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Boosting the Oxygen Reduction Electrocatalytic Performance of Non-Precious Metal Nanocarbons via Triple Boundary Engineering using Protic Ionic Liquids Mo Qiao, Guillermo A. Ferrero, Leticia Fernández Velasco, Wei Vern Hor, Yan Yang, Hui Luo, Peter Lodewyckx, Antonio B Fuertes, Marta Sevilla, and Maria-Magdalena Titirici ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18375 • Publication Date (Web): 28 Feb 2019 Downloaded from http://pubs.acs.org on March 2, 2019

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ACS Applied Materials & Interfaces

Boosting the Oxygen Reduction Electrocatalytic Performance of Non-Precious Metal Nanocarbons via Triple Boundary Engineering using Protic Ionic Liquids Mo Qiao[a,e], Guillermo A. Ferrero[b], Leticia Fernández Velasco [c], Wei Vern Hor[a], Yan Yang[a], Hui Luo[a], Peter Lodewyckx[c], Antonio B. Fuertes[b], Marta Sevilla[b], Maria-Magdalena Titirici*[a, d, e]. [a] School of Engineering and Materials Science, Queen Mary University of London, London, E1 4NS, UK [b] Instituto Nacional del Carbon (CSIC), P.O. Box 73, Oviedo 33080, Spain [c] Department of Chemistry, Royal Military Academy, Avenue Renaissance 30, 1000 Brussels, Belgium [d] Materials Research Institute, Queen Mary University of London, London, E1 4NS, UK [e] Department of Chemical Engineering, Imperial College London, South Kensington Campus, London, SW7 2AZ, UK Keywords: Non-precious metal catalysts, electrocatalysis, triple phase boundary, interface reaction, oxygen reduction reaction

Abstract The oxygen reduction reaction in aqueous media plays a critical role in sustainable and clean energy technologies such as Polymer Electrolyte Membrane (PEM) and Alkaline Fuel Cells 1 ACS Paragon Plus Environment

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(AFC). In this work we present a new concept to improve the ORR performance by engineering the interface reaction at the electrocatalyst-electrolyte-oxygen triple-phase boundary (TPB) using a protic and hydrophobic ionic liquid and demonstrate the wide and general applicability of this concept to several Pt-free catalysts. Two catalysts, a Fe-N co-doped and a metal-free N-doped carbon electrocatalysts are used as proof of concept. The ionic liquid layer grafted at the nanocarbon surface creates a water-equilibrated secondary reaction medium with a higher O2 affinity towards oxygen adsorption, promoting the diffusion towards the catalytic active site, while its protic character provides sufficient H+/H3O+ conductivity, and the hydrophobic nature prevents the resulting reaction product water from accumulating and blocking the interface. Our strategy brings obvious improvements in the ORR performance in both acid and alkaline electrolytes, while the catalytic activity of FeNC-nanocarbon outperforms commercial Pt-C in alkaline electrolyte. We believe that this research will pave new routes towards the development of high-performance ORR catalysts free of noble-metals via careful interface engineering at the triple point.

1. Introduction The literature on the design of highly active and stable electrocatalysts for oxygen reduction reaction (ORR) for next-generation energy technologies, such as fuel cells and metal-air batteries, has rapidly expanded.1-3 ORR involves complicated multiple proton-electron transfers, and is kinetically sluggish due to the high-energy barrier to break the O-O bond.4-5 Despite intensive research in ORR to date, precious metals, especially Pt, are still an indispensable component for achieving a good electrocatalytic performance, especially under acidic conditions. However, the high cost, scarce reserves, and poor

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stability of Pt in acidic media brings about tremendous challenges, hindering their sustainable commercialisation. Non precious metal carbon electrocatalysts have been considered a promising alternative due to their low cost, tuneable textural properties, easy functionalization and high electrical conductivity.6-9 In order to achieve an ideal nanocarbon electrocatalyst, much effort has been focused on the improvement of the intrinsic active sites including heteroatoms, edges, and defects,10-12 as well as the critical matrix properties such as conductivity, surface area and porosity.13 Nitrogen doped carbons incorporating transition metals such as Fe, Co, Ni have demonstrated comparable ORR activity to commercial Pt-C in both acid and alkaline electrolytes.14-18 However, most of the carbon-based electrocatalysts show a limited catalytic performance in acid electrolyte as a result of the lack of effective active sites and leaching of the transition metal during the electrochemistry process.19-20 Since ORR is a gas-solid-liquid process, the reaction takes place at the triple-phase boundary (TPB) of the solid electrocatalyst, liquid electrolyte, and oxygen, where several steps significantly influence the overall reaction: i) mass diffusion of oxygen, proton and the produced water towards/from the electrochemical surface in electrolyte; ii) surface reaction involving the adsorption of reactants, interfacial charge transfer, and the desorption of intermediates/products near active sites.21-24 Therefore, the improvement of the TPB interface, where the ORR reaction actually happens, is critical. Recently, hydrophobic ionic liquids (ILs) with a high O2 solubility were employed to modify Pt nanoparticles, leading to improved kinetics, selectivity, and durability in ORR performance.25-29 The IL layer can favour the O2 transport towards the catalyst’s surface, preserve the active sites from water covering and protect the Pt from oxygenated species or poisoning, leading to an impressive activity enhancement.29 However, the optimization of the catalyst/electrolyte/oxygen gas interface in nanocarbon catalysts has been rarely explored in the ORR literature up to date. Much effort has been devoted to 3 ACS Paragon Plus Environment


