Supporting information

Supporting information Constructing a Triple-Phase Interface in Micropores to Boost Performance of Fe/N/C Catalysts for Direct Methanol Fuel Cells Yu-Cheng Wang, Long Huang,, Pu Zhang, Yi-Ting Qiu, Tian Sheng, Zhi-You Zhou,, * Gang Wang, Jian-Guo Liu,, * Muhammad Rauf, Zheng-Qiang Gu, Wei-Tai Wu, Shi-Gang Sun, * State Key Laboratory of Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China Kunming Sino-Platinum Metals Catalyst Co. Ltd., Kunming 650106, China National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China 1. Materials and methods 1.1 Synthesis of Fe/N/C catalyst The Fe/N/C catalyst was prepared through high-temperature pyrolysis of the mixture of the inorganic iron salts (FeCl 3 ) and PmPDA-coated carbon black, according to the previous method. s1 In brief, to the mixture of KJ600 carbon black (2 g) grafted with sulfophenyl group, m-phenylenediamine (m-pda, 15 g) and concentrated HCl (36%, 50 ml), as well as (NH 4 ) 2 S 2 O 8 (2 M, 140 ml) and FeCl 3 (1 M, 40 ml), were added to form a Pm-PDA-coated carbon black. After centrifugal separation and washing, the obtained powder (0.3 g) was mixed with FeCl 3 (1 M, 0.9 ml). The obtained precursor was pyrolyzed at 950 C in Ar atmosphere for 1 h. The pyrolyzed sample was acid leached in 1 M HCl solution at 80 C for 8 h followed by centrifugation and washing. Finally, the obtained powder was again pyrolyzed at 950 C for 3 h to obtain the final catalyst. 1.2 Fabrication of MEA Preparation of DMFC anode Commercial Pt-Ru/C (20 mg, 40 wt% Pt and 20 wt% Ru, Johnson Matthey) was dispersed in the mixture of deionized water (200 µl) and isopropanol (400 µl), followed by adding 217 µl of 5% Nafion solution, and then ultrasonically dispersed for 15 min in an ice bath. The obtained anode ink was coated on a gas diffusion layer (1.21 cm 2 ) consisting of a PTFE-pretreated Toray 060 carbon paper with a microporous layer. The total metal (Pt+Ru) loading of the anode was 4.0 mg cm -2. S1

Preparation of Fe/N/C-based DMFC cathode In a typical procedure, the Fe/N/C catalyst (25 mg), isopropanol (1 ml), 5% Nafion solution (550 µl) and DMS (50 mg) was mixed and ultrasonically dispersed in an ice bath. The resulting cathode ink was coated on a gas diffusion layer described above and finally dried at 80 o C under vacuum. The Fe/N/C loading was 5.0 mg cm -2. The optimum ratio of DMS to Fe/N/C was 2:1. Preparation of Pt/C cathode for DMFC The cathode ink was prepared by mixing commercial Pt/C catalyst (60wt%Pt, Johnson Matthey), Nafion solution (5wt%), and moderate solvent, then ultrasonic mixed for 10 min in an ice bath. The cathode ink was dispersed on a gas diffusion layer through silk-screen process. The Pt loading was controlled at 2 mg cm -2. Hot-pressing MEA The membrane electrode assembly (MEA) was fabricated by hot-pressing a cathode, an anode, a Nafion membrane (Dupont NRE 211 or 115, corresponding to the thickness of 25.4 or 127 µm, respectively), and required size of gasket at 135 o C and 3 M Pa for 2 min. The compression ratio of the total MEA was 35%. The Nafion membrane had been pre-treated with 3% H 2 O 2 and 1 M H 2 SO 4 for 1 h to remove impurities, and preserved in deionized water prior to hot-pressing. 1.3 Fuel cell performance test Polarization curves and durability curves were measured on a Fuel Cell Test System (Arbin Instrument Corporation). Non-humidified oxygen (99.999%) or air was fed as the oxidant at a flow rate of 100 sccm. Methanol solution was passed at 1 ml min -1 and pre-heated to 60 o C or 80 o C before the inlet of the anode. The performance of DMFC was conducted at ambient pressure and the active area of MEA was 1.21 cm -2. After keeping the MEA under these conditions for about half an hour, a constant open circuit voltage (OCP) was obtained, then followed by cell ohmic resistance test and the polarization performance test. The polarization data were recorded in a galvanostatic polarization mode and each point was stayed for 1 minute to get the steady state data. The cell ohmic resistance was tested again after the polarization performance test to make sure the integrity of MEA. 1.4 Electrochemical measurements The ORR polarization curves were obtained on a CHI-760D bipotentiostat equipped with a rotating ring-disk electrode. A standard three-electrode system, including a glassy carbon (GC, φ = 5.61 mm) disk-pt ring RRDE coated with catalyst, a GC plate counter electrode, and a reversible hydrogen reference (RHE) electrode. The scanning rate was set to 10 mv s -1. The electrolyte solution was purged for at S2

