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Platinum terpyridine metallopolymer electrode as cost-effective replacement for bulk platinum catalysts in ORR and HER Sait Elmas, Wesley Beelders, Siobhan Julie Bradley, Renee Kroon, Geoffry Laufersky, Mats Andersson, and Thomas Nann ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02198 • Publication Date (Web): 25 Sep 2017 Downloaded from http://pubs.acs.org on September 27, 2017

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ACS Sustainable Chemistry & Engineering

Platinum Terpyridine MetallopolMetallopolymer electrode as costcost-effective rereplacement for bulk platinum catacatalysts in ORR and HER Sait Elmas1, Wesley Beelders1, Siobhan J. Bradley2, Renee Kroon3, Geoffry Laufersky2, Mats Andersson1,3 and Thomas Nann*1,2. 1) Future Industries Institute, University of South Australia, Mawson Lakes Campus, Adelaide SA 5095, Australia. [email protected], [email protected], [email protected]

2) The MacDiarmid Institute, School of Chemical and Physical Sciences, Victoria University of Wellington, Laby 410 Gate 6 Kelburn Parade, Kelburn, Wellington 6140, New Zealand. [email protected], [email protected], [email protected]

3) Chalmers University of Technology, Department of Chemistry and Chemical Engineering, 41296, Gothenburg, Sweden. [email protected]

KEYWORDS: Oxygen reduction reaction, hydrogen evolution reaction, electrocatalyst, conducting polymer, terthiophene, metal complex, low metal loading, fuel cells. ABSTRACT: Conducting polymers consisting of metal-selective coordination units and a highly conductive backbone – so-called metallopolymers – are interesting materials exposing single atoms for photo/electrocatalysis and thus represent a potential low-cost alternative for bulk or nanoparticulate platinum group metals (PGMs). We synthesised and fully characterised an electropolymerisable monomer bearing a pendant terpyridine unit for the selective complexation of PGMs. Electrocatalytic tests of the resulting metallopolymer, poly-[(tThTerpy)PtCl]Cl, revealed activity both in the oxygen-reduction reaction (ORR) and hydrogen evolution reaction (HER). Rotating disk experiments (RDEs) showed the direct four-electron reduction of molecular oxygen to water at low angular velocities of the rotating electrode. Furthermore, the fabrication of Pt metallopolymers proved to be simple, non-hazardous and versatile. This proof-of-concept opens up the possibility for developing future low-cost electro- and photocatalysts to replace current systems.

INTRODUCTION The high cost and low natural abundance of platinum group metals (PGMs) is a major obstacle to their broad technical application calling for more sustainable structures/designs/formulations. Such solutions involve replacement of the PGMs by less expensive and earth abundant elements or reduction of the amount of noble metal to make “every atom count” by using single-site metal centres to maintain or exceed the performance of the bulk metal equivalent. In energy conversion applications such as electrocatalytic watersplitting, the use of platinum and its alloys is gaining traction.1–5 However the high content of expensive platinum materials impedes the widespread application of this technology. Metallopolymers - conducting polymers doped with metal centers in a well-defined coordination environment - are emerging as smart materials in a broad range of photo/electronic devices as they have the potential to combine the processability of soft organic materials with the redox properties of metals.6–10 In electrocatalytic applications, such as wa-

ter-splitting and oxygen reduction reaction (ORR), the metallopolymer or metallosupramolecular polymers can therefore provide hybrid functionalities. This makes it possible to embed superior catalytic properties of transition metals into a conducting backbone that consists of a less expensive and abundant organic material.11–14 In an economic sense, the bulk inorganic electrode material that is not explicitly involved in catalysis (“buried catalyst material”) can be replaced by organic polymers without affecting the conducting ability of the electrode. Furthermore, the mechanical flexibility of the polymer and the metal selectivity of tailor-made coordination environments can address the major technical hurdle: the leaching of catalyst material during mechanical stress. We designed a monomer consisting of an easily electropolymerisable backbone and a pendant, chelating unit that has a strong binding character to the late transition metals, particularly the platinum group metals. The terpyridine (terpy) ligand is one of the most well-known tridentate pincer-type15–18 and chelating ligands. Its planarity, the donor set N^N^N and the

