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Free-Standing 3D Porous N-Doped Graphene Aerogel Supported Platinum Nanocluster for Efficient Hydrogen Production from Ammonia Electrolysis Yufei Zhou, Guoquan Zhang, Mingchuan Yu, Xiaojing Wang, Jinling Lv, and Fenglin Yang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00586 • Publication Date (Web): 30 May 2018 Downloaded from http://pubs.acs.org on May 30, 2018

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ACS Sustainable Chemistry & Engineering

Free-Standing 3D Porous N-Doped Graphene Aerogel Supported Platinum Nanocluster for Efficient Hydrogen Production from Ammonia Electrolysis Yufei Zhou, Guoquan Zhang*, Mingchuan Yu, Xiaojing Wang, Jinling Lv, and Fenglin Yang

Key Laboratory of Industrial Ecology and Environmental Engineering Ministry of Education, School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, P.R. China

KEYWORDS: N-doped graphene aerogel; platinum nanocluster; ammonia electrolysis cell; hydrogen production

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ABSTRACT

Ammonia oxidation reaction (AOR) is an environmentally friendly electrochemical technology for hydrogen production. Nowadays, exploiting low-costing, high-performance and robust catalysts for AOR is essential to improve the overall efficiency of ammonia electrolysis cell (AEC). Here, we report the synthesis and characterization of a novel free-standing threedimensional (3D) porous N-doped graphene aerogel (NGA) anchored with Pt nanocluster (Pt/NGA) monolithic material as a high-performance and robust electrocatalyst for hydrogen production from AEC. The NGA substrate was facilely fabricated through a self-assemble process into the randomly arranged 3D porous backbone structure with graphene oxide (GO) and poly(oxypropylene)diamine D400 as precursors in a pure water solvent. Then, the Pt nanocluster-like structures were uniformly dispersed and embedded onto NGA through the simple electrodeposition method. The as-prepared Pt/NGA monolithic materials exhibited a higher ammonia electro-oxidation activity with the mass activity of 1.77 mA µg-1Pt and the specific activity of 0.64 mA cm-2ECSA, due mainly to the uniformly dispersed Pt nanocluster-like morphology, the improved electrical conductivity, the 3D porous NGA networks as well as the N-doping structure in graphene framework. When the 3D Pt/NGA monolith was directly used as working electrodes of AEC, a considerable hydrogen volume of 8.5 mL (about 1.90 mL mg-1Pt) was produced at 0.8 V in 3 h. This novel free-standing 3D porous NGA monolith is expected to be a potential and promising material for application in the fields of electrocatalysis and electrochemical energy transform.


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Introduction Under double pressures of environmental pollution and energy crisis, a sustainable clean energy is urgently required to solve the above issues. It is well known that hydrogen, with the advantages of high energy density and environment-friendly, is a potential source to replace of the conventional fossil fuel.1 Except electrochemical water splitting,2-4 ammonia electrolysis also holds a great promise for scalable commercial production of the clean hydrogen fuel from pure ammonia or even ammonia-rich wastewaters.5-9 Botte group5-7,9 believed that ammonia electrolysis (Eq. 1-3) is more fascinating than water electrolysis, because it can produce hydrogen in-situ at lower over-potentials and simultaneously releases eco-friendly nitrogen and hydrogen. More significantly, ammonia-rich wastewaters can even be utilized as raw material for hydrogen production from ammonia electrolysis.6 Anode: ammonia oxidation reaction (AOR) 2NH3(aq)+6OH-→N2(g) +6H2O+6e-

E0 = -0.77 V/SHE

(1)

Cathode: hydrogen evolution reaction (HER) 6H2O+6e-→3H2(g) +6OH-

E0 = -0.83 V/SHE

(2)

Overall: ammonia electrolysis cell (AEC) 2NH3(aq)→3H2(g) +N2(g)

E0 = 0.06 V/SHE

(3)

In general, AOR rather than HER dominates AEC efficiency. Although some noble-metal-free catalysts were developed and offered excellent electrocalytic efficiency for AOR, the most effective catalysts are mostly limited in Pt/Pt-based electrocatalysts.10-13 In addition to the active component, the supporting material is another essential factor affecting the electrocatalytic AOR activity. An appropriate support with a large surface area, high electrical conductivity and good stability can achieve a high dispersion of Pt or Pt-based alloying nanoparticles and the maximum


