Probing the Oxygen Reduction Reaction Active Sites over Nitrogen

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Probing the ORR Active Sites Over Nitrogen-doped Carbon Nanostructures (CNx) in Acidic Media Using Phosphate Anion Kuldeep Balram Mamtani, Deeksha Jain, Dmitry Y. Zemlyanov, Gokhan Celik, Jennifer Luthman, Gordon Renkes, Anne C. Co, and Umit S. Ozkan ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01786 • Publication Date (Web): 15 Sep 2016 Downloaded from http://pubs.acs.org on September 15, 2016

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ACS Catalysis

Probing The ORR Active Sites Over Nitrogen-Doped Carbon Nanostructures (CNx) In Acidic Media Using Phosphate Anion Kuldeep Mamtani1, Deeksha Jain1, Dmitry Zemlyanov2, Gokhan Celik1, Jennifer Luthman1, Gordon Renkes3, Anne C. Co3, Umit S. Ozkan1* 1

William G. Lowrie Department of Chemical and Biomolecular Engineering, The Ohio State University, Columbus OH 43202 2

Birck Nanotechnology Center,

Purdue University, West Lafayette, IN 47907 3

Department of Chemistry and Biochemistry,

The Ohio State University, Columbus OH 43210

* Corresponding Author [email protected] 614-292-6623

ACS Paragon Plus Environment

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Abstract To probe the active sites of nitrogen-doped carbon nanostructures (CNx), the effect of dihydrogen phosphate (H2PO4-) anion on their ORR performance was investigated by adding increasing concentrations of phosphoric acid in half-cell measurements. A linear decrease in specific kinetic current at 0.7 V was noted with increasing phosphate anion concentration. It was also found that the adsorption of phosphate species on CNx was strong and the corresponding ORR activity was not recovered when the catalyst was reintroduced to a fresh HClO4 solution. Trends similar to those noted upon addition of H3PO4 in half-cell were observed when CNx catalysts were soaked in phosphoric acid. Adsorption of dihydrogen phosphate ions on the surface of CNx exposed to phosphoric acid was verified by transmission infrared (IR) and Raman spectroscopy as well as Xray photoelectron spectroscopy (XPS). XPS results also showed a decrease in the surface concentration of pyridinic-N species accompanied by an increase of equal magnitude in the surface fraction of quaternary-N species, which would include the pyridinic-NH sites. A linear correlation was observed between the loss in pyridinic-N site density and that in ORR activity. The observed poisoning phenomenon is consistent with the two possible active site models, i.e., pyridinic-N sites, which would be rendered inactive by protonation or the C sites neighboring pyridinic-N species. These latter species would be poisoned by a site blocking effect if they strongly adsorb the phosphate ions. Strong adsorption of negatively charged phosphate ions on neighboring C atoms would also stabilize the pyridinic-NH sites. By identifying a poison that can be used as a probe, this study provides a first step towards identification and quantification of active sites in CNx catalysts.


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Keywords: ORR, CNx, phosphate anion, adsorption, probe, active sites


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Introduction The slow kinetics of oxygen reduction reaction (ORR) in both proton or anion exchange membrane fuel cells necessitates the use of platinum-based catalysts, which are expensive and rare. Widespread deployment of fuel cells requires the development of abundant, affordable and reliable non-noble metal-based cathode (NNMC) catalysts. Carbon-based materials are promising alternatives to Pt-based catalysts. A class of these carbon-based catalysts, termed as “MeNC”, has been extensively studied by Dodelet and collaborators.1-5 Here, “Me” represents a transition metal such as Fe that can be derived from a macrocycle or a simple metal salt such as iron acetate. It has been well established that the ORR active site on these MeNCs is the metal-center.5-11 On the other hand, there also exists another class of carbon-based ORR catalysts, referred to as “CNx”, where the transition metal remains encased in the carbon nanostructure and is inaccessible to oxygen.12-19 For example, CNx materials can be synthesized using chemical vapor deposition of a C and N source (such as CH3CN) over a metal-doped support (such as MgO, SiO2, or Al2O3) followed by an acid-washing step to remove the exposed metal and the oxide support. Thus, the metal merely acts to initiate the growth of CNx, but is not accessible to participate in the ORR.20-30 Our group has previously synthesized both MeNC and CNx and clearly demonstrated that MeNC and CNx are indeed two different materials with very different ORR active sites.17, 31 Identification of the actual ORR active sites in each of these two materials (MeNC and CNx) has been the focus of many studies. There is growing consensus that planar FeN4 with Fe2+ ion in low spin state and coordinated to four pyrrolic nitrogen groups in a carbon matrix is an active site for MeNC or FeNC catalysts in particular.18-19,