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bite-angle tailor the ligand for coordination to a series of late transition metals. Additionally, it can be easily combined with a series of co-ligands to tailor its activity. The backbone of the ligand is ideally a unit that is easily electropolymerisable onto any electrode surface where the as-prepared polymer film provides good conductivity behaviour. Among the conductive polymers polythiophene, (PTh) and its derivatives are the most well-studied and applicable candidates.19 They are highly conductive20,21, easily synthesised and they have already been pursued in applications such as organic solar cells (OSCs)22,23, field-effect transistors24 and electrocatalysis.25,26 Ideally, the as-prepared polymer film is then easily loaded with the metal of choice by dipping the electrode into a solution of the metal salt without any further complicated synthetic routes for complexation (Figure 1A). The electrochemically-synthesised metallopolymer, loaded with platinum (II), was then tested for the electrocatalytic hydrogen evolution reaction (HER) and oxygen reduction reactions (ORR). Figure 1B illustrates the concept of replacing a bulk platinum electrode by a poly(terpyridinyl)terthiophene metallopolymer, poly[(tThTerpy)PtCl]Cl, with distinct platinum(II) catalytic centres for the oxygen reduction reaction (ORR).

A

B

Figure 1. A - Illustration of the complexing of platinum (II) ions by dipping of the polymer in a solution of potassium tetrachloroplatinate(II); B Concept of the platinum-metallopolymer with single-site catalytic centres for the ORR.

EXPERIMENTAL SECTION Materials and Methods All chemicals and solvents were obtained from Sigma Aldrich without further purification. All intermediates involving cross-coupling reactions were synthesised under standard Schlenk techniques using argon as protecting gas. The monomer was synthesised under ambient and reflux conditions without further precautions. NMR spectra were recorded with a Bruker Avance II 300 MHz spectrometer by using a triple-resonance 1H, nBB inverse probe head. Unambiguous assignment of the 1H and 13C resonances was achieved from 1H COSY, 13C APT, HSQC and HMBC spectra. All 2D experiments were performed under

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standard pulse sequences from the Bruker pulse program library. The chemical shifts are quoted relative to TMS. XPS measurements were performed using monochromatized Al Kα X-rays (1486.7 eV) at a power of 225 W on a Kratos Axis-Ultra spectrometer (160 eV analyzer pass energy for survey scans, 20 eV for high-resolution scans). The analysis spot size was ~300 × 700 µm. Core electron binding energies are given relative to an adventitious hydrocarbon C 1s binding energy of 284.7 eV. All XPS spectra were processed with CasaXPS (ver. 2.3.16 PR 1.6) data processing software using a Shirley background correction. Single crystals were mounted in paratone-N oil on a plastic loop. X-ray diffractions were collected with Mo-Kα radiation (λ = 0.7107 Angstroem) on an Oxford Diffraction X-calibur single crystal X-ray diffractometer. Data sets were corrected for absorption using a multi-scan method. The structures were solved and refined by direct methods using OLEX software package.27 All non-hydrogen atoms were refined with anistropic displacement parameters. The figures were generated with OLEX. The electropolymerisation of the monomer was performed in a three-electrode configured electrochemical cell on an AUTOLAB potentiometer using a gold slide (microscope slide with 100 nm Au and 40 nm Ti sublayer, obtained from Deposition Research Laboratory, Inc) as working electrode, a platinum mesh (1x1 cm, 1mm Ø) as counter electrode and a silver wire as pseudo-reference electrode. The working and counter electrodes were cleaned in a piranha solution (30% H2O2/98% H2SO4 = 1/3 (v/v)) before use. For the oxygen reduction reaction (ORR) experiments a rotating disc electrode (glassy carbon, 0.197 cm2) was electrochemically deposited with polymer and submerged in a K2[PtCl4] (40 mg in 20 mL H2O) for 1 h at 50 °C. The rotating disc voltammograms were taken on a RSI potentiostat at angular velocities between 200 and 2000 rounds per minute (rpms) using the rotating disc electrode deposited with metallopolymer as working electrode (WE), Ag|AgCl in a saturated KCl solution as reference electrode (RE) and a platinum loop in proton membrane as counter electrode (CE). Hydrogen evolution reactions were performed in 0.1M KCl solution in an electrochemical cell (EC) using the metallopolymers A and B on Au substrates as working electrode (WE), a Pt rod as counter electrode (CE) and Ag|AgCl (3M KCl) as reference electrode. Prior to sweeping between 0 and -1.5 the electrolyte solution was saturated with argon gas. After 3-5 potentiometric sweeps, the evolved hydrogen was taken from the headspace of the EC and detected by gas chromatography (SRI Gas Analyzer with thermal detector) using N2 as carrier gas. Synthesis of poly-[(tThTerpy)PtCl]Cl 2,5-Dibromo-3-thiophenecarbaldehyde: Following the procedure in the literature28, a mixture of bromine (2.45 mL, 47.88 mmol) and 7.5 mL of aqueous HBr (48%) were added dropwise to a mixture of 2 mL thiophene-3-carboxaldehyde (22.83 mmol) and 10 mL HBr in 20 mL Et2O at 0 C using an ice bath. The reaction mixture was then refluxed at 50 °C for overnight, cooled to room temperature, quenched with 25 mL of saturated NH4Cl solution (aq) and extracted three times with water. The combined organic phases were washed with brine and dried over anhydrous Na2SO4. All solvents and volatiles were removed on the rotary evaporator and the product was isolated via column chromatography using an eluent mixture of ethyl acetate and cyclohexane in ratio 19/1. After re-


