Preparation, Characterization and Properties of Pt-Cu Co-reduced and ...

Journal of New Materials for Electrochemical Systems 9, 73-81 (2006) © J. New Mat. Electrochem. Systems

Preparation, Characterization and Properties of Pt-Cu Co-reduced and Pt-on-Cu Skin Type Bimetallic Carbon-Supported (Vulcan XC72) Electrocatalysts† K.S. Nagabhushana1, C. Weidenthaler1, *S. Hočevar2, D. Strmčnik2, M. Gaberšček2, A. L. Antozzi3 and G. N. Martelli3 1

Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, D 45470, Mülheim an der Ruhr, Germany. 2 National Institute of Chemistry, Hajdrihova 19, P.O.Box 660, SI-1001 Ljubljana, Slovenia 3 De Nora Tecnologie Elettrochimiche S.r.l., Via Bistolfi, 35, I-20134 Milano, Italy Received: March 21, 2005, Accepted: July 12, 2005 Abstract: Pt/Cu salt co-reduction and, subsequent reduction of Cu(acac)2 and PtCl2 allows to generate either alloyed Pt-Cu or skin type Pt-on-Cu carbon-supported (20 wt%, Pt:Cu = 50:50 a/o on Vulcan XC72) electrocatalysts. An examination by TEM revealed that the co-reduced Pt-Cu catalyst have well dispersed bimetallic nanoparticles (av. particle size 3.6 nm). The skin type Pt-on-Cu catalyst shows tiny Pt clusters (1-2 nm) decorating the surface of larger Cu particles (6-8 nm). XRD pattern of the co-reduced Pt-Cu catalyst shows weak and broad diffraction peaks consistent with a predominantly alloyed composition (plus a few Pt crystallites). Pattern of the skin type Pt-on-Cu/C catalyst reveals larger nanoparticles and points to the formation of (surface) alloy. SEM/EDAX showed a uniform metal distribution present in both Pt-Cu systems. XPS measurements indicated that in both cases only Pto is present. In co-reduced alloy catalyst a higher amount of Cu2+ was present at the nanoparticle surface (Cuo/ Cu2+ = 0.6), while on the surface of the skin type Pt-on-Cu system Cuo and Cu2+ exist in equal amounts (Cuo/ Cu2+ = 1.0). Both types of Cu containing catalysts have higher mass specific activity in hydrogen oxidation reaction (HOR) than the industrial benchmark Pt/C catalyst. The electrocatalytic properties depend on morphological structure subtleties. Keywords: Fuel cell, Electrocatalysis, Bimetallic catalysts, Pt-Cu, Hydrogen oxidation reaction

1. INTRODUCTION

tive cracking catalyst [8]. Few attempts have also been directed so far towards examining the utility of these bimetallic systems as electrocatalysts [9,10]. The colloidal precursors approach [11] provides an easy access to a novel type of nanoscopic electrocatalysts having advantageous properties [12,13]. Here we describe a straightforward approach to platinum-copper based nanocatalysts of two different morphologies which combine an advantageous performance with reduced content of Pt, e.g. by using Cu as the inner layer of Pt-coated nanoparticles.

Pt based catalysts for polymer electrolyte membrane fuel cell (PEMFC) applications are well established [1-5]. However, in order to finally introduce PEMFC-based energy conversion systems into stationary and vehicle traction technology or into the even more demanding market niches of small electronic devices such as laptops or cell phones, fundamental research efforts and technical developments are still needed. Since Pt is a very costly noble metal, recently numerous studies were directed towards reducing the Pt content in PEMFC electrodes without compromising the electrochemical performances [6,7]. Pt-Cu is used e.g., as selec-

2. EXPERIMENTAL All the experiments were done under anaerobic Schlenk conditions. The solvents were thoroughly dried before use. Commercial chemicals were used without further purifica-

*To whom correspondence should be addressed: Email: [email protected] †

Dedicated to Prof. Dr. Helmut Bönnemann on his 65th birthday. This work, in part, was performed under his supervision.