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utilization of noble materials.14-16 Recently, three dimensional (3D) graphene aerogels have attracted tremendous attention and been widely utilized in the electrochemical energy storage/release system17-19, due to their exceptionally high catalytic activity, large surface area, better electrical conductivity and strong mechanical properties. Chen et al.20 synthesized a 3D Ndoped graphene hydrogel/NiCo double hydroxide electrcatalyst, which exhibited an excellent catalytic efficiency for oxygen evolution reaction associated with the multi-dimensional conductive network, rich macro/mesoporosity, high wettability and N-doping structure. Zhou et al.21 reported the electrocatalytic hydrogen evolution by using 3D hierarchical MoS2 nanosheets anchored on graphene oxide (GO), in which the better electrocatalytic performance were ascribed largely to 3D frameworks for fast charge transport and collection aa well as the synergistic effect between MoS2 and oxygen-containing groups on GO nanosheets. Additionally, the 3D graphene frameworks embedded with Pt/Pt-based active components were also fabricated and applied for the electro-oxidation of methanol and ethanol.22-24 In this work, we facilely synthesized a novel free-standing 3D porous N-doped graphene aerogel (NGA) monolithic material anchored with Pt nanocluster. During the synthesis of the 3D NGA monolith, the poly(oxypropylene)diamine D400 (D400) content, GO concentration and pH played the remarkable roles on the morphologies and properties of NGA. Under the optimal conditions, the finely dispersed Pt nanocluster-like structures were successfully anchored onto the 3D NGA. The synthesized Pt/NGA monoliths were characterized by scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD) and Xray photoelectron spectroscopy (XPS). The Pt/NGA monolithic electrodes presented a high catalytic activity for hydrogen production through AOR, due to its 3D porous interpenetrated network structure, large specific surface area, high electrical conducting and N-doping structure.


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These outstanding properties are beneficial to achieving the lower over-potential for AOR and HER, which is favor for reducing energy consumption and enhancing hydrogen production efficiency of AEC. Unquestionably, this novel free-standing 3D porous NGA monolithic material has potential application in electrocatalysis and electrochemical energy transformation system. Experimental Section Synthesis of 3D NGA and Pt/NGA The 3D NGA monolith was synthesized through a hydrothermal treatment by using GO and D400 as precursors. Here, GO was prepared from natural graphite powders using an improved method.25 In brief, 20 mL GO suspension (5 mg/mL) with adjusting pH by NaOH solution and a certain amount of D400 were added into a Teflon-lined stainless autoclave, which were then sealed and placed in an oven at 180 °C for 12 h. After a hydrothermal reaction, the obtained products were firstly treated with ethanol and water for 6 h to remove the impurities, and then freeze-dried at -80°C for two days. In the end, a free-standing 3D porous NGA monolith was obtained. To serve as control, various NGA monolithic materials were also fabricated by using the same process under the condition of different D400 contents, GO concentrations and pH values. To improve the conductivity and surface morphology of the as-prepared NGA monolith, an annealed treatment was carried out at a certain temperature for 2 h under Ar atmosphere in a tube furnace. The annealed material was termed as NGA-x (x represents the carbonization temperature). The as-prepared NGA monolith was used as a working electrode and immersed in 0.5 M H2SO4 solution containing 5 mM H2PtCl6·6H2O precursor, meanwhile the Pt foil and SCE were used as counter and reference electrodes, respectively. With the aid of the stirring, a constant


