Two-layer anode electrode with non-noble catalysts as CO tolerant structure for PEM fuel cell

Two-layer anode electrode with non-noble catalysts as CO tolerant structure for PEM fuel cell

international journal of hydrogen energy xxx (xxxx) xxx Available online at www.sciencedirect.com ScienceDirect journal homepage: www.elsevier.com/l...

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international journal of hydrogen energy xxx (xxxx) xxx

Available online at www.sciencedirect.com

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Two-layer anode electrode with non-noble catalysts as CO tolerant structure for PEM fuel cell S. Kheradmandinia, N. Khandan*, M.H. Eikani Department of Chemical Technologies, Iranian Research Organization for Science and Technology (IROST), P.O. Box 3353136846, Tehran, Iran

highlights  Two-layer anode Electrode with Non-noble metals and commercial Pt/C is studied.  Anode electrode resistance against CO gas evaluated by CV test in half-cell.  Results show removal of carbon monoxide in the outer layer at lower potentials.  Electrode with Sn20.Co80/C and Pt/C has permanent function after 100 cycles.

article info

abstract

Article history:

Many studies on platinum-based dual-metal or multi-metal alloys catalysts are underway

Received 7 April 2019

to strengthen the resistance of the anode catalyst layer against hydrogen fuel contami-

Received in revised form

nation, especially carbon monoxide. The change in the structure of the catalyst layer can

21 August 2019

also be a new and effective way to remove Carbon Monoxide. In the few multi-layer

Accepted 2 September 2019

electrode structure reported cases, ruthenium metal is used in the outer layer, for

Available online xxx

sieving the Carbon Monoxide molecules before reaching to the Pt/C catalyst in the inner layer. In this study, we make two-layer catalyst anode electrode with SnO2/C and Sn20.Co80/

Keywords:

C in outer layer and commercial Pt/C in inner layer. The performance of these electrodes

PEM fuel cell

for Carbon Monoxide electro-oxidation evaluate by cyclic voltammetry and the results

Anode structure

compare with the activity of an electrode with commercial platinum catalyst only. Our

Two-layer catalyst

two-layer electrodes have the same efficiency as commercial platinum electrodes and even

Non-noble metals

more for pure hydrogen oxidation and much better activity for pure Carbon Monoxide

CO electro-oxidation

electro-oxidation at low potentials, in half-cell media. These electrodes have better stability for Carbon Monoxide oxidation after 100 cycles along with Carbon Monoxide gas bubbling and electrode with bi-metal catalyst in outer layer has almost no loss in performance. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Polymer electrolyte (or proton exchange) membrane (PEM) fuel cells are promising energy sources and

environmentally friendly devices with a high power density and low pollution, which make them a suitable choice for stationary, automotive industry and portable applications [1]. Despite all these benefits, Performance, durability and

* Corresponding author. E-mail address: [email protected] (N. Khandan). https://doi.org/10.1016/j.ijhydene.2019.09.031 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Kheradmandinia S et al., Two-layer anode electrode with non-noble catalysts as CO tolerant structure for PEM fuel cell, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.031