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K.S. Nagabhushana et al. / J. New Mat. Electrochem. Systems

tion. Transmission electron microscopy (TEM) measurements were performed using a Hitachi H 7500 Instrument (cold field emission electron source). Specimens were prepared by placing a small amount of the catalyst dispersed in tetrahydrofurane (THF) on a nickel grid. XPS measurements were performed with a Kratos HSi spectrometer with a hemispherical analyzer. The monochromatized Al Kα X-ray source (E=1486.6 eV) was operated at 15 kV and 15 mA. For the narrow scans, analyzer pass energy of 40 eV was applied. The hybride mode was used as lense mode. The base pressure in the analysis chamber was 6×10-10 Torr. In order to take into account charging effects, all spectra have been referred to C 1s at 284.5 eV. XRD studies were carried out with a Stoe STADIP θ-θ diffractometer equipped with a linear position selective detector in transmission geometry using Cu Kα1 radiation. Both Pt-Cu catalysts were measured in glass capillary tubes having 0.5 mm diameter. SEM was measured on a HITACHI S-3500N instrument. A paste of the catalyst in ethanol was placed on the Al sample holder and fixed with conductive glue. EDX measurements were performed using the hardware from OXFORD (England) with a software INCA (version 4.02) and a Pentafet detector having resolution of 133 eV at 5.9 keV. 2.1. SYNTHETIC PROCEDURES 1. Homogeneous Pt/Cu catalyst preparation via coreduction of copper acetylacetonate and platinum chloride: In a 2L- two neck flask fitted with a dropping funnel and a vacuum adapter, 1.07 g of platinum chloride (4.022 mmol) and 1.06 g of copper acetylacetonate (4.022 mmol) were suspended in 600 ml of dry THF and stirred heavily under a steady flow of Argon. To this mixture, 10.0 ml of 1.63 molar LiBet3H in THF solution diluted by 50 ml of dry THF was added dropwise over night. The solution was stirred at 80 oC for 8 hours upon addition of 4.16 g of Vulcan XC 72 and then cooled to room temperature. After leaving the suspension for 2 hours without mixing, the colourless supernatant was pressed off, the carbon supported nanoparticles were rinsed twice with 100 ml of dry THF and then dried in vacuum (10—2 mbar). This supported catalyst was then activated by conditioning at 300 oC using argon, oxygen and then hydrogen for 30 minutes each. 2. Skin type Pt-on-Cu catalyst preparation via stepwise

reduction of copper acetylacetonate and platinum chloride: In a 1L two neck flask equipped with a dropping funnel under argon, 2.08 g of Vulcan XC72 was dispersed in 300 ml of dry THF. To this well dispersed solution, 530 mg of Copper acetylacetonate (2.011 mmol) was introduced and the whole mixture was stirred at room temperature for half an hour. To this solution, 2.5 ml of 1.63 molar LiBet3H diluted in 30 ml of THF was introduced over a period of 2 hours. Metal copper on carbon was then isolated by pressing off the colourless supernatant and then heating the flask to 200 oC under a flow of Argon. This material was re-dispersed in 300 ml of fresh THF and to this, a suspension of 530 mg of PtCl2 (2.011 mmol) in 50 ml THF, sonicated for 15 minutes,

