Suppression Effect of Small Organic Molecules on Oxygen Reduction

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Suppression Effect of Small Organic Molecules on Oxygen Reduction activity of Fe/N/C Catalysts Yu-Cheng Wang, Yu-Jiao Lai, Li-Yang Wan, Hong Yang, Jiao Dong, Long Huang, Chi Chen, Muhammad Rauf, Zhi-You Zhou, and Shi-Gang Sun ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b00516 • Publication Date (Web): 18 May 2018 Downloaded from http://pubs.acs.org on May 19, 2018

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ACS Energy Letters

Suppression Effect of Small Organic Molecules on Oxygen Reduction activity of Fe/N/C Catalysts Yu-Cheng Wang, † Yu-Jiao Lai, † Li-Yang Wan, † Hong Yang, † Jiao Dong, † Long Huang, ‡ Chi Chen, † Muhammad Rauf, † Zhi-You Zhou, †, * and Shi-Gang Sun† †

State Key Laboratory of Physical Chemistry of Solid Surfaces, Collaborative innovation center

of Chemistry for Energy Materials, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, China ‡

Sino-Precious Metals Holding Co., Ltd., 650106, Kunming, Yunnan, China

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Z.Y.Z.)

ACS Paragon Plus Environment

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ACS Energy Letters

ABSTRACT: Pyrolyzed Fe/N/C catalysts have high selectivity for catalyzing oxygen reduction reaction (ORR), and exhibit no activity toward the electrooxidation of small organic molecules (SOMs). As such, their tolerance toward SOMs has rarely been questioned. Unexpectedly, we found that SOMs can greatly suppress the ORR activity of microporous-type Fe/N/C catalysts in alkaline medium. Such suppression effect is strengthened as molecular polarity decreases or molecular weight increases, and can be attributed to the blockage of micropore transport channels for ORR-related species by SOMs adsorption. Interestingly, such suppression effect by micropore filling disappears in the acidic medium. High tolerance to SOMs in acidic medium is attributed to the protonation of pyridinic N, which increases the polarity of micropore surrounding, and inhibit the adsorption of low-polarity SOMs. This study is helpful to guide the synthesis of porous Fe/N/C electrocatalysts, especially when they are applied in an alkaline medium.

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ACS Energy Letters

Oxygen reduction reaction (ORR) is a key reaction for electrochemical energy conversion and storage devices such as fuel cells and metal-air batteries. High-performance ORR cathode requires not only high catalytic activity but also fast mass transport.1 If the mass-transport channels are blocked, the apparent ORR activity will be suppressed severely although the intrinsic reaction kinetics is fast.2 Excellent resistance to small organic molecules (SOMs, such as CH3OH, C2H5OH, and HCOOH) is another important property for ORR catalysts.3 Volatile organic compounds from the air can negatively affect the performance of the oxygen electrode by adsorption on Pt surface, decreasing the area of reactive surface.3 Especially in liquid-feed fuel cells with SOMs as fuel, the resistance of SOMs crossover from the anode to cathode becomes more critical.4-5 SOMs can be readily oxidized on Pt catalysts, so that they can markedly decrease the performance of Pt-based cathode.6 Pyrolyzed Fe/N/C catalysts have been considered as the most promising non-precious metal (NPM) catalyst for ORR.7-10 One of the advantages is that Fe/N/C catalysts have no catalytic activity toward SOMs.11 As such, their tolerance to SOMs has rarely been questioned. However, this recognition is only based on the intrinsic reaction kinetics of ORR, and the effects of SOMs on mass transport have not been considered. Pore structure is a critical property for mass transport of ORR catalysts.12 Unlike conventional Pt/C catalysts, whose active sites are on the surface,13 highly active Fe/N/C catalysts need abundant micropores to host active sites.14-17 Such microporous structure makes Fe/N/C catalysts similar to activated carbon. It is well known that activated carbon materials show strong adsorption toward SOMs.18-19 This implies that the micropore mass-transport channels of Fe/N/C catalysts have the risk of being blocked by SOMs adsorption.


