lightharvesting units based on semiconductors or dyes in direct contact with the electrolyte.4-8 Due to their inherent flexibility and constructional simplicity, the photoelectrochemical systems are sometimes considered potentially more efficient and cheaper than a conventional electrolyzer driven by a photovoltaic cell array.5,9-10 In particular, photoelectrochemical devices comprising of a tandem of photocathodes and photoanodes with optimized optical (bandgap), photoelectrochemical (quasi-Fermi levels, current matching), and surface catalytic properties should be able to provide solar-to-hydrogen efficiencies of around 25%.5,11-13 Importantly, if the light absorber itself is not catalytically active, the photogenerated charges (electrons and holes) must be transferred to electrocatalysts which catalyze the water-splitting reactions. An effective coupling of well-designed redox cocatalysts to light absorbers is therefore of crucial importance for the function of photoelectrochemical devices.14-18 This is particularly true in case of photoanodes for water splitting in which the light absorber must typically be modified by a water oxidation catalyst catalyzing the highly complex multi-electron transfer reactions required for water oxidation to dioxygen.19 However, our understanding of the catalysis of water oxidation at the absorber/cocatalyst interface is still limited. This can be exemplified by several recent studies on cocatalyst-modified metal oxide semiconductors (e.g., Fe2O3, BiVO4) which have, rather surprisingly, shown a substantial evidence that cobalt oxide-based cocatalysts (e.g., Co-Pi)20 deposited on the light absorber surface often do not primarily act as “true” catalysts enhancing the rate of water

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oxidation but instead function rather as hole extracting layers improving charge separation,21-22 or “surface passivation agents” which diminish surface recombination23-24 and/or modify the band bending in the semiconductor25 (for a helpful discussion of this issue see also Refs. 26 and 27). Obviously, in order to develop our understanding of various factors influencing the operation of cocatalysts in photoanodes for water splitting, there is a need for studies of photoanodes in which the cocatalyst acts as a genuine oxygen evolution catalyst.

Figure 1. Concept of TiO2-PH hybrid photoanodes: (a) nanocrystalline anatase TiO2 deposited on a transparent conductive fluorine-doped tin oxide (FTO) glass substrate, and subsequently modified at the surface with a thin (< 1-5 nm) layer of polyheptazine (PH, melon, “g-C3N4”) and loaded with an oxygen evolution cocatalyst; (b) a simplified potential scheme illustrating the visible light photoactivity of TiO2-PH hybrids based on direct optical excitation of an electron from the HOMO (valence band) of PH (with contributions of molecular orbitals formed upon interaction of PH with TiO2) into the conduction band of TiO2; for a conclusive experimental evidence for the formation of a charge transfer complex between TiO2 and PH see our previous publications;28-29 note that the presence of a cocatalyst is necessary to induce complete water oxidation to dioxygen.

This holds in particular for a distinct class of photoanodes based on “soft” light-harvesting absorbers (dyes, for example) which are coupled to an oxygen evolution cocatalyst (e.g., colloidal IrOx nanocrystals) and at the same time attached to a nanocrystalline wide-bandgap metal oxide acting as electron collector.6,30-35 The advantage of such architectures is that the photogenerated electrons are collected at a relatively negative potential which is determined by the quasi-Fermi level of the electroncollecting metal oxide (for TiO2 at –0.15 V vs. RHE).36 In an ideal scenario, this should allow for photooxidation of water at potentials much more negative than at conventional metal oxide photoanodes like Fe2O3 or BiVO4, reducing significantly the requirement of further bias provided either externally or by a photocathode in a tandem. 4 Environment ACS Paragon Plus

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Following a similar strategy, we have recently investigated incorporation of various cocatalysts into porous photoanodes (Figure 1)28-29,37-41 based on an inorganic-organic hybrid system of TiO2 and polyheptazine ("PH", more precisely poly(aminoimino)heptazine or melon, a CNxHy polymeric sheptazine derivative, also often referred to as "graphitic carbon nitride" or "g-C3N4" in the literature).4246

In this type of visible-light active photoanodes the photogenerated holes are localized in the

catalytically inactive organic layer and cannot induce complete water oxidation to dioxygen unless an additional water cocatalyst is present.28-29,37-41 This type of porous photoanodes represents also a highly sensitive testing system for the quality of coupling between the hybrid absorber and the cocatalyst since poor coupling leads inevitably to accumulation of holes in the organic PH layer and intense photocorrosion.40 Thus, these hybrid photoanodes represent an excellent model system of “soft” photoelectrocatalytic architectures for light-driven water splitting, in which the cocatalyst unambiguously acts as a water oxidation catalyst (no oxygen evolution without a cocatalyst). In this paper, we report on porous hybrid photoanodes loaded with ultrasmall (1-2 nm) CoO(OH)x nanoparticles which significantly outperform – both in terms of activity and stability – photoanodes comprising conventional cobalt-based cocatalysts (Co-Pi).20,29,47-56 Furthermore, the beneficial effects of structural and optical properties of the nanosized cocatalysts, the enhancement of activity and stability in borate electrolyte, and the cocatalyst transformations during photoelectrocatalysis (followed by X-ray absorption spectroscopy) are elucidated and discussed.

