Metallic Two-dimensional Nanoframes: Unsupported Hierarchical

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Metallic Two-dimensional Nanoframes: Unsupported Hierarchical Nickel-Platinum Alloy Nanoarchitectures with Enhanced Electrochemical Oxygen Reduction Activity and Stability Jose Fernando Godinez-Salomon, Ruben Mendoza-Cruz, Maria Josefina Arellano-Jiménez, Miguel José-Yacamán, and Christopher P. Rhodes ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 12 May 2017 Downloaded from http://pubs.acs.org on May 12, 2017

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ACS Applied Materials & Interfaces

Metallic Two-dimensional Nanoframes: Unsupported Hierarchical Nickel-Platinum Alloy Nanoarchitectures with Enhanced Electrochemical Oxygen Reduction Activity and Stability Fernando Godínez-Salomón,a Rubén Mendoza-Cruz,b M. Josefina Arellano-Jimenez, b Miguel Jose-Yacaman, b and Christopher P. Rhodes a,*

a

Department of Chemistry and Biochemistry, Texas State University 601 University Dr., San Marcos TX 78666, USA

b

Department of Physics and Astronomy, University of Texas at San Antonio One UTSA Circle, San Antonio, TX 78249, USA

*Corresponding author: voice: 512.245.6721; email: [email protected]

Keywords: Nanoframes, two-dimensional materials, electrocatalysts, oxygen reduction reaction, fuel cells

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Abstract Electrochemical oxygen reduction reaction (ORR) catalysts that have both high activities and long-term stabilities are needed for proton-exchange membrane fuel cells (PEMFCs) and metal-air batteries. Two-dimensional (2D) materials based on graphene have shown high catalytic activities, however carbon-based materials result in significant catalyst degradation due to carbon oxidation that occurs at high electrochemical potentials. Here, we introduce the synthesis and electrochemical performance of metallic 2D nanoframes which represent a new approach to translate 2D materials into unsupported (carbon-free) electrocatalysts that have both significantly higher ORR catalytic activities and stabilities compared with conventional Pt/carbon electrocatalysts. Metallic Ni-Pt 2D nanoframes were synthesized by controlled thermal treatments of Pt-decorated Ni(OH)2 nanosheets.

The

nanoframes consist of a hierarchical 2D framework composed of a highly catalytically active Pt-Ni alloy phase with an interconnected solid and pore network that results in threedimensional molecular accessibility. The inclusion of Ni within the Pt structure resulted in significantly smaller Pt lattice distances compared to those of Pt nanoparticles. Based on the unique local and extended structure, the ORR specific activity of Ni-Pt 2D nanoframes (5.8 mA cmPt-2) was an order of magnitude higher than Pt/carbon. In addition, acellerated stability testing at elevated potentials up to 1.3 VRHE showed that the metallic Ni-Pt nanoframes exhibit significantly improved stability compared with Pt/carbon catalysts. The nanoarchitecture and local structure of metallic 2D nanoframes results in high combined specific activity and elevated potential stability. Analysis of the ORR electrochemical reaction kinetics on the Ni-Pt nanoframes supports that at low overpotentials the first electron transfer is the rate determining step and the reaction proceeds via a four electron reduction process. The ability to create metallic 2D structures with 3D molecular accessibility opens up new opportunities for the design of high activity and stability carbon-free catalyst nanoarchitectures for numerous electrocatalytic and catalytic applications. 2 ACS Paragon Plus Environment