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engineering the catalyst surface by tuning wettability in order to improve the accessibility of reactants to active centres, and therefore increase the surface and mass utilization efficiency of catalysts.30-32 The reported tuning methods are mostly based on an heteroatom doping technique which inevitably changes the matrix properties, such as porosity and surface chemistry. In addition, these techniques introduce more active sites, so that the independent role of wettability or aerophilicity remains elusive. Sun and coworkers have been the only group to propose the “superaerophilic” surface concept to create more threephase contact points in a micro-/nanostructured electrode using Co/N-containing carbon nanotube arrays.33 They used a poly(tetraﬂuoroethylene) modification of the interface to increase the ORR performance in both acid and base media, to a level comparable to commercial Pt/C films. Several groups have recently reviewed the modification of the interface to promote the gas-involved reaction in electrocatalysts.21, 24, 34 The criteria for a suitable second component for surface modification is strict: i) it should be electrochemically stable under the tested voltage range; ii) it must have a high O2 solubility to facilitate the mass transfer; iii) it must be hydrophobic but with reasonable water solubility to facilicate the water transfer path; iv) it should be protic to facilicate the proton transfer for ORR reaction in acid electrolyte. Our group has previously reported a first initial study using oxygenophilic and hydrophobic IL to improve the TPB interface reaction of metal-free graphite nanocarbon catalysts, by promoting the oxygen adsorption and facilitating a water-equilibrated hydrophobic layer to expel the produced water from poisoning the active sites.3 In this current study, we further extend this concept to prove its general character towards porous Fe-N-C and N-C amorphous carbon catalysts. Oxygenophilic, hydrophobic and protic ionic liquid, 1-Ethylimidazolium bis(trifluoromethylsulfonyl) imide, has been applied to conduct in-depth investigations for a better understanding of the underlying mechanisms. 2. Results and discussion


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1-ethylimidazolium bis(trifluoromethylsulfonyl) imide was selected in this study to modify the TPB microenvironment between electrocatalyst, electrolyte and O2 molecule. Its chemical structure is depicted in Fig. S1. The schematic diagram in Fig. 1 shows the TPB interface between solid, liquid and gas phases. The ionic liquid forms a water-equilibrated secondary layer that promotes the mass transfer towards and from the catalyst surface. A comparison between the SEM images in Fig. 1 and the TEM images in Fig. S2 before and after the IL coating shows similar carbon spheres with a diameter of ca. 300 nm. However, the surface of the spheres in the inset in Fig. 1b is visibly smoother than the rough surface in Fig. 1a, which provides some evidence of the existence of the coating layer of IL. In fact, in the case of the FeNC catalyst with IL layer (FeNC-IL), it is more difficult to obtain a sharp focus during the SEM characterization operation, which is mainly attributed to the relative low electronic conductivity of the IL layer. This observation directly demonstrates the existence of a thin and homogeneous organic layer on the surface of the carbon electrocatalyst. The presence of the IL layer is further supported by a slight decrease in the mesopore volume, as shown by N2 physisorption (Fig. 2a and Fig. S3). The specific surface area (SSA, calculated based on the mass of carbon) and pore volume are summarized in Table S1. After coating with IL, the negative shift of the peak from pore widths of ca. 7.6 nm to ca. 4.2 nm (Fig 2a), indicate the presence of a thin IL layer of ca. 3 nm, which was consistent with the reduction of mesopore volume and consistent with our previous report on metal free graphite catalysts.3 On the other hand, as can be seen in Fig. S4, the water contact angle increased from 127º for pristine carbon to 143º for the ILcoated carbon surface, indicating improved hydrophobicity. This result is further quantitatively confirmed by the water adsorption isotherms in Fig. 2b. The upswing in the water adsorption isotherm of FeNC-IL is displaced by 10% to the higher relative humidity compared to FeNC, demonstrating the increased hydrophobicity introduced by the IL layer, and there is a reduction in the total water uptake from 135% (FeNC) to 122% (FeNC-IL) due to the changes induced in both the textural and surface properties of the catalyst surface after the incorporation of the IL. Both materials show negligible water uptake in the low relative humidity (RH) region, which shows their hydrophobic character.35-36 5 ACS Paragon Plus Environment


