Novel Pd-Co Electrocatalyst Supported on Carbon Fibers with

a clean energy source. Water electrolysis is an attractive approach to producing hydrogen, but high energy cost limits its application. Coal is consid...
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Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

Novel Pd−Co Electrocatalyst Supported on Carbon Fibers with Enhanced Electrocatalytic Activity for Coal Electrolysis To Produce Hydrogen Ping Yu,*,† Junchao Ma,† Rong Zhang,† Jin Z. Zhang,*,‡ and Gerardine G. Botte§ †

Department of Chemical Engineering, Shenyang University of Chemical Technology, Shenyang 110142, P. R. China Department of Chemistry and Biochemistry, University of California, Santa Cruz, California 95064, United States § Center for Electrochemical Engineering Research, Chemical and Biomolecular Engineering Department, Ohio University, Athens, Ohio 45701, United States ‡

ABSTRACT: Production of hydrogen from electrolysis of coal slurry is a new approach combining the clean utilization of coal with hydrogen production. A novel Pd−Co nanoelectrocatalyst supported on carbon fibers (PdCo/CFs) prepared by impregnation and reduction was developed and first applied to coal electrolysis for hydrogen generation. The performance of the PdCo/CFs electrocatalysts is superior to that of pure Pd/CFs catalyst with an improvement of about 16.9%. The improved performance can be attributed to modification of surface electronic structure due to alloying. The materials synthesized herein are promising anode catalysts for coal electrolysis to produce hydrogen. KEYWORDS: coal slurry, hydrogen production, Pd−Co, electrocatalyst, fuel cell

H

those of Pt. Pd has a crystal structure and atomic size identical to platinum. It was reported that Pd catalysts can oxidize catalytically CO and hydrocarbons.14 It is known that Pd alloyed with 3d-transition metals, such as Co, Ni, Fe, etc., often demonstrates improved catalytic activity and stability compared to those of pure Pd. This new type of material has demonstrated unusual properties due to the modification of the Pd electronic structure.15−18 However, there has been no report on H2 production from coal electro-oxidation with Pd− Co electrocatalyst as an anode in acidic solution. On the other hand, carbon fibers as a support can afford a larger surface area for the catalyst.10 In this study, Pd−Co electrocatalyst on carbon fibers was developed first and then applied for coal electrolysis to produce hydrogen in acidic solution. Carbon fiber supported Pd−Co electrocatalysts (PdCo/CFs) were synthesized by an impregnation and reduction method. In brief, 20 cm long carbon fibers wound around the titanium gauze (1 cm × 1 cm, 20 mesh) as the electrode substrate. The amount of Co(NO3)2·6H2O (from Sinopharm Chemical Reagent Co., Ltd., 98.5+% purity) was dissolved in 0.3 M PdCl2 (from Sinopharm Chemical Reagent Co., Ltd., 59.0% metals basis) aqueous solution according to the given Pd−Co atomic ratios. The mixture was then ultrasonically blended in an ultrasonic cleaner (VGT-1620QTD) for 20 min. This is followed by impregnation of the substrate in the solution for 4−6 h. For comparison, the pure Pd electrocatalyst was prepared following the same steps without Co(NO3)2·6H2O. The sample was then dried in a drying oven (GZX-GF101-1-

ydrogen is a promising clean and renewable energy source for applications including electricity generation.1,2 Since there is little or no pollution to the environment, it is considered as a clean energy source. Water electrolysis is an attractive approach to producing hydrogen, but the high energy cost limits its application. Coal is considered to be a relatively cheap resource on earth. However, its utilization by combustion generates serious pollution. A clean and more efficient approach is hydrogen production via electrolysis of coal slurry.3,4 In 1979, Coughlin and Farooque proposed the following reactions for coal electrolysis:5,6 at the anode: C + 2H 2O → CO2 + 4H+ + 4e−

(1)

at the cathode: 4H+ + 4e− → 2H 2

(2)

the overall reaction: C + 2H 2O → CO2 + 2H 2

(3)