32-38

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However, there is limited information on the ORR active sites in CNx catalysts. Techniques such as Mössbauer spectroscopy or X-ray Absorption Spectroscopy (XAS) are bulk techniques, which are of little use for confirming the absence of a metalcentered active site on the surface of CNx. Furthermore, there are contradicting reports in the literature about the nature of active sites. A correlation of ORR activity with the pyridinic-N content has been reported, but it is not clear if the pyridinic-N species are the active sites26-28, 39-46, or are “markers” for the active sites.13, 16, 21, 47 CNx active sites suggested in the literature include pyridinic-N species24-26, 36-43, quaternary-N species40, 48-50

, and C sites adjacent to pyridinic-N species.13, 16, 21, 47 This latter proposal would be

consistent with the pyridinic-N sites being a “marker”. The argument for it is that pyridinic-N sites have a high affinity for electrons, leaving adjacent C atoms at an electronegative state.23, 30, 47, 51-53 Our previous work, as well as of others, has shown that CNx catalysts are not poisoned by molecules such as CO, CN- and H2S25,

53-56

containing a metal-centered ORR active site.6-7,

which are known to poison catalysts 57

This makes it more challenging to

identify the ORR active site on CNx. Thus, clearly there is a need to develop strategies for identifying and quantifying the ORR active sites in CNx materials. One strategy is to first identify probe molecules which will indeed poison the CNx catalysts. The insights thus gained can eventually be used to provide valuable information on ORR active sites in these CNx catalysts.


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To our knowledge, no known poison has been reported for CNx materials to date. In this article, we report phosphate anion as a probe molecule that poisons the ORR active sites on CNx. Interaction of phosphate ions in phosphoric acid with Pt surfaces has been extensively studied.58-66 It is known that phosphate anions adsorb on Pt surfaces and have detrimental effects on ORR activity. There is no such report available for NNMC catalysts except one from Zelenay and co-workers.67 However, the objective of that study by Zelenay and coworkers was to evaluate the possible application of polyaniline (PANI)-based catalysts for phosphoric acid fuel cells and not to use phosphate anion adsorption as a probe. In fact, the authors did not observe a decrease in ORR activity of these PANI-based catalysts except a decrease in the limiting current. Here, we demonstrate that ORR activity of CNx catalysts decreases significantly after phosphate anions adsorb on the catalyst. This observation was found to be valid during ORR in half-cell measurements, where phosphoric acid (H3PO4) was added to the electrolyte (0.1 M HClO4) at various concentrations as well as when the most active form of CNx was soaked in 0.1 M H3PO4. Results from the characterization experiments using,

X-ray

Photoelectron

Spectroscopy

(XPS),

Transmission

Infrared

(IR)

Spectroscopy and Raman Spectroscopy support findings from the ORR activity measurements.


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Experimental Catalyst Synthesis Pristine CNx: These catalysts were prepared through incipient wet impregnation technique on MgO (Aldrich) support by using 2 weight% iron (from iron (II) acetate, Aldrich) dissolved in deionized water, the volume of which was decided based on the pore volume of the support. The sample was then kept overnight in an oven for solvent evaporation and then ball-milled at 200 rpm for 3h using a rotary ball-mill. The ballmilled precursor then went through an acetonitrile pyrolysis step for 2h at 900 °C. The sample was then subjected to an acid-washing step in 1 M HCl for 1h at 60 °C and then vacuum-filtered, washed and dried in an oven at 80 °C. The final product collected from the oven is denoted as pristine CNx. Soaked CNx: This sample was synthesized in an identical manner to that of pristine CNx except for one additional step. To prepare soaked CNx sample, pristine CNx catalyst was soaked in 0.1 M H3PO4 for 1h at room temperature followed by vacuum filtration and drying in an oven at 80 °C. The sample collected from the oven was denoted as soaked CNx (0.1 M H3PO4). Similarly, soaked CNx (0.3 M H3PO4), soaked CNx (0.5 M H3PO4) and soaked CNx (1.0 M H3PO4) samples refer to samples synthesized by soaking pristine CNx in 0.3 M, 0.5 M and 1.0 M H3PO4, respectively. Electrochemical Testing Oxygen reduction reaction (ORR) activity was measured in a standard three-electrode system comprising of a working electrode (glassy carbon disk, 5.61 mm and 0.2472 cm2), a hydrogen reference electrode (ET070 Hydroflex) and a counter electrode (Pt coil). The electrolyte used was 0.1 M HClO4. To prepare the catalyst ink, 95 µL of 5 wt%