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Figure 1. Fabrication process of 3D porous Pt/NGA monolithic material potential of -0.3 V vs. SCE for 60 min was applied to prepare the Pt/NGA monolithic material (Figure 1). To deeply investigate the AOR activity of Pt/NGA materials, the as-prepared NGA monolith was grinded into powder and mixed with deionized water/ethanol/Nafion solution (4.5:4.5:1, v:v:v) under ultrasonication to form a NGA suspension (ca. 1.0 mg/mL of concentration). Then, the 5 µL of suspension was dropped onto the glassy carbon (GC) electrode. After dried in N2 atmosphere, the NGA/GC electrode was electrodeposited with Pt to prepare Pt/NGA modified GC electrode for AOR in a 0.1 M NH3 + 1 M KOH aqueous solution. The loading amount of Pt on NGA was measured through an inductively coupled plasma technique (ICP, Optima 2000DV, PerkinElmer) after dissolving the active component (Pt) from the substrate in a mixed solution of NO3:HCl:HF=1:3:0.05 (volume ratio) for 12 h. The Pt loading content on NGA powder and NGA monolith (ca. 82.1 mg) were ca. 0.022 mg/mg and 0.055 mg/mg, respectively. Physicochemical and electrochemical characterizations Surface morphologies of the synthesized materials were observed using a scanning electron microscope (SEM, NOVA Nano SEM 450) equipped with energy dispersive X-ray spectroscopy (EDX) detector and transmission electron microscope (TEM, JEM-2000 EX). Crystallographic


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pattern was obtained by X-ray diffraction (XRD, Rigaku diffraction, D/MAX-2400) using CuKα radiation. X-ray photoelectron spectroscopy (XPS) was used to characterize surface atomic structure and chemical state. Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and chronoamperometry (CA) technologies were used to measure the electrocatalytic activities of samples for AOR. The Pt foil and Hg/HgO was used as counter and reference electrodes, respectively. The reference electrode was calibrated with a reversible hydrogen electrode (RHE) in 1 M H2-saturated KOH aqueous solution (VRHE = VHg/HgO + 0.8525 V). Operation and efficiencies calculations of AEC The structure of AEC used in this work was shown in Figure S1. The anode chamber and cathode chamber with the volume of about 30 mL were separated by a proton exchange membrane (PEM). The electrolyte in both cells was 0.1 M NH3 + 1 M KOH aqueous solution. In AEC system, the Pt/NGA monolith materials were directly used as anode and cathode to investigate its electrocatalytic activity for AOR and hydrogen production. The cell efficiency (ε), current efficiency and power consumption were calculated to evaluate the AEC performance (Eq. 4-6). Cell efficiency ε, %=

3×∆

(4)

×∆NH3 ×××

where ∆ is the lower heating value of hydrogen (242.7 kJ/mol), ∆ is the lower heating value of ammonia (320.1 kJ/mol), F is Faraday’s constant (96,480 C/mol), and E is the AEC voltage. Current efficiency %=

MH2, act MH2, theo

×100

(5)

where MH2,act and MH2,theo are mass of hydrogen based experimental and Faraday’s law. Power consumption Wh gNH3 = -1

I×E×t MNH3

(6)


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where I is the current applied in the coupled system, E is the AEC applied voltage, MNH3 is the molecular weight of ammonia (17.03 g), and t is the operation time. Results and discussion Synthesis and characterization of 3D NGA materials NGA monoliths were synthesized through a chemical cross-linking method with GO and D400 as precursors in a pure water solvent. As a cross-linking agent, D400 can react with the reactive oxygen-containing functional groups of GO to generate a gel (Figure 1). Generally, the crosslinking reaction forms a graphene oxide monolith within 60 s at 90°C.26 Herein, a hydrothermal reaction (180°C, 12 h) was selected for the one-step synthesis of NGA monolith, in which the D400 served not only as a cross-linking agent, but also as the nitrogen source during the formation of NGA monoliths. Various parameters such as GO/D400 mass ratios, GO concentration and initial pH of GO suspension have significant effects on the morphology and mechanical property of NGA. As seen in Figure 2a, a 3D cylindrical NGA monolith with well mechanical strength was obtained under the condition of GO/D400=1:1, GO concentration = 5.0 mg/mL and pH = 3.0. The NGA monoliths synthesized under other conditions were either collapsed or shrunken in volume (Figure 2a and Table S1). In this reaction, D400 plays a great role in forming 3D aerogel structures because its -NH2 group could crosslink with GO sheets through the covalent bond (-CN or -OC-NH-).26 After introduction of D400, the agglomerating phenomenon of GO nanosheets resulting from strong π-stacking and hydrophobic interactions is significantly suppressed. A critical concentration of GO suspension was also obtained, which controls the formation of robust 3D NGA aerogel. Less GO concentration (< 4 mg/mL) could not form a substantial NGA aerogel, because it is difficult for less GO nanosheets to react with D400 constructing the whole