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FT-IR was conducted to investigate the surface chemistry of the electrocatalyst (Figure 2c). After incorporation of the IL, additional absorption bands are observed between 10001300 cm-1, which can be ascribed to SO2 and CF3 vibrations. Strengthened absorption peaks at 3139, 3075 and 2979 cm-1 result from the C-H stretching vibration mode of the imidazole ring and the alkyl chains.37 The surface composition of the different elements was investigated by means of XPS measurements and is given in Figure 2d and Table S2. The N 1s spectra can be deconvoluted into pyridinic-N, Fe-Nx, graphitic-N and oxidized-N at the binding energies of 398.3, 399.6, 400.8 and 403.6 eV, respectively (Fig. S5, Table S3). Fe-Nx, pyridinic- and quaternary-N are usually considered as effective active sites.13, 38 Compared to FeNC, the incorporation of IL introduces N, S, F and the imidazole ring, which results in the shift of the binding energies of C, O and N components in FeNC-IL (Table S2). Moreover, new signals have been detected for S, F and an increased ratio of N has been detected in FeNC-IL, which is consistent with the results from FT-IR. These evidences further prove that the IL interacts with the catalyst surface and alters its surface properties. The catalytic activity of pristine catalyst has been proved by the cyclic voltammetry (CV) curves in N2 and O2- saturated electrolytes in our previous report,39-40 herewith the ORR performance before and after the IL modification is investigated. Oxygen reduction reaction was first measured in 0.1 M HClO4 solution, in which carbon-based electrocatalysts usually fail to perform.41 iR correction calculated from the EIS results (Fig. S6a) was applied to compensate for the ohmnic potential drop caused by the system resistance.42-43 As shown in Fig. 2e, all the electrocatalysts demonstrate comparable limiting current density to that of commercial Pt-C of the same mass loading. Remarkably, the half-wave potential improves with the IL incorporation, from 574 mV for FeNC to 620 mV for FeNC-IL. This value of halfwave potential is comparable to the best Fe, N-doped carbon catalysts reported so far in the literature (Table S4). Furthermore, the Tafel slope decreases from 87.1 mV dec-1 for FeNC to 66.3 mV dec-1 for FeNC-IL, which is much closer to Pt-C (56.6 mV dec-1), demonstrating faster ORR kinetics (Fig. S7a). It 6 ACS Paragon Plus Environment

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is worth noting that so far only a few carbon-based materials have provided a comparable ORR performance with Pt-C in acid electrolyte.41 Long-term stability is another critical issue for metal-based highly-active catalysts due to the acid etching effect and poisonous by-products during the reaction.44 Herewith chronoamperometric experiments have been performed to test the long-term durability of both FeNC and FeNC-IL. After 25000 s, the FeNC-IL exhibits a current retention of 70%, which is higher than that of FeNC (i.e., 53%), and remarkably superior to that of commercial Pt-C (~ 20 %, Fig. S6b). This improvement can be ascribed to the hydrophobic properties of the IL layer which impede the produced H2O and H2O2 from building up locally and therefore oxygenate or poison the active sites. RRDE was employed to accurately measure the electron transfer number during the ORR process. As shown in Fig. S6c, at 0.8 V which is close to the kinetic control region, FeNC-IL exhibits an increment in the electron transfer number from 3.7 (for pristine FeNC catalyst) to 3.9. Meanwhile, the H2O2 yield decreases from 15.7 % to 7.3 %. The differences in electron transfer number and H2O2 yield between FeNC and FeNCIL exist through the whole voltage range. At 0.2 V, which is close to the diffusion control region, the electron transfer number of FeNC-IL increases to 3.96, while the H2O2 yield decreases to 2 %, which is proof for a typical 4 electron reaction pathway. Moreover, MeOH tolerance of FeNC-IL was demonstrated to be comparable with the pristine FeNC catalysts, and superior to Pt-C (Fig. S6d). ORR was also measured in 0.1 M KOH (Fig. 2f, Fig. S8). Fig. 2f shows the LSV curves after the iR correction calculated based on the EIS results (Fig. S8a), the corresponding Tafel plots of iR corrected curves being shown in Fig. S7b. As can be seen in Fig. 2f, both the limiting current density and half-wave potential of the pristine FeNC catalyst match those of commercial Pt-C due to enhanced accessibility to efficient Fe-N and N active sites through the mesoporous structure as has been discussed in our previous work.39 Meanwhile, the half-wave potential of the FeNC-IL catalyst is 901 mV, which implies a positive voltage shift of 41 mV with respect to that of FeNC (860 mV), and outperforms the Pt-C catalyst (854 mV). The half-wave potential of the pristine FeNC catalyst demonstrated comparable behaviour to many 7 ACS Paragon Plus Environment


