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Elucidating Surface Restructuring-Induced Catalytic Reactivity of Cobalt Phosphide Nanoparticles Under Electrochemical Conditions Zishan Wu, Quan Gan, Xiaolin Li, Yiren Zhong, and Hailiang Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11843 • Publication Date (Web): 16 Jan 2018 Downloaded from http://pubs.acs.org on January 17, 2018

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The Journal of Physical Chemistry

Elucidating Surface Restructuring-Induced Catalytic Reactivity of Cobalt Phosphide Nanoparticles under Electrochemical Conditions Zishan Wu,†,‡ Quan Gan,†,‡,§ Xiaolin Li, †,‡,⁑ Yiren Zhong,†,‡ Hailiang Wang*,†,‡ †

Department of Chemistry, Yale University, New Haven, Connecticut 06511, United States

‡

Energy Sciences Institute, Yale University, West Haven, Connecticut 06516, United States

§

Department of Chemistry, Nankai University, Tianjin 300071, The People’s Republic of China

⁑

College of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400030, The

People’s Republic of China *

email: [email protected]

Abstract Probing and understanding surface restructuring-induced electrocatalytic reactivity is an essential but challenging step toward rational prediction of electrocatalytic properties and design of high-performance catalysts. Cobalt phosphide (CoP) nanoparticles are state-of-the-art electrocatalysts for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). However, the structure-reactivity correlations are not straightforward because the nanoparticles will restructure under working conditions. Employing a protective sample transfer procedure, we use simple lab XPS to unveil the changes in oxidation state and composition of the nanoparticle surface induced by electrochemical reaction conditions. CoP nanoparticles are naturally oxidized on their surface. In alkaline electrolyte under HER conditions, a Co-rich phosphide surface is generated as a result of polyphosphate dissolution and reduction of the oxidized P and Co species. In alkaline electrolyte under OER conditions, an oxidation and dissolution process occurs and the surface evolves into hydroxide/oxide with a subtle amount of phosphate residue. In acidic electrolyte under HER conditions, the surface oxidation layer is dissolved by the electrolyte and a fresh CoP surface is exposed. These surface restructuring results help rationalizing the electrocatalytic reactivity of CoP nanoparticles for water splitting under various electrochemical conditions.

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Introduction Alternative energy sources are extensively pursued for replacing the widely used fossil fuels which are limited in supply and produce carbon dioxide emissions.1-6 Hydrogen gas generated from electrochemical water splitting is considered as a carbon-neutral and environmentally-friendly energy carrier with high gravimetric energy density. To improve the energy efficiency of electrochemical hydrogen production from water, a huge volume of electrocatalysts have been designed to catalyze the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), the two half reactions of water electrocatalysis.5, 7-8 Among them, metal phosphides, for instance CoP nanoparticles, are an important type of catalyst materials with bifunctional activity and good functional durability for both the HER and OER.9-12 However, how metal phosphide nanostructures, particularly their surfaces, adapt to the electrochemical conditions and lead to the observed catalytic properties, remains poorly understood. On one hand, CoP nanoparticles under ambient conditions possess an oxidized surface layer,13-15 which may undergo reduction reactions under HER conditions. On the other hand, CoP nanoparticles may continue to be oxidized under OER conditions to generate the oxide/hydroxide active species. Neither of the restructuring processes has been studied in detail yet.16 Since electrochemical reactions occur on the surface of an electrocatalyst, studying the catalyst surface in the electrocatalytic environment is an essential step toward unravelling the underlying structure-reactivity correlations.16-18 X-ray photoelectron spectroscopy (XPS) is a useful technique in studying surface composition and chemical environment. However, XPS characterization under electrochemical conditions is highly challenging because high vacuum is usually required for XPS measurement whereas most electrochemical reactions are conducted under ambient conditions.19 Postmortem measurement can be problematic because exposing the post-reaction catalyst surface to air may oxidize the reduced species generated under the electrochemical conditions and thus make it difficult to trace back to the active species for the reaction. Although ambient-pressure XPS has been developed for in-operando surface characterization of electrocatalysts for electrochemical water splitting,20 such measurements require special facilities at synchrotron X-ray sources and the higher-energy incident X-ray photons could make the measurement less surface sensitive. In this work, we devise a simple lab-based yet powerful quasi-in-situ XPS method that allows for probing post-reaction catalyst surface structures without exposure to air, rationalizing restructuring processes under electrochemical conditions, and correlating catalyst reconstruction with electrocatalytic properties. The method is utilized to characterize the surface restructuring of CoP nanoparticles under both HER and OER conditions. Under ambient conditions, CoP nanoparticles are naturally oxidized on their surface. Upon soaking in 1 M aqueous KOH without any applied potential, the polyphosphate anions 2 ACS Paragon Plus Environment