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Introduction The development of nanostructured oxygen reduction electrocatalysts with high activity, long-term stability and low cost is key, unmet challenge for the wide scale adoption of high efficiency, clean proton-exchange membrane fuel cells (PEMFCs) for vehicles as well as metal-air batteries.1 Under acidic conditions, electrochemical oxygen reduction on Pt exhibits slow and complicated kinetic behavior attributed to site blocking and electronic effects of adsorbed species.2 The sluggish oxygen reduction reaction (ORR) kinetics impose the use of higher catalysts loadings (i.e. Pt) at the cathode side.3 Targets for Pt mass activity (>0.44 A/mgPGM at 0.9 VIR-free; PGM: platinum group metal), catalyst loading (≤0.125 mgPGM/cm2), cost per power (≤$15/kW), and stability (100 % of their initial electrochemical active surface areas after 1000 cycles at elevated potentials (1.3 VRHE). The stability of the 2D nanoframe catalysts is significantly higher than carbon-supported Pt catalysts under the same accelerated stability testing parameters. The specific activity of NiPt-300 slightly decreased from 5.8 to 5.4 mA cmPt-2 after 1000 cycles, however the specific activity after cycling remained significantly higher than reported values for unsupported Pt-Ni catalysts and numerous carbon-supported PtNi catalysts. Compared with catalysts that are distinct, isolated nanoparticles or nanoframes that require a carbon support, the metallic 2D nanoframe structure combines a local and extended structure that limits Pt dissolution, migration, and agglomeration resulting in catalysts with high activity and stability. The Ni-Pt 2D nanoframes provide a substantial improvement in combined activity and stability over current catalysts, particularly for significant and important elevated voltage region where carbon-supported catalysts, even with very high activities can suffer significant degradation in performance. Further work is aimed at increasing the Pt surface composition, increasing the surface area and porosity, evaluating the stability of the 2D-nanoframes under long-term electrochemical cycling, and developing rapid synthesis methods. Additional work is needed to understand the changes occurring during extended cycles in both the load and start-up/shutdown voltage ranges, evaluate the specific surface structure and its role on the activity and stability, further quantify metal dissolution during the activation step and after stability testing, and investigate chemical leaching to enable use within MEA configurations. The understanding of how controlled temperature/atmosphere treatments can be used obtain metallic 2D nanoframes with variable metal composition, surface structure, crystallite size, and porosity opens up new avenues for the design of high activity and stability carbon-free electrocatalyst


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nanoarchitectures for oxygen reduction as well other important electrochemical energy conversion reactions. Materials and Methods Chemicals: Potassium hexachloroplatinate (K2PtCl6, 99.98%) was obtained from Sigma-Aldrich. Nickel nitrate hexahydrate (Ni(NO3)2.6H2O, 98 %) and urea (N2COH4, 99.3%), were purchased from Alfa Aesar. Ethyleneglycol, isopropanol (HPLC grade), and ACS grade water (18 MΩ-cm) were obtained from VWR Analytical. All electrochemical measurements were carried out in 0.1 M HClO4 prepared with 70% HClO4 (Veritas Doubly Distilled) (0.000001% Cl-). All reagents were used without further purification. Synthesis of α-Ni(OH)2 nanosheets: The α-Ni(OH)2 nanosheets were synthesized using a hydrothermal process adapted from a prior report.77 To prepare the α-Ni(OH)2 nanosheets, 1.06 g (3.64 mmol) of Ni(NO3)2⋅6H2O was combined with 0.6597 g (10.98 mmol) of urea in 20 mL of ultrapure water (18 MΩ-cm) and 25 mL of ethyleneglycol (EG). The solution was then transferred into a 50 mL Telfon-lined autoclave reactor (Parr Instruments). The reaction vessel was placed in a 120 °C pre-heated oven and allowed to react for 4.5 h without active stirring. After the reaction time, the autoclave was removed from the oven and allowed to cool at room temperature. The light green powder was recovered by centrifugation (3000 RPM, 3 min, Thermo, Sorvall ST16) and subsequently rinsed five times with water and two times with isopropanol using the same centrifuge conditions. Following the rinsing steps, the powder was dried under vacuum at 60 °C overnight. Based on analysis discussed in the text, the dried powder was notated as α-Ni(OH)2 nanosheets. Decorated Ni(OH)2@Pt nanosheets: The α-Ni(OH)2 nanosheets were decorated with Pt by modification of a previously reported synthesis method.42 To deposit Pt on the α-Ni(OH)2 nanosheets, 0.10382 g of the prepared Ni(OH)2 was dispersed in 25 mL of ultrapure water, and subsequently 0.02730 g of K2PtCl6, corresponding to 20 wt. % of metallic Pt vs α-Ni(OH)2, 31 ACS Paragon Plus Environment


