ORR electrocatalysts

Accepted Manuscript Title: Cu and Pd nanoparticles supported on a graphitic carbon material as bifunctional HER/ORR electrocatalysts Author: Marta Nunes Diana M. Fernandes M.V. Morales I. Rodr´ıguez-Ramos A. Guerrero-Ruiz Cristina Freire PII: DOI: Reference:

S0920-5861(18)31271-9 https://doi.org/doi:10.1016/j.cattod.2019.04.043 CATTOD 12138

To appear in:

Catalysis Today

Received date: Revised date: Accepted date:

19 September 2018 20 March 2019 10 April 2019

Please cite this article as: M. Nunes, D.M. Fernandes, M.V. Morales, I. Rodr´iguezRamos, A. Guerrero-Ruiz, C. Freire, Cu and Pd nanoparticles supported on a graphitic carbon material as bifunctional HER/ORR electrocatalysts, Catalysis Today (2019), https://doi.org/10.1016/j.cattod.2019.04.043 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Graphical Abstract

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Pd NPs

Cu NPs

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HSAG

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Cu and Pd NPs supported on HSAG were tested as bifunctional ORR/HER

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•

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electrocatalysts; •

The composites showed good electrocatalytic performances and an moderate

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durability;

A small loading of Pd NPs was able to improve the ORR/HER electrocatalytic

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activity.

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•

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Highlights

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Cu and Pd nanoparticles supported on a graphitic carbon material as bifunctional HER/ORR electrocatalysts

Guerrero-Ruizb and Cristina Freirea

REQUIMTE/LAQV, Departamento de Química e Bioquímica, Faculdade de Ciências,

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a

Departamento de Química Inorgánica y Química Técnica, Facultad de Ciencias,

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UNED, Senda del Rey 9, 28040, Madrid, Spain.

Instituto de Catálisis y Petroleoquímica, CSIC, C/ Marie Curie 2, Cantoblanco, 28049,

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c

Madrid, Spain.

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Authors with equal contribution

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1

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Universidade do Porto, 4169-007 Porto, Portugal. b

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Marta Nunes,a,1,* Diana M. Fernandes,a,1,* M. V. Morales,b I. Rodríguez-Ramos,c A.

*Corresponding author: Dr. Marta Nunes ([email protected]) and Dr. Diana M.

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Fernandes ([email protected])

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ABSTRACT The development of efficient, available and robust substitutes for the Pt-based electrocatalysts is very important for a sustainable energetic future. Herein, we report a series of composites based on Cu, Pd and Cu-Pd nanoparticles (NPs) supported on high

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surface area graphite (HSAG), as electrocatalysts for the energy-related reduction

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reactions – oxygen reduction (ORR) and hydrogen evolution (HER) reactions.

All the composites showed interesting ORR electrocatalytic activities in alkaline

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medium. The Pd/HSAG and Cu-Pd/HSAG composites exhibited the most promising performances, with onset potentials of 0.84 and 0.91 V and current densities of

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jL, 0.3 V, 1600 rpm = -3.5 and -4.2 mA cm-2, respectively. All the composites showed

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selectivity for the 4-electron process and Tafel slopes in the range 48 - 77 mV dec-1. The metal/HSAG composites revealed a great tolerance to methanol and moderate

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electrochemical stability.

In highly acidic medium (0.5 mol dm-3 H2SO4, pH = 0.3) only the Cu-Pd/HSAG

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and Pd/HSAG electrocatalysts presented electrocatalytic activity towards HER, with relative low overpotentials (η10 = 0.145 and 0.063 V, respectively), small Tafel slopes (75 and 42 mV dec-1) and similar exchange current densities (0.43 and 0.57 mA cm-2).

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These electrocatalysts also showed moderate electrochemical stability, in particular CuPd/HSAG for which overpotential only changed between 0.033 and 0.038 V for j = 40 mA cm-2.

The results showed that only small loading of Pd NPs (1 wt.% Pd) was able to improve significantly the ORR/HER electrocatalytic activity, which is a very important outcome to rationalise the design of efficient and cost-effective electrocatalysts in future.

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Keywords: Oxygen reduction reaction; Hydrogen evolution reaction; Palladium nanoparticles; Copper nanoparticles; Bifunctionality.

1. INTRODUCTION

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One of humanity’s greatest challenges today is to create a worldwide sustainable energy system that preserves the environment.[1] At this point, a series of

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electrochemical processes – hydrogen evolution reaction (HER), oxygen evolution

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reaction (OER) and oxygen reduction reaction (ORR) - are the core of potential renewable energy conversion and storage technologies, such as water electrolysis, fuel

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cells and rechargeable metal-air batteries.[2],[3] Due to the sluggish kinetics of the HER and ORR reactions, currently platinum(Pt)-based electrocatalysts are used.[4] However,

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Pt scarcity (37 ppb in the Earth’s crust) and its high price (that accounts for over 55 % of the total device cost)[5] inhibit a large commercialisation of the renewable energy

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technologies based on these reactions. To overcome this handicap and to become these technologies competitive with the fossil fuels-based power sources, high efficient, eco-

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friendly and less costly electrocatalysts are desirable.[6] The solution is to reduce the Pt metal loading or replace it by other metal. Palladium(Pd)-containing electrocatalysts are possible candidates to substitute Pt,

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once Pd metal is cheaper and more abundant than Pt, with relatively good electrocatalytic activity.[7],[8] Nonetheless, the United States Department of Energy (DOE) set a Pt-group metal (PGM) target of 0.1 mgPGM cm-2 for cathode’s loading in 2020.[9],[10] Thus, mixtures of Pd with different non-noble metals can be an effective way to reduce its amount, decreasing the electrocatalyst cost while keeping the electrocatalytic ability.[7] Several Pd-based mixtures with Fe,[6] Ni,[11] Ag,[6],[7] Au[6],[12] have been reported so far, and showed superior electrocatalytic performance and increased long-term stability compared with pure Pd catalyst. In particular, Cu 5 Page 5 of 38

metal-transition can be a good candidate, due to its rich redox chemistry and because Cu compounds exhibit biomimetic chemistry with O2 (e.g. reductive activation of O2 in enzymes and catalysis of the 4-electron reduction of O2 to water by protein laccase), which could be particularly attractive for ORR.[13] In fact, a great effort has been made

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about the ORR electrocatalytic ability of Pd-Cu alloys;[14],[15] a Pd-Cu alloy was also tested as HER electrocatalyst, revealing superior activity and durability than Pt/C.[16]

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On the other hand, the electrocatalyst performance is also affected by the support

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material, which should have large porosity and high surface area to promote a homogeneous distribution of the metal particles, high electrical conductivity and

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sufficient mechanical stability, leading to synergetic effects that enhance the electrocatalytic performance.[6],[17] The carbon-based materials, such as activated

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carbons, carbon nanotubes and graphene are ideal candidates.[3],[18],[19] Thereby, here we report the application of metal NPs – Pd, Cu or Cu-Pd -

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supported on high surface area graphite (HSAG), as ORR/HER electrocatalysts. The selected HSAG support is characterized by disordered graphite layers of small particles

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size, which confers a noticeable mesoporous character[20] and motivates its utilization as support for ORR (and OER) electrocatalysts.[21] The metal/HSAG composites were prepared by wetness impregnation technique, followed by generation of metal

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nanoparticles using NaBH4 as reducing agent.[22] According to our knowledge, this work reports for the first time the application of Cu, Pd or Cu-Pd/HSAG composites as bifunctional ORR/HER electrocatalysts. The results showed that a small loading of Pd NPs (1 wt.% Pd vs. 5 wt.% Cu in Cu-Pd/HSAG) allowed improving significantly the bifunctional ORR/HER electrocatalytic performance, which is a very important outcome to rationalise the design of efficient, cheaper and robust electrocatalysts in future.

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2.

EXPERIMENTAL SECTION

2.1

Reagents and solvents The reagents used in electrocatalysts preparation, specifically High Surface Area

Graphite (HSAG, Timcal, SBET = 490 m2/g), Cu(NO3)2.3H2O (Aldrich) and

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Pd(NO3)2.2H2O (Aldrich) were used as received. Potassium chloride (KCl, Merck, ≥ 99.5 %), potassium hexacianoferrate (III) (K3[Fe(CN)6], Merck, ≥ 99 %),

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aminoruthenium (III) chloride ([Ru(NH3)6]Cl3, Aldrich, 98.0 %), potassium hydroxide

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(KOH, Riedel-de-Häen), sulfuric acid (H2SO4, Merck, 95-97 %), Nafion (Aldrich, 5 wt% solution in lower aliphatic alcohols and water), 2-propanol (Aldrich, 99.5 %),

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methanol (Fisher Scientific, > 99.99 %) and 20 wt% Pt/C (HiSPEC® 3000, Alfa Aesar) were used as received. Ultra-pure water (resistivity 18.2 M cm at 25 ºC, Millipore)

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was used.

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2.2 Preparation and characterization of materials

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The materials were prepared by a procedure reported elsewhere [22]. Briefly, Cu and/or Pd metal nanoparticles (NPs) were deposited on the HSAG carbon support by the incipient wetness impregnation method. Aqueous solutions of the metal precursors Cu(NO3)2.3H2O or Pd(NO3)2.2H2O were used to prepare Cu/HSAG and Pd/HSAG

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electrocatalysts, respectively, with the suitable concentration to achieve a metal loading of 5 wt.%. A bimetallic electrocatalyst, Cu-Pd/HSAG, was prepared by impregnation with the same Cu and Pd precursors, but the metal loading was 5 wt.% for Cu and 1 wt.% for Pd. After impregnation, the samples were dried in air at 383 K overnight. Then, the electrocatalysts were activated: 100 mg of the impregnated electrocatalysts were reduced using 100 mL of 50 mM NaBH4 solution under stirring for 15 min at room temperature. Finally, they were filtered, washed with distilled water and dried.

