Ligin-derived graphene-like ultrathin carbon materials for ORR catalysis

Low-cost nitrogen-doped carbon materials with graphene-like structures were designed and prepared from extensive renewable biomass waste lignin. The obtained carbon materials exhibited well-designed ultrathin two-dimensional structures with high specific area and mesoporous characteristics. Benefiting from the well-designed structures with extensive catalytic active sites and efficient mass transfer and diffusion, as well as the modified electronic characteristics from nitrogen doping, the lignin-derived carbon materials exhibited pronounced ORR catalytic activity and methanol tolerance. Considering these, the lignin-derived carbon materials hold great application potentials as ORR catalysts and catalyst supports.


Introduction
Hydrogen energy, as a typical green secondary energy, has been gradually becoming a important carrier of global energy transformation and development.Fuel cell is one of the key technologies of hydrogen energy application.Due to its superiorities in high energy conversion efficiency, clean process, almost zero emission, long life and wide range of hydrogen fuel sources, it holds great application prospects in important scientific and technological fields such as vehicles, drones, robots and communication stations.As the core material of hydrogen fuel cell, the cost of precious metal platinum catalyst accounts for about 40% of the reactor system, which seriously limits its large-scale commercial application.Therefore, in order to cope with the development of hydrogen energy industry, achieve breakthroughs in hydrogen fuel cell technology, and comply with the development trend of high performance, multi-function and green development of new materials, designing and developing high-performance non-noble metal catalysts is the key topic and key technology of the current hydrogen energy industry and hydrogen fuel cell research, which has important theoretical research and practical application significance.
Due to the large specific surface area, rapid ion diffusion and expanded surface active sites, two-dimensional materials have attracted extensive attention in electrochemical applications, especially catalysis.Recently, graphene-like nitrogen-doped carbon ultrathin two-dimensional materials are regarded as a new generation of high-performance oxygen reduction reaction (ORR) catalysts that are expected to replace traditional precious metal platinum catalysts [1][2][3].At the same time, as a catalyst carrier, the large specific surface area of graphene-like nitrogen-doped carbon ultrathin two-dimensional materials promotes high density exposure of catalytic active sites and high conductivity advantages, which also provides more possibilities for designing non-noble metal catalysts.In order to avoid the complex preparation procedure of traditional nitrogen-doped carbon materials and dependence on toxic raw materials, the use of low-cost, widely sourced, environmentally friendly and renewable biomass raw materials for derivative preparation is regarded as a promising alternative [4].Lignin is the second largest reserves of natural biomass polymer on the earth, with a unique polymer structure and rich functional group characteristics (phenolic polymer functional group, phenyl skeleton, unique C3 chain structure and highly compact bonding form).It is a high-quality precursor for preparing graphene-like ultrathin carbon, nitrogen-doped carbon and other high-performance carbon materials with excellent physical and mechanical strength and chemical activity [5].
Based on these considerations, nitrogen-doped carbon materials with graphene-like structures were designed and prepared based on extensive renewable biomass waste lignin, which showed good ORR catalytic activity and had great potential in the application of catalysts and catalyst carriers for hydrogen fuel cell cathode.

Materials and methods
The raw material of lignin powder was carefully purified in aqueous solution of 20 wt % KOH and 1 M HCl in sequence to remove the residual inorganic species.To prepare carbon materials, 0.2g lignin and 0.2 g Ni powders were mixed uniformly in agate mortar, then calcine the mixed powders (in porcelain boat) in tube furnace with controlled temperature and gas atmosphere step by step: 1) 400℃ for 2h (heating rate: 10 ℃ min -1 ) with 100 sccm He gas, 2) 600℃ for 2h (heating rate: 10 ℃ min -1 ) with 50 sccm He gas, 3) 600℃ for 2h (heating rate: 10 ℃ min -1 ) with 50 sccm He gas, 4) 1000℃ for 0.5 h (heating rate: 20 ℃ min -1 ) with 50 sccm He gas, 5) rapidly cool to room mate with 50 sccm He gas.Finally, the obtained powders were added into 2M Fe(NO3)3 aqueous solution, stirred at 80℃ for 12h, then the final solid products were filtered and dried at 45℃ overnight.

Characterization
X-ray diffraction (XRD, Rigaku D/max-2200PC) was used to characterize the crystallographic structure of the lignin-derived product.The microstructure and detailed morphology was observed using field-emission scanning electron microscope (SEM, JEOL JSM-7500F) and transmission electron microscope (TEM, FEI Gatan F30).Brunauer-Emmett-Teller (BET) measurements were performed on an ASAP-2010 analyzer.X-ray photoelectron spectroscope (XPS, AXIS UTLTRADLD) was used to analyze the elemental composition.A Reishaw Microscope System (RM2000) was used for Raman measurements (at 532 nm).

Performance measurements
PARSTAT 2273 electrochemistry system was used for electrochemical performance measurements of the product with a conventional three-electrode electrochemical cell.The catalysts-loaded glassy carbon rotating disk electrode was used as the working electrode, graphite rod was selected as counter electrode, the reference electrode was Ag/AgCl electrode, and the electrolyte was 0.1 M KOH aqueous solution.All electrochemical measurements were conducted at room temperature.