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was added. The slurry was then transferred to the autoclave, stirred 12 h, and then heated at 80 °C for 4 days without stirring. Finally, a grey powder was recovered, rinsed, and dried as described above. 2D NiPt nanoframes: The 2D Ni(OH)2@Pt catalyst was thermally treated at 200 or 300 °C for 20 minutes under 100 mL/min flowing Ar/H2 (95/5 vol. %) using a ramp rate of 10 °C/min starting from room temperature. The synthesized samples were notated as α-Ni(OH)2, Ni(OH)2@Pt, NiPt-200, and NiPt-300, for the as-prepared precursor nanosheets, nanosheets after platinum deposition, and samples after thermal treatment at 200 or 300 °C respectively. Samples were also characterized after electrochemical conditioning (AEC) which consisted of cyclic voltammetry (CV), carbon monoxide (CO)-stripping, and linear voltammetry over the ORR potential region. Details of the specific experimental parameters used for each of these steps is described in the following paragraphs. The sample were labeled with the suffix “AEC” to denote the characterization after the electrochemical treatment steps: Ni(OH)2@Pt-AEC, NiPt-200-AEC and NiPt-300-AEC. Physical and Structural Characterization: The catalyst composition was determined using inductively coupled plasma atomic emission spectroscopy (ICP-AES) carried out by Galbraith Laboratories Inc. (Knoxville, TN).

From ICP analysis, the as-prepared NiPt-200

catalyst was determined to be 25.5 ± 5.1 wt % Pt, and the NiPt-300 catalyst was determined to be 29.9 ± 1.3 wt % Pt. Powder X-Ray diffraction (XRD) measurements were conducted using a Bruker AXS D8 Advance powder X-Ray diffractometer with a Cu Ka (λ = 1.5406 Å) radiation source, operating at 40 kV and 25 mA and a high resolution energy dispersive 1D Linxeye XE detector. The scan range of 2Ɵ was 5° < 2Ɵ < 85 ° with a 0.01° increment. Brunauer–Emmett– Teller (BET) surface areas were obtained from nitrogen adsorption/desorption isotherms measured using a Micromeritics ASAP 2020 surface area and porosimetry analyzer. Samples were degassed at 120 °C for 16 h prior to characterization. 32 ACS Paragon Plus Environment

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The structure, morphology, crystallinity, and elemental distribution of the catalysts were determined by high-resolution electron microscopy (HR-TEM) and selected area electron diffraction (SAED) using a 2010-F (200 kV, JEOL). SAED measurements were calibrated using the lattice distance from 2D-Ni-nanoframes obtained by heating α-Ni(OH)2 nanosheets at 300 °C under H2/Ar. Cs-corrected scanning transmission electron microscopy (STEM) was carried out by using a JEM-ARM200F (200 kV, JEOL) equipped with an energy dispersed spectrometer (EDAX Silicon Drift detector). High angle annular dark field STEM (HAADF-STEM) was obtained with a convergence angle of 26 mrad and a collection semi-angles from 50 to 180 mrad. The probe size used was about 0.09 nm with the probe current of 22 pA. Rotating Disk Electrochemical Characterization. The electrochemical measurements were conducted at constant temperature (298 K) in a three electrode cell using a rotating disk electrode (RDE) or rotating ring disk electrode (RRDE) configuration with an Autolab PGSTAT128N bipotentiostat and rotation control (Pine Instruments). A Pt mesh and a freshly prepared reversible hydrogen electrode (RHE) were used as counter and reference electrodes respectively. The potential of the RHE electrode was confirmed by measuring the potential versus a commercial silver/silver chloride reference electrode (0.199 VNHE). The catalyst was suspended within an ink that was prepared by combining the catalyst, Nafion suspension (Aldrich, 5 wt %, 1100 g equivalent weight), isopropanol, and ultrapure water, following procedures from previous reports.57-58 The catalyst ink was prepared by mixing 3 mg of the catalyst with 3 mL of a stock solution consisting of 7.96 mL ultra-pure water (18.2 MΩ), 2 mL of isopropanol, and 40 µL of 5 wt% Nafion solution (Sigma-Aldrich).58 The same ink formulation was used for all catalysts. The catalyst inks were sonicated in an ultrasonic bath (Fisher, 40 kHz) for 5 minutes while maintaining the temperature around 25 °C. The catalyst ink was deposited onto a mirror polished glassy carbon electrode (0.196 cm2) which was used as the working electrode. To control the catalyst loading for the activity tests, ink volumes of 33 ACS Paragon Plus Environment


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20 uL (Pt/C), 18 uL (Ni(OH)2:Pt), and 18 uL (NiPt-200 and NiPt-300) were deposited onto the glassy carbon electrode. After depositing the ink, the working electrode was dried at room temperature within a fume hood.