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cost are the most important obstacles for PEM fuel cells commercialization [2]. All these challenges aggregated in the fuel cell catalyst layer. Platinum, the best-known catalyst for PEM fuel cells, shows high catalytic activity in conventional PEM fuel cells operation conditions [3], but the main issue is catalyst poisoning by carbon monoxide (CO), particularly when reformed hydrogen consist of 50e100 ppm of CO is used as the anode reactant. CO strong adsorption occurs on the anode Pt catalyst active sites, which prevents the adsorption and oxidation of hydrogen [1,4]. Additional fuel processing is one solution to mitigate the CO poisoning effect. This method involves high processing costs and could decrease the CO concentration in fuel only below 100 ppm, which still is harmful for Pt catalyst active sites [5]. As another way, several studies were accomplished to synthesize and develop catalysts with enhanced CO tolerance, which could decrease the need for H2 purification demand [6,7]. According to these studies, more resistance against CO poisoning is achievable by decreasing the adsorption of CO molecules or facilitating the oxidation of CO on the catalyst surface. These studies focused on reinforcement of catalyst layer composition using alloyed or core-shell catalysts. Normally one or two metal participate with Pt base to make alloy or core-shell structure catalysts [8,9] and a few works studied the non-platinum base catalysts [10e12]. Mousavi Ehteshami et al. reviewed articles, listed many of bi and tri metallic catalysts with enhanced CO tolerance, and described CO resisting parameters and catalytic activity of these catalysts [7]. Presence of additional materials expedite the CO electro-oxidation by lowering the oxidation over potential or weakening the PteCO band. These two known mechanism called bi-function and electronic ligand effect respectively [4,13,14]. Pt50Ru50/C is best known bi-metal catalyst which commercially used as CO tolerant anode catalyst in PEM fuel cell. Whereas this catalyst could tolerate less than 100 ppm CO concentrate in fuel, enhanced anode catalyst layer is still needed for elimination of CO pollutant [8,15]. A simple solution to overcome to the CO problem is improving electrode structure and using two or multi-layer anode electrode. First Haug et al. (2002) [16] used three-layer Pt sputter deposited electrode to increase performance and catalyst utilization. At the same period, Yu et al. (2002) [17] designed a two-layer anode structure, which removed CO at the outer layer by PtRu/C electrocatalyst in advance, hence the purer hydrogen reaches the inner platinum catalyst layer. Results shows better performance for new composite electrode at the presence of both pure hydrogen and CO/H2 fuel stream than common PtRu/C electrodes. Based on Yu's study, Wan studied multi-layer anode structures with three outer layer [15]. He used combination of deposited Ru layers and a printed Pt50eRu50 layer as outer layers and printed Pt as inner layer. Results showed better performance in the presence of 50 ppm CO comparing with conventional Pt50.Ru50/C layer and more CO tolerance than Huag's structure. Due to fact that, optimum current density in PEM fuel cells is yielded at low potentials about 0.4e0.6 V and CO gas is electro-oxidized by Pt catalyst at >0.7 V so, the key factor in

this structure is oxidizing of CO molecules by outer layer catalyst at the range of cell working potentials. As discussed, ruthenium is the most prevalent element added to Pt catalyst, but its capability for CO oxidation is limited, whereas more loading in catalyst layer is not practical because it is precious metal. Also Ru atoms has poor stability in PEM acidic media and high potential operation [18,19]. Further noble metals, non-noble metals such as Sn [13,20e23], Co [13,24,25] and Ni [4,13,26,27] widely studied to improve the anode catalyst CO endurance. From the other side, almost all researches worked on addition of one or two metal to the Pt/C catalyst, whereas in multi-layer structure each metal individually could oxidize the CO impurity, thus capability of proper elements for CO electro-oxidation should be studied independently. So in the previous works, we synthesized carbon supported Ni, SnO2 and CoO none noble catalysts and among them SnO2/ C and CoO/C showed considerable capability for the CO electro-oxidation reaction at low potentials [28]. Although activities of these non-noble metal base catalysts are not comparable to that of Pt/C, but they may be suitable candidate for outer layer of two-layer anode catalyst. Therefore, we made two-layer anode catalyst structure with SnO2/C and Co80Sn20/C in outer and commercial Pt/C in inner layer and evaluated the performance of these electrodes for CO electro-oxidation by using cyclic voltammetry (CV) along with continues CO gas bubbling. Ultimately, results were compared with that of conventional commercial Pt/C single layer anode.

Experimental Materials In-house synthesized SnO2/C and Co80Sn20/C are used from our previous works. Sulphuric acid (98%), ethanol and isopropyl alcohol are prepared from Merck. Commercial Pt/C and carbon paper (TorayTM Carbon paper) are purchased from the Fuel Cell Store. Carbon Vulcan X-72 is purchased from Cabbot. Nafion Solution LQ-1115, 1100 EW 15%wt is from Ion Power and 30% teflon solution is from ElectroChem. SEM image are taken by MIRA II TESCAN to insure the multi-layer structure of anode electrode and each layer thickness.

