Ultra-Thin Layer Structured Anodes for Highly Durable Low …Ultra-Thin Layer Structured Anodes for...

Nano Res

1

Ultra-Thin Layer Structured Anodes for Highly Durable

Low-Pt Direct Formic Acid Fuel Cells

Rongyue Wang1,§, Jianguo Liu2,§, Pan Liu3, Xuanxuan Bi1, Xiuling Yan1,4, Wenxin Wang1, Yifei Meng2,

Xingbo Ge1, Mingwei Chen3, and Yi Ding1 ()

Nano Res., Just Accepted Manuscript • DOI 10.1007/s12274-014-0517-9

http://www.thenanoresearch.com on June 16, 2014

© Tsinghua University Press 2014

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Nano Research

DOI 10.1007/s12274-014-0517-9

Ultra-Thin Layer Structured Anodes for Highly

Durable Low-Pt Direct Formic Acid Fuel Cells

Rongyue Wang1, § , Jianguo Liu2, § , Pan Liu3,

Xuanxuan Bi1, Xiuling Yan1,4, Wenxin Wang1, Yifei

Meng2, Xingbo Ge1, Mingwei Chen3, and Yi

Ding1()

1 Center for Advanced Energy Materials & Technology

Research (AEMT), and School of Chemistry and

Chemical Engineering, Shandong University, Jinan

250100, China.

2 Eco-Materials and Renewable Energy Research Center,

Department of Materials Science and Engineering,

National Laboratory of Solid State Microstructures,

Nanjing University, Nanjing 210093, China.

3 WPI Advanced Institute for Materials Research, Tohoku

University, Sendai 980-8577, Japan.

4 Resources and Ecologic Research Institute, School of

Chemistry and Bioscience, Yili Normal University,

Xinjiang 835000, China.

§These authors contribute equally to this work.

By confining highly active nanoengineered catalysts into an

ultra-thin catalyst layer, a dramatic decrease in Pt usage down

to 3 microgram per cm-2 is achieved in direct formic acid fuel

cell anode while maintaining impressive electrode activity,

durability and power performance in both single cells and

multi-cell stacks.

Ultra-Thin Layer Structured Anodes for Highly

Durable Low-Pt Direct Formic Acid Fuel Cells



§

Received: day month year

Revised: day month year

Accepted: day month year

(automatically inserted by

the publisher)

© Tsinghua University Press

and Springer-Verlag Berlin

Heidelberg 2014

KEYWORDS

Direct formic acid fuel

cells, Low-Pt loading,

Core/shell structures,

Nanoporous gold,

Dealloying

ABSTRACT

Direct formic acid fuel cells (DFAFCs) allow highly efficient low temperature

conversion of chemical energy into electricity and are expected to play a vital

role in our future sustainable society. However, the massive precious metal

usage in current membrane electrode assembly (MEA) technology greatly

inhibits their actual applications. Here we demonstrate a new type anodes

constructed by confining highly active nanoengineered catalysts into an

ultra-thin catalyst layer of order 100 nm. Specifically, an atomic layer of

platinum was firstly deposited onto nanoporous gold (NPG) leaf to achieve

high utilization of Pt and easy accessibility of both reactants and electrons to

active sites. Those NPG-Pt core/shell nanostructures were further decorated by

sub-monolayer of Bi to create highly active reaction sites for formic acid

electro-oxidation. Thus obtained layer-structured NPG-Pt-Bi thin films allow a

dramatic decrease in Pt usage down to 3 micrograms per cm-2, while

maintaining very high electrode activity and power performance at sufficiently

low overall precious metal loading. Moreover, this kind of electrode materials

show superior durability during half-year test in actual DFAFCs, with

remarkable resistance to common impurities in formic acid, which together

imply their great potential in actual device instrumentation.

1. Introduction

When powered by liquid fuels, polymer electrolyte

membrane fuel cells [1, 2] show great potential to

support portable electronic devices and off-grid

facilities [3, 4]. Compared with methanol, formic acid

has received increasing attention recently due to its

advantages of being non-flammable, less toxic,

higher electromotive force, and lower fuel crossover

[5-7]. Although some prototype examples have been

demonstrated, the commercialization of direct formic

acid fuel cells (DFAFCs) has been hampered by the

massive usage of precious metals such as Pt [8] and

Pd, which is typically on the order of several

milligrams per square centimeter area at anode (see

Table S1).

