Benzene Adsorption: A Significant Inhibitor for the Hydrogen Oxidation

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Benzene Adsorption - A Significant Inhibitor for the Hydrogen Oxidation Reaction in Alkaline Conditions Ivana Matanovi#, Hoon Taek Chung, and Yu Seung Kim J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b02228 • Publication Date (Web): 25 Sep 2017 Downloaded from http://pubs.acs.org on September 26, 2017

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The Journal of Physical Chemistry Letters

Benzene Adsorption - A Significant Inhibitor for the Hydrogen Oxidation Reaction in Alkaline Conditions Ivana Matanovic,‡[a,b] Hoon Taek Chung‡[c] and Yu Seung Kim*[c] [a] Department of Chemical and Biological Engineering, University of New Mexico, Albuquerque, New Mexico 87231 USA [b] T-1: Physics and Chemistry of Materials, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 USA [c] MPA-11: Materials Synthesis and Integrated Devices, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 USA AUTHOR INFORMATION Corresponding Author *[email protected]

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ABSTRACT: Slow hydrogen oxidation reaction (HOR) kinetics on Pt under alkaline conditions is a significant technical barrier for the development of high-performance hydroxide exchange membrane fuel cells. Here we report that benzene adsorption on Pt is a major factor responsible for the sluggish HOR. Furthermore, we demonstrate that bimetallic catalysts, such as PtMo/C, PtNi/C and PtRu/C, can reduce the adsorption of benzene, and thereby improve HOR activity. In particular, the HOR voltammogram of PtRu/C in 0.1 M benzyl ammonium showed minimal benzene adsorption. Density functional theory calculations indicate that the adsorption of benzyl ammonium on the bimetallic PtRu is endergonic for all four possible orientations of the cation, which explains the significantly better HOR activity observed for the bimetallic catalysts. The new HOR inhibition mechanism described here provides insights for the design of future polymer electrolytes and electrocatalysts for better performing polymer membrane-based fuel cells. TOC GRAPHICS


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Carbon-neutral power utilization for automotive power, stationary power generation and energy storage medium is an important area of research for minimizing greenhouse gas emissions and diversifying the nation’s reliance of foreign energy sources.1 Hydrogen, derived from steam reforming of natural gas or renewable sources, is an important domestically produced energy resource. Fuel cells that run on hydrogen are capable of transforming the chemical energy contained in hydrogen to electrical energy. In particular, proton exchange membrane fuel cells (PEMFCs) are the most suitable fuel cell for automobile power engines thanks to a fairly low running temperature (~ 80 °C), quick start-up times (< 30 sec), as well as good durability (> 5000 h). However, PEMFCs currently require a high loading of expensive Pt catalysts, ~ 0.2 g kW-1, which is, in part, limiting their widespread commercialization.2 While PEMFCs operate under acidic conditions, hydroxide exchange membrane fuel cells (HEMFCs) have the potential to use earth-abundant elements, e.g., silver, cobalt, carbon, etc., as catalysts because their oxygen reduction reaction activity under alkaline conditions is comparable to platinum group metals.3,4 However, we note that with only a few specific and notable exceptions,5,6 the HEMFC performance reported in most literature is extremely poor, ca. < 500 mW cm-2 peak power density under H2/O2 conditions.7,8 One of the significant causes for low HEMFC performance is a sluggish hydrogen oxidation reaction (HOR) at the anode.9 The sluggish HOR activity of Pt in alkaline solutions (as opposed to acidic solutions) was first investigated by Sheng et al.10 According to their work, the measured HOR exchange current density of polycrystalline Pt and Pt/C nanoparticles in 0.1 M KOH is about 100 times lower than that in 0.1 M HClO4. Based on the higher HOR activity of Ir and Pt-Ru alloy catalysts compared to Pt in alkali metal solutions, Strmcnik et al. suggested that a combination of H atom and hydroxide adsorption sites are required to enhance HOR activity.11 This HOR activity improvement has been achieved by