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Nafion® solution and 350 µL ethanol (200 proof) were added to 10 mg of catalyst weighed in a 2 mL vial. The vial was then kept for ultrasonication in an ice bath until the catalyst was well-dispersed at which time 9 µL of the ink was pipetted onto the glassy carbon disk resulting in a catalyst loading of 800 µg/cm2. Cyclic voltammograms (CVs) were first performed at 50 mV/s from 1.2 V to 0 V to 1.2 V with the working electrode rotating at 1000 rpm until reproducible CVs were noted in the oxygen saturated electrolyte. Slow CVs at 10 mV/s were then collected at 400, 800, 1000, 1200 and 1600 rpm on the disk again from 1.2 V to 0 V to 1.2 V. CVs were also collected in an argon saturated electrolyte serving as a blank. ORR performance was evaluated by comparing (i) potential at a background-subtracted current density of -0.1 mA/cm2, (ii) half-wave potential (E1/2), (iii) specific kinetic current (iK) at 0.7 V, (iv) rate constant (k) for ORR. ORR performance measurements on at least 3 other CNx catalysts for the same experimental conditions gave kinetic current and E1/2 values that are within 10%. Calculation of specific kinetic current and subsequently the rate constant for ORR was made using the Koutechy-Levich equation (equation I). 1 1 1 = + () where i is the measured current density, iK is the kinetic current density defined by = and ilim is the limiting current density defined by = 0.62 / /


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Here, n is the number of electrons transferred per molecule of oxygen, F is the Faraday’s constant (96485 C/mole of electrons), k is the rate constant for ORR, is the bulk concentration of oxygen (1.18 x 10-6 mol/cm3)68-69, is the diffusion coefficient of oxygen (1.93 x 10-5 cm2/s)68-70, is the kinematic viscosity of the electrolyte (1.009 x 10-2 cm2/s).68, 70 In-addition, Tafel analysis was performed to gain insights into the rate-determining step for ORR using equation II.

= + ! "#$ %

(II)

&

where V0 is the equilibrium potential (1.23 V), b is the Tafel slope and i0 is the exchange current density. All potentials referred to in this work are referenced with respect to a reversible hydrogen electrode (RHE) scale. The selectivity for water formation for pristine and H3PO4 soaked CNx samples was measured using two different techniques: (i) using Koutecky-Levich technique where half-cell measurements are performed at different rotating speeds to obtain n, the total number of electrons passed during the ORR reaction and (ii) using the rotating ring disk electrode (RRDE) experiments where the Pt-ring around the glassy carbon disk was held at a constant potential of 1.2 V to detect the H2O2 intermediate formed on the rotating disc during the ORR process on the disk. The selectivity was calculated using equation (III). A value of n=4 corresponds to a complete reduction of oxygen to water, whereas a value of n=2 is equivalent to 100 % hydrogen peroxide formation.


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=

'()

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(III)

+

() * -,

where n is the number of electrons transferred per oxygen molecule ID is disk current, IR is the ring current and N is the collection efficiency (37% as specified by the manufacturer). Electrochemically accessible surface area (EASA) values reported in this study were obtained by measuring electrical double layer capacitance (C). CVs were collected in argon saturated electrolyte at 5, 10, 25, 50, 100 and 200 mV/s by sweeping the potential across a non- Faradaic region, where the capacitive current, Icapacitive, is linearly proportional to the scan rate (ν) as shown by equation (IV). (IV)

./0/.123 = 4

EASA defined in equation V was then obtained using the measured C values. For our calculations, we compared the capacitance of CNx to the specific capacitance (Cs) value of 20 µF/cm2 based on literature to obtain the EASA.67, 71-75 8

(V)

5676 = 8

9

Roughness factor (RF) was calculated as a ratio of EASA on the electrode to the geometric area of the disk electrode.


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Characterization Surface

areas

and

pore

size

distributions

were

measured

using

Brunauer−Emmett−Teller (BET) analysis on a Micromeritics ASAP 2010 instrument. Experiments were conducted at liquid nitrogen temperature (77 K) with nitrogen as the adsorbent. Samples were degassed overnight at 140 °C under vacuum before being analyzed. Transmission infrared (IR) spectra were acquired for pristine CNx and CNx soaked in 0.1 M H3PO4. For this, Thermo NICOLET 6700 FTIR spectrometer equipped with a liquidnitrogen-cooled MCT detector and a KBr beam splitter was employed. The sample was diluted with KBr and pelletized using a 13 mm die. The spectra were collected by scanning over 500 interferograms with a resolution of 4 cm-1. The spectrum collected for pristine CNx was used as a background spectrum. Raman spectroscopy experiments were performed at room temperature on a Renishaw inVia Raman microprobe system using 10 mW 514 nm laser excitation and line focus with a 50X objective and ca. 50 µm long line. Ten accumulations with 10 seconds exposure time each were collected to improve the signal-to-noise ratio. assignments were based on the literature.

The peak

Data analysis and curve fitting was

performed using the vendor’s WIRE program. X-ray Photoelectron Spectroscopy (XPS) was used to analyze the composition of the surface species. A Kratos Axis Ultra DLD Spectrometer was used with Al Kα monochromatic X-ray radiation (1486.6 eV). The spectra were collected at room temperature with 20 eV pass energy. Binding energy (B.E.) values were referenced to


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the standard C 1s binding energy of 284.5 eV. CasaXPS program was used for data analysis

and

curve

fitting.

Shirley-type

background

and

Lorentzian-Gaussian

combination were used for data processing. P 2p region was fitted using a doublet with 2p3/2 and 2p1/2 components with the binding energy separation between the two fixed at 0.84 eV and the area ratio as 2:1.