least 30 min with high-purity N 2 or O 2 before each test. ORR polarization curves were conducted with potential cycles at potentials between 0.2 V and 1.0 V at 10 mv s -1 in N 2 or O 2 -saturated 0.1 M H 2 SO 4 at 30 o C. Double layer current was corrected and solution ohmic drop was compensated for ORR polarization curves. As for the preparation of catalyst ink, the bare Fe/N/C catalyst (6 mg) was dispersed in a mixture of de-ionized water (0.5 ml) and ethanol (0.5 ml). For the Fe/N/C-DMS catalyst ink, isometric isopropanol (1 ml) used as solvent to disperse Nafion and DMS. And then the dispersion was ultrasonically dispersed for 1 h. The ink was coated on the GC disk electrode, and dried at room temperature. The loading of Fe/N/C catalyst on the electrode surface was maintained at 0.6 mg cm -2. 1.5 Physical characterizations Contact angle of water and methanol solution droplet on the surface of Fe/N/C catalyst layer were conducted on SDC-100 contact angle measurement instrument (Shengding Precision Instrument Co., Ltd., China) at room temperature. The Ar adsorption/desorption isotherm was tested on ASAP 2020 physisorption analyzer (Micromeritics). Micropore size distribution was obtained by Horvath Kawazoe (HK) method, and the mesopore size distribution by Barret Joyner Halenda (BJH) method. The adsorption branch data were used. Laser diffraction particle size distribution of DMS in isopropanol was carried on Zetasizer Nano ZS (Malvern, UK). Reference [s1] Wang, Q.; Zhou, Z. Y.; Lai, Y. J.; You, Y.; Liu, J. G.; Wu, X. L.; Terefe, E.; Chen, C.; Song, L.; Rauf, M.; et al. Phenylenediamine-Based FeN X /C Catalyst with High Activity for Oxygen Reduction in Acid Medium and Its Active-Site Probing. J. Am. Chem. Soc. 2014, 136, 10882-10885. S3

2. Performance comparison of direct methanol fuel cells Table S1. Performance comparison of DMFC employing non-precious metal catalyst as the cathode. Cathode C MeOH / M T cell / o C Current density at 0.4 V / ma cm -2 Peak power density / mw cm -2 Ref. Fe/N/C-DMS 3 80 230 130 60 200 102 This work Fe-N-rGO-900 0 C 0.5 75 130 56 1 Fe-CA-N 2 60 65 56 2 Co-Ppy/MWCNT 1 90 120 55 3 Fe-Nx-C-THT 5 90 75 50 4 CoTMPP 1.1 70 60 45 5 FeAAPyr 5 90 28 35 6 Fe-ABZIM 5 90 30 22 7 FeNP-C 2 50 35 21 8 Fe-N-C 2 90 20 20 9 Fe-N/CNN 2 90 8 15 10 Air was used in cathode. Reference for Table S1 1. Li, Q.; Wang, T.; Havas, D.; Zhang, H.; Xu, P.; Han, J.; Cho, J.; Wu, G. High- Performance Direct Methanol Fuel Cells with Precious-Metal-Free Cathode. Adv. Sci. 2016, 3, 1600140. 2. Yang, W.; Chen, S.; Lin, W. Oxygen Reduction on Non-Noble Metal Electrocatalysts Supported on N-Doped Carbon Aerogel Composites. Int. J. Hydrogen Energy 2012, 37, 942-945. 3. Reddy, A. L. M.; Rajalakshmi, N.; Ramaprabhu, S. Cobalt-Polypyrrole-Multiwalled Carbon Nanotube Catalysts for Hydrogen and Alcohol Fuel Cells. Carbon 2008, 46, 2-11. S4