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In order to determine the bulk metallic content of the samples thermogravimetric analyses (TGA) were carried out in a SDT Q 600 apparatus under air atmosphere (100 mL/min). Structural properties of the support and the electrocatalysts were determined by X-ray diffraction (XRD) at room temperature over the range 2θ=10-90º with a

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Polycristal X’Pert Pro PANalytical diffractometer with Ni-filtered Cu/Kα radiation (λ= 0.1544 nm) operating at 45 kV and 40 mA. With the aim to obtain information on the

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morphological characteristics such as shape and size of metal crystallites, the reduced

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catalysts were subjected to a detailed Transmission Electron Microscopy (TEM) study. TEM micrographs were obtained on a JEOL 2100F electron microscope operated at

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200 kV and equipped with an Energy-Dispersive X-Ray detector (EDX). The samples were milled and suspended in ethanol by ultrasonic treatment and a drop of the fine

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suspension was placed on a carbon-coated nickel grid to be loaded into the microscope. The Scanning Transmission Electron Microscopy (STEM) was done using a spot size of

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1 nm. X-ray photoelectron spectroscopy (XPS) was performed in a Kratos AXIS Ultra HSA spectrometer with VISION software for data acquisition using monochromatized

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Al Kα radiation (1486.6 eV) operating at 15 kV (90 W) in FAT (Fixed Analyser Transmission) mode. All binding energies (BE) were referenced to the C 1s line at 284.6 eV. Spectra were analyzed with Casa XPS software by fitting after Shirley

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background correction.

2.3 Electrochemical characterization of modified electrodes The

cyclic

voltammetry

(CV)

studies

were

performed

on

a

potentiostat/galvanostat Autolab PGSTAT 30 (EcoChimie B.V.) with the software GPES. A 3-electrode cell was used to perform the measurements, where the working electrode was a glassy carbon electrode, GCE, (d=3 mm, MF-2012, BAS), the counter

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electrode was a platinum wire (7.5 cm, MW-1032, BAS) and the reference electrode was Ag/AgCl (sat. KCl) (MF-2052, BAS). The CV tests were carried out in N2 saturated solution and at room temperature. Prior

to

modification,

the

working

electrode

first

went

through

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cleaning/polishing process, using diamond pastes (MetaDi II, Buehler) of 6, 3 and 1 µm and Al2O3 (Buehler) of 0.3 µm particle size, on a microcloth polishing pad (BAS). To

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finish, the electrode was rinsed with ultra-pure water. The modified electrodes were

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prepared through depositing a 7.5 μL drop of the electrocatalysts onto the working electrode using dispersions of the selected materials (HSAG, Cu/HSAG, Pd/HSAG, Cu-

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Pd/HSAG). Then, the drop dried under a flux of air. The dispersions were prepared using 1 mg of material in a 125 μL water, 125 μL 2-propanol and 20 μL Nafion mixture

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which was sonicated for 20 min to obtain a homogeneous ink.

CV of the redox probes [Ru(NH3)6]Cl3 and K3[Fe(CN)6] was performed using

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solutions containing 1.0 × 10-3 mol dm-3 of probe in 1.0 mol dm-3 KCl. Depending on the solution used, the potential range used varied between 0.90 and -0.50 V or 0.40 V

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and -0.60 V vs. Ag/AgCl, at different scan rates from 0.010 to 0.500 V s-1 for bare, HSAG and Pd/HSAG electrodes and 0.010 to 0.090 V s-1 for Cu/HSAG and CuPd/HSAG.

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The electroactive surface areas were determined using the Randles-Sevcik

equation, Equation (1), assuming that the electrode process is diffusion controlled: ipa = 2.69 × 105 n 3/2 A Dx 1/2 C ν1/2

(1)

where n - number of electrons involved in the process (here n = 1), A - electrode surface area (cm2), Dx - diffusion coefficient (6.30 × 10-6 cm2 s-1 for [Fe(CN)6]3-/4- [23] and 6.20 × 10-6 cm2 s-1 for [Ru(NH3)6]3+/2+[23]), C - concentration of the specie (mol cm-3), ipa anodic peak current (A) and v - scan rate (V s-1) [24] .

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2.4

Evaluation of the ORR and HER electrocatalytic activities The ORR and HER performances of each material were evaluated in a 3-electrode

cell at room temperature. The electrochemical measurements were made using a potentiostat/galvanostat Autolab PGSTAT 302N (EcoChimie B.V.), controlled by

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NOVA 2.0 software. The electrodes used were: working - glassy carbon (d = 3 mm, Metrohm) rotating disk electrode (RDE), reference - Ag/AgCl (3 mol dm-3 KCl,

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Metrohm) and counter - glassy carbon rod (d = 2 mm, Metrohm, for ORR studies) or a

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platinum wire (d = 0.6mm, 0.5 m, 99.99+%, Goodfellow, for HER). The counter electrode for HER experiments was separated from the electrolyte and RDE through a

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glass frit to avoid platinum dissolution. For studies in bipotentiostat mode, a rotating ring disk electrode (RRDE) of glassy carbon disk (d = 5 mm) with a platinum ring (d =

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375 m, Metrohm) was used as working electrode. The modified electrodes were prepared as described above, yielding an electrocatalyst loading of ≈ 840 g cm-2.

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The ORR electrocatalytic tests were accomplished in N2- or O2-saturated 0.1 mol dm-3 KOH solution (purged for 30 min before the measurements). Cyclic voltammetry

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(CV) experiments were carried out at the scan rate of 0.005 V s-1 and the linear sweep voltammetry (LSV) at 0.005 V s-1 for distinct rotation speeds from 400 to 3000 rpm. The ORR current was achieved by subtracting the current measured in N2-saturated

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electrolyte from the current measured in O2-saturated electrolyte. The onset potential (Eonset) was calculated considering the potential at which the current density reaches 5 % of the diffusion-limiting current density.[25] LSV data was investigated though Koutecky-Levich (K-L) equation (2). The number of electrons transferred per O2 molecule (

) in the ORR process was calculated from the slopes of the K-L plot:[23]

(2) 10 Page 10 of 38

where j, jL and jk are the experimentally measured current density, the diffusion-limiting current density and the kinetic current density, respectively, and  is the angular velocity; B is related with the diffusion limiting current density and is expressed by the Equation (3): F

where F is the Faraday constant (96 485 C mol-1),

is the bulk concentration of O2

is the diffusion coefficient of O2

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(1.15×10-3 mol dm-3 in 0.1 mol dm-3 KOH),

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(3)

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B = 0.2

(1.95×10-5 cm2 s-1 in 0.1 mol dm-3 KOH) and v is the kinematic viscosity of the

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electrolyte (0.008977 cm2 s-1). The constant 0.2 was adopted (rotating speed in rpm).[23]

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For the bipotentiostat measurements, the modified disk-electrode was swept cathodically at the scan rate of 0.005 V s-1 and 1600 rpm in O2-saturated 0.1 mol dm-3

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KOH solution, while the potential of the Pt ring was kept constant at E = 0.2 V vs. Ag/AgCl to assure the oxidation of the peroxide species formed during the ORR. The

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peroxide percentage (% H2O2) was calculated based on Equation (4):[26] % H2O2 =

(4)

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where iD and iR are the disk and ring currents, respectively, and N is the current collection efficiency of the Pt ring (calculated as N = 0.25).[27] The ORR Tafel plots (ERHE vs. log jk) were obtained by the correction of the

measured currents in LSV for diffusion, to give the kinetic currents in the mixed activation-diffusion region, through:[28] (5)

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The mass transport correction was made using the diffusion-limiting current density jL, calculated by combination of equations (2) and (3). The evaluation of the long-term electrochemical stability by chronoamperometry was performed in O2-saturated 0.1 mol dm-3 KOH solution, with the RDE electrode at E = -0.45 V vs. Ag/AgCl and 1600 rpm 20 000 s.

The

methanol-tolerance

evaluation

was

also

made

by

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during

chronoamperometry, considering the addition of 1.0 mol dm-3 methanol after ≈ 1000 s.

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The evaluation of the electrocatalytic activity towards HER was performed by

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LSV in H2SO4 (0.5 mol dm-3, pH = 0.3) at 0.005 V s-1 and 1500 rpm. The obtained currents were normalized to the superficial geometric area (0.07068 cm2) to give the

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current density, j. The Tafel slopes were obtained through the application of the Tafel equation (η = a + b log j, where Tafel slope = b) to the linear portion of the Tafel plot (η

density at η = 0 were also determined.

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vs. logarithm of j). The exchange current densities (j0) which correspond to the current

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The potential values obtained vs. Ag/AgCl were converted to vs. RHE (reversible

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hydrogen electrode) according to the Nernst equation: ERHE = EAg/AgCl + 0.059 pH + EºAg/AgCl

(6)

where ERHE is the converted potential vs. RHE, EAg/AgCl is the experimentally measured

3.