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In this study, we found that methanol and ethanol can considerably suppress the ORR performance of a microporous-type Fe/N/C catalyst in alkaline medium, which is rarely observed before. By systematically investigating the selection of suppression effect toward different organic molecules and various types of Fe/N/C catalysts, we found the suppression level depends on molecular size and polarity of organic molecules, as well as the microporous surface of Fe/N/C catalysts. Based on the above results, we concluded the suppression effect originates from micropore filling by SOMs adsorption, which can block mass transport channels of ORRrelated species and suppress the ORR activity. Furthermore, this suppression effect highly depends on the solution pH values: microporous-type Fe/N/C catalyst is susceptible to SOMs in alkaline medium, but is insensitive in acidic medium. This dependence comes from the change of microporous surface polarity caused by protonation/deprotonation of pyridinic-N under different pH environment, thus altering the adsorption capacity of low-polarity SOMs in micropores. The microporous-type Fe/N/C catalyst investigated in this study was prepared through hightemperature pyrolysis with phenylenediamine (PDA) as nitrogen surface, according to the previous method,20 and denoted as PDA-Fe/N/C. We first tested the suppression effect of SOMs on ORR activity in alkaline medium, since Fe/N/C catalyst can perform stably in alkaline medium, and the degradation itself can be neglected. It is generally accepted that alcohols (e.g., methanol and ethanol) have little effect on ORR performance of Fe/N/C catalysts. Figure 1a shows the ORR polarization curves of PDA-Fe/N/C in 0.1 M NaOH solution with and without the addition of 0.5 M methanol and ethanol. Obviously, the suppression effect of methanol on the ORR activity of PDA-Fe/N/C is quite serious, with a 32 mV negative shift of half-wave potential (E1/2) and a drop of 60% of instinct kinetic current at 0.9 V (RHE) after mass-transfer correction by Koutecky–Levich equation, as shown in Figure 1b. While for ethanol, the suppression


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ACS Energy Letters

phenomenon was even more severe with a negative shift of E1/2 by 46 mV and kinetic current drop of 80%. Such severe suppression phenomenon occurred on the PDA-Fe/N/C is contrary to the excellent resistance to alcohols of non-Pt ORR catalysts reported previously.21-24 Note that the effect of SOMs on the H2O2 yield is difficult to be evaluated due to massive oxidation of SOMs on the ring electrode. To make clear the mechanism of suppression effect occurred on PDA-Fe/N/C, a series of SOMs with different molecular size and polarity were tested. Figure 1c compares the kinetic current at 0.90 V of PDA-Fe/N/C measured in 0.1 M NaOH solution with the presence of 14 organic molecules, relative to the blank solution. The concentration of organic molecules was fixed at 20 mM to avoid the significant change in the physical properties of the aqueous solution (i.e., permittivity). Surprisingly, all organic molecules can decrease the ORR performance of PDA-Fe/N/C. This result suggests that the suppression effect is unlikely from a specific toxic interaction between active sites and organic molecules. Moreover, we found that the suppression effect highly depends on the molecular polarity and size of SOMs. Strong-polarity molecules, such as methanol, ethanol, dimethylsulfoxide (DMSO), and acetonitrile, had less suppression effect, while low-polarity molecules, such as chloroform and ethyl ether, can substantially deactivate the catalyst. For alcohols molecules, the suppression effect increases with increasing the length of the carbon chain (labeled by red in Figure 1c), namely molecular size. Such dependence of suppression effect on molecular polarity and size is consistent with the adsorption capacity of organic molecules on activated carbon.18-19 Therefore, the suppression effect seems to come from the physical adsorption of organic molecules in the micropores of PDA-Fe/N/C catalyst, thus blocking the transport channels of ORR-related reactants. To verify whether the observed activity loss come from the physical adsorption of organic molecules in the micropores


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of PDA-Fe/N/C catalyst, a washing experimental was carried out. In this experiment, the deactivated electrode was washed in deionized water for 3-5 min to remove physically adsorbed organic molecules (ethanol and THF in this case), followed by re-tested in 0.1 M NaOH without SOMs. The ORR activity of PDA-Fe/N/C could be almost recovered after washing (Figure S1).

Figure 1. (a) Polarization curves of PDA-Fe/N/C in O2-saturated 0.1 M NaOH solution with and without 0.5 M methanol or ethanol. PDA-Fe/N/C catalyst loading: 0.60 mg cm-2; Scan rate: 10 mV s-1; Rotating rate: 900 rpm. (b) Kinetic current at 0.90 V of PDA-Fe/N/C catalyst in 0.5 M methanol and ethanol relative to the blank solution. (c) Kinetic current of PDA-Fe/N/C at 0.9 V in 0.1 M NaOH solution with the addition of 14 organic molecules (20 mM) relative to the blank solution. C4H8O2: 1,4-dioxane; primary alcohols were used. Inconsistent with previously reported NMP catalysts, PDA-Fe/N/C catalyst is extremely sensitive to SOMs. To figure out the key factor that determines the suppression effect, we tested another three Fe/N/C catalysts prepared from different nitrogen sources, that is, polyaniline (PANI),8 aminothiazole (AT),25 and Schiff base networks (SNW).26 The phase structure and morphology of these four catalysts were investigated by XRD and SEM (Figure S2). Among the four catalysts, PANI-Fe/N/C and AT-Fe/N/C contained some crystalline FeS, while no