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advanced Fe, N-doped carbon catalysts reported in the literature, while our strategy further improves the ORR activity towards an even more positive half-wave potential, which is superior than most of the Fe, N- doped carbon catalysts reported so far (Table S5). The Tafel slope in Fig. S7b decreases from 55.4 mV dec-1 for FeNC to 44.3 mV dec-1 for FeNC-IL, demonstrating faster ORR kinetics after IL coating. Although the process in alkaline medium depends on electron transfer instead of proton transfer, the synergetic effect of oxygenophilic and hydrophobic properties has been shown to improve the ORR performance by promoting the O2 adsorption and therefore enhancing the overall active sites utilization. The accurate electron transfer number and H2O2 yield were analysed by RRDE (Figure S8b). At 0.8 V close to the kinetic control region, both the electron transfer number and H2O2 yield before and after IL incorporation are similar to Pt-C, which is close to an ideal 4e- pathway. However, after incorporation of the IL layer, the electron transfer number is slightly reduced, and the H2O2 yield increased, which is probably the result from a synergistic effect of improved hydrophobicity and decreased electronic conductivity of the IL surface. Both FeNC and FeNC-IL demonstrated better tolerance to MeOH and superior long-term stability compared to commercial Pt-C (FigureS8c, d). All the above results prove that our strategy brings significant improvements on the catalytic activity and long-term stability of carbon-based catalysts. This can be ascribed to the fact that the waterequilibrated secondary phase formed by the IL facilitates the TPB microenvironment as follows: i) it promotes the oxygen adsorption and proton transfer to active centres, thus improving the utilization of the active sites, ii) its hydrophobic nature expels the produced water and prevents it against blocking the active sites, iii) it protects the effective active sites from being poisoned, and therefore improves the longterm stability. Moreover, as previously reported, the nanocarbon surface interacts with the IL through CH hydrogen bonding,3 so that this strategy improves the ORR interface reaction without disturbing the structural properties of the catalyst matrix (e.g. no additional heteroatom doping into carbon matrix). The improvement of ORR catalytic activity in both acid and alkaline electrolyte proves that the oxygen access 8 ACS Paragon Plus Environment

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towards the active centres is the rate-limiting step in ORR in this case. It could be concluded that in the case of the pristine FeNC electrocatalyst the active sites on the nanocarbon catalysts have not been fully utilized because of the limited supply of oxygen from the aqueous electrolyte, so that the significant role of active sites on nanocarbon catalysts has not been fully performed. Compared with the numerous efforts on improving the active sites on a catalysts matrix, these findings shed light on a new research pathway to optimize the TPB microenvironment and thus ensure that the existing active sites are fully functional and utilized. To further understand the mechanism through which the IL layer facilitates the TPB, various ratios of IL (0.5 and 1.5 times of the original ratio) were further applied to investigate the influence of the IL layer, named as FeNC-IL0.5 and FeNC-IL1.5, respectively. The different IL ratio is supposed to result in the half/ 1.5 times higher IL layer thickness on pristine catalyst compared with FeNC-IL. LSV curves with iR compensation and corresponding Tafel plots calculated based on iR corrected LSV curves in both acid and alkaline electrolytes are shown in Fig. 3a, b, Fig S9 and Fig S10 respectively. In HClO4 electrolyte, the FeNC-IL0.5 catalyst exhibits similar onset potential and limiting current density to those of pristine FeNC. Increasing the IL content in FeNC-IL provides an obvious positive shift from 578 mV in FeNC, 574 mV in FeNC-IL0.5 to 620 mV in FeNC-IL at half-wave potential, with very similar current density under the diffusion-controlled region. However, further increase on the IL content (FeNC-IL1.5) leads to a decrease in both the half-wave potential (590 mV), and the limiting current density at 0.2 V (4.8 mA/cm2). A similar trend has also been spotted in KOH electrolyte (Fig. 3b, Fig S10), the LSV curves after iR correction show also comparable ORR activity for FeNC and FeNC-IL0.5. An increase in the IL ratio in FeNC-IL brings visible improvements on the catalytic activity, with an up-shift of the half-wave potential from 878 mV for FeNC-IL0.5 to 900 mV for FeNC-IL. Further improvement in the IL ratio to FeNC-IL1.5 leads to a reduction of the catalytic activity, as deduced by the similar half-wave potential and decreased limiting current density compared with FeNC-IL. In both acid and alkaline electrolytes, a 9 ACS Paragon Plus Environment