Results and Discussion Electrocatalytic Measurements The poisoning effect of phosphoric acid (H3PO4) on the ORR activity of CNx catalysts was examined by performing half-cell measurements in 0.1 M HClO4 before and after exposing the catalyst-coated disk electrode to 0.1 M H3PO4 (Fig 1).

The sample

exhibited significantly lower ORR activity in 0.1 M H3PO4 electrolyte than in 0.1 M HClO4. The onset potential decreased from 0.76 to 0.70 V and E1/2 from 0.59 to 0.50 V. The kinetic current also reduced from 0.83 to 0.14 mA/mg catalyst with a corresponding decrease in ORR rate constant (k) (Table 1). When the ORR activity of this very same catalyst-coated disk electrode (exposed to 0.1 M H3PO4) was re-measured in a fresh 0.1 M HClO4 electrolyte, a partial recovery in the ORR activity was noted. However, the ORR activity still remained significantly lower compared to those unexposed to H3PO4 as shown in Table 1. We also compared the ORR activities of pristine CNx and soaked CNx (0.1M H3PO4) in 0.1 M HClO4. Results are presented in Figure 2 (a). The soaked CNx (0.1M H3PO4) sample exhibited significantly lower ORR activity than the pristine CNx sample that was


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not exposed to H3PO4. The onset potential and the half-wave potential values of the soaked CNx (0.1 M H3PO4) were respectively found to be 50 mV and 80 mV lower than its pristine CNx counterpart (Table 2). The specific kinetic current (iK) and the ORR rate constant values also decreased to about 1/5th of its original values as a result of H3PO4 soaking. To investigate if H3PO4 soaking alters the ORR pathway, we measured the selectivity of the pristine CNx catalyst and the one soaked in 0.1 M H3PO4. Figure 2 (b) presents the corresponding Koutechy-Levich plots at 0.2 V. The plots were found to be linear and parallel at potentials in the range of 0.1 to 0.3 V. The slope of these plots were used to obtain selectivity (n) which was found to be close to 4 for both samples. This observation suggests that H3PO4 soaking does not alter the ORR pathway and dominant pathway remains the four-electron pathway even after H3PO4 soaking. Inaddition, the selectivity was found not to change with potential both before and after H3PO4 soaking as shown in inset of Figure 2(b). The selectivity values reported here using Koutechy-Levich analysis were in close agreement to those obtained from rotating ring disk electrode (RRDE) experiments. Figure S1 presents the typical disk and the ring current plots for the pristine CNx sample. However, it should be noted that the “observed” selectivity depends on the catalyst loading.76 It is likely that the catalyst loading used here is high enough to further reduce any hydrogen peroxide formed to water which would in fact correspond to a two-step 2 + 2e- reduction of oxygen. Capacitance measurements were also made for the two catalyst samples. Figure 3 (a) and 3 (b) present the CVs at various scan rates for the pristine CNx and soaked CNx (0.1 M H3PO4) samples in an argon-saturated 0.1 M HClO4 solution. These CVs were


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collected in a potential range (from 1.2 to 1.0 to 1.2 V) where no Faradaic processes occur. The current in this window is linearly proportional to the scan rate. Capacitance is obtained from the slope of the current as a function of sweep rate shown in Figure 3 (c).The capacitance measured for pristine CNx is 5.7 mF. Electrochemically accessible surface area (EASA) was estimated by comparing the capacitance of a pristine CNx to the specific capacitance, Cs, of 20 uF/cm2. The EASA calculated for pristine CNx was 145 m2/gcatalyst resulting in a roughness factor of 1159. The capacitance measured for CNx soaked in H3PO4 was 1.6 mF which corresponds to EASA of 40 m2/gcatalyst and a roughness factor of 359. The lower EASA and roughness factor suggests that there as fewer available sites for ORR. While the CNx soaked in H3PO4 gave lower EASA and RF values based on the capacitance measured, it is worth noting that the EASA and RF values for these two catalysts may not be directly comparable since adsorbed species on the surface would lead to changes in the capacitance, which may or may not alter the electrochemically accessible area as described in equation V.

As shown in the following sections,

soaking CNx in a 0.1 M H3PO4 solution resulted in H2PO4- ion adsorption on the surface of CNx. Adsorbed anions (such as sulfates and phosphates) generally increase the interfacial capacitance. On the other hand, if the interface is composed of capacitive sites connected in series, it is possible that the experiment will result in a smaller measured total interfacial capacitance. Therefore while the EASA and RF values for H3PO4-exposed CNx are reported, we are aware of the assumptions made to obtain these numbers and caution the over interpretation of the EASA and RF for the poisoned CNx catalyst.


