Evaluation of the Optimum Composition of Low-Temperature Fuel Cell

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Review

Evaluation of the optimum composition of low-temperature fuel cell electrocatalysts for methanol oxidation by combinatorial screening. Ermete Antolini ACS Comb. Sci., Just Accepted Manuscript • DOI: 10.1021/acscombsci.6b00080 • Publication Date (Web): 19 Dec 2016 Downloaded from http://pubs.acs.org on December 21, 2016

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ACS Combinatorial Science

Evaluation of the optimum composition of low-temperature fuel cell electrocatalysts for methanol oxidation by combinatorial screening.

Ermete Antolini Scuola di Scienza dei Materiali, Via 25 aprile 22, 16016 Cogoleto, Genova, Italy e-mail: [email protected]

Abstract Combinatorial chemistry and high throughput screening represent an innovative and rapid tool to prepare and evaluate a large number of new materials, saving time and expense for research and development. Considering that the activity and selectivity of catalysts depend on complex kinetic phenomena, making their development largely empirical in practice, they are prime candidates for combinatorial discovery and optimization. This review presents an overview of recent results of combinatorial screening of low-temperature fuel cell electrocatalysts for methanol oxidation. Optimum catalyst compositions obtained by combinatorial screening were compared with those of bulk catalysts, and the effect of the library geometry on the screening of catalyst composition is highlighted.

Keywords: Fuel cells, combinatorial and high-throughput screening, catalysts, platinum, methanol oxidation.

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Introduction Direct methanol fuel cells, (DMFC) are promising environmentally friendly devices to generate electricity by direct electrochemical conversion of methanol and oxygen into water, carbon dioxide and/or partial alcohol oxidation products.1 Platinum is the most effective metal for methanol oxidation at low temperatures (< 100 oC) in acidic environments, so platinum nanoparticles are widely used as DMFC anode electrocatalysts. The high cost of platinum and the poisoning of platinum by strongly adsorbed species coming from the dissociative adsorption of methanol represent major drawbacks to the widespread use of DMFCs. Thus, to decrease both the poisoning of Pt and the cost of the electrode, binary, ternary and quaternary Pt-based anode catalysts have been extensively investigated.2,3 Combinatorial chemistry is a systematic approach to the discovery of new materials with improved properties. As a complex enterprise, catalysis is both susceptible to hypothesis-driven improvement4-7 and is a prime candidate for combinatorial exploration. Reddington et al.8 first applied combinatorial synthesis and high throughput screening to the field of electrocatalysts, followed by the development of different deposition methods for the synthesis of combinatorial libraries, such as physical vapour deposition, electro-deposition, solution dispensing and thin film sputtering. However, the characteristics of combinatorial materials made on milligram scale in small spots in a deposition library or gradient can be quite different than those of corresponding large-scale bulk materials, due to various reasons.9 For example, the reduction or elimination of stirring time of catalyst precursors in the preparation of spot materials can result in nonhomogeneous samples. Similarly, shorter reaction times of the precursors with the reducing agents in the spot synthesis can result in incomplete reduction and smaller particles than those formed in bulk samples. In addition, co-sputtering of materials using multi-sample targets can suffer from contamination, resulting in varied compositional spreads with time, and electrochemical gradients formed during electrosynthesis of spot materials may be a limitation and a drawback.


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More promising has been the development of diverse high-throughput electrocatalyst screening methods, including optical screening, scanning electrochemical microscopy (SECM), and multielectrode half cell and multielectrode full cell measurements. Jeon et al.10 have analyzed the advantages and disadvantages of the different screening techniques, and the state-of-the-art in combinatorial synthesis and screening techniques for the characterization of electrocatalysts has been assessed in several excellent reviews.9-12 Usually, catalysts for methanol oxidation are formed from 2, 3 or 4 metals, with a main metal, commonly Pt, as the active catalyst, and a second metal, in most cases Ru, as the main co-catalyst. A third or fourth metal usually acts as a modifier of some characteristic of the binary catalysts, such as the lattice parameter of the alloy and the dissolution of the nonprecious metal, affecting the activity and stability of the catalyst. On these bases, combinatorial screening is a powerful tool to optimize the composition of ternary and, particularly, quaternary catalysts. The multicomponent nature of these systems makes them attractive for combinatorial development, particularly of ternary and quaternary catalysts.