was introduced over 2 hours. The entire mixture was then reduced by adding another 2.5 ml of LiBet3H in 25 ml of THF. After stirring the total mass for 2-3 hours, the solution was stirred for another two hours at 80 oC. At the end of this time, the reaction mass was cooled to room temperature, the clear supernatant was pressed off, solid product was rinsed twice with 100 ml of THF to remove any lithium chloride still present, and then dried. The supported catalyst was further activated by conditioning at 300 oC using argon, oxygen and hydrogen for 30 minutes each. 2.2. ELECTROCHEMICAL MEASUREMENTS The intrinsic electrocatalytic activity of the three catalyst samples in hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) was determined using Rotating Disc Electrode (RDE) method. Solartron 1286 Electrochemical Interface and EG&G PARC Model 616 with standard glassy carbon RDE (A = 0.1256 cm2) was used for this purpose. Carbon-supported electrocatalyst deposited via proprietary method on the glassy carbon, was used as working electrode (WE). Mercury sulphate electrode (MSE) Hg/Hg2SO4 (Radiometer analytical REF621) with Luggin capillary was used as a reference electrode (RE) and Pt wire was used as a counter electrode (CE). A 100 ml experimental cell (PAR K0064 Jacketed cell bottom) was filled with 0.5 M H2SO4 electrolyte and bubbled with reactant or purge gases (Ar, H2, 2% CO in Ar) at a constant flow rate of 300 sccm /min. In HOR, each electrocatalyst was tested on two parallel, equally prepared samples for three times each. Anodic polarization was performed first at a rotational rate of 2500 rpm using potential sweep rate of 20 mV/s in the potential window from approximately 0 V to 1.2 V vs NHE. Then the anodic polarization curves at six different rotation rates (100, 400, 900, 1600, 2500 and 3600 rpm) were scanned with the same potential sweep rate and the kinetic parameters of the reaction were determined from the Koutecky-Levich plots. CO tolerance of the three catalysts was determined using a CO stripping procedure in which the catalyst was first poisoned by bubbling the electrolyte solution with pure CO and then purged with argon until CO was removed from electrolyte solution. The CO adsorbed on catalyst was then electro-oxidized during potential sweep at a rate of 20 mV/s in the potential window between 0 and 1.2 V vs. NHE. The second potential sweep was performed in order to obtain the baseline. After subtracting the baseline the peak integration was carried out between the two points with potential value equal to zero. In ORR, each electrocatalyst was tested on two parallel, equally prepared samples for three times each. Cathodic polarization was performed first at a rotational rate of 3600 rpm. The current value at 0.76 V vs NHE, and the potentials at 200 μA (E@200μA) and at half of the highest current value (E1/2) were determined. Then, the cathodic polarization curves were scanned at six different rotational rates and the kinetic parameters for ORR were determined from Koutecky-Levich plots at 0.66 V and 0.36 V potentials vs

Preparation, Characterization and Properties of Pt-Cu Co-reduced and Pt-on-Cu Skin Type Bimetallic Carbon-Supported (Vulcan XC72) Electrocatalysts / J. New Mat. Electrochem. Systems

75

a)

Figure 2. TEM of the platinum on copper catalyst (20 wt%, 1:1 a/o). Inset shows platinum (1-2 nm) decorating copper nanoparticles (6-8 nm).

b) 10 8

are generated in the absence of any protecting shell, the size distribution is not unimodal. However, agglomeration is prevented because the particle formation takes place in the presence of Vulcan support. After deposition, such unprotected particles show a reduced mobility on Vulcan during the conditioning process, but limited agglomeration of particles in close to the support surface cannot be excluded [14].

6 4 2 0 1

2 2.7 3 3.3 3.6 3.9 4.2 4.5 6

1

Figure 1. TEM of Cu-Pt co-reduced nanoparticles (20 wt% 1:1 a/o) on Vulcan XC 72. The average particle size is 3.6 nm.

NHE. Current densities are referenced to either the geometric area of the glassy carbon electrode (A = 0.1256 cm2) or the metal loading (mass specific). The industrial catalyst used for comparison was 28.6 wt. % Pt/ Vulcan XC 72 (HP) belonging to lot number A0280203. According to specifications, this catalyst has average Pt crystallite size of 2.5 nm (determined by XRD), Pt specific surface area of 112 m2/g with 47% of electroactive Pt. 3. RESULTS AND DISCUSSION Reduction of platinum and copper salts by lithium triethylborohydride resulted in a uniform distribution of bimetallic metal particles on the support. Since the metal nanoparticles