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interpenetrated network structure.27 In addition, solution pH also plays a major part in forming steady 3D NGA monolith through adjusting the functional groups on GO nanosheet surfaces. In an alkaline condition, the transformation of -COOH groups into -COO- groups can effectively inhibits the agglomeration and facilitates the gelation of GO nanosheets, however, higher pH value would result in electrostatic repulsion to hinder hydrogel formation due to the charge variation of GO nanosheets.28,29 After freeze-drying, the disorderly interlaced GO nanosheets stacked hierarchically forming 3D macroporous structures with pore diameter of ca. 0.5-2.0 µm (Figure 2b). Few oxygen content exists on NGA monolith from EDX spectra (Figure S2), suggesting that GO was reduced and most oxygen-containing groups were removed through the hydrothermal treatment.30,31 TEM image shown in Figure 2c exhibits the wrinkled and transparent graphene sheets. Moreover, two new peaks at ca. 1260 and 796 cm-1 was observed in FT-IR spectrum of NGA (Figure 2d) when compared to that of GO, indicating the formation of new C-N bond between GO and D400.26 Other characterizations including TGA, XRD, Raman and N2 adsorption/desorption isotherm were shown in Figure S3 and Figure S4, all of which demonstrate that a stable NGA monolith was formed. To obtain high electrochemical and catalytic activity, NGA monoliths were synthesized under various annealed treatments (0, 400, 600, 800°C) and GO/D400 ratios (4:1, 2:1, 1:1, 1:2 and 1:4). As shown in Figure S5 and Figure S6, all the resultant NGA-x (x represents the annealed temperature) materials remain their original shapes even after calcination at 800°C. At the same time, the steady free-standing 3D structures result in stable and rich macroporous morphologies. The conductivity of NGA materials are improved gradually with the increasing calcination temperature from 2.13×10-2, 1.43, 5.95 to 9.90 mS cm-1 for NGA-0, NGA-400, NGA-600 and


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Figure 2. a) Digital images of graphene hydrogen/aerogel with various GO/D400 ratios, GO concentration and initial pH of GO suspension; b), c) and d) SEM, TEM images and FT-IR spectrum of NGA monolith synthesized under the conditions of GO/D400=1:1, GO=5.0 mg/mL and pH=3.0. NGA-800, respectively. The enhanced conductivity leads to a better electrocatalytic activity due to the atomic rearrangement degree in NGA (Figure S7). Additionally, the introduction of nitrogen not only brought about a dramatic variation in porous morphology of NGA (Figure S8), but also plays a profound influence on NGA’s electrochemical activity (Figure S9). It has been demonstrated that the electroactivity of N-doped graphene is much higher than that of the undoped graphene material, because N doping can effectively tune the electronic density so as to improve the electro-transfer efficiency.32,33 Finally, a NGA monolith with higher electrochemical activity was obtained under the conditions of GO/D400 = 1:1, GO concentration = 5.0 mg/mL, initial GO suspension pH = 3.0 and annealed temperature = 800°C. As shown in Figure S10, the NGA monolith synthesized at the optimum condition possessed excellent mechanical property and can recover original configuration state after compressing. More importantly, this free-


