Research Article www.acsami.org
Highly Functional Bioinspired Fe/N/C Oxygen Reduction Reaction Catalysts: Structure-Regulating Oxygen Sorption Yingfang Yao,†,‡,# Yong You,†,# Gaixia Zhang,§ Jianguo Liu,*,†,∥ Haoran Sun,† Zhigang Zou,*,†,‡ and Shuhui Sun*,§ †
National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences, and Collaborative Innovation Center of Advanced Microstructures, and ‡Department of Physics, Nanjing University, 22 Hankou Road, Nanjing 210093, China § Institut National de la Recherche Scientifique − Énergie, Matériaux et Télécommunications, 1650 Boulevard Lionel-Boulet, Varennes, Quebec J3X 1S2, Canada ∥ Kunshan Sunlaite New Energy Co., Ltd., 1699# South Zuchongzhi Road, Suzhou, Kunshan, 215347, China S Supporting Information *
ABSTRACT: Tuna is one of the most rapid and distant swimmers. Its unique gill structure with the porous lamellae promotes fast oxygen exchange that guarantees tuna’s high metabolic and athletic demands. Inspired by this specific structure, we designed and fabricated microporous graphene nanoplatelets (GNPs)-based Fe/N/C electrocatalysts for oxygen reduction reaction (ORR). Careful control of GNP structure leads to the increment of microporosity, which influences the O2 adsorption positively and desorption oppositely, resulting in enhanced O2 diffusion, while experiencing reduced ORR kinetics. Working in the cathode of proton-exchange membrane fuel cells, the GNP catalysts require a compromise between adsorption/desorption for effective O2 exchange, and as a result, appropriate microporosity is needed. In this work, the highest power density, 521 mW·cm−2, at zero back pressure is achieved. KEYWORDS: proton-exchange membrane fuel cells, oxygen reduction reaction, nonprecious metal catalyst, graphene nanoplatelets, microporosity
1. INTRODUCTION Proton-exchange membrane fuel cells (PEMFCs) are treated as one of the major alternative power suppliers to replace fossil fuel for internal combustion engines.1,2 However, the scarcity and high cost of Pt, used as the electrocatalysts of PEMFCs, has highly hindered wide commercialization of PEMFCs.3 Recent biochemical inspiration of hemoglobin in red blood promoted the development of Fe/N/C oxygen reduction reaction (ORR) catalysts,4−7 which have effective electrochemical catalytic performance and relatively good stability and therefore is regarded as one of the most promising nonprecious metal catalysts to replace Pt. The kinetics of ORR on catalytic surfaces should counterbalance two opposite effects, that is, relatively strong adsorption behavior of reactive oxygen species and relatively low coverage by nonreactive oxygenated species via desorption.8−10 Therefore, a high-performance Fe/N/C electrocatalyst should obtain effective oxygen exchange. In nature, fast oxygen exchange of hemoglobin is usually obtained in lungs of mammals11 and gills of fishes,12−14 which arises from the high specific area that allows effective O2 adsorption into blood and CO2 desorption into air. As the members of the most rapid and distant swimmers, scombrids and billfishes, specifically tunas, require © XXXX American Chemical Society
high-rate branchial gas transfer for their metabolic requirements and athletic demands. Tunas’ gill structures consist of groups of lamellae bestrewed with sub-micrometer pores (Figure S1, see Supporting Information) that facilitate the interaction between hemoglobin and O2. This structure offers the largest relative surface area of any fish groups14 to allow efficient oxygen exchange (Figure 1), and resultantly vigorous aerobic activities.14,15 Inspired by this biological structure, we attempted to fabricate Fe/N/C catalysts with lamellar porous structure. This structural configuration should influence the gas adsorption/desorption properties of the catalyst nanomaterials, which are expected to improve the electrochemical performance of GNP-based Fe/N/C catalysts. Among all types of carbon substrates,16−25 microporous graphene nanoplatelets (GNPs) should be a great catalyst candidate. Recently, although graphene-based nonprecious metal ORR catalysts have been extensively studied,20,26−33 it seems a quite difficult task to study the porous structureinduced oxygen sorption behavior and its influence on the Received: December 18, 2015 Accepted: February 23, 2016