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potential vs. Ag/AgCl and EºAg/AgCl = 0.1976 at 25 ºC.[29]

RESULTS AND DISCUSSION

3.1 Materials characterization The XRD patterns of HSAG support and respective composites with Cu and/or Pd NPs are shown in Figure 1. The diffractogram of the HSAG exhibited the characteristic diffraction pattern of graphitic materials, with a pronounced peak at 26 degrees, due to the (002) graphitic basal plane reflection. No changes in the graphitic

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crystalline structure were detected in any catalyst after the incorporation of the metallic phases. The diffraction peaks observed for Cu/HSAG composite indicated the presence of Cu0 monoclinic phase, with a crystallite size of 9.8 nm (estimated from the peak at 2θ = 50.4º, Table 1). For Pd/HSAG, it was observed a peak at 2θ = 50.4º assigned to the

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diffraction of (111) plane of face-centered cubic crystal lattice, and the crystallite size was estimated as 5.2 nm. In the XRD pattern of the bimetallic Cu-Pd/HSAG composite,

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the peaks associated with the metal phases were not detected, indicating a high

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dispersion of the metal NPs on the surface, in the amorphous phase or below the

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detection limit of the XRD.[22]

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Cu-Pd/HSAG

0

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Intensity (a.u)

Pd

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36

10

20

30

40

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0

Pd/HSAG 38

40

42

0

Cu

40

45

50

55

Cu/HSAG

HSAG

50

60

70

80

90

2

Figure 1. XRD patterns of the HSAG and the HSAG supported catalysts.

Table 1: Structural and compositional properties of the composites: metal nanoparticles crystallite sizes (estimated from XRD and TEM) and XPS data (adapted from Ref. [22]). Sample

Metallic residue (metal or metal oxide)a / wt.%

Crystallite size / nm

XPS

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XRDb

TEM

Pd/Cu ratio

Cu/HSAG

6.2

9.8

n.d.

-

Pd/HSAG

5.6

5.2

2.6

-

Cu-Pd/HSAG

7.5

n.d.

n.d.

0.22

a

Metallic residue obtained by TGA after burning away (air, 100 mL/min) the graphitic support at 900 oC.

b

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Calculated by the Scherrer equation. n.d. denotes not detected or poorly defined particles, whereby mean particle size could not be estimated.

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The micrographs obtained by microscopy characterization of the composites are

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presented in Figure 2. In Cu/HSAG (Figure 2 a) and b)), the Cu NPs were poorly defined, probably due to the Cu oxidation and the poor degree of crystallization of Cu

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NPs induced by the reduction process at room temperature, whereby it was not possible the estimation of the average particle size; nonetheless, the EDX analysis confirmed the

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presence of Cu on the catalyst’s surface (inset in Figure 2 b)). For Pd/HSAG (Figure 2

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c)), the TEM analyses indicated the presence of small Pd particles (2.6 nm, Table 1), homogeneously dispersed on the support. For the bimetallic composite (Figure 2 d), 2e)

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and 2f)), two types of species were identified: the more defined and smaller elements correspond to Cu-Pd particles, while the other barely defined correspond to Cu NPs. This indicates that Pd and Cu atoms are close in the smaller particles, although the

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formation of alloy cannot be confirmed taking into account it was not detected by the XRD (Table 1). Moreover, the alloy formation was not expected, once the applied reduction treatment with NaBH4 in the preparation step was performed at room temperature. The barely defined and larger Cu nanoparticles appear as segregated entities, as evidenced in Figure 2 f).[22]

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(b)

(c)

(d)

ed

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cr

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(a)

(f)

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(e)

Figure 2. Microscopy images of the catalysts: a) TEM image of Cu/HSAG, b) STEM image and corresponding EDX spectra (inset) of Cu/HSAG, c) TEM image of Pd/HSAG, d) TEM image of Cu-Pd/HSAG, e) STEM image and EDX spectra (inset) of Cu-Pd/HSAG in a region where Cu and Pd are detected simultaneously) and f) STEM

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image and EDX spectra (inset) of Cu-Pd/HSAG in a region where only Cu is detected. Further images can be found in Ref. [22].

The XPS general spectra of the prepared composites and the corresponding core level XPS spectra of the main elements are represented in Figure 3. First of all, it is

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worth mentioning that the only elements detected in the survey spectra were: C, O, Cu

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and/or Pd, and Na. The detection of this last element indicates the existence of sodium

residues likely originated from the reducing agent NaBH4 employed during the

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synthesis stage; nonetheless, its atomic content is negligible (lower than 0.3 at.%). The Cu 2p core level spectrum of Cu/HSAG (Figure 3a) exhibits two major peaks centered

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at binding energies of 934.9 and 954.8 eV, attributed to Cu 2p3/2 and Cu 2p1/2 doublets

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characteristic of copper, respectively. The presence of the satellite peaks at 963.6 and 943.5 eV confirmed the existence of Cu(II) species. The 2p3/2 peak was deconvoluted

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into two components: at 932.5 eV attributed to Cu(0)/Cu(I) species and at 934.5 eV attributed to Cu(II). The existence of Cu(0) is in accordance with the XRD results, and

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the partial re-oxidation of Cu surface can explain the presence of more oxidized Cu species. The XPS spectrum of Pd/HSAG (Figure 3b) in the Pd 3d region presented two doublets associated with 3d5/2 and 3d3/2, each one deconvoluted into two peaks

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associated with different oxidation states of Pd. For 3d5/2, the contributions appeared at 335.9 and 337.9 eV and are assigned to Pd(0) and Pd(II), respectively. The XPS spectra of Cu-Pd/HSAG composite in Cu 2p and Pd 3d regions are similar to those of respective monometallic composites. Nonetheless, the relative percentage of Pd(0) to Pd(II) is higher in the Cu-Pd/HSAG, compared with the monometallic Pd/HSAG (78 % vs. 54 %). The Pd 3d5/2 binding energy also shifted to lower values, while the Cu 2p3/2 binding energy shifted to higher values (in comparison with the monometallic composites), which indicated an interaction between Pd and Cu atomic orbitals due to 16 Page 16 of 38

their close proximity in the bimetallic complex, corroborating the results obtained by STEM/EDX. For Cu-Pd/HSAG, the calculated Pd/Cu atomic ratio (0.22) is almost twice the expected value (0.12), meaning that some Pd NPs could be deposited

Pd 3d

(b)

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(a)

C 1s

C 1s

surrounding Cu NPs.[22]

1000

800

600

400

200

0

1200

cr 335

330

800

600

400

200

0

M

940

930

920 350

ed

1200

1000

345

340

335

330

Cu LMM O 1s Na 1s

950

O KLL

960

Na 1s

970

C 1s

Pd 3d

Cu 2p

an

B.E. (eV)

C KLL

CPS (a.u.)

340

O KLL

1000

B.E. (eV) (c)

345

O 1s Na KLL

350

Na 1s

920

C KLL

930

Pd MNN

940

CPS (a.u.)

950

Cu LMM O 1s Na KLL

Na 1s

960

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1200

970

O KLL

C KLL

CPS (a.u.)

Cu 2p

800

600

400

200

0

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B.E. (eV)

Figure 3. XPS survey spectra of the HSAG supported catalysts: a) Cu/HSAG and Cu 2p region (inset), b) Pd/HSAG and Pd 3d region (inset) and c) Cu-Pd/HSAG, Cu 2p region

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(left inset) and Pd 3p region (right inset).

3.2 Electrochemical characterization of the modified electrodes The electrochemical characterization of all modified electrodes was initially

performed by cyclic voltammetry in 1.0 mol dm-3 KCl. The cyclic voltammograms (CVs) of bare GCE and HSAG, Cu/HSAG, Pd/HSAG and Cu-Pd/HSAG modified electrodes are compared in Figure 4 (a). A rectangular voltammogram was observed for the HSAG modified electrode, suggesting a capacitive behaviour for this material while

17 Page 17 of 38

the other three presented some redox processes. The Cu/HSAG electrode showed an oxidation peak at ≈ -0.163 V vs. Ag/AgCl which can be attributed to the reduction of copper (Cu(II) →Cu(I))[30, 31] while the Pd/HSAG electrode showed a reduction peak at ≈ 0.059 V vs. Ag/AgCl and an oxidation one at 0.574 V vs. Ag/AgCl, which can be

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attributed to redox processes of different palladium species (Pd (0) and Pd(II)) in the electrocatalysts.[22]

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The electrochemical features of all electrocatalysts were also explored in the

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presence of [Fe(CN)6]3-/4- and [Ru(NH3)6]3+/2+, Figure 4 (b) and (c), respectively. The CVs for [Fe(CN)6]3-/4- showed one pair of peaks at Epc ≈ 0.242 V and Epa ≈ 0.308 V vs.