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visible decrease of the Tafel slope between the pristine and IL coordinated catalysts can be spotted (Fig S9a, c), indicating increased kinetics after the addition of ionic liquid; however, there is no significant difference within the ionic liquid samples (FeNC-IL0.5, FeNC-IL, FeNC-IL1.5). In alkaline media, a slightly increased content of IL (FeNC-IL0.5, FeNC-IL) leads to a decreased charge resistance due to a synergistic effect of limited electric conductivity and improved mass transfer of oxygen (Fig S9d). However, as the content of ionic liquid further increases, the charge resistance obviously increased in both acid and alkaline electrolyte (Fig S9b, d). In alkaline electrolyte where proton transfer does not involve in the reaction process, the influence of viscosity can be observed (Fig S10): compared with FeNC-IL, the FeNC-IL1.5 which possesses thicker IL coating shows obvious diffusion-controlled behavior at lower overpotential with lower current density. This is because the oxygen diffusion decreased in a more viscous medium such as ionic liquid. As the IL layer become thicker, the effect of oxygen viscosity exceeds the effect of oxygen solubility and controls the diffusion process. Therefore FeNC-IL1.5 showed similar overpotential and lower current density at ~0.8 v, but the limited current density is similar with FeNC-IL at higher over potential when the limiting current density become steady. When the IL layer is thin (FeNCIL0.5), the IL layer is too thin to show viscosity limited control, in which the high O2 solution overcontrol the diffusion process, and therefore FeNC-IL0.5 shows higher limiting current density compared with FeNC-IL. These results suggest that an excess of IL will bring negative effects, which is most probably a result from the increased thickness of the IL layer which leads to a lower electric conductivity and higher viscosity for oxygen to diffuse to catalyst surface. Further, this increased water-equilibrated secondary layer, which hinders the electronic transfer, prevents the produced water from diffusing into the electrolyte. The obstructed water molecules therefore accumulate and block the active sites from contacting the reactants such as oxygen and proton (Fig. 3c). This investigation further elucidates the role of the IL in ORR electrocatalysis: the perfluorinated side chains of [NTf2]- anions in the IL provide a high affinity for O2. The solubility of O2 in the ionic liquid 10 ACS Paragon Plus Environment

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with [NTf2]- is ca. 2.2 mM,29 which is almost twice that in aqueous electrolytes (ca. 1.1 mM in 0.10 M KOH45 and ca. 1.2 mM in 0.1 M HClO425). The IL layer on the catalyst surface facilitates a higher O2 diffusion compared with exterior aqueous electrolytes, and thus increases the residence time, intensifies the O2 diffusion to the active centres, and consequently improves the utilization of the active sites which facilitates an improved interface reaction.30 Furthermore, the extra “N−H” acidic site of the imidazolium cation in [C2im]+ in the IL facilitates the water mole fraction solubility (ca. 0.6). This hydrogen bonding largely determines the water solubility in the IL structure.46 It has been reported that the dissolved water at this mole fraction formed an interconnected through structure in the IL layer that facilitated the transportation of the produced water so that it does not accumulate and block the active sites.28 When the pathway was enlarged by the thicker IL layer, the water molecules expulsion was indirectly hindered, so that the balance between the production and transportation of the water molecules was broken. Therefore, in both alkaline and acid electrolyte, the hindered water molecules stay at the surface of the active site reducing the accessibility of the reactants (protons/electrons, O2, etc) to the active centres, so that the catalytic activity of FeNC-IL1.5 decreases compared to FeNC-IL. Moreover, the IL layer plays a role as a reaction medium that partially replaces the aqueous electrolyte and thus can protect the nanocarbon catalysts from being or poisoned by exterior aqueous electrolyte (0.1 M HClO4 or 0.1 M KOH).29, 47 Considering the benefits and side-effects of the IL layer, the IL ratio in the system should be carefully controlled, so that those advantages can be maximized to optimize the TPB microenvironment and bring about a substantially improved ORR catalytic activity and stability. NMR measurements have been conducted to investigate the stability of the ionic liquid layer on the nanocarbon surface. Both HClO4 and KOH electrolytes before and after a long-term stability test for FeNC and FeNC-IL1.5 were collected, condensed via evaporation, and then analysed by NMR. After the long-term stability test in both alkaline and acid electrolyte, the electrolytes of FeNC and FeNC-IL1.5 demonstrated a similar composition (Fig. S11 a-d), while the zoomed-in spectra of FeNC-IL1.5-HClO4 11 ACS Paragon Plus Environment