A
DOI: 10.1021/acsami.5b11870 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Material Characterizations. The morphology of as-prepared samples was characterized by field emission scanning electron microscopy (FE-SEM) (FEI NOVA NanoSEM230, USA) and transmission electron microscopy (TEM) (JEOL 3010, Japan). The crystal structures of the samples were characterized by a powder X-ray diffractometer (XRD, Ultima III, Rigaku Corp., Japan) using Cu Kα radiation (λ = 1.54178 Å, 40 kV, 40 mA). The surface composition of the samples was characterized by using X-ray photoelectron spectroscopy (XPS, ESCALAB 250); the nonmonochromatic Mg Kα X-ray was chosen as the X-ray source. The specific surface area was measured through Brumauer−Emmett−Teller (BET) method by N2 adsorption (TriStar-3000, Micromeritics, USA) and O2 adsorption (ASAP2020-M +C, Micromeritics, USA). Raman spectra were collected by a Jobin Yvon HR800 Raman scattering system with an excitation wavelength of 488 nm. Thermogravimetric (TG) analysis was carried out using Netzsch STA 449C equipment with a heating rate of 10 °C/min under an air atmosphere. O2 temperature-programmed desorption (O2-TPD) was measured using the Micromeritics Autochem II2920. Electrochemical Characterization. Catalyst ORR performance and H2O2 selectivity were measured using rotating ring-disk electrode (RRDE) technique in 0.1 M HClO4 aqueous solution. All measurements were conducted in a thermostat-controlled, standard threeelectrode cell at room temperature in which a platinum foil, a reversible hydrogen electrode (RHE), and a RRDE were used as the counter electrode, the reference electrode, and the working electrode, respectively. The catalyst “ink” was prepared by ultrasonically mixing a mass of 10 mg of catalyst with 95 μL of a 5 wt % Nafion solution (Aldrich) and 350 μL of ethanol for 1 h. Then an aliquot of 4.5 μL was pipetted onto the RRDE (diameter = 4 mm), resulting in a catalyst loading (iron plus carbon black) of around 716 μg·cm−2. The catalyst film was dried in air at room temperature. Meanwhile, 0.1 M HClO4 aqueous solution saturated with oxygen by bubbling O2 for 30 min served as the supporting electrolyte. Steady cyclic voltammetry was performed in a potential range of 0−1.2 V vs RHE at a scan rate of 50 mV·s−1. For the ORR measurements using the RRDE technique, the catalysts-modified gas chromatrography electrode was rotated at a speed of 900 rpm in O2 saturated 0.1 M HClO4 aqueous solution, and the RRDE current voltage curves were measured at a potential scan rate of 10 mV·s−1. A potential of 1.20 V vs RHE was applied to the Pt ring electrode for measuring the H2O2 production. The four-electron selectivity of catalysts was evaluated based on the H2O2 yield, calculated from the following the equation:
Figure 1. Schematic illustration of the sorption behavior of oxygen for graphene nanoplatelet (GNP) catalysts analogous to the oxygen exchange in tunas’ gills. The sub-micrometer pores on the gill lamellae facilitates the oxygen exchange between the vessel blood and seawater. Similarly, the micropores on the GNPs play an important role in the sorption behavior of oxygen species in the ORR process.
catalytic performance. This is partly because of the obstacle of structural control with limited chemical change. In our work, we produced the GNP-based Fe/N/C catalysts with the pyrolysis of highly dispersed polyimide. And molten salts (MS) were induced as the dispersing matrixes. In the fabrication process, the chemical similarity is ensured with the same precursor, and the porous structures can be simply controlled by changing the ratio of MS to reactive precursors.34−37 We found that similar to tunas’ porous lamellar gills, at the sites of which the fast oxygen exchange occurs between the vessel blood and seawater, proper amount of micropores could facilitate both adsorption and desorption of oxygen species, and thus improve the kinetic current and diffusion-limiting current density, and finally increase the electrochemical performance of GNP-based ORR catalysts.
