Fe-P: A New Class of Electroactive Catalyst for Oxygen Reduction Reaction

Supporting Information Fe-P: A New Class of Electroactive Catalyst for Oxygen Reduction Reaction Kiran Pal Singh, Eun Jin Bae, and Jong-Sung Yu* Department of Energy Systems Engineering, Daegu Gyeongbuk Institute of Science & Technology (DGIST), Daegu, 711-873, Republic of Korea Experimental Preparation of Fe-P doped carbon: In a typical synthesis, 1.1 g of FeCl 3 was added to 10 g of phytic acid followed by 15 min stirring of the mixture. After stirring, a turbid brown colored thick liquid is formed, which was then kept at 160 0 C for 12 h. The resulting ferric phytate brown colored powder is then grinded and carbonized at various temperatures of 800, 900 and 1000 0 C for 3 h in nitrogen gas atmosphere to obtain Fe-P-800, Fe-P-900, Fe-P-1000. The heat-treated sample was then pre-leached in 0.5 M H 2 SO 4 at 80 C for 8 h to remove unstable and inactive species from the catalyst, and thoroughly washed in de-ionized water. Finally, the catalyst was heat-treated again in nitrogen gas atmosphere at 800, 900 and 1000 0 C for Fe-P-800, Fe-P-900, Fe-P-1000, respectively for 3 h. Preparation of P-doped carbon: For synthesis of P-doped carbon, 10 g of phytic acid is directly kept at 160 0 C for 12 h. After heating, very sticky glue is obtained, which is directly kept for carbonization at 900 0 C 3 h in nitrogen gas atmosphere to obtain PA-900. Materials Characterization: Surface morphologies of all the prepared Fe-P catalysts were examined by ultra-high resolution UHR-SEM, attached with energy dispersive spectroscopy (EDS analyzer operated at an acceleration voltage of 20 kv) and UHR-TEM techniques by using Hitachi S- 5500 microscopes with an acceleration voltage of 30 kv. To confirm the crystallinity and uniformity of FePO 4 in Fe-P catalyst, X-ray diffraction (XRD) patterns were recorded on a Rigaku Smartlab X-ray diffractometer attached with Cu Kα radiation operating at 30 ma and 40 KV and with a diffracted beam monochromator. The elemental composition and their bonding properties were analyzed by an X-ray photoelectron spectroscopy (XPS, ESCALAB 250 XPS system using a monochromated Al KR X-ray S1

source). Nitrogen adsorption-desorption isotherms were analyzed with ASAP 2020 Physisorption Analyzer (Micromeritics, USA) at 77 K to obtain surface textural properties. The specific surface areas and pore size distributions were calculated using the conventional Brunauer-Emmett-Teller (BET) and density functional theory (DFT) methods, respectively. Pore size distribution in mesopore range was determined by Barrett-Joyner-Halenda (BJH). Before performing the measurements, all the samples were outgassed under vacuum at 200 o C for 12 h until the pressure was less than 5µm Hg. Electrochemical Measurements: All of the electrochemical analyses were carried out at room temperature in a conventional three-electrode arrangement using rotating ring-disk electrode (RRDE) connected to an electrochemical analyser (BioLogic VMP3). A glassy carbon RRDE (GC-RRDE) coated with as-synthesized Fe-free P-doped carbon, Fe-P carbon or commercial Vulcan XC-72-supported Pt catalyst was used as a working electrode, whereas an Ag/AgCl with saturated KCl and a Pt wire were used as reference and counter electrodes, respectively. All the potentials are reported in terms of reversible hydrogen electrode (RHE). For this purpose the Ag/AgCl reference electrode was calibrated with respect to RHE on the potential cycle measured at scan rate of 1mV/s in H 2 -saturated /HClO 4. The catalyst ink was prepared by dispersing 5.0 mg of corresponding Fe-free P-doped carbon, Fe-P carbon or Pt/C (20 wt%) catalyst, respectively, in a mixture of 0.1 ml 5.0 % nafion solution and 0.9 ml deionized water. The dispersed ink was drop casted over GC-RRDE and dried at 50 o C in oven. Catalyst loading amount was kept 39.5 µg cm -2 for all the Fe-free P-doped carbon, Fe-P carbon and Pt/C samples. The cyclic voltammetry (CV) profiles were recorded in O 2 -saturated or HClO 4 solution for ORR with scan rate of 10 mv/s in the potential range from +1.46 to -0.034 V in alkaline medium or from +1.35 to -0.056 V in acid medium, respectively. Before recording, the working electrodes were cycled for 20 cycles for stabilization of current density. RRDE measurements were performed by recording linear sweep voltammetry (LSV) curves in the oxygen-saturated or HClO 4 solution. The LSV curves for ORR were recorded between +1.166 and -0.034 V in alkaline medium or +1.256 and -0.056 V in acid medium at potential scan rate of 10 mv/s, where the Pt ring potential was measured at set potential of +0.5 V. For long term cycling performance, the ORR forward peak maximum currents were recorded during the repeated potential cycling at the scan rate of 50 mv/s in O 2 -saturated HClO 4 solution. Electrochemical impedance spectroscopy (EIS) measurement was carried out on samples before and after cycling performance at a frequency range of 10 KHz to 0.1 Hz. The electron number (n) and HO - 2 percentage were calculated by following equations; n= I D 4I D I + R N S2