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Next, the ORR activity loss is correlated with the concentration of phosphoric acid added. A series of half-cell experiments were conducted where increasing amounts of H3PO4 were added to the 0.1 M HClO4 electrolyte and corresponding ORR activities were measured. Figure 4 presents the mass-transport corrected Tafel plots corresponding to various concentrations of H3PO4 added in the main electrolyte. These plots were extracted from the polarization curves shown in the inset of Figure 4 using the specific kinetic current (iK) values obtained from the Koutechy-Levich equation (equation I). It is evident from Figure 4 that a steady decrease in ORR activity takes place with increase in H3PO4 concentration. Comparison of the ORR kinetic parameters with different concentrations of H3PO4 is also presented in Table 3. When the specific kinetic current (iK) at 0.7 V was plotted as a function of the dihydrogen phosphate (H2PO4-) anion concentration (Fig 5a), a linear correlation with a negative slope was observed, i.e., the specific kinetic current was found to decrease linearly with increasing H2PO4- anion concentration. A sample calculation for the phosphate anion concentration is presented in the Supplementary Information. It should also be mentioned that increase in H3PO4 concentration beyond 1 M did not appreciably affect iK suggesting that surface saturation was reached (Figure S2). Figure 5b shows the change in limiting current density (ilim) with increasing dihydrogen phosphate (H2PO4-) anion concentration. An interesting observation from Figure 5 is that both kinetic current (Fig 5a) and limiting current (Fig 5b) decrease linearly with increasing phosphate ion concentration. If Figure 5a is considered alone, one would question if the poisoning effect is due to the loss of intrinsic activity of each site, or if it is due to a decrease in the abundance of accessible sites.


Figure 5b answers this

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question by showing that the limiting current is also decreasing with increasing phosphate ion concentration, indicating that there is a decrease in the abundance of active sites. If the poisoning effect was due only to the loss of intrinsic activity for each site, the limiting current densities would not have changed with increasing dihydrogen phosphate (H2PO4-) anion concentration. It is interesting to note that the loss in iK correlates with the loss in ilim caused by addition of H3PO4 at various concentrations to the half-cell (Figure 5 (c)). These three figures also show that the active sites on a pristine electrode are equally accessible and poisoned in the same proportion whether they are on the exterior geometric surface of the electrode (close to the mouth of the pores) or not. Similar Tafel slope values obtained at different dihydrogen phosphate (H2PO4-) anion concentrations (Table 3) suggest that the rate-determining step is not altered in the ORR mechanism as a result of H3PO4 addition. The half-cell experiments all point to a decrease in the number of active sites with increasing phosphoric acid addition. One could argue that the decrease in the limiting current could also be explained by increased kinematic viscosity, decreased solubility of oxygen and decreased diffusion coefficient of oxygen with increasing H3PO4 concentration, as suggested by Li, et al.67 in the study where the authors observed a decrease in the limiting current upon H3PO4 addition to the electrolyte for PANI-based catalysts. However, the range of H3PO4 concentration used in this study will not significantly alter the O2 diffusivity, solubility and electrolyte kinematic viscosity. For example, the kinematic viscosity and oxygen solubility are reported to be 0.01 cm2/s


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and 1.18 mol/m3 for 0.1 M HClO4 as well as 1 M H3PO4 at room temperature.77-78 Similarly, the diffusion coefficient of oxygen in 0.1 M HClO4 and 0.1 M H3PO4 has been reported as 1.93 x 10-5 cm2/s.79 Furthermore, the magnitude of decrease in iK and ilim reported here is more significant than that can be explained by changes in viscosity, oxygen solubility and diffusion coefficient upon H3PO4 addition. Also, the experiments where half-cell measurements were made in phosphate ion-free electrolytes after exposure to phosphoric acid show that the decrease observed in the limiting current cannot be explained by any change in viscosity or oxygen diffusivity. However, it must be noted that such effects may be important at higher H3PO4 concentrations where significantly higher viscosity, lower oxygen solubility and diffusion coefficient values are reported.78, 80

Characterization Surface Area Measurements The adsorption-desorption isotherms for pristine CNx and soaked CNx (0.1 M H3PO4) are compared in Figure S3 (a) in the supporting information section. The isotherms or the hysteresis loops of CNx were not affected by H3PO4 soaking.

Both samples

exhibited a H3 type hysteresis loop with underlying type II isotherm. Furthermore, pore size distributions of the two samples were similar (Figure S3 (b)). Both samples were found to be mesoporous with pore diameters varying between 30 - 50 Å. The cumulative pore volumes and specific BET surface areas for the two samples were also found to be comparable (~115m2/g). Thus, these results imply that the decrease in ORR


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activity after H3PO4 soaking as observed before cannot be attributed to changes in surface area or pore volume. Transmission IR Spectroscopy The transmission IR spectra for soaked CNx (0.1 M H3PO4) is presented in Figure 6. For this figure, the spectrum for pristine CNx is used as the background. Three distinct bands were observed. The vibrational band at 1612 cm-1 can be associated with O-H -1

bending mode.81-82 Two bands around 1070 cm

-1

and 945 cm are attributed to anti-

symmetrical and symmetrical stretching of H2PO4 species83-87 in the soaked CNx (0.1 M -