3.1. CHARACTERIZATION BY TEM, XRD, XPS AND SEM TEM and histogram of the co-reduced Pt-Cu supported catalyst are presented in Figure 1a,b. It is evident from this Fig.1 that the particles have no unimodal distribution (see histogram). The particle sizes, however, range predominantly between 2 and 4 nm. Occasional sintering of the particles giving particles of 5-7 nm in size is also observed. More important, the particles appear uniformly distributed on the support. The reduction of copper salt using lithium triethylborohydride in the absence of Pt generates relatively larger Cu nanoparticles. The particle size range of 2-4 nm observed for Pt-Cu co-reduced nanoparticles obtained by co-reduction indicates that initially formed Pt nanoparticles serve as a “seed” in the course of the coreduction process. This becomes more evident in the case of Pt-on-Cu (skin type) supported catalyst (Fig.2). In this case (where copper salt was first reduced) an inspection of the TEM shows that Vulcan XC72 support is covered with large Cu nanoparticles (6-8 nm). This is in agreement with the literature value quoted for the tetraoctylammonium stabilized copper (8.3 nm) [15]. Since the platinum is laid on the already supported copper nanoparticles, the size distribution is found to be broader than in case of the co-reduced catalyst.


76

a llo y

P t0

u n id e n t if ie d p h a s e [4 - 8 0 2 ] P t / P la tin u m

o n io n t y p e

P t0 P t0

c o -re d u c e d

2 0 .0

3 0 .0

4 0 .0

5 0 .0

6 0 .0

7 0 .0

8 0 .0

2 T h e ta /°

Figure 3. Comparison of the XRD of the 20 wt% co-reduced Pt-Cu/C and Pt on Cu skin /C catalysts.

The relatively broad particle size distribution found in the case of the skin type Pt-on-Cu catalyst is due to very few pure platinum particles (1-2 nm) present on the support along with bimetallic nanoparticles composed of different number of platinum nanoparticles decorating the bigger copper (6-8 nm) nanoparticles (see inset in Fig.2). Thus the sequential reduction pathway obviously leads to broad distribution of particle size. Since no three dimensional picture can be derived from TEM, the “real size” of the skin type Pt-onCu particles cannot be determined. From the twodimensional picture (Fig.2), particle sizes of up to 12 nm were evidenced. A comparative XRD of both the co-reduced (Fig.3, bottom) and the skin type (top) Pt-Cu catalysts are provided in Figure 3. The characteristic diffraction peaks of pure Pt (111) at 39.76 o2Ө, Pt (200) 46.24o 2Ө and Pt (220) at 67.46o 2Ө are slightly shifted to higher 2Ө values indicating the successful reduction of Pt to its metallic state. Modification of its crystal lattice proves alloying with Cu. The broad peaks that appear at 25o 2Ө are attributed to the presence of Vulcan XC 72 carbon as the supporting material and the scattering contribution of the amorphous glass capillary. The XRD patterns show that the skin type Pt-Cu catalyst contains larger metal particles than the co-reduced Pt-Cu catalyst. The fullwidth-half-maximum of the reflections in the skin type catalyst is much smaller whereas for the co-reduced type catalyst the refections are very broad with overlapping of signals. Moreover, a clearly pronounced shoulder on the low angle side of the main peak reveals that the skin type catalyst material contains both reduced Pt0 and Pt-Cu alloyed nanoparticles. From the powder pattern of the co-reduced catalyst sample, a slightly different result is obtained. Very sharp but weak reflections exactly at the reflection positions for pure platinum are positioned on top of the broad reflections of the alloy. This indicates the presence of a small number of larger platinum crystals along with much smaller Pt-Cu nanoparticles. Due to extensive overlapping of the reflections belonging to two different phases, determination of the particle sizes were not possible. In the Pt-on-Cu skin type catalyst, additional signals were