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Ag/AgCl for all modified electrodes that are assigned to Fe3+/Fe2+ electronic transfer. The anodic to cathodic peak-to-peak separation values (∆Ep) varied between 0.065 and

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0.077 V vs. Ag/AgCl, which are slightly smaller than the obtained for the bare GCE (0.083 V). Additionally, the Cu/HSAG and Cu-Pd/HSAG presented another pair of

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peaks with Epc ≈ 0.766 V and Epa ≈ 0.736 V vs. Ag/AgCl (∆Ep ≈ 0.035 V). This new pair of peaks may be attributed to another possible redox process of copper (Cu(I)

ce pt

→Cu(0)).[32]

The CVs with [Ru(NH3)6]3+/2+ for HSAG and Pd/HSAG are very similar to the CV obtained for the bare GCE in terms of peak potentials, with Epc ≈ -0.210 V and Epa ≈

Ac

-0.135 V vs. Ag/AgCl (∆Ep ≈ 0.075 V). This peak is attributed to Ru3+/Ru2+ electronic transfer. For the Cu/HSAG and Cu-Pd/HSAG, besides the previous peak it is also observed an additional redox process with Epc ≈ -0.012 V and Epa ≈ 0.273 V vs. Ag/AgCl. These results suggest that the Cu-containing electrocatalysts interact with both redox probes. Figures S1 and S2 (Supporting Information, SI) show the CVs at different scan rates at the different electrodes for [Fe(CN)6]3-/4- and [Ru(NH3)6]3+/2+ and the respective

18 Page 18 of 38

plots of ipa vs. ν1/2. In the experimental timescale used (v in the range 0.010 to 0.500 V s1

for bare, HSAG and Pd/HSAG electrodes and 0.010 to 0.090 V s-1 for Cu/HSAG and

Cu-Pd/HSAG) both Epc and Epa varied less than 0.005 V with all electrodes for [Fe(CN)6]3-/4- and 0.005 V (GCE, Cu/HSAG, Pd/HSAG, Cu-Pd/HSAG) and 0.019 V

ip t

(HSAG) for [Ru(NH3)6]3+/2+. Additionally, peak currents are proportional to ν1/2, suggesting diffusion-controlled processes.[24]

cr

The electroactive surface areas of bare and modified GCEs were obtained from

us

the CVs of 1 × 10-3 mol dm-3 K3[Fe(CN)6] and [Ru(NH3)6]Cl3 in KCl 1 mol dm-3 and the values are summarized in Table 2. The electroactive surface areas determined, with

an

both redox probes, for all modified electrodes are higher (up to a 55% increase) than those obtained for bare GCE, suggesting that electrode modification with HSAG,

M

Cu/HSAG, Pd/HSAG and Cu-Pd/HSAG provides a more conductive pathway for electron transfer. It was also observed that the modification of the bare electrode with

ed

the Cu-containing electrocatalysts led to higher electroactive surface areas with an increase of ≈ 40% for Cu/HSAG with both redox probes and an increase between 30

200.0

GCE HSAG Cu/HSAG Pd/HSAG Cu-Pd/HSAG

(a)

Ac

150.0

ce pt

(Ru(NH3)6]3+/2+) and 55% ([Fe(CN)6]3-/4-) for the Cu-Pd/HSAG.

100.0

60.0 40.0

(b)

20.0

i / µA

i / µA

50.0

GCE HSAG Cu/HSAG Pd/HSAG Cu-Pd/HSAG

0.0

-50.0

0.0 -20.0

-100.0

-40.0

-150.0

-60.0 -1.0

-0.5

0.0

0.5

1.0 -3

E / V vs. Ag/AgCl (3 mol dm KCl)

-0.6 -0.4 -0.2 0.0

0.2

0.4

0.6

0.8

1.0

-3

E / V vs. Ag/AgCl (3 mol dm KCl)

19 Page 19 of 38

20.0

GCE HSAG Cu/HSAG Pd/HSAG Cu-Pd/HSAG

15.0 10.0

(c)

i / µA

5.0 0.0 -5.0 -10.0

ip t

-15.0 -20.0 -25.0 -0.4

-0.2

0.0

0.2

0.4

cr

-0.6

-3

E / V vs. Ag/AgCl (3 mol dm KCl)

us

Figure 4. CVs of bare GCE, HSAG, Cu/HSAG, Pd/HSAG and Cu-Pd/HSAG modified electrodesin: (a) 1.0 mol dm-3 KCl and in the presence of 1.0×10-3 mol dm-3 redox probe

M

an

in 1.0 mol dm-3KCl: (b) K3[Fe(CN)6] and (c) [Ru(NH3)6]Cl3; at v = 0.05 V s-1

Table 2: Electroactive surface areas (A/cm2) determined using the Randles-Sevcik

ed

equation.

Electroactive surface area / cm2

Sample

ce pt

[Fe(CN)6]3-/4-

Ru(NH3)6]3+/2+

0.0648 (± 0.0027) 0.0719 (± 0.0017)

0.0603 (± 0.0012)

Cu/HSAG

0.0888 (± 0.0071)

0.0849 (± 0.0083)

Pd/HSAG

0.0700 (± 0.0019)

0.0748 (± 0.0075)

Cu-Pd/HSAG

0.1004 (± 0.0033)

0.0765 (± 0.0098)

GCE

Ac

HSAG

0.0676 (± 0.0016)

20 Page 20 of 38

3.3 ORR electrocatalytic activity The ORR electrocatalytic ability of the prepared materials was evaluated in 0.1 mol dm-3 KOH supporting electrolyte. In Figure 5 (a) are shown the CVs obtained for Pd/HSAG in N2- and O2-saturated solutions. The CVs obtained in similar

ip t

experiments for the other materials (HSAG, Cu/HSAG, Cu-Pd/HSAG and 20 wt.% Pt/C reference) are shown in Figure S3.

cr

No electrochemical processes were observed in N2-saturated solution, besides

us

those characteristics of materials, due to the redox processes of Cu (Cu(II)/Cu(I)/Cu(0)) and Pd (Pd(0)/Pd(II)) metal centres, or the combination of both in the bimetallic

an

composite. The Cu/HSAG showed a pair of redox peaks at Epa= 0.87 V and Epc= 0.68 V, the Pd/HSAG exhibited very low intense cathodic peaks at Epc= 0.69 and 0.84 V and

M

the Cu-Pd/HSAG showed a pair of peaks at Epa= 0.87 V and Epc= 0.69 V. In O2saturated solution, all materials showed a new irreversible cathodic electrochemical

ed

process, revealing their electrocatalytic ability for ORR. As depicted in Figure 5 (b), the HSAG modified electrode showed a defined cathodic peak at Epc= 0.72 V, the

Ac

0.79 V.

ce pt

Cu/HSAG at Epc= 0.77 V, the Pd/HSAG at Epc= 0.83 V and the Cu-Pd/HSAG at Epc=

21 Page 21 of 38

0.2

0.2

(b) 0.1

0.0

0.0 -2

0.1

-0.1

j / mA cm

-0.2 -0.3

HSAG Cu/HSAG Pd/HSAG Cu-Pd/HSAG

-0.1 -0.2 -0.3

N2

-0.4

-0.4

O2 -0.5 0.2

0.4

0.6

0.8

-0.5

1.0

0.2

0.4

E / V vs. RHE

3.5

-4.0 -5.0 0.6

0.8

E / V vs. RHE

2.5

1.0

HSAG Cu/HSAG Pd/HSAG Cu-Pd/HSAG

2.0

1.5

1.0

M

Bare GCE HSAG Cu/HSAG Pd/HSAG Cu-Pd/HSAG Pt/C

an

nO2

j / mA cm

-2

3.0 -2.0 -3.0

(d)

us

-1.0

0.4

1.0

cr

4.0

0.2

0.8

E / V vs. RHE

(c)

0.0

0.6

ip t

j / mA cm

-2

(a) Pd/HSAG

0.35

0.40

0.45

0.50

0.55

0.60

E / V vs. RHE

ed

Figure 5. ORR results in KOH 0.1 mol dm-3 (v = 0.005 V s-1): (a) CVs of Pd/HSAG in N2and O2-saturated solutions, (b) CVs of the as-prepared electrocatalysts in O2-saturated

ce pt

solutions, (c) LSV in O2-saturated solution at 1600 rpm and (d) number of electrons transferred per O2 molecule (nO2) at different potential values.

Ac

Figure 5 (c) shows the ORR polarization plots of the prepared electrocatalysts, at 1600 rpm. The HSAG support showed an Eonset value of 0.78 V and a diffusion-limiting current density of jL, 0.3 V, 1600 rpm = -3.0 mA cm-2. The Cu/HSAG and Cu-Pd/HSAG composites displayed similar ORR performances, with Eonset = 0.81 and 0.84 V and jL, 0.3 V, 1600 rpm = -3.3 and -3.5 mA cm-2 for Cu/HSAG and Cu-Pd/HSAG, respectively. The Pd/HSAG presented the most promising ORR performance, with a more positive Eonset of 0.91 V and a higher diffusion-limiting current density of jL, 0.3 V, 1600 rpm = 4.2 mA cm-2. These results revealed the advantage of modify the electrode with these 22 Page 22 of 38

electrocatalysts, once all presented more positive Eonset and higher diffusion-limiting current densities than the bare GCE (Eonset = 0.68 V and jL, 0.5 V, 1600 rpm = -1.7 mA cm-2). The modified-electrodes presented larger electroactive surface areas than bare GCE (Table 2) which should favour the ORR, with the incorporated metals offering more

ip t

efficient electroactive centres for ORR, in particular the Pd NPs due to its noble-metal characteristic and similar behaviour to Pt. The main rule of HSAG support should be to

cr

stabilize the metal NPs and assist in the electron-conduction, as already assumed for

us

other carbon-based supports.[33] Furthermore, the results obtained with the CuPd/HSAG composite revealed that a small doping of Pd (1 wt.% vs. 5 wt.% Cu)

an

improved the ORR performance, in comparison with the non-noble metal composite Cu/HSAG, bringing it more close to the performance of noble-metal composite