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ACS Energy Letters

crystalline Fe species were found in PDA-Fe/N/C and SNW-Fe/N/C. Figure 2a-2d show the ORR polarization curves of the PDA-Fe/N/C, PANI-Fe/N/C, AT-Fe/N/C, and SNW-Fe/N/C, in O2-saturated 0.1 M NaOH solution with different concentration (0.02 ~ 0.5 M) ethanol. The polarization curve recorded in the blank solution without ethanol was also shown for comparison. Clearly, the PDA-Fe/N/C exhibited the highest ethanol suppression effect with a negative shift of E1/2 by 46 mV, which is nearly twice larger than that of PANI-Fe/N/C in 0.5 M ethanol solution. By contrast, no obvious activity loss was observed for both AT-Fe/N/C and SNW-Fe/N/C catalysts. Similarly, the sensitivity of the four catalysts to 1,4-dioxane also follows the same order of PDA-Fe/N/C > PANI-Fe/N/C > AT-Fe/N/C ≈ SNW-Fe/N/C (Figure S3). Considering the suppression phenomenon may be related to the transport of ORR-related species, the pore structures of the four catalysts were tested. Figure 2e-2h show the N2/Ar adsorption-desorption isotherms of the four Fe/N/C catalysts. PDA-Fe/N/C shows type-I isotherm (Figure 2e) with large adsorption capacity in the low pressure region, implying it is a typical microporous material. The isotherm of PANI-Fe/N/C is similar to that of PDA-Fe/N/C except the quick increase of nitrogen adsorption at a relatively high pressure, which indicates the coexistence of micropores and mesopores. By contrast, both AT-Fe/N/C and SNW-Fe/N/C show the type-IV isotherms with a remarkable hysteresis loops, indicating the nature of the mesopores. Quantitatively, Brunauer-Emmett-Teller (BET) surface areas of PDA-Fe/N/C, PANI-Fe/N/C, AT-Fe/N/C, and AT-Fe/N/C were 650, 229, 929, and 935 m2 g-1, respectively. And micropore surface areas were 498, 129, 120, and 191 m2 g-1, respectively, according to the t-plot analysis. We found that the loss of ORR activity in 0.5 M ethanol can be well correlated with the fraction of micropores surface (i.e., the ratio of micropore surface area to BET surface area), as shown in Figure 3a.


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Figure 2. (a-d) Polarization curves of (a) PDA-Fe/N/C; (b) PANI-Fe/N/C; (c) AT-Fe/N/C; (d) SNW-Fe/N/C in 0.1 M NaOH solution with the addition of ethanol at concentration ranging from 0.02 to 0.5 M. Catalyst loading: 0.60 mg cm-2; Scan rate: 10 mV s-1; Rotating rate: 900 rpm. (e) Ar adsorption/desorption isotherm of PDA-Fe/N/C; (f-h) N2 adsorption/desorption isotherm of (f) PANI-Fe/N/C; (g) AT-Fe/N/C; (h) SNW-Fe/N/C. Note that, in the case of PDA-Fe/N/C, almost all of pores were micropores, thus Ar was used as adsorbate in isotherm test to avoid N2 quadrupole interactions. There are massive reports of methanol tolerance of Fe/N/C catalysts. To better understand the correlation between micropore surface fraction and the SOMs suppression effect, we collected a series of methanol tolerance data of Fe/N/C catalysts in alkaline medium reported previously,9, 23, 27-32

as well as our test result for above four Fe/N/C catalysts. The relationship between the

suppression effect and micropore surface fraction is displayed in Figure 3b. Obviously, at micropore surface fraction smaller than 0.37, the Fe/N/C catalysts exhibit excellent resistance to methanol (0.5 to 3M) with above 90% of the kinetic current remained. When the micropore surface fraction surpasses a critical value of 0.37, the relative kinetic current of Fe/N/C catalysts