H3PO4) sample. The IR spectrum confirms the presence of adsorbed H2PO4 species on the soaked CNx (0.1 M H3PO4) and support results from the electrochemical testing. Raman Spectroscopy The Raman spectra for pristine and soaked CNx (0.1 M H3PO4) are shown in Figure 7. Pristine CNx exhibited presence of first-order D and G bands centered at 1346 and 1576 cm-1 as also reported previously.26, 88-89 The D band arises due to disorder whereas the G band is attributed to the presence of graphitic carbon. These two primary bands were also observed in the Raman spectra for soaked CNx (0.1 M H3PO4) sample with a slight down-shift compared to its pristine counterpart. Furthermore, the two bands were sharper in the H3PO4- soaked CNx sample suggesting that the disorder is reduced after soaking.90 The soaked CNx (0.1 M H3PO4) sample exhibited an additional shoulder around 1606 cm-1, which corresponds to defect-induced D’ band.88 The Raman band at 996 cm-1 present in the soaked CNx (0.1 M H3PO4) sample can be assigned to


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asymmetric stretching vibrations of the P-O bond.91 Previous studies in the literature92 have also linked Raman bands in this range to adsorbed H2PO4- ions on the surface. XPS The nature of the surface species before and after H3PO4 soaking was studied using XPS. A range of H3PO4 concentrations (0.1 M to 1 M) were used. For comparison, spectra for pristine sample and sample soaked in 0.1M H3PO4 are presented in Figures 8-10. The surface elemental composition revealed that there was a significant amount of carbon on the surface of all samples with the other elements being nitrogen and oxygen. The H3PO4-soaked CNx samples additionally exhibited phosphorus on the surface. No detectable iron was noted on the surface of any of the samples, providing further support to the assertion that no metal contributes to the ORR activity of CNx materials. As expected, no phosphorus was detected on the surface of pristine CNx. All soaked CNx samples exhibited a distinct 2p3/2 peak at 133.2 eV which represents P-O bonds on the surface and is associated with phosphate-type species. Furthermore, P was seen to exist in the +5 valence state in the soaked CNx samples.29, 93-98 The P 2p region XPS spectra for pristine CNx and soaked CNx (0.1 M H3PO4) are shown in Figure 8. Figure 9 compares the O 1s region of the XPS spectra for pristine CNx and soaked CNx (0.1 M H3PO4). The broad O 1s peak for pristine CNx suggests presence of several species on the surface including carbonyl species ( -C=O) and C-O/OH groups as evident by the peaks around 531.1 eV and 532.40 eV respectively.99-100 The soaked CNx (0.1 M H3PO4) sample exhibited peaks at 530.5 eV and 532 eV which can be


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associated with the presence of oxygen in the form of P=O and P-O-H, respectively.101102

This is consistent with the findings from Raman and IR experiments that showed the

presence of H2PO4- ions in soaked CNx (0.1 M H3PO4). All soaked samples displayed O 1s features similar to the ones shown in Figure 9b. The distribution of the nitrogen species noted for pristine and H3PO4 soaked CNx samples is presented in Table 4. All samples exhibited three types of nitrogen functionalities namely pyridinic-N (398.3 eV)25,

103

, quaternary-N (400.7 – 400.8 eV)40

and pyridinic-N+O- (402.4 - 402.5 eV)104 although with somewhat varying relative distributions. One point to note about the XPS results is that the summation of the atomic percentages of the pyridinic-N and quaternary-N remained similar although the former decreased while the latter increased with increased H3PO4 concentration of the soaking medium. A comparison of the N 1s XP spectra for pristine CNx and soaked CNx (0.1 M H3PO4) is presented in Figure 10. When the change in ORR activity was examined as a function of pyridinic-N content of the catalysts, an interesting correlation was observed (Figure 11). %loss in ORR activity as represented by the loss of kinetic current at 0.7 V was seen to decrease linearly with decreasing pyridinic-N content. Thus, XPS and RDE results discussed here provide valuable insights into the sites poisoned by H2PO4- leading to the loss of ORR activity on CNx. As pointed out earlier, pyridinic-N sites are believed to be either the ORR active sites24-26, 36-43 or “markers” for ORR active sites13, 16, 21, 47 in CNx catalysts by several researchers. If pyridinic N species are indeed the active sites, their transformation to inactive species will cause a


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decrease in ORR activity. In particular, these pyridinic-N species can be protonated to pyridinic-NH functionalities as a result of H3PO4 soaking. The protonation of pyridinic-N sites was proposed by Popov and co-workers to explain the deactivation of similar catalyst materials in acidic environment.40,