To decrease precious metal usage while

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2 Nano Res.

maintaining high enough performance, the specific

parameters contributed to the fuel cell voltage decay

should be understood clearly and avoided in the

membrane electrode assembly (MEA) fabrication [9].

For DFAFC anode, three main factors lead to the fuel

cell voltage decline (over-potential): 1) poisoning

intermediate formation on catalyst surface; 2)

insufficient electron conduction; and 3) limits in

proton transportation or reactants diffusion. It is

widely known that pure Pt catalyst is prone to be

poisoned by CO intermediate in formic acid

electro-oxidation [10] which results in large

over-potential. To solve this problem, a variety of

approaches have been developed to inhibit the

formation of poisoning intermediates and improve

the catalytic activity of Pt [11-13]. Although orders of

magnitude catalytic activity enhancements have been

demonstrated in electrochemical measurements, it is

rarely seen under real fuel cell testing conditions

[14-25]. This is related to the other two factors which

are associated with undesired electrode structure

involved in the current MEA construction

methodology, which is based on carbon supported Pt

or Pt-based alloy nanoparticle catalysts. As shown in

Scheme 1a-c, within a typical MEA, carbon

supported Pt nanoparticles are mixed with Nafion

ionomers and pasted onto diffusion layer to make a

catalyst layer with thickness of order 10 µm [26](up

to 40 µm for most reported DFAFCs). The

contribution of catalysts to fuel cell performance

would be restricted by either electron/reactant

transportation (for catalysts near the membrane side)

or proton transportation (for catalysts near the

diffusion layer side) through the ~40 µm thick

catalyst layer (see Scheme. 1a). The Pt efficiency

could be increased by simply decreasing the

thickness of catalyst layer, but cell performance will

then be sacrificed [19, 20], not to mention that only

20-30% Pt was reported to actually contribute to the

apparent performance in a real fuel cell [27]. These

problems could only be solved by concentrating high

performance nanocatalysts in an ultra-thin electrode

layer. The recently reported 3M’s (Minnesota Mining

and Manufacturing Company) nanostructured thin

film catalysts have shown the advantage of this

concept when functioning as a H2-PEMFC cathode,

although their electrochemical surface areas are

much lower than Pt/C catalysts [28, 29]. Direct

growth of Pt or Pd nanoparticles onto carbon paper

would also contribute to performance improvement.

However, with much larger particle size around

15-25 nm, the precious metal utilization was low [18,

30].

An ideal anode electrode in DFAFCs should

simultaneously fulfill the following desired

properties: high Pt utilization, high catalytic activity

toward formic acid electro-oxidation and high active

site concentration in an ultra-thin catalyst region. In

this work, we present such an electrode based on

nanoporous gold (NPG) leaf supported Pt

nanoarchitectures (see Scheme. 1d-f). With three

dimensional bicontinuous porous structure and ~100

nm order thickness, NPG leaf made by dealloying

could be an ideal catalyst support within which its

inter-connected ligament facilitates the collection and

transportation of electrons from the catalyst surface

to current collectors and the open porous structure

allows the surfaces easily accessible by the reactants

[31-34]. Using electrochemical method and/or

chemical method, one can deposit catalytically active

materials such as Pt on the ligament surface of NPG

uniformly at atomic layer precision [32, 35, 36]. In

principle, the Pt utilization of NPG supported Pt

nanocatalysts could achieve a perfect value of 100%

for a monolayer structure (see Scheme. 1e). Different

from carbon materials on which the deposited Pt

atoms tend to aggregate into nanoparticles to

minimize the surface energy, NPG would stabilize

the Pt adlayers by strong metallic bonds (alloying).

Further decorating the Pt surface with

sub-monolayer Bi, one can greatly improve the

catalytic activity of Pt by changing the reaction paths

or facilitating the removal of CO poisoning

intermediate (see Scheme. 1f). By confining NPG leaf

supported Pt/Bi catalysts into an ultra-thin layer, we

demonstrate a high performance DFAFC anode with

extremely low Pt loading down to 3 micrograms per

cm-2. Both single cell and stack tests demonstrated

that these rationally designed anodes possessed very

high activity and durability.

Scheme 1. Schematic illustration of membrane electrode assembly (MEA) structures. a-c) MEA structure of Pt/C based anode. d-f)

MEA structure of NPG-Pt-Bi based anode. For simplicity, only anode side was highlighted.