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adding an oxophilic component, such as Ir or Ru, to Pt.11,12 Others proposed that the strong binding energy of H on Pt in alkali metal electrolytes is responsible for the slow HOR activity.13,14 This interpretation was used to explain the higher HOR activities of Pt nanotubes15 and Pt-based bimetallic catalysts.10,16,17 However, the H-binding energy as a sole descriptor of alkaline HOR activity has remained elusive as others have shown that the pH-dependent shift in the H-binding energy in cyclic voltammetry of Pt(111) is small (if observable at all). Thus, the nature of the ‘hydrogen’ peaks on polycrystalline Pt is unlikely to be associated with the adsorption of hydrogen alone but also includes the effect of the adsorption of oxygenated species.18,19 McCrum et al. proposed that alkali metal cations can favorably adsorb onto Pt and disrupt the solvation of adsorbed hydroxide consequently driving the hydrogen adsorption peak observed in cyclic voltammetry to higher potentials.20 Our group and others have investigated the impact of cation-hydroxide-water co-adsorption on the HOR activity of Pt using alkali organic cation solutions.21-24 The authors found that cation-hydroxide-water co-adsorption on the surface of Pt is a time-dependent chemisorption process that inhibits HOR by limiting hydrogen access to the Pt surface. While those mechanistic studies explain certain aspects of sluggish alkaline HOR processes on Pt, they do not readily explain the large overall performance differences between HEMFCs. The inadequacy of the mechanistic explanation of alkaline HOR becomes more obvious when one also considers that alkali metal solution circulating fuel cells developed in the early 2000s, which showed excellent kinetic performance, e.g. >1.0 A cm-2 at 0.9 V surpassing the PEMFC performance at that time.25,26 To explain the current poor HEMFC performance, we hypothesize that the adsorption of aromatic groups in the polymer electrolytes is a major inhibitor of alkaline HOR activity due to its presence in polymer electrolytes required for HEMFCs. The adsorption


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of benzene and other aromatic species on Pt surfaces has been extensively investigated over the past several decades using cyclic voltammetry, electrochemical mass and infrared spectroscopy, and radiotracer methods.27-35 However, most studies on the adsorption of benzene and other aromatic species were performed in acidic conditions as Pt is particularly sensitive to impurities under acidic conditions.30,36 Fortunately, the adverse impact of the benzene adsorption on Pt has not been an issue in PEMFCs, because benzene-free perfluorinated polymer electrolytes such as Nafion® have been predominantly used for these acid systems. In contrast, avoiding benzenecontaining polymer electrolytes is difficult for HEMFCs because no alkaline-stable and benzenefree perfluorinated polymer electrolytes are currently available.37 To the best of our knowledge, the impact of benzene adsorption on HOR activity in alkaline conditions has not been reported to date. In this study, we compare HOR and cyclic voltammograms (CVs) of carbon supported Pt in the 0.1 M tetramethylammonium hydroxide (TMAOH) and benzyltrimethyl ammonium hydroxide (BTMAOH) solutions to identify the effect of benzene adsorption on HOR activity under alkaline conditions. Furthermore, the impact of benzene adsorption on HOR activity of Pt bimetallic catalysts is also investigated. Density functional theory (DFT) calculations on Pt and Pt-based bimetallic catalysts are used to explain the difference in the HOR activity between Pt and Pt-based bimetallic catalysts. Finally, we demonstrate the importance of benzene adsorption on alkaline HOR activity by comparing it to HOR inhibition mechanisms and using benzene adsorption to explain the disparate HEMFC performance data reported in the literature. Two electrolytes, 0.1 M TMAOH and BTMAOH, were used in this study to identify the effect of benzene adsorption on HOR activity under alkaline conditions. The chemical structures of the two alkali organic molecules are the same except that BTMAOH has a benzyl group that



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replaces one of the methyl groups in TMAOH. Figure 1(a) compares the HOR voltammograms of a Pt/C in 0.1 M TMAOH and BTMAOH. The HOR voltammogram of Pt/C in 0.1 M TMAOH shows a typical HOR shape: the HOR current increases with the potential and reaches the limiting current density of 1.6 mA cm-2. On the other hand, the HOR voltammogram of Pt/C in 0.1 M BTMAOH shows a peculiar behavior: current density drops suddenly at 0.028 V, and then starts to increase again at 0.06 V before reaching the limiting current at ~ 0.3 V.

We measured CVs in both electrolytes (Figure 1(b)). The CV of Pt/C in 0.1 M TMAOH displays typical characteristics of Pt catalysts in alkaline electrolytes.14 However, in 0.1 M BTMAOH there appears an inflection point at ~ 0.05 V similar to the HOR voltammogram. In addition, the CV of Pt/C in BTMAOH is very different from that in TMAOH with the hydrogen desorption peak at 0.29 V is shifted to a more negative potential, ca. 0.19 V. This negative potential shift of the hydrogen desorption peak on Pt is known to be caused by benzene