H 2O2 (%) = 200 ×
2. EXPERIMENTAL SECTION Synthesis of Catalysts. MA (melamine) (99 wt %) and PMDA (pyromellitic dianhydride) (99 wt %) were purchased from Aladdin Industrial Corporation (America) in China. FeCl3·6H2O (analytical grade), ZnCl2 (99.9 wt %), and KCl (99 wt %) were purchased from Shanghai Chemical Reagent Co. (China). All the above chemicals were used as received. In a typical process, for sample GNP_15_1, 0.5 mmol of FeCl3· 6H2O, 2.5 mmol of MA, 3.75 mmol of PMDA, and metal chloride salts (KCl/ZnCl2 = 51/49 by molar) with the weight ratio of MS/reactant weight ratio = 15:1 (MS:KCl/ZnCl2) were thoroughly mixed and homogenized with ball milling for ∼30 min. The homogeneous mixture was added into a porcelain crucible and then transferred to a tube furnace with N2 atmosphere. The reaction chamber was heated to 325 °C at 10 °C/min and dwelled for 4 h to allow the polymerization. Then the furnace temperature was increased to 900 °C and kept at this temperature for 1 h to allow complete pyrolysis. After natural cooling to ambient temperature, the products were ultrasonically rinsed with distilled water and 0.5 M H2SO4 at 80 °C for 12 h to remove the salts and other impurities. The wet carbon sample collected by filtration was dried in a vacuum at 80 °C for 24 h. Finally, the product was then heat-treated at 900 °C in a N2 atmosphere for another 2 h to obtain GNP series Fe/N/C catalysts. The samples GNP_25_1 and GNP_50_1 with variable MS/reactant weight ratios of 25:1 and 50:1, respectively, were synthesized under identical conditions (see Supporting Information).
IR /N (IR /N ) + ID
Here, the ID and IR are the disk current and ring current, respectively, and the N is the ring collection efficiency of ∼0.42. Fuel Cell Tests. A MEA (membrane electrode assembly) was fabricated by decal transfer method. The GNP series Fe/N/C catalyst samples were dispersed in a mixture of Nafion solution (5 wt %, Ion Power Inc.), ethanol, and deionized water. The dry catalyst/Nafion ratio was 2/1. The obtained catalyst “ink” was then dropwisely transferred to the gas diffusion layer (Sunrise Power Inc., China) until the cathode catalyst loading reached 4 mg·cm−2. The anode catalyst was 60 wt % Pt/C from Johnson Matthey with a loading of 0.2 mgPt·cm−2, and the Nafion content in the anodic catalyst layer was 33 wt %. Then the MEA was prepared by hot-pressing the electrodes and Nafion 211 membrane with an active area of 1.0 cm−2. Fuel cell polarization curve was tested at 80 °C on a fuel cell test system (Model 850e, Scribner Associates Inc.). H2 and O2 flow rates were set at 300 sccm at 100% relative humidity, and no back pressure was applied (i.e., the O2 and H2 partial pressure was about 0.53 bar since the saturation water vapor pressure at 80 °C is ca. 0.47 bar).
3. RESULTS AND DISCUSSION To compare the influence of porosity on the catalytic performance of GNP series Fe/N/C catalysts, rational control of material characteristics is required. The goal is to control the variation of pore numbers and to keep the similarity on the B
DOI: 10.1021/acsami.5b11870 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
of chemical composition of as-prepared samples. Furthermore, the phase composition and structure of the samples were examined by XRD shown in Figure 3C. For all samples, a broad peak found at 2θ ≈ 26.5° can be associated with graphitic carbon. The weak peaks at around 2θ = 45.0° might be indexed with Fe3C. The low signal strength of XRD patterns are probably because of the amorphous phase of Fe species, or low content of Fe, which can be demonstrated in XPS survey in Table 1. Despite the morphological and compositional similarity, the ORR activities of different GNP series Fe/N/C catalysts vary significantly. The ORR performance of the catalysts was investigated via RRDE as a function of the MS/reactant weight ratio. As shown in Figure 4, two tendencies can be observed. First, the kinetic current decreases with increasing the MS/reactant weight ratio. The onset potential declines from 0.97 V (vs RHE) for GNP_15_1, to 0.94 V for GNP_25_1, and to 0.93 V for GNP_50_1. And the kinetic current density at 0.8 V decreases from −1.9 mA·cm−2 for GNP_15_1, to −1.3 mA·cm−2 for