Electrolysis of coal slurries requires only 0.21 V standard potential, which is much less than the 1.23 V required for water electrolysis.7−9 However, the process of coal electrolysis to produce hydrogen is not kinetically favorable.10 Platinum is the effective electrocatalysts for coal electro-oxidation to produce hydrogen. However, the high cost of Pt limited its widespread commercial application.11 To lower the Pt loading, many methods are applied such as using Pt-based alloys or core−shell electrocatalysts.2,12 Pd is less expensive and more available than Pt.13 Recently, Pd catalysts have attracted much attention as non-Pt catalysts due to Pd’s properties which are similar to © XXXX American Chemical Society

Received: October 30, 2017 Accepted: January 19, 2018 Published: January 19, 2018 A

DOI: 10.1021/acsaem.7b00085 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials

Figure 1. XRD patterns (a) and its detailed XRD patterns (b) for PdCo/CFs electrocatalysts with various Co contents.

BS) at 120 °C for 2 h. The reduction process occurred in a tubular furnace (from YuDian Automation Technology Co., Ltd.) in a mixed atmosphere of H2 (100 mL min−1) and Ar (500 mL min−1) at 450 °C for 2 h, which was first poured with N2 (600 mL min−1) for 30 min to remove the air. The sample was finally cooled down in Ar atmosphere. The phase structures of the catalysts were identified by X-ray diffraction (XRD, X’Pert PRO) at a scan rate of 1.6° min−1. The catalysts were characterized by scanning electron microscopy and energy dispersive X-ray analysis (SEM/EDS, Philips FEI-Inspect F) to determine the surface morphology and composition. Transmission electron microscopy (TEM, Jeol JEM 2010F) was used to characterize the microstructures of the catalysts. The surface properties of the catalysts were characterized by an X-ray photoeletron spectroscopic (XPS, ESCALAB250) using an Al Kα source at 150 W. The structure of the coal after electrolysis was analyzed by FTIR spectroscopy (NEXUS, American Thermo Co.). The total carbon content of the coal sample after electrolysis was detected using a carbon− sulfur analyzer (SC-144DR, American Leco Co.). The hydrogen and oxygen contents were analyzed using an oxygen−nitrogen−hydrogen analyzer (ONH836, American Leco Co.). Electrochemical experiments were performed in an H-type electrolytic cell at 80 °C using an electrochemistry station (from Shanghai Chenhua Instruments Limited Co., CHI660E). The Pd−Co electrocatalyst on carbon fibers (PdCo/CFs) served as the anode. The pure Pt electrode (3 cm × 3 cm, 99.99+% purity) was applied to the counter electrode. The anodic solution was Yanzhou coal slurry (≤88 μm) with 4.80 g of coal in a 120 mL compartment in 1.0 M H2SO4 with 0.040 M Fe2+/Fe3+. A blank solution without coal was used for comparison. The cathodic compartment was 1.0 M H2SO4. A constant current density of 20 mA cm−2 was used for electrochemical measurements under galvanostatic conditions. The measurements continued until the cell potential reached 1.17 V to avoid the electrolysis of water at higher potential. Amperometric experiments of PdCo/CFs were performed for coal slurry in 1.0 M H2SO4 at 1.0 V potential. All measurements were conducted with constant stirring to ensure effective contact between the electrocatalyst and the coal. The composition of the Pd−Co catalyst with nominal Pd/Co ratios of 7:3, 1:1, 3:7 was analyzed by energy dispersive spectroscopy (EDS) giving approximately 2.8:1, 1:1.2, and 1:2.6, respectively. XRD analysis of PdCo/CFs with various compositions reveal typical Pd face-centered cubic (fcc)

features, as shown in Figure 1a. With the Co content increasing, the diffraction peaks of PdCo/CFs shift to higher angles compared to those of the Pd/CFs, as shown in Figure 1b. The angular shifts indicated the lattice contraction due to the incorporation of Co into the Pd crystalline structure to form the Pd−Co alloy phase.16,19 The lattice parameters of the catalysts were calculated on the basis of the (220) diffraction peaks, as shown in Table 1. Smaller lattice parameters for the Table 1. XRD Results of PdCo/CFs with Various Co Contents Pd/Co atomic ratio