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adsorption under alkaline conditions.38 At potentials from 0.6 up to 1.2 V, a significant oxidation current was observed, which can be mainly attributed to benzene oxidation.39 No changes in impedance during the CV cycling indicates that the benzene oxidation does not lead to a cleavage of the aromatic ring to produce CO2.40 During the negative sweep, the peak originating from benzene reduction starts to appear at ~ 0.2 V.39 These HOR voltammograms and CV results suggest that the benzene starts to adsorb at a relatively low potential at ~ 0.02 V and does not desorb from the Pt surface at the higher CV potentials. The HOR voltammogram of several Pt bimetallic alloys were also measured in 0.1 M BTMAOH (Figure 2(a)). The current density inflection point observed in the Pt/C sample was reduced in Pt bimetallic alloys. This inflection point becomes more evident for the Pt-alloys in the order of Mo, Ni and Ru, respectively. In fact, the HOR voltammogram of Pt1Ru1/C in 0.1 M BTMAOH becomes identical to that of Pt/C in 0.1 M TMAOH (benzene free solution) with only a slightly lower HOR current density (1.17 mA cm-2 for Pt1Ru1/C as opposed to 1.35 mA cm-2 for Pt/C at 0.05 V). The charge transfer coefficient (α) calculated from the Butler-Volmer equation, which indicates the balance of the anodic and cathodic charge transfers in HOR and HER, was determined as 0.46 for the Pt1Ru1/C, representing the nearly equal anodic and cathodic charge transfer (Figure 2(b)). This charge transfer coefficient is similar to the value of 0.5 that was obtained in 0.1 M KOH.10 In contrast, the charge transfer coefficient for the Pt/C catalyst was 0.07, far off from 0.5, reflecting the benzene adsorption onto Pt only in the HOR potential region.



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Figure 2(c)-(d) compares the CVs for the Pt-based bimetallic catalysts with that of Pt/C in 0.1 M BTMAOH. For the Pt1Ru1/C, a notable reversible oxidation and reduction can be attributed to Ru oxidation/reduction as observed in the CV of Pt1Ru1/C in 0.1 M NaOH solution (Figure S1). More importantly, there is no inflection point during the positive scan, nor a negatively shifted desorption peak, which is consistent with the benzene-free alkali metal solutions.16 This data clearly supports the explanation that the benzene adsorption was greatly suppressed on the Pt1Ru1/C at the HOR potentials. For the Pt1Ni1/C catalysts, the oxidation peak after 0.6 V and the reversible reduction peak below 0.2 V are much smaller than those observed in Pt/C,


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demonstrating that the oxidation of benzene is minimal for Pt1Ni1/C. However, a notable inflection point at ~ 0.05 V indicates that benzene adsorption still occurs with the Pt1Ni1/C. For the Pt3Mo1/C, more distinct inflection point at ~ 0.05 V was observed, indicating that the benzene adsorption is more pronounced compared to the Pt1Ni1/C catalyst. Benzene oxidation and reduction on Pt3Mo/C are also more pronounced compared to what was observed with Pt1Ni1/C. Pt/C shows the highest benzene oxidation/reduction currents. This benzene oxidation current inversely follows the inflection current observed in Figure 2(a), supporting our claim that the oxidation current observed in the CVs is due to benzene adsorption. To confirm that benzene oxidation and reduction is related to benzene adsorption, we performed CVs of amorphous carbon (XC-72) in TMAOH and BTMAOH (Figure S1). The CV of amorphous carbon in BTMAOH is similar to that in TMAOH with smaller double layer capacity. No benzene oxidation/reduction peaks and inflection current due to benzene adsorption was found with BTMAOH. This confirms that that the oxidation and reduction of benzene is related to the benzene adsorption. Density functional theory (DFT) was used to understand the adsorption behavior of BTMA onto the Pt and Pt-alloys. Four different orientations of BTMA were considered (Figure 3(a)) and the corresponding Gibbs free energies of adsorption at zero potential were calculated (Table 1). DFT results show that the benzene ring of the benzyl group parallel to the Pt(111) surface has the highest Gibbs free energy of adsorption, −0.78 eV, which indicates that this is the most favorable orientation of the BTMA cation on the Pt(111) surface. This result is in a good agreement with findings of Soriaga and Hubbard,29,41 who demonstrated that aromatic compounds can easily and non-dissociatively adsorb onto Pt surfaces, primarily in the orientation



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in which the aromatic ring is parallel to the metal surface due to the favorable interaction of aromatic π-electrons of benzene with the electronic cloud around the metal atoms.

Table 1. Gibbs free energy for BTMA cation adsorption (in eV) calculated using DFT with PW91 functional on Pt(111), Pt3Mo1(111), Pt1Ni1(111), and Pt1Ru1(111) surfaces.