GNP_25_1, and to −0.9 mA·cm−2 for GNP_50_1 (“−” indicates the reduction process). Second, at more negative potential zone where mass transfer dominates the current density (i.e., diffusion-limiting current density), a reduction peak is observed in the potential range of 0.65− 0.75 V for each polarization curve. Cyclic voltammetry of GNP_50_1 was performed in a potential range of 0−1.2 V vs RHE at a scan rate of 50 mV·s−1 in the electrolyte solution saturated with N2 (Figure S3, see Supporting Information). No oxidation peak or reduction peak appeared around 0.65∼0.75 V, indicating that the peak did not correspond to the reduction of Fe3+ to Fe2+. This peak is probably due to the thick layer of the catalyst on RDE tips and the microporosity of the catalysts as well. Although all the samples share the same mass per unit, that is, 716 μg·cm−2, this peak grows stronger with the increase of MS/reactant ratio, indicating the enhanced capability of oxygen uptake in GNP matrixes. When MS/ reactant weight ratio decreases to 10/1 or 5/1, the ORR activity (both the onset potential and the kinetic current density at 0.8 V) of the catalysts decreased successively, as shown in Figure S4 (see Supporting Information). The H2O2 yield with all GNP series Fe/N/C catalysts remains below 3% at all potentials, signaling virtually complete O2 reduction to H2O in a four-electron process. The avoidance of two-electron reaction that related to the undesirable H2O2 production matches or exceeds the four-electron selectivity of Pt-based catalysts.40 Further, the half-wave potential of GNP series catalysts only dropped 18 mV after 10 000 cycle of CV tests from 0.6 to 1.0 V (Figure S5, see Supporting Information), and the major potential drop occurred in the first 1000 cycles, indicating good stability of the GNP Fe/N/C catalysts. The gradation of electrocatalytic performance of GNP series Fe/N/C catalysts should operationally originate from the weight ratio changes of MS/reactant during sample preparations. With such changes, the specific surface area and porosity of GNPbased Fe/N/C products could be easily controlled. The apparent surface areas and pore size distributions were determined with nitrogen sorption measurements (Figure 5A, B) by applying the BET method. For all the GNP-based Fe/N/C samples, the isotherms show subtle hysteresis as relative pressure higher than p/p0 ∼ 0.44, which is probably caused by the macropores, and the interstitial pores of neighboring nanoplatelets. The N2 adsorption isotherms are of type I, dominated by micropores and the typical graphene-like surface layer sorption.37 The isotherms
other characteristics, such as morphology, chemical composition, and pore size. Thanks to the incorporation of MS, the comparatively lower eutectic temperature of KCl/ZnCl2 at 230 °C and a higher decomposition temperature (over 1000 °C) make the polycondensation and carbonization of reactants dilute in the circumstance of salty liquids. If good miscibility of the carbonizing polymer in the liquids environment is retained over a main part of the reaction pathway, the formed intermediates should be surrounded by salt clusters at the initial stage. Therefore, relatively microporous graphene nanoplatelets could be expected after the chloride salts are removed by simple washing with distilled water and 0.5 M H2SO4 solution. The porosity can be readily controlled by the cooperative interaction of chloride salts and the polymerized reactors during the pyrolysis process.34,37 SEM photographs illustrate the morphology of graphene nanoplatelets for all prepared samples (Figure S2, see Supporting Information). These lamellae are generally several micrometers in size, twisted, and wrinkled somehow, which is probably caused by the thermal stress during heat treatment. TEM images also confirm the similar GNP morphologies (Figure 2) for all samples. The high-resolution TEM (HRTEM, Figure 2D) illustrates highly microporous structure of the as-prepared samples, which is analogous to the branchial structure.
Figure 2. TEM images of (A) GNP_15_1, (B) GNP_25_1, and (C) GNP_50_1. HRTEM image of GNP_50_1 was also illustrated at (D), in which the numerous pores as the white areas could be observed.
