Small Addition of Boron in Palladium Catalyst, Big Improvement in

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Small Addition of Boron in Palladium Catalyst, Big Improvement in Fuel Cell’s Performance: What may Interfacial Spectroelectrochemistry Tell? Kun Jiang, Jinfa Chang, Han Wang, Sylvain Brimaud, Wei Xing, R. Jürgen Behm, and Wen-Bin Cai ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b00416 • Publication Date (Web): 03 Mar 2016 Downloaded from http://pubs.acs.org on March 5, 2016

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ACS Applied Materials & Interfaces

Small Addition of Boron in Palladium Catalyst, Big Improvement in Fuel Cell’s Performance: What may Interfacial Spectroelectrochemistry Tell?

Kun Jiang,a Jinfa Chang,b Han Wang,a Sylvain Brimaud,c Wei Xing,b R. Jürgen Behmc and Wen-Bin Cai*,a a

Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Collaborative Innovation Center of Chemistry for Energy Materials, Department of Chemistry, Fudan University, Shanghai 200433, China

b

State Key Laboratory of Electroanalytical Chemistry, Jilin Province Key Laboratory of Low

Carbon Chemical Power Sources, Laboratory of Advanced Power Sources, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China c

Institute of Surface Chemistry and Catalysis, Ulm University, Ulm D-89069, Germany

KEYWORDS: boron doped palladium, electrocatalysis, direct formic acid fuel cell, attenuated total reflection infrared spectroscopy, interfacial chemistry

ACS Paragon Plus Environment

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ABSTRACT

Direct formic acid fuel cell (DFAFC) with Pd-based catalyst anode is a promising energy converter to power portable devices. However, its commercialization is entangled with insufficient activity and poor stability of existing anode catalysts. Here we initially report that a DFAFC using facilely synthesized Pd-B/C with ca. 6 at.% B doping as the anode catalyst yields a maximum output power density of 316 mW cm-2 at 30 °C, twice as that with a same DFAFC using otherwise the state-of-the-art Pd/C. More strikingly, at a constant voltage of 0.3 V, the output power of the former cell is ca. 9 times as high as that of the latter after 4.5 h’s continuous operation. In situ attenuated total reflection infrared spectroscopy is applied to probe comparatively the interfacial behaviors at Pd-B/C and Pd/C in conditions mimicking those for the DFAFC anode operation, revealing that the significantly improved cell performance correlates well with a substantially lowered CO accumulation at B-doped Pd surfaces.


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1. Introduction In times of growing global energy and environmental concerns, developing alternative power sources is of central interest, and seeking for proper energy storage and conversion techniques is then of research focus. Among various candidates for sustainable energy supply, low temperature polymer electrolyte fuel cells (PEFCs) are attractive as they can directly transform chemical energies into electricity.1-2 Formic acid (FA), recognised as a natural biomass with a neutral CO2 emission (recycled from CO2 reduction), can be employed conveniently as a potent energy vector in the anode fuel of PEFCs. It is commonly acknowledged that FA electrooxidation proceeds via two main pathways on Pt group metals, i.e., the dehydrogenation pathway to form CO2 and the dehydration pathway to form CO followed by further oxidation to CO2 at higher potentials.3 On Pd surfaces, the dehydrogenation pathway predominates. Besides, Pd instead of Pt as the main catalytic metal for FA oxidation may further improve the cost-effectiveness of direct formic acid fuel cells (DFAFCs).3-6 In fact, Pd-anode catalyst-based DFAFCs are a very promising energy carrier and supplier for portable devices, alternative to direct methanol fuel cells using Pt-based anode catalysts in consideration of facile anode kinetics and less fuel cross-over in DFAFCs. Nevertheless, monometallic Pd is prone to deactivation with time due to surface poisoning in formic acid solution. The main tactics to tackle the problem is to form Pd-based alloy catalysts with a second metal or to form multi-component Pd-based composite catalysts by taking advantage of the bi-functional, electronic and/or geometric effects to improve the overall catalyst performance.7-11 Yet, there is a large space to upgrade the activity and durability of the existing catalysts, especially when they are applied in real DFAFC environments.12 On the other hand, despite many fundamental investigations on the decomposition of formic acid at Pd bulk and