found which so far cannot be unambiguously assigned to individual Pt and Cu or their combinations. It is well understood that Cu and Pt are completely miscible, and at low temperatures three ordered phases (Cu3Pt, CuPt and CuPt3) are found. [16] Considering the synthetic protocol and literature data on the possible alloy formation, it can be expected that the co-reduced Pt-Cu catalyst should exist predominantly in an alloy form. During conditioning process, temperatures of 300oC are applied and under these conditions, non-alloyed inter-metallic Pt-Cu compositions are not thermodynamically stable. During the sequential reduction of Cu and Pt salts (generating skin type catalyst), alloy formation can occur via the diffusion of the Pt atoms into the Cu bulk resulting in a disordered Pt-Cu surface alloy after conditioning procedure [17]. The alloy composition was found to be temperature dependent. The formation of relatively stable alloys in local equilibrium over Cu surface layers is determined by the segregation and re-ordering processes. The presence of pure Pt signals in addition to Pt-Cu alloys clearly indicates Cu3Pt might be a very favourable composition in this bimetallic catalyst type. It should be mentioned here that with powder XRD the crystallite size can be determined if the reflections of x-ray amorphous structures are relatively insignificant. The surface characterization of both catalyst types was performed by XPS. The Pt 4f 7/2 signal for the co-reduced PtCu had a binding energy value of 71.3 eV and a binding energy of 71.4 eV for the skin type Pt-on-Cu catalyst. These species were assigned to Pt(0). Additionally, the peaks were very asymmetric with a shoulder at the high energy side (between 76 and 78 eV), which indicates the presence of a Pt species with a higher oxidation state, e.g. Pt(II). Quantification of the two species was not performed. The quantitative analyses of the surface Cu of both catalyst types are presented in figures 4 and 5. From this analysis, copper in the co-reduced Pt-Cu catalyst exists predominantly in the +2 oxidation state (with Cuo/ Cu2+ = 0.6) while in the case of the skin type catalyst, an equal amount of oxidized and metallic copper was seen at the surface with Cuo/ Cu2+ = 1.0. The quantitative analysis of the elements present in the coreduced sample indicated an element ratio of C: Pt: Cu = 95: 2: 3. The skin type catalyst showed the corresponding elements in the atomic ratio 98: 1: 1. The variation in the amount of oxidized copper (and their relative abundance at the surfaces) can be attributed specifically to the morphological differences of the two catalyst systems. Presence of higher amounts of copper at the surface of the co-reduced sample indicates either formation of Cu3Pt alloys (along with segregated pure platinum as also suggested by XRD) or by assuming segregation of copper in the bimetallic nanoparticle. The presence of equal amounts of Pt and Cu in the skin type catalyst implies incomplete coverage of the Cu surface by Pt. This was also confirmed by both TEM and XRD analysis. SEM analysis (not shown here) with element mapping clearly indicates a uniform distribution of Pt and Cu in both catalyst types. EDX suggested a Pt: Cu atomic ratio of 50:50 a/o in the case of the co-reduced catalyst and a Pt: Cu ratio of 55:45 a/o for the skin type catalyst. This result in comparison with the XPS analysis indicates the surface composi-


Cu0 : Cu2+= 0.6

Cu 2p1/ 2 II

965

960

Cu 2p1/ 2 I

955

Cu 2p3/ 2 species II

Cu 2p3/ 2 species I

934.5 eV, Cu2+

932.1eV,Cu0

Cu sat

950

945

77

Cu sat

940

935

930

925

Bi ndi ng E ner gy [ eV ]

Figure 4. XPS analysis of Cu in the Pt-Cu co-reduced, supported catalyst. The ratio of relative intensities of Cu0 / Cu2+ is 0.6. Cu 2p3/ 2 species I 932.1eV, Cu0

Cu0 : Cu2+=1 Cu 2p3/ 2 species II 934.4 eV, Cu2+ Cu 2p1/ 2 I Cu sat

Cu 2p1/ 2 II

965

960

955

950

Cu sat

945

940

935

930

925

9

Bi ndi ng E ner gy [ eV ]

Figure 5. XPS analysis of Cu in the Pt on Cu supported catalyst. The ratio of relative intensities of Cu0 / Cu2+ is 1. 0.45

I @ 2500 rpm (mA)

0.40 0.35

Pt/C (28.6 wt.% benchmark) Pt/C (20 wt.% recalculated) Pt-on-Cu (20 wt.% total metal) PtCu co-reduced (20 wt.% total metal)

0.30 0.25 0.20

0.0

0.2

0.4

0.6

0.8

1.0

1.2

E vs SHE (V)

Figure 6. Anodic polarization curves for three catalysts. The dashed line represents the current for 28.6 wt.% Pt/Vulcan XC 72 industrial benchmark catalyst; the dash-dot line represents the current for the same catalyst normalized to 20 wt.% of Pt; dotted line represents the current for the 20 wt.% Pt-on-Cu 50:50 a/o catalyst on Vulcan XC 72 carbon and full line represents the current for 20 wt.% Pt-Cu coreduced on Vulcan XC 72 carbon.