Furthermore, the identical amount of reactive precursors and conditions of heat treatment ensures the similar chemical composition of the products. GNP series Fe/N/C catalysts were probed by XPS. As shown in Figure 3A and Table 1, the survey spectra of the samples show that with MS/reactant ratios varying from 15/1, 25/1, to 50/1, trivial change of element composition is observed. For example, nitrogen contents only vary from 3.99 at. %, to 3.41 at. %, and to 3.03 at. %, respectively. It has been well-recognized that the nitrogenbonding configurations in Fe/N/C catalysts are expected to play an important role in the ORR electrocatalytic performances and highly depend on the condition of heat treatment.17,31,38,39 Analysis of N 1s spectra reveals the presence of pyridinic-N and graphitic-N for all the prepared samples, corresponding to the binding energies of 398.6 and 401.0 eV, respectively, as shown in Figure 3B. The percentages of pyridinic-N and graphitic-N in different samples are almost the same, for example, 50.5%/49.5% for GNP_15_1, 51.6%/48.4% for GNP_25_1, and 51.9%/48.1% for GNP_50_1, demonstrating the similarity C
DOI: 10.1021/acsami.5b11870 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 3. Characterization of chemical composition. (A) XPS survey spectra of GNPs. (B) XPS N 1s scan spectra of GNP-based Fe/N/C catalysts. (C) Powder XRD patterns of GNPs compared with XRD pattern of Fe3C.
Table 1. XPS Analysis of GNP-Based Fe/N/C Catalysts N 1s composition samples GNP_15_1 GNP_25_1 GNP_50_1
C O N Fe pyrindinic-N graphatic-N (at. %) (at. %) (at. %) (at. %) (at. %) (at. %) 84.68 84.66 83.84
11.10 11.69 12.88
3.99 3.41 3.03
0.24 0.24 0.25
50.5 51.6 51.9
49.5 48.4 48.1
reflect that the eutectic salts presumably acted as a “molecular template” during pyrolysis, either forming ion pairs or little salt clusters, resulting in the pores under the scale of ∼2 nm. Besides, the GNP-based Fe/N/C catalysts showed a strengthened nitrogen adsorption and a higher specific surface area with the increase of MS/reactant weight ratio. This is because the reactive precursors are more dilutedly dispersed in the eutectic KCl/ ZnCl2, leading to thinner nanoplatelets and a larger amount of micropores, as demonstrated by the BET surface areas and pore distributions in Figure 5B. Fitting the BET equation41 to the N2 adsorption isotherms of GNP-based Fe/N/C samples give estimated surface areas of 561.6, 882.8, and 1230.6 m2·g−1, respectively. The Dubinin−Radushkevich equation gives estimated pore volumes of 0.51, 0.60, and 0.67 cm3·g−1 for GNP_15_1, GNP_25_1, and GNP_50_1, respectively. We also noted that the GNP series Fe/N/C materials share almost the same pore size distribution, with the dominant pore diameters at ∼1.0 nm and average pore diameter in the range of 1.5 ± 0.2 nm, as illustrated in Figure 5B,C. The comparison (Figure S6, see Supporting Information) of N2 adsorption isotherms and pore size distributions of the catalyst prepared without MS with that of GNP series catalysts also provide evidence to corroborate the claim of molecular template during pyrolysis. As the effective ORR catalysts, GNP-based Fe/N/C materials were assumed to behave differently in O2 adsorption compared with that in N2 adsorption. Therefore, the O2 sorption
Figure 4. Steady-state ORR polarization curves (bottom) and H2O2 yield (top) of GNP-based Fe/N/C catalysts in O2-saturated 0.1 M HClO4 solution. Rotating speed, 900 rpm; scan rate, 10 mV s−1; catalysts loading, 0.71 mg·cm−2; 25 °C water bath. For the ORR polarization curves, the voltage at 0.01 mA·cm−2 was treated as the onset potential.
measurements were also carried out and shown in Figure 5. Despite only relatively small differences in the kinetic diameters between O2 (3.46 Å) and N2 (3.64 Å),42,43 with values much smaller than the measured pore sizes of GNPs (1.0 nm), slight hysteresis loops of O2 desorption is observed with P/P0 ≥ 0.16 (Figure 5A), which is smaller than 0.44 for N2, reflecting the D
DOI: 10.1021/acsami.5b11870 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 5. (A) Comparison of N2/O2 sorption measurements of the samples with different MS/reactant weight ratios at 77 K with the samples degassed at 423 K under vacuum; (B) Corresponding pore size distributions from N2 sorption measurements. Inset: Dependence of apparent surface area on the MS/reactant weight ratio. (C) Corresponding pore size distributions from O2 sorption measurements.