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standing 3D porous NGA material can be facilely fabricated in a large-size through a simple hydrothermal self-assemble-calcination process. It is widely accepted that platinum is the most effective electrocatalyst for AOR.10-12 In present work, a convenient and green potentiostatic electrodeposition method was adopted to prepare Pt/NGA-800 electrocatalysts. The SEM images shown in Figure 3a and 3c exhibit the typical laminar NGA structure with the homogeneous dispersion of Pt nanoparticles after the electrodeposition process. Figure 3b displayed the EDX results with mapping model, which implies the existence of nitrogen and platinum atoms. From the magnified SEM image of Figure 3c, a nanocluster-like Pt particle with many interconnected branches can be clearly observed anchoring onto NGA surface. TEM image (Figure 3d) further reveals that the nanocluster-like Pt particles present the hierarchical and multi-branched morphologies. The dendritic length of Pt nanocluster is approximately 70 nm (Figure 3d insert), which is consisted with the SEM image. This unique nanocluster-like morphology and multi-branched structure of Pt particles is maybe derived from the slow reductive rate of Pt precursors due to the presence of D400 on NGA surface,34 which can be proved by the decreasing reduction current during the electrodeposition process (Figure S11). The lattice fringes of d-spacing shown in Figure S12 is approximately 0.22 nm, corresponding to the (111) planes of face-centered cubic (fcc) Pt.34,35 As seen from Figure 3e, the XRD pattern of Pt/NGA-800 sample presents the characteristic (111), (200) and (220) lattice plane of Pt,36 and the grain size of Pt (111) is ca. 6.2 nm. The wide-scan XPS spectrum of Pt/NGA-800 shown in the insert of Figure 3f confirms the presence of C, N, O and Pt, which agrees well with the EDX element mapping results in Figure 3c. The high-resolution Pt4f XPS spectrum of Figure 3f exhibits the characteristic peaks at 71.6, 74.8, 75.5 and 75.8 eV, which can be assigned to the Pt4f energy level of Pt0 and Pt2+, respectively.37 Compared to the


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Figure 3. a)-c) SEM images and EDX element mapping, d) TEM image, e) XRD spectrum and f) wide-scan XPS survey and high-resolution Pt4f XPS spectra of the Pt/NGA-800 synthesized under the conditions of GO/D400=1:1, GO concentration=5.0 mg/mL, initial pH of GO suspension=3.0 and calcination temperature=800°C. general Pt/C catalysts, the Pt4f XPS peaks of Pt/NGA-800 have an obvious positive shift, implying the variation of the charge transfer after N doping into graphene aerogel structure.38 Catalytic activity of Pt/NGA materials in AOR To deeply evaluate the electrocatalytic activity of Pt/NGA materials for AOR, the Pt/NGA power (Pt loading content is ca. 0.022 mgPt/mgNGA) modified glassy carbon electrode was prepared. Figure 4a shows the effect of the annealed temperature of NGA on AOR activity. All samples exhibit a traditional ammonia oxidation peak at 0.625 V vs. RHE, which is accordance with our previous work.15 The Pt/NGA-800 modified GC electrode displays higher AOR current density than other electrodes, indicating the improved conductivity and electroactivity of Pt/NGA due to the atomic re-arrangement of NGA after annealed at 800°C. The Nyquist plot shown in Figure 4b reflects the charge-transfer resistance (Rct) of various Pt/NGA modified GC electrode during AOR. Combining with the equivalent circuit (inset of Figure 4b and Table S2), the Pt/NGA-800 possesses smaller Rct, which suggests that the higher conductivity can


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efficiently facilitate AOR. On the other hand, the GO/D400 ratio would also exert significant roles on the electrocatalytic activity of Pt/NGA for AOR. It can be seen from Figure 4d that the Pt/NGA samples synthesized with GO/D400 ratio of 4:1 and 1:1 give higher AOR current densities than that of Pt/GA (without adding D400), indicating the existence of synergetic and additive effect between D400 and GO nanosheets during the cross-linking gelation process. The Nyquist plots displayed in Figure 4e demonstrate that the Rct value of Pt/NGA decreases with the increasing D400/GO ratio: 0 (no D400, GA) < 1:4 < 1:1 < 4:1 (Table S2), which is consistent with the results of the conductivity sequencing, i.e. 11.12, 10.67, 9.90 and 3.85 mS cm-1 for D400/GO ratio of 0, 1:4, 1:1 and 4:1, respectively. The adverse tendency of Rct along with the increasing D400 content indicated the efficient reduction of GO sheets by the nitrogencontaining groups of D400 on NGA surface. But the effect parameters of AOR activity are rather than the conductivity and Rct. The N-doped structure, especially the pyridinic-N, porphyric-N and graphitic-N, can generally induce the charge variation of the adjacent carbon atoms after their doping into carbon material skeleton, and thus results in a significant improvement of electrocatalytic AOR activity.39 That is the reason why the N-doped samples (GO/D400 ratio of 4:1 and 1:1) exhibit better AOR activity than the sample with no D400 (Pt/GA). However, excessive D400 (GO/D400 of 1:4) would lead to the decreased AOR current density. On one hand, too much N-doped structure will cause the rich defects into the reduced GO sheets, thus hindering the charge transform.40 On the other hand, more nitrogen-containing chained compounds formed from the calcination of D400 may enclose the reduced GO nanosheets, destroying the stability of 3D NGA structure.26 The electrocatalytic activity and stability of various Pt/NGA samples synthesized under different GO/D400 ratio and calcination temperature were investigated by CA technique at 0.62 V vs. RHE for 3600 s