% HO = 200 2 I D I R N I R + N where I D, I R and N = 0.424 are the disk current, ring current and collection efficiency, respectively for the employed RRDE. For Tafel plot, the kinetic current was calculated from mass transport correction of LSV curve by following equation: where J is the measured current density on the glassy carbon disk, J K and J L are the kinetic and diffusion limiting current densities, respectively. From above equation, i K is obtained by multiplying J k with the geometric area of the glassy carbon disk, which was then normalized with the electrode mass, 4.9 µg, to obtain the mass activity, i m of the electrodes. 1) Topographic Study on Fe-P/C samples Figure S1. (a, b) Typical SEM images and (a', b') TEM images of Fe-P-800 and Fe-P-1000, respectively. S3

Figure S1'. Typical SEM and TEM images of (a, a') Fe-P-800, (b, b') Fe-P-900 and (c, c') Fe-P1000. Explanation for morphology change: As can be seen from the SEM and TEM images of the prepared catalysts, PA-900 without Fe reveals particle-like morphology, whereas Fe-P-x samples with incorporated Fe possess sheet-like morphology, which gets more prominent on increasing the temperature. The observed variation in the morphology is basically due to the difference in the precursors for PA-900 and Fe-P-x samples. As we have discussed in the manuscript for preparation of catalyst samples, Fe-P carbons were generated from FeCl3 and phytic acid, whereas phytic acid alone was directly carbonized to generate only p-doped carbon, PA-900. Due to this reason an obvious change in morphology is observed. At high enough temperature it is found that the Fe particles react with the carbon matrix to form Fe carbide, which on further increasing the temperature again dissociate into graphitic carbon and Fe particles.1 As the graphitization effect of Fe particle is localized, various graphitic carbons of small craystallite size are supposed to be form. However, in the present scenario, the amount of Fe is very high, and hence it can be surmised that the catalytic graphitization might be also very high and extensive. Furthermore, the graphiticity of carbon is highly dependent on the operating temperature and therefore increases with increase in temperature, which can also be seen from the SEM and TEM images. Hence, with high operating temperature as well as presence of high amount of Fe catalyst, the degree of graphitization enhances, which eventually results in the observed changes in the morphology of the catalysts. Iron + Carbon+ 900 0C Iron Carbide + < 1000 0C Carbongraphitic + Fe particles S4

2) Chemical composition of Fe-P/C samples Figure S2. XPS peak survey spectra of PA-900, Fe-P-800, Fe-P-900, and Fe-P-1000. Table S1: Percentage of different P species found in prepared PA-900, Fe-P-800, Fe-P-900, and Fe-P-1000 catalysts Sample Name % of P Species P-C (P1) P-O (P2) P-O-Fe (P3) Metaphosphates (P4) PA-900 63.3 36.7 - - Fe-P-800 3.3 11.5 52.1 33.1 Fe-P-900 77.7 6.4 11.4 4.5 Fe-P-1000 82.6 12.3 5.1 - S5

Figure S3. EDS survey spectra of (a) Fe-P-800, (b) Fe-P-900, (c) Fe-P-1000 and (d) Fe-P-900 (after cycling test). Figure S4. (a) Nitrogen adsorption-desorption isotherms, (b) the corresponding pore size distribution curves and (c) Barrett-Joyner-Halenda (BJH) pores size distribution curves of PA- 900, Fe-P-800, Fe-P-900 and Fe-P-1000. S6

Table S2: Nitrogen sorption data of PA-900, Fe-P-800, Fe-P-900 and Fe-P-1000 Sample Physical Characteristics BET total surface area (m 2 g -1 ) Micropore surface area (m 2 g -1 ) Pore volume (cm 3 g -1 ) PA-900 577 416.14 0.35 Fe-P-800 1068 903.28 0.52 Fe-P-900 1371 1075.38 0.75 Fe-P-1000 1659 1262.56 1.11 Explanation for the upsurge in the surface area: It can be seen from Table S2 that on carbonizing PA at 900 o C, highly porous structure with surface area of 577 m 2 /g is obtained. Formation of this porous structure can be attributed to the high amount of acidic functional groups present on the precursor PA structure. PA at high temperature can release six molecules of orthophosphate such as hydrogen phosphate (HPO 4 2- ) and dihydrogen phosphate (H 2 PO 4 - ), which at high enough temperature can form phosphorus pentoxide (P 2 O 5 ), which is a strong oxidant. Therefore, during carbonization, these formed moieties oxidizes carbon surface, which escapes the surface in the form of CO 2 gas, leaving behind pores, mainly micropores in carbon framework. 2-4 Furthermore, carbonization of Fe-P samples at high temperature also leads to the thermal oxidation of unreacted FeCl 3 precursor to form iron oxides imbedded in carbon. Hence, during acid leaching these iron oxide particles are etched out of the carbon matrix, which eventually leads to such a high upsurge in the surface area of resultant Fe-P samples. Moreover, as we have shown in our Raman and XRD analysis, at higher temperature, Fe-metaphosphate species decomposes, which also leads to the further improvement in surface area on increasing the temperature as shown in Table S2. S7