4. Sebastián, D.; Serov, A.; Artyushkova, K.; Gordon, J.; Atanassov, P.; Aricò, A. S.; Baglio, V. High Performance and Cost-Effective Direct Methanol Fuel Cells: Fe-N-C Methanol-Tolerant Oxygen Reduction Reaction Catalysts. ChemSusChem 2016, 9, 1986-1995. 5. Piela, B.; Olson, T. S.; Atanassov, P.; Zelenay, P. Highly Methanol-Tolerant Non- Precious Metal Cathode Catalysts for Direct Methanol Fuel Cell. Electrochim. Acta 2010, 55, 7615-7621. 6. Sebastián, D.; Baglio, V.; Aricò, A. S.; Serov, A.; Atanassov, P. Performance Analysis of a Non-Platinum Group Metal Catalyst Based on Iron-Aminoantipyrine for Direct Methanol Fuel Cells. Appl. Catal. B-Environ. 2016, 182, 297-305. 7. Sebastián, D.; Serov, A.; Artyushkova, K.; Atanassov, P.; Aricò, A. S.; Baglio, V. Performance, Methanol Tolerance and Stability of Fe-Aminobenzimidazole Derived Catalyst for Direct Methanol Fuel Cells. J. Power Sources 2016, 319, 235-246. 8. Hu, Y.; Zhu, J.; Lv, Q.; Liu, C.; Li, Q.; Xing, W. Promotional Effect of Phosphorus Doping on the Activity of the Fe-N/C Catalyst for the Oxygen Reduction Reaction. Electrochim. Acta 2015, 155, 335-340. 9. Osmieri, L.; Escudero-Cid, R.; Videla, A. H. M.; Ocón, P.; Specchia, S. Performance of a Fe-N-C Catalyst for the Oxygen Reduction Reaction in Direct Methanol Fuel Cell: Cathode Formulation Optimization and Short-Term Durability. Appl. Catal. B-Environ. 2017, 201, 253-265. 10. Negro, E.; Videla, A. H. A. M.; Baglio, V.; Aricò, A. S.; Specchia, S.; Koper, G. J. M. Fe N Supported on Graphitic Carbon Nano-Networks Grown from Cobalt as Oxygen Reduction Catalysts for Low-Temperature Fuel Cells. Appl. Catal. B Environ. 2015, 166, 75-83. S5

3. Effect of the ratio of DMS(14kD) to Fe/N/C catalyst on the DMFC performance Cell voltage / V 0.8 0.6 0.4 0.2 DMS:Fe/N/C=1:10 DMS:Fe/N/C=1:1 DMS:Fe/N/C=2:1 DMS:Fe/N/C=4:1 0.0 0.0 0.1 0.2 0.3 0.4 0.5 Current density / A cm -2 Figure S1. Polarization curves of Fe/N/C-based DMFC using cathode with different content of dimethyl silicon oil (weight ratio) at 60 o C. Methanol concentration: 3 M; DMS molecular weight:14 kd. Obviously, the highest performance was obtained under DMS: Fe/N/C = 2:1 (mass ratio). S6

4. Performance comparison of DMFC with Fe/N/C and Pt/C cathodes at 80 o C Cell voltage / V 0.8 0.6 0.4 0.2 Pt/C Fe/N/C-DMS(14kD) 0.0 0.00 0.0 0.1 0.2 0.3 0.4 0.5 Current density / A cm -2 Figure S2. Polarization curves (left) and power density curves (right) of DMFC with Fe/N/C-DMS(14kD) and Pt/C catalyst as cathodes. The test temperature was 80 o C. Thin Nafion 211 membrane was used for Fe/N/C, and thick Nafion 115 for Pt/C; anode catalyst: Pt-Ru/C (40wt% Pt, 20wt% Ru) with 4.0 mg metal cm -2 ; cathode catalyst loading: 5 mg cat cm -2 for Fe/N/C and 2 mg pt cm -2 for Pt/C cathode; DMS: Fe/N/C catalyst = 2: 1 (weight ratio); pure O 2 was used under ambient pressure. Clearly, both performances are very close. 0.15 0.12 0.09 0.06 0.03 Power density / W cm -2 S7

5. Cell ohmic resistance of Fe/N/C-based DMFC Ohmic Resistance / Ω cm 2 0.20 0.15 0.10 0.05 0.00 Fe/N/C 3.8kD 6kD 14kD 28kD Figure S3. Cell ohmic resistance of Fe/N/C-based DMFC with different DMS molecular weights ranging from 3.8kD to 28kD. The data of bare Fe/N/C cathode was also shown for comparison. Thin Nafion 211 membrane was used. Anode catalyst: Pt- Ru/C (40wt% Pt, 20wt% Ru) with 4.0 mg metal cm -2 ; cathode catalyst loading: 5 mg cat cm -2 for Fe/N/C; DMS: Fe/N/C catalyst = 2 : 1 (weight ratio); pure O 2 was used under ambient pressure. S8