RESULTS AND DISCUSSION The deposition and structure of the cocatalyst For the deposition of ultrasmall CoO(OH)x nanoparticles into the porous TiO2-PH photoelectrodes we have developed a two-step impregnation process. First, the electrodes were immersed into an aqueous solution of cobalt nitrate (0.1 M) for 10 minutes, and dried in air. Subsequently, the electrodes were quickly dipped into aqueous ammonia solution (25%), and dried again. The second step, addition of ammonia solution acting as a weak base, facilitates formation of CoO(OH)x and was found to be necessary to achieve higher activity and stability of our photoelectrodes. The as-deposited CoO(OH)x nanoparticles are very small, and can be best displayed by elemental mapping in a scanning transmission electron microscope (STEM-EELS elemental mapping). In this technique, electron energy-loss (EELS) spectra are taken point-by-point over a set scan region, indicated by the white rectangle in Figure 2b. By integrating the intensities under the Ti L2,3, the O-K and the Co L2,3 edges (see the summed EELS spectrum from the scan region in Figure 2c), an elemental map can be 5 Environment ACS Paragon Plus

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generated for each of the elements of interest. This map, shown as inset to Figure 2c, shows that the CoO(OH)x nanoparticles are randomly distributed in the porous structure of TiO2-PH and their size is in the range of 1-2 nm.

Figure 2. HAADF-STEM images (a, b) of TiO2-PH loaded with CoO(OH)x nanoparticles; (c) Summed EELS spectrum taken from the entire scan region indicated by the white rectangle in (b); the insets show the resulting STEM-EELS elemental map and simultaneously acquired ADF-STEM image. (d) Cobalt K-edge EXAFS spectra of CoO(OH)x and Co-Pi in TiO2-PH hybrid photoanodes measured ex-situ after photoelectrocatalysis (PEC) and compared to electrochemically deposited Co-Pi and bulk heterogenite (a highly ordered CoO(OH) phase).

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In order to estimate and compare the behaviour of our ultrasmall CoO(OH)x nanoparticles, photoelectrodes loaded with conventional cobalt-based electrocatalyst (Co-Pi) were taken as a benchmark.20,47-51 The benchmark Co-Pi cocatalyst was deposited photoelectrochemically in phosphate buffer (pH 7) containing Co(II) ions, as described in our previous work.29 Notably, the chemical structure of both ultrasmall CoO(OH)x nanoparticles and Co-Pi is very similar, as exemplified by the extended X-ray absorption fine structure (EXAFS) data shown in Figure 2d. In both cases, the EXAFS spectra suggest a highly disordered structure reminiscent of CoO(OH) heterogenite phase, which is in line with previous results on active cobalt-based electrocatalysts.47,57-60 However, in contrast to the very small average size of CoO(OH)x nanoparticles (1-2 nm), Co-Pi cocatalyst comprises also larger nanoparticles with a size of ~5nm (see Supporting Information, Figure S1). Photoelectrocatalytic water oxidation in phosphate buffer While the catalytic activity for oxygen evolution of extremely small cobalt oxide nanoparticles61-62 or even isolated cobalt ion sites63 is well established, their catalytic activity has been mostly studied in photocatalytic systems utilizing sacrificial oxidizing agents like peroxodisulfate.61-63 As it is known that in such systems the oxygen evolution activity is often dictated by the reactivity of the sacrificial reagents rather than by kinetics of oxygen evolution from water,64 investigations of truly photoelectrocatalytic systems without any additional oxidizing agents are of paramount importance.

Figure 3. Photoaction spectra (incident photon-to-current efficiencies, IPCE) under intermittent (5 s light, 5 s dark) monochromatic irradiation (a), oxygen evolution (b), and photocurrent transients (c) under polychromatic visible light irradiation (cut-off filter > 420 nm) recorded in a phosphate buffer (0.1 M; pH 7) at 1.12 V vs. RHE; all samples were measured at three different electrodes and the error bars are taken as 2σ.

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In order to assess the activity and stability of our ultrasmall CoO(OH)x nanoparticles as a “true” cocatalyst in hybrid photoanodes for photoelectrocatalytic water oxidation, an initial series of photoelectrochemical experiments was performed in phosphate buffer (pH 7). First, we measured the external quantum efficiencies (incident photon-to-current efficiencies, IPCE) under short-term intermittent irradiation at different wavelengths (Figure 3a). The photoaction spectra show photocurrent response in the visible range down to ~550 nm, corresponding well with the optical absorption edge of the TiO2-PH hybrid (~2.3 eV, ~540 nm).28-29,38,40 As compared to photoanodes loaded with Co-Pi, the TiO2-PH/CoO(OH)x photoanodes exhibit slightly better performance, whereby the enhancement becomes more pronounced in the visible range (> 400 nm). The photocurrent response exhibited a strong dependence on applied potential with photocurrents continuously diminishing when lowering the bias potential (see Supporting Information, Figure S2). Furthermore, at lower bias potentials the spike-like shape of photocurrent transients became more pronounced, indicating enhanced surface recombination.40 Both of these findings confirm that even in the presence of the CoO(OH)x cocatalyst the photoactivity is limited by primary recombination in the TiO2-PH hybrid. In other words, in the absence of sufficient electric bias, the back electron transfer from TiO2 to PH occurs on a faster time scale than the reaction of photogenerated holes with water. Nevertheless, it is noteworthy that anodic photocurrents are still detectable at bias potentials as negative as 0.1 V vs. RHE, which confirms that the electrochemical potential of photogenerated electrons (quasi-Fermi level) is close to the conduction band edge of TiO2, as predicted by the energy scheme in Figure 1. Here, it should be noted that dye-sensitized and hybrid photoanodes often suffer severe photocorrosion during water photooxidation because a significant portion of the oxidative equivalents (holes) generated under irradiation does not oxidize water to dioxygen, but instead induces oxidative transformations in the light absorber. For example, the Faradaic efficiency for oxygen evolution of ~20% and correspondingly poor stability has been reported for a ruthenium dye-sensitized TiO2 photoelectrode with a colloidal IrOx cocatalyst.30 Hence, long-term photoelectrochemical experiments with oxygen evolution detection are very important for the assessment of photoanode performance. In the next step, we have therefore carried out prolonged (1 hour) photoelectrocatalytic experiments (Figure 3 b,c). During these experiments the photoanodes were irradiated by polychromatic visible light (using a cut-off filter > 420 nm) in order to effectively shut off the photoresponse due to intrinsic bandto-band transition in TiO2 (bandgap of ~3.2 eV, ~390 nm), allowing us to study exclusively photoelectrocatalysis driven by the visible-light photoresponse of the TiO2-PH hybrid (see Figure 1). In 8 Environment ACS Paragon Plus