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nearly decreases linearly with increasing the micropore surface fraction, further demonstrating

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Figure 3. (a) Relative kinetic current at 0.9 V with the addition of 0.5 M ethanol vs. microporous surface fraction of Fe/N/C catalysts. The activity obtained in blank solution was used as the reference. (b) Relative kinetic current at 0.9 V with the addition of methanol (0.5 ~ 3 M) vs micropore surface fraction of Fe/N/C catalysts. The red squares represent four catalysts tested in this study. The black squares were extracted from previously published literature.9, 23, 2732

On basis of the above results, we proposed a possible model to illustrate the selective suppression phenomenon of organic molecules for microporous-type Fe/N/C catalysts (Figure 4). As for microporous-type Fe/N/C catalysts, such as PDA-Fe/N/C, low-polarity organic molecules can be readily adsorbed into the micropores. Although Fe/N/C catalysts have no catalytic activity toward organic molecules, the physical adsorption of organic molecules can fill the micropores to block the mass transport channels, thus impeding ORR-related reactants access to active sites that are mostly located in micropores (Figure 4a). Because of the lack of high specificity of


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physical adsorption in micropores, almost all organic molecules can, more or less, suppress the ORR performance of the microporous-type Fe/N/C catalysts. Meanwhile, due to the low polarity of the carbonaceous surface, low-polarity molecules can be adsorbed preferentially. Thus, they exhibit severe suppression effect. Besides molecular polarity, molecular size is another factor that can influence the suppression level. As for organic molecules adsorption in carbon materials, the adsorption capacity will increase with increasing molecular size due to large van der Waals' force. Large molecules are also easy to block the mass transport channels in micropores. So that, it is reasonable to observe the suppression phenomenon shown in Figure 1c: The lower polarity or the larger the molecular size of SOMs, the more severely the ORR activity of target catalyst declines. However, on the scale of mesopores and macropores, SOMs have a low possibility to block transport channels (Figure 4b). Therefore, increasing mesopores and macropores of Fe/N/C catalysts will reduce the suppression effect. Moreover, as illustrated in Figure 4c, the incorporation of mesopores can also alleviate the suppression responses of microporous active sites for SOMs, because more entrances are created for micropores, and transport channels become more open. Consequently, as the micropores surface fraction decreases to a critical value (i.e., 0.37 for methanol), the suppression effects begin to disappear.


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ORR

Figure 4. Proposed models of suppression effect by small organic molecules. (a) Micropores can be filled by SOMs (pink box) adsorption, thus impeding ORR-related reactants access to active sites (red point). ORR activity is suppressed. (b) On the scale of mesopores, SOMs have a low possibility to block transport channels. (c) The incorporation of mesopores can alleviate the suppression effect by creating more entrances for micropore channels, as indicated by yellow arrows. Interestingly, we found that the suppression effect of SOMs toward Fe/N/C catalysts highly depends on the solution acidity/alkalinity. We selected the PDA-Fe/N/C catalyst and three organic molecules (i.e., ethanol, 1,4-dioxane, and n-butanol) with different molecular polarity and size, to evaluate the suppression phenomenon in 0.1 M H2SO4. The i-t curves at near the E1/2 were recorded for about 100 s, and then organic molecules were injected to reach a concentration of 20 mM to observe the response of ORR current (Figure 5a). The advantage of the i-t curve test is the self-degradation of Fe/N/C catalysts in the acid medium can be minimized. The responses of the PDA-Fe/N/C toward organic molecules in the alkaline medium were also tested by the