Table 1 Direct methanol fuel cell anode materials screened by combinatorial methods. TM = transition metal. Anode materials

Formula

Binary Pt-based catalysts

Pt1-xMx

Ternary Pt-Ru-based catalysts

Pt1-x-yRuxMy

Quaternary Pt-Ru-based catalysts

Pt1-x-y-zRuxMyM’z

Ternary Ru-free Pt-based catalysts

Pt1-x-yMxM’y

Quaternary Ru-free Pt-based catalysts

Pt1-x-y-zCoxCryVz

Screened parameters Metal/amount M/x M = TM, Sn M/x,y M = TM, Sn M,M’/x,y,z M = TM M,M’/x,y M = TM, Bi, Pb x,y,z

The screening of multimetallic catalysts has usually been carried out by keeping the active metal (main catalyst) fixed and varying the amount and/or the kind of the other metals. This overview highlight results of combinatorial screening of catalysts for the methanol oxidation reaction (MOR). 3 Environment ACS Paragon Plus


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In most cases, optimum compositions have been compared with those of bulk catalysts, and the effect of the library structure on the optimum composition is discussed. DMFC anode materials screened by combinatorial methods are shown in Table 1, and are discussed below.

Screening of MOR electrocatalysts by combinatorial methods Pt-M binary catalysts The problem of platinum poisoning by strongly adsorbed species coming from the dissociative adsorption of methanol has been addressed most commonly by the addition of cocatalysts, such as Ru, Mo, Sn, W, and Ni to Pt. Among them, the binary Pt-Ru system has been the most promising candidate and is the current state-of-the art MOR catalyst.13 However, direct methanol fuel cells using Pt-Ru anode catalysts operate at power densities about 10 times lower than that of a polymer electrolyte membrane fuel cell (PEMFC) fueled with hydrogen, if the same noble metal loading is used. Therefore, the efficiency of DMFCs with Pt-Ru alloys as anode catalysts is still insufficient for practical application. To improve the activity of the DMFC anode catalysts, a number of laboratories have performed combinatorial screening for optimization of the composition of Pt1xRux

catalysts and Ru-alternative co-catalysts. In one study of Pt-M binary catalysts, 18 Pt-M

binary (M = Sn, Ta, W, Mo, Ru, Fe, In, Pd, Hf, Zn, Zr, Nb, Sc, Ni, Ti, V, Cr, Rh) thin film libraries were deposited at low M concentrations using magnetron sputtering and screened for methanol electrooxidation by a fluorescence method.14 The films with M = Sn, Zn, In, Fe, and Ru showed the highest MOR activity. Pt-M (M = Sn, Zn, In) showed the highest activity at compositions below 5 at% M with a high fraction of Pt fcc(111) texturing, while Pt-Fe showed the best activity at 10 at% Fe. Regarding Pt-Ru, the lowest MOR onset potential was observed at 35 at% Ru with only slight texturing of the film. Other studies reported combinatorial screening of Pt-Ru libraries, and an optimum MOR activity was observed at a lower Ru content.15,16 Combinatorial screening of a Pt1-xRux library (x = 0-0.8) by Strasser revealed a Pt88Ru12 composition as the most active at room temperature in 1M CH3OH solution.15 A rapid decrease of the activity for methanol oxidation 4 Environment ACS Paragon Plus

