Supplementary data

Formic acid electrooxidation activity of Pt and Pt/Au catalysts: effects of

surface physical properties and irreversible adsorption of Bi

Jae Kwang Yooa, Mihwa Choib, Seugran Yangb, Bonggeun Shongc, Hee-Suk Chungd, Youngku Sohna, and Choong Kyun Rheea,*

aDepartment of Chemistry, Chungnam National University, Daejeon, 34134, Korea

bCreative Future Laboratory, KEPCO Research Institute, Daejeon, 34056, Korea

cDepartment of Chemical Engineering, Hongik University, Seoul 04066, KoreadJeonju Center, Korea Basic Science Institute, Jeonju, 54907, Korea

*Corresponding author E-mail address: ckrhee@cnu.ac.kr (C.K. Rhee).

1

1. TEM micrographs

Fig. S1. Typical TEM micrographs of (a) Au NP and (b) commercial Pt NP.

2. Surface properties of nanoparticles of different sizes

In evaluation of the results acquired using nanoparticles of different sizes, the size difference

should be carefully considered. For example, the surface properties such as surface area per

mass (m2/g) and mass-based performance (W/gcatalyst) should be compared under a condition

of same nanoparticle size because larger nanoparticles have lower surface areas per mass.

Therefore, such surface properties measured with one of the being-evaluated nanoparticle are

required to convert to the values of a hypothetical nanoparticle whose size is equivalent to

that of the other nanoparticle being compared with. The aim of this section is to demonstrate

conversion of such properties of a nanoparticle of one size to that of a nanoparticle of a

different size but identical shape when the total mass is constant.

The particle number ratio of nanoparticle A and B differing in size are described under a

condition of a constant total mass (i.e., mtotal = constant) as below:

(nB/nA) = [mtotal/{ρBVB}]/[mtotal/{ρAVA }] = (ρAVA)/(ρBVB), where n, V, and ρ are the number of

nanoparticles, the volume of a nanoparticle, and the density of constituent element,

respectively.

On the other hand, the total surface area ratio (SB/SA) is expressed with the number of

nanoparticles and the surface area of a nanoparticle as follows: SB/SA = (nBsB)/(nAsA) = (nB/nA)2

(sB/sA) = {(ρA×VA)/(ρB×VB)}(sB/sA) = (ρAVA/sA)/(ρBVB/sB), where the small letter s is the

surface area of a nanoparticle. In general, the ratio of volume to surface area, i.e., V/s, can be

expressed kR, where R is the radius of a nanoparticle and k is a constant depending on the

shape of nanoparticle. For instance, if a nanoparticle is spherical, k is 1/3 because V = 4πR3/3

and s = 4πR2. Another example is octahedron in which k = 1/3√3 because V=4R3/3 and

s=4√3R2. Therefore, SB/SA = (ρAkARA)/(ρBkBRB).

In Fig. 4(b) in the main text, the Au nanoparticle took a shape of icosahedron, so that the

Au nanoparticles are assumed to be icosahedrons regardless of their sizes. On the other hand,

the Pt nanoparticle in the commercial Pt NP was ill-defined to specify its shape (Fig. 4(a));

however, the shapes of the real Pt nanoparticles employed in this work are presumed to be

icosahedrons for simplicity in calculation. In other word, if all the nanoparticles under

consideration have an identical shape, the final equation concerning the total surface area

ratio becomes simpler by cancelling out the constant k’s as follows: SB/SA = (ρARA)/(ρBRB) =

(ρADA)/(ρBDB), where D is the size of nanoparticles measured using TEM.

Thus, the above relation allows conversion of a surface property/mass of nanoparticles of

one element and one size to that of another element and another size under the conditions that

the total mass is constant and that the shape of nanoparticles are identical. The condition that

the total mass (mtotal) of metal is constant is met, because the actual metal loading amounts of

Pt NPs and Au NPs are 10 wt%. Only the assumption is that all the nanoparticles under

consideration take an identical shape.