The uniformity of the thickness and distribution of the

catalyst layer on the glassy carbon working electrodes used for RDE and RRDE tests are critical factors that affect the accurate measurement of electrochemical catalytic activity since nonuniform film thicknesses and distributions can influence mass transport processes within the catalyst layer.57-58 For films used for activity measurements, the current within the potential region of 0.2 VRHE < E < 0.7 VRHE obtained from linear voltammetry ORR measurements was within ~10% of the theoretical diffusion limited current of -5.7 mA cm-2geo, which was previously put forth as a reasonable metric used as part of assessing film quality.57-58 Variations in measured activities for the same catalyst are attributed to differences in film thickness and distribution of the catalyst film on the glassy carbon working electrode, and specific and mass activities are reported as averages with associated standard deviations of multiple measurements. The Pt loading was optimized based on the maximum specific and mass activity for each catalyst in order to mitigate sources of error during the determination of specific activities.57 The resulting Pt loading for Pt/C was LPt= 16 µgPt cm-2geo which is in similar to previously reported Pt loadings (14.3-20 µgPt cm-2geo) for supported catalysts.58 For the unsupported catalysts, the resulting Pt loadings were 31 µg cm-2geo (Ni(OH)2:Pt) and 35 µg cm2

geo

(NiPt-200 and NiPt-300). We note that the unsupported NiPt-200 and NiPt-300 catalysts

have lower ECSAPt values (5.3-7.4 m2 g-1Pt) compared with Pt/C (72.8 m2 g-1Pt) as presented in Table S1 and required higher loadings on the electrode compared with Pt/C. Previously reported ORR activity tests using RDE also showed that a catalyst with a low ECSAPt of 8 m2/gPt required higher loadings (~25-75 µgPt cm-2geo) compared with Pt/C in order to cover the whole geometric area of the glassy carbon working electrode,57 and the loading of NiPt-200


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and NiPt-300 catalysts used within this study (35 µg cm-2geo) is in a similar range as the loadings (~25-75 µgPt cm-2geo) used within this prior report. The electrolyte used for RDE and RRDE tests was 0.1 M HClO4 (70 wt %, Fisher, ultrapure) in ultrapure water. After preparation, the electrodes were immersed in deaerated 0.1 M HClO4 under potential control at 0.1 V. IR-compensation and background subtraction were utilized to obtain electrochemical parameters.78 The catalysts were conditioned by cycling 40 times between 0.05 and 1.2 VRHE at a scan rate 50 mV s-1 within Argon-purged 0.1 M HClO4 in order to produce a stable electrode surface. The hydrogen underpotential deposition (Hupd) charge over the H desorption region of the 40th cycle was integrated to obtained the Pt electrochemical surface area (ECSAPt-Hupd) using a specific charge of 210 µC cm-2. For the Hupd charge for NiPt-300, the potential window used for analysis was restricted between 0.08 and 0.4 V in order to avoid the hydrogen evolution reaction (HER) feature, as described in the text. CO stripping voltammetry was also carried out to complement the ECSAPt-Hupd obtained by Hupd. The electrochemical surface area obtained from CO stripping, ECSAPt,CO, was calculated using a scan rate of 20 mV s-1 and using 420 µC cm-2 as the charge corresponding to a monolayer of adsorbed CO. For evaluation of the oxygen reduction activity, CVs were obtained from 0.05 to 1.05 VRHE using a scan rate 20 mV s-1 and a rotation rate of 1600 rpm in oxygen-saturated 0.1 M HClO4. Based on the very similar values of ECSAPt,CO and ECSAPt-Hupd (Table S1), the ECSAPt used for the calculation of the ORR specific activity was determined from the ECSAPt,Hupd obtained from the third cycle of the CV measured after the ORR activity testing. The voltammetric curves in the ORR region were corrected by background subtraction of the CVs obtained under Ar to account for the capacitive contribution. Tafel plots were obtained at 25 °C from the positive scan direction at 1600 rpm, and Tafel slopes were determined using a voltage