Results and discussion
Lignin exhibits a typical structure of aromatic rings with rich functional group characteristics (phenolic polymer functional group, phenyl skeleton, unique C3 chain structure and highly compact bonding form), which can serve as a potential precursor for nitrogen-doped carbon synthesis.Fig. 1a presents the XRD pattern of the obtained carbon materials, which presents two broad characteristic peaks at ~23° and 44.0°, as well as a weaker and broader peak at ~15°.The low-intensity characteristic peaks can be indexed to the (002) and (100) planes of graphite with low degree of graphitization [6].And the weak peak at ~15° indicates the existence of N incorporation.According to literatures, nitrogen doping can modify the electronic characteristics of carbon and modulate the local charge density distribution to realize superior ORR activity [7,8].To further verify the existence of N incorporation in the product, XPS measurements were conducted.According to the XPS survey spectra in Fig. 1b, the presence of C, N and O was observed in the lignin-derived product, thus confirming the composition as N-doping carbon [9].While the existence of O element may be ascribed to the physicochemical absorbed oxygen and the unavoidable oxidation reactions of the product when exposed to the air [10].Meanwhile, Raman spectrum was also obtained to further reveal the composition and structure features.As shown in Fig. 2, the G band (~1580 cm -1 ) refers to the E2g vibration mode of graphitic sp 2 carbon domain, which further confirms the graphene-like structures of this lignin-derived product.While the obvious D band (~1360 cm -1 ) with high density of ID/IG~0.92 corresponds to the partially disordered structures, indicating the existence of massive structural defects caused by nitrogen doping [11].
Fig. 2 Raman spectrum of the lignin-derived nitrogen-doped carbon.
The detailed morphology and microstructure of the lignin-derived nitrogen-doped carbon were investigated by SEM and TEM.As shown in the SEM image in Fig. 3, the obtained carbon materials exhibit obvious morphology of interconnected ultrathin nanosheets, which are almost transparent and curved.More clearly, according to the TEM image in Fig. 4a, the obtained product are assembled with two-dimensional graphene-like ultrathin flexible nanosheets.As shown in the magnified TEM image in Fig. 4b, the ultrathin flexible nanosheets exhibit almost transparent morphology and shows obvious wrinkles or corrugation.Meanwhile, according to the further-magnified TEM image of Fig. 4c, obvious porous structures of ~2 nm in size were observed in the surface of the flexible nanosheets, suggesting their mesoporous characteristics.In order to calculate the specific surface area and confirm the porous features of the obtained carbon materials, full nitrogen adsorption/desorption isotherms were also collected.As shown in Fig. 5a, a type IV curve with a H3-type hysteresis loop was observed clearly in the isotherm, further indicating the mesoporous structures existed in the graphene-like carbon materials [12].Meanwhile, according to the adsorption data, the specific surface area of the the carbon materials can be calculated to be 289.37 m 2 g -1 .Furthermore, as shown in Fig. 5b, obvious peaks at the range of 2-4 nm can be observed in the pore size distribution pattern, corresponding to the mesoporous channels in the product.These results are consistent with the TEM images discussed above.Considering the graphene-like structure with high specific area and mesoporous characteristics, and the advantageous composition with nitrogen-doping, the lignin-derived nitrogen-doped carbon materials are expected to be promising in ORR catalysis.To confirm the catalytic activities of the lignin-derived carbon materials, cyclic voltammetry (CV) was performed at a scan rate of 50 mV s -1 .As shown in Fig. 6a, the obtained nitrogen-doped carbon exhibits an obvious cathodic peak in O2-saturated electrolyte of 0.1 M KOH aqueous solution, thus confirming its ORR catalytic activities.In contrast, in N2-saturated electrolyte, featureless peak currents are observed in the CV curve of our products.The linear sweep voltammetry (LSV) curves obtained using the RDE technique at a rotation rate of 1600 rpm in Fig. 6b also verify the considerable ORR catalytic performance, although not comparable to the commercial Pt/C catalyst in both onset potentials and current densities.Moreover, when adding 3.0 M methanol into the O2-saturated electrolyte, no obvious changes can be observed in the CV curve of the lignin-derived nitrogen-doped carbon, suggesting its excellent tolerance to methanol.Moreover, as shown in Fig. 7, in long term test, the lignin-derived nitrogen-doped carbon exhibit remarkable stability in contrast to commercial Pt/C catalyst.Only slight current attenuation can be observed in the ORR current-time chronoamperometric response curve of the lignin-derived nitrogen-doped carbon when adding 3.0 M methanol into the O2-saturated electrolyte.In fact, this methanol tolerance and catalytic stability is of great significance for its applications in practical hydrogen fuel cells.

Conclusions
With the addition of Ni metal as structural template and a facile pyrolysis procedure of biomass waste lignin, we successfully synthesized a low-cost and renewable carbon material.Characterization results confirmed that the obtained carbon material exhibited remarkable graphene-like two-dimensional structures, with a specific surface area of 289.37 m 2 g -1 and unique mesoporous features, which are beneficial to realize high density of catalytic active sites, and efficient mass transfer and diffusion in ORR catalysis.Due to the nitrogen-containing functional group of lignin, nitrogen doping was verified in the lignin-derived carbon, which can modify the electronic characteristics and modulate the local charge density distribution to realize superior ORR activity.Benefiting from these well-designed structures and compositions, the lignin-derived carbon material exhibited pronounced ORR catalytic activity and tolerance to methanol, thus holding great application potentials as ORR catalysts and catalyst supports.

Fig. 6
Fig. 6 (a) CV curves of the lignin-derived nitrogen-doped carbon in O2-saturated electrolyte, N2-saturated electrolyte and O2-saturated electrolyte with 3.0 M CH3OH; (b) LSV curves of the lignin-derived nitrogen-doped carbon and Pt/C in O2-saturated electrolyte at a rotation rate of 1600 rpm.

Fig. 7
Fig. 7 Current-time chronoamperometric response of the lignin-derived nitrogen-doped carbon and Pt/C in O2-saturated electrolyte at a rotation rate of 1600 rpm, in which the arrow indicate the introduction of 3.0 M methanol.