Working electrode preparation 20 mg of catalysts, 1 mL distilled water, 1.5 mL iso-propyl alcohol and 67 ml 15 %wt Nafion solution are mixed and sonicated (BANDELIN UW3200, Germany) for 30 min to get uniform catalyst ink. 150 mL of prepared mixture is spread on Teflonized carbon paper by a micro-sampler in two or three steps and dried in a 60  C oven between each step. Electrode is weighed carefully to prepare 0.4 mgcm-2 of net metal catalyst on each layer. 1 mgcm-2 thin film GDL is applied between two catalyst layers. GDL ink formed by 35% teflon and 65% carbon Vulcan. Final catalyst spot will have about 9 mm diameter. Three working electrode is built as Table 1.

Please cite this article as: Kheradmandinia S et al., Two-layer anode electrode with non-noble catalysts as CO tolerant structure for PEM fuel cell, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.031

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Table 1 e List of electrodes and their catalyst structure. Electrode

Structure

S1 S2 S3

Carbon Paper-Pt/C Carbon Paper-Pt/C-GDL-SnO2/C Carbon Paper-Pt/C-GDL- Sn20.Co80/C

Electrochemical test set-up An in-house half-cell is made with a 100 mL total volume. Exposed catalyst surface with 0.5 cm2 area and 8 mm diameter is foreseen via a hole in the bottom of the cell. Gas stream is led to the working electrode surface by PTFE tube. Adequate bores in the cell cap are designed to hold the reference electrode, counter electrode and gas leading tube. The Ag/AgCl reference electrode and platinum rod electrode as counter electrode is purchased from AZAR Electrode (Isfahan, IRAN), and EMSTATE 3þ Potansioastate from Palm Sense Co. (Netherland) is used for electrochemical tests. Generated data are collected by PC and recorded by PSTRACE4 version 4.6.1 (Palm Sense Co.) interface software. Fig. 1 presents Half-cell and electrochemical test devices and schematic diagram.

Electrochemical measurements Cyclic voltammetry employed to evaluate the electrochemical activity of the catalysts. CV tests carried out in a conventional three-electrode half-cell cell, at room temperature. Before each experiment, the half-cell washed with distillate water and 0.5 M Sulphuric acids to assure elimination of possible impurities, thereafter a working electrode is fixed and the cell filled with approximately 60e70 ml electrolyte. Cell purged by nitrogen gas for 15 min to deaerate the catalyst and electrolyte media. CV tests executed from 0.0 to 1.0 V with a 20 mVs1scan rate. For each test first, Blank CVs done to remove any possible impurities from catalyst and test cell. After that, CVs

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carried out under continuous bubbling of hydrogen gas on the working electrode surface as a H2 CVs to check the H2 electrooxidation capability and activate the catalyst simultaneously. Finally, after insurance of the catalyst complete activation, first five blank cycles are done as before test blank CVs to record the background electrochemical activity of catalyst and test media. Then 110 CVs are set, 100 cycles under continuous bubbling of CO gas to the working electrode surface as a CO CVs and then gas stream stopped to have 10 CVs as after test blank CVs. The blank CVs continued until the CV curves repeated on itself. The last one considered as last blank CV after the CO CV test. Net charge amount for CO electrooxidation calculated by subtracting the positive amounts of Blank CV charge from CO CV charge at the same potentials.

Results and discussion Composite anode structure Catalysts characteristics presented in previous work [28]. Fig. 2 shows the working electrode sectional SEM image which three-layer structure is distinctive. In this image, upper thick layer is carbon paper; the next layer attached to the carbon paper is the Commercial Pt/C catalyst, then the GDL and bimetallic non-noble catalyst are placed respectively.

Electrochemical tests results Pt/C catalyst in regular lab condition and without potential implementation, oxidize H2, so as expected, between 0.0 and 1.0 V, all 3 electrode have high H2 electro-oxidation activity which showed in Fig. 3. Results in Fig. 3 proves that, new composite anodes capability to H2 fuel oxidation in fuel cell is the same as conventional catalyst even more. Considering high activity of Pt species to H2 oxidation, mass transfer in electrode catalyst layer will be determinant for charge yield, so results proved

Fig. 1 e Electrochemical test setup image and anode structure schematic diagram. Please cite this article as: Kheradmandinia S et al., Two-layer anode electrode with non-noble catalysts as CO tolerant structure for PEM fuel cell, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.031