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moval of the solvents and volatiles 5 g of yellow product (95 % yield) were obtained. 1H NMR (300 MHz, CDCl3, δ): 10.03 (1H, s, CHO), 7.56 (1H, 1, H4). 13C NMR (75 MHz, CDCl3, δ): 183.6 (1C, CHO), 139.7 (1C, C3), 129.2 (1C, C4), 124.7 (1C, C2), 113.8 (1C, C5). The 1H and 13C APT NMR spectra of the compound are depicted in Fig. S2A/B, supporting information. 3’-Formyl-2’:2’,5’:2”-terthiophene: To a stirred mixture of 2,5-dibromo-3-thiophenecarbaldehyde (5 g, 18.5 mmol) and tetrakis(triphenylphosphinepalladium [Pd(PPh3)4] (1.125 g, 1.11 mmol) in 1,2-dimethoxyerhane (300 mL) were added 2thiophene boronic acid (5.42g, 42.3 mmol) and a solution of 1M Na2CO3 (110 mL). The reaction mixture was degassed and flushed three times with argon, and refluxed for 5 h. After that, another portion of 2-thiophene boronic acid was added under protection gas and the mixture was refluxed for another 8 h. The reaction mixture was then cooled down to room temperature and all solvents and volatiles were removed on the rotary evaporator. The crude product was redissolved in CH2Cl2, washed three times with deionised water and the combined organic phases dried over anhydrous Na2SO4. After filtration on a silica pad, the product was obtained as bright yellow solid via column chromatography using ethyl acetate/cyclohexane (1/1) as eluent (3.6 g, 90%). 1H NMR (300 MHz, CDCl3, δ): 10.08 (s, 1H, CHO), 7.56 (s, 1H, H4’), 7.50 (dd, 2H, H5), 7.32 (dd, 2H, H3), 7.29 (dd, 1H, H5”), 7.23 (dd, 1H, H3”), 7.16 (dd, 1H, H4), 7.05 (dd, 1H, H4”). Full 1H NMR spectrum of the compound is depicted in Fig. S3, supporting information. 4-(terthiophenyl)-terpyridine:29 2-acetylpyridine (490 mg, 4 mmol) was added into a solution of 3’-Formyl-2’:2’,5’:2”terthiophene (540 mg, 2 mmol) in EtOH (100 mL). KOH pellets (220 mg, 4 mmol) and NH3 (10 mL, 30 %)were then added into the solution. The mixture was stirred overnight. The formed yellow-brown solid was removed and washed two times with cold EtOH/H2O (1/1) leading to 15.6 % of yellow product. Alternative synthesis method:30 2-acetylpyridine (200 mg, 1.65 mmol) was combined with 3’-Formyl-2’:2’,5’:2”terthiophene (204 mg, 0.83 mmol) and KOH pellets (93 mg, 1.65 mmol) and ground for 10 minutes using a mortar and pestle. The homogenous crude mixture was then transferred with 50 mL acetic acid into a round-bottom flask and refluxed with excess of NH4OAc at 110 C for 20 hours. The dark red solution was then reduced to half volume on the rotary evaporator and the crude product precipitated with water. After several steps of filtration over silica gel using a series of solvents the product was then isolated via column chromatography in ethyl acetate and cyclohexane (1/6) resulting in yields < 5%. 1 H NMR (300 MHz, CDCl3 δ): 8.66-8.68 (m, 2H, Terpy H), 8.62-8.65 (m, 2H, Terpy H), 8.52 (s, 2H, Terpy H), 7.82-7.88 (m, 2H, Terpy H), 7.32 (m, 2H, Terpy H), 7.29-7.34 (m + s, 3H, Thiophene H + Terpy H), 7.23-7.27 (m, 2H, Thiophene H), 7.16-7.18 (m, 1H, Thiophene H), 7.03-10 (m, 2H, thiophene H). 6.90-6.93 (m, 1H, Thiophene-H); 13C NMR (75 MHz, CDCl3, δ): 156.4, 156.1, 149.5, 146.4, 137.4, 137.1, 136.9, 136.4, 136.1, 135.1, 128.3, 127.7, 127.4, 126.7, 126.6, 125.3, 124.5, 124.1, 121.8, 121.6. 1H, 13C APT, H,H-COSY, HSQC and HMBC NMR spectra of the compound are depicted in Fig. S4A,B, S5A,B and S6A, supporting information. 4-(terthiophenyl terpyridine dichloro platinum (II): 21 mg (0.05 mmol) Potassium tetrachloroplatinate, K2[PtCl4], dissolved in 2 mL H2O were combined with 24 mg (0.05 mmol) 4-(terthiophenyl) terpyridine ligand in 1 mL acetonitrile and