Figure 6. (A) Raman spectra, (B) O2-TPD of GNP-based Fe/N/C catalysts, and (C) TG analysis of representative GNP_15_1; temperature rate: 10 °C·min−1.
stronger affinity of O2 to GNPs than N2 does.42,44 Higher dependence of O2 sorption upon the MS/reactant weight ratio is also observed. The O2 adsorption of GNP_50_1 exceeds N2, for example, 603 cm−3·g−1 O2 vs 449 cm−3·g−1 N2 at P/P0 = 0.8.
This value dramatically dropped with the decrease of MS/ reactant weight ratio. O2 adsorption is almost the same as N2 for GNP_25_1, for example, 327 cm−3·g−1 O2 vs 330 cm−3·g−1 N2 at P/P0 = 0.8, while it is even slightly lower than N2 for E
DOI: 10.1021/acsami.5b11870 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces GNP_15_1, for example, 209 cm−3·g−1 O2 vs 230 cm−3·g−1 N2 at P/P0 = 0.8. We believe such remarkable sensitivity of O2 adsorption is mainly caused by the amount of exposed oxytropic sites, composed of either Fe species45 or N ligands.46,47 When the MS/reactant ratio is as high as 50/1, almost all the oxygenic active sites are exposed in the micropores, facilitating the chemical contact with oxygen that provides much larger chemi-/ physisorption capacity of O2 than physisorption of N2. However, when the MS/reactant ratio decreases to 25/1, or even 15/1, the chemical active sites are embedded in the carbonaceous frameworks instead of interacting with O2. As a result, the O2 adsorption behavior of GNP-based Fe/N/C catalysts with small MS/reactant ratios can be lower than that of N2 adsorption, reflecting the adjustability of O2 chemisorption/physisorption of GNP-based Fe/N/C catalysts. According to the investigations mentioned above, we could ascribe the enhanced oxygen reduction peaks in the diffusionlimiting zone (Figure 4) to the improved oxygen adsorption of different GNP-based Fe/N/C catalysts (Figure 5), mainly caused by the higher microporosity and surface area. We also found that such microporous structure which impedes the integrity of carbon framework leads to the reduction of kinetic current. As shown in the Raman spectra (Figure 6A), two peaks emerging near 1590 and 1352 cm−1 can be assigned to the G band (graphitic carbon) and D band (disordered carbon), respectively. With increasing the MS/reactant weight ratio from 15/1, to 25/1, and to 50/1, the G band becomes blunt and the intensity ratio of the G to the D band (IG/ID) decreases from 1.08, to 0.98, and to 0.94, demonstrating the microporeintroduced disorder of carbon structures. Such structural disorder, on the one hand, reduces the electric conductivity of GNPs;48−50 on the other hand, it also baffles the desorption of oxygenate species of GNPs during ORR catalysis. To qualitatively analyze the desorption behavior of GNPs, O2 temperature-programmed desorption (O2-TPD) measurements was carried out, as shown in Figure 6B. The TG analysis (the mass loss under the 120 °C in TG analysis was due to the removal of water that was adsorbed in catalysts) in oxygen (Figure 6C) ensures the stability of GNPbased Fe/N/C catalysts at a temperature lower than ∼350 °C, and over such temperature combustion of carbon substrates occurs. Therefore, the O2-TPD measurements were carried out in the temperature range of 25−300 °C. The O2 desorption starts from 90 °C for GNP_15_1, to 123 °C for GNP_25_1, and to 226 °C for GNP_50_1, respectively. The O2-TPD results indicate that the desorption of oxygenate species gets more difficult on the GNP-based Fe/N/C catalysts with higher microporosity (prepared with higher MS/reactant weight ratio). In brief, the loss of both electric conductivity and desorption activity caused by the increase of microporosity leads to the reduction of catalytic kinetics of ORR. Finally, the MEAs were fabricated, and the H2/O2 PEMFCs performances were tested. These MEA consisted of GNP-based Fe/N/C catalysts (4.0 mg·cm−2) as the cathode and Pt/C (0.2 mgPt·cm−2) as the anode. During fuel cell tests, no back pressure was applied. Figure 7A illustrates the polarization curves and power density plots of the PEMFCs with GNPbased Fe/N/C catalysts. The highest power density (Pmax) of GNP_25_1 reaches 521 mW·cm−2 at 0.35 V, slightly higher than Pmax of 501 mW·cm−2 at 0.36 V for GNP_15_1, and Pmax of 473 mW·cm−2 at 0.37 V for GNP_50_1. To the best of our knowledge, this is one of the highest Pmax at zero back pressure reported so far for PEMFCs with Fe/N/C as cathode catalysts. After iR-correction, Tafel plots were obtained (Figure 7B).