46

It should be noted that XPS cannot

distinguish the quaternary-N species bonded to three C atoms in the basal plane from the pyridinic-NH species40. Hence our results which showed that the pyridinic-N fractions decreased and quaternary-N fractions increased after H3PO4 soaking while the summation of the two fractions remained similar are consistent with this model. Although one can ask why these sites were not protonated when they were in other highly acidic media (H2SO4, HClO4, HCl), it is conceivable that dissociative adsorption of H3PO4 on adjacent C and N sites may lead to a protonated pyridinic-N site which may be stabilized by the presence of a neighboring H2PO4- ion with a negative charge. On the other hand, when non-adsorbing anions such as ClO4- are involved, this protonation is avoided. To validate this hypothesis, we performed control experiments where instead of soaking the pristine CNx in 0.1M H3PO4, we soaked it in HClO4 or HCl or H2SO4 solutions (all 0.1M) using identical experimental conditions employed in H3PO4 soaking. All of these are strong acids and will provide a sufficient concentration of protons in the medium. No decrease in ORR activity was noted relative to pristine CNx when the sample was soaked in these strong acids. In fact, an increase in activity is seen when the catalyst is first soaked in HCl (Figure S4). It is possible that the higher electronegativity of Cl may lead to a net positive charge on the carbon and facilitate O2 adsorption on the surface. Furthermore, exchange of some pyridinic-N sites with Clbased sites, which may contribute to activity, is also possible.


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These results would also be consistent with the assertion that pyridinic-N sites are markers for the active sites (i.e., adjacent C atoms which are made more electronegative by the high affinity of pyridinic-N for electrons). Adsorption of H2PO4ions would block the active sites and the dissociative adsorption of phosphoric acid on adjacent C and N sites would also result in a decrease of the pyridinic-N species. A secondary effect of protonation of pyridinic-N sites would be the alteration of the electronic interaction between pyridinic-N and adjacent C sites. Pyridinic-NH sites would not be likely to make the adjacent C atoms more electronegative, and hence affect the ORR activity. Additionally, presence of phosphate species on the surface of soaked CNx samples as evident from P 2p and O 1s regions suggests that ORR activity is reduced after H3PO4 soaking due to site blocking. The electrochemical testing results where a decrease in kinetic current was accompanied by a decrease in the limiting current after soaking also support the site blocking hypothesis. The two active site models discussed above are also corroborated by the fact that the decrease in pyridinic-N content (as determined by XPS) as a result of H3PO4 soaking was found to be linearly correlated with the corresponding loss in ORR activity (Figure 11). Conclusions Interaction of phosphoric acid with surface of CNx catalysts was studied. It was found that H3PO4- anions adsorbed on the CNx catalyst surface and led to a marked decrease in ORR activity. The specific kinetic current (iK) as well as the limiting current density (ilim) decreased linearly with increase in H2PO4- ion concentration in the half-cell. The adsorption of the phosphate anions was found to be strong and only partially reversible,


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but noted not to alter the ORR pathway. XPS and transmission IR experiments provided evidence of phosphate species in the H3PO4-soaked CNx sample as well as a decrease in pyridinic N content accompanied by an increase in quaternary N content, which may be due to formation of protonated pyridinic N-sites.

The loss in ORR activity as

represented by the kinetic current at 0.7 V was also seen to vary linearly with the decrease in pyridinic nitrogen content. The observed poisoning phenomenon is consistent with the two possible active site models, (i) pyridinic-N sites, which would be rendered inactive by protonation and (ii) C sites neighboring pyridinic-N species. For the first active site model, transformation of pyridinic-N functionalities on the surface to inactive forms such as pyridinic-NH species would be expected to lead to a direct decrease in the density of these sites. Such a transformation would also alter the electronic interaction between pyridinic-N and adjacent C sites, if the active sites are C species adjacent to pyridinic-N sites (second active site model), i.e., if pyridinic-N sites are “markers” for the active sites. These latter species would be poisoned by a site blocking effect if they adsorb the phosphate ions. Strong adsorption of negatively charged phosphate ions on neighboring C atoms would also stabilize the pyridinic-NH sites. Thus, by successfully finding a probe molecule that poisons CNx catalysts, the insights from this study constitute a major step towards identification and quantification of actual ORR active sites in CNx catalysts.


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Acknowledgements This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-FG02-07ER15896. We would also like to thank Ohio Coal Research Consortium for their financial support under Subcontract No. OCRC-C-04. The authors would like to thank Dr. Jeffrey Miller for facilitating the XPS experiments.

Associated Content Supporting Information BET adsorption-desorption isotherms and pore size distributions before and after H3PO4 soaking; methodology used for calculation of H2PO4- ion concentration; ORR activity measurements with H3PO4 additions beyond 1M; RRDE measurements; half-cell testing after soaking in different strong acids. This material is available free of charge via the internet.


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Table 1. Comparison of ORR kinetic parameters for pristine CNx catalyst before and after exposing the catalyst-coated electrode to 0.1 M H3PO4.