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occurred up to 25 at% Ru, followed by a slow MOR activity decrease with increasing Ru content. A similar result was reported by Cooper and McGinn,16 who observed a peak in methanol oxidation rate at a Pt:Ru composition of 90:10. An effect of methanol concentration on the optimum Pt-Ru composition was reported by Jiang et al.17 They deposited Pt-Ru catalysts of well controlled Ru content over a wide range of compositions by radio-frequency magnetron co-sputtering. The optimal Pt:Ru compositions of the catalysts at room temperature in 1 M and 16.6 M CH3OH solutions were 76:24 and 54:46, respectively. Pt-Ru-based ternary and quaternary catalysts Considerable research has been done to improve the performance of Pt-Ru binary catalysts by the incorporation of a third or fourth metal.2,3,13 The presence of a third/fourth component can significantly enhance the catalytic activity for methanol oxidation, supporting the oxidation of intermediate species and/or reducing their adsorption on Pt.13 Measurements of activity for methanol oxidation in half cells and/or in DMFCs showed that many ternary Pt-Ru-based catalysts better perform than the standard Pt-Ru catalyst.3 Considering the many variables in the composition, preparation, and use of multimetallic catalysts, many optimization studies have involved combinatorial and high-throughput screening methods.15,16,18-27 In their pioneering effort, Reddington et al.8 tested combinations of five elements (Pt, Ru, Os, Ir, and Rh) and found many effective DMFC anode catalysts in unexpected regions of composition space. A quaternary catalyst (Pt44Ru41Os10Ir5) was identified with significantly higher activity than the state-of-the-art binary PtRu catalysts. More generally, combinatorial screening of several different ternary/quarternary catalysts, with the notable exception of Pt-Ru-Sn,21 provided compositions with higher MOR activity than Pt-Ru.14-20,22-26 A comparative study of experimental (sputter-deposition ternary thin-film electrocatalysts) and theoretical (density functional theory) methods was later undertaken by Strasser et al. for the development of ternary Pt-Ru-M catalysts.18 In this case, experimental and theoretical results were closely aligned with each other, revealing very similar trends in electrocatalytic activity as a 5 Environment ACS Paragon Plus


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function of catalyst composition. An interesting example is provided by the combinatorial screening of a Pt-Ru-Cu thin film library, demonstrating that it is hard to identify active compositions if an insufficient number of them are explored.19 The compositional map of MOR activity of such a library is shown in Fig. 1; note that a single region of highest activity (more active than Pt-Ru) was found over a relatively small range of compositional space centered at Pt66Ru17Cu17. Except for Pt20Ru20Cu60 catalyst, all compositions having Pt amounts less than 60 at% showed relatively poor performance. Binary Pt-Cu compositions also showed no activity improvement.

Figure 1 Peak current densities of a Pt-Ru-Cu library during potential cycling between −0.06 and 1.34 V at a scan rate of 10 mV s-1. Reproduced from Ref. 19, copyright 2008, with permission from Elsevier.

From combinatorial screening of different ternary/quaternary Pt-Ru-M/Pt-Ru-M-M’ catalysts (Table 2),14-26 the frequencies of Pt, Ru and other transition metals in optimal formulations are shown in Figure 2A. Ruthenium and MT are most often each found in the 20-40 at% range, with Pt making up the remainder (ranging from 20 to 60 at%) in optimized catalysts. As expected, the frequency of the compositions in the range 0-19 at% (low metal content) increased in the order Pt < Ru < MT. Another way to illustrate the “focusing” of compositional ratios is shown in Fig. 2B. The MT/(Ru+ MT) and, in a less marked way, MT/(Pt+ MT) ratios showed most frequent optimized values in the range 26-50 at%. By combining the information from both histograms in Figure 2, it is possible to restrict the range of compositions on which to focus combinatorial screening: to a rough approximation, an optimum Pt:Ru:MT composition should be around 40:35:25. 6 Environment ACS Paragon Plus

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Table 2 The optimum fuel cell MOR catalyst composition for different systems by combinatorial screening. TM: transition metals. System

Optimum catalyst composition

References

Pt1-xMx, (M = TM,Sn) Pt-Ru-Co Pt-Ru-Cu Pt-Ru-W Pt-Ru-Fe Pt-Ru-Mo Pt-Ru-Sn Pt-Ru-Ni Pt-Ru-Os-Ir Pt-Ru-Co-W Pt-Ru-Mo-W Pt-Ru-Ir-Ni Pt-Ru-Ni-Zr

M = Ru, Sn, Zn, In, Fe. xRu = 0.1 Pt18Ru20Co62, Pt12Ru50Co38 Pt66Ru17Cu17 Pt44Ru12W44 Pt31Ru48Fe21, Pt22Ru41Fe37 Pt34Ru39Mo27, Pt26Ru36Mo38 Pt80Ru20 Pt60Ru30Ni10 Pt44Ru41Os10Ir5 Pt43Ru33Co14W10 Pt77Ru17Mo4W2 Pt34Ru30Ir13Ni23, Pt42Ru34Ir12Ni12, Pt45Ru30Ni25 Pt33Ru23Ni31Zr13