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film electrodes,13-14 least attention was directed to the practical Pd-based catalysts in a condition mimicking anode operation of a DFAFC. As a result, the correlation between the fundamental results and the DFAFC performance has not been well established. Therefore, it is challenging but enticing to identify an effective surface chemical “descriptor” that links directly to the enhanced or suppressed performance of a Pd-based catalyst in a real DFAFC, so as to guide the rational design of practical catalysts.15-18 In order to address the above concerns, inspired by our preliminary work on facile synthesis of B-doped Pd catalyst and half-cell assessment of this material toward formic acid oxidation using conventional electrochemical methods,19 we herein aim to explore its feasibility in a real fuel cell with a new focus on the interplay of the resulting fuel cell performance and the relevant interfacial chemistry. The Pd-B/C catalyst is initially applied in the anode of a real DFAFC, leading to a cell performance far superior to that using the state-of-the-art Pd/C anode catalyst. What’s more, highly sensitive in situ attenuated total reflection infrared spectroscopy (ATR-IR) is applied to characterize the interfacial chemistry occurring at Pd-B/C and Pd/C catalysts under the conditions mimicking the anode operation of DFAFC, in order to provide a molecular level insight into the remarkably enhanced fuel cell performance.

2. Experimental Section 2.1. Catalyst synthesis and characterization Carbon supported Pd-B catalyst with a targeted Pd loading of 20 wt.% was prepared through a wet chemical synthesis with dimethylamine borane (DMAB) as the reducing agent.19-20 Typically, 50 mg of NH4F, 250 mg of H3BO3 and 315 µmol of Na2PdCl4 was mixed in 40 mL of Millipore H2O (18.2 MΩ·cm), and kept under vigorous stirring with high-purity Ar bubbling for


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15 min. The pH was adjusted to ca. 9 with aqueous ammonia. Then, 134 mg of activated Vulcan XC-72 carbon was added to form carbon slurry and further sonicated for 30 min. Under vigorous stirring, 20 mL of freshly prepared ice-cold 0.1 M DMAB aqueous solution was dropwisely added to the suspension at 0.5 mL min-1 controlled by a peristaltic pump. The whole reduction procedure was carried out in an ice-water bath for 2 h. The resulted suspension was further stirred at 30 °C for 2 h. Finally, it was filtered, washed with copious amounts of Millipore water and dried in a clean vacuum oven at 70 °C overnight. Commercial Pd/C from Johnson Matthey (Lot No: E18S029) with a constant Pd loading of 20 wt.% was used a reference for comparison without further treatment. The morphology and dispersion of Pd-based/C catalysts were characterized by (high resolution) transmission electron microscopy (HR-TEM) on a JEOL JEM-2100F microscope. The chemical compositions were analysed by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) on a Thermo Elemental IRIS Intrepid. The fine crystalline structures were examined by X-ray diffraction (XRD) on a D8 Advance and Davinci Design X-ray diffractometer with the Cu Kα radiation from 35° to 50°, at a scan rate of 0.005° per step and a holding time of 0.5 s per step. The electronic structures were analysed by X-ray photoelectron spectroscopy (XPS), carried out on a Physical Electronics PHI 5800 Multi ESCA System, using a monochromatic Al-Kα radiation (1486.6 eV). The binding energies were calibrated by referencing to the C 1s peak at 284.6 eV.

2.2. Preparation of membrane electrode assemblies and fuel cell measurement Nafion 117 (DuPont) was used as the proton exchange membranes and the pre-treatment of the Nafion membrane was accomplished by successively treating it at 80 °C in a 5 wt.% H2O2