tion of metals in the samples is different from their bulk composition. In addition, a smaller amount of chloride was detected (as lithium chloride, formed as a by product of the reaction, could not be completely washed away by THF). 3.2. ELECTROCHEMICAL CHARACTERIZATION Data of HOR polarization curves for different catalysts measured at 2500 rpm were used to compare the catalysts activities in HOR as the currents at 0.1V vs NHE. Examples of such plots are given in Figures 6 for the reference Pt/C catalyst, Pt-Cu co-reduced/C type catalyst and the skin type Pt-on-Cu /C catalyst. The anodic polarization curves obtained at different rpm of RDE for two samples of Pt-Cu co-reduced/C catalyst studied are presented in Figure 7a and 7b. Data of HOR polarization curves for Pt-Cu co-reduced catalyst obtained at six different rotational rates were used to construct the so-called Koutecky-Levich plot: the inverse current density i-1 (cm2/μA) measured in the diffusion limited potential region (i.e. at 0.1V vs NHE) was plotted against inverse of square root of the rotational rate ω-0.5 (rpm-0.5). The KouteckyLevich plot constructed from the data in Figure 7a and 7b is shown in Figure 8. The analogue graphs of anodic polariza-


78

0.55

0.55

0.50

a

3600 rpm

0.50

0.45 0.35 0.30 900 rpm

0.25 0.20 0.15 0.10

2500 rpm

0.40

1600 rpm

I (mA)

I (mA)

0.45

2500 rpm

0.40

0.35

1600 rpm

0.30 900 rpm

0.25

400 rpm

0.20

100 rpm

0.15

400 rpm 100 rpm

0.10

0.05 -0.8

-0.6

-0.4

-0.2

0.0

b

3600 rpm

0.2

0.4

0.05

0.6

-0.8

-0.6

-0.4

E vs. MSE (V)

-0.2

0.0

0.2

0.4

0.6

E vs. MSE (V)

Figure 7. Anodic polarization curves for two samples (a and b) of 20 wt.% Pt-Cu co-reduced on Vulcan XC72 catalyst at six rotational rates. Experimental conditions are described in text.

Sample a Sample b Linear fit

0.6

0.6 0.5

i (cm /mA)

0.5

0.4

2

0.4

2

i (cm /mA)

Sample a Sample b Linear fit

0.7

0.7

-1

-1

0.3

0.3

0.2

0.2

0.1

0.1

0.0 0.00

0.01

0.02 -0.5

ω

0.03

(rpm

-0.5

0.04

0.0 0.00

0.05

-0.5

)

0.50

a

3600 rpm

0.03

(rpm

0.04

0.05

-0.5

)

Figure 10. HOR over 20 wt.% Pt-on-Cu skin type on Vulcan XC72 catalyst. Koutecky-Levich plot. Each experimental point of the two samples (a and b) represents an average over 3 measurements.

b

3600 rpm

0.45

0.45 0.40

2500 rpm

0.35

1600 rpm

0.35

0.30 900 rpm

0.25 0.20

400 rpm

1600 rpm

0.30 0.25

900 rpm

0.20

400 rpm

0.15

0.15 100 rpm

0.10

2500 rpm

0.40

I (mA)

I (mA)

0.02 ω

Figure 8. HOR over 20 wt.% Pt-Cu co-reduced on Vulcan XC72 catalyst. Koutecky-Levich plot. Each experimental point of the two samples (a and b) represents an average over 3 measurements. 0.50

0.01

100 rpm

0.10 0.05

0.05 -0.8

-0.6

-0.4

-0.2

0.0

E vs. MSE (V)

0.2

0.4

0.6

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

E vs. MSE (V)

Figure 9. Anodic polarization curves for two samples (a and b) of 20 wt.% Pt-on-Cu skin type on Vulcan XC72 catalyst at six rotational rates. Experimental conditions are described in text.