Case 1: Surface areas of Au and Pt NPs

The surface areas per mass of Au NP and Pt NP are measured to be 42 and 108 m2/g. Thus,

the measured ratio of surface areas (SPt/SAu) is 2.6 (= 108 (m2/g)/42 (m2/g)). On the other

3

hand, a ratio of surface areas per mass was calculated to be 2.2 by inserting the densities of

Au and Pt (ρAu= 19.30 g/cm3 and ρPt= 21.45 g/cm3) and the measured diameters (DAu= 5.2 nm

and DPt = 2.1 nm) into the above relation. Considering the uncertainties in measuring surface

area and the assumption of icosahedron Pt nanoparticles, the measured ratio is reasonably

close to the calculated one.

Case 2: Bi/Pt atomic ratios of Bi/Pt NPs determined using ICP-AES

Fig. S2. Conversion of the Bi/Pt atomic ratios of the real Bi/Pt NPs (2.1 nm) (denoted Pt NP') to those of the hypothetical Bi/Pt NPs (5.2 nm) (designated Pt NP).

In comparison of deposited Bi coverages of Bi/Pt/Au NPs and Bi/Pt NPs (equivalent to Bi/Pt

atomic ratios) determined using ICP-AES, the different sizes of Au NPs and Pt NPs should be

considered. Because the whole catalysts were digested into solution phase, the measured

Bi/Pt atomic ratio is equivalent to the mass ratio of the Bi on surface to the whole Pt. Namely

the ratio is surface amount of Bi per Pt mass. Under a condition that the total mass of Pt in Pt

NP is kept constant, the Bi/Pt ratio of a hypothetical Pt NP of 5.2 nm should be lower than

that of the real Pt NP of 2.1 nm. In comparison of Bi/Pt atomic ratios of Bi/Pt/Au NPs and

4

Bi/Pt NPs on the same particle size base, the values of real Bi/Pt NPs (diameter = 2.1 nm)

should be converted to those of hypothetical Bi/Pt NPs (diameter = 5.2 nm) which are

comparable to Au NP in size. The relation between the Bi/Pt atomic ratios of real Pt NPs

((Bi/Pt)2.1 nm) and hypothetical Pt NPs ((Bi/Pt)5.2 nm) is (Bi/Pt)5.2 nm = (Bi/Pt)2.1 nm/2.5 after the

following manipulation:

(Bi/Pt)5.2 nm/(Bi/Pt)2.1 nm SPt NP,5.2 nm/SPt NP, 2.1 nm = (ρPt/ρPt)(D2.1 nm/D5.2 nm) = 2.1 (nm)/5.2 (nm) =

1/2.5.

Fig. S2 demonstrates conversion of the Bi/Pt atomic ratios of the real Bi/Pt NPs

(denoted Pt NP') to those of the hypothetical Bi/Pt NPs (designated Pt NP).

Case 3: Catalyst-mass performances of formic acid oxidation

In this work the catalytic performances of various Pt surfaces in single-cell tests are discussed

in terms of total catalyst mass (gcatalyst), surface area (cm2) and Pt amount (gPt). The catalytic

performance of a catalyst of interest is defined as enhancement factor, i.e., a power ratio of

the catalyst referring to plain Pt NP in the three different terms. Especially, in comparing the

catalyst-mass-base enhancement factors the difference in the sizes of Pt NPs and Au NPs

should be accommodated: the measured catalyst-mass-base performances of real Au NP

substrate (diameter = 5.2 nm) would be smaller than those of hypothetical Au NP substrate

(diameter = 2.1 nm) so that the catalyst-mass-base performances of the catalysts using real

Au NP substrates should be corrected to be compared with those of Pt NP substrates. The

catalyst-mass-base performances (EFAu NP,5.2 nm) on Au NP substrates are to those on

hypothetical Au NP (EFAu NP,2.1 nm) is carried out by EFAu NP,2.1 nm = EFAu NP,5.2 nm×2.5 as

manipulated as follows: EF5.2 nm/EF2.1 nm S5.2 nm/S2.1 nm = (ρAu/ρAu)(D2.1 nm/D5.2 nm) = 2.1 (nm)/5.2

(nm) = 1/2.5.

5

3. EDS images of Bi(5.0)/Pt(3)/Au NP

Fig. S3. EDS images of Bi(5.0)/Pt(3)/Au NP: (a) TEM image of Bi(5.0)/Pt(3)/Au NP, and (b) EDS images of C, Au, Pt and Bi in the area boxed in (a).