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Fig. 2 e Working electrode sectional SEM image.

that, new structure doesn't have any negative effect on gas stream properties in macro and micro scale and additional outer layers does not limit the mass transfer to the inner Pt/ C layer. After H2 gas CVs and complete activation of catalyst layers, CVs along with CO gas continuous bubbling carried out and results, for Pt/C electrode for instance, recorded in following sequence; Blank CV before the CO test is recorded then 100 CVs is done under contentious CO gas stream. CV Cy. 5 and CV Cy. 100 considered as test beginning and finishing CO CVs respectively. Finally, gas bubbling stopped and CVs continued

until CV no. 109 is repeated on CV 108 exactly, so CV No. 108 is considered as after test Blank CV. In Fig. 4 the results for commercial Pt/C is depicted. The blank CVs before the CO test and after the test (Cy. 108) are almost conform which means that, test system background activity and catalyst structure is not change during the test and all current charges between these two cycle is related to electrode activity in the presence of CO molecules in test media. After starting the CO CV test, four cycle passed to insure the experiment condition stabilization and then cycle 5 considered as CO CV beginning current charge and cycle 100 as test end data. When cycle 100 accomplished, CO gas bubbling ceased and CVs recorded to the last one. The current charge obtained from the first cycle after cutting CO gas stream (CV Cy 101) is important because it is recorded in CO saturated media without gas stream force draft. The beginning and end CO CVs, last blank CV and CV 101 for two layer electrodes is showed in Figs. 5 and 6. In order to correctly compare the results of the experiments, the values of Last blank CV charges, in the positive region and upward going potential, subtracted from the Beginning and End CO CV values in the same potentials. Negative values of Last blank CV considered as zero. Resulted curves depicted in Fig. 7. As expected, commercial Pt/C has highest net charge values in the beginning of CO CV test and potentials above 0.7 V. Whereas at the potentials lower than 0.7 V, two layer electrodes especially S2 have better activity which means that, the presence of non-noble metals in the outer layer has led to an increase in the oxidation of CO in the lower potentials. In other words, CO contaminant is removed in the outer layer and the net fuel is evenly absorbed into the inner layer. Another important point in this figure is reduction of current charges for electrodes S1 and S2 after 100 CO CV cycles. This decrease is more pronounced in potentials higher than 0.6 V

Fig. 3 e H2 electro-oxidation current charge curves for commercial Pt/C and two layer anodes. Please cite this article as: Kheradmandinia S et al., Two-layer anode electrode with non-noble catalysts as CO tolerant structure for PEM fuel cell, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.031

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Fig. 4 e CO CVs and Blank CVs before and after CO test for commercial Pt/C.

Fig. 5 e CO CVs and Last blank CV for two-layer electrode S2.

and is higher for commercial Pt/C electrode. While the current charge values for the S3 electrode are approximately the same at the beginning and end of the test throughout the potential range. PEM fuel cells operating potential rang is between 0.4 and 0.8 V, it is seen from Fig. 7 in this interval of potential the net current charge from CO electro-oxidation by two layer electrodes is significantly greater than the platinum electrode current and the current charge after 100 CV for the Pt/C electrode decreased from about 20% at 0.6 V to about 0.75% at 0.8 V. This poisoning effect is more intense in the voltage range of 0.7e0.75 V, and the catalyst in this range has more

than 90% performance loss. Also, the performance loss of the S2 electrode from the beginning to the end of the test starts from about 22% at 0.55 V and continues to about 28% at 0.8 V. This article confirms the toxicity of platinum in the S1 electrode as well as tin oxide in the outer layer of the S2 electrode. But the results of the S3 electrode indicate that the Sn20.Co80/C bi-metal catalyst layer has a high performance for oxidation of CO, while maintaining a high degree of sustainability in the long run. These results confirm our previous findings that capability of non-noble metals for CO electro-oxidation at low potentials [28] and also bimetal catalysts of tin-cobalt have a good stability in terms of PEM fuel cell operation conditions.