79

Table 1. Analysis of Koutecky-Levich plots for HOR over Pt on Vulcan XC72 industrial benchmark, over Pt-Cu co-reduced on Vulcan XC72 and over 20 wt.% Pt-on-Cu on Vulcan XC72 catalysts. Sample

Metal content (wt.%) 28.6 20 20

Pt(ind. bench.) Pt-Cu co-red. Pt-on-Cu

Intercept (cm2/mA) 0.0547 0.0384 0.0395

Slope (cm2/mA) 9.91 13.21 13.80

[email protected] V @ 2500 rpm (mA) 498 414 398

Mass spec. act. (A/gPt) 174.1 257 263

Table 2. Electrocatalytic activity in HOR of catalysts samples as obtained experimentally in two different laboratories. Sample description

TMc content

Pt content

(wt.%)

(wt.%)

L1 Relat. activity

L1 RDEa

i lim

Pt-Cu 50:50 a/o co-reduced Pt-on-Cu 50:50 a/o skin type

L2 Relat. activity

Interlab. average of relat. act.

i lim

(A/g Pt) Pt (ind. benchm.)

L2 RDEb

(A/g Pt)

28.6

28.6

83.6

1.00

174.1

1.00

1.00

20

15.1

129

1.54

257

1.48

1.51±0.042

20

15.1

133

1.59

263

1.51

1.55±0.057

a

WE = glassy carbon (A = 0.1256 cm2); RE = Hg/HgSO4 (SME), 0.682 V vs NHE; CE = Pt wire (high surface area); electrolyte: 0.5M H2SO4; T = 298K; H2 flow rate = 230 mL/min; potential sweep rate = 20 mV/s; potential window: from —0.7 V SHG (-0.039 V NHE) to 0.5 V SHG (+1.161 V NHE) b WE = glassy carbon (A = 0.1256 cm2); RE = Hg/HgSO4 (SME), 0.682 V vs NHE; CE = Pt wire; electrolyte: 0.5M H2SO4; T = 295K; H2 flow rate = 300 mL/min; potential sweep rate = 20 mV/s; potential window: from —0.2 V (-0.0204 V NHE) to 1.0 V (+1.1796 V NHE). c TM = total metal content

Table 3. HOR kinetic parameters for the three electrocatalysts as obtained from Koutecky-Levich plots of RDE kinetic data measured in two different laboratories. Sample Total metal L1 L2 L1 L2 Pt content 1/Interceptb 1/Slope 1/Slope content 1/Intercepta Description

Pt (ind. benchm.) Pt-Cu 50:50 a/o co-reduced Pt-on-Cu 50:50 a/o skin type a,b

(wt.%)

(wt.%)

(mA/cm2)

(mA/cm2)

(mA/cm2)

(mA/cm2)

28.6

28.6

8

18.3

0.0434

0.1009

20

15.1

20

26.0.

0.0325

0.0757

20

15.1

18

25.3

0.0325

0.0725

See footnotes to Table 2

is presented in Figure 10. Figures 8 and 10 demonstrate a very good reproducibility of kinetic measurements on both catalyst samples. The HOR kinetic parameters obtained from KouteckyLevich plots for all three studied catalysts are given in Table 1. The current at 0.1 V vs NHE (obtained potentiodynamically with a potential sweep 20 mV/s and at 2500 rpm rotational rate) is the first parameter used to compare the activity of different catalysts. The other two parameters are intercept and slope obtained from the Koutecky-Levich plot. The

inverse value of the intercept gives the limiting current at infinite rotational rate that corresponds to pure electrontransfer limitation and is proportional to the kinetic constant of the HOR. Consequently: the smaller the intercept, the better are the catalytic performances. The inverse value of the slope is proportional to the boundary-layer diffusionlimited current density, which is determined by mass transport properties at RDE. Therefore, the smaller is the slope, the better are the catalytic performances [18, 19]. As it follows from Figure 6 and Table 1, the Pt-Cu co-reduced cata-