2. Experimental section

2.1 Sample preparation

NPG was prepared by dealloying 12 carat AuAg

alloy leaves in concentrated nitric acid for 30 minutes

at room temperature. For comparison purposes,

AuAg alloy foils with larger thickness were also

selected. To control the Pt loading, NPG-Pt samples

could be prepared by depositing Pt onto the

ligament surface of NPG using either chemical

deposition or Cu-UPD (under potential deposition)

mediated deposition process. The detailed

preparation procedures of NPG and NPG-Pt samples

could be found in previous literatures [14, 31, 35].

The NPG-Pt catalysts were then immersed into 5 mM

Bi(III) containing 0.1 M HClO4 solution for 5 min to

allow Bi decoration. After cleaning with ultra pure

water, NPG-Pt-Bi samples were reduced by

sweeping the potential to 0 V from open circuit

potential. The Bi(III) containing solution was

prepared by dissolving proper amount of Bi2O3 (5M)

in 0.1 M HClO4. The electrochemical and

electrocatalytic properties of Pt/C, NPG-Pt, and

NPG-Pt-Bi were characterized by cyclic voltammetry

(CV) in 0.5 M H2SO4 and 0.5 M H2SO4+1 M HCOOH

solution. The catalytic activities of different catalysts

were normalized to Pt loadings by dividing the data

in CV curves by Pt mass on the electrodes,

respectively.

2.2 Fuel cell testing

NPG-Pt-Bi catalysts were brushed onto carbon paper

for use as anodes. Pt/C catalysts were also brushed

onto carbon paper to prepare cathode electrodes.

Both anodes and cathodes were hot pressed onto

each side of Nafion® 115 to make an MEA. The MEA

was fixed into a fuel cell to test the performance. The

fuel cell was activated by CV and tested at 40 oC by

supplying 3 M formic acid to the anode (about 2 ml

min-1) and dry air (about 120 ml min-1) to the cathode.

The outlet formic acid temperature was used as

indicator of the stack temperature. Cell discharging

and polarization tests were conducted on a fuel cell

station equipped with a Kikusui PLZ30 (Japan)

electronic load.

2.3 Characterization

The morphology of NPG-Pt-Bi anode and cross

section of MEA were examined by scanning electron

microscopy (FEI NOVA NanoSEM 230). The

composition and actual precious metal loading of

these nanoelectrodes were determined by ICP-AES

(IRIS Advantage). The chemical states of Au, Pt and

Bi in NPG-Pt and NPG-Pt-Bi samples were analyzed

with a VGESCALAB X-ray photoelectron

spectrometer (XPS), using monochromatized Mg Ka

X-ray as the excitation source, and choosing C 1s

(284.60 eV) as the reference line. The microstructure

characterization was performed with a 200 kV

JEM-2100F electron microscope (JEOL) equipped

with two aberration correctors (CEOS GmbH) for the

image- and probe-forming lens systems and X-ray

energy-dispersive spectroscopy (JED-2300T, JEOL)

for chemical composition analysis.

3. Results and discussion

3.1 Half-cell demonstration

To demonstrate the structural and catalytic

uniqueness of these rationally designed catalysts, we

fabricated an NPG supported monolayer Pt catalyst

using copper under potential deposition method [35]

and further decorated it with sub-monolayer Bi

atoms. The cyclic voltammetric (CV) curve of NPG

supported Pt (NPG-Pt) was shown in Figure 1a.


4 Nano Res.

Although only one monolayer Pt was deposited,

nearly the entire surface of NPG was covered by Pt

as demonstrated by the absence of Au reduction

peak at 1.2 V [35]. This also means almost all Pt

atoms are exposed on the surface thus are accessible

to reactants and current collector simultaneously.