[14-16] [15,16] [19] [16] [20] [20] [21] [22] [8] [23] [24] [25] [26]

Pt-Rh-Os Pt-Bi-Pb Pt-Ni-Cr Pt-Co-Cr Pt-Co-Cr-V

Pt62Rh25Os13 Pt61Bi12Pb27 Pt28Ni36Cr36 Pt30Co30Cr40 Pt25Co25Cr25V25

[8] [49] [51] [52] [53]

Figure 2 (A) Frequency of the Pt, Ru and MT (MT = M for ternary catalysts and = (M + M’) for quaternary catalysts) in optimized catalysts

obtained by combinatorial screening. (B) Frequency of compositional ratios in optimum

composition of ternary/quaternary catalysts by combinatorial screening and in a triangular library. In MT/(A + MT) A stands for Pt or Ru. All the frequencies have been obtained by optimum compositions from literature data.14-26



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Comparison of Pt-Ru-MT catalysts by combinatorial screening vs. Pt-Ru-MT bulk catalysts. The main difference between combinatorial spot and large-scale bulk catalysts is essentially related to the amount of prepared materials, very low and high for spot and bulk catalysts, respectively. As reported in the Introduction, the characteristics of combinatorial materials made on milligram scale in small spots in a deposition library or gradient can be quite different than those of corresponding large-scale bulk materials:9 For large-scale bulk and spot materials in the same composition, bulk materials may segregate, and their surface composition, which determines the catalytic activity, can be different from that of spot materials [28]. Moreover, the difference between large-scale bulk and spot materials can come from the gap in preparation and evaluation techniques. As we are aware of, combinatorial libraries cannot be prepared by the identical techniques employed for the well tailored bulk catalysts. High-throughput electrocatalyst screening methods, such as optical screening, SECM, multielectrode half cell and multielectrode full cell, were used to evaluate the catalytic activity of the materials. The large use of the optical screening method is related to the rapidity, low cost, and relatively easy to implement. But this method measures the MOR activity indirectly and the evaluation environment is quite different from real fuel cell conditions. In the case of SECM, the results can be approximate and only semiquantitative. The exact optimum composition could not be determined from the SECM data. But rather these data can suggest more compositionally specific investigations, such as “compositionally zoomed” libraries which expand the active regions, or further characterization by multichannel microelectrode systems and bulk particles of the optimum compositions. The multielectrode half cell provides advantages of the control of each composition and direct measurement of the MOR activity, but stationary set-up of the electrochemical testing station suffers from possibility of crosscontamination. The problem regarding the use of these methods, but this is a problem regarding also the bulk catalysts, the screening is generally carried out in liquid electrolytes and at room temperature, that is, under quite different conditions that those in an operating fuel cell. Thus, it is 8 Environment ACS Paragon Plus

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still challenging to successfully apply these techniques to the discovery of efficient electrocatalysts. In this regard, the multielectrode full cell technique can be considered the best method as it can provide an MOR activity of each spot in real fuel cell conditions. Finally, an important difference between the use of combinatorial spot and conventional bulk methods to evaluate the optimum catalyst composition is the number of composition tested, very high and generally low for combinatorial spot and

conventional large-scale bulk methods, respectively. The optimum

composition of large-scale bulk ternary and quaternary catalysts was evaluated by comparing the MOR activity of carbon supported catalysts (from 3 to 15 compositions in the various dataset) prepared in the same way and tested in the same conditions.29-43 The frequencies of Pt, Ru and other transition metals in optimal formulations are shown in Figure 3A. In optimized catalysts Pt, Ru and MT are most often each found in the 40-59, 20-39 and 0-19 at% ranges, respectively. Compared to the optimized catalyst compositions by combinatorial screening, a higher Pt content and a lower MT content can be observed.. As can be seen in Fig. 3B, the MT/(Pt+ MT) ratios showed most frequent optimized values in the range 0-25 at%, while the optimized values MT/(Ru+ MT) ratio are distributed in a wider 0-50 at% range. As in the case of combinatorial screening, by combining the information from both histograms in Fig. 3, it is possible to restrict the range of compositions on which to focus combinatorial screening:

to a rough approximation, an optimum Pt:Ru:MT

composition should be around 57:35:13, that is, a higher Pt and a lower MT content than the optimum composition by combinatorial screening.