Orientation/ System Pt Pt3Mo1 Pt1Ni1 Pt1Ru1

(1)

(2)

(3)

(4)

−0.78 −0.09 +0.07 +0.20

−0.63 −0.15 −0.04 +0.10

+0.01 +0.53 +1.05 +1.15

−0.63 −0.19 0.00 +0.13


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The adsorption energy of BTMA decreases with the increase in the electric field, as increasing the applied electric field will create repulsion between a positive cation and the positively charged electrode (Figure 3(b)). For orientations 2 and 4 similar dependence on the electric field was found. For clarity, only orientation 2 is representatively shown on Figure 3(b). Notably at an electric field > ~ 0.1 V/Å, cation adsorption becomes more favorable than benzene adsorption. Thus, in experiments, the change in orientation of BTMA is expected at potential of ~ 0.3 V if we assume that the electric field acts over a Helmholtz layer with a thickness of 3 Å.42 This result indicates that the recovering HOR current after 0.3 V shown in Figure 1(a) may originate from the configurational change of the adsorbed BTMA molecule. This hypothesis is supported by the fact that the HOR current of Pt with a polymer electrolyte having benzene groups but no cationic groups did not fully recover until Pt oxidation occurred due to the absence of the configurational change.23 In contrast to Pt where benzene adsorption is the strongest, in the case of bimetallic Pt-alloys cation and the orientations in which both trimethylammonium and benzyl group interact with the surface (orientation 4) have the largest free energies of adsorption. For all surfaces, the orientation in which the benzene ring of benzyl group is perpendicular to the surface (orientation 3) is the least probable way that BTMA cation interacts with the metal surface. More importantly, Table 1 also shows that regardless of its orientation, Pt(111) surface has the strongest interaction with the BTMA, followed by the Pt3Mo1(111), Pt1Ni1(111), and Pt1Ru1(111) surfaces. In particular, adsorption of BTMA on Pt1Ru1 is endergonic for all four possible orientations of BTMA and, thus, a negligible poisoning of HOR by BTMA is expected for the Pt1Ru1 bimetallic catalyst. This trend is in excellent agreement with the data observed on the HOR voltammograms shown in Figure 2(a).



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The origin of less-aromatic adsorption on the Pt alloyed catalysts was further investigated by studying the change in the electronic structure of Pt and the electron transfer between BTMA and the metal surfaces induced by alloying. Projected density of states (PDOS) shown in Figure 3(c) demonstrate that alloying Pt with Mo, Ni, and Ru pushes the center of the d-band of surface Pt atoms towards more negative values relative to the Fermi level. The good correlation between the change in the d-band center and the adsorption energies of BTMA suggests increased filling of antibonding orbitals formed by the interaction between BTMA molecular orbitals and the metal surface,43 which in turn decreases the adsorption energy of BTMA on the alloyed catalysts. The effect is most pronounced in the case of alloying Pt with Ru followed by alloying with Ni and Mo, which is again consistent with our experimental data. Charge transfer of 0.97e, 0.88e, 0.85e, and 0.81e was calculated between Pt, Pt3Mo, Pt1Ni1 and Pt1Ru1 surface and BTMA in the orientation with the highest adsorption energy, which correlates well with the changes in the dband and the adsorption energies (Table S1). This result also indicates that there is a binding interaction between BTMA and the metal surface for all the studied systems, which in some cases is not strong enough to compensate for the entropy decrease in the adsorption process. The RDE experiments and DFT calculations indicated that benzene adsorption significantly impacts the HOR activity of Pt-based catalysts. Therefore, it is of particular interest to compare the impact of HOR inhibition by benzene adsorption with other HOR inhibition mechanisms. Figure 4(a) shows the HOR voltammograms of the Pt/C and Pt1Ru1/C in 0.1 M NaOH, which demonstrate the HOR activity difference between the two catalysts. The half-wave potential of Pt1Ru1/C was negatively shifted ca. 1 mV, when compared to that of Pt/C in 0.1 M NaOH, which is only a marginally higher HOR activity for Pt1Ru1/C compared with that of Pt/C. The exchange current density of Pt1Ru1/C is 1.94 mA cm-2 vs. 1.76 mA cm-2 of Pt/C, which is only a 9 %


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difference. Considering that the negative potential shift from Pt/C to Pt1Ru1/C in 0.1 M BTMAOH is 83 mV at the current density at which the half-wave potential was obtained for Pt1Ru1/C, the mechanistic impact of the Pt1Ru1/C catalyst on the HOR activity seems to be insignificant. It has been shown that the half-wave potential of the state-of-the-art Ir/PdRu/C catalyst is also negatively shifted by only 15 mV from the Pt/C control in H2 saturated 0.1 M KOH,44 which is still small compared to the HOR activity improvement observed for the lessbenzene adsorbing Pt1Ru1/C catalyst.