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stark contrast to the relative similarity of IPCE values, the oxygen evolution rate was more than doubled at photoanodes comprising ultrasmall CoO(OH)x nanoparticles as compared to benchmark TiO2-PH/Co-Pi electrodes (Figure 3b). Importantly, this enhancement is mainly due to increased apparent Faradaic efficiency65 for dioxygen evolution which was 34±3% and 17±3% at TiO2PH/CoO(OH)x and TiO2-PH/Co-Pi, respectively. In other words, while the overall charge passed during photoelectrocatalysis was similar in both cases (Figure 3c), in photoelectrodes loaded with CoO(OH)x the amount of photoholes contributing to oxygen evolution was higher than in case of Co-Pi by the factor of 2. Two further points should be noted. Firstly, no oxygen evolution was observed in the absence of cocatalyst (Figure 3b) though significant current still could be observed (not shown). Without a cocatalyst for oxygen evolution the photocurrent only contributes to oxidative photocorrosion of the TiO2-PH hybrid light absorber. Secondly, the higher Faradaic efficiencies in the presence of CoO(OH)x translate directly into enhanced stability of photocurrent under prolonged irradiation (Figure 3c). In other words, the more effectively the photoholes are transferred to the cocatalyst and their accumulation in the organic component (PH) of the TiO2-PH hybrid is avoided, the better the stability of the hybrid photoelectrode. Why is the performance of photoelectrodes loaded with CoO(OH)x nanoparticles better as compared to Co-Pi? In order to address this question, we first estimated the amount of cobalt in the porous photoanodes by energy dispersive X-ray (EDX) elemental mapping. It was found that the overall amount of cobalt in TiO2-PH/CoO(OH)x was higher by the factor of ~2.1 as compared to TiO2-PH/CoPi. Moreover, the distribution of CoO(OH)x was more homogeneous than in case of Co-Pi, as apparent from the cross-sectional EDX elemental mapping (Figure 4a,b). In the next step, we estimated the amount of electrochemically active cobalt sites in CoO(OH)x and Co-Pi by integrating the overall charge passed during the cathodic scan in cycling voltammograms (Figure 4c). The amount of electrochemically active sites in CoO(OH)x nanoparticles was found to be even higher – by the factor of ~3.3 – as compared to Co-Pi, which is expected as the average size of CoO(OH)x nanoparticles is smaller than in Co-Pi. These results indicate that one of the reasons for the enhanced performance of TiO2-PH/CoO(OH)x is clearly the higher content of catalytically active cobalt sites within the porous structure of the hybrid photoelectrode. However, we note that not only the absolute amount of cobalt active sites but also the size and optical properties of the cocatalyst play a crucial role. Maximizing the photoelectrocatalytic performance (i.e., achieving the optimum photocurrent, Faradaic efficiency of oxygen evolution, and stability at the same time) of porous photoanodes loaded with a cocatalyst is determined by a complex interplay between providing sufficient amount of cocatalyst while, at the

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same time, avoiding undesired parasitic light absorption by the cocatalyst particles. The corresponding figure-of-merit has been recently coined “optocatalytic efficiency” by Boettcher et al.16 In this context, it should be emphasized that the amount of Co-Pi cocatalyst deposited in our TiO2-PH/Co-Pi photoanodes was optimized for the highest photocurrents by adjusting the photodeposition time. This means that simply depositing higher amounts of Co-Pi to achieve higher Faradaic efficiencies is not a viable alternative as it results in drastic decrease of photocurrents.29 This is because relatively larger Co-Pi particles absorb strongly light in the visible range, and block thus the excitation of the TiO2-PH light absorber. In contrast, in ultrasmall CoO(OH)x nanoparticles the band structure cannot fully develop which leads to their higher transparency in the visible range. The difference in optical properties between hybrid photoanodes loaded with CoO(OH)x and Co-Pi is well visible by naked eye (Figure 4d). While photodeposition of Co-Pi leads to dark coloring of the photoelectrode, the deposition of ultrasmall CoO(OH)x does not make any visible difference. We point out that slight darkening of TiO2-PH/CoO(OH)x is observable after photoelectrocatalysis (Figure 4d), which is ascribed to increased average oxidation state of Co after photoelectrocatalysis (see the discussion of XAS data below). However, even after photoelectrocatalysis the absorption of CoO(OH)x in the visible range is much lower (by the factor of ~2 in terms of Kubelka-Munk function) as compared to Co-Pi, as revealed by analysis of differential electronic absorption spectra (see Supporting Information, Figure S3). This is a striking difference, given the much larger overall Co content (by the factor of ~2.1) and amount of electrochemically active Co sites (by the factor of ~3.3) in TiO2-PH/CoO(OH)x than in TiO2PH/Co-Pi. By way of preliminary conclusion, we can say that: i) though the chemical structure and the intrinsic electrocatalytic activity (per active site) of CoO(OH)x and Co-Pi is similar, ii) the major advantage of the ultrasmall CoO(OH)x nanoparticles as cocatalyst is the possibility to achieve higher loading of porous photoanodes with cobalt catalytic sites, while, at the same time, minimizing the undesired parasitic light absorption by the cocatalyst.