M

Pd/HSAG. This can be a good outcome for the design of ORR electrocatalysts, which can combine non-noble metals (e.g. Cu) with just small loadings of noble metals (e.g.

ed

Pd), to allow the reduction of the amount of noble metals typically used, without significantly compromise the electrocatalytic performance. The appointed strategy is

ce pt

valuable to reduce the electrocatalysts cost and potentiate their large-scale application. However, in comparison with the Pt/C reference (Eonset = 0.97 V and jL, 0.5 V, 1600 rpm = 5.1 mA cm-2), all of the prepared materials presented a less positive Eonset and smaller

Ac

current densities, with a Eonset = 0.06 V between the Eonset of Pt/C and the composite with better performance, Pd/HSAG. This result indicated that the design of the prepared composites needs to be further optimized, in order to achieve the standard Pt/C activity. When compared with results described in literature, the prepared electrocatalysts showed Eonset values very competitive to those of other Cu-based catalysts, as the bioinspired Cu(3,3’-diaminobenzidine) polymeric complex assembled on carbon black (Eonset ≈ 0.85 V vs. RHE) [34] or the pyrolized Cu-based catalysts within graphene

23 Page 23 of 38

(Eonset = 0.80-0.85 V vs. RHE) [33], for which Cu-N was identified as active site, or even Pd-Cu alloys deposited on Vulcan (Eonset ≈ 0.85 V vs. RHE) [15]. The current densities were also similar: for PdCu NPs mixed with carbon (XC-72), the jL values reported were between -4.0 and -5.0 mA cm-2.[14]

ip t

The ORR kinetics of all materials was evaluated by the K-L plots (j-1 vs. -1/2), at various potential values, using the linear sweep voltammograms acquired at different

cr

rotation rates (Figure S4). As depict in Figure S5, the K-L data exhibited good linearity,

us

suggesting a first order electrocatalytic reduction of O2 in relation to the concentration of O2 dissolved, in the potential range explored.[35] Furthermore, the K-L plots of the over the potential range,

an

composites had similar slopes, indicating an identical

a high dependency of

M

otherwise to what was observed for HSAG K-L plots, whose different slopes anticipated with potential. Figure 5 d) shows the

values at several

for the HSAG carbon support increased from

2.1 electrons to

ce pt

above, the

ed

potential values, estimated through the K-L plots (Equations 2 and 3). As presumed

3.0 electrons as the potential became more negative, indicating that this electrocatalyst was not selective for either the 2 or 4 electron process. A similar

Ac

behaviour was already observed by our group for other pristine carbon materials (biomass-derived activated carbons) [23] and in literature for carbon aerogels, for example [36, 37]. For the nanocomposites, range between 0.35 and 0.55/0.60 V, with a

was almost constant in the potential average of

= 3.7, 3.4 and 3.1

electrons for Cu/HSAG, Pd/HSAG and Cu-Pd/HSAG, respectively. This data indicated a trend of these electrocatalysts for the 4 electron process and were close to the achieved for the Pt/C reference (

= 3.7 electrons in the potential range between 0.50 24 Page 24 of 38

and 0.80 V). The

values are also similar to those reported in literature for similar

materials, such as PdCu NPs mixed with carbon ( Cu-based catalysts within graphene (

≈3.8 electrons) [14] and pyrolized

≈3.2-4.0 electrons) [33].

ip t

To validate the data obtained from the K-L plots, the ORR mechanistic pathways of the prepared electrocatalysts were explored through the RRDE measurements. Figure

cr

6 (a) shows the voltammograms obtained at the electrode disk and ring. The estimated

us

% H2O2 for each modified electrode was calculated through Equation 4 and is presented

HSAG Cu/HSAG Pd/HSAG Cu-Pd/HSAG Pt/C

0.6 ring (Pt)

HSAG Cu/HSAG Pd/HSAG Cu-Pd/HSAG Pt/C

(b)

60

50

M

0.0 -2.0 -4.0 -6.0

70

% H 2O2

0.2

(a)

ed

j / mA cm-2

0.4

an

in Figure 6 (b).

disk (glassy carbon)

0.4

0.8

ce pt

0.6

E / V vs. RHE

1.0

40

30

20 10 0 0.4

0.6

0.8

1.0

E / V vs. RHE

Figure 6. (a) Linear sweep voltammograms recorded with RRDE in O2-saturated

Ac

0.1 mol dm-3 KOH solution, at 1600 rpm; the disk potential was swept at v = 0.005 V s-1 and the ring potential was kept at E = 0.2 V vs. Ag/AgCl. (b) Estimated percentage of H2O2 formed.

Together with the most promising ORR performance (more positive Eonset and the highest jL value) among the prepared electrocatalysts, Pd/HSAG also showed the lowest generated density current at the ring (jR, 0.3 V = 0.13 mA cm-2) which, in the potential range explored (mainly between 0.6-0.8 V), are similar to those of Pt/C. The 25 Page 25 of 38

Cu/HSAG and Cu-Pd/HSAG composites exhibited higher jR values (jR, 0.3 V = 0.27 and 0.31 mA cm-2, respectively) with identical profiles between them, followed by HSAG support, whose substantially higher jR value (jR, 0.3 V = 0.48 mA cm-2) appoints for a superior amount of H2O2 produced at the disk. Indeed, for Pd/HSAG modified electrode

ip t

the % H2O2 did not exceed 10 % in the potential range between 0.30 and 0.90 V vs. RHE, while for Cu/HSAG and Cu-Pd/HSAG the % H2O2 reaches almost 25 % at 0.45 V

cr

vs. RHE, slightly decreasing after this value. For HSAG, the % H2O2 increased

us

significantly in comparison with the composites, reaching 62 % at ≈ 0.52 V vs. RHE, and decreasing at more negative potential values. These results represent a decrease of

an

at least 2.5 times in the amount of H2O2 produced after the immobilization of the metal nanoparticles on HSAG support, and are consistent with the

estimated from the K-L

M

plots. While the HSAG support was associated with an indirect pathway with formation

ed

of a large amount of H2O2, the immobilization of Cu, Pd or a mixture of Cu-Pd nanoparticles changed the mechanism for an almost 4-electron pathway.

ce pt

The Tafel plots of the prepared electrocatalysts and of Pt/C reference in the low current density region, ranging from E = 0.7 to 1.0 V vs. RHE, are shown in Figure 7. In this region, the determined Tafel slopes were 48.7, 58.8, 77.2 and 121.3 mV dec-1 for

Ac

Cu-Pd/HSAG, Cu/HSAG and Pd/HSAG composites and Pt/C, respectively. The similarity of the Tafel slope values indicated an identical ORR mechanism in the prepared HSAG-based nanocomposites. Typically, ORR on metal-containing materials proceeds through a dissociative 4-electron pathway, due to the adequate adsorption ability for O2.[38] By this mechanism, in alkaline medium O2 is initially adsorbed, followed by break of O-O bond and formation of adsorbed O* species (O2+2*→2O*, where * represents a surface catalytic site). Then the specie gains two electrons and two protons and form directly the final product OH- (2O*+2e-+2H2O→2OH*+2OH-; 26 Page 26 of 38

2OH*+2e-→2OH-+2*).[38] As the obtained Tafel slope values were smaller than that of Pt/C, this can suggests that O2 is easily adsorbed and activated at the nanocomposites surfaces.[39]

ip t

77.2 mV dec-1

1.0

121.3 mV dec-1 -1

cr

0.8 Cu/HSAG Pd/HSAG Cu-Pd/HSAG Pt/C

0.7 -2.0

-1.5

us

E / V vs. RHE

48.7 mV dec 0.9

58.8 mV dec-1

-1.0

-0.5 -2

0.0

an

log |jk / mA cm |

0.5

M

Figure 7. ORR Tafel plots in low current density region, obtained from LSV data in

The

electrochemical

ed

Figure 1 (c) at 1600 rpm.

stability

of

the

catalysts

was

evaluated

by

ce pt

chronoamperometry in O2-saturated 0.1 mol dm-3 KOH solution during 20 000 s, Figure 8 (a). At the end of this time, the Pd/HSAG chronoamperogram exhibited a current decay to 82 % and those of Cu/HSAG and Cu-Pd/HSAG showed a small current

Ac

attenuation for 87 and 89 %, respectively. The values are lesser than the achieved for Pt/C reference (97 %) and appoint to a moderate durability of the prepared nanocomposites. The composites with Cu (Cu/HSAG and Cu-Pd/HSAG) showed to be more stable than the composite only with the noble metal (Pd/HSAG). In particular, the superior stability of Cu-Pd/HSAG in comparison with the other prepared composites, indicated that can occur a synergetic effect between the Cu and Pd metal centres (explained by their close proximity, according with the TEM/EDX and XPS characterizations), which promotes the enhancement of bimetallic composite stability. 27 Page 27 of 38

Contrary to what happened in the LSVs electrocatalytic tests, these results indicated that the performance of the bimetallic nanocomposite can be explained by a synergetic effect and not the simple combination of individual contributions. The tolerance of the electrocatalysts to the methanol crossover was evaluated by

ip t

chronoamperometry, through the addition of methanol during the experiment. The j-t responses shown in Figure 8 (b) revealed that the addition of methanol did not caused a

cr

significant current decrease for the metal/HSAG catalysts, meaning that they are

us

tolerant to methanol and have high selectivity for ORR. The Cu-containing composites showed smaller current decreases, which did not surpassed the 10 % for Pd/HSAG, the

an

prepared composite with the lowest methanol tolerance. In contrast, the commercial Pt/C catalyst exhibited a drastically current attenuation for ≈ 40 % after the addition of

M

methanol, confirming its known high electroactivity towards methanol oxidation that subdues the ORR performance.[40] The high tolerance to methanol exhibited by the

ed

prepared metal/HSAG catalysts is a promising advantage and allows overcoming an

ce pt

intrinsic problem of the Pt-based ORR electrocatalysts.