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Figure 4. AOR activity of various Pt/NGA at different annealed temperatures: a) CV curve, b) Nyquist plot and equivalent circuit, and c) CA curve; AOR activity of various Pt/NGA synthesized at different GO/D400 ratios: d) CV curve, e) Nyquist plot, and f) CA curve; Comparisons between Pt/NGA and other Pt-based materials: g) mass activity and specific activity, h) Nyquist plot and CA curves normalized by i) mass activity and j) specific activity. (Figure 4c and 4f). It is clearly seen that the activity sequencing is in accordance with the results of CV and Nyquist. Compared to other GO/D400 ratios and calcination temperatures, the Pt/NGA prepared under the conditions of GO:D400=1:1 and 800℃-calcination presents better electrocatalytic activity for AOR, due to the appropriate N doping amount. Figure 4g shows the comparison of the mass and specific activity of various Pt-based electrocatalysts for AOR. It can be obviously observed that the Pt/NGA modified electrode presents a superior electrocatalytic activity with the mass activity and specific activity of ca. 1.77 mA µg-1Pt and 0.64 mA cm-2ECSA, respectively, which is much higher than those of the other Pt-


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based electrocatalysts. The Nyquist plots (Figure 4h) for the different Pt-based electrocalysts recorded during the electrocatalysis of AOR also demonstrated an interesting tendency that the better electrocatalytic activity for AOR accompanying the smaller Rct. Beyond all doubt, Pt/NGA exhibits the best electrochemical performance. Furthermore, the CA measurements, which are normalized by mass activity and specific activity, also makes clear that the Pt/NGA modified electrode possesses superior activity and sustainability for AOR to other Pt-based electrocatalysts. This outstanding performance can be attributed to the appropriate N-doping, the unique Pt nanocluster-like morphology and the 3D porous network structure, which provide larger specific surface area, better electronic conductivity and more active sites. To further understand the effect of synthesis condition of Pt/NGA on its electrocatalytic activity for AOR, XPS spectra measurements of various NGA supports were conducted to investigate their chemical states and composition information. As can be seen from Figure 5a, all the high-resolution C1s XPS spectra of NGA (GO/D400=1:1) annealed at different temperature present the characteristic peaks at about 284.8, 286.0, 286.7, 288.1 and 289.0 eV, which should be assigned to C-C/C=C, C-N, C-O, C=O and O=C-O, respectively. The O/C ratio decreases with the increasing calcination temperature (Figure S14), which implies that the improved charge-transfer efficiency and electrocatalytic activity of Pt/NGA shown in Figure 4 should be resulted mostly from the non-oxygen-containing groups. Taking this into account, the effects of calcination temperature and GO/D400 ratio on N1s XPS spectra were also investigated. The results displayed in Figure 5a and Table S3 suggest that the main N species are nitrogencontaining chained groups, including C-N=C (397.8 eV), C-NH-C (399.1 eV), -NH2/-NH (399.8 eV) and N-(C3) (401.8 eV) 26,41-44 for 0℃- and 400℃-annealed samples. However, the N-doped