The catalytic activity of NPG-Pt was tested by CV in

0.5 M H2SO4 + 1 M HCOOH mixed solution at room

temperature and the results were presented in Figure

1b, where the data acquired from the commercial

Pt/C catalyst were also listed for comparison. As

shown in Figure 1b, the NPG-Pt catalyst exhibits

similar CV curves as Pt/C catalyst toward formic acid

electro-oxidation. In the forward scan, the NPG-Pt

catalyst shows small oxidation current as the surface

was passivated by CO [37]. However, the Pt mass

specific activity was greatly improved due to much

improved Pt utilization. To further improve the

catalytic activity, NPG-Pt catalyst was decorated

with Bi. And this process was realized by an

irreversible spontaneous adsorption of Bi by simply

immersing the NPG-Pt electrodes into 5 mM Bi(III)

containing 0.1 M HClO4 solution for 5 min. The Bi

coverage could be easily controlled by Bi(III)

concentration and the immersion time [38]. The

successful deposition of Bi was confirmed by CV

curves shown in Figure 1a where the hydrogen

adsorption and desorption charges become evidently

smaller. Different from NPG-Pt and Pt/C catalysts,

NPG-Pt-Bi exhibits a dramatic negative-shift in onset

potential to below 0.2 V and huge anodic peaks at

around 0.6 V as showed in Figure 1b, indicative of

successful inhibition for the formation of poisoning

CO intermediate due to the selection of the direct

reaction path induced by the “third body” effect [14,

15, 38, 39]. That also indicates the continuous Pt

surface has been divided into small Pt ensembles

which are too small for the formation of poisoning

CO intermediate but large enough for the direct

reaction path to occur. By successfully constructing

onto a single platform all three key functions, i.e. (i)

high Pt utilization, (ii) great tolerance to poisoning,

(iii) high conductivity and accessibility, these novel

nanostructures demonstrated very high

electrochemical performance of NPG-Pt-Bi toward

formic acid electro-oxidation, and at 0.6 V the

NPG-Pt-Bi catalyst could exhibit over 570-fold

enhancement in Pt mass specific activity as

compared with the Pt/C catalyst (103.4 vs. 0.18 A

mg-1). The Pt mass specific activity on NPG-Pt-Bi

catalyst is to our knowledge the highest value

reported to date.

Figure 1. Electrochemistry and electrocatalytic activities of three electrodes. (a) Cyclic voltammetric (CV) curves of Pt/C, NPG-Pt, and

NPG-Pt-Bi electrodes in 0.5 M H2SO4. (b) Pt mass specific catalytic activities of Pt/C (enlarged by a factor of ten for better comparison),

NPG-Pt, and NPG-Pt-Bi in 0.5 M H2SO4+1 M HCOOH. The positive potential was restricted to 0.8 V for NPG-Pt-Bi sample to avoid Bi

oxidation. Sweep rate: 50 mV s-1.

3.2 Structural characterization

While half-cell testing demonstrated the tremendous

catalytic activity enhancement for NPG-Pt-Bi

electrode as compared with the commercial Pt/C

catalyst, we are interested in their actual

performance in real fuel cells. To make larger

amount of catalysts, we first mass-produced NPG-Pt

catalysts using the previously developed electroless

plating method [31], on which Bi was further

deposited. The prepared sample was examined by

transmission electron microscopy (TEM), high

resolution TEM (HRTEM), scanning TEM (STEM)

under high-angle annular dark-field (HAADF) and

X-ray photoelectron spectroscopy (XPS). TEM

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5 Nano Res.

images (Figure 2a&b) demonstrate interconnected

ligaments of the NPG-Pt-Bi nanocomposites with

pore size around 20 nm. As shown in Figure 2c, the

entire ligament surface is uniformly covered by a

layer of nanoparticles with size around 1-2 nm. As

the pristine NPG ligament is characterized by a

smooth surface, these small nanoparticles could be

ascribed to Pt. The formation of Bi nanoparticles is

quite unlikely because the loading of Bi is

significantly lower than that of Pt which will be

discussed later. As shown in Figure 2d-f, these Pt

nanoparticles are grown on the surface of NPG in an

epitaxial mode which is consistent with our previous

results [31]. It should be noted that there might exist

an ultra-thin Pt layer between Pt nanoparticles and

NPG surface because electrochemical potential

cycling did not reveal evident Au signals.

Figure 2. Electron micrographs of NPG-Pt-Bi catalysts. (a) TEM image. (b, c) HAADF TEM images. (d-f) HAADF HRTEM images.

To determine the distribution of each element, we

performed EDS mapping on one ligament under

HAADF mode. As shown in Figure 3b, the intensity

of gold is higher in the middle of ligament than the

edge areas which is reasonable as the cross section of

NPG ligament is nearly a circle. However, the

distribution of Pt follows the opposite way where

there are higher intensities along the edge areas. This

is obviously caused by a core/shell type NPG-Pt

structure which could be seen more clearly in the

EDS mapping overlay for Au, Pt and Bi shown in

Figure 3e. The atomic percentage of Au, Pt and Bi is

93.12±0.68, 6.01±0.30 and 0.87±0.18%, respectively.