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solution, distilled water, 8 wt.% H2SO4 solution and then in distilled water again, for 30 min each step. Prior to the fabrication of hot-pressed membrane electrode assembly and catalyst-coated membrane, a carbon cloth was used as gas diffusion layer for current collector and assisting in water management. Membrane electrode assemblies (MEAs) with a 3×3 cm2 active cell area were fabricated using a ‘direct paint’ technique applied to the catalyst layer.21 The catalyst inks were prepared by dispersing the catalyst nanoparticles into appropriate amounts of Millipore water and a 5% recast Nafion solution. For all MEAs in this study, the cathode was constitutive of unsupported platinum black nanoparticles (27 m2 g−1, Johnson Matthey) at a standard loading of 4 mg cm−2. The anode was constitutive of carbon supported Pd catalysts. A carbon cloth diffusion layer (ETeK) was placed on top of both the cathode and anode catalyst layers. Both sides of the cathode carbon cloth were Teflon coated for water management. A single cell test fixture consisted of machined graphite flow fields with direct liquid feeds and gold plated copper plates to avoid corrosion (Fuel Cell Technologies Inc.). Hot-pressing was conducted at 140 °C, 10 M Pa for 90 s. Two different anode catalysts were investigated in this study: (I) 20 wt.% Pd-B/C and (II) 20 wt.% commercial Pd/C. The anode catalyst loading was fixed at 6 mg cm-2 including the mass of carbon support, or 1.2 mg cm-2 of metallic Pd. The MEA was fitted between two stainless steel plates in a punctual flow bed. The polarization curves were obtained using a Fuel Cell Test System (Arbin Instrument Corp.) under the operation conditions of 30 °C. High purity O2 (99.99 %) was applied as the oxidant at a flowing rate of 500 ml min-1 as the cathode atmosphere and 3.0 M formic acid (diluted from 95% formic acid, reagent grade, Sigma-Aldrich) as the reactant fed on the anode side at 200 ml min-1. Both sides are under ambient pressure.


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2.3. in situ ATR-IR measurement The ATR-IR measurement was run on a Pd-based/C catalyst layer casted on Au film chemically deposited on the basal plane of a hemicylindrical Si prism using a Varian 3100 FT-IR Excalibur spectrometer equipped with an MCT detector, at a spectral resolution of 8 cm-1 with an unpolarized IR radiation at an incidence angle of ca. 65°. 125 µL of the catalyst ink was transferred onto the electrochemically pre-cleaned Au film via a pipette. All the spectra are shown in the absorbance unit as -log (I/I0), where I and I0 represent the intensities of the reflected radiation of the sample and reference spectra, respectively. The electrolyte solution preparation is the same as that described in the supporting information for half-cell electrochemical measurement. Experimental details including chemical deposition of the Au films, and catalyst overlayer casted on Au films, setup of the ATR cell etc., can be found elsewhere.14, 20

3. Results and Discussion 3.1. ex situ characterization of B-doping in Pd Pd-B/C catalyst (20 wt.% Pd) was home-made via a facile aqueous chemical reduction route in one-pot, in which dimethylamine borane served as both the reducing agent and boron source.19-20 No post-treatment procedures like annealing or surfactant removal were needed, ensuring a potential scale-up synthesis of this catalyst upon requirement. ICP-AES analysis suggests a B content of ca. 6 at.% in Pd-B. Typical TEM images for as-prepared Pd-B/C and commercial Pd/C (20 wt.% Pd, J.M. as received) are shown in Figure 1A and 1B, both Pd-B (ca. 4.1±0.4 nm) and Pd (ca. 3.0±0.7 nm) nanoparticles are well dispersed on carbon black.


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The commercial Pd/C displays X-Ray diffraction peaks featuring pure Pd (PDF#65-6174) as shown in Figure 1C, while the diffraction peaks for Pd-B/C shift negatively, e.g., the Pd(111) diffraction peak shifts from 40.1° to 39.0°, corresponding to an enlarged Pd-Pd lattice distance with B doping in the interstitial region. The electronic structure modification on Pd by B-doping can be confirmed by the shift of Pd 3d core-level binding energy in the XPS spectra as shown in Figure 1D.

Figure 1. TEM images for 20 wt.% (A) Pd-B/C and (B) Pd/C catalysts with the same scale bar of 20 nm, together with (C) XRD patterns and (D) XPS spectra for Pd 3d regions. Note: A larger portion of Pd(II) detected on the as-received Pd/C indicates the formation of surface oxide layer


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possibly due to a post-annealing treatment by the original provider, which can be readily reduced upon contact with formic acid at open circuit potential.