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Figure 5. a) High-resolution C1s and N1s XPS spectra of NGA synthesized at GO/D400 ratio of 1:1 and different calcination temperatures, b) high-resolution N1s XPS spectra of NGA synthesized at different GO/D400 ratios and 800℃. species such as pyridinic-N, pyrrolic-N and graphitic-N were observed in graphene structure at 398.3 eV, 400.4 eV and 401.6 eV, respectively, when the calcination temperature was increased up to above 600℃. The N content of 800℃-annealed NGA even reached about 14.2 wt% according to the XPS spectra. It is worth noting that high N content, in especial the pyridinic N and graphitic N those at the edge of or in graphene plane of carbon materials, can efficiently regulate the variation of electric density, which is beneficial to reactants adsorption and electroactalytic activity.32,33,43 The persistence of some nitrogen-containing chained groups such as C-N=C (397.3 eV) and C-NH-C (399.2 eV) in 600 and 800℃-annealed NGA indicated the strong crosslink of these nitrogen-containing chained groups with graphene nanosheets. For NGA samples obtained at various GO/D400 ratios, the high-resolution N1s XPS spectra in Figure 5b can be divided into five peaks at 397.5, 398.4, 399.3, 400.2 and 401.6 eV,


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corresponding to the C-N=C, pyridinic N, C-NH-C, pyrrolic N and graphitic N, respectively. However, the nitrogen contents in these N-containing species calculated and summarized in Figure S15 and Table S4, are quite different. The higher ratio of pyridinic-N and pyrrolic-N to graphitic-N was obtained for NGA synthesized at GO/D400=1:1 and 1:4, which would perform the higher catalytic activity.39 Above results demonstrated that high calcination temperature can efficiently decrease the O/C ratio and achieve N doping in graphene nanosheets using D400 as a crosslinking agent, which is responsible for the improved conductivity, more activity sites and high electrocatalytic activity of Pt/NGA due to the synergistic effect between the unique Pt nanocluster-like structure and 3D porous NGA networks. Performances of 3D Pt/NGA monoliths as electrodes in AEC The gas production performance of AEC using the Pt/NGA-800 (0.055 mg/mg of Pt loading amount) as anode and cathode was studied in-detail. Figure 6a shows the volume of nitrogen and hydrogen generated in AEC at different applied cell voltages. It is clearly seen that the cumulative gas productivity are considerable when the applied cell voltage is larger than 0.5 V, and the maximum volume of hydrogen and nitrogen, which is ca. 8.5 mL (1.90 mL mg-1Pt) and 3.9 mL (0.87 mL mg-1Pt), respectively, was obtained at 0.8 V (the Video in Supporting Information). The changes of potentials and current during 180 min operation of AEC were also recorded and the results are shown in Figure 6b and 6c. Obviously, the variation tendency of electrode potential and current of AEC is to keep at a constant value finally. For example, the cathode potentials can maintain at -1.0 V vs. Hg/HgO for all the AEC applied voltages, while the stable anode potentials was achieved at more positive values when 0.9 and 1.0 V of AEC cell voltages were applied. Compared to other the applied AEC voltages, more positive anode potentials can lead to anodes poisoning45 and thus the notable decreasing in electrocatalytic


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Figure 6. Operation profiles of the Pt/NGA monolith as both anode and cathode in AEC at different applied voltage: a) gas production, b) variation of anode and cathode potential, c) current, d) current and cell efficiencies and power consumption. Comparisons of the fresh and the used Pt/NGA monolithic electrodes: e) Nyquist plots and f) high-resolution N1s spectra. activity, which were clearly proved by the result that more gases are generated at 0.8 V than that at 0.9 and 1.0 V of the applied AEC voltages. By contrast, the commercial 20%Pt/C-based AEC was also operated at 0.8 V of the applied cell voltages. As can be seen from Figure S16, the electrochemical activity of the commercial Pt/C was inferior to that of the Pt/NGA-based AEC. Moreover, the Pt/NGA-based AEC efficiencies and power consumption were calculated in Figure 6d. As seen, there is slight decreasing trend for the cell efficiency as the AEC applied voltage is increasing, but it remains at ca. 80% finally, which suggests that the larger applied voltage resulted in the increased overpotential. However, the current efficiency exhibits an opposite tendency, indicating that the larger applied voltage can provide higher current to promote AOR. The power consumption performs a slow downward trend with the increasing of applied voltage, and finally stabilized at ca. 0.7-0.9 Wh g-1ammonia when the applied voltage exceeds 0.7 V, which implies that the power consumption of the efficiently operated AEC is