100

100

80

87 % 82 %

Cu/HSAG Pd/HSAG Cu-Pd/HSAG Pt/C

Relative current / %

89 %

90

Ac

Relative current / %

97 %

80

methanol

60 40

Cu/HSAG Pd/HSAG Cu-Pd/HSAG Pt/C

20

(a)

70

(b)

0 0

5000

10000

Time / s

15000

20000

0

500

1000

1500

2000

2500

Time / s

Figure 8. Relative j-t chronoamperometric responses of the electrocatalysts at E = -0.45 V vs. Ag/AgCl and 1600 rpm in O2-saturated 0.1 mol dm-3 KOH solution: (a) long-term

28 Page 28 of 38

electrochemical stability during 20 000 s and (b) with the addition of 1.0 mol dm-3 methanol after ≈ 1000 s. 3.4 HER electrocatalytic activity The HER electrocatalytic properties of HSAG, Cu/HSAG, Pd/HSAG, Cu-

ip t

Pd/HSAG and Pt/C were evaluated in 0.5 mol dm-3 H2SO4 (pH = 0.3) at 0.005 V s-1 and 1500 rpm. In Figure 9 (a) are shown the polarization curves obtained by LSV for the

cr

bare RDE, and RDE modified with HSAG, Cu/HSAG, Pd/HSAG, Cu-Pd/HSAG and

us

Pt/C. It can be observed that neither HSAG nor Cu/HSAG exhibit HER activity

an

however, Pd/HSAG and Cu-Pd/HSAG show HER features similar to Pt/C.

0.4

2.0

(a)

644mV/dec

0.0

-8.0 -10.0 -0.6

-0.4

-0.2

0.0

0.2

 / V vs. RHE

-6.0

ce pt

-4.0

ed

RDE HSAG Cu/HSAG Pd/HSAG Cu-Pd/HSAG Pt/C

-2

j / mA cm

M

0.3

-2.0

0.2

635mV/dec

0.1

75mV/dec

29mV/dec

-0.1 -4.4

0.4

-4.0

10.0

Pd/HSAG

-2

Initial After 500 cycles After CA

-10.0

-20.0

-2.8

-2.4

-2.0

-1.6

-2

Cu-Pd/HSAG

Initial After 500 cycles After CA

-10.0

-20.0

-30.0

-30.0

-40.0 -0.6

-3.2

(d)

0.0

j / mA cm

j / mA cm

-2

0.0

-3.6

log j / A cm

Ac

(c)

42mV/dec

0.0

E / V vs. RHE

10.0

Pt/C HSAG Cu/HSAG Pd/HSAG Cu-Pd/HSAG

(b)

-0.4

-0.2

0.0

E / V vs. RHE

0.2

0.4

-40.0 -0.6

-0.4

-0.2

0.0

0.2

0.4

E / V vs. RHE

29 Page 29 of 38

Figure 9. LSVs for bare RDE, HSAG, Cu/HSAG, Pd/HSAG and Cu-Pd/HSAG electrocatalysts and Pt/C (a), the corresponding Tafel plots (b) and the stability tests for Pd/HSAG (c) and Cu-Pd/HSAG (d). All in 0.5 mol dm-3 H2SO4 at a ν = 0.005 V s-1.

ip t

In general, to evaluate the electrocatalysts HER performances, the potential value for a current density j = 10 mA cm-2 (overpotential - η10) is a key parameter as

cr

solar-light-coupled hydrogen evolution devices commonly operate at 10 - 20 mA cm-2

us

under typical conditions (AM 1.5, 1 sun)[41]. Consequently, here the overpotentials were determined for j = 10 and 30 mA cm-2. The Cu-Pd/HSAG showed an overpotential

an

of η10 = 0.145 V while for Pd/HSAG this value was η10 = 0.063 V. For the Pt/C reference the value of η10 = 0.024 V was obtained, under the same experimental

M

conditions, which is in good agreement with previously reported works [42]. To achieved a current density j = 30 mA cm-2, Cu-Pd/HSAG and Pd/HSAG need

ed

overpotentials of about 0.222 V and 0.136 V vs RHE. These values are higher than that obtained for Pt/C (0.046 V), and somewhat higher, as excepted, than the values for a

ce pt

current density of 10 mA cm-2.

The Tafel slope, which indicates the electrodes kinetic activity, is another important parameter to evaluate HER features of electrocatalysts. The Tafel plots for

Ac

HSAG, Cu/HSAG, Pd/HSAG, Cu-Pd/HSAG and Pt/C RDE modified electrodes are depicted in Figure 9 (b). The Pd/HSAG electrocatalysts presented the lowest Tafel slope with a value of 0.042 V dec-1, which is quite similar to that obtained for the Pt/C (0.029 V dec-1) which is considered the best HER electrocatalyst. The Cu-Pd/HSAG also showed a low Tafel slope of 0.075 V dec-1, unlike the Cu/HSAG and HSAG electrocatalysts with values of 0.644 and 0.635 V dec-1, respectively. The results clearly show that even a small doping of Pd NPs (weight metal loading: 5 wt.% Cu to 1 wt.%

30 Page 30 of 38

Pd in Cu-Pd/HSAG) is able to improve significantly the non-noble metal composite (5 wt.% Cu in Cu/HSAG) electrocatalytic activity. The obtained slopes suggest a Volmer-Heyrovsky HER mechanism, implying that for Cu-Pd/HSAG the rate-limiting step can be either the discharge reaction or the hydrogen desorption, since the slope

ip t

obtained is between the established values for each one (0.118 and 0.039 V dec -1, respectively[43]), while for the Pd/HSAG the hydrogen desorption is the rate-limiting

cr

step.

us

The exchange current densities (j0) were also determined using the Tafel plots. It is obtained when η is assumed to be zero by an extrapolation method and it describes

an

the intrinsic electrocatalytic activity of the electrocatalyst under equilibrium conditions. The values obtained for Pd/HSAG and Cu-Pd/HSAG were j0 = 0.57 and 0.43 mA cm-2,

M

respectively. Even though Pt/C presented better exchange current density (j0 = 1.12 mA cm-2), the Pd/HSAG and Cu-Pd/HSAG showed much better j0 values than the

ed

majority of published results with similar electrocatalysts, like the Pd71/Cu29 nanoparticles supported on carbon black (j0 = 0.38 mA cm-2)[44] or the Pd modified Pt

ce pt

supported on tungsten carbide nanocrystals (j0 = 0.12 - 0.41 mA cm-2)[45], as can be seen in Table 3.

The electrochemical stability of the electrocatalysts is also of huge importance

Ac

for future practical applications. Stability tests were performed for the Pd/HSAG and Cu-Pd/HSAG electrocatalysts using two methods: a) performing a chronoamperometry (CA) for 13000 s and the linear sweep voltammograms (LSVs) were acquired before and after this and b) performing 500 LSV cycles and comparing the 1st and 500th. Figure 9 (c) and (d) show the results obtained for Pd/HSAG and Cu-Pd/HSAG, respectively. The results show that for Pd/HSAG there is only a slight change of 0.033 V for j = 40 mA cm-2 after 500 cycles while, after the chronoamperometry test the overpotential

31 Page 31 of 38

changed by 0.095 V. For the Cu-Pd/HSAG there are no significant changes between the LSVs before and after CA and between the 1st and 500th potential sweep. Actually, the overpotential changed between 0.033 and 0.038 V for j = 40 mA cm-2. As for the ORR, the non-noble metal, Cu/HSAG, presented higher stability than the noble metal based

ip t

composite, Pd/HSAG, which reinforces the Cu’s role in the Pd NPs stabilization in the Cu-Pd/HSAG composite. For example, the Pd nanoparticles supported on graphene,

cr

presented a decrease of 15% in current density after only 1000 s.[46]

us

These results thus demonstrate the good durability in a highly acidic medium

Ac

ce pt

ed

M

an

(0.5 mol dm-3 H2SO4) of the as-prepared electrocatalysts.

32 Page 32 of 38

cr

ip t Catalyst

Loading

Electrode

Electrolyte

Pd/HSAG 0.03

GCE

0.5M H2SO4

M

Cu-Pd/HSAG

η (mV vs.

j0 (mA cm-2)

Tafel slope

Reference

(mV dec-1)

RHE) 63

0.57

42

145

0.43

75

10

This work

0.28

GC

0.5M H2SO4

10

80

0.24

30

[47]

Cu–Cu2ONPs@Cb

0.10

CPE

0.4M H2SO4

10

~672

N/A

93

[48]

Cu3P NW/CFc

15.2

-

10

143

0.18

67

[49]

75

0.38

48

189

N/A

150

331

N/A

126

225

N/A

123

10

~100

N/A

46

0.12

28

N/A

N/A 0.41

21

0.10 Pd27Cu73/C Pd/C

PtPd-WC/Cf

N/A

GCE

N/A

0.1M H2SO4

GCE

0.5M H2SO4

Graphite

0.1M H2SO4

rod

0.5M H2SO4

Ac c

Pd-graphenee

0.5M H2SO4

ep te

Pd40Cu60/C

d

Pd NPAsa

Pd71Cu29/Cd

a

j (mA cm-2)

an

(mg cm-2)

us

Table 3. Comparative overview of HER electrocatalyst performance.