6.Oxygen gain graph of Fe/N/C-based DMFC 0.4 Oxygen gain / V 0.3 0.2 0.1 Fe/N/C-DMS(3.8kD) Fe/N/C Fe/N/C-DMS(14kD) 0.0 0.00 0.05 0.10 0.15 0.20 0.25 0.30 Current density / A cm -2 Figure S4.Oxygen gain graph of Fe/N/C-based DMFC with and without DMS. Thin Nafion 211 membrane was used. Anode catalyst: Pt-Ru/C (40wt% Pt, 20wt% Ru) with 4.0 mg metal cm -2 ; cathode catalyst loading: 5 mg cat cm -2 for Fe/N/C; DMS: Fe/N/C catalyst = 2 : 1 (weight ratio); pure O 2 and air were used under ambient pressure, respectively. S9

7.ORR polarization curves of Fe/N/C with and without DMS (14kD) 0 j / ma cm -2-2 -4 Fe/N/C Fe/N/C-DMS(14kD) 0.4 0.6 0.8 1.0 E / V (RHE) Figure S5. ORR polarization curves of Fe/N/C with and without DMS in the rotating disk electrode test. Conditions: solution, O 2 -saturated 0.1 M H 2 SO 4 ; rotating speed, 900 rpm; potential scan rate, 10 mv s 1 ; Fe/N/C catalyst loading, 0.6 mg cm 2. In aqueous solution test, where water flooding effect is not a consideration, DMS(14kD) also enhanced the ORR activity. This enhancement mainly comes from higher oxygen solubility in DMS phase than in water phase. S10

8. Size distribution of DMS in isopropanol at different concentrations a) Intensity / percent 40 30 20 10 DMS(3.8kD) DMS(6kD) DMS(14kD) DMS(28kD) 5 mg ml -1 b) Intensity / percent 0 30 20 10 0 1 10 100 1000 10000 Size / d. nm DMS(3.8kD) DMS(6kD) DMS(14kD) DMS(28kD) 0.2 mg ml -1 1 10 100 1000 1000 Size / d. nm Figure S6. Laser diffraction particle size distribution of DMS with different molecular weights dispersed in isopropanol at different concentration: a) 5 mg ml -1 ; b) 0.2 mg ml -1. S11

9. Ar adsorption-desorption isotherm of the Fe/N/C with different DMS content a) Quantity Absorbed (cm 3 /g STP) 100 80 60 40 20 DMS(3.8kD)-0.5 Vpore DMS(14kD)-0.16 Vpore 0 0.0 0.2 0.4 0.6 0.8 1.0 Relative pressure (P /P 0 ) b) dv /dw / cm 3 g -1 nm -1 0.10 0.05 0.00 DMS(3.8kD)-0.5 Vpore DMS(14kD)-0.16 Vpore 0.8 1.2 1.6 2.0 Pore Width / nm c) 0.02 dv /dw / cm 3 g -1 nm -1 0.01 0.00 DMS(3.8kD)-0.5 Vpore DMS(14kD)-0.16 Vpore 10 100 Pore Diameter / nm Figure S7. (a) Ar adsorption-desorption isotherm of the Fe/N/C catalyst with the addition of DMS(3.8kD) and DMS(14kD). The volume of DMS(3.8kD) was equal to 50% of the total pore volume of Fe/N/C; while 16% for DMS(14kD). (b) micropore (HK method) and (c) mesopore (BJH method) size distributions. S12

10. Methanol resistance of the Fe/N/C-DMS(14kD) cathode 0-1 Blank 1M CH 3 OH j / ma cm -2-2 -3-4 0.4 0.6 0.8 1.0 E / V (RHE) Figure S8. Comparison of ORR polarization curves of Fe/N/C catalyst recorded in 0.1 M H 2 SO 4 and 0.1 M H 2 SO 4 + 1 M CH 3 OH solution. Both solutions were O 2 - saturated. Rotating speed, 900 rpm; potential scan rate, 10 mv s -1 ; Fe/N/C catalyst loading: 0.6 mg cm -2. Obviously, the Fe/N/C catalyst is insensitive to methanol under ORR condition. S13