Figure 4(b) shows the impact of cation-hydroxide-water co-adsorption on HOR activity of Pt/C. The Pt/C electrode with the co-adsorbed layer was obtained by exposing the clean Pt/C electrode for 120 min in 0.1 M TMAOH at 0.1 V. The half-wave potential difference between the clean and co-adsorbed electrodes is 27 mV, which is more than 3 times smaller than the potential difference between the Pt/C and Pt1Ru1/C catalysts in 0.1 M BTMAOH. It is hard to



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assess which inhibition mechanism is more significant because the cation-hydroxide-water coadsorption is time-dependent; thus, further HOR activity loss is expected with longer exposure of the Pt/C electrode at 0.1 V. However, it can be concluded that benzene adsorption is the major HOR inhibition mechanism that impacts the initial HEMFC performance when the benzene containing polymer electrolyte is used with the Pt catalyst. Our conclusion that benzene adsorption plays a major role in HOR inhibition appears to be consistent with other’s results: (1) substantially better HEMFC performance with commercial PtRu alloy anode catalysts;16,45 (2) excellent HEMFC performance with radiation grafted poly(ethylene-co-tetrafluoroethylene),45,46 low density polyethylene47 or perfluorinated ionomeric binder,48 which do not have benzene groups in their backbone structure; and (3) consistently poor HEMFC performance when aromatic polymer electrolytes are used with Pt/C catalyst, i.e., the peak power density < 400 mW cm-2.7,8,49,50 Based on our results, the design of advanced anodes for HEMFCs would benefit from: (1) the development of HOR catalysts with less benzene-adsorbing characteristics, (2) the synthesis of polymer electrolytes with less or no hydrogen functionalized aromatic groups, and (3) a better understanding of the structure-adsorption property relationship between benzene and benzene derivatives and the catalytic metal surfaces in alkaline conditions. In conclusion, we have shown for the first time that the HOR activity of Pt is inhibited by benzene adsorption. Comparing the HOR voltammograms representing other HOR inhibition mechanisms, our results demonstrate that benzene adsorption in the orientation in which the benzene ring is parallel to the Pt surface has a major influence on the HOR activity. The adsorbed benzene parallel to the Pt surface can desorb at hydrogen evolution potential, ca. < 0.0 V vs. RHE, due to hydrogen generation, as well as at higher hydrogen oxidation potentials, ca. > 0.3 V vs. RHE, due to the configurational change driven by the ammonium adsorption, which is


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a more favorable mode at the high potential. This suggests that polymer electrolytes without cation-substituted benzene groups in the polymer backbone49,51, may have a more negative impact on HEMFC performance than the ammonium substituted benzyl group. The results from this research suggest that the HEMFC performance with properly designed electrode materials can demonstrate excellent performance that is even superior to PEMFC performance, as reported by others.5,6 The results from this research may also be applicable to other fuel cell systems, such as phosphoric acid-doped high temperature polymer electrolyte membrane fuel cells52,53 and low temperature polymer electrolyte membrane fuel cells using polyaromatic electrolytes54, which showed relatively poor performance by the possible adsorption of the aromatic moiety onto Pt surface. Further studies regarding the effect of benzene concentration, substituents, and other aromatic or heterocyclic compounds with π electrons are needed for a more precise interpretation of the acidic fuel cell performance in the presence of benzene adsorption. Moreover, the effect of benzene adsorption on the oxygen reduction reaction needs to be better understood as some reports indicate that aromatic impurities of cathode air stream adversely impact the PEMFC performance.35,55,56

ASSOCIATED CONTENT Supporting Information. Details of the experimental and computational works. CVs, , XRD patters of Pt/C, Pt3Mo/C, Pt1Ni1/C, and Pt1Ru1/C, impedance spectra and adsorption energies for BTMA cation. Notes ‡

These authors contributed equally.

The authors declare no competing financial interests. 15 ACS Paragon Plus Environment


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ACKNOWLEDGMENT This work was supported by the US Department of Energy, Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office. Los Alamos National Laboratory is operated by Los Alamos National Security, LLC under Contract DE-AC52-06NA25396. Computational work was performed using the computational resources of Los Alamos National Laboratory, NERSC, which is supported by the Office of Science of the U.S. Department of Energy and CNMS, sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.

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