stirred overnight at 50 C. 10 mL H2O was then added to the reaction mixture and the complex was extracted with CH2Cl2 (3 x 10 mL). After combining all organic phases and removing the solvent on rotary evaporator the complex obtained was an orange-brown solid. 1H NMR (300 MHz, CDCl3 δ): 8.46 – 8.48 (m, 3H,), 8.06 (s, 2H, Terpy H), 7.78 – 7.91 (m, 4H), 7.30 – 7.34 (m, 2H), 7.11 (s, 1H, Thiophene H), 6.89 – 6.90 (m, 1H), 6.87 – 6.84 (m, 2H), 6.64 – 6.66 (m, 1H), 6.49 – 6.53 (m, 1H). 1H NMR spectrum of the complex compound is depicted in Fig. S7, supporting information. Electropolymerisation of 4-(terthiophenyl) terpyridine on gold substrates: In an electrochemical cell 50 mg of 4(terthiophenyl) terpyridine were dissolved with 870 mg tetrabutylammonium hexafluorophosphate (TBAPF) in 20 ml CH2Cl2 and the solution was swept with variable numbers of sweeps between -0.8 and +1.5 V at scan rate of 100mV/s. The deposited red polymer film on gold was then washed thoroughly with CH2Cl2 and gently dried under nitrogen stream. RESULTS AND DISCUSSION To synthesise the monomer, we followed a protocol involving a four-step synthetic route (see Scheme 1). The 3thiophene carboxaldehyde was brominated with HBr28 (i) in near-quantitative yield. In the second step the intermediate terthiophenyl aldehyde was synthesised via Suzuki coupling31 using 2-thiophene boronic acid as second reagent (ii). Following a green route proposed by Raston and Cave30, the resulting formyl-terthiophene was then reacted with 2 equivalents of 2acetylpyridine via aldol condensation and subsequent Michael addition (iii) and cyclised in AcOH/NH4OAc at elevated temperature or stirred in an ammonia/EtOH solution at room temperature (iv).

Scheme 1: Synthetic route of the electropolymerisable monomer 4(terthiophenyl) terpyridine: i) Br2/HBr, EtOH, 60 °C, 2h, ii) Suzukicoupling using 2-thiophene boronic acid; iii) 2 eq. 2-acetylpyridine, 2 eq. NaOH, grinding using mortar and pestle, rt; iv) 10 eq. NH4OAc, AcOH, 110 °C, 12-15 h; v) cyclic electropolymerisation between -0.8 and +1.5 V in a 0.01 M monomer and 0.1 M TBAPF electrolyte solution.

The monomer was isolated via column chromatography and its chemical structure was elucidated using 1H-, 13C-APT -, H,H-COSY-, HSQC- and HMBC-NMR spectroscopy. All relevant NMR spectra of the monomer and precursors are depicted in Figure S2A – S6A (see Supporting Information). Single crystals of the monomer were obtained by slow evaporation of CH2Cl2 at 4 °C and the solid-state structure determined (see molecular structure in Figure 2 or Figure S1 in Supporting Information, tThTerpy). Two planes formed by the terpyridine unit and the terthiophene backbone dominate the molecular structure. The notably high tilt angle (dihedral angle) of 75.12° between both planes is caused by steric repul-



