0.2 V Electrolysis Voltage-Driven Alkaline Hydrogen Production with

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0.2 V Electrolysis Voltage-Driven Alkaline Hydrogen Production with Nitrogen-Doped Carbon Nanobowls Supported Ultrafine Rh Nanoparticles of 1.4 nm Nan Jia, Yanping Liu, Lei Wang, Pei Chen, Xinbing Chen, Zhongwei An, and Yu Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b13586 • Publication Date (Web): 29 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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ACS Applied Materials & Interfaces

0.2 V Electrolysis Voltage-Driven Alkaline Hydrogen Production

with

Nitrogen-Doped

Carbon

Nanobowls Supported Ultrafine Rh Nanoparticles of 1.4 nm Nan Jia, Yanping Liu, Lei Wang, Pei Chen,* Xinbing Chen,* Zhongwei An, and Yu Chen Key Laboratory of Applied Surface and Colloid Chemistry (MOE), Shaanxi Key Laboratory for Advanced Energy Devices, Shaanxi Engineering Lab for Advanced Energy Technology, School of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710062, P. R. China. ABSTRACT: The development of high effective and low cost electrocatalyst for energy-saving hydrogen production via water splitting is still a great challenge. Herein, porous nitrogen-doped carbon nanobowls (N-CBs) has been designed and used for the controlled growth of ultrafine rhodium (Rh) nanoparticles. With the aid of interfacial bonding of Rh and N, ultrafine Rh nanoparticle with an average size of 1.4 nm have been successfully immobilized on the N-CBs. This Rh/N-CBs electrocatalyst shows superior activity and highly stability for the hydrogen evolution reaction (HER) and hydrazine oxidation reaction (HzOR). More importantly, the Rh/N-CBs exhibits highly activity for hydrogen production from water electrolysis, marking with a cell voltage of 0.2 V to achieve the current density of 31>" >!"?3 when it serves as cathodic electrocatalyst for HER and anodic electrocatalyst for HzOR in 0>% KOH with 14@>% hydrazine. The density functional theory (DFT) calculations demonstrate that a near-zero hydrogen adsorption free energy brought by the chemical bonding of Rh with the pyrrole-N doped in N-CBs is responsible to the excellent HER activity of Rh/N-CBs electrocatalysts. KEYWORDS: Nitrogen-doped carbon nanobowls, ultrafine Rh nanoparticles, low voltage, hydrogen production, bifunctional electrocatalyst.

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1. INTRODUCTION

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conductivity plays a distinctly subsidiary role in the boosting the catalytic activity for the HER by a strong interaction with Rh or Rh2O3 nanoparticles. Benifited from the unique nanostructures, this material showed a superior activity for the HER in alkaline media relative to Pt/C. In order to achieve the highly dispersed metal nanoparticles, Wang and coworker developed a room-temperature mortar grinding method, by which Rh nanoparticles with a average size of 2 nm were dispersed on the commercial XC-72 Vulcan carbon (Rh NP/C) via in situ reduction of RhCl3 with sodium borohydride.33 Thanking for the uniform distribution, small size, clean surface, and high intrinsic activity of Rh NP, the activity of Rh NP/C for the HER in alkaline media is outperforming the Pt/C electrocatalyst. Cheng and coworkers used ultrathin MoS2 nanosheet to support Rh nanoparticles. The obtained Rh-MoS2 nanocomposite exhibited a higher activity for the HER in acidic suolution than that of Pt/C. The authors attributed the dramatically improved electrocatalytic performance of RhMoS2 nanocomposites to the hydrogen spillover from Rh to MoS2.34 All the above work demonstrates that the substrates play an important role not only in controlling the size and uniform dispersion of Rh nanoparticle, but also in generating synergistic catalysis effect for the HER. Hence, designing novel substrates is important to achieve highly active Ru-based HER catalysts. Carbon nanobowl, developed by our group, has large surface area, porous structure and good conductivity.35 Moreover, its special bowl shape is greatly in favor of increasing the compacted density by stacking bowls together,36 thus raising the number of active site per unit volume when it is used as electrode catalyst. Further, various organic agents can be used as carbon source to synthesize the carbon nanobowl, which provides possibility for the synthesis of hetero-atom doped carbon material. On one hand, the doped hetero-atom, especially N, can trigger the catalytic activity of carbon materials to the HER in alkaline or acid media by inducing the charge rearrangement of carbon materials,37-39 on the other hand, the doped nitrogen can efficiently archor and stabilize metal nanoparticles (e.g. Pd, Pt) by a strong interaction with the metal species, consequently downsizing the metal nanopaticles, inhibiting their agglomeration, minimizing their leaching and loss of catalytic activity.40-44 Most importantly, many researches have demonstrated that there is a synergistic effect of the doped N and metal nanoparticles (such as Pt, Ru ) in catalyzing various reactions, thus significantly enhancing the catalysis performance.45-49 However, to our best knowledge, few study forced on loading Rh nanopaticles on N-doped carbon substrate. Taking advantage of the strong coordination of chitin with Rh3+ ions, Cao and coworkers prepared Rh nanopaticles loaded N-doped carbon catalyst (Rh/N–C) by thermal cracking the mixture of chitin and (NH4)3RhCl6, and the Rh/N–C showed excellent catalytic activity for the hydrogenation of benzoic acid to cyclohexane carboxylic acid.50 Xia and coworkers have successfully archored NiRh alloy nanoparticles on nitrogen-doped porous carbon derived from metal-organic frameworks (NiRh/NPC-900) through the

Hydrogen (H2), as high efficiency and clean energy, is one of effective solutions for the energy shortage and environment pollution problems. In recent years, hydrogen production by water electrolysis has attracted increasing attention due to zero pollution. Howerver, the serious polarization of