4. Determination of Bi and Pt coverages

Fig. S4. Schematic illustration of adsorbed Bi atoms on Pt substrates: (a) Pt NP and (b) Pt deposits on Au NP.

Fig. S4 schematically illustrates adsorbed Bi atoms on Pt NP and Pt deposits on Au NP.

The coverages of Bi in the main text are defined in two points of view. One is ‘deposited Bi

coverage’ standing for a number ratio of all the deposited Bi atoms to the substrate atoms

within the sampling depth of employed analytical technique. The other is ‘surface Bi

6

coverage’ referring to a number ratio of Bi atoms contacting with an electrolyte to the

substrate surface atoms. Thus, the deposited coverage relates to all the deposited atoms

described with the filled and open circles in Fig. S4, while the surface coverage concerns

only the surface atoms presented with the filled circles.

Deposited Bi coverages of Bi/Pt NPs and Bi/Pt/Au NPs are Bi/Pt atomic ratios derived

from XPS and ICP-AES measurements. However, the two methods defer from each other in

terms of sampling depth. In XPS measurements, large fraction of the photoelectrons

generated from deposited species on the surfaces (Bi and Pt) can be assumed to be detected,

while the photoelectrons from only a few surface layers of the substrate would reach the

detector. In ICP-AES analysis, on the other hand, the whole sample is dissolved, and thus

providing virtually infinite sampling depth of the substrate. For an identical sample, thus, the

deposited Bi coverage (i.e., the Bi/Pt atomic ratio) derived from XPS would be higher than

that from ICP-AES, because Bi exists mostly on the top of the catalyst surface. Indeed, Fig. 3

in the main text demonstrates this argument.

The strategies estimating surface Bi coverages of Bi/Pt NPs and Bi/Pt/Au NPs defer from

each other.

Surface Bi coverages of Bi/Pt NPs are estimated by measuring the hydrogen stripping

charges. The hydrogen charge of Pt NP without Bi (QH, w/o Bi) decreases upon adsorption of Bi

(QH, w/ Bi) due to a blockage of three Pt atoms per one adsorbed Bi atom. Thus, the surface Bi

coverage is calculated as follows: (QH, wo/ Bi – QH, w/ Bi)/(3QH, wo/ Bi).

7

On the other hand, a different method is employed for determination of surface Bi

coverages of Bi/Pt/Au NPs. A huge background capacitive current of the carbon support

obscures analysis of small current decrements in hydrogen stripping charge after adsorption

of Bi on Pt/Au NP. Instead, a linear relationship between surface Bi coverages and deposited

Bi coverages (i.e., the atomic ratio observed with XPS) on Bi/Pt NPs is available as shown in

Fig. S5. Using the calibration line (the dashed line in Fig. S5), assuming the XPS cross-

section of Bi on Bi/Pt NPs or Bi/Pt/Au NPs are same, the XPS Bi/Pt atomic ratios of

Fig. S5. A plot of XPS atomic ratio of Bi/Pt as a function of surface Bi coverage. The dashed line is a calibration line using the XPS atomic ratios and Bi coverages of Bi/Pt NPs, and is utilized to estimate surface Bi coverages of Bi/Pt/Au NPs.

Bi/Pt/Au NPs are converted to surface Bi coverages. It is noted that the surface Bi coverages

estimated as above correspond to Bi/Pt ratios, not Bi/Au ratios.

Surface Pt coverages of Pt/Au NPs are determined with the stripping charges of CO

adsorbed on Pt deposits. The reason for using CO stripping charge (QCO) instead of hydrogen

charge is that the hydrogen charge overlaps with the background capacitive charge of the

8

carbon support. The number of Au atoms at the surface is estimated employing the reduction

charge of Au oxides (QAu) formed by voltammetric scan to 1.38 V with 10 mV/sec. Then

surface Pt coverage of a Pt/Au NP is a ratio of CO charge to Au reduction charge considering

corresponding conversion factors: (QCO/420 μC/cm2)/(QAu/400 μC/cm2).