Potential (V vs. RHE

Half-wave potential

iK

Electrolyte

@ -0.1 mA/cm2geometric)

(V vs. RHE)

(mA/mgcatalyst) @ 0.7 V vs. RHE

0.1 M HClO4

0.76

0.59

0.83

0.19

0.1 M H3PO4

0.70

0.50

0.14

0.03

0.1 M HClO4 after H3PO4 exposure

0.75

0.54

0.22

0.05

E1/2


Rate constant k (x 100) (cm3/s.gcatalyst)

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ACS Catalysis

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Table 2. Comparison of ORR kinetic parameters for pristine CNx and soaked CNx (0.1 M H3PO4) samples. Potential

Sample

(V vs. RHE @ 0.1 mA/cm2geometric)

Pristine CNx

Soaked CNx (0.1 M H3PO4)

Half-wave potential E1/2 (V vs. RHE)

iK (mA/mgcatalyst) @ 0.7 V vs. RHE

Rate constant k (x 100) (cm3/s.gcatalyst)

0.76

0.59

0.97

0.22

0.71

0.51

0.19

0.04


30

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ACS Catalysis

Table 3. Effect of addition of H3PO4 at various concentrations to 0.1 M HClO4 on kinetic parameters for ORR.

Concentration of Measured Concentration of H3PO4 added pH of the H2PO4- (M) solution (M)

Rate constant k iK (mA/mgcatalyst) at 0.7 V vs. RHE

(x 100) (cm3/s.gcatalyst)

Tafel Slope (mV/dec)

0.00

1.01

0.000

0.99

0.23

-86

0.10

0.97

0.007

0.86

0.20

-85

0.30

0.76

0.018

0.76

0.19

-84

0.50

0.60

0.028

0.58

0.18

-82

0.75

0.43

0.039

0.53

0.13

-86

1.00

0.22

0.048

0.39

0.12

-84


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Table 4. N 1s distribution for pristine CNx and soaked CNx samples as determined by XPS. Sample

Relative % distribution pyridinic-N

quaternary-N*

pyridinic N+O-

(398.3 eV)

(400.7- 400.8 eV)

(402.4 - 402.5 eV)

Pristine CNx

44

47

9


34

56

10


24

63

13


23

63

14


20

70

10

* can also refer to pyridinic-NH species for the soaked sample


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ACS Catalysis

Graphical Abstract


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Figure 1. ORR polarization curves of CNx catalyst in 0.1 M H3PO4 and 0.1 M HClO4 before and after exposing the catalyst-coated electrode to 0.1 M H3PO4. (O2 saturated 0.1 M HClO4, 1600 rpm, 10 mV/s and 800 μgcatalyst/cm2geometric).


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ACS Catalysis

Figure 2. (a) and (b) Polarization curves and Koutechy-Levich plots at 0.2 V vs. RHE, respectively, of CNx catalyst before and after soaking in 0.1 M H3PO4 .The theoretical line corresponding to selectivity (n) of 4 is also included in (b). Inset in (b) represents selectivity (n) as a function of potential (V) (O2 saturated 0.1 M HClO4, 1600 rpm, 10 mV/s and 800 μgcatalyst/cm2geometric).


ACS Catalysis

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Figure 3. Cyclic voltammograms at various scan rates for (a) pristine CNx and (b) soaked CNx (0.1 M H3PO4) collected in 0.1 M HClO4 across a potential window without Faradaic processes. The capacitive current at 1.1 V as a function of scan rate for both catalysts is presented in (c).


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ACS Catalysis

Figure 4. Mass-transport corrected ORR polarization curves for CNx catalyst at various concentrations of H3PO4 added to 0.1 M HClO4. Inset represents the high potential region of a 3-electrode half-cell data. (O2 saturated 0.1 M HClO4, 1600 rpm, 10 mV/s and 800 μgcatalyst/cm2geometric).


ACS Catalysis

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Figure 5. Effect of H2PO4- ion concentration on (a) specific kinetic current at 0.7 V vs. RHE (iK) and (b) limiting current density (ilim) of CNx catalyst for ORR. (c) The correlation between % loss in ilim and that in iK . (O2 saturated 0.1 M HClO4, 1600 rpm, 10 mV/s and 800 μgcatalyst/cm2geometric).


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ACS Catalysis

Figure 6. Transmission IR spectrum for CNx soaked in 0.1 M H3PO4 at room temperature. The spectrum for pristine CNx was used as the background.


ACS Catalysis

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Figure 7. Raman spectra for CNx catalyst before (blue) and after (red) soaking in 0.1 M H3PO4.


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ACS Catalysis

Figure 8. P 2p region XPS spectra for CNx catalyst before (a) and after (b) soaking in 0.1 M H3PO4


ACS Catalysis

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Figure 9. O 1s region XPS spectra for CNx catalyst before (a) and after (b) soaking in 0.1 M H3PO4


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ACS Catalysis

Figure 10. N 1s region XPS spectra for CNx catalyst before (a) and after (b) soaking in 0.1 M H3PO4


ACS Catalysis

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Figure 11. Correlation between the loss in iK and loss in pyridinic-N site density as a result of H3PO4 exposure


Probing the Oxygen Reduction Reaction Active Sites over Nitrogen

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