Please cite this article as: Kheradmandinia S et al., Two-layer anode electrode with non-noble catalysts as CO tolerant structure for PEM fuel cell, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.031

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Fig. 6 e CO CVs and Last blank CV for two-layer electrode S3.

Fig. 7 e Comparing net current charge obtained from beginning and end of CO CVs.

As already mentioned, after ceasing the CO gas bubbling, the Blank cycle 101 is swept through the CO saturated medium. The only difference between cycles 100 and 101, is removal of gas stream force draft, which leads to an increase in the effective collisions of the gas molecules in the catalyst boundary layer. To calculate the net current charge obtained in Cycle 101, the same procedure is applied and last blank amounts is subtracted. The results of Last blank CV fractionation from cycle 101, Beginning and End CO CV for all three electrodes is compared in Fig. 8 at the potential range of 0.4e0.8 V.

The smooth shape of 101 curves in Figs. 4e6 and 8 proves that the rough nesses in CO CV curves are due to the gas flow force drat and gas stream draft reduces the effective contact time between CO molecules and catalyst particles. Therefore, assuming that we do not have mass transfer problem in cycle 101, Output current will be limited just by reaction rate. It could be seen in Fig. 8 that the cycle 101 current charge for S1 and S2 is lower than the starting CO CV charge at the potentials below 0.8 V which means that the current rate is limited by electro oxidation rate and mass transfer does not affected the reaction at this potential rang. For S1 and S2 electrodes, up to 0.7 V the

Please cite this article as: Kheradmandinia S et al., Two-layer anode electrode with non-noble catalysts as CO tolerant structure for PEM fuel cell, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.031

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Fig. 8 e Comparing the Cycle 101, CO Beginning & End CV net current charge for S1, S2 & S3.

CV 101 current values are the same as the end CV current and at more than 0.7 V the current is more than the end of the cycle. It means that, in the case of both electrodes, despite of the good transfer conditions, the CO poisoning has caused, that the current decreases compared to the start cycle at the whole potential ranges. In the potentials above 0.7 V the current charge increases compared to the last cycle which shows that, the reaction rate is determined by mass transfer. Note that the current values still do not reach the initial CVs value. In the case of electrode S3 as depicted in Fig. 8, up to potential of 0.55 V, there is no significant difference between the beginning, end, and the cyclic 101 charge current values. Higher than the 0.55 V potential, the current values increases slightly, even more than the values of the beginning of the test. These results prove that the S3 electrode does not have an impressive current drop after 100 cycles and is stable against the CO contamination. Also, the effect of mass transfer on the overall reaction rate at voltages higher than 0.55 V is very high. This problem may be caused by the structure of the catalyst layers of the electrode, which will be eliminated in the continuous and laminar flow regime of the PEM fuel cell. Comparing the results of the 101 cycle for all three electrodes in Fig. 8 shows that the anode structure with two catalyst layers has both good resistances to carbon monoxide poisoning and has a better performance than anode with a platinum catalyst alone. Also, the two-layer anode structure with the outer layer of the bi-metal catalyst, although less efficient than the single-metal layer for CO oxidation, still has better stability at long time operation and at lower potentials, has a good resistance to carbon monoxide poisoning.

Conclusion In addition to research on the synthesis of fuel pollutionresistant anode catalysts, the use of the double-layer or

multi-layer anode structure is a creative and effective way to remove carbon monoxide at low potentials, before reaching Pt/C layer. The use of non-noble metals in the outer layer, while significantly reducing the CO content of anode fuel, does not have a significant effect on the cost of the electrode. It is also possible to load them in excessive amounts. The results of these experiments showed that the presence of the second layer does not only prevent the transfer of hydrogen mass to the inner platinum layer, but also increase the catalytic activity of the anode electrode in general. The removal of carbon monoxide in the outer layer leads to an increase in the overall resistance of the electrode against CO poisoning effect comparing to the commercial platinum electrodes. Electrode with outer layer of the bimetallic catalyst Sn20.Co80/C and inner layer of commercial Pt/C, has almost no functional loss after one hundred cycles, while maintaining better performance than conventional electrodes.

references

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Please cite this article as: Kheradmandinia S et al., Two-layer anode electrode with non-noble catalysts as CO tolerant structure for PEM fuel cell, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.031