From the low percentage of Bi, we can conclude that

the nanoparticles on the ligament surface could be

ascribed to Pt. Moreover, the fine EDS mapping

results on one ligament surface also indicate the

correspondence of particle like structures to Pt

signals which are shown in Figure S1. In addition to

the very low percentage of Bi, electrochemical result

shows hydrogen adsorption/desorption on Pt after Bi

deposition which together demonstrate the

sub-monolayer structure of Bi in NPG-Pt-Bi catalyst.

As demonstrated by XPS results (Figures S2&3),

there is no noticeable change for Au and Pt binding

energies after Bi decoration. In contrast to the

metallic state of Au and Pt, Bi species in NPG-Pt-Bi

catalyst was found to be in the oxidation state and

could be ascribed to Bi2O3 [40].

Figure 3. HAADF TEM image of NPG-Pt-Bi catalyst and the corresponding EDS mappings. (a) HAADF TEM image. (b) EDS

mapping for Au. (c) EDS mapping for Pt. (d) EDS mapping for Bi. (e) EDS mapping overlay for Au, Pt and Bi.

3.3 Fuel cell performance

Nanoporous gold based electrodes are robust

enough to be readily incorporated into a fuel cell

MEA by either attaching onto a Nafion® 115

membrane directly or being first dispersed into a

solution and then brushing the mixed slurry with

Nafion® onto carbon paper just like the process to

make an MEA using the traditional Pt/C catalyst.

Both methods resulted in an ultra-thin catalyst layer

in MEA. Figure 4a shows one such MEA using the

second fabrication procedure which is composed of

NPG-Pt-Bi small sheets attached on top of carbon

paper. The porous structure of NPG was effectively

reserved during processing (Figure 4b&c). This

structure ensures the easy accessibility of

catalytically active sites by reactants. And the direct

connection between catalyst sheets and carbon paper

also facilitates the electron transportation. As shown

in Figure 4d, the thickness of the NPG-Pt-Bi based

catalyst layer was estimated to be ~300 nm

depending on the catalyst loading. A closer look into

the catalyst region resolves single, double or triple

NPG-Pt-Bi layers in the cross-sectional SEM images

(Figure S4), indicating the catalyst layer thickness in

the range of 100-500 nm. The respective energy

dispersive X-ray spectroscopy (EDS) results shown

in Figure 4d demonstrate that the anode catalyst was

composed of Au and Pt (the Bi content is too low to

be detectable). In comparison, the catalyst layer with

2.2 mg cm-2 Pt loading is about 40 μm in thickness

for the Pt/C based MEA (Figure 4e), almost two

orders of magnitude thicker.

Unlike the insufficient electron transportation paths

in the traditional catalyst layer, electron

transportation from reaction sites to external circuit

is instantaneous for the ultra-thin NPG-Pt-Bi based

anode, considering that the electron conductivity of

metals is at least 3-4 orders of magnitude higher than

traditional carbon materials. In addition, the open

nanoporosity of the catalyst layer also favors the fast

diffusion of formic acid molecules. This structural

feature was also proved by the fact that galvanic


7 Nano Res.

replacement reaction between Pt ions in solution and

copper monolayer on ligament surface of NPG leaf

could result in very uniform and nearly complete

coverage of entire porous surfaces with Pt monolayer

as aforementioned. It should be noted that

construction of an ultra-thin catalyst layer would not

necessarily lead to high fuel cell performance if the

concentration of active sites is not high enough to

guarantee a low over-potential for formic acid

electro-oxidation. The electrochemical surface areas

(ECSA) of Pt in anodes were evaluated using

hydrogen adsorption/desorption method. With 40

μm thick catalyst layer, the 2.2 mg Pt/C anode

exhibits an ECSA of 1298 cm2 which equals ~3.2 cm2

Pt surface in every 100 nm thick catalyst layer. In

contrast, a 100 nm thick NPG-Pt-Bi anode shows an

ECSA of 11 cm2. In another word, the reaction site

concentration in NPG-Pt-Bi anode is almost 3.4 times

higher than that in Pt/C based anode. Considering

each individual Pt atom in NPG-Pt-Bi is far more

active than that in Pt/C, the fuel cell performance of

NPG-Pt-Bi anode is thus expected to be very high

even at a much lower Pt loading.

