Grafting Poly(acrylic acid) from PEDOT To Control the Deposition and

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Grafting poly(acrylic acid) from PEDOT to control the deposition and growth of platinum nanoparticles for enhanced electrocatalytic hydrogen evolution Alissa J. Hackett, Jenny Malmstrom, and Jadranka Travas-Sejdic ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b02013 • Publication Date (Web): 08 Jan 2019 Downloaded from http://pubs.acs.org on January 11, 2019

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ACS Applied Energy Materials

Grafting poly(acrylic acid) from PEDOT to control the deposition and growth of platinum nanoparticles for enhanced electrocatalytic hydrogen evolution Alissa J. Hackett,ab Jenny Malmström,bc Jadranka Travas-Sejdic*ab aPolymer

Electronics Research Centre, School of Chemical Sciences, University of Auckland, New Zealand

bMacDiarmid cDepartment

Institute for Advanced Materials and Nanotechnology, New Zealand

of Chemical and Materials Engineering, University of Auckland, New Zealand *Email: [email protected]

Abstract Electrochemical water splitting is a sustainable, environmentally friendly method of hydrogen generation for green energy. However, the ideal electrocatalyst, platinum, is limited by cost and scarcity. Thus, new approaches are needed to minimise the amount of platinum required for efficient hydrogen production, while retaining its electrocatalytic activity. In this work, we report the development of a novel electrocatalyst based on platinum nanoparticles (PtNPs) deposited on conducting poly(3,4-ethylenedioxythiophene) (PEDOT), grafted with poly(acrylic acid) (PAA) chains. The presence of PAA controls the PtNP size and prevents aggregation during electrodeposition. The composite materials demonstrated enhanced electrocatalytic activity for the hydrogen evolution reaction (HER) in acid, with onset potentials as low as -84 mV vs RHE and exchange current densities up to 161 µA cm-2.Thus, these composite electrodes show promise as a straightforward, cost-effective alternative to conventional platinum HER catalysts. Furthermore, this novel fabrication approach

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shows great potential for the development of future electrocatalysts, providing an infinitely tailorable substrate for nanoparticle deposition thanks to the versatility of grafted conducting polymer films.

Keywords: hydrogen evolution; electrocatalysis; platinum; nanoparticles; conducting polymer; graft polymer

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Introduction

In the search for clean, renewable alternatives to fossil fuels, hydrogen stands out as an ideal fuel due to its high energy density and clean combustion products.1 One of the simplest methods of hydrogen production is the electrocatalytic splitting of water; however, this method requires suitable catalysts to lower the energy requirements of the reaction and thus reduce energy costs.2 The hydrogen evolution reaction (HER) occurs at the cathode as part of the electrochemical water splitting reaction, and proceeds via the following overall equation in acid: 2H + + 2𝑒 ― ⇌H2 The generally accepted steps of HER in acid are adsorption of a H intermediate Hads (Volmer step), followed by either electrochemical desorption (Heyrovsky step) or chemical desorption (Tafel step),3-4 each with characteristic Tafel slopes: Volmer: Heyrovsky: Tafel:

H + + 𝑒 ― ⇌Hads

120 mV decade-1

Hads + H + + 𝑒 ― ⇌H2

40 mV decade-1

Hads + Hads⇌H2

30 mV decade-1


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The rate-limiting step, and hence the mechanism of hydrogen evolution on a catalyst, may be determined from its experimentally-determined Tafel slope. Platinum (Pt) is considered an ideal HER electrocatalyst, combining a low onset potential with high electrocatalytic activity. However, its use is hindered by its high cost, and global scarcity,5 To overcome this limitation, many recent catalytic materials rely on the use of nanomaterials to increase the surface area available for catalysis, thus increasing efficiency while reducing amount of the bulk material required. Platinum nanoparticles on activated carbon (Pt/C) are already commercially available, displaying electrocatalytic activity comparable to Pt metal electrodes, but at a fraction of the cost.6-7 Other PtNP composite materials show similar results.8-13 Of all the methods of producing PtNPs,14-16 electrochemical deposition from a platinum salt electrolyte is one of the fastest and most straightforward.17 This one-pot system provides instant nanoparticle growth, reducing the need for stabilising agents to prevent aggregation, and thus does not require subsequent purification steps beyond electrode rinsing.18-19 Conducting polymers (CPs) have shown great promise as electrocatalysts in recent years, due to their conductive nature, high surface area, relative ease of preparation, and low cost.20 Their use as catalysts for hydrogen generation has not been widely explored; nevertheless, previous studies of certain CPs and CP-composites demonstrate their potential in this area.21-24 When used as a substrate for catalyticallyactive metal deposition, the CP provides a large surface area for metal loading, acts as a network for efficient electron transfer between the active metal sites, and can help to stabilise metal intermediates during catalysis.24-26 These properties can be further improved via doping, grafting or blending the CP with other polymers.22, 27


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In this work, we report a novel series of catalytic materials based on conducting polymers decorated with PtNPs. PtNPs were deposited electrochemically on to poly(3,4-ethylenedioxythiophene (PEDOT) films that had been grafted with poly(acrylic acid) (PAA) chains to improve surface hydrophilicity. The PAA chains were found to enhance surface coverage and reduce aggregation of the deposited NPs compared to ungrafted samples. The polymer/PtNP films were investigated as catalysts for electrochemical hydrogen evolution, demonstrating promising electrocatalytic activities comparable to previously-reported HER catalysts. Importantly, this fabrication method proved to be straightforward and cost-effective, with almost infinite opportunities for tailoring the materials to specific applications. The expanding library of CPs ready for grafting,28-30 combined with the vast body of research on graft copolymers,31-32 provide an obvious route for substrate customisation. Meanwhile, nanoparticle properties may be controlled through both substrate choice and electrodeposition conditions.33-35 2

Materials and methods

2.1 Materials Deionised water (Milli-Q, 18 MΩ cm) was used in all aqueous solutions. 3,4-Ethylenedioxythiophene (EDOT) and hydroxymethyl EDOT were purchased from AK Scientific and used as received. All other chemicals were purchased from Sigma Aldrich and used as received, unless otherwise stated. Copper(I) bromide (CuBr) was purified by suspending in glacial acetic acid overnight, washed with absolute ethanol and diethyl ether, dried under vacuum, and stored under nitrogen until use. tert-Butyl acrylate (tBA) was passed over an activated basic alumina column before use.