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sion between the terpyridine unit and the terthiophene backbone indicating the feasibility of the chain growing at the terminal positions of the terthiophene moiety. Note, that the monomer with a single thiophene backbone unit was not electropolymerisable due to steric repulsion (see Xray molecular structure of ThTerpy in S1). To obtain the metallopolymer, two different methods were used. Method A involved pre-complexation of the monomer and electropolymerisation of the metalorganic compound (sample A), whereas in method B the electropolymerised polymer (Scheme 1, step v)) was dipped into an aqueous metal salt solution (post-complexation, sample B). The precomplexed monomer with Pt(II) was obtained by treatment with K2[PtCl4] in AN/H2O (1/1) solution at 50 °C. Figure 3A shows the stacked 1H-NMR spectra of the monomer and the [(tThTerpy)PtCl]Cl complex in CDCl3. After complexation, all protons corresponding to the terthiophene backbone and terpy unit experience strong high-field shifting while the terpy protons became more distributed. The highest shielding effects are observed at the protons adjacent to the coordination centre and they are in line with a series of reported terpyridine platinum chloro complexes.32,33 The complexation with Pt(II) is indicated by the platinum satellites (24 Hz) which appear alongside the proton singlet of the central pyridine ring (8.11 ppm).

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The XPS measurements of the Pt-metallopolymer sample B showed that the 4f binding energies were centered at 75.6 and 72.3 eV for 4f7/2 and 4f5/2, respectively (Figure 3B). They are in the expected range of platinum(II)-terpyridine complexes.34 Additional peaks at higher binding energies for 4f7/2 and 4f5/2 are assigned to aqua complexes34, which might come from ligand-exchange species of poly-[(tThTerpy)PtCl]Cl with water molecules forming poly-[(tThTerpy)Pt(OH2)]Cl2. Hence, the major metallopolymer species in the analysed film are poly-[(tThTerpy)PtCl]Cl and poly-[(tThTerpy)PtOH2]Cl2 indicating a ligand exchange reaction between the chlorine coligand and water. However, trace amounts of zero-valent states of Pt were detected too. This is not unlikely as the XPS experiments conducted at room temperature often cause reduction of metals.35,36 In a similar work, the terpyridine ligand was incorporated into a conducting poly(p-phenylenevinylene) polymer chain via a Heck coupling reaction, but the reaction required precomplexation with the palladium in order to keep the catalyst active for the copolymerization reaction.37 In contrast, with our study we show as a proof concept, that the pre- and postcomplexation of the polymer are easily achievable allowing binding of the terpyridine to metals of choice.

A

B

Figure 2: The resolved and refined molecular structure of the synthesised monomer terthiophenyl terpyridine (tThTerpy) obtained by Xray diffraction on single crystals; Nitrogen, sulfur, carbon, and hydrogen atoms are represented by blue, yellow, dark grey and white, respectively.

Alternatively, the monomer was pre-polymerised and the obtained polymer film on a solid gold electrode was dipped into an aqueous K2[PtCl4] solution for 3 h at 50 °C (sample B) as shown in Fig. 1A. After washing off all precursor and inorganic salts from the surface with deionised water, the composition on the electrode surface was analysed by X-ray photoelectron spectroscopy (XPS). The atomic ratio of nitrogen to platinum of exactly 3/1 revealed that all possible terpyridine units are coordinated to platinum.

Figure 3: A – stacked 1H NMR spectra of the monomer tThTerpy and its complex [(tThTerpy)PtCl]Cl in CDCl3; B – High resolution Pt 4f7 spectrum showing the bonding environment of the platinum in the metallopolymer (sample B) deposited on a gold substrate.


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Electropolymerisation of the monomer To obtain the conducting polymer, the monomer was swept in an acetonitrile solution between -0.8 and 1.5V using cyclic voltammetry methods (vide infra for experimental details). The electropolymerisation showed a quasi-reversible redox event at 0.9 V vs Fc/Fc+ that is typical in the range of electropolymerisable thiophene and its derivatives.19 With subsequent cycling the onset oxidation potential shifted to 0.70 V (Figure 4A) indicating the growth of a polymer film. After the first potentiostatic sweep, the formation of a red-orange film was already visible on the working electrode. A photographic picture of the polymer film on a gold substrate (working electrode) is depicted in the UV-vis spectrum, Figure S8A (supporting Information). The UV-vis spectrum of the polymer dissolved in acetone shows a bathochromic shift of 80 nm compared to the monomer recorded in dichloromethane (Fig. S8A and B, Supporting Information). Note that monomer and polymer were not soluble in same solvents making a comparative discussion of their UV-vis absorbance spectra difficult. However, with a subsequent increase in the potentiometric sweep, the characteristic oxidation peak didn’t grow any further. The anodic oxidation started at about 0.75 V, and showed a strong corresponding reduction peak in the range of 0.25 and 1.25V. During the subsequent cycles (2nd and 3rd) another oxidation peak appeared in the anodic range between 0.5 and 0.9 V accompanied by a broader reduction event between 0.25 and 0.75. Additionally, all redox events decreased with increasing number of cycles. The electropolymerisation of the pre-complexed monomer (sample A) showed more redox features in the potential range -1.4 to 1.5 V (Fig. 4B), which are attributed to the metallopolymer species. Similarly to the electropolymerisation of the pure monomer the first potentiostatic sweep of the monomerplatinum complex also led to the formation of a red film on the gold substrate. The decrease of all peak currents (Fig. 4B) within increasing numbers of the potentiometric sweeps clearly indicates the decrease of the conductivity of the working electrode. Thus, a single cycling between the selected potential ranges suffices to obtain a metallopolymer film with good conductivity.