Figure 4. (a-c) Plan-view SEM image of catalyst layer prepared with NPG-Pt-Bi. Cross-sectional SEM images and EDS results of

MEAs with (d) NPG-Pt-Bi and (e) Pt/C anode.

Figure 5a shows the current-voltage (C-V) and

current-power (C-P) polarization curves of

NPG-Pt-Bi catalysts with different Pt loadings. An

MEA made with the commercial Pt/C catalyst (2.2

mg Pt cm-2) was also presented for comparison.

Typically, the NPG-Pt-Bi based anodes showed both

higher open circuit voltage and maximum power

output, although the Pt loading is orders of

magnitude lower. For example, the sample with Pt

loading as low as 3 micrograms (44 mW cm-2)

already exhibited larger maximum power than the

Pt/C catalyst (40 mW cm-2). The maximum power

output for NPG-Pt-Bi 0.02 mg catalyst is ~80 mW

cm-2, which is twice of that for Pt/C catalyst even the

catalyst layer is only 100 nm thick for this specific

sample. The apparent fuel cell power performance

also compares favorably to most literature results,

although here the Pt loading is typically two orders

of magnitude lower (Table S1). Further increasing the

Pt loading didn’t result in apparent increase in fuel

cell performance which may be caused by the

saturation of active sites in the catalyst layer (Figure

S5). The apparent maximum powers were

normalized to the amount of Pt used on anodes and

the results were shown in Figure 5b. The Pt loading

specific maximum power densities of NPG-Pt-Bi

samples are in the range of 4-14.7 W mg-1 with the

highest value for the catalyst with 3 micrograms Pt.

In contrast, the data in literatures are typically in the


8 Nano Res.

range of 10-280 mW mg-1 (the fuel cell performance

varies under different conditions, see Figure S6 for

example), indicating two orders of magnitude

improvement in specific power efficiency for these

new electrodes. Considering the relatively limited Pt

reserve on our planet, the results of NPG-Pt-Bi

catalysts are of considerable significance since their

anode efficiency even approaches those of hydrogen

fuel cells which are typically below 20 W mg-1 [41].

Even when the mass of NPG substrate was

considered, the anode specific power density

normalized to the total precious metal loading

reaches 667 mW mg-1 for the NPG-Pt-Bi (0.02 mg Pt +

0.1 mg Au) catalyst which is still much higher than

the reported ones. The fuel cell performance can also

be boosted to over 160 mW cm-2 by feeding dry

oxygen as the oxidant (Figure S6). We are now

further optimizing its performance under various

conditions and also trying to replace the gold

substrate by using less precious nanoporous metals.

Figure 5. Single-cell performances of NPG-Pt-Bi and Pt/C based MEAs. (a) C-V and C-P polarization curves for Pt/C catalyst (red)

with 2.2 mg Pt loading and NPG-Pt-Bi samples with Pt loading of 0.003, 0.013, and 0.02 mg per cm2 in the anode. The actual Pt loading

in NPG-Pt-Bi catalyst was measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES). (b) Metal loadings (Pt or

Pd) and mass specific maximum power densities of literature results listed in Table S1 and that of NPG-Pt-Bi catalysts. (c) Durability

test of NPG-Pt-Bi 0.09 mg catalysts. The fuel cell was first discharged at 100 mA cm-2 for 10 h, after which the fuel cell was stored in

air and further tested in the following 6 months, with each test for more than 4 h. (d) C-V and C-P polarization curves of NPG-Pt-Bi

0.09 mg anode using 3M formic acid without (red squares) or with 100 ppm CH3OH (green circles), HCOOCH3 (magenta triangles),

and CH3COOH (blue triangles) impurities. Fuel cells were operated at 40 (a, b, c) or 50 oC (d), using the same Pt/C (2.2 mg cm-2 Pt)

cathode, with formic acid (3 M) as the fuel and dry air as the oxidant.

The long-term stability of NPG-Pt-Bi catalysts was

tested in a fuel cell with Pt loading of 0.09 mg cm-2.

Operating the fuel cell at 100 mA cm-2 for 10 h did

not see obvious voltage decline. After discharging

test, the cell was stored at room temperature in air.