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Figure 3 (A) Frequency of the Pt, Ru and MT in optimized large-scale bulk catalysts (B) Frequency of compositional ratios in optimum composition of ternary/quaternary large-scale bulk catalysts. All the frequencies have been obtained by optimum compositions from literature data.29-43

The disagreement between combinatorial and bulk optimum compositions for the MT/(Pt+ MT) ratio could be ascribed to two counteracting factors: 1) the high number of Pt-poor compositions of the library, as we will discuss in the next section, and 2) the disposition of the electrochemists to compare compositions with high Pt content (Pt is the main catalyst): indeed, among Pt, Ru and MT, platinum was the metal present in the highest content in a large part of the bulk compositions evaluated. In various cases, the best compositions by combinatorial screening were evaluated in bulk form and compared with the state-of-the-art catalyst, showing higher activities than Pt-Ru. Being this comparison not fully satisfactory, we have compared the MOR activity of spot and bulk catalysts in a wide range of compositions. In Figures 4A and B we have compared the dependence of the Pt-RuM (M = Fe and Mo) to Pt-Ru MOR activity of spot and bulk catalysts on the M/(M + Ru) weight ratio, at 70 wt% Pt for Pt-Ru-Fe and at Pt/Ru weight ratio = 2 for Pt-Ru-Mo. Both the spot data were obtained by Lee and Jeon,20 whereas the bulk data for Pt-Ru-Fe were obtained by Scofield et al.,38 and the bulk data for Pt-Ru-Mo were obtained by different sources.44-47 For both ternary catalysts, the relative MOR activity vs. M/(M+Ru) ratio plots of spot and bulk catalysts showed the same parabolic trend, but with different relative activity. For both Pt-Ru-Fe and Pt-Ru-Mo, the relative activity of the bulk catalysts was higher than that of the spot catalysts (47 and 36 % higher 10 Environment ACS Paragon Plus

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at the maximum value, respectively). The M/(M + Ru) weight ratio at the maximum relative activity was near the same for the Pt-Ru-Fe catalysts, but different for the Pt-Ru-Mo (ca. 0.3 and 0.4 for spot and bulk catalysts, respectively). In the latter case, the disagreement between combinatorial spot and large-scale bulk material optimum compositions could be due to surface segregation, that is, for bulk and spot materials with the same composition, the surface composition of the bulk catalyst may be different from that of the spot catalyst, resulting in a different optimum composition.

(A)

(B) Figure 4 Dependence of the Pt-Ru-M (M = Fe (Fig. 4A) and Mo (Fig. 4B)) to Pt-Ru MOR activity of spot and bulk catalysts on the M/(M + Ru) weight ratio, at fixed 70 wt% Pt for Pt-Ru-Fe and at fixed Pt/Ru weight ratio = 2 for Pt-Ru-



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Mo. Both the spot data were obtained by Lee and Jeon; 20 bulk data for Pt-Ru-Fe by Scofield et al.,38 and bulk data for Pt-Ru-Mo by different sources.