2θ (220), deg

domain size, nm

lattice param, nm

1:0 7:3 1:1 3:7

68.406 68.433 68.471 68.559

27.20 26.47 14.94 33.04

0.3876 0.3874 0.3873 0.3868

PdCo/CFs catalysts were obtained compared to Pd/CFs, indicating lattice contraction upon formation of the Pd−Co alloy.20 It was also obviously seen that the Co diffraction peak appeared on the PdCo/CFs (1:1) and PdCo/CFs (3:7). Figure 2a−d shows SEM images of PdCo/CFs electrocatalyts with loading of 0.65 mg cm−1 of carbon fibers with various atomic ratios. Pd−Co particles were well-distributed on the surface of carbon fibers. Figure 2e,f shows the typical TEM images of PdCo/CFs (1:1). The images indicated that the dspacing of the catalyst was 0.263 nm corresponding to the (200) planes of fcc crystals, suggesting the formation of Pd−Co alloy.20 Survey spectrum and Pd 3d XPS spectra of Pd/CFs and PdCo/CFs (1:1) catalysts were shown in Figure 3a,b. The formation of the Pd−Co alloy was further confirmed. The presence of both Pd and Co can be clearly seen in Figure 3a. Two obvious peaks of pure Pd/CFs in the Pd 3d spectrum were located at 335.3 and 340.4 eV, attributed to Pd 3d3/2 and Pd 3d5/2, respectively. However, for the PdCo/CFs catalyst, it was interesting to detect the XPS binding energies of Pd 3d3/2 and Pd 3d5/2, located at 335.7 and 340.9 eV. The Pd 3d peaks of PdCo/CFs shifted toward higher bonding energy, compared to those of pure Pd/CFs. The positive shift of binding energy indicated Pd−Co alloy formation, which is consistent with that reported in the literature.21−23 The electrochemical performance of PdCo/CFs and pure Pd/CFs was measured under galvanostatic conditions. At least three samples with the same ratio of Pd and Co were measured B

DOI: 10.1021/acsaem.7b00085 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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compositions of Pd/Co 1:0, 7:3, 1:1, and 3:7 was 38 615 ± 330, 42 305 ± 200, 45 145 ± 410, and 40 170 ± 270 s, respectively. Compared to that of pure Pd/CFs, the efficiency of electrooxidation for coal was increased by PdCo/CFs catalyst up to 9.6%, 16.9%, and 4.0%, respectively. The PdCo/CFs (1:1) catalyst owned the best catalytic activity for coal electrolysis among the PdCo/CFs catalysts. It can be attributed to the atomic optimization of the Pd−Co relationship. When the amount of Co was increased up to the Pd and Co atomic ratio of 3:7, coal electrochemical conversion efficiency decreased compared to PdCo/CFs (1:1), possibly due to the decreased utilization of active Pd sites for electrocatalytic reactions with more Co in the Pd−Co catalyst. However, the polarization time of PdCo/CFs (3:7) was still longer than that of pure Pd/CFs, indicating that the PdCo/CFs (3:7) has better electrocatalytic activity than Pd/CFs. Further evaluation of the electrocatalytic activity of the PdCo/CFs catalysts was performed under the amperometric conditions with constant potential 1.0 V in coal slurry. The I−t curves of electrodes with different Pd/Co ratios were shown in Figure 4e. The results demonstrated that the current density of PdCo/CFs was much higher than that of pure Pd/CFs catalyst. This indicated that the PdCo/CFs exhibited higher electrocatalytic activity for coal electrolysis. The PdCo/CFs (1:1) catalyst showed the highest current density and hence the highest electrocatalytic activity among the PdCo/CFs catalysts. This result is in agreement with the galvanostatic measurement shown in Figure 4a−d. The PdCo/CFs (1:1) catalyst showed the slowest current decay with time among the Pd/CFs, PdCo/ CFs (7:3), and PdCo/CFs (3:7) catalysts, indicating that the PdCo/CFs (1:1) catalyst has a much better durability. Further work on the stability of PdCo/CFs on longer time scales should be done in the future. Figure 4f shows the FTIR spectra of the coal proximate and ultimate electrolysis under galvanostatic conditions. The peak at 3406 cm−1 was assigned to the N−H and hydroxyl groups, such as alcohol, phenol groups in the organic fraction of coal, whose peak intensity decreased after electrolysis. The bands in the 2820−2900 cm−1 range are attributed to C−H stretching vibration of CH, CH2 and CH3 groups,28 whose intensities decreased after electrolysis. These groups have high reactivity and hence can be oxidized easily. In general, a decrease of peak intensity of C−H group was accompanied by an increase in intensity of the carbonyl group representing oxidation.29 A