With the C 1s peak calibrated at 284.6 eV, the Pd0 3d5/2 binding energy is 335.1 eV for Pd/C, and is blue-shifted to 336.1 eV for Pd-B/C. Therefore, a charge transfer effect can be expected between B and Pd components, leading to electron-enriched Pd surfaces.22-24 The electron transfer direction is consistent with the work function order of Pd (5.22 eV) and B (4.45 eV).25 Notably, according to the detailed explanation on XPS results of Pt and Pt-based alloys by Watanabe’s group,22 the positive-shift of Pt XPS doublet peaks (or the downshift of Pt core-level) corresponds to the obtainment of partial electron by Pt from an alloying metal, unlike the usual explanation on the XPS peak shifts for different valences of metals. In addition, the Pt core-level shift is commensurate with its d-band centre shift. By adding up the ICP-AES, XRD and XPS data, we are able to confirm an effective B doping for the Pd-B/C via a one-pot aqueous synthesis. It fact, a recent model DFT calculation by Nørskov and Studt et al.26 indicates B is stabilized in the subsurface layers of Pd(111).

3.2. Fuel cell performance evaluation For the DFAFC performance evaluation, Pd/C or Pd-B/C catalyst was integrated into the anode of a MEA with a same Pt black-based cathode via hot-pressing. The steady-state polarization and output power-density curves of the two sets of DFAFC single cells are shown in Figure 2A. The maximum power density of 159 mW cm-2 on the Pd/C-applied fuel cell was achieved, consistent with an early report.27 In contrast, the Pd-B/C-applied fuel cell yielded a maximum power density of 316 mW cm-2, in agreement with their different electrochemical


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behaviors in conventional half-cell measurement (Figure S1). Moreover, Pd-B/C-applied fuel cell also showed a tremendously improved stability in discharge process at a constant voltage of 0.3 V. After 4.5-h non-stop operation, the power of this fuel cell is 9-fold as high as that on the Pd/C-applied one. To our knowledge, the DFAFC incorporating the home-synthesized Pd-B/C is top ranking among those reported so far in terms of the maximum output power under similar operation conditions except the anode catalysts (Table S1).28-30

Figure 2. (A) Polarization and power-density curves, together with (B) discharge curves at 0.3 V at 30 oC for two sets of DFAFCs using Pd-B/C and Pd/C as the anode catalysts, respectively. The flowing rate of 3 M FA was 200 mL min-1 and the flowing rate of O2 was 500 mL min-1. Note: in the polarization measurement of the fuel cell (A), the open circuit duration for refueling 3 M FA was ca. 300 s.


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In order to understand the much higher output power of the Pd-B/C-applied fuel cell, we firstly examine the effect of electrochemical active surface area (ECSA). CO monolayer anodic stripping curves shown in Figure S2 were used to evaluate the ECSAs, giving an ECSA of 97.4 m2 g-1Pd for Pd/C and that of 79.8 m2 g-1Pd for Pd-B/C, respectively. Hence, the above enhanced fuel cell performance with Pd-B/C should otherwise arise from a higher specific activity toward electrochemical dehydrogenation of formic acid due to boron-doping as discussed below. Along the same line of consideration as set by Watanabe’s group,22 the downshift of the Pd 3d core level for Pd-B/C (as indicated by the positive shift of 3d3/2 and 3d5/2 doublet peaks in XPS) is commensurate with a downshift of Pd d-band center. The down-shift of Pd d-band center may be due to a superior charge transfer effect overriding the lattice expansion effect.19, 24, 31 It is known an appropriate downshift of Pd d-band center favors the electrocatalytic formic acid oxidation on Pd surface.32 A weakened binding strength for poisoning species on surface Pd sites thus can be achieved, as illustrated from a negatively shifted onset CO oxidation potential of ca. 40 mV on Pd-B/C vs. that on Pd/C (Figure S2). In fact, the latest microkinetic model and DFT calculation on decomposition of formic acid on B-doped Pd(111) by Studt’s group gave a strong support to our experimental results and discussions.26 Their calculation results suggested that the boron-doping effect may destabilize carbon-bound species such as *CO and *COOH on the catalyst surface, and leaving more free sites for acitve intermediates to boost the dehydrogenation pathway during FA decomposition.