10

[44]

[46] [45]

Pd nanoparticle assemblies copper-cuprous oxide nanoparticles in the carbon matrix c self-supported Cu3P nanowire arrays on commercial porous copper foam d carbon black loaded PdxCu100-x nanoparticles e Pd nanoparticles supported on graphene f Pd modified Pt supported on tungsten carbide nanocrystals b

33 Page 33 of 38

4. CONCLUSIONS Composites based on Cu, Pd and Cu-Pd NPs supported on HSAG were electrochemically characterized and successfully applied as bifunctional ORR/HER electrocatalysts. The electrochemical characterization of the prepared materials in the

ip t

presence of certain redox probes revealed high electroactive surface coverages, suggesting a very conductive pathway for electron transfer on modified electrodes and

cr

showing the advantage of the electrode modification.

us

All the materials showed interesting ORR electrocatalytic activities in alkaline medium. The Pd/HSAG composite exhibited the most promising ORR electrocatalytic

an

ability (Eonset = 0.91 V vs. RHE, jL, 0.3 V, 1600 rpm = -4.2 mA cm-2), followed by CuPd/HSAG (Eonset = 0.84 V vs. RHE, jL, 0.3 V, 1600 rpm = -3.5 mA cm-2). The metal/HSAG

M

composites followed a first order O2 reduction kinetics, with a trend for the 4-electron process. The bipotentiostatic measurements validated these assumptions, with the

ed

Pd/HSAG showing a very small amount of generated H2O2 (less than 10 % vs. 62 % for HSAG support). The composites exhibited similar Tafel slopes (48-77 mV dec-1), lower

ce pt

than that for Pt/C, indicating that O2 is easily adsorbed and activated at the composites’s surface. Furthermore, the prepared composites revealed a moderate electrochemical stability (current attenuation for 82-89 % after 20000 s), but a great tolerance to

Ac

methanol, with a current decrease less than 10 % after the methanol addition. From the prepared composites, the bimetallic Cu-Pd/HSAG composite showed to be most stable and tolerant to methanol, probably due to a synergetic effect between the Cu and Pd NPs. Nevertheless, the prepared nanocomposites should be further optimized, in order to achieve the standard ORR electrocatalytic performance of Pt/C catalysts and improve the electrochemical stability.

34 Page 34 of 38

With respect to HER, only the Pd-containing composites showed electrocatalytic activity, with the Pd/HSAG showing the small overpotential (η10 = 0.063 V) and Tafel slope (0.042 V dec-1), which are very close to those of the state-of-the-art electrocatalyst. Both Pd/HSAG and Cu-Pd/HSAG composites showed similar j0 values

ip t

(0.43 and 0.57 mA cm-2), and moderate durability with only slight changes on overpotential after 13000 s or 500 LSV cycles.

cr

For both reactions, the obtained results showed that the presence of Pd noble-

us

metal is crucial to improve the electrocatalytic activity. More interestingly, even a small loading of Pd NPs (1 wt.% Pd in Cu-Pd/HSAG) was able to improve significantly the

an

non-noble metal composite (Cu/HSAG) electrocatalytic activity towards ORR/HER and also the durability. This is a very important outcome to rationalise the design of

M

efficient, cheaper and robust ORR/HER electrocatalysts, by the loading of composites

ACKNOWLEDGMENTS

ed

based in non-noble metals with small amounts of noble-metals.

(FCT)/MEC

ce pt

This work was financially supported by Fundação para a Ciência e a Tecnologia and

EU

under

FEDER

founds

(POCI/01/0145/FEDER/007265),

Programme PT2020 (Project UID/QUI/50006/2013) and by Project UNIRCELL -

Ac

POCI-01-0145-FEDER-016422 – funded by European Structural and Investment Funds (FEEI) through - Programa Operacional Competitividade e Internacionalização COMPETE2020.

REFERENCES [1] Z.W. Seh, J. Kibsgaard, C.F. Dickens, I.B. Chorkendorff, J.K. Norskov, T.F. Jaramillo, Combining theory and experiment in electrocatalysis: Insights into materials design, Science, 355 (2017) 146-+.

35 Page 35 of 38

Ac

ce pt

ed

M

an

us

cr

ip t

[2] Y.P. Zhu, C.X. Guo, Y. Zheng, S.Z. Qiao, Surface and Interface Engineering of Noble-MetalFree Electrocatalysts for Efficient Energy Conversion Processes, Accounts Chem. Res., 50 (2017) 915-923. [3] H.X. Li, Y.P. Huang, H.H. Zhou, W.J. Yang, M.B. Li, Z. Huang, C.P. Fu, Y.F. Kuang, One step insitu synthesis of Co@N, S co-doped CNTs composite with excellent HER and ORR bi-functional electrocatalytic performances, Electrochim. Acta, 247 (2017) 736-744. [4] J.T. Zhang, L.T. Qu, G.Q. Shi, J.Y. Liu, J.F. Chen, L.M. Dai, N,P-Codoped Carbon Networks as Efficient Metal-free Bifunctional Catalysts for Oxygen Reduction and Hydrogen Evolution Reactions, Angew. Chem.-Int. Edit., 55 (2016) 2230-2234. [5] L.X. Luo, F.J. Zhu, R.X. Tian, L. Li, S.Y. Shen, X.H. Yan, J.L. Zhang, Composition-Graded PdxNi1-x Nanospheres with Pt Monolayer Shells as High-Performance Electrocatalysts for Oxygen Reduction Reaction, ACS Catal., 7 (2017) 5420-5430. [6] M. Martins, B. Sljukic, O. Metin, M. Sevim, C.A.C. Sequeira, T. Sener, D.M.F. Santos, Bimetallic PdM (M = Fe, Ag, Au) alloy nanoparticles assembled on reduced graphene oxide as catalysts for direct borohydride fuel cells, J. Alloy. Compd., 718 (2017) 204-214. [7] K. Park, H. Matsune, M. Kishida, S. Takenaka, Carbon-supported Pd-Ag catalysts with silicacoating layers as active and durable cathode catalysts for polymer electrolyte fuel cells, Int. J. Hydrog. Energy, 42 (2017) 18951-18958. [8] L.L. Zhang, Q.W. Chang, H.M. Chen, M.H. Shao, Recent advances in palladium-based electrocatalysts for fuel cell reactions and hydrogen evolution reaction, Nano Energy, 29 (2016) 198-219. [9] C.V. Boone, G. Maia, Pt-Pd and Pt-Pd-(Cu or Fe or Co)/graphene nanoribbon nanocomposites as efficient catalysts toward the oxygen reduction reaction, Electrochim. Acta, 247 (2017) 19-29. [10] A. Kongkanand, M.F. Mathias, The Priority and Challenge of High-Power Performance of Low-Platinum Proton-Exchange Membrane Fuel Cells, J. Phys. Chem. Lett., 7 (2016) 1127-1137. [11] J.W. Chen, J.D. Chen, D.N. Yu, M. Zhang, H. Zhu, M.L. Du, Carbon nanofiber-supported PdNi alloy nanoparticles as highly efficient bifunctional catalysts for hydrogen and oxygen evolution reactions, Electrochim. Acta, 246 (2017) 17-26. [12] X.X. Lin, X.F. Zhang, A.J. Wang, K.M. Fang, J.H. Yuan, J.J. Feng, Simple one-pot aqueous synthesis of AuPd alloy nanocrystals/reduced graphene oxide as highly efficient and stable electrocatalyst for oxygen reduction and hydrogen evolution reactions, J. Colloid Interface Sci., 499 (2017) 128-137. [13] W.J. Fan, Z.L. Li, C.H. You, X. Zong, X.L. Tian, S. Miao, T. Shu, C. Li, S.J. Liao, Binary Fe, Cudoped bamboo-like carbon nanotubes as efficient catalyst for the oxygen reduction reaction, Nano Energy, 37 (2017) 187-194. [14] J.F. Wu, S.Y. Shan, J. Luo, P. Joseph, V. Petkoy, C.J. Zhong, PdCu Nanoalloy Electrocatalysts in Oxygen Reduction Reaction: Role of Composition and Phase State in Catalytic Synergy, ACS Appl. Mater. Interfaces, 7 (2015) 25906-25913. [15] K.Z. Jiang, P.T. Wang, S.J. Guo, X. Zhang, X. Shen, G. Lu, D. Su, X.Q. Huang, Ordered PdCuBased Nanoparticles as Bifunctional Oxygen-Reduction and Ethanol-Oxidation Electrocatalysts, Angew. Chem.-Int. Edit., 55 (2016) 9030-9035. [16] J. Li, F. Li, S.X. Guo, J. Zhang, J.T. Ma, PdCu@Pd Nanocube with Pt-like Activity for Hydrogen Evolution Reaction, ACS Appl. Mater. Interfaces, 9 (2017) 8151-8160. [17] X. Liu, W. Liu, M. Ko, M. Park, M.G. Kim, P. Oh, S. Chae, S. Park, A. Casimir, G. Wu, J. Cho, Metal (Ni, Co)-Metal Oxides/Graphene Nanocomposites as Multifunctional Electrocatalysts, Adv. Funct. Mater., 25 (2015) 5799-5808. [18] X. Zhang, R.R. Liu, Y.P. Zang, G.Q. Liu, G.Z. Wang, Y.X. Zhang, H.M. Zhang, H.J. Zhao, Co/CoO nanoparticles immobilized on Co-N-doped carbon as trifunctional electrocatalysts for oxygen reduction, oxygen evolution and hydrogen evolution reactions, Chem. Commun., 52 (2016) 5946-5949. 36