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range of 0.9 to 1.0 VRHE. Carbon-supported Pt nanoparticles (Pt/C, Etek, 20 wt% Pt) were used as a reference for the CVs, CO stripping, and ORR activity tests. The H2O2 production was evaluated using a rotating ring disk electrode (RRDE) configuration (disk area of 0.237 cm2). The disk potential was scanned between 0.05 and 1.05 VRHE at 20 mV s-1 using a rotation rate of 1600 rpm while the ring was potentiostated at 1.4 VRHE. The collection efficiency was calculated using a deaerated electrolyte KOH 0.1 M (Alfa Aesar) with 10 mmol L-1 K3Fe(CN)6 (Alfa Aesar).79 A Pt wire and mercury-mercury oxide electrode (E°Hg/HgO = 0.131 V vs SCE; ΔE = 0.009 V), were used as counter and reference electrode respectively, however all the potentials are reported with respect to RHE (ERHE = EHg/HgO-measured + 0.059 pH + E°Hg/HgO; ERHE-corrected = 0.898 V) For the collection efficiency experiment, the disk potential was scanned at 20 mV s-1 while the ring was potentiostated at 1.5 VRHE. The collection efficiency was determined according to the equation N = - IR/ID, where IR and ID correspond to the ring and disk currents respectively. The average collection efficiency obtained at 1600 rpm was 0.43 ± 0.05. The accelerated stability test was carried out using the RDE configuration decribed above within an O2-saturated 0.1 M HClO4 electrolyte, without stirring by cycling the electrode over 1000 cycles from 0.6 to 1.3 VRHE at scan rate of 100 mV s-1 at 25 °C. In order to increase the detection of small amounts of Pt within the electrolyte using ex-situ ICP-MS, for the accelerated stability tests the Pt loading was increased to 22 µg cm-2geo (Pt/C), 43 µg cm-2geo (NiPt-200) and 58 µg cm-2geo (NiPt-300). Prior to the evaluation, the surface of the commercial catalysts Pt/C was cleaned and the Ni-Pt catalysts were electrochemical dealloyed (activation step) by cycling from 0.05 V to 1.2 V at a scan rate of 100 mV s-1 (60 scans) and 20 mV s-1 (20 scans) within an Ar-saturated 0.1 M HClO4 electrolyte. In addition, after the activation step, the electrolyte was replaced with a fresh electrolyte solution, which was used for the stability tests. During the stability testing, every 200 scans the potential was swept between 0.05 and 1.2 VRHE in Ar-saturated electrolyte to determine the electrochemical surface area (ECSAPt,H-upd). The 36 ACS Paragon Plus Environment

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ECSAPt,H-upd was determined from the electrochemical double layer-corrected charge obtained from the H desorption peak, using the details described above. The normalized remaining ECSAPt was determined according to the equation (Qx/Q1*100) where Q1 is the charge at the beginning of the test (after the activation step) and Qx the charge determined at the corresponding number of scans during the stability test (i.e. 200, 400, etc.). The Pt content of the electrode and electrolyte after accelerated stability testing were determined using ICP-MS measurements (Galbraith Laboratories). ASSOCIATED CONTENT Supporting Information: SEM, TEM, STEM, and SAED images of Ni(OH)2, Ni(OH)2@Pt,

NiPt-200, NiPt-300 samples; Electrochemical data (CVs and Tafel plots) of catalysts. This material is available free of charge via the Internet at http://pubs.acs.org

AUTHOR INFORMATION Corresponding author *

Email: [email protected]; Voice: 512.245.6721

ACKNOWLEDGEMENT C.P. Rhodes acknowledges support from the Office of Naval Research (Grant No. N00014-161-2777) and National Science Foundation PREM (Grant No. DMR-1205670). M. JoseYacaman and M. J. Arellano-Jimenez acknowledge the National Center of Research Resources (5G12RR013646-12), the National Institute on Minority Health and Health Disparities (G12MD007591) NIH, and the Welch Foundation (Grant No. AX-1615).

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Table of Contents Graphical Abstract Two-dimensional (2D) Metallic Ni-Pt Nanoframes 2 nm

4H+

1 µm

4e-

O2

2H2O

Pt Ni


Metallic Two-dimensional Nanoframes: Unsupported Hierarchical

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