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on the nanoparticle surface are replaced by hydroxide ions to render a hydroxide-dominant surface. Under HER conditions, both the oxidized P and Co species are electrochemically reduced while the polyphosphate species are simultaneously dissolved in the electrolyte, resulting a Co-rich phosphide surface. Under OER conditions, the subsurface or even the inner species of CoP nanoparticles are electrochemically oxidized and the generated polyphosphate species continuously leach into the electrolyte, leading to a relatively thick hydroxide/oxide layer. Further investigation reveals that CoP nanoparticles can be oxidized even at a potential below the theoretical value of water oxidation. In 0.5 M aqueous H2SO4, the oxidized surface layer of CoP nanoparticles is rapidly removed by acid to expose the underlying CoP surface. Under all the conditions studied, there exists a small fraction of phosphate or (hypo)phosphite species on the nanoparticle surface. These surface restructuring results provide deeper insights into understanding the electrocatalytic reactivity of metal phosphide nanoparticles.

Results and Discussion CoP nanoparticles supported on mildly-oxidized multiwall carbon nanotubes (CNTs) were synthesized following a two-step process (see Supporting Information for experimental details), in which CoOx/CNT was first synthesized via a hydrolysis reaction and then converted to CoP/CNT by reacting with PH3 gas generated via the decomposition of NaH2PO2·H2O. The synthesized CoP/CNT material features nanoparticles with the size of 10-40 nm supported on CNTs (Figure 1a). The composition and phase of the nanoparticles are confirmed by energy dispersive X-ray spectroscopy (EDX) (Figure 1b) and X-ray diffraction (Figure 1c), respectively. Note that the as-made CoP nanoparticles (stored under ambient conditions for ~5 h) are already partially oxidized on the surface, as evidenced by the Co-O and P-O components in the Co 2p and P 2p XPS spectra (Figure 1d). As the nanoparticles are longer exposed in ambient air, the surface oxidation becomes increasingly substantial (Figure S1). Similar phenomena of natural surface oxidation have been observed for many transition metal pnictide and chalcogenide nanomaterials.21-24 The CoP nanoparticles manifest almost excellent catalytic activity for water electrolysis (Table S1).13, 22, 25 The overpotentials at an HER current density of 10 mA/cm2 are determined to be 87 and 114 mV in 0.5 M H2SO4 and 1 M KOH aqueous solutions, respectively, and that for OER is 238 mV in 1 M KOH. In pursuit of a deeper understanding of this “bifunctional” catalytic behavior, we seek to probe the surface restructuring of CoP nanoparticles under reaction conditions and to identify the real catalytically active species for the HER and OER.



Figure 1. CoP nanoparticles with bifunctional electrocatalytic activity for HER and OER. The material was stored under ambient conditions for ~5 h after it was synthesized. (a) Scanning electron microscopy (SEM) image, (b) EDX spectrum, (c) XRD pattern, (d) Co 2p and P 2p core-level XPS spectra of CoP/CNT. (e) Electrocatalytic performance of CoP/CNT for HER in 0.5 M H2SO4 and 1 M KOH. (f) Electrocatalytic performance of CoP/CNT for OER in 1 M KOH. The catalyst mass loadings were 1.6 and 0.8 mg/cm2 for the HER and OER measurements, respectively. All the linear sweep voltammograms were recorded at a scan rate of 5 mV/s and iR corrected.

CoP nanoparticle surface is sensitive to air, which makes it difficult to examine the real catalyst surface especially the reduced species formed under electrochemical conditions. To overcome this difficulty, we devised a method to protect the reduced species on catalyst surface from being oxidized by 4 ACS Paragon Plus Environment

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air during the transfer from the electrochemical reactor to the XPS spectrometer (Figure 2a). After electrochemical measurement in an Ar-purging cell placed in the air, the catalyst electrode, with a droplet of electrolyte covering the surface to protect the catalyst underneath from directly contacting the air, is immediately immersed in Ar pre-purged ethanol, which is then transferred into an Ar-filled glove box where the electrode is cleaned, dried and mounted on an XPS sample holder, ready to be transferred into the XPS spectrometer with a commercially available vacuum transfer vessel. The validity of this method is demonstrated with the following controlled experiment. Al foil was first soaked in a 1 M NaOH aqueous solution to remove the native oxide layer. When the foil was transferred into the XPS spectrometer without protection against the air, the Al surface was re-oxidized (Figure 2b). When the aforementioned protective transfer procedure was adopted, the cleaned Al surface could be maintained (Figure 2b).

Figure 2. Quasi-in-situ XPS measurement enabled by a protective sample transfer procedure. (a) Schematic illustration for the protective sample transfer procedure. (b) Al 2p core-level XPS spectra of Al foil cleaned in 1 M NaOH and transferred to XPS measurement with and without using the protective transfer procedure illustrated in panel (a).