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Figure 4. Scanning electron micrographs and EDX elemental mapping of TiO2-PH/CoO(OH)x (a) and TiO2PH/Co-Pi (b) photoanodes. (c) Cycling voltammograms (without iR-drop correction) with integrated charges recorded in a phosphate buffer (0.1 M; pH 7) at CoO(OH)x and Co-Pi deposited on FTO electrodes covered by a porous TiO2 support layer; note that in this case Co-Pi was deposited electrochemically in a phosphate buffer (0.1 M; pH 7) containing Co(NO3)2 (0.3 mmol/L) at 1.72 V vs. RHE; the assignment of redox waves was done according to Risch et al.58 (d) Optical images of TiO2-PH photoelectrodes without and with cocatalysts.

Enhancement of stability in borate electrolytes As it is known that the structure, activity, and stability of cobalt-based electrocatalysts depends on the electrolyte composition,66-70 we further investigated the photoelectrocatalytic activity in borate electrolytes. Since electrolytes with the optimum pH for borate buffer (9.2) were found to facilitate photocorrosion of our TiO2-PH hybrid absorber (see Supporting Information, Figure S4), we chose a lower pH value (7.7) for our experiments.

Figure 5. Photocurrent transients (a) and oxygen evolution (b) under polychromatic visible light irradiation (cut-off filter > 420 nm) recorded in different electrolytes at 1.12 V vs. RHE.

Interestingly, during long-term photoelectrocatalytic experiments (irradiation for 4 hours) in borate electrolyte the TiO2-PH/CoO(OH)x photoelectrodes exhibited so far unprecedented performance, both in terms of apparent Faradaic efficiency (>40%) and stability (Figure 5). Notably, the oxygen evolution onset was determined to be at 0.56 V vs. RHE, the most negative value for detectable oxygen onset obtained at our TiO2-PH photoanodes so far (see Supporting Information, Figure S5). The enhanced performance in borate electrolyte is in line with recent report by Spiccia et al. who found superior performance of TaON photoanodes modified with CoOx nanoparticles (5-15 nm) in borate buffer (pH 9.2) as compared to phosphate buffer (pH 7.0).70 However, apart from much smaller cocatalyst particle size in our case (1-2 nm), two further facts are surprising. Firstly, as our optimum performance pH is 11 Environment ACS Paragon Plus

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~7.7-8.0 (see Supporting Information, Figure S4), the enhancement is achieved even at pH rather far from the pKA of borate buffer (pH 9.2) which is typically believed to dictate also the optimum activity pH range for cobalt-based electrocatalysts.69 Secondly, given the similar structure of Co-Pi and ultrasmall CoO(OH)x nanoparticles, it is surprising that though the activity of Co-Pi is also enhanced in borate as compared to phosphate, the stability of TiO2-PH/Co-Pi is not significantly improved in borate, in stark contrast to the behavior of TiO2-PH/CoO(OH)x (Figure 5a). In order to examine the difference in the stability of photoelectrode performance, we first carried out a series of photoelectrocatalytic cycling experiments using TiO2-PH/CoO(OH)x and TiO2-PH/Co-Pi in borate electrolyte (pH 7.7; see Supporting Information, Figure S6). During four consecutive irradiation cycles for 20 minutes, the apparent Faradaic efficiency at TiO2-PH/CoO(OH)x was nearly constant (~40%), while in case of TiO2-PH/Co-Pi it dropped drastically from 45% to 24%. This clearly shows that in borate Co-Pi has similar initial activity but is less stable than CoO(OH)x. Under prolonged irradiation, this lower stability of Co-Pi translates directly into lower Faradaic efficiency at the TiO2PH/Co-Pi photoanode. This, in turn, results in diminished stability of the photoanode since larger portion of photogenerated holes does not oxidize water but induces photocorrosion of the TiO2-PH absorber instead.

Figure 6. Cobalt K-edge XAS data of CoO(OH)x (a-c) and Co-Pi (d-f) in TiO2-PH hybrid photoanodes measured before and after photoelectrocatalysis (PEC; irradiation by visible light, cut-off filter > 420 nm) in borate

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solution (pH 7.7) at 1.12 V vs. RHE for 1 hour). Data are depicted as XANES (a,d; the insets show the pre-edge region), EXAFS (b,e), and Fourier Transform of the EXAFS (c, f).