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Glass microscope slides coated with gold (50 nm thickness, plus 40 nm Ti adhesion layer) were purchased from Deposition Research Lab Inc (DRLI). Slides were cut into 1 x 2.5 cm pieces and electrochemically cleaned in 0.5 M H2SO4 before use.

2.2 Synthesis of EDOT monomer containing an ATRP (atom transfer radical polymerisation)-initiating site: (3,4ethylenedioxythiophene)methyl 2-bromopropanoate (BrEDOT) (3,4-ethylenedioxythiophene)methyl 2-bromopropanoate (BrEDOT) was synthesized from hydroxymethyl EDOT according to a previously-reported method.36 The crude product was purified using a neutral alumina column (7:1 hexane/ethyl acetate) to give BrEDOT as a yellow oil (484 mg, 54.3% yield). 1H NMR (400 MHz, CDCl3): d 1.83 (3H, d, CH3), 4.04−4.47 (6H, m), 6.35 (2H, s, Ar).

2.3 Electropolymerisation of PEDOT films In this work, all PEDOT films consist of a two layers – an adhesion layer of PEDOT, and a thin coating of PBrEDOT containing the ATRP-initiating sites. The films were synthesised on gold-coated glass slides by cyclic voltammetry in two deposition steps, as previously described.37 A three-electrode setup was used with the gold-coated glass as the working electrode, a stainless steel mesh counter electrode and an Ag/AgCl (3 M KCl) reference electrode. First, an adhesion layer of PEDOT was deposited from a solution containing 0.02 M EDOT and 0.1 M sodium p-toluenesulfonate in 3:1 acetonitrile/water (-0.3 to 1.4 V vs Ag/AgCl (3 M KCl), 100 mV s-1, 3 cycles). After rinsing with acetonitrile, a layer of PBrEDOT was deposited on top from 0.02 M BrEDOT/0.1 M LiClO4 in acetonitrile (-0.3 to 1.4 V, 100 mV s-1, 2 cycles). Films were then rinsed with acetonitrile and water and dried with nitrogen.


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2.4 Grafting of PAA chains from PEDOT to produce PEDOT-g-PAA films Grafting of PAA from PEDOT substrates was carried out through polymerisation of tert-butylacrylate (tBA) monomer via ARGET (activators regenerated by electron transfer) ATRP38 to PtBA, followed by acid hydrolysis to PAA. CuCl2 (1.2 mg), N,N,N’,N”,N”-pentamethyldiethylenetriamine (65 µL), tBA (4.4 mL) and acetone (4 mL) were added to a PEDOT substrate and stir bar in a 10 mL round bottom flask. After 30 min purging with N2, tin ethylhexanoate (0.3 mL) was added to the reaction mixture. The flask was sealed and left at 60 °C for 4 h, at which time the reaction was stopped by exposing it to air.39 The substrates were then thoroughly washed in acetone and dried with N2. Acid hydrolysis was carried out by immersing the PtBA-grafted substrates in dischloromethane containing 1% methanesulfonic acid for 30 min at room temperature.36 Following this, the substrates were washed thoroughly in dichloromethane, acetone and deionised water, and dried with N2.

2.5 Electrodeposition of Pt nanoparticles A three-electrode electrochemical cell was set up, using PEDOT or PEDOT-g-PAA substrates as the working electrode, stainless steel mesh as the counter electrode, and Ag/AgCl (3 M KCl) as the reference. K2PtCl4 (1 mM in 0.5 M H2SO4) was used as the electrolyte. A constant potential of -100 mV vs Ag/AgCl was applied for a predefined length of time (0-30 s), reducing the electrolyte to Pt0 on the surface of the polymer film.17, 40 The total charge passed during electrodeposition was used to calculate the Pt mass loading according to Faraday’s law, assuming 100% deposition efficiency.


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2.6 Physical characterisation 2.6.1 Water contact angle Water contact angle measurements were carried out using 10 µL drops of deionised water on ungrafted PEDOT and grafted PEDOT-g-PAA substrates. Images were recorded and analysed using a CAM 100 contact angle meter (KSV Instruments). Mean contact angles and standard deviations were recorded from a total of eight measurements for each substrate.

2.6.2 SEM/EDS High resolution SEM images were recorded under vacuum using a Philips FEI (Philips) XL30 SFEG scanning electron microscope fitted with an EDAX Phoenix EDS detector for simultaneous elemental analysis. Samples were not sputter-coated prior to analysis. PtNP particle analysis was carried out using backscattered electron images loaded into ImageJ software (details in SI).

2.6.3 AFM AFM height, amplitude and phase data were recorded simultaneously using a Cypher ES AFM (Oxford Instruments) in tapping mode in air. Measurements were recorded in air using Tap 150 Al-G cantilevers from Budget Sensors (tip radius

Grafting Poly(acrylic acid) from PEDOT To Control the Deposition and

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