Before investigating the surface restructuring of CoP nanoparticles under electrocatalytic reaction conditions, we first characterize the surface structural changes of the nanoparticles after being soaked in electrolyte without any bias applied. Upon soaking in 1 M KOH, the material shows a growing and gradually dominating component at 780.8 eV in the Co 2p XPS spectrum (Figure 3a), corresponding to hydroxide-bound Co species.26 Consistently, the O 1s peak shifts to a lower binding energy position which is typically assigned to the hydroxide group (Figure 3c).20 In the P 2p spectrum, the component at 134.2 eV, which is assigned to polyphosphate species according to the literature,27 gradually diminishes (Figure 3b), whereas the phosphate component at 133.2 eV and the phosphide component at 130.0 remain largely unchanged (Figure 3b and S2).28 Considering the concomitant spectral changes of the Co 2p and O 1s electrons, we can conclude that in KOH solution, the surface polyphosphate anions of CoP nanoparticles are replaced by hydroxide ions. The replacement reaction is thermodynamically favorable considering the differences in the solubility products of cobalt hydroxides and cobalt phosphate.29 This 5 ACS Paragon Plus Environment


process changes the nanoparticle surface from being P-rich to Co-rich (Figure 3d). Based on the XPS spectral evolution, the surface restructuring process of CoP nanoparticles in KOH solution completes within 2 min.

Figure 3. Restructuring of CoP nanoparticles in alkaline electrolyte without potential applied. (a) Co 2p, (b) P 2p, and (c) O 1s core-level XPS spectra and (d) surface atomic percentages of CoP nanoparticles (stored under ambient conditions for 18 days after synthesis) before and after being soaked in 1 M KOH solution for various amounts of time (1, 2, 5, and 10 min). The samples were transferred from the electrolyte to the XPS spectrometer using the protective transfer procedure.

Now we consider the effects of applying a reductive potential under HER conditions. A series of identical CoP/CNT electrodes were held at the potential of -150 mV vs RHE in 1 M KOH for various amounts of time before quasi-in-situ XPS measurements were conducted. As the HER proceeds, the CoO component at 782.1 eV in the Co 2p spectrum disappears, whereas the Co-P peak originally positioned at 779.3 eV increases in intensity and slightly shifts to lower binding energy (Figure 4a). In the P 2p spectrum, the P-Co component at 130.0 eV grows at the expense of the P-O component at 134.2 eV (Figure 4b). Note the remaining weak feature at 132.9 eV (Figure 4b), which could be assigned to phosphite or hypophosphite residue present on the catalyst surface under the HER conditions (Figure S2).27 These spectral changes indicate that most of the oxidized species on CoP nanoparticle surface can 6 ACS Paragon Plus Environment

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be reduced under the HER conditions. Consistently, the intensity of the O 1s spectrum is greatly reduced (Figure 4c). Electrochemical reduction of the CoP nanoparticles is also clearly revealed by the chronoamperogram which records the time-dependent current density of the CoP/CNT electrode at -150 mV vs RHE. Following the capacitive current drop, a reduction current peak is clearly visible at 14 s before the stable HER current density is reached (Figure S3). If we stop and then resume the electrode potential, no reduction features superimposed on the HER current can be observed in the second chronoamperogram (Figure S3), which confirms that the reduction current peak indeed corresponds to the reduction of the oxidized species on the CoP nanoparticle surface and the reduced species can be maintained after the electrode potential is removed. The surface compositions of the CoP nanoparticles after catalyzing HER are plotted in Figure 4d. As the HER proceeds, the catalyst surface changes from being P-rich to Co-rich, but not so much as in the case without applying the reductive potential. This phenomenon is understood as a result of the aforementioned hydroxide-replacing-polyphosphate process taking place simultaneously with the surface-reduction process. The resulting Co-rich and reduced surface agrees well with the Co 2p XPS peak at 778.5 eV which can be assigned to Co2P-like species.30 As an interesting comparison, CoOx nanoparticles that have been placed under the same HER conditions for 20 min show no sign of reduction in either the XPS spectra or the chronoamperogram (Figure 4a, 4c, and S3), indicating that the oxide cannot be reduced under low-overpotential conditions and is thus not capable of catalyzing HER.



Figure 4. Restructuring of CoP nanoparticles under HER conditions in alkaline electrolyte. (a) Co 2p, (b) P 2p, and (c) O 1s core-level XPS spectra and (d) surface atomic percentages of CoP nanoparticles (stored under ambient conditions for 25 days after synthesis) before and after catalyzing HER at -150 mV vs RHE in 1 M KOH solution for various amounts of time. The samples were transferred from the electrolyte to the XPS spectrometer using the protective transfer procedure.