Repeatedly operating this cell in the following six

months (each for more than 4 h) always generated a

power density around 70 mW cm-2, indicating the

excellent durability of this catalyst (Figure 5c). The

influence of possible impurities in formic acid was

also tested. The maximum power decreased very

slightly after adding 100 ppm contaminants (CH3OH,

HCOOCH3 or CH3COOH) into formic acid solution

(Figure 5d). When fuel cell operated at constant

current density of 100 mA cm-2, the negative effects

of CH3OH and HCOOCH3 are almost negligible

(Figure S7a). While it seems the influence of

CH3COOH is a little larger than other two

contaminants, the potential drop is within 10% of its

initial potential (Figure S7a). More importantly, even


9 Nano Res.

the impurity concentrations increased to 500 ppm,

the fuel cells still showed stable voltage-time curves

at constant current density of 100 mA cm-2 and the

potential drop was within 12% of the initial potential

(Figure S7c). These results demonstrated the

NPG-Pt-Bi catalyst is robust enough to be fueled by

common formic acid (contaminant concentrations

are often in a range of 15-180 ppm) [42]. The stability

of NPG-Pt-Bi catalyst could be highlighted by

comparing with Pd catalyst. Although Pd/C catalyst

with a Pd loading of 3 mg cm-2 has similar initial

catalytic activity, but the performance declined

quickly even fueled with pure formic acid without

any contaminants (see Figures S8&S9).

3.4 Stack performance

Featured by high activity, superior stability and ultra

low Pt loading, NPG-Pt-Bi catalysts are highly

promising for real fuel cells. To demonstrate the

possibility, we prepared a stack with ten cells (each

cell has an area of 68 cm2, see insert in Figure 6b)

and investigated the performance. Figure 6a shows

the C-V and C-P polarization curves of this stack. The

maximum power density achieved 40.9 W at a

current of 12.9 A, indicating an area specific power

density of 85 mW cm-2 which is a little higher than

that in a single cell. This may be caused by the slight

increase in temperature during stack operation.

When the fuel cell stack was operated at 4.8 A, there

was almost no voltage decline (around 4.5 V) in 10 h

which again proved its high stability. Moreover, the

voltage of each individual cell varied very little

around 0.5 V (Figure 6b&c), demonstrating the

potential of making even larger fuel cells in a

repeatable manner.

Figure 6. Stack performance of NPG-Pt-Bi based MEA. (a) C-V and C-P polarization curves for a 10-cell stack (each cell: 6 cm 8 cm,

with Pt loading of 0.09 mg cm-2) using NPG-Pt-Bi catalysts as anodes. (b) Voltage-time curves at a constant current density of 100 mA

cm-2. Insert is a digital photo of this stack. (c) The voltages of single cells during the constant current discharging at 100 mA cm-2. The

fuel cell stack was operated at room temperature, with formic acid (3 M) as the fuel and dry air as the oxidant, the flow rate of formic

acid and air was 10 ml min-1 and 2 L min-1, respectively. (d) Current–time curves of DFAFC stack under constant voltage discharging at

5 V with 300 ml formic acid solution at different concentration.

Figure 6d shows the variation of discharging current

of DFAFC stack at a constant voltage (5 V) using 300

ml formic acid solution of different concentrations

from 1.0 to 5.0 M. The discharging current was found

to increase rapidly in the early stages for all solutions,

reach a peak value and decrease gradually toward

zero as the formic acid in the fuel tank was

consumed. It was found that the transient stack

operating temperature varied with a trend similar to

the transient discharging current. This behavior


10 Nano Res.

indicates that the current differences among the

different concentrations of formic acid can be

attributed to the differences in the temperature

variation and mixed potential caused by the different

rates of formic acid crossover.

To investigate the fuel utilization, we define the fuel

efficiency as:

η= Discharging capacity (Ah)/Theoretical

discharging capacity (Ah)

In addition, the energy efficiency of DFAFC can be

defined as:

ξ= Discharging energy(Wh)/Theoretical discharging

energy(Wh)

The fuel efficiencies of the DFAFC stack at 1.0, 2.0,

3.0, 4.0, and 5.0 M are then calculated to be 78, 65, 64,

48, and 39%, respectively, which are much higher

than that of DMFC [43] due to its lower formic acid

crossover. The corresponding energy efficiencies are

26, 22, 21, 16, and 13%, respectively, comparable with

reported values [4]. The above results clearly

demonstrated that these rationally designed low Pt

loading NPG-Pt-Bi catalysts could be used in real

fuel cells.