3.3. in situ ATR-IR spectroscopy on interfacial chemistry


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In order to provide a molecular level insight, in situ EC-ATR-IR was applied to monitor the interfacial behaviors14,

33-34

at Pd-B/C and Pd/C at both open circuit potential (OCP) and

positively going multi-step potentials under conditions to some extent mimicking the anode operation in the real DFAFCs, including the incorporation of Nafion isomers in the catalyst ink, the use of 3 M formic acid, the application of similar potential and time variation patterns. The comparison of ATR-IR spectra of saturated CO adsorption on Pd/C, Pd-B/C and Pd film electrodes indicates a sufficiently high surface sensitivity of this technique for the present measurement (Figure S3).

Figure 3. Time-resolved ATR-IR spectra recorded on (A) Pd/C and (B) Pd-B/C surfaces after fast injection concentrated FA to a final concentration of 3.0 M HCOOH + 0.5 M H2SO4. The reference spectrum was recorded in 0.5 M H2SO4 prior to FA injection at OCP, and the acquisition time for each single-beam spectrum is 5 s.


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Figure 3 is the time-evolved ATR-IR spectra upon introducing 3.0 M FA + 0.5 M H2SO4 solution to replace 0.5 M H2SO4 solution in a spectroelectrochemical cell at OCP, mimicking a refueling process for a DFAFC at its open-circuit voltage. The strong bands located at 1720, 1400 and 1214 cm-1 in both figures are safely ascribed to νC=O, δCOH /δHCO, and νC-O vibrations of formic acid molecules near the electrode surfaces, respectively. These band intensities gradually grow in initial 30 s as a result of the diffusion of bulk formic acid to the catalyst surface. On Pd/C, a significant accumulation of bridge-bonded CO species (COB) was detected at the region from ca. 1810 to 1860 cm-1 in Figure 3A. The gradual growth of wavenumbers for COB suggests an increasing CO coverage on Pd surface during FA decomposition at OCP.13-14 While on PdB/C, it’s interesting to see the predominant pathway of FA dehydrogenation. The interfacial CO2 species at 2345 cm-1 was observed throughout the measurement, together with negligible CO signals in the region of 1800 to 1900 cm-1. The largely suppressed CO accumulation and a higher CO2 selectivity on Pd-B versus Pd surfaces in 3.0 M FA at OCP is in good harmony with our previous report for H2 liberation in formic acid-formate system20 and a subsequent DFT calculations by Studt et al.26


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Figure 4. Potential-dependent ATR-IR spectra recorded on (A) Pd/C and (B) Pd-B/C surfaces in 3.0 M FA + 0.5 M H2SO4 right after the duration of OCP for 300 s. The duration for each potential was about 120 s, the same as that used in measuring the polarization curve in Figure 2A, and the spectra taken at 1.25 V were used as the references, respectively.


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Figure 5. The aqueous CO2 bands in potential dependent ATR-IR spectra for Pd/C and Pd-B/C in 3.0 M FA + 0.5 M H2SO4, which are replotted from Figure 4 using otherwise the spectra taken at 0.05 V (vs. RHE) as the references, respectively.