However, why is Co-Pi in borate solution less stable than CoO(OH)x? In order to address this question, we have performed an analysis of X-ray absorption spectroscopy (XAS) data collected at a synchrotron beamline on photoelectrodes before and after photoelectrocatalysis in borate electrolyte (Figure 6). (It should be noted that since Co-Pi was deposited photoelectrochemically, in case of TiO2-PH/Co-Pi the difference in XAS before and after photoelectrocatalysis also reflects the difference between photoelectrocatalysis in phosphate and borate electrolyte.) The most remarkable change apparent in case of both CoO(OH)x and Co-Pi is the increase in average oxidation state of Co in the resting state of the cocatalyst after photoelectrocatalysis in borate electrolyte (Figure 6a,d). The best interpretation is that a highly disordered mixed valence (Co(II)/Co(III)) phase (or phase mixture) is deposited in both cases. After photoelectrocatalysis this highly disordered phase is partially oxidized to yield slightly less disordered CoO(OH)-like phase (see also Figure 2d). The increase of the average oxidation state also explains the slight darkening of CoO(OH)x after photoelectrocatalysis (Figure 4d), since the average size of CoO(OH)x nanoparticles remained in the range of 1-2 nm after photoelectrocatalysis (see Supporting Information, Figure S7). Further structural insights are available from the analysis of the extended X-ray absorption fine structure (EXAFS). Comparison of Co-Pi to CoO(OH)x before photoelectrocatalysis reveals only minor differences. However, the CoO(OH)x phase is definitely more ordered after photoelectrocatalysis as compared to Co-Pi, which is clear from the relative increase in the intensity of the Fourier transform and also of the EXAFS in both cases (Figure 6b,c,e,f). We speculate that this increased structural order of CoO(OH)x may explain its higher stability in borate as compared to Co-Pi. In this context, it is noteworthy that Hocking et al. recently prepared cobalt oxide electrocatalysts which systematically differed in their degree of disorder (as revealed by EXAFS) and phosphate content, and observed that the more ordered samples (containing less phosphate) were slightly more active catalysts for water oxidation than the disordered samples (containing more phosphate).60 It should be also noted that an increase in long-range order of cobalt-based electrocatalysts in dependence on the electrolyte anion (chloride > acetate > phosphate) has been observed by Dau et al.68 Besides low solubility of cobalt salts in phosphate buffers, already reported in electrochemical studies,71 there might be more fundamental reasons for lower structural disorder observed in the presence of borate anions. First, we suggest that such differences can be interpreted within the framework of Pearson’s concept of hard and soft acid and bases (HSAB).72 As Co(III) and Co(IV) formed during the 13 Environment ACS Paragon Plus

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(photo)electrocatalytic action are hard Lewis acid, the presence of hard bases (e.g., HPO42– and H2PO4– anions present in phosphate buffer at pH 7) will lead to increased structural disorder in cobalt oxide due to stronger interaction between Co(III/IV) and the anions. Conversely, in the presence of soft bases (e.g., chloride) more ordered structures will be favored. Now, in a relatively concentrated (0.1 M) borate solution at pH 7.7, the largest portion of anions will be tetraborates (i.e., B4O5(OH)42– and HB4O5(OH)4–). These anions are relatively large and represent thus softer bases than HPO42– and H2PO4–, which would explain the relatively increased order in CoO(OH)x after photoelectrocatalysis in borate electrolyte. Quantum-chemical calculations (see Supporting Information for details) performed for a series of complexes of general formula [Co(H2O)6L]n– (where L = Cl–, BO33–, HBO32–, B4O5(OH)42– and HPO42–, and Co runs oxidation states from +2 to +4) demonstrate that tetraborate anion bonds with Co much less covalently than the rest of the ligands, which accords qualitatively with HSAB conclusions. This is a rule of thumb that solids with higher degree of covalent bonding tend to form more structurally disordered solids upon precipitation than that with ionic ones due to entropy reasons (ionic bonds do not require activation energies for redirection, therefore they are more flexible).73 Accordingly, formation of hydrogen phosphate-containing compounds during the dynamical process of dissolution-reprecipitation of CoO(OH)x in phosphate buffer should lead to a more structurally disordered oxide phase. Second, as the same calculations demonstrate, the HPO42– anion is the only one from the series which stabilizes Co(IV) in solution, while for the rest of complexes an oxidation of water or ligand is observed, often with consequent formation of Co–OH groups, being a step towards reprecipitation of CoO(OH)x. Therefore, hydrogen phosphate anion covalently bonded to Co would with higher probability precipitate along with cobalt hydroxide and, thus, cause disordering stresses in CoO(OH)x phase. Our results suggest that similar increased order occurs in Co-Pi to a much smaller extent (Figure 6b,c,e,f), without any significant enhancement of stability (Figure 5a). While reasons for this are unclear, one might speculate that this is possibly due to the larger average particle size in Co-Pi and due to the fact that Co-Pi was formed in phosphate buffer and phosphate anions are most probably still present in its structure.47 At any rate, these results suggest that the impact of preparative methods and catalyst history on cocatalyst performance may be highly significant. CONCLUSION

