High-Performance Oxygen Reduction Electrocatalysts based on Cheap Carbon Black, Nitrogen, and Trace Iron

High-Performance Oxygen Reduction Electrocatalysts based on Cheap Carbon Black, Nitrogen, and Trace Iron Jing Liu, Xiujuan Sun, Ping Song, Yuwei Zhang, Wei Xing, and Weilin Xu * Due to the energy crisis and increasing scarcity of noble metals, sustainable non-precious metal or metal-free electrocatalysts for oxygen reduction reactions (ORR) are attracting more and more attention for fuel cell systems. [ 1 6 ] Heteroatom (N, B, S, P, Fe, Co, or Mn)-doped carbon materials, such as carbon nanotubes (CNTs), [ 7 9 ] graphene, [ 10 ] graphitic arrays, [ 11 ] and amorphous carbon, [ 12 14 ] have been found to exhibit excellent electrocatalytic performance for ORR. Compared with traditional Pt-based catalysts, non-precious metal-based catalysts for ORR possess several advantages, such as higher activity, long-term operation stability, tolerance to poisons, and, most importantly, sustainability. Among all these carbon-based non-pt catalysts, very few are on a competitive level with platinum. [ 1,3,6,9,13,15 ] One family of these most promising alternatives to Pt are mainly the iron or cobalt-doped carbon materials with a relatively high optimal metal content of about 3 10 wt%. [ 1 3,5,15 19 ] Here, by tuning the Fe content and co-doping with nitrogen on cheap carbon black (CB) over a wide range from 0.02 to 20 wt%, we found, in addition to the previously reported optimal Fe content in a high concentration range, that there is a second or real optimal value in an extremely low concentration range. The new-found optimal catalyst (CB-NFe) with a trace Fe content down to 0.05 wt% showed a superior high performance compared with the other non-pt electrocatalysts for ORR. It was comparable in acidic medium [ 3,6,15 ] and better in alkaline medium [ 1,2,9 ] than commercial Pt/C validated unambiguously by the alkaline direct methanol fuel cell tests. These CB-NFe catalysts are among the most efficient electrocatalysts for ORR. Since the cost of CB is only one in ten thousandth the cost of Pt, these CB-NFe electrocatalysts are the most promising alternatives to Pt for ORR in fuel cells to date. The synthesis of CB-NFe was based on a simple procedure with cheap chemicals BP2000 (BP), melamine (C 3 H 6 N 6 ), and FeCl 3 (Supporting Information). For comparison, BP, BP-N, and BP-Fe were also obtained in a similar way. In order to characterize the performance of different ORR electrocatalysts, generally two parameters of onset potential ( E onset ) and half-wave potential ( E 1/2 ) from linear sweep voltammetry (LSV) in O 2 -saturated 0.1 M KOH on a rotating disk electrode (RDE) are used for comparison. [ 1 ] Figure 1 a shows the effect of pyrolyzing temperature on the performance of the catalyst. It can be seen that the catalyst shows best performance with a pyrolyzing temperature of about 900 C. We further varied the initial melamine/ carbon (M/C) weight ratio to tune the final nitrogen content in the catalyst. As shown in Figure 1 b and c, the catalyst activity shows a volcano-shaped dependence on N content or the initial M/C ratio. The optimal N content was found to be about 2.4 wt% with an initial M/C mass ratio of 10. It is worth noting that when the initial M/C weight ratio is higher than 5, the final N contents in the catalysts are almost saturated, but the activities vary over a wide range, indicating the key to high ORR electrocatalytic activity is not directly related to the apparent N content, but to the doping pattern of N and Fe in the carbon. Based on the above optimal conditions, we studied the effect of Fe content on the catalyst performance by varying Fe J. Liu, [+] X. Sun, [+] Dr. P. Song, Dr. Y. Zhang, Prof. W. Xing, Prof. W. Xu State Key Laboratory of Electroanalytical Chemistry Changchun Institute of Applied Chemistry Chinese Academy of Science 5625 Renmin Street, Changchun, 130022, PR China E-mail: weilinxu@ciac.jl.cn J. Liu, X. Sun, Dr. P. Song, Dr. Y. Zhang, Prof. W. Xing, Prof. W. Xu Jilin Province Key Laboratory of Low Carbon Chemical Power Changchun Institute of Applied Chemistry Chinese Academy of Science 5625 Renmin Street, Changchun, 130022, PR China X. Sun Graduate University of Chinese Academy of Science Beijing, 100049, PR China [+] These authors contributed equally to this work. Figure 1. The optimization