Page 36 of 38

Ac

ce pt

ed

M

an

us

cr

ip t

[19] M. Zeng, Y.L. Liu, F.P. Zhao, K.Q. Nie, N. Han, X.X. Wang, W.J. Huang, X.N. Song, J. Zhong, Y.G. Li, Metallic Cobalt Nanoparticles Encapsulated in Nitrogen-Enriched Graphene Shells: Its Bifunctional Electrocatalysis and Application in Zinc-Air Batteries, Adv. Funct. Mater., 26 (2016) 4397-4404. [20] M.R. Cuervo, E. Asedegbega-Nieto, E. Diaz, S. Ordonez, A. Vega, A.B. Dongil, I. RodriguezRamos, Modification of the adsorption properties of high surface area graphites by oxygen functional groups, Carbon, 46 (2008) 2096-2106. [21] M. Sakthivel, S. Bhandari, J.F. Drillet, On Activity and Stability of Rhombohedral LaNiO3 Catalyst towards ORR and OER in Alkaline Electrolyte, ECS Electrochem. Lett., 4 (2015) A56A58. [22] M.V. Morales, M. Rocha, C. Freire, E. Asedegbega-Nieto, E. Gallegos-Suarez, I. RodriguezRamos, A. Guerrero-Ruiz, Development of highly efficient Cu versus Pd catalysts supported on graphitic carbon materials for the reduction of 4-nitrophenol to 4-aminophenol at room temperature, Carbon, 111 (2017) 150-161. [23] M. Nunes, I.M. Rocha, D.M. Fernandes, A.S. Mestre, C.N. Moura, A.P. Carvalho, M.F.R. Pereira, C. Freire, Sucrose-derived activated carbons: electron transfer properties and application as oxygen reduction electrocatalysts, RSC Adv., 5 (2015) 102919-102931. [24] A.J.F. Bard, L.R. Faulkner, Electrochemical Methods, Fundamentals and Applications, New York, 2001. [25] S.Y. Gao, H. Fan, S.X. Zhang, Nitrogen-enriched carbon from bamboo fungus with superior oxygen reduction reaction activity, J. Mater. Chem. A, 2 (2014) 18263-18270. [26] L. Delmondo, G.P. Salvador, J.A. Munoz-Tabares, A. Sacco, N. Garino, M. Castellino, M. Gerosa, G. Massaglia, A. Chiodoni, M. Quaglio, Nanostructured MnxOy for oxygen reduction reaction (ORR) catalysts, Appl. Surf. Sci., 388 (2016) 631-639. [27] A.T. Swesi, J. Masud, M. Nath, Nickel selenide as a high-efficiency catalyst for oxygen evolution reaction, Energy Environ. Sci., 9 (2016) 1771-1782. [28] M. Jahan, Z.L. Liu, K.P. Loh, A Graphene Oxide and Copper-Centered Metal Organic Framework Composite as a Tri-Functional Catalyst for HER, OER, and ORR, Adv. Funct. Mater., 23 (2013) 5363-5372. [29] S. Hoang, S.W. Guo, N.T. Hahn, A.J. Bard, C.B. Mullins, Visible Light Driven Photoelectrochemical Water Oxidation on Nitrogen-Modified TiO2 Nanowires, Nano Lett., 12 (2012) 26-32. [30] Y.C. Lan, C. Gai, P.J.A. Kenis, J.X. Lu, Electrochemical Reduction of Carbon Dioxide on Cu/CuO Core/Shell Catalysts, ChemElectroChem, 1 (2014) 1577-1582. [31] W.K. Han, J.W. Choi, G.H. Hwang, S.J. Hong, J.S. Lee, S.G. Kang, Fabrication of Cu nano particles by direct electrochemical reduction from CuO nano particles, Appl. Surf. Sci., 252 (2006) 2832-2838. [32] CRC Handbook of Chemistry and Physics, 92nd Edition, Editor-in-chief W. H. Haynes, CRC Press by Taylor & Francis Group, 2011. [33] H.H. Wu, H.B. Li, X.F. Zhao, Q.F. Liu, J. Wang, J.P. Xiao, S.H. Xie, R. Si, F. Yang, S. Miao, X.G. Guo, G.X. Wang, X.H. Bao, Highly doped and exposed Cu(I)-N active sites within graphene towards efficient oxygen reduction for zinc-air batteries, Energy Environ. Sci., 9 (2016) 37363745. [34] F. He, L. Mi, Y.F. Shen, X.H. Chen, Y.R. Yang, H. Mei, S.Q. Liu, T. Mori, Y.J. Zhang, Driving electrochemical oxygen reduction and hydrazine oxidation reaction by enzyme-inspired polymeric Cu(3,3 '-diaminobenzidine) catalyst, J. Mater. Chem. A, 5 (2017) 17413-17420. [35] J.H. Guo, Y.T. Shi, X.G. Bai, X.C. Wang, T.L. Ma, Atomically thin MoSe2/graphene and WSe2/graphene nanosheets for the highly efficient oxygen reduction reaction, J. Mater. Chem. A, 3 (2015) 24397-24404. [36] S.A. Wohlgemuth, R.J. White, M.G. Willinger, M.M. Titirici, M. Antonietti, A one-pot hydrothermal synthesis of sulfur and nitrogen doped carbon aerogels with enhanced electrocatalytic activity in the oxygen reduction reaction, Green Chem., 14 (2012) 1515-1523. 37

Page 37 of 38

Ac

ce pt

ed

M

an

us

cr

ip t

[37] S.A. Wohlgemuth, T.P. Fellinger, P. Jaker, M. Antonietti, Tunable nitrogen-doped carbon aerogels as sustainable electrocatalysts in the oxygen reduction reaction, J. Mater. Chem. A, 1 (2013) 4002-4009. [38] Y. Jiao, Y. Zheng, M.T. Jaroniec, S.Z. Qiao, Design of electrocatalysts for oxygen- and hydrogen-involving energy conversion reactions, Chem. Soc. Rev., 44 (2015) 2060-2086. [39] Z.Q. Jiang, Z.J. Jiang, T. Maiyalagan, A. Manthiram, Cobalt oxide-coated N- and B-doped graphene hollow spheres as bifunctional electrocatalysts for oxygen reduction and oxygen evolution reactions, J. Mater. Chem. A, 4 (2016) 5877-5889. [40] M.Y. Song, H.Y. Park, D.S. Yang, D. Bhattacharjya, J.S. Yu, Seaweed-Derived HeteroatomDoped Highly Porous Carbon as an Electrocatalyst for the Oxygen Reduction Reaction, ChemSusChem, 7 (2014) 1755-1763. [41] J.D. Benck, T.R. Hellstern, J. Kibsgaard, P. Chakthranont, T.F. Jaramillo, Catalyzing the Hydrogen Evolution Reaction (HER) with Molybdenum Sulfide Nanomaterials, ACS Catal., 4 (2014) 3957-3971. [42] D.M. Fernandes, M.P. Araujo, A. Haider, A.S. Mougharbel, A.J.S. Fernandes, U. Kortz, C. Freire, Polyoxometalate-graphene Electrocatalysts for the Hydrogen Evolution Reaction, ChemElectroChem, 5 (2018) 273-283. [43] M. Zeng, Y.G. Li, Recent advances in heterogeneous electrocatalysts for the hydrogen evolution reaction, J. Mater. Chem. A, 3 (2015) 14942-14962. [44] X. Zhang, D.F. Wu, D.J. Cheng, Component-dependent electrocatalytic activity of PdCu bimetallic nanoparticles for hydrogen evolution reaction, Electrochim. Acta, 246 (2017) 572579. [45] M. Wu, P.K. Shen, Z.D. Wei, S.Q. Song, M. Nie, High activity PtPd-WC/C electrocatalyst for hydrogen evolution reaction, Journal of Power Sources, 166 (2007) 310-316. [46] S. Ghasemi, S.R. Hosseini, S. Nabipour, P. Asen, Palladium nanoparticles supported on graphene as an efficient electrocatalyst for hydrogen evolution reaction, Int. J. Hydrog. Energy, 40 (2015) 16184-16191. [47] S.L. Liu, X.Q. Mu, H.Y. Duan, C.Y. Chen, H. Zhang, Pd Nanoparticle Assemblies as Efficient Catalysts for the Hydrogen Evolution and Oxygen Reduction Reactions, European Journal of Inorganic Chemistry, (2017) 535-539. [48] P. Muthukumar, V.V. Kumar, G.R.K. Reddy, P.S. Kumar, S.P. Anthony, Fabrication of strong bifunctional electrocatalytically active hybrid Cu-Cu2O nanoparticles in a carbon matrix, Catalysis Science & Technology, 8 (2018) 1414-1422. [49] J.Q. Tian, Q. Liu, N.Y. Cheng, A.M. Asiri, X.P. Sun, Self-Supported Cu3P Nanowire Arrays as an Integrated High-Performance Three-Dimensional Cathode for Generating Hydrogen from Water, Angew. Chem.-Int. Edit., 53 (2014) 9577-9581.

38 Page 38 of 38