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ACS Catalysis

To conclude, we achieved an effective loading of porous photoanodes for water photooxidation with ultrasmall (~1-2 nm) CoO(OH)x nanoparticles and investigated the photoelectrocatalytic performance of the visible-light active hybrid photoanodes in which the CoO(OH)x nanoparticles act unambiguously as oxygen evolution cocatalysts. While the chemical structure and the intrinsic electrocatalytic activity of CoO(OH)x was found to be similar to benchmark Co-Pi catalyst comprising larger (~5 nm) particles, the porous hybrid photoanodes loaded with ultrasmall CoO(OH)x nanoparticles outperformed significantly photoanodes loaded with Co-Pi in terms of Faradaic efficiency of oxygen evolution and stability. The reason for this enhancement is the very small size and correspondingly high transparency in the visible range of CoO(OH)x nanoparticles, which allows for achieving higher loading of porous photoanodes with cobalt cocatalytic sites, while minimizing the undesired parasitic light absorption by the cocatalyst. Given the simplicity of our cocatalyst deposition method, we expect that it will find immediate application in fabrication of various nanostructured photoelectrodes in which the size and optical properties of the cocatalyst particles must be optimized to avoid losses due to suboptimal light management.74 Furthermore, the stability of the photoelectrocatalytic performance was significantly increased in borate electrolyte as compared to phosphate, which was found to be due to increased stability of CoO(OH)x in borate. Based on X-ray absorption data recorded before and after photoelectrocatalysis, we relate the improved stability of CoO(OH)x in borate electrolyte to increased structural ordering of the initially highly disordered CoO(OH)x nanoparticles. This ordering is dictated by the chemistry of the electrolyte anions and their interaction with cobalt ions in higher oxidation states generated during photoelectrocatalytic oxygen evolution, as corroborated by DFT calculations. More generally, our results highlight the significant impact of structural and optical properties of cocatalysts on their performance in photoelectrodes, as well as the strong influence of the electrolyte composition on the activity and stability of photoelectrocatalytic systems comprising transition metal oxide electrocatalysts, especially when operated at moderate pH, which is often necessary due to the light absorber stability issues. ACKNOWLEDGMENTS Financial support by the MIWFT-NRW within the project “Anorganische Nanomaterialien für Anwendungen in der Photokatalyse: Wasseraufbereitung und Wasserstoffgewinnung“, by the EU-FP7 Grant “4G-PHOTOCAT” (Grant No. 309636), and by the DFG (BE 5102/4-1) is gratefully acknowledged. We thank the Sachtleben company for providing Hombikat UV 100, and Sandra Schmidt and Stefan Klink for help with SEM/EDX measurements. The support of the Center for Electrochemical Sciences (CES) is gratefully acknowledged. We acknowledge facility support from the 15 Environment ACS Paragon Plus

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Australian synchrotron and thank beamline scientists Drs Peter Kapen, Bernt Johannesonn and Chris Glover for assistance during beamtime. The authors also acknowledge the computer time supported by the state of Baden-Württemberg through the bwHPC project and the DFG through grant number INST40/ 467-1 FUGG.

REFERENCES 1. 2. 3.

4. 5. 6. 7. 8. 9.

10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.

Lewis, N. S.; Nocera, D. G., Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15729-15735. van de Krol, R.; Grätzel, M., Photoelectrochemical Hydrogen Production. Springer: 2012. Faunce, T. A.; Lubitz, W.; Rutherford, A. W.; MacFarlane, D.; Moore, G. F.; Yang, P.; Nocera, D. G.; Moore, T. A.; Gregory, D. H.; Fukuzumi, S.; Yoon, K. B.; Armstrong, F. A.; Wasielewski, M. R.; Styring, S., Energy Environ. Sci. 2013, 6, 695-698. Peter, L. M., Electroanalysis 2015, 27, 864-871. Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S., Chem. Rev. 2010, 110, 6446-6473. Swierk, J. R.; Mallouk, T. E., Chem. Soc. Rev. 2013, 42, 2357-2387. Hisatomi, T.; Kubota, J.; Domen, K., Chem. Soc. Rev. 2014, 43, 7520-7535. Sivula, K.; van de Krol, R., Nat. Rev. Mater. 2016, 1, 15010. Xiang, C.; Weber, A. Z.; Ardo, S.; Berger, A.; Chen, Y.; Coridan, R.; Fountaine, K. T.; Haussener, S.; Hu, S.; Liu, R.; Lewis, N. S.; Modestino, M. A.; Shaner, M. M.; Singh, M. R.; Stevens, J. C.; Sun, K.; Walczak, K., Angew. Chem., Int. Ed. 2016, 55, 12974-12988 Ager, J. W.; Shaner, M. R.; Walczak, K. A.; Sharp, I. D.; Ardo, S., Energy Environ. Sci. 2015, 8, 2811-2824. Nozik, A. J., Appl. Phys. Lett. 1976, 29, 150-153. Bolton, J. R.; Strickler, S. J.; Connolly, J. S., Nature 1985, 316, 495-500. Hu, S.; Xiang, C.; Haussener, S.; Berger, A. D.; Lewis, N. S., Energy Environ. Sci. 2013, 6, 2984-2993. Yang, J.; Wang, D.; Han, H.; Li, C., Acc. Chem. Res. 2013, 46, 1900-1909. Lin, F.; Boettcher, S. W., Nat. Mater. 2014, 13, 81-86. Trotochaud, L.; Mills, T. J.; Boettcher, S. W., J. Phys. Chem. Lett. 2013, 4, 931-935. Bi, W.; Zhang, L.; Sun, Z.; Li, X.; Jin, T.; Wu, X.; Zhang, Q.; Luo, Y.; Wu, C.; Xie, Y., ACS Catal. 2016, 6, 4253-4257. Nellist, M. R.; Laskowski, F. A.; Lin, F.; Mills, T. J.; Boettcher, S. W., Acc. Chem. Res. 2016, 49, 733-740. Dau, H.; Limberg, C.; Reier, T.; Risch, M.; Roggan, S.; Strasser, P., ChemCatChem 2010, 2, 724-761. Kanan, M. W.; Nocera, D. G., Science 2008, 321, 1072-1075. Klahr, B.; Gimenez, S.; Fabregat-Santiago, F.; Bisquert, J.; Hamann, T. W., J. Am. Chem. Soc. 2012, 134, 16693-16700. Carroll, G. M.; Gamelin, D. R., J. Mater. Chem. A 2016, 4, 2986-2994.