A

B

Figure 4: Cyclic voltammograms of the tThTerpy (A) and [(tThTerpy)PtCl]Cl monomers (B) in AN at a scan rate of 100mV/s using 0.1M TBAPF as electrolyte. For clarity reasons, only three potentiometric sweeps are shown in the cyclic voltammogram A.

In contrast to our work, a similar type of outer-sphere metallopolymer with a Pt-[NCN] pincer was reported by Holliday et al.38 They showed smooth growth of the film, but the platinum centres are in immediate contact with the polymer backbone. The reported electropolymerisable behaviour of the Pt-[NCN] pincer metallopolymer is similar to inner-sphere metallopolymers, where the metal centers play a structural role allowing smooth film growth during the cyclic electropolymerisation.11,12,39–41 We hypothesise that due to the sterical effects of the outersphere terpy units (Figure S1) the coupling of the preorganised film on the surface with new monomer molecules was hindered. Since the terpy units are not electropolymerisable, the formed polymer during the initial potentiometric sweeps must be organised in such a way that all thiophene units are shielded from further electrochemical coupling with the incoming monomer. This hypothesis is backed up by the stoichiometric platinum content in XPS survey spectrum (vide infra and Fig. S6B in supporting information) and the monothiophene terpyridine monomer, which was not electropolymerisable (Figure S1, Supporting Information). Consequently, in subsequent investigations we pursued the metallopolymer deposition via method B where only three potentiometric



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sweeps were applied to the monomer solution to obtain adherent films on the electrode surface. For practical purpose, the complexation (metal loading) appeared to be more convenient. Scanning Electron Microscopy (SEM) images of the polytThTerpy and poly-[(tThTerpy)PtCl]Cl (see Figure S9A,B, Supporting Information) didn’t show significant changes in the surface topology of the film after submerging the polymer film in an aqueous K2[PtCl4] solution. The minor swelling effects of the resulting metallopolymer in Fig. S9B are presumably caused either by formation of aqua complexes or the resulting metallopolymer itself. Nonetheless, the SEM analysis corroborated the full coverage of the gold substrates with poly-tThTerpy and poly-[(tThTerpy)PtCl]Cl, respectively. Additional EDX analysis during image scanning showed Pt being present in the metallopolymer sample (Fig. S10, supporting information).

Electrocatalytic oxygen reduction reaction (ORR) at rotating disk electrode (RDE) To obtain the first indications of its electrocatalytic activity for the oxygen reduction reaction, the platinummetallopolymer (Pt MP) was deposited on a glassy carbon electrode according to method B and swept in an air- and argon-saturated 0.1 M KOH solution between -1.0 and +1.0 volts, each. Knowing that the gold electrode is the most suitable surface for a good adhesion of our metallopolymer, the glassy carbon material allowed for a direct comparison with benchmark catalysts. Latter electrode has asserted itself as most established electrode in fuel cell research. In the air-saturated electrolyte solution the cathodic reduction of molecular oxygen starts at -0.25 volts resulting in a peak current density of -0.6 volts - four times higher than the current density at the same peak potentials obtained from the argon-saturated solution (Figure 5A). Below -1.0 V molecular hydrogen evolution begins.

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A

B

Figure 5: A – Cyclic voltammograms of the Pt-metallopolymer electrode in an air- and argon-saturated solution in 0.1M KOH and a scan rate of 50 mV/s; B – Rotating disc voltammograms of the oxygen-reduction reaction (ORR) at different angular velocities in 0.1M KOH and with a scan rate of 10 mV/s (for clarity, only half cycles are shown).