Table S1. Surface Pt and Bi coverages of Pt/Au NPs and Bi/Pt/Au NPs. The surface Bi coverages are presented in Fig. 3(b).

substrate method surface Pt coverage

Au NP deposition number 1 2 3

0.42±0.03 0.59±0.04 0.72±0.09 CO charge

surface Bi coverage[Bi3+] (mM) 0.50 2.0 5.0

Pt NP 0.16±0.01 0.26±0.02 0.33±0.01 H charge

Pt(1)/Au NP 0.37 0.39 0.62 XPSPt(2)/Au NP 0.21 0.34 0.52 XPS

Pt(3)/Au NP 0.17 0.28 0.37 XPS

5. Unit cells used in DFT calculations

Fig. S6. Unit cells used to build the slab model surfaces. 2×2 unit cells were used for (111) and (100) surfaces with 4 atomic layers of Pt or Au and more than 15 Å of vacuum separating the slabs. The grey, yellow and purple balls represent atoms of Pt, Au and Bi, respectively.

9

6. Cyclic voltammograms of Bi/Pt NPs

Fig. S7. Cyclic voltammograms of Bi/Pt NPs in 0.50 M H2SO4 solution. Scan rate: 10 mV sec-1.

7. Cyclic voltammograms of Pt/Au NPs

Fig. S8. Cyclic voltammograms of Pt/Au NP in 0.50 M H2SO4 solution. Scan rate: 10 mV sec-

1.

10

8. Cyclic voltammograms of Bi/Pt/Au NPs

Fig. S9. Typical cyclic voltammograms of Bi(5.0)/Pt/Au NPs in 0.50 M H2SO4 solution. Scan rate: 10 mV sec-1.

9. Typical XPS spectra of Bi/Pt/Au NP

Fig. S10. XPS spectra of Bi(5.0)/Pt(3)/Au NP.

10. Curve-fitted voltammograms

11

Curve-fitting of the voltammograms shown in Fig. S11 was performed using a program

(Fityk 0.8.6) available at the public site of http://fityk.nieto.pl/. Curve-fitting started with two

peaks whose potentials are ~0.4 V for dehydrogenation path and ~0.6 V for oxidized Bi path

for Bi/Pt NPs; and ~0.2 V for dehydrogenation path and ~0.6 V for oxidized Bi path for

Bi/Pt/Au NPs. The respective peak potentials for dehydrogenation path on Bi/Pt NPs and

Bi/Pt/Au NPs were estimated from the voltammograms of Bi/Pt NPs and Bi/Pt/Au NPs

whose amounts of Bi were far low not to show any currents via oxidized Bi path. For best

fittings, the shapes and tilting of the peak components were adjusted; but the potentials and

widths of the component peaks were not fixed during regressions.

Fig. S11. Curve-fitted voltammograms of (a) the positive-going trace of Bi(2.0)/Pt NP (Fig. 4(a)), (b) the positive-going trace of Bi(5.0)/Pt(2)/Au NP (Fig. 5(d)), and (c) the negative-going trace of Bi(5.0)/Pt(2)/Au NP (Fig. 5(d)). The purpose of curve fitting is to distinguish the currents of dehydrogenation (blue dotted lines) and oxidized Bi path (pink dashed lines).

12

11. Enhancement factors of Bi/Pt NPs and Bi/Pt/Au NPs in half cell tests

Table S2. Formic acid oxidation peak currents of negative-going traces of the first voltammetric scan in a solution of 2.0 M formic acid and 0.50 M H2SO4 when the positive-going scan limit is 0.4 V at the scan rate of 10 mV/sec. The enhancement factor is defined as a ratio of a formic acid oxidation current observed during negative-going excursion (following a positive-going scan to 0.4 V) to that acquired on plain Pt NP.