Figure 7. (A) Polarization and power density plots for H2/O2 PEMFCs with GNP-based Fe/N/C as cathode catalysts at 80 °C. Zero back pressure was applied; flow rate, 300 sccm at cathode/anode; MEA active area, 1.0 cm2; NRE 211 membrane; cathode catalyst loading, 4.0 mg·cm−2; anode catalyst, Pt/C (60 wt %, JM) with 0.2 mgPt·cm−2. (B) Corresponding iR-corrected polarization plots of H2/O2 PEMFCs.
The Tafel slopes based on fuel cell data are 91 mV dec−1 for GNP_15_1, 104 mV dec−1 for GNP_25_1, and 96 mV dec−1 for GNP_50_1. The variation of Tafel slopes may be attributed to various factors, such as MEA fabricating conditions, turbulence of testing temperature, humidity, gas feeding rate, water management, and so forth. Despite minimum change between PEMFCs assembled with different GNP-based Fe/N/C catalysts, slight performance superiority can still be observed with the fuel cell consisting of GNP_25_1, possibly because of the counterbalance between oxygen adsorption and desorption on the cathode side, suggesting moderate microporosity analogous to tunas’ gill structure leads to effective oxygen exchange and thus high ORR activity, as indicated with Sabatier principle.8−10,51
4. CONCLUSION Conclusively, we systematically designed and developed GNPbased Fe/N/C catalyst materials with microporous structure as Fe/N/C ORR catalysts. This type of material shows welldefined controllability of specific surface area and microporosity, and accordingly structural sensitivity of oxygen adsorption/ desorption. As the material-dependent chemisorption has recently been a hot spot of the global investigation, and is treated as one of the major factors to influence ORR activities,8−10 we focused our F
DOI: 10.1021/acsami.5b11870 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
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research on the structure-deciding sorption behavior, and found out that the increase of microporosity affects the ORR activity on the aspects of not only the O2 adsorption that facilitates reactive gas diffusion ability but also the hindered kinetics probably caused by the desorption of oxygenate species. Resultantly, it is understandable that GNP-based Fe/N/C cathodic catalysts for a high-performance PEMFC require the counterbalance between the adsorption and desorption, that is, the structural microporosity and integrity for the effective oxygen exchange during the catalytic process. The GNP-based Fe/N/C materials have good catalytic performance and stability, and thus possess high potential for PEMFC commercialization. Moreover, the concept and results discovered in this work may shed light on the development novel materials for other applications, such as photo-/photoelectrical water splitting, metal air batteries, and gas sensors.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b11870. Preparation of GNP_25_1/GNP_50_1 catalyst; SEM images of tuna’s gill structure, GNP_15_1, GNP_25_1, and GNP_50_1; cyclic voltammetry test of presentative GNP_50_1; steady-state ORR polarization curves of GNP series catalysts; the lifetime test of presentative GNP_15_1 ORR electrocatalyst; comparison of N2 sorption measurements of the samples with different MS/reactant weight ratio (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*Tel.: +86 25 83621219. Fax: +86 25 83686632. E-mail:
[email protected]. *Tel.: +86 25 83686630. Fax: +86 25 83686632. E-mail:
[email protected]. *Tel.:+1-514-228-6919. Fax: +1-450-929-8102. E-mail:
[email protected]. Author Contributions #
These two authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by Natural Science Foundation of China (21176111, 21476104), National Basic Research Program of China (973 Program, 2013CB632404), Natural Science Foundation of Jiangsu Province (BK2012217), and Priority Academic Program Development of Jiangsu Higher Education Institutions. Jianguo Liu also is thankful for the support of Jiangsu Province Natural Science Foundation for Distinguished Young Scholars (BK20150009), the Fundamental Research Funds for the Central Universities, and Qing Lan Project of Jiangsu Province, China.
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REFERENCES
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DOI: 10.1021/acsami.5b11870 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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DOI: 10.1021/acsami.5b11870 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