80

Table 4. Analysis of Koutecky-Levich plots for ORR over 20 wt.% Pt-on-Cu on Vulcan XC72 and over 20 wt.% Pt on Vulcan XC72 catalysts. Selected voltage and current values at 3600 rpm are presented. Note that the voltages are given with respect to NHE. E1/2 is the potential corresponding to the half of the highest current value. Levich-Koutecky parameters at 0.66 and at 0.36 V, respectively are given in the last two columns. In all cases, averages of measurements on two samples, each measured three times are given. The corresponding standard deviations are also presented. E1/2 (V)

E@200mA (V)

[email protected] (mA)

0.486±0.005

0.532±0.008

158±24

Sample 20 wt.% Pt-on-Cu

0.513±0.010

20 wt.% Pt

0.561±0.006

236±17

Potential V vs. NHE

Intercept (cm2/mA)

Slope (cm2/mA)

0.66

0.170±0.005

9.31±0.17

0.36

0.050±0.002

9.42±0.08

0.66

0.106±0.007

8.03±0.21

0.36

0.036±0.004

8.14±0.11

0 -100

I/microA

-200 -300 -400 -500 -600

100 rpm 400 rpm 900 rpm 1600 rpm 2500 rpm 3600 rpm

-700 0.0

0.2

0.4

0.6

0.8

1.0

E/V vs. Ag/AgCl

Figure 11. Catodic polarization curves for 20 wt.% Pt-on-Cu on Vulcan XC72 catalyst at six rotational rates. Experimental conditions are described in text.

lyst and the Pt-on-Cu catalyst are much better than the pure Pt catalyst if compared at the same total metal loading of 20 wt.%. The catalytic activity of the same Pt-on-Cu (skin type) catalyst was also compared with Pt-Cu co-reduced catalyst and with the industrial benchmark catalyst in two different laboratories. Table 2 summarizes the results. First, it can be noticed that the relative activities of the Cu containing catalysts are more than 50% higher with respect to the industrial benchmark catalyst. Second, the relative activities obtained in the two laboratories scatter by less than 4%. This gives high confidence that the newly synthesized Pt-Cu coreduced/C and Pt-on-Cu skin type/C catalysts are much better than the industrial benchmark Pt/C catalyst. Among the three catalysts, the Pt-on-Cu skin type/C performs best. This is confirmed by the analysis of the Koutecky-Levich plots given in Table 3. The intercepts of the Cu containing catalysts are lower than the intercepts of the industrial benchmark Pt/C catalyst despite the fact that the Pt loading in the benchmark catalyst is nearly twice that in Cu containing catalysts. Also, the slopes for the Cu containing catalysts are substantially higher than the slope for the industrial benchmark Pt catalyst. This again confirms that the Cu containing catalysts are much better than the industrial benchmark Pt/C catalyst in HOR.

Figure 12. ORR over 20 wt.% Pt-on-Cu on Vulcan XC72 catalyst. Koutecky-Levich plots.

Both Cu containing catalysts have been demonstrated to be more resistant towards CO poisoning than the pure Pt catalyst. The CO striping peak of Pt-Cu co-reduced catalyst is 0.2 V lower than the CO striping peak of pure Pt catalyst and the CO oxidation potential (ignition potential) of Pt-Cu co-reduced catalyst is 0.1 V lower than that for pure Pt catalyst. The Pt-on-Cu catalyst is less CO tolerant than the PtCu co-reduced catalyst. It seems that in Pt-Cu co-reduced catalyst the CO oxidation is promoted by the affinity of Cu towards CO adsorption. The affinity of adjacent Cu for OH may enable the Langmuir-Hinshelwood type mechanism of CO oxidation. The data of ORR polarization curves obtained with the Pton-Cu catalyst at six different rotational rates are illustrated in Figure 11 and the Koutecky-Levich plots at two cathode potentials are represented in Figure 12. The ORR kinetic parameters obtained from Koutecky-Levich plot are given in Table 4 and they are compared with the data obtained on pure 20 wt.% Pt/C catalyst prepared in a similar way. It can be noticed that the Pt-on-Cu catalyst is less active than the pure Pt catalyst for the ORR: .it has a lower current at 0.76 V vs NHE and 3600 rpm rotational rate and a higher intercept and slope than pure Pt catalyst at both potentials, 0.66 V and 0.36 V vs NHE.


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