of the CB-NFe catalysts. a) Pyrolyzing temperature dependence of the catalyst performance, all these catalysts were obtained with an M/C ratio of 10 and a Fe loading of 0.05 wt%. b,c) M/C ratio dependence of the catalyst activity and fi nal N content, all these catalysts were obtained with heat-treatment under 900 C and 0.05 wt% Fe. The error bar in (c) is the s.d. based on three groups of independent samples.inset is the fi nal N-content dependence of catalyst activity. d) Fecontent dependence of BP-NFe catalysts obtained with heat-treatment under 900 C and an M/C ratio of 10. The values of Fe contents in these catalysts were obtained with ICP-MS. 1

www.materialsviews.com Figure 2. Structural characterization of BP-based catalysts. a)upper: Typical TEM image of pure BP; Lower: Typical TEM image of optimal BP-NFe. b) Nitrogen sorption isotherm of optimal BP-NFe. content from 0.02 to 20 wt%. As shown in Figure 1d, there are two apparent optimal peaks. Peak I at Fe 8 wt% is similar to that observed by many other groups.[18] Surprisingly, at an extremely low Fe content of 0.05 wt%, there is a real optimal catalyst possessing the highest activity (Peak II). To our best knowledge, this phenomenon has never been observed before for Fe-doped ORR electrocatalysts. The optimal trace Fe content of 0.05 wt% observed here is consistent with a previous report that only a small amount of Fe would be expected to participate in atomically dispersed active sites, 0.2 wt.% in ORR electrocatalysts.[19] The reason why the optimum point shows excellent ORR performance could possibly be attributed to the fact that the specific nature and surface density of different active sites present in the final catalysts strongly depend on the choice of precursor materials and exact synthesis procedures utilized.[25d] As an example, the BP-based optimal catalyst (BP-NFe with 0.05 wt% Fe and 2.4 wt% N) at Peak II was introduced in detail as follows. The morphology was investigated by means of transmission electron microscopy (TEM). As shown in Figure 2a, the pure BP shows an amorphous, spherical morphology, while the BP-NFe catalyst shows an amorphous porous structure with thin-layer graphene-like nanosheets which can be seen at the particle edge. The change of morphology is consistent with a recent observation that Fe could act as a catalyst for the formation of a graphene-like structure.[27] The iron nanoparticles couldn t be found by TEM in this catalyst probably due to the extremely low Fe content. The porous nature of BP-NFe was assessed by nitrogen adsorption desorption analysis (Figure 2b). The type-iv isotherm of BP-NFe indicated a mesoporous structure. The Brunauer Emmett Teller (BET) surface area of the optimal BP-NFe was 1184.8 m2 g 1, slightly smaller than the 1391.3 m2 g 1 for BP. This small difference could probably be attributed to the filling of micropores by carbon formed from the pyrolyzing of melamine. To assess the catalytic activity of these BP-based catalysts for ORR, we performed cyclic voltammetry (CV) and LSV on a RDE. As Figure 3a and b shows, the pure BP in 0.1 M KOH is sluggish for ORR, evident by the low onset potential ( 0.22 V) and the fact that it is a two-step two-electron process (Figure S1, Supporting Information). The pure Fe-doping shows very little promotion of the ORR process compared with the pure N-doping, which promotes the ORR process 2 Figure 3. Electrochemical characterization of BP-NFe. a) CVs of pure BP, BP-N, BP-Fe, and BP-NFe in O2-saturated 0.1 M KOH with scan rate of 50 mv s-1. b) RDE polarization curves of pure BP, BP-N, BP-Fe, BP-NFe, and Pt/C in O2-saturated 0.1 M KOH with a scan rate of 5 mv s 1 and rotation speed of 1600 rpm. c) Voltamperograms for oxygen reduction on BP-NFe in O2-saturated 0.1 M KOH at various rotation speeds with scan rate of 5 mv s-1. d) Diffusion-corrected Tafel plots for BP-NFe and Pt/C extracted from (b). The loadings of catalysts are 0.39 mg cm 2 for doped carbon catalysts and 0.15 mg cm 2 for commercial Pt/C. greatly as indicated by the higher onset potentials ( 0.15 V) (Figure 3a,b). But its performance is still not on the same level as commercial Pt/C. Expectedly, when BP was co-doped by N and Fe, the ORR activity was greatly enhanced due to a synergetic effect between doped N and Fe atoms.[2,7] CV shows a peak potential at 0.19 V, which is slightly higher than that ( 0.2 V) on commercial 20 wt% Pt/C (E-TEK), suggesting a pronounced electrocatalytic activity of BP-NFe. The high ORR electrocatalytic activity of BP-NFe is also gleaned from its much higher onset potential ( 0.045 V) and half-wave potential (E1/2 0.089 V) (dotted red line in Figure 3b), which are slightly higher than that on commercial Pt/C (blue in Figure 3b). Interestingly, it was found that the optimal BP-NFe shows an improved performance with larger diffusion-limiting current (green in Figure 3b) when its ink was re-tested after 30 days. The increase of diffusion-limiting current could be attributed to the increase of oxygen diffusion coefficient in the microenvironment of the catalyst layer or the exposure of more active sites.[19b] Although the pure BP has a larger BET surface area (1391.3 m2 g 1) than BP-NFe (1184.8 m2 g 1), the CV of the BP-NFe electrocatalyst is much thicker than for BP (Figure 3a), indicating a much higher electrochemical surface area of BP-NFe, bearing a higher number of active sites than BP. This fact indicates the high-density active sites were mainly created by the co-doping of N and Fe on BP. Typical current potential curves of BP-NFe in an oxygen-saturated 0.1 M KOH electrolyte are shown in Figure 3c. The current shows a typical increase with rotation rate due to the shortened diffusion layer.[13] Analysis of the steady-state diffusion plateau currents by Kouteckey Levich plots (Figure S2, Supporting Information) reveals a four-electron process (n 3.9) of the ORR

on BP-NFe, with water as the main product, as is the case for Ptbased catalysts. Such a high n value indicates good four-electron selectivity of the BP-NFe catalyst in alkaline aqueous medium by either a direct four-electron route or a two + two route. [ 5 ] The performance of BP-NFe was further evaluated for mechanistic and kinetic performance using diffusion-corrected Tafel plots (Figure 3 d). The Tafel slope in the low current density region on BP-NFe is 68 mv decade 1, which is close to that (67 mv decade 1 ) on a Pt/C surface. This reveals the transfer of the first electron on both of these two catalysts is the ratedetermining step under Temkin conditions for the adsorption of intermediates. [ 20 ] In the high current density region, the Tafel slop is 92 mv/decade, which is slightly smaller than that (96 mv decade 1 ) on Pt/C surface. This result is attributed to a change in the mechanism of ORR from Temkin to Langmuir adsorption conditions when the current density increases. [ 20a ] From a mechanistic point of view, this would imply the ORR mechanisms on BP-NFe and Pt-based catalysts are similar in an alkaline medium. [ 12 ] In addition, the smaller Tafel slop of BP-NFe than Pt/C at high current density reveals that the overpotential increases slowly with current density, leading to better ORR activity of BP-NFe. [ 20b ] A sixteen-times higher exchange current density (1.6 10 3 ma cm 2 ) of BP-NFe was obtained from Tafel plots when compared with the exchange current density (1.0 10 4 ma cm 2 ) of commercial 20 wt% Pt/C, indicating a much higher intrinsic activity of BP-NFe for the ORR than commercial Pt/C (Table S1, Supporting Information). The tolerance of BP-NFe to methanol or CO was also assessed with LSV in an O 2 saturated electrolyte containing methanol (3 M ) or CO. As shown in Figure 4 a, no activity Figure 4. The tolerance and stability of BP-NFe and Pt/C. LSVs of BP-NFe (a) and Pt/C (c) in O 2 -saturated (black), 3 M methanol O 2 -saturated (red), CO- and O 2 -saturated (green) 0.1 M KOH with a scan rate of 5 mv s 1. b) RDE polarization curves of BP-NFe with a scan rate of 5 mv s 1 be0fore (black) and after 30 000 potential cycles (red) in O 2 -saturated 0.1 M KOH, and then KCN was added (blue). For comparison, the RDE of Pt/C in O 2 -saturated 0.1 M KOH was also added (magenta). d) RDE polarization curves of Pt/C with scan rate of 5 mv s 1 before and after 6000 potential cycles in O 2 -saturated 0.1 M KOH. specific to methanol or CO was observed on BP-NFe as the characteristic peaks of ORR are maintained. These results indicate that the BP-NFe can easily catalyze the reduction of O 2 but is tolerant to methanol and CO. On Pt/C (Figure 4 c) the electrooxidation of methanol or CO seriously retards the ORR process, as indicated by the disappearance of the oxygen reduction peak. This fact indicates that the as-prepared BP-NFe is a nice alternative to Pt for alkaline direct methanol fuel cells as a cathode. Based on the US Department of Energy s accelerated durability test protocol we assessed the durability or stability of the BP-NFe catalyst by cycling the catalyst between 1.2 and 0.2 V at 200 mv s 1 in an O 2 saturated 0.1 M KOH. [ 3 ] As shown in Figure 4 d, a 32 mv negative shift of half-wave potential E 1/2 after 6000 cycles shows the deterioration of Pt occurred on Pt/C. This could be attributed to the migration/aggregation of Pt nanoparticles caused by continuous potential cycling and subsequent loss of the specific catalytic activity. [ 9 ] BP-NFe showed a much smaller negative shift (12 mv) of E 1/2 (Figure 4b) after 30 000 continuous cycles, thus exhibiting excellent long-term operation stability. [ 1 ] In order to assess the role of iron in forming active ORR catalytic sites on BP-NFe catalysts, we investigated the ORR activity of BP-NFe in 0.1 M KOH containing 10 m M KCN (Figure 4 b) after 30 000 cycles. CN ions are known to coordinate strongly to iron and poison the iron-centred catalytic sites for ORR. [ 2,21 ] With the addition of CN, the ORR half-wave potential of the BP-NFe catalyst decreases significantly by more than 100 mv (from red curve to blue curve indicated by the blue arrow), with a decrease in the diffusion-limiting current, suggesting blocking of the iron sites by CN ions. It is worth noting that the activity of CN -poisoned BP-NFe almost equals that of BP-N as shown in Figure 3 b, which is close to commercial Pt/C. The high residual activity of the poisoned BP-NFe could be attributed to the Fe-free ORR active sites from the doped-n centers, which is inert to the poison of CN ions. [ 2 ] All these results suggest that both Fe-centered and N-centered active sites are important for the high ORR electrocatalytic activity observed in BP-NFe catalysts. Based on this fact it can be inferred that these two types of active sites always co-exist in NFe-co-doped catalysts. In order to further substantiate the high performance of BP-NFe observed above in an alkaline solution; we performed alkaline direct methanol fuel cell (ADMFC) tests with BP-NFe and commercial Pt/C as cathodes, respectively (Supporting Information). As shown in Figure 5 a, the ADMFC with BP-NFe as cathode catalyst (3 mg cm 2 or 1.5 μg Fe cm 2 ) shows a better performance than that with commercial Pt/C (60 wt%, 3 mg Pt cm 2 ) as cathode. Under similar conditions, the open circuit voltage of 0.8 V for the ADMFC with BP-NFe is higher than that of 0.73 V for the cell with Pt/C, indicating a much better methanol tolerance of BP-NFe for ORR. The maximum power density with BP-NFe is 16.6 mw cm 2 at 60 C, compared to 13 mw cm 2 for commercial Pt/C. The potential of the BP-NFe cell shows a small decrease of about 3% after 24 h at 37 C at a fixed current of 200 ma, while the Pt/C cathode experiences a potential decrease of 10%, indicating a better stability for the BP-NFe over Pt/C. All these data from ADMFCs further substantiate the high performance of BP-NFe as an ORR catalyst in alkaline medium, and unambiguously indicate 3

www.materialsviews.com Figure 5. a) The voltage and power density of ADMFCs at 60 C with (square) optimal BP-NFe (3 mg cm 2 ) and (star) Pt/C (60 wt%, 3 mg Pt cm 2 ) as cathodes, respectively. Anode: Pt/C (60 wt%, 3 mg Pt cm2 ) with 2 M methanol in 2 M KOH with a flow rate of 5 ml min 1, cathode: dry oxygen with flow rate of 100 ml min 1. b) XPS survey scan of optimal BP-NFe. c) High-resolution XPS spectrum of N 1s from optimal BP-NFe. d) Raman spectrum for BP and BP-NFe catalysts. The smooth curves are the multiple-peak fi tting results. the BP-NFe is an excellent alternative to Pt as a cathode catalyst in alkaline fuel cells, whether it be a performance or cost point of view. The obtained performance of ADMFC with BP-NFe as cathode is on par with that obtained with Pt black as a cathode (Figure S3, Supporting Information). [ 22 ] We propose some reasons for the mechanism for the high activity of the BP-NFe. Some clues can be found from the high resolution X-ray photoelectron spectroscopy (XPS) spectra of N and Fe. As shown in Figure 5 b, the survey scan for optimal BP-NFe shows the existence of N 1s, O 1s, and C 1s. Due to the extremely low content of Fe in this optimal catalyst, Fe could not be detected. Figure 5 c shows a single bonding configuration of N atoms, indicated by a symmetric peak of N 1s at 399.4 ev corresponding to pyrrole-like nitrogen. [ 23 ] For nitrogen-doping, certain types of N-containing functional groups, such as pyrrolyic and pyridinic groups, especially those at graphitic edge plane sites, have been claimed to be responsible for the high ORR activity. [ 12 ] The high activity of BP-NFe could then be partially attributed to the high content of pyrrolelike N. The Fe 2p signal shown in Figure S4, Supporting Information, was from another BP-NFe catalyst with Fe 0.3 wt%. The two peaks at about 709 ev for Fe 2+ and 711 ev for Fe 3+ indicate that the Fe exists in the form of Fe 3 O 4. [ 24 ] According to a previous report, [ 17 ] the catalytic activity of pure oxidized iron species for ORR is very low, just like that shown in Figure 3 b for BP-Fe. So, if the Fe in the optimal catalyst also exists in the form of Fe 3 O 4, then the extremely high activity of BP-NFe could be attributed to a synergetic effect between doped-n and -Fe atoms indicated by the much higher onset potential and E 1/2 on BP-NFe compared with those on BP-N or BP-Fe (Figure 3 b). The higher onset potential and E 1/2 on BP-NFe indicate each active site on it is a complex of doped-n, -Fe, and carbon atoms. The new complex active site possesses much higher intrinsic activity for ORR than a single N- or Fe-doped active site due to a synergetic effect. [ 2,7 ] According to previous results, these new complex active sites are most probably the in-plane Fe-N 4 centers embedded in a graphene-type matrix since the optimal catalyst contains mainly the pyrrole-like N and trace iron. [ 25 ] According to quantum calculation, [ 26 ] Fe-N 4 centers can activate the ORR process by the significant decrease of oxygen adsorption energy and extension of the O O bond. Raman spectroscopy is the most effective and non-destructive technique to characterize the structure and quality of carbon materials, in particular to determine defects and disordered structures. Figure 5 d shows the Raman spectrum of pristine BP and BP-NFe catalysts. With the NFe co-doping on BP, there are no significant shifts or line broadening, suggesting that the carbon structure is mostly retained after the doping. In addition, the intensity ratio between G- and D-bands increased slightly from 0.63 to 0.70 after doping, [ 25,26 ] indicating a more ordered structure after NFe co-doping due to the formation of thin, transparent graphene-like nanosheets as observed in Figure 2 a [ 27 ] although pyrrolyic-n breaks the sp 2 -hybridized bond because of a pentagonal structure. This fact indicates the formation of thin, transparent graphene-like nanosheets dominates the morphology variation due to the low content of doped-n. A small Raman peak at about 1131cm 1, attributed to the v (CO) stretching modes, [ 28 ] was also observed, confirming the existence of oxygen species on the surface as shown in Figure 5 b. The above optimal BP-NFe also show high electrocatalytic performance in acidic conditions for the ORR process with an onset potential of about 0.6 V vs. a scanning calomel electrode (SCE) as shown in Figure S5, Supporting Information. Its performance is on the same level as that obtained based on carbon nanotube graphene complexes. [ 1 ] At the present very few reports on non-pt electrocatalysts have shown high performance for ORR in both alkaline and acidic conditions. In conclusion, a family of sustainable high-performance ORR electrocatalysts (CB-NFe) was reported for the first time based on cheap CB, nitrogen, and trace iron. The optimal CB-NFe (with trace Fe of 0.05 wt%) electrocatalyst shows higher ORR performance than traditional catalysts with a higher content of Fe. The ADMFC with the best CB-NFe as cathode (1.5 μ g Fe cm 2 ) outperforms the one with a Pt-based cathode (3 mg Pt cm 2 ), substantiating that the optimal BP-NFe is one of the most promising alternatives to Pt or other rare materials as an ORR electrocatalyst in alkaline fuel cells. Due to the low cost and abundance of CB, these CB-NFe electrocatalysts possess the best price/performance ratio for ORR to date. Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements Work was funded by the National Basic Research Program of China (973 Program, 2012CB932800, 2012CB215500), National Natural 4

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