Now we turn to the restructuring behavior under OER conditions. A series of identical CoP/CNT electrodes were held at the potential of 1.67 V vs RHE in 1 M KOH for various amounts of time before XPS measurements were performed. The chronoamperogram exhibits an oxidation current peak superimposed on the steady OER current (Figure S4), indicating oxidation of the CoP nanoparticles. As the OER proceeds, the Co-P feature in the Co 2p XPS spectrum is replaced by Co-O (Figure 5a). In the P 2p spectrum, the P-Co peak positioned at 130.0 eV disappears within 30 s and the P-O component almost diminishes after 2 min (Figure 5b). The O 1s spectrum evolves into a major hydroxide component at 531.5 eV and a minor lattice oxygen component (O-Co) centered at 530.0 eV (Figure 5c).20, 31 Taken together, the results suggest that under the OER conditions, the phosphide ions of the CoP nanoparticles are oxidized to polyphosphate-like species and then dissolve in the electrolyte, converting the catalyst surface to hydroxide/oxide-like species (Figure 5d). The restructuring process is likely to go deeper 8 ACS Paragon Plus Environment

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beyond the nanoparticle surface, and may completely transform the original CoP nanoparticles if they are small in size or the formed hydroxide/oxide layer is “leaky”. These findings rationalize the restructuring process of CoP nanoparticles under OER conditions and provide additional evidence for the previous observations of phosphide conversion to oxide and leaching of high oxidation-state P into electrolyte under OER conditions.10, 13, 16, 32-33 In addition, it is worth noting that even after 10 min of continuous OER catalysis, P residue (phosphate-like species) can still be probed by XPS (Figure 5b).34 This supports the hypothesis that such P residues make phosphide-derived oxide more active for OER than the pristine oxide, possibly by enhancing proton transfer.35-36 Our further studies reveal that oxidation of CoP nanoparticles can occur at 1.07 V vs RHE (Figure S5), which is below the equilibrium potential for the OER (1.23 V vs RHE).

Figure 5. Restructuring of CoP nanoparticles under OER conditions in alkaline electrolyte. (a) Co 2p, (b) P 2p, and (c) O 1s core-level XPS spectra and (d) surface atomic percentages of CoP nanoparticles (stored under ambient conditions for 2 days after synthesis) before and after catalyzing OER at 1.67 V vs RHE in 1 M KOH solution for various amounts of time.

The restructuring process of CoP nanoparticles in strong acid is much more straightforward than that in alkaline solution. Upon soaking in 0.5 M H2SO4 for 5 min, the nanoparticle surface shows no oxidized Co species and a very low fraction of oxidized P species (Figure S6), indicating that the surface 9 ACS Paragon Plus Environment


oxidized species can be easily dissolved by H2SO4 and a nearly clean CoP surface is exposed, which conforms with a very recent finding based on Raman and X-ray absorption spectroscopy studies that the oxidized components on a CoP film will disappear under HER conditions in 0.5 M H2SO4.37 Applying a reductive potential does not make noticeable difference to the XPS spectra, because electrochemical reduction will not happen if the oxidized species have already been removed by the electrolyte.

Figure 6. Graphical illustration of surface restructuring of CoP nanoparticles under electrochemical conditions. A natural oxidation layer exists on the surface of CoP nanoparticles. In acidic electrolyte, the oxidized layer will be removed whereas soaking in alkaline electrolyte transforms the layer to a hydroxide-dominant surface. Under HER conditions in alkaline electrolyte, electrochemical reduction of the oxidized species and dissolution of the oxidized P species will together render a Co-rich phosphide surface. Under OER conditions in alkaline electrolyte, a cobalt hydroxide/oxide surface will be formed as a result of combined oxidation and anion substitution.

Conclusions In summary, adopting a protective sample transfer procedure for convenient lab XPS measurement, we have successfully probed the surface restructuring processes of CoP nanoparticle catalysts under HER and OER conditions, as summarized in Figure 6. In alkaline electrolyte, the nanoparticle surface changes to Co-rich phosphide under HER conditions, and to hydroxide/oxide under OER conditions. In acidic electrolyte with or without a reductive potential, clean CoP surface is exposed upon dissolution of the native oxidation layer.

Supporting Information Experimental details and additional display items are available as supporting information. 10 ACS Paragon Plus Environment

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Acknowledgements This work is partially supported by the Petroleum Research Funds from the American Chemical Society and the Global Innovation Initiative from the Institute of International Education. Q.G. thanks Nankai University for financial support. X.L. thanks the Chinese Scholarship Council for financial support.

Author Information Corresponding Author *

[email protected]

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