The catalytic performance of the platinum-metallopolymer for the reduction of molecular oxygen was then tested on a rotating disk electrode (RDE). The obtained rotating disc voltamogramms in 0.1 M KOH show cathodic current densities as a function of the angular velocity of the RDE resulting in a plateau at -0.5 to -0.6 volts (Figure 5B). The linearity in the calculated Koutecky-Levich plots (Figure 6A) and their similar slopes indicate first-order kinetics towards the oxygen reduction in the potential range of -0.35 and -0.55 V. The kinetic current densities, obtained from the intercept of the K-L plots, are 0.6 mA/cm2 at -0.4 V, 0.893 mA/cm2 at -0.45 V, 1.03 mA/cm2 at -0.50 V and 1.04 mA/cm2 at -0.55 V vs Ag|AgCl. As expected, the obtained kinetic currents normalised to the geometric surface area are lower compared to Pt/C benchmark catalysts.42,43 However, in our study the ICP MS analysis of in total six Pt MP films prepared on the RDE surface revealed Pt contents of 36µg/cm2 to 76 µg/cm2, which are three to five times lower than the amount of noble metal used in the benchmark systems.43 The ICP-MS results are summarised in Table S2 (Supporting Information). The calculated mass activities were 0.014 - 0.029 A/mgPt. From the slope of the K-L plot the number of electrons involved in the ORR were calculated at each rotational speed. As shown in Figure 6B the molecular oxygen undergoes a


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four-electron reduction at lower angular velocities in the potential range of -0.4 to -0.55 volts. Above 800 rpm a twoelectron reduction is calculated suggesting that longer residence time for the oxygen molecules at the catalytic centres is required to reduce them to H2O.

in Figure 7A is associated with trapped air in the polymer matrix of the as-prepared metallopolymer film and, the “big jump” between the first (1st scan) and the subsequent sweeps in Figure 7B,C are caused by diffusion limits of molecular oxygen at the electrode-solution interface.

A

B

Figure 6: A – Koutecky-Levich plot calculated at the reduction potentials -0.35, -0.45 and -0.55 V vs. Ag|AgCl(3M KCl) and B – calculated electron numbers involved in the ORR at selected reduction potentials and angular velocities of the RDE.

And last, a stability test of the platinum (II) metallopolymer (Pt MP) was conducted in an air-saturated 0.1 molar KOH solution for 6.45 hours in total. The stability test was split into three consecutive steps, where the as-prepared Pt MP on the rotating gold electrode (0 rpm) was initially swept at a scan rate of 20 mv/s for 2.15 hours between 0 and -0.8 volts, then washed, dried and swept for another 2.15 h under same conditions in the consecutive experiments, each. The respective cyclic voltammograms compared to the N2-saturated solution are shown in Figure 7 below. All three cyclic voltammograms show a stable end current of 30µA/cm2 at -0.6 volts after 2.15 h sweeping indicating good stability of the hybrid film on the rotating gold electrode. The first voltammogram (Figure 7A) shows a smooth decay in the current at the selected voltage within the increasing numbers of potentiometric sweeps. After washing and drying in the subsequent steps the currents at -0.6 volts remained in the initial and last potentiometric sweeps unchanged at -60µA/cm2 (Figure 7B and C) suggesting no loss of activity in the ORR at all. The smooth decrease in the current in the first experiment

Figure 7: Cyclic voltammograms of the (A) as prepared Pt metallopolymer film and (B), (C) after two steps of washing and drying. The experiments were conducted in a 0.1M KOH solution under air- and nitrogensaturation for 2.15 hour, each.