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and FeNC-IL1.5-KOH did not show the typical peaks of the pure IL (Fig. S11e). Although the exchange between the electrolyte and IL is in theory possible to happen, our results prove the stability of the IL layer on the catalyst surface by indicating no additional element in the electrolyte of FeNC-IL1.5 compared with FeNC. Finally, a metal-free N-doped carbon catalyst using a similar synthesis strategy was also manufactured and tested in order to demonstrate the universality of our strategy. The same ionic liquid was added to a different starting metal-free nitrogen-doped porous carbon, labelled as NC, in order to obtain sample NCIL. The N2 adsorption results are shown in Fig 4a and b, the surface area and total pore volume of NC and NC-IL have been summarized in Table S6. The visibly reduced N2 uptake and pore volume indicates similarly to the FeNC-IL material that a layer of IL has been successfully formed in the pores. The water contact angle improved from 133º (NC) to 144º (NC-IL), indicating the improvement of surface hydrophobicity (Fig S12). This improvement in surface hydrophobicity was further revealed by the water adsorption isotherm. In comparison with the case of FeNC-IL, the effect of the addition of the IL is more clearly seen There is less interaction between the catalyst and water as demonstrated by the lower slope at low relative humidity, the micropore filling starts at higher RH and the water uptake is decreased (from 163 to 136 %), which is in line with the corresponding nitrogen adsorption isotherms. The FT-IR in Fig. 4d confirmed the change of surface chemistry by demonstrating the additional adsorption signals of NCIL compared with that of NC: the peaks at wavenumbers between 10001300 cm-1 and 30003200 cm-1 can be attributed to the SO2 and CF3 vibrations, and the C-H stretching vibration mode of imidazole ring and alkyl chains.37 This demonstrated the existance of IL on NC surface, simily to the case of the FeNCIL samples. Improved ORR catalytic activity in both acid and alkaline electrolyte has been recorded (Fig 5a, b): in acid electrolyte, the halfwave potential positively shifted from ca. 454 mV (NC) to ca. 508 mV (NC-IL). In alkaline electrolyte, the halfwave potential shifted from ca. 830 mV (NC) to ca. 844 mV (NCIL). The corresponding Tafel plots , as can be seen in the inset of Fig 5a and 5b, respectively, slightly 12 ACS Paragon Plus Environment

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decreased, demonstrating faster ORR kinetics after IL coating in both acid and alkaline electrolytes. The improved catalytic activity of NC after IL modification successfully demonstrated the universal applicability of this concept to different carbon materials. 3. Conclusions In summary, we have demonstrated an effective strategy to improve the ORR activity and long-term stability, by controlling the TPB to optimize the interface reaction of electrocatalyst surface microenvironment without changing the catalyst matrix. An oxygenophilic and hydrophobic ionic liquid was applied to serve as a water-equilibrated secondary medium on the electrocatalyst surface to facilitate mass transfer, so that the reactant O2 and protons can easier access the active centres and therefore improve the utilization of the active sites. After IL modification, our electrocatalysts outperformed the commercial Pt-C in both catalytic activity and long-term stability, at a lower cost compared with the scarce Pt. Furthermore, this IL layer served also as a reaction medium, expelling the water and any by-products of the ORR process. This has impeded such products from building up locally and therefore protected the active sites from being oxygenated or poisoned. This led to a remarkable improvement in the long-term stability of the active sites. Our study also demonstrates that the hydrophobic layer can have different effects on the reaction system, both positive and negative. Hence the ratio of IL needs to be carefully considered and balanced. Furthermore, this work indicates that the active sites of current electrocatalysts are not fully utilized, and the adsorption of oxygen on the active sites is the rate determining step in ORR. This new discovery is essential for future optimisation of nanocarbon ORR electrocatalysts. While pursuing the transformation to improve the critical property of the catalyst itself, we should also pay attention to whether these enhanced properties are fully utilized. Future studies need to be conducted using spectroscopic and modelling techniques to quantify the interactions between the IL and O2 on the carbon catalysts as well as the role of water in the ORR reaction along with the O2 concentration and the nature of the intermediates. 13 ACS Paragon Plus Environment