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

23. 24. 25. 26. 27. 28.

29. 30. 31. 32. 33.

34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46.

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Cummings, C. Y.; Marken, F.; Peter, L. M.; Tahir, A. A.; Wijayantha, K. G. U., Chem. Commun. 2012, 48, 2027-2029. Ma, Y.; Le Formal, F.; Kafizas, A.; Pendlebury, S. R.; Durrant, J. R., J. Mater. Chem. A 2015, 3, 20649-20657. Barroso, M.; Cowan, A. J.; Pendlebury, S. R.; Grätzel, M.; Klug, D. R.; Durrant, J. R., J. Am. Chem. Soc. 2011, 133, 14868-14871. Gamelin, D. R., Nat. Chem. 2012, 4, 965-967. Sivula, K., J. Phys. Chem. Lett. 2013, 4, 1624-1633. Bledowski, M.; Wang, L.; Ramakrishnan, A.; Khavryuchenko, O. V.; Khavryuchenko, V. D.; Ricci, P. C.; Strunk, J.; Cremer, T.; Kolbeck, C.; Beranek, R., Phys. Chem. Chem. Phys. 2011, 13, 21511-21519. Bledowski, M.; Wang, L.; Ramakrishnan, A.; Bétard, A.; Khavryuchenko, O. V.; Beranek, R., ChemPhysChem 2012, 13, 3018-3024. Youngblood, W. J.; Lee, S.-H. A.; Kobayashi, Y.; Hernandez-Pagan, E. A.; Hoertz, P. G.; Moore, T. A.; Moore, A. L.; Gust, D.; Mallouk, T. E., J. Am. Chem. Soc. 2009, 131, 926-927. Youngblood, W. J.; Lee, S.-H. A.; Maeda, K.; Mallouk, T. E., Acc. Chem. Res. 2009, 42, 19661973. Moore, G. F.; Blakemore, J. D.; Milot, R. L.; Hull, J. F.; Song, H.-e.; Cai, L.; Schmuttenmaer, C. A.; Crabtree, R. H.; Brudvig, G. W., Energy Environ. Sci. 2011, 4, 2389-2392. Zhao, Y.; Swierk, J. R.; Megiatto, J. D.; Sherman, B.; Youngblood, W. J.; Qin, D.; Lentz, D. M.; Moore, A. L.; Moore, T. A.; Gust, D.; Mallouk, T. E., Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 15612-15616. Swierk, J. R.; McCool, N. S.; Saunders, T. P.; Barber, G. D.; Strayer, M. E.; Vargas-Barbosa, N. M.; Mallouk, T. E., J. Phys. Chem. C 2014. Yu, Z.; Li, F.; Sun, L., Energy Environ. Sci. 2015, 8, 760-775. Beranek, R., Adv. Phys. Chem. 2011, DOI:10.1155/2011/786759. Bledowski, M.; Wang, L.; Ramakrishnan, A.; Beranek, R., J. Mater. Res. 2013, 28, 411-417. Wang, L.; Bledowski, M.; Ramakrishnan, A.; König, D.; Ludwig, A.; Beranek, R., J. Electrochem. Soc. 2012, 159, H616-H622. Mei, B.; Byford, H.; Bledowski, M.; Wang, L.; Strunk, J.; Muhler, M.; Beranek, R., Sol. Energy Mater. Sol. Cells 2013, 117, 48-53. Bledowski, M.; Wang, L.; Neubert, S.; Mitoraj, D.; Beranek, R., J. Phys. Chem. C 2014, 118, 18951-18961. Khavryuchenko, O. V.; Wang, L.; Mitoraj, D.; Peslherbe, G. H.; Beranek, R., J. Coord. Chem. 2015, 68, 3317-3327. Wang, X.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G.; Carlsson, J. M.; Domen, K.; Antonietti, M., Nat. Mater. 2009, 8, 76-80. Wang, Y.; Wang, X.; Antonietti, M., Angew. Chem., Int. Ed. 2012, 51, 68-89. Martin, D. J.; Qiu, K.; Shevlin, S. A.; Handoko, A. D.; Chen, X.; Guo, Z.; Tang, J., Angew. Chem., Int. Ed. 2014, 53, 9240-9245. Ong, W.-J.; Tan, L.-L.; Ng, Y. H.; Yong, S.-T.; Chai, S.-P., Chem. Rev. 2016, 10.1021/acs.chemrev.6b00075. Lin, L.; Ou, H.; Zhang, Y.; Wang, X., ACS Catal. 2016, 6, 3921-3931.

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47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65.

66. 67. 68. 69. 70. 71. 72. 73.