3) Electrochemical study on Fe-P/C samples Figure S5. (a) Mass activity profiles of PA-900, Fe-P-800, Fe-P-900, Fe-P-1000 and 20 wt. % Pt/C and (b) comparative diagram portraying the dependence of ORR activity on P-C, Fe-P and surface area of all the samples in O 2 -saturated acid medium. S8

Figure S6. Electrochemical activity studied using RRDE technique at a rotation rate of 1,600 rpm in O 2 -saturated solution for PA-900, Fe-P-800, Fe-P-900, Fe-P-1000, and 20% Pt/C (E-TEK). (a) Effect of carbonization temperature on LSV curves for Fe-loaded and Fe-free PA samples, (b) electron number transferred per oxygen molecule, (c) %HO 2 - production, and (d) current produce on ring electrode during ORR. S9

Figure S7. Cyclic voltammetry curve for PA-900, Fe-P-800, Fe-P-900, and Fe-P-1000 recorded in O 2 -saturared (a) acidic electrolyte, (b) alkaline electrolyte, and in N 2 -saturated (c) acidic electrolyte, (d) alkaline electrolyte and LSV curves of (e) Fe-P-800 and (f) Fe-P-1000 in O 2 and N 2 -saturated 0.1M HClO 4 electrolyte. S10

4) Evaluation of long term cycling test on Fe-P/C samples Figure S8. (a) Relative J t responses at 0.36 V for Fe-P-900 and 20 wt. % Pt/C (E-TEK) electrodes, and (b) EIS Nyquist plots measured for the Fe-P-900 before and after cycling samples in O 2 -saturated HClO 4 solution. Figure S9. (a) SEM and (b) TEM images of Fe-P-900 catalyst after long term cycling tests shown in Figure S8a. S11

Figure S10: (a) XPS peak survey for Fe-P-900 after cycling test and (b) deconvolution profiles of P 2p spectra of Fe-P-900 before and after cycling test. Table S3: Percentage of different P species found in Fe-P-900 before and after cycling test shown in Figure S10b. Sample Name % of P Species P-C P-O P-O-Fe metaphosphates pyrophosphates (P1) (P2) (P3) (P4) (P5) Fe-P-900 before testing Fe-P-900 after testing 77.70 6.40 11.40 4.50 70.36 13.01 7.13-9.50 S12

Table S4. Comparison of ORR activities and kinetics of reported heteroatom-doped carbon catalysts with Fe-P-900. Catalyst Preparation method/ Electrolyte Pyrolysis temperature Onset potential, V vs. RHE Limiting Current density (ma/cm -2 ) Electron transfer number, n Surface area (m 2 g -1 ) Reference Fe-P-C Fe-P-C Phosphorusdoped graphite Layers Fe-N-C catalyst Fe N/C catalyst Carbonsupported Fe-N catalysts Co and Fe loaded N- doped carbon Pyrolysis of phytic acid based polymer in presence of Fe salt at 900 O C Pyrolysis of phytic acid based polymer in presence of Fe salt at 900 O C Pyrolysis of toluene and triphenylphosphine at 1000 O C Pyrolysis of bidppz molecule with FeSO 4 at 800 O C Pyrolysis of 4,40,400- s-triazine-1,3,5- triyltri-paminobenzoic acid with FeCl 2 at 800 O C Pyrolysis of 2,4,6- tris(2-pyridyl)-1,3,5- triazine with (NH 4 ) 2 Fe(SO 4 ) 2 at 800 O C Pyrolysis of 2,6- diaminopyridine in presence of Co(NO) 3 and Fe(NO) 3 at 700 O C in NH 3 HClO 4 HClO 4 H 2 SO 4 0.05 M H 2 SO 4 0.84 5.9 3.83 1371 Present study 0.95 5.01 3.61 1371 Present study 0.85 5.4 3.00 398.6 5 ~ 0.9 6.09 3.92 56 6 0.91 4.1 3.70 -- 7 0.88 4.3 3.80 -- 8 0.87 4.9 3.96 635 9 Fe-N doped carbon nanofibers Fe-N-C catalyst atmosphere Pyrolysis of Fe-PAN (ironpolyacrylonitrile) fibres at 1000 O C Pyrolysis of carbendazim and FeCl 3 mixed with silica template at 800 O C 0.5 M H 2 SO 4 ~ 0.94 ~ 5.5 3.6 377 10 ~ 0.89 ~ 5.5 -- 600 11 Nitrogendoped M/C (M = Fe, Co, Ni) Pyrolysis of N,N9- bis(salicylidene)- ethylenediamin (salen), with metal nitrate at 1000 O C 0.96 ~ 3.75 3.96 483 12 S13

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