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4. Experimental Section Preparation of electrocatalyst The FeNC electrocatalyst was synthesized according to previous report.39 Briefly, silica particles with a hollow core and a mesoporous shell (SCMS) were synthesized as reported by Unger et al.48 Afterwards, the SCMS particles were impregnated by the dropwise impregnation technique with a 2 M FeCl3 ethanol solution (around 0.27 g FeCl3/g silica). Then, the impregnated sample was exposed to pyrrole (Aldrich, 99%, distilled) vapors at 25 °C for 22 h in a closed vessel. The dark solid thus obtained was heated under N2 to 850 °C (3 °C min−1) for 1 h. Finally, the carbonized composite was treated with hydrofluoric acid for 4 days to dissolve the silica framework and most of the iron species generated during the heattreatment. The carbon capsules with nitrogen functionalities and iron traces have been designated as FeNC. The NC electrocatalyst was synthesized according to previous report.[40] To be more specific, 3 g of (C6H5O7)2Ca34H2O (Sigma Aldrich) was heat-treated in a stainless steel reactor up to 800 ºC at a heating rate of 3 ºC min-1 in the presence of nitrogen and held at this temperature for 1 h. The resulting black solid was then washed with diluted HCl (10%). Finally, the carbon particles were collected by filtration, washed with abundant distilled water and dried at 120 ºC for several hours. Afterwards, the mesoporous carbon was mixed with melamine, using a ratio of melamine/carbon as 4:1. Finally, the mixture was heat treated under nitrogen up to 800 ºC (heating rate of 3 ºC min-1) for 1 h. The as-obtained N-doped mesoporous samples are denoted as NC. Surface modification of Electrocatalysts via ionic liquids 1-Ethylimidazolium bis(trifluoromethylsulfonyl) imide ([C2im][NTf2]) was purchased from IoLiTec. All the chemicals were used as obtained without further treatment. Deionized water was applied for all the experiments. 14 ACS Paragon Plus Environment

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4 mg of electrocatalysts (FeNC or NC) was mixed with 300 µL ethanol. Then 1.1 µL of IL was carefully added into the solution. The mixture was then ultrasonicated for 10 mins to homogeneously disperse the ionic liquids (ILs) into a slurry, the mixture was then anchored on a shaker to shake for 60 hours to make the ILs fully interacted with the electrocatalysts. FeNC and NC without addition of ILs were treated following the same process for comparison. The obtained slurry was then frozen at -20 °C, followed by a freeze dry process for 72 h. The obtained samples were named as FeNC, FeNC-IL, NC, NC-IL, respectively. To study the influence of different IL ratio in FeNC, FeNC-IL0.5 and FeNC-IL1.5 were synthsized using the same procedure, but the volume of IL added was 0.55 µL and 2.2 µL, respectively. Electrocatalytic performance measurements The electrode materials were prepared by mixing 4 mg catalysts, 264 µL ethanol, 700 µL H2O, and 36 µL nafion solution (5% w/w). The mixture was then sonicated for 10 min with probe ultrasonicator to obtain a homogeneous slurry, then 14 µL of the slurry was deposited onto the glassy carbon disk (5.0 mm in diameter) of rotating disk electrode (RRDE). All tests were measured using a standard three-electrode cell on electrochemical workstation (Metrohm Autolab PGSTAT204), where an Ag/AgCl in 3M KCl solution was served as the reference electrode, and platinum wire as the counter electrode. The catalyst loading was ca. 0.285 mg/cm2 (ionic liquid was not included in the calculation). Commercial Pt/C (20 wt %, Sigma-Aldrich, 738549) of the same mass loading was prepared as electrodes for comparison. 0.10 M KOH solution and 0.10 M HClO4 solution were prepared as alkaline and acid electrolytes, respectively. Linear sweep voltammograms (LSV) were performed at a rotation rate of 1600 rpm and a scan rate of 10 mV s-1. The current-time chronoamperometric responses were recorded after the first 1000 s activation period for stabilizing the electrodes at half-wave potential at a rotation rate of 800 rpm in O2-saturated 0.1 M KOH and 0.1 M HClO4, respectively. The following equation was used to calculate the percentage of H2O2 released during the reaction: 15 ACS Paragon Plus Environment


% 𝐻2𝑂2 = 100

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2𝐼𝑅/𝑁 I𝐷 + (𝐼𝑅 /𝑁)