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relative stable. The electrocatalytic mechanism of 3D Pt/NGA-based AEC for AOR and HER was displayed in Figure 7. Compared with other Pt-based AECs, the Pt/NGA-based AEC takes advantages in hydrogen production and resistance to poisoning (Table S5). The Pt/NGA-based AEC operated at a high current can continuously produce ca. 8.5 mL H2 (about 1.90 mL mg-1Pt) in 3 h, which is more than or comparable to other Pt/C-based AEC with the similar power consumption. More importantly, the Pt/NGA-based AEC possesses better sustainability and stability in hydrogen produce. To clarity this excellent characteristics of Pt/NGA, the Nyquist plot (Figure 6e), XPS spectra of the fresh and used Pt/NGA (Figure S17) were measured. Compared with the fresh Pt/NGA material, a slight increase of Rct was observed for the used Pt/NGA, and meanwhile, the high-resolution N1s XPS spectrum shown in Figure 6f revealed that the nitrogen content of the used Pt/NGA is reduced. Above results should be attributed to the fact that some poisonous substances such as nitrogen atoms would adsorb onto the Pt/NGA surface, which occupied the reactive sites and led to an increase of the Rct during AOR process.45 Compared with the other Ptbased electrocatalysts, the unique 3D Pt/NGA materials with high specific surface area can efficiently restrain this poisoning phenomenon, due to its interpenetrated porous network structure and N-doping. Moreover, the adsorbing of poisonous substance at reactive sites would also be restrained, because many nitrogen-containing chained substrates derived from D400 crosslinking agent can be firmly bond onto graphene nanosheets, resulting in a positive corelevel shift in Pt4f.46


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Figure 7. The electrocatalytic mechanism of 3D Pt/NGA monolith electrodes for AOR and HER in an AEC. Conclusions In summary, a high-performance and stable free-standing 3D porous Pt/NGA material was successfully synthesized and applied in an AEC for hydrogen production. The NGA substrate was prepared through a self-assemble process with GO and D400 as precursors in a pure water solvent. The SEM, TEM, XRD and XPS techniques show that NGA substrate is a suitable material to anchor Pt nanoparticles. Through a simple electrodeposition method, the Pt nanoparticles with the well-defined nanocluster-like structure were homogeneously distributed on NGA substrate. The Pt/NGA synthesized under the conditions of GO/D400=1:1, pH=3.0 and calcination temperature=800℃ exhibited a superior AOR activity over other Pt-based electrocatalysts, which was attributed to N doping in graphene framework and the uniform deposition of unique nanocluster-like morphology of Pt particles on the 3D porous NGA substrate, that is characterized by large special surface area, high conductivity, abundant reaction cites and efficient mass flow pathways in the randomly arranged porous backbone structure. Approximate 3.9 mL of N2 and 8.5 mL of H2 was obtained from AEC using 3D porous Pt/NGA


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monolith as cathode and anode at 0.8 V of cell voltage in 3 h, with the cell efficiency of 68.5%, current efficiency of 76.2% and power consumption of 2.9 Wh g-1ammonia. Furthermore, the 3D porous Pt/NGA monolith anode offered a high resistance to the poisoning effect and ensured the sustainable and stably generation of hydrogen in an AEC. Thus, our free-standing 3D porous NGA monolith would be a potential material for application in electrocatalysis and electrochemical energy transform fields. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. The preparation process of Pt/NGA monolith and the effects of different parameters on NGA materials; the construction of AEC system; the characterization data including SEM-EDX, TGA, XRD, Raman, N2 adsorption/desorption isotherm, XPS spectra and digital photographs of the NGA materials; the comparison of the electrochemical activity of the NGA materials and other Pt-C based electrode for AOR. AUTHOR INFORMATION Corresponding Author *Guoquan Zhang, E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT


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TOC/Abstract graphic: For Table of Contents Use Only

Synopsis A novel 3D porous Pt/NGA monolithic material was synthesized through a hydrothermalannealed method and applied in an ammonia electrolysis cell for hydrogen production.


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Free-Standing 3D Porous N-Doped Graphene ... - ACS Publications

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