4. Conclusions

In conclusion, using nanoporous gold leaves as

substrates, we designed and fabricated high

performance anode electrocatalysts for formic acid

fuel cells with ultra low Pt loading and excellent

stability. By concentrating highly active catalysts in

an ultra-thin catalyst layer, the electron

transportation, and reactant diffusion in the

electrode were optimized, which resulted in

unparalleled performance in DFAFCs. With the

development of advanced membrane technology and

poisoning-resistant cathode materials, the

commercialization of various PEMFCs seems on a

steady way to becoming reality, within which

DFAFCs may become an attractive portable power

source. Guiding by the efficiency loss contribution

rules in various clean-energy technologies, the

electrode design strategy demonstrated in this work

may shine light on the development of new

generation, high performance electrodes used for

example in metal-air batteries.

Acknowledgements

This work was sponsored by the National 973

(2012CB932800) Program Project of China, and the

National Science Foundation of China (51171092,

20906045). Y. D. is a Tai-Shan Scholar supported by

the Fundamental Research Funds of Shandong

University. The assistance of Shangling Tian,

Wenliang Zhang, and Zezhong Li in fuel cell testing

is gratefully acknowledged.

Electronic Supplementary Material:

Supplementary material (DFAFC performances in

literatures; EDS mappings of NPG-Pt-Bi catalyst;

XPS results of NPG-Pt and NPG-Pt-Bi samples; SEM

images of NPG-Pt-Bi based catalyst layer; DFAFC

performances of NPG-Pt-Bi and Pd based anodes) is

available in the online version of this article at

http://dx.doi.org/10.1007/s12274-***-****-*

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Electronic Supplementary Material

Ultra-Thin Layer Structured Anodes for Highly Durable

Low-Pt Direct Formic Acid Fuel Cells



§

Supporting information to DOI 10.1007/s12274-****-****-* (automatically inserted by the publisher)


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Table S1. DFAFC performances and detailed parameters in literatures. *It is not of our intention to show all

literature results in this table and only most related data were listed. N/A: data not available. RT: room

temperature.


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Figure S1. HAADF TEM image of NPG-Pt-Bi catalyst and the corresponding EDS mappings. (a) HAADF TEM

image. (b) EDS mapping for Au. (c) EDS mapping for Pt. (d) EDS mapping for Bi. (e) EDS mapping overlay for

Au, Pt and Bi.


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Figure S2. XPS results of NPG-Pt and NPG-Pt-Bi samples.


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Figure S3. XPS result of NPG-Pt-Bi sample.


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Figure S4. Cross sectional SEM images of NPG-Pt-Bi based catalyst layer.


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Figure S5. Current–voltage and current-power polarization curves for NPG-Pt-Bi 0.09 mg catalyst. The fuel cell

was operated at 40 oC, using the same Pt/C (2.2 mg cm-1 Pt) cathode, with formic acid (3 M) as the fuel and dry

air as the oxidant.


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Figure S6. Current–voltage and current-power polarization curves for NPG-Pt-Bi 0.09 mg catalyst. The fuel cell


oxygen as the oxidant.


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Figure S7. a and c, Voltage-time curves at a constant current density of 100 mA cm-2 with impurities of CH3OH,

HCOOCH3, and CH3COOH of 100 and 500 ppm, respectively. b, Current–voltage and current-power

polarization curves with impurities of CH3OH, HCOOCH3, and CH3COOH of 500 ppm. Fuel cells were

operated at 50 oC, using the same Pt/C (2.2 mg cm-2 Pt) cathode, with formic acid (3 M) as the fuel and dry air

as the oxidant.


Nano Res.

Figure S8. Current–voltage and current-power polarization curves for Pd 3 mg anode catalyst. The fuel cell was

operated at 40 oC, using the same Pt/C (2.2 mg cm-2 Pt) cathode, with formic acid (3 M) as the fuel and dry air

as the oxidant.


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Figure S9. Voltage-time curves for Pd 3 mg catalyst at constant current density of 100 mA cm-2. The fuel cell


air as the oxidant.

.

Address correspondence to Yi Ding, email: [email protected]

mailto:[email protected]

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