Where IR refers to the faradaic current at the ring, ID is the faradaic current at the disk, N is the H2O2 collection coefficient at the ring, which was provided by the manufacturer, N is 0.249 to this RRDE working electrode. Characterization The morphology and structure of the samples were characterized using an FEI Inspect-F scanning electron microscope (SEM), and a JEM 2010 (JEOL Ltd., Tokyo, Japan) transmission electron microscope (TEM). Water sorption isotherms were carried out at 20 ºC using a gravimetric water sorption analyzer (Aquadyne DVS, Quantachrome Instruments). Before each sorption measurement, the samples were outgassed at 120 ºC overnight. Each point of the gravimetric isotherm is obtained after an equilibrium time corresponding to 0.0004% of mass change per minute (cut-off criterion) at a given relative humidity. The physical adsorption-desorption isotherm was measured using an Autosorb-IQ system (Quantachrome Instruments, USA). The specific surface area was calculated by the multipoint Brunauer–Emmett–Teller (BET) method, and the pore-size distribution was calculated based on Quenched Solid Density Function Theory (QSDFT) using the adsorption branch. Fourier transform infrared spectroscopy (FTIR) spectra was measured on Tensor27 (Bruker). X-ray photoelectron spectroscopic data (XPS) was collected using a Thermo Scientific K-Alpha* instrument (Thermo Fisher Scientific, East Grinstead, UK) equipped with a monochromated aluminium K-alpha X-ray source. The charge compensation system on the instrument was used for the analysis, and the monatomic and gas cluster ion source (MAGCIS) was used in cluster mode for a necessary sample cleaning. The contact angle of electrocatalyst in this contribution was determined on Dataphysics OCA20. Supporting Information 16 ACS Paragon Plus Environment

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The chemical structure of the ionic liquid (1-Ethylimidazolium bis(trifluoromethylsulfonyl) imide); More characterizations on electrocatalysts’ morphology and surface chemistry; Additional electrochemical experiments.

Corresponding Author Maria-Magdalena Titirici [email protected]; [email protected] Author Contributions M.M. Titirici and M. Qiao contributed to the idea of this work. M. Qiao designed the experiments and wrote the manuscript. M. Qiao, G. A. Ferrero and W. V. Hor did the experiments. L. F. Velasco contributed to the test and results analysis of water adsorption measurement. Y. Yang contributed to the test and results analysis of NMR measurement. H. Luo contributed to the TEM measurement. All the coauthors contributed to the revision of this manuscript. Acknowledgment Magdalena Titirici would like to acknowledge EPSRC grants EP/R021554/1 and EP/N509899/1 and the EU CIG 631092. Mo Qiao, Yan Yang, Hui Luo acknowledge the CSC for a PhD scholarship.



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Figure 1. a) Schematic diagram of the triple-phase interface between solid, liquid and gas phases. The inset SEM images correspond to the a) pristine FeNC electrocatalyst and b) FeNC electrocatalyst after coating with IL layer.



Figure 2. a) QSDFT pore size distributions of FeNC and FeNC-IL from N2 adsorption isotherms. b) H2O vapour sorption isotherm at 20 ºC for samples FeNC and FeNC-IL. c) FT-IR spectra of the samples. The insets show zoomed spectra in the region 28003300 cm-1. The labels of inset Xaxis and Y-axis are wavenumber (cm-1) and relative intensity, respectively. d) XPS surveys of FeNC and FeNC-IL. e) LSV curves obtained in O2-saturated 0.10 M HClO4 solution. f) LSV curves obtained in O2-saturated 0.10 M KOH solution. Figures e and f show the ORR performance


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of the IL-modified FeNC catalyst after iR compensation, the rotating rate was 1600 rpm, with a scan rate of 10.0 mV s-1.

Figure 3. ORR performance of the FeNC and IL-modified FeNC catalysts with different ionic liquid content. (a) LSV curves obtained in O2-saturated 0.10 M HClO4 solution, and (b) LSV curves obtained in 0.10 M KOH O2-saturated solution. The rotating rate was 1600 rpm, with a scan



rate of 10.0 mV s-1. The dash line in (a, b) show the LSV curve after iR correction. c) Schematic diagram illustrating the influence of different IL ratios on the TPB microenvironment.

Figure 4. a) QSDFT PSDs of NC and NC-IL. b) N2 sorption isotherms of NC and NC-IL.c) H2O vapor sorption isotherm at 20 ºC for the samples NC and NC-IL. The insets show zoomed isotherm at low relative humidity. The labels of inset X-axis and Y-axis are relative humidity (%) and mass water content (%), respectively d) FT-IR spectra of the samples. The inset shows zoomed spectra in the 28003300 cm-1 range. The labels of inset X-axis and Y-axis are wavenumber (cm-1) and relative intensity, respectively


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Figure 5. ORR performance for pristine and IL-modified NC catalysts. a) LSV curves in O2saturated 0.10 M HClO4 solution and b) LSV curves in O2-saturated 0.10 M KOH solution (rotating rate = 1600 rpm, scan rate = 10.0 mV s-1). The insets show the corresponding Tafel plots derived from the LSV results.



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