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same method, as shown in Figure 5b for comparison. Clearly, all of three organic molecules have little influence on the ORR activity of PDA-Fe/N/C in acidic medium, which is greatly different from that observed in alkaline medium. The excellent resistance to organic molecules implies that organic molecules are difficult to be adsorbed into micropores in acidic medium. We found that such pH-dependent suppression effect may be correlated with the protonation/deprotonation of pyridinic N (Figure 5c). It is known that pyrolyzed Fe/N/C catalysts contain abundant pyridinic N that has alkalinity from lone-pair electrons. As for PDA-Fe/N/C, the weight content of pyridinic N was about 1.3%.20 In alkaline medium, these pyridinic N species are not protonated (i.e., electric neutrality), and thus the microporous surface of the catalysts has a low polarity. Therefore, low-polarity organic molecules can be readily adsorbed into micropores, thus impeding the transport of ORR-related reactants. While in acid medium, pyridinic N species will be protonated to form pyridinium ion (Py + H+ → PyH+), as demonstrated by the fact that the corresponding N1s XPS peak of pyridine N at 398.2 eV was disappeared after sulfuric acid was added (Figure S4). The surface charge increases significantly the polarity of micropore surrounding, so that the adsorption of low-polarity organic molecules will be suppressed greatly. Therefore, PDA-Fe/N/C exhibits excellent resistance to organic molecules in the acid medium. It is worth noting that electrode potential can also change surface charge density, thus changing surface polarity and the adsorption amount of the SOMs.33 Typically, the adsorption amount of SOMs reaches a maximal at the potential of zero charge (PZC). Away from the PZC, the adsorption amount of SOMs decrease quickly. However, the suppression effect of SOMs on ORR performance of PDA-Fe/N/C could be observed in a widerange potential region in alkaline medium. So that, the suppression effect of SOMs may mainly come from physical adsorption of SOMs, not from electrochemical adsorption.


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To verify the proposed models, we tested the adsorption capacity of organic molecules on the PDA-Fe/N/C in both alkaline and acidic media (Figure 5d). The concentration of organic molecules before and after the adsorption by the PDA-Fe/N/C catalyst was analyzed by 1H nuclear magnetic resonance (NMR) (Figure S5). The adsorption capacity of PDA-Fe/N/C catalyst toward different SOMs (5 mM) in 0.1 M NaOH solution follows the order: ethanol (0.10 mmol gcat-1) < 1,4-dioxane (0.16 mmol gcat-1) < n-butanol (0.34 mmol gcat-1). Obviously, the order of adsorption capacity is positively correlated with the suppression effect, further confirming that the observed ORR activity loss originates from the adsorption of SOMs in alkaline medium. While in 0.1 M H2SO4 solution, the adsorption capacity of SOMs greatly decreases, which is in good agreement with the excellent resistance of PDA-Fe/N/C toward SOMs. To confirm the pHdependent adsorption capacity caused by the protonation/deprotonation of pyridinic N, we further tested the adsorption capacity of n-butanol on a microporous activated carbon (KJ600 carbon black, BET area of ~1500 m2 g-1) without the pyridinic N (Figure S6). As expected, the adsorption amount in 0.1 M H2SO4 solution is close to that in 0.1 M NaOH solution (Figure 5d).


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Figure 5. Chronoamperometric responses of PDA-Fe/N/C in O2-saturated 0.1 M H2SO4 at 0.80 V (a) and 0.1 M NaOH at 0.9 V (b) with the addition of 20 mM organic molecules at 100 s. Catalyst loading: 0.60 mg cm-2; Scan rate: 10 mV s-1; Rotating rate: 900 rpm. (c) Proposed mechanism for pH-dependent suppression phenomenon of microporous-type Fe/N/C catalysts by organic molecules. In acidic solution, the protonation of pyridinic N will greatly increase the polarity of micropore surrounding, which inhibits the adsorption of low-polarity organic molecules. (d) Adsorption capacity of three organic molecules on PDA-Fe/N/C and KJ600 carbon black in 0.1 M NaOH and 0.1 M H2SO4 solution. Test conditions: 30 mg of PDA-Fe/N/C or KJ600 carbon black in 3.0 mL solution containing 5 mM organic molecules. In conclusion, this study provides new insight into the effect of small organic molecules on Fe/N/C catalysts for ORR, as well as its dependence on catalyst porosity and solution pH value. Small organic molecules can suppress seriously the ORR performance of microporous-type


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Fe/N/C catalysts through micropore filling in alkaline medium, to block mass transport channels. Much less suppression effect observed in acidic medium is attributed to the protonation of pyridinic N, which increases the polarity of micropore surrounding, and inhibits the adsorption of organic molecules. The gained knowledge is helpful to guide the synthesis of porous Fe/N/C electrocatalysts in the aspect of pore structures. ASSOCIATED CONTENT Supporting Information. The supporting Information is available free of charge on the ACS Publications website at DOI: Additional ORR data and physical characterization curves. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Z.Y.Z.) Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This study was supported by grants from National Key Research and Development Program of China (2016YFB0101202) and NSFC (91645121 and 21621091).

REFERENCES 1. O’Hayre, R.; Barnett, D. M.; Prinz, F. B., The Triple Phase Boundary. J. Electrochem. Soc. 2005, 152, A439-A444.


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Suppression Effect of Small Organic Molecules on Oxygen Reduction

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