Current per substrate surface areasurface Bi/Pt NPs Bi/Pt/Au NPs

Pt deposition number 1 2 3

[Bi3+] (mM) current(mA/cm2) EF* current

(mA/cm2) EF* current(mA/cm2) EF* current

(mA/cm2) EF*

0.0 0.61±0.05 1.00 2.67±0.53 4.37 2.85±0.64 4.67 1.89±0.36 3.050.5 5.75±0.59 9.44 2.86±0.40 4.69 3.98±0.88 6.52 3.23±0.57 5.302.0 8.30±0.78 13.60 3.60±0.50 5.90 4.72±1.29 7.74 4.01±0.39 6.575.0 6.25±0.87 10.25 2.37±0.00 3.89 3.69±0.27 6.05 3.98±0.40 6.52

*EF = enhancement factor

Current per Pt surface area

*EF = enhancement factor

Current per Pt mass

*EF = enhancement factor

13

surface Bi/Pt NPs Bi/Pt/Au NPs

Pt deposition number 1 2 3

[Bi3+] (mM) current(A/m2) EF* current

(A/m2) EF* current(A/m2) EF* current

(A/m2) EF*

0.0 6.1 1.0 63.6 10.4 48.3 7.9 26.3 4.30.5 57.5 9.4 68.1 11.2 67.5 11.1 44.9 7.42.0 83.0 13.6 85.7 14.1 80.0 13.1 55.7 9.15.0 62.5 10.3 56.4 9.2 62.5 10.2 55.3 9.1

surface Bi/Pt NPs Bi/Pt/Au NPs

Pt deposition number 1 2 3

[Bi3+] (mM) current(A/mg) EF* current

(A/mg ) EF* current(A/mg ) EF* current

(A/mg ) EF*

0.0 0.66 1.0 14.6 22.1 8.1 12.3 3.7 5.60.5 6.21 9.4 15.1 22.9 11.4 17.3 8.4 12.72.0 8.96 13.6 24.0 36.4 14.7 22.3 9.5 14.45.0 6.75 10.3 12.6 19.1 11.0 16.7 10.6 16.1

12. Enhancement factors of Bi/Pt NPs and Bi/Pt/Au NPs in single cell tests

Table S3. Formic acid oxidation catalytic performances in terms of catalyst mass, substratesurface area and Pt mass derived from single cell tests. Enhancement factor in single cell test is defined as a ratio of maximum power density referring to Pt NP.

catalystcatalyst mass substrate surface area Pt mass

W/gcatalyst EF* mW/cm2 EF* W/gPt EF*

Pt NP 23.00 1.00 0.2115 1.00 230 1.00Bi(2.0)/Pt NP 44.67 1.94 0.4108 1.94 472.3 2.05Bi(5.0)/Pt NP 24.00 1.04 0.2208 1.04 200.6 0.87Pt(1)/Au NP 25.67 1.12 0.6101 2.88 2700.6 11.74Pt(2)/Au NP 37.33 1.62 0.8875 4.19 2123.4 9.23Pt(3)/Au NP 29.33 1.28 0.6972 3.30 1620.3 7.04Bi(0.5)/Pt(2)/Au NP 33.67 1.46 0.8003 3.78 1822.8 7.93Bi(2.0)/Pt(2)/Au NP 26.33 1.14 0.6259 2.96 1752.4 7.62Bi(2.0)/Pt(3)/Au NP 41.00 1.78 0.9746 4.61 1726.7 7.51Bi(5.0)/Pt(3)/Au NP 31.33 1.36 0.7448 3.52 1563.5 6.80

*EF = enhancement factor

13. Open circuit potentials and cell voltages of Bi/Pt NPs and Bi/Pt/Au NPs in single cell tests

Table S4. Open circuit potentials and cell voltages at 100 mA/cm2 of Bi/Pt NPs and Bi/Pt/Au NPs in single-cell tests.

catalyst open circuit potential (V) cell voltage (V)at 100 mA/cm2

Pt NP 0.736 0.388

Bi(2.0)/Pt NP 0.838 0.688

Bi(5.0)/Pt NP 0.794 0.543

Pt(1)/Au NP 0.793 0.380

Pt(2)/Au NP 0.813 0.432

Pt(3)/Au NP 0.733 0.376Bi(0.5)/Pt(2)/Au NP 0.815 0.479

Bi(2.0)/Pt(2)/Au NP 0.808 0.489

Bi(2.0)/Pt(3)/Au NP 0.855 0.583

Bi(5.0)/Pt(3)/Au NP 0.814 0.539

14