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Electrocatalytic hydrogen evolution (HER) reaction For the electrocatalytic water splitting, the polymer was loaded with the platinum (II) catalyst via two different methods: The electropolymerised polymer on the gold electrode was placed in an aqueous potassium tetrachloroplatinate (K2PtCl4) solution at 50 °C for 3 hours and washed thoroughly with deionised water to remove the platinum salt residues from the surface (Sample B). During the polymer complex formation and washing procedures no polymer leaching from the gold surface was observed indicating good adhesive properties of the polythiophene backbone for gold. Surface analysis of the metallopolymer sample by XPS revealed that all accessible terpy units are entirely occupied by platinum as the XPS data showed a platinum-to-nitrogen ratio of exactly 1/3 (Figure S6B). Alternatively, the monomer was pre-complexed with Pt(II) and obtained monomer platinum(II) complex, [(tThTerpy)PtCl]Cl, was electropolymerised in an organic electrolyte solution (Sample A). The metallopolymer sample A was then swept between 0 and -1.5 V in a 0.1 molar potassium chloride (0.1M KCl, pH 6.2) solution at a scan rate of 50 mV/s. Sample B was swept up to -1.8 V showing stabilities at higher cathodic potentials. Both cyclic voltammograms are shown in Figure 8. For comparison, a platinum mesh, the gold electrode and the metal free polymer were measured under the same conditions in a fresh 0.1M KCl solution, each. As the bare Au electrode and the polymer showed slight gas evolution at higher cathodic potentials (onset potentials started at -1.3 V) vigorous gas evolution at both metallopolymer electrodes (poly-[(tThTerpy)PtCl]Cl A and B) were observed between -1.0 and -1.5V. As shown in Figure 8 the onset potential for the hydrogen evolution started at -1.1 V. Hydrogen evolution began at slightly higher cathodic potentials for the pre-complexed sample, but showed almost the same cathodic current density at -1.5V as obtained by sample A. The platinum mesh showed the highest current density at -1.5V with HER beginning at -1.0 V. The comparably higher activity of the pure platinum mesh is not surprising since it possesses the highest platinum density per surface area.

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CONCLUSION The ease of processability and synthetic accessibility of conducting metallopolymers make them good candidates for replacing bulk noble metals in renewable energy applications. By using a commercially available platinum precursor the metallopolymer was easily obtained by dipping the polymer electrode into an aqueous metal salt solution under mild conditions. The catalytic activities of the simple metallopolymer were shown for both the electrocatalytic oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER). Considering the negligible surface area compared to the Pt mesh, the activities in HER are almost as competitive as the pure platinum. On the other hand, the activities in the ORR were below the benchmark systems as shown in the calculated kinetic current densities and mass activities. However, these results were not surprising since the number of Pt atoms exposed to molecular oxygen (also known as electrochemically active surface area, ECSA) is a direct function of the specific activities in mA/cm2 Pt. In our study, the amount of Pt used was far lower than conventional catalyst systems. Chlorine coligands are known to be disruptive factors in catalysis, which opens up potential for significant improvement of the catalytic activity of metallopolymers in future work, by replacing these co-ligands with non-or weakly coordinating molecules. In conclusion, metallopolymers with tailored functional groups show great potential in replacing bulk noble metals in electrocatalytic applications by cheap and easily processable conducting polymers.

ASSOCIATED CONTENT Supporting Information Supporting Information of current work contains crystallographic data of the monomers ThTerpy and tThTerpy, both, as well as 1H, 13C APT-, H,H-COSY-, HMBC, HSQC-NMR spectra and additional XPS and EDX data. Crystallographic cif-files are included as separate supporting information. The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author * [email protected]

ACKNOWLEDGMENT We thankfully acknowledge Ass. Prof. Christopher Sumby, University of Adelaide, for supporting to conduct X-ray diffraction experiments on single crystals and data collection. This work was performed in part at the South Australian node of the Australian National Fabrication Facility under the National Collaborative Research Infrastructure Strategy.

ABBREVIATIONS Figure 8: Cyclic voltammogram of the polymer poly-tThTerpy and the metallopolymers poly-[(tThTerpy)PtCl]Cl (A, B) swept between 0 and 1.5 V vs. Ag|AgCl (3M KCl) and compared to Pt mesh and bare Au substrate. The pH of the solution was 6.2.

PGMs, platinum group metals; ThTerpy, 4-thiophenyl terpyridine; tThTerpy, 4-(terthiophenyl) terpyridine; poly-tThTerpy, poly(terthiophenyl) terpyridine, poly-[(tThTerpy)PtCl]Cl, poly-[(terthiophenyl) terpyridine dichloro platinum (II)]; TLC, thin layer chromatography; TMS, tetramethyl silane; TBAPF tetrabutyl ammonium hexafluorophosphate; AN, acetonitrile; EtOH, etha-


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nol; AcOH, acetic acid; rt, room temperature; Pt MP platinum (II) metallopolymer.

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For Table of Contents Use Only

SYNOPSIS: Every atom counts when a coordinating, conducting polymer is loaded with platinum. Since catalysis (for example in fuel cells) happens only at the surface of the platinum, metallopolymers were studied as a lower cost alternative.


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Platinum Terpyridine Metallopolymer Electrode as ... - ACS Publications

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