After the 300-s OCP duration, we proceeded to acquire ATR-IR single-beam spectra with the potential moving stepwise at an interval of 0.1 V from 0.05 V to 1.25 V (vs. Reversible Hydrogen Electrode, RHE) for both catalysts. Figure 4 shows potential dependent spectra with reference to the single-beam spectrum at 1.25 V. The most strikingly different spectral feature regarding surface species is that throughout the potential range from 0.15 to 1.05 V, a significant COB band with a typical Stark effect at potentials higher than 0.15 V was found on Pd/C, whereas it was hardly detected on Pd-B/C. The apparently higher ν(COB) frequencies at 0.05 and 0.15 V in Figure 4A could be due to the co-adsorption of Had. Other spectral features are assigned according to Refs. 13 and 20. The spectral feature suggests a greatly suppressed CO accumulation on Pd-B/C as compared to that on Pd/C under a mimicking DFAFC discharge condition. In other words, the surface CO species on a Pd-based anode catalyst may serve as a key “negative” chemical “descriptor” reflecting its dehydrogenation activity and thus the overall performance of a DFAFC. Meanwhile, replotting the CO2 absorption data of Figure 4 using otherwise the single-beam spectra taken at 0.05 V (vs. RHE) as the respective references are shown in Figure 5. Notably, unlike in the case of a traditional IRRAS measurement with a thin-layer electrolyte structure, the CO2 produced at the catalyst surfaces in the ATR-IR measurement may diffuse readily to the bulk solution, the more CO2 produced the more pronounced diffusion. Since the evanescent wave penetration depth in ATR mode is limited to several hundred nanometers, the ATR-IR


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detection method may to some extent underestimate the total amount of CO2 produced in the solution. Nevertheless, the CO2 band intensities approximately reflect relative dehydrogenation activities of Pd-B/C and Pd/C catalysts in 3.0 M formic acid. A more pronounced CO2 band intensity for Pd-B/C with increasing oxidation potential can be seen, especially at relative lower oxidation potentials (0.l5 to 0.55 V vs. RHE, corresponding to the practical voltage range of 0.30 to 0.70 V for an operating DFAFC, given the working potential of the ORR cathode to be ca. 0.85 V vs. RHE). This is in agreement with the enhanced dehydrogenation on Pd-B/C found by our electrochemical and fuel cell measurements as well as by the recent model calculation.26

4. Conclusions In summary, we have demonstrated a superb DFAFC performance can be achieved in terms of maximum and durable output power densities through the use of home-made Pd-B/C as its anode catalyst in replacement of the state-of-the-art commercial Pd/C. Effective B-doping in Pd lattice interstices leads to a net down-shift of Pd d-band center and thus weakened adsorption strength for surface poisoning species like CO, as confirmed by XRD, XPS and anodic CO stripping characterizations. By using high sensitivity in situ ATR-IR spectroscopy, we have found a very good correlation between the enhanced output power density of the Pd-B/C-applied DFAFC and the better CO-resistance of Pd-B/C in simulated working conditions. This interactive investigation combining fuel cell performance evaluation with non-traditional catalysts and highly sensitive surface analytic techniques may further promote the development of PEFCs.

ASSOCIATED CONTENT


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Supporting Information Additional electrochemical evaluation, spectral analysis and fuel cell performance comparison. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENT We gratefully acknowledge the support from the 973 Program of MOST (No. 2015CB932303), NSFC (Nos. 21473039 and 21273046), as well as the fellowship program of the Ministry of Science, Research and the Arts of Baden-Württemberg (AZ 6221.-CHN-2/310/1) for international collaboration.

REFERENCES 1. Gasteiger, H. A.; Markovic, N. M., Just a Dream-or Future Reality? Science 2009, 324 (5923), 48-49. 2. Shao, M., Electrocatalysis in Fuel Cells. Catalysts 2015, 5 (4), 2115-2121. 3. Jiang, K.; Zhang, H.-X.; Zou, S.; Cai, W.-B., Electrocatalysis of Formic Acid on Palladium and Platinum Surfaces: from Fundamental Mechanisms to Fuel Cell Applications. Phys. Chem. Chem. Phys. 2014, 16 (38), 20360-20376. 4. Ha, S.; Larssen, R.; Zhu, Y.; Masel, R. I., Direct Formic Acid Fuel Cells with 600 mA cm(-2) at 0.4 V and 22 degrees C. Fuel Cells 2004, 4 (4), 337-343. 5. Yu, X.; Pickup, P. G., Recent Advances in Direct Formic Acid Fuel Cells (DFAFC). J. Power Sources 2008, 182 (1), 124-132. 6. Grasemann, M.; Laurenczy, G., Formic Acid as a Hydrogen Source - Recent Developments and Future Trends. Energ. Environ. Sci. 2012, 5 (8), 8171-8181.


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Table of Contents Graphic


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Small Addition of Boron in Palladium Catalyst, Big Improvement in

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