44-47

Pt-based Ru-free ternary and quaternary catalysts A major drawback regarding the use of Pt-Ru-based catalyst is Ru dissolution during DMFC operation, its transport across the membrane and reduction on the cathode electrode.48 Thus, to avoid DMFC performance degradation, the achievement of active fuel cell anode Ru-free catalysts is highly desirable. Moreover, to decrease the cost of Pt-based anode catalysts for DMFCs, maintaining a high catalytic activity, a way is the replacement of the expensive Ru, the most effective co-catalyst, with a couple of non-precious metals. Research on Ru-free MOR catalysts is not extensive but encouraging results were obtained. Only as an interesting example of Ru-free ternary Pt-based catalyst for the MOR, being the cost of Rh and Os higher than that of Ru, Reddington et al.8 found that the best catalyst in a small ternary Pt-Os-Rh array, that is, Pt62Rh25Os13, was only slightly inferior to Pt50Ru50. It has to be denoted that this catalyst lies in a ternary region bounded by wholly inactive binaries, Pt-Os and Pt-Rh, where it is hard to expect to find active catalysts. Based on the MOR activity of Pt-Pb, Jin et al.49 screened the ternary Pt-Bi-Pb system for DMFC anode electrocatalysts by a combinatorial method. Thin films with continuous composition spreads of the three elements by magnetron sputtering were deposited at temperatures from ambient to 510 °C. By fluorescence screening, the Pt61Bi12Pb27 catalyst deposited at 400 °C showed the highest MOR activity. Low Bi contents decreased the onset potential by up to 50 mV with respect to Pt-Pb. Considering the improved MOR activity of Pt by the addition of Ni or Co,50 the research group of McGinn51,52 evaluated the effect of the addition a third metal, such as Cr, on the catalytic activity of Pt-Ni and Pt-Co catalysts by combinatorial screening. Thin-film sputtering and a multielectrode half cell system were used for synthesis and characterization of Pt-Ni-Cr and Pt-Co-Cr triangular libraries. Both the optimized ternary compositions, Pt28Ni36Cr36 and Pt30Co30Cr40, had a relatively 12 Environment ACS Paragon Plus

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low Pt content, a high stability and, in a large-scale powder form, a higher MOR activity than PtRu. In the case of the Pt-Ni-Cr system, the region with high current density was very narrow near the Pt28Ni36Cr36 composition, meaning that the MOR activity of the ternary Pt-Ni-Cr catalysts is highly dependent on composition. Alloying of vanadium into optimum Pt-Co-Cr compositions further improved the MOR activity.53 Pt25Co25Cr25V25 showed the highest catalytic activity. Due to higher dissolution rate of Co and Cr than V, the chemical ratio of Cr:Co:V in Pt25Co25Cr25V25 changed from 1:1:1 to 0.74:0.86:1.00 after voltammetry. A Pt-rich Pt-V binary compositional surface (Pt:V = 89.:11) took place after electrochemical testing.

Effect of triangular library on the optimum composition of catalysts by combinatorial methods. Generally, triangular libraries, represented schematically in Figure 5, have been used to screen fuel cell ternary systems.8,15,16,19,20-22,51-55 The circles on the face of the triangle represent ternary catalysts, while the circles at each corner and on the edge represent single and binary catalysts, respectively. The number of compositions (Nc) in a triangular library of a ternary system is given by: Nc = Σ(n) (n + 1),

0 ≤ n (integer) ≤ (∆c)-1

(1)

where ∆c is the minimum difference between two compositions of the same element. For example a commonly used library is Pt1-x-yRuxMy with 0≤x,y≤1 and 0≤x+y≤1, at ∆c = 0.1. In this case, the amount of the three metals is varied between 0 and 100 at% in steps of 10 at% (x,y = 0, 0.1, 0.2, 0.3, etc, n = 10, and Nc = 66). If one wishes to survey catalysts at only high Pt levels (such as ≥ 70 at%), only three ternary compositions (80:10:10, 70:20:10 and 70:10:20) would be required. Of course, more fine tuning of the compositional space rapidly increases the number of measurements required; for ∆c = 0.05 n =20 and Nc = 231. In this case, for Pt ≥ 70 at%, fifteen ternary compositions would have to be screened.



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Figure 5 Combinatorial triangular array map of ternary Pt-Ru-M catalysts for ∆c = 0.1.

Excluding single metals and binary compositions, the frequencies of M/(A+ M) (A = Ru or Pt) atomic ratios for a triangular library are shown in Fig. 2B. In a triangular library the number of ternary compositions with low Pt content (< 50 at%) is higher than that of ternary compositions with high platinum content, as can be seen in Fig. 6A, where the green circles represent ternary compositions with the Pt content < 40 %, and the green + blue circles ternary compositions with Pt content < 50, and in the histogram in Fig. 6B, where the values of the NcPt

Evaluation of the Optimum Composition of Low-Temperature Fuel Cell

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