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Kanan, M. W.; Yano, J.; Surendranath, Y.; Dinca, M.; Yachandra, V. K.; Nocera, D. G., J. Am. Chem. Soc. 2010, 132, 13692-13701. McAlpin, J. G.; Surendranath, Y.; Dinca, M.; Stich, T. A.; Stoian, S. A.; Casey, W. H.; Nocera, D. G.; Britt, R. D., J. Am. Chem. Soc. 2010, 132, 6882-6883. Surendranath, Y.; Kanan, M. W.; Nocera, D. G., J. Am. Chem. Soc. 2010, 132, 16501-16509. Wang, L.-P.; Van Voorhis, T., J. Phys. Chem. Lett. 2011, 2, 2200-2204. Steinmiller, E. M. P.; Choi, K.-S., Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 20633-20636, S20633/1-S20633/2. Seabold, J. A.; Choi, K.-S., Chem. Mater. 2011, 23, 1105-1112. Zhong, D. K.; Gamelin, D. R., J. Am. Chem. Soc. 2010, 132, 4202-4207. McDonald, K. J.; Choi, K.-S., Chem. Mater. 2011, 23, 1686-1693. Zhong, D. K.; Cornuz, M.; Sivula, K.; Gratzel, M.; Gamelin, D. R., Energy Environ. Sci. 2011, 4, 1759-1764. Sun, J.; Zhong, D. K.; Gamelin, D. R., Energy Environ. Sci. 2010, 3, 1252-1261. Risch, M.; Khare, V.; Zaharieva, I.; Gerencser, L.; Chernev, P.; Dau, H., J. Am. Chem. Soc. 2009, 131, 6936-6937. Risch, M.; Ringleb, F.; Kohlhoff, M.; Bogdanoff, P.; Chernev, P.; Zaharieva, I.; Dau, H., Energy Environ. Sci. 2015, 8, 661-674. Bergmann, A.; Martinez-Moreno, E.; Teschner, D.; Chernev, P.; Gliech, M.; de Araújo, J. F.; Reier, T.; Dau, H.; Strasser, P., Nat. Commun. 2015, 6, 8625. King, H. J.; Bonke, S. A.; Chang, S. L. Y.; Spiccia, L.; Johannessen, B.; Hocking, R. K., ChemCatChem 2017, 9, 511-521. Jiao, F.; Frei, H., Angew. Chem., Int. Ed. 2009, 48, 1841-1844. Zidki, T.; Zhang, L.; Shafirovich, V.; Lymar, S. V., J. Am. Chem. Soc. 2012, 134, 14275-14278. Ahn, H. S.; Yano, J.; Tilley, T. D., Energy Environ. Sci. 2013, 6, 3080-3087. Schneider, J.; Bahnemann, D. W., J. Phys. Chem. Lett. 2013, 4, 3479-3483. Herein, we use the term “apparent faradaic efficiency” in order to highlight that in our setup only oxygen dissolved in the electrolyte was determined, not accounting for the oxygen in the cell headspace (see Supporting Information for more details). Surendranath, Y.; Dinca, M.; Nocera, D. G., J. Am. Chem. Soc. 2009, 131, 2615-2620. Gerken, J. B.; McAlpin, J. G.; Chen, J. Y. C.; Rigsby, M. L.; Casey, W. H.; Britt, R. D.; Stahl, S. S., J. Am. Chem. Soc. 2011, 133, 14431-14442. Risch, M.; Klingan, K.; Ringleb, F.; Chernev, P.; Zaharieva, I.; Fischer, A.; Dau, H., ChemSusChem 2012, 5, 542-549. Klingan, K.; Ringleb, F.; Zaharieva, I.; Heidkamp, J.; Chernev, P.; Gonzalez-Flores, D.; Risch, M.; Fischer, A.; Dau, H., ChemSusChem 2014, 7, 1301-1310. Gujral, S. S.; Simonov, A. N.; Higashi, M.; Fang, X.-Y.; Abe, R.; Spiccia, L., ACS Catal. 2016, 6, 3404-3417. Pavliuk, M. V.; Mijangos, E.; Makhankova, V. G.; Kokozay, V. N.; Pullen, S.; Liu, J.; Zhu, J.; Styring, S.; Thapper, A., ChemSusChem 2016, 9, 2957-2966. Pearson, R. G., J. Am. Chem. Soc. 1963, 85, 3533-3539. Khavryuchenko, O. V.; Lesnyak, V. V.; Khavryuchenko, V. D., Quantum chemical simulation of amorphism of solids. In Physics, Chemistry And Application Of Nanostructures: Reviews and 18 Environment ACS Paragon Plus

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Short Notes, Borisenko, V. I.; Gaponenko, S. V.; Gurin, V. S., Eds. World Scientific: 2009; pp 426-429. Osterloh, F. E., Chem. Soc. Rev. 2013, 42, 2294-2320.

ASSOCIATED CONTENT Supporting Information. Experimental details; potential dependence of photocurrent transients; HRTEM and STEM-EELS data; differential UV-Vis spectra of cocatalysts; oxygen onset measurements; pH dependence of photocurrent and oxygen evolution in borate electrolytes; stability measurements; details of theoretical calculations. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * Radim Beranek, Institute of Electrochemistry, Ulm University, Albert-Einstein-Allee 47, 89081 Ulm, Germany Fax: +49-731-5025409; Tel: +49-731-5025402; E-mail: [email protected]

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