Figure 2. SEM images of PdCo/CFs electrocatalyts with various atomic ratios: (a) 1:0, (b) 7:3, (c) 1:1, (d) 3:7. (e, f) TEM image of the PdCo/CFs (1:1) nanoparticles.

for reproducibility. Figure 4a−d shows the relationship between the cell potential and the polarization time of Pd/CFs with various Co compositions. Previous research has found that the amount of coal that is electrochemically oxidized is proportional to the polarization time.24,25 The results indicated that the polarization time of coal electrolysis was longer than that of the electrolysis of the blank solution with only Fe2+/ Fe3+. This can be attributed to the electrochemical conversion of coal, which agrees with the previous reports.24,26,27 All of the PdCo/ CFs eletrocatalysts exhibited longer polarization time than pure Pd/CFs catalyst, which was attributed to the alloying of Pd with Co when Co is added to the Pd catalyst undergoing H2 atmosphere heat treatment. On the basis of Figure 4a−d, the polarization time of PdCo/CFs electrodes with various Co

Figure 3. (a) Survey spectra XPS analysis of Pd/CFs and PdCo/CFs (1:1) electrocatalysts. (b) Pd 3d X-ray photoelectron spectra of Pd/CFs and PdCo/CFs electrocatalysts. C

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Figure 4. Galvanostatic measurements of the PdCo/CFs electrocatalyts with various atomic ratios: (a) 1:0, (b) 7:3, (c) 1:1, (d) 3:7. (e) I−t curves of the PdCo/CFs catalysts with various compositions at 1.0 V potential. (f) FTIR spectra of the coal sample before and after electrolysis. (g) Proximate and ultimate analysis of C, H, and O contents in coal sample. (h) Hydrogen and carbon dioxide evolution at the cathode and the anode. D

DOI: 10.1021/acsaem.7b00085 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials small increase in intensity at 1700 cm−1 assigned to the vibration of carbonyl indicated the oxidation of coal. The peak of 1437 cm−1 is assigned to the vibration of C−O functional groups. These oxygen-containing groups were easily oxidized after electrolysis. Proximate and ultimate analyses of C, H, and O content in coal samples under galvanostatic conditions were determined to prove the coal conversion, as shown in Figure 4g. The results revealed that the carbon and hydrogen content decreased after electrolysis, but the oxygen content increased. The decreased carbon and hydrogen content after electrolysis further indicated that the coal electro-oxidation consumed C and H content in coal.9 It was reported that parts of carbon were oxidized into gaseous and liquid products.4 Some carbonyl and carboxyl functional groups were intermediates that could accumulate on the coal.5 The results are in agreement with FTIR analysis above. This can also explain the observation that the potential increased more quickly toward the end of galvanostatic measurement, as shown in Figure 4a−d. It was supposed that the formation of films on the surface of the coal hindered the polarization, thereby reducing the active sites for the coal to react on the electrode.10 The gases generated from the anode and the cathode were collected under galvanostatic conditions. The volume of H2 and CO2 generated at the cathode and the anode was 14 and 2 mL, respectively, as shown in Figure 4h. The volume ratio of H2 and CO2 was 7, which is higher than the theoretical ratio of 2 according to reaction 3. This is possibly due to some film formed on the coal surface and the large portion of carbon oxides that remained in coal. The evidence is consistent with that of previous reports.5,24 In summary, an effective PdCo/CFs catalyst for coal electrolysis has been demonstrated to produce hydrogen in an acidic medium, which was prepared by an impregnation and reduction method. The performance of the PdCo/CFs electrocatalysts is superior to that of pure Pd/CFs catalyst. The PdCo/CFs (1:1) showed the best electrocatalytic performance with an improvement of about 16.9%. The improved performance is attributed to modification of surface electronic structure due to alloying. This work shows that PdCo/CFs materials are promising electrocatalysts for hydrogen generation through coal electrolysis.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Ping Yu: 0000-0003-0801-1620 Jin Z. Zhang: 0000-0003-3437-912X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Science and Technology Funds from Liaoning Education Department (No. LFD2017002) and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry.

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DOI: 10.1021/acsaem.7b00085 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX