HE Wang-Tao, MA Ru-Guang, ZHU Yu-Fang, YANG Ming-Jie, WANG Jia-Cheng

Renewable Porous Carbons Prepared by KOH Activation as Oxygen Reduction Electrocatalysts

HE Wang-Tao1,2, MA Ru-Guang2, ZHU Yu-Fang1, YANG Ming-Jie3, WANG Jia-Cheng2

(1. School of Materials Science and Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China; 2. State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China; 3. Department of Chemical Engineering, Monash University, Victoria 3800, Australia)

The nitrogen-doped porous carbon applied for oxygen reduction reaction (ORR) has aroused extensive interests due to its unique physical and chemical properties. However, the complicated nitrogen-doping strategy and high cost limit its extensive application. In this work, a series of nitrogen-doped porous carbons were prepared by a facile pyrolysis process coupling with subsequent KOH activation using renewable N-enriched biomass potato as carbon source. Effects of activation temperature and KOH amounts on the textural properties and electrocatalytic ORR activities of the final samples were investigated in detail. The KOH activation treatment results in a high specific surface area (SSA) and hierarchical porous structure, which is beneficial for improved ORR performance. The optimized NPC-750 possesses a high SSA of 1134.2 m2∙g–1, developed hierarchical pores as well as moderate nitrogen content (1.57at%). It also exhibits a positive onset potential of 0.89 V (RHE) and half-wave potential of 0.79 V (RHE). Simultaneously, the advanced long-time stability and methanol-tolerance capacity were also obtained, implying that these biomass-derived porous carbons are potential low-cost ORR electrocatalysts. Moreover, these porous carbons show great potential in various fields including supercapacitors, adsorption/separation, catalysis and batteries as well.

biomass; porous; nitrogen-doped carbons; KOH activation; oxygen reduction reaction

The low kinetics of oxygen reduction reaction (ORR) on the cathode is a main barrier for the commercial proton- exchange membrane fuel cells (PEMFCs)[1-2]. Pt is generally considered as one of the best catalysts towards ORR. However, high-cost and low stability limits its widespread application[3-5]. Therefore, it is urgent to find new low-cost and high activity alternatives for Pt-based catalysts. Recently, nitrogen-doped porous carbon materials (NPCs) have been recognized as a promising type of ORR catalyst for PEMFCs[6]. The carbon matrix possesses excellent electrical conductivity and stability which can promote electronic transmission process during ORR[7]. On the other hand, the introduction of nitrogen atoms can create more active sites due to the changes of electron structure of carbon framework, leading to improvement of catalytic performance towards ORR[8-10].

Recently, the use of agricultural by-products as precursor materials to prepare nitrogen-doped carbons attracted much attention[11-13]. Precursor materials have lots of advantages such as environment-friendliness, low-cost and abundance on the earth. Moreover, there are various hetero-elements such as N, S, Fe,in biomass materials[14-15]. Therefore, it is convenient to obtain nitrogen- doped carbons by direct pyrolysis of biomass materials without additional nitrogen sources[16]. However, the specific surface area (SSA) and porosity of nitrogen doped carbons obtained from simply pyrolysis of biomass materials are too low to match the demand of exposing enough active sites, leading to poor electrochemical performance[17-18]. Thus, it is highly desirable to improve the SSA and porosity of nitrogen doped carbons derived from biomass.

To our knowledge, there are various activating agents and templates applied in preparation of porous carbons with high SSA and high porosity, such as CO2, ZnCl2, KOH, H3PO4and SiO2[19-23]. In particular, KOH is one of the most common activating agents. The porous carbons obtained from KOH activation have a high SSA with considerable micro-pores[24]. Simultaneously, biomass derived carbons usually possess lots of macropores. Thus, KOH activated nitrogen-doped porous carbons always have well-developed hierarchical pore structure which could significantly enhance the transport of electrons[25]. Moreover, KOH activation process is also suitable for large scale production of porous carbons due to its mature technology and convenient operation. Compared with other activating reagents, KOH activation prefers to obtain porous carbons with high SSAs and lots of micro-pores.

Herein, we report a facile strategy of preparing KOH activated nitrogen-doped porous carbons using agriculture products potato as a precursor which possesses abundant nitrogen atoms and developed porous texture. The optimal catalyst NPC-750 obtains a high SSA of 1134 m2∙g–1and a suitable nitrogen content of 1.57at% which provides sufficient active sites for ORR electro- catalysis. Moreover, the well-defined hierarchical pore structure boosts the transport of ORR-relevant species. Their combination endows it outstanding ORR activity in an O2-saturated 0.1 mol∙L–1KOH solution. It exhibits an onset potential of 0.89 V (RHE) and a half-wave potential of 0.79 V (RHE) which is comparable to most state-of-the-art non-metal catalysts for ORR in an alkaline media. In addition, it has only 12% degradation of current density after 5 h of ORR, indicating excellent long-time stability.

1 Experimental

1.1 Synthesis of nitrogen-doped carbons (NCs)

The nitrogen doped carbons (NCs) were prepared from biomass potato as a single carbon and nitrogen sou­rce. Typically, a steamed potato was grounded into mas­hed potatoes and then dried in an oven at 60 ℃. Subse­quently, 3 g dried mashed potatoes were put into a porcelain boat, and transferred to the tube furnace, followed by carbonization under argon atmosphere at 900 ℃ for 2 h with a heating rate of 5 ℃∙min–1. The final product was named as nitrogen-doped carbons (NCs).

1.2 Synthesis of nitrogen-doped porous carbons (NPCs)

NPCs were synthesizedchemical activation of NCs using KOH as activating reagent. For instance, 3 g KOH was dissolved in 50 mL deionized water, and 1 g NC was added into the KOH solution followed by stirring for 30 min to ensure uniform dispersion. After drying at 80 ℃, the black powder was annealed in argon atmosphere at 700, 750 and 800 ℃ for 1 h with a heating rate of 3 ℃∙min–1, respectively. Finally, the product was washed by 1 mol∙L–1HCl solution and deionized water for several times in order to remove residual KOH and other impurities. The final products were donated as NPC-(means the activating temperature, ℃).

1.3 Characterization

The morphologies of samples were characterized by using scanning electron microscopy (SEM, Magellan 400) at an accelerating voltage of 2 kV. The transmission electron microscopy (TEM, JEM-2100F) was carried out at an accelerating voltage of 200 kV. Raman spectra were recorded by Raman spectrometer (LABRAM HR800). The pore structure properties of samples were studied by ASAP 2010 instrument with the nitrogen adsorption tem­perature of 77 K. The specific surface area was calcula­ted from the Brunner-Emmett-Teller (BET) method. The X-ray photoelectron spectroscopy (XPS) was investig­ated on the X-ray photoelectron spectrometer (ESCALAB 250).

1.4 Electrochemical characterization

The electrochemical properties of samples were performed on an electrochemical station of bipotentiostat (Pine instrument Co. LTD. USA). The electro-catalytic ORR performance was measured in a three-electrode system using a saturated calomel electrode (SCE) as reference electrode and a Pt plate as counter electrode in O2-saturated 0.1 mol∙L–1KOH aqueous solution. Working electrode was the glassy carbon electrode coated with catalyst ink. Typically, 5 mg samples and 20 μL Nafion (5wt%) were dispersed into the solution containing 500 μL deionized water and 500 μL ethanol followed by ultrasonic treatment for 1 h to form homogenous ink. Subsequently, 20 μL ink was dropped on the surface of glassy carbon electrode, followed by drying to form the working electrode. Electrochemical measurements including cyclic voltammetry (CV), rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) were tested at a potential range from –1.0 to 0.1 V (SCE).

2 Results and discussion

The morphology of as-prepared samples was chara­cterized by SEM and TEM. As shown in Fig. 1(a), the SEM image of NC particle reveals a smooth surface with a few small holes. After subjecting the subsequent activation by KOH (NC/KOH=1/3), the smooth surface became rough and porous, as evidenced by increased density of small holes on the surface which resulted from the combination of micropores. Moreover, the porous structure especially in the range of nanoscales of NPC particles was more obvious with the activating temperature increasing (Fig. 1(b)). TEM image of NPC-750 in Fig. 1(c) also shows the existence of large amount of pores within carbon matrix. Such a porous texture is beneficial for mass transfer during ORR, thereby improving the electrocatalytic performance of NPCs[26]. In addition, the HRTEM image in Fig. 1(d) as well as the SAED (inset in Fig. 1(d)) reveals the amo­rphous structure of the activated sample (NPC-750).

Fig. 1 SEM images of (a) NC and (b) NPC-750; (c) TEM and (d) HRTEM images of NPC-750 (inset in (d): corresponding SAED pattern of NPC-750)

Raman spectra of NPCs in Fig. 2(a) exhibit two strong peaks located at about 1305 and 1596 cm–1. The peak at 1596 cm–1(G band) is attributed to E2gmode of graphite and related to the vibration of sp2-bonded carbon atoms in a 2D hexagonal lattice, while the peak at 1305 cm–1(D band) corresponds to vibrations of carbon atoms with dangling bonds in plane terminations of diso­rdered[27-28]. Therefore, the lower intensity ratio of D band to G band (D/G) suggested higher graphitization degree of carbon materials. As we all know, KOH activation can generate a lot of micropores and defects into the carbon matrix[29]. As shown in Fig. 2(a), the higherD/Gvalues (1.15–1.16) of NPCs activated by KOH reveals increasing defects in carbon frameworks compared with NCs (~1.01), which results in high porosity and surface area. The defects could act as adsorption and active sites during the ORR process, thus leading to enhanced ORR performance.

Fig. 2(b) exhibits the N2adsorption and desorption isotherms of all samples. NCs prepared from direct car­bonization of mashed potatoes have no obvious uptake of N2, suggesting its low porosity and specific surface area (SSA). However, the NPCs all show the type IV iso­therms with H3 hysteresis loops at 0.4

Fig. 2 (a) Raman spectra, (b) nitrogen adsorption-desorption isotherms and (c) pore size distribution curves of NPCs with inset in (c) showing the enlarged part

Table 1 Summary of porosity parameters of all samples

As shown in Fig. 3(a), XPS spectra reveals the different composition of surface atoms for all of the samples. Peaks located at 285, 400, 532 eV suggest C1s, N1s and O1s, respectively[32], and the N contents of NPCs decreases with increasing temperature because the KOH activation could lead to the loss of the doped heteroatoms. Moreover, the NPC-750 has a most suitable N content of 1.57at% which could provide sufficient active sites for ORR process. The high resolution N1s spectra of NPCs (Fig. 4(b-d)) can be deconvoluted into four peaks at 398.77, 399.7, 401.58 and 403.69 eV[33-36], which can be assigned to pyridinic N, pyrrolic N, graphitic N and oxidized nitrogen species (N-O), respectively. As we all know, KOH activation could destroy the graphene layer and expose more edges in the carbon framework. Furthermore, pyridinic and pyrrolic N atoms bonded with two sp2carbon atoms will contribute electron lone pairs into carbon conjugated systems, consequently inducing electron donor effects. On the other side, graphitic N atoms take the position inside an aromatic ring with sp2hybridization could further enhance the electrical conductivity of the carbon material (Fig. 3(f)). Therefore, improved ORR performance is expected for NPC-750 due to its sufficient pyridinic N content of 33.2at% and suitable graphitic N content of 19.7at% (Fig. 3(e)). Moreover, the existence of O functionalities could enable participation in Faradaic reactions and the combined effect between N and O groups is also beneficial for capacitance enhancement[37-38].

Fig. 3 (a) XPS survey spectra of NPCs, high resolution N 1s spectra of (b) NPC-700, (c) NPC-750 and (d) NPC-800; (e) Contents of pyridinic N, graphitic N and pyrrolic N for NPCs at different activation temperature; (f) Illustration of three types of nitrogen in NPCs

The ORR activity and four electron selectivity of all samples were investigated by rotating disk electrode (RDE) and rotating ring disk electrode (RRDE) in a three-electrode system, respectively. Fig. 4(a) shows CV curves of NPC-750 in 0.1 mol∙L–1O2or N2saturated KOH aqueous solution. An evident peak was obtained in O2saturated electrolyte, while it disappeared in N2saturated electrolyte at the same conditions, suggesting the obvious ORR occurred on the electrode. As shown in Fig. 4, the linear sweep voltammetry (LSV) curves at the rotating speed of 1600 r∙min–1for all samples were recorded. NC shows poor activity for ORR as expected due to its low SSA and low porosity which limit the exposure of the surface active sites. In contrast, the enhanced ORR activity with the increase of activating temperature for NPCs was observed, indicating that more nanopores introduced by KOH activation in carbon matrix lead to larger SSA and porosity. However, it shows a dramatically decrease of ORR activity owing to the absence of nitrogen while the temperature exceeds 800 ℃. Therefore, NPC-750 (KOH/carbon=3/1) exhibits the best catalytic performance for ORR with a positive onset potential of 0.89 V (RHE) and half-wave potential of 0.79 V (RHE) (Fig. 4(c)). The limited current density of NPC-750 also reaches 5.53 mA∙cm–2, indicating a higher density of surface active sites as well as the best ORR performance. Simultaneously, the KOH/carbon mass ratio dependent activity for NPCs catalyst was also determined. When the KOH/carbon mass ratio keeps 3, the NPC-700 shows the best ORR performance.

Fig. 4 (a) CV curves of NPC-750 in N2 and O2-saturated electrolyte; (b) LSV curves of all samples at 1600 r∙min–1; (c) Bar plots of EOnset and EHalf-Wave for all samples; (d) LSV curves of NPC-750 at different rotating rates from 400 to 2025 r∙min–1 with inset showing the corresponding Kouteckey-Levich plots at different potentials; (e) RRDE polarization curves of NPC-750 at 1600 r∙min–1; (f) Electron transfer number and hydrogen peroxide yield of NPC-750 ORR performance was recorded in 0.1 mol∙L–1 KOH solution

The four electron selectivity of NPCs catalyst was evaluated by Koutechy-Levich(K-L) equation and RRDE measurement[39-40]. Fig. 4(d) shows the LSV curves of NPC-750 at different rotating speeds from 400 to 2025 r∙min–1. Obviously, the increased current density was observed as the enhanced rotating speed due to the shortened diffusion layer for electrode material. Furthermore, the electron transfer number of NPC-750 calculated by the K-L equation is 3.66 (inset in Fig. 4(d)), indicating the ORR process mainly transformed through a four electron pathway which is important for fuel cells, because peroxides produced by a two-electron process could poison the cellscorroding the membrane and catalyst layer[41-42]. As shown in Fig. 4(e-f), the RRDE measurement was also taken to comprehend the reaction pathway of NPC-750. The ring current is close to zero, implying its weak selectivity of two electron pathway for ORR. The electron transfer numbers () is 3.08–3.81 at a potential range of 0–1.0 V (RHE) and the hydrogen peroxide yield is 9.5%–45.2%. It suggests that the ORR process catalyzed by NPC-750 is a combination of two electron and four electron reaction pathway, although the four electron process is dominant.

The long-time stability is a key factor for excellent ORR catalysts. As shown in Fig. 5(a), after 5 h of ORR process, there are 88% of the initial current remained for NPC-750, which is far superior to Pt/C. Moreover, the methanol-tolerance capacity of NPC-750 was also evaluated (Fig. 5(b)). Pt/C suffered a dramatic decline of the current when 3 mol∙L–1methanol was added at 200 s during the chronoamperometric measurement. In contrast, a slight increase of the current was observed for NPC-750, indicating the excellent methanol-tolerance capacity which could meet the demands of the practical applica­tions.

3 Conclusions

In summary, nitrogen-doped porous carbons were prepared from renewable nitrogen-rich potato by the direct pyrolysis process followed by KOH activation. High temperature pyrolysis leads to the outstanding electrical conductivity. The KOH activation treatment results in a high SSA and increased porosity, which could facilitate the mass transfer during ORR process. The high specific surface area of 1134.2 m2∙g–1, hierarchical porous structure and moderate nitrogen content of 1.57at% make NPC- 750 own an excellent oxygen reduction reaction performance. It shows a positive onset potential of 0.89 V (RHE) and half-wave potential of 0.79 V (RHE). The limited current density of NPC-750 reaches 5.53 A∙cm–2. Furthermore, NPC-750 exhibits superior long-time stability and methanol-tolerance capacity to commercial Pt/C catalyst, implying that potato-derived NPC-750 has great potential as an excellent catalyst for oxygen reduction reaction in the future practical appli­cations.

Fig. 5 (a) Long-time stability, (b) methanol-tolerance capacity of NPC-750 and Pt/C in 0.1 mol∙L–1 KOH solution

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氢氧化钾活化制备可再生多孔碳及其电催化氧还原性能研究

何王涛1,2, 马汝广2, 朱钰方1, 杨明杰3, 王家成2

(1. 上海理工大学 材料科学与工程学院, 上海 200093; 2. 中国科学院 上海硅酸盐研究所, 高性能陶瓷和超微结构国家重点实验室, 上海 200050; 3. 莫纳什大学 化学工程系, 维多利亚 3800, 澳大利亚)

氧还原反应缓慢的动力学过程严重限制了燃料电池的能量转换效率, 而商用Pt/C催化剂成本太高、资源稀缺、稳定性差, 需要寻找合适的材料来取代商用的Pt/C催化剂。近年来, 氮掺杂多孔碳材料因其独特的物理和化学特性吸引了大量的关注。本文使用富含氮元素的可再生土豆作为生物质前驱体, 通过简单的一步热解过程和KOH活化方法相结合制备出了一系列氮掺杂多孔碳电催化剂; 并系统研究了KOH用量和活化温度对碳基体孔结构和电催化性能的影响。结果表明, 当活化温度为750 ℃、KOH与碳的质量比为3/1时, 所制备的催化剂(NPC-750)的氧还原活性最高, 起始电位和半波电位分别达到0.89和0.79 V (RHE), 极限电流密度达到5.53 mA∙cm–2。NPC-750优异的氧还原催化活性主要归因于其发达的孔结构、高的比表面积(1134.2 m2∙g–1)和合适的氮含量(1.57at%)。同时, 优异的循环稳定性和抗甲醇中毒性能进一步说明这些生物多孔碳材料是潜在的低成本氧还原电催化剂。此外, 这些高比表面积多孔碳在超级电容、吸附/分离、催化以及电池等领域也具有潜在的应用前景。

生物质; 多孔; 氮掺杂碳; 氢氧化钾活化; 氧还原反应

TQ174

A

2019-01-17;

2019-03-04

National Natural Science Foundation of China (51572172, 51602332); Equipment Research Program (6140721050215); One-Hundred Talent Plan of Chinese Academy of Sciences

HE Wang-Tao(1994-), male, candidate of Master degree. E-mail: 1402250556@qq.com

ZHU Yu-Fang, professor. E-mail: yfzhu@usst.edu.cn; WANG Jia-Cheng, professor. E-mail: jiacheng.wang@mail. sic.as.cn

1000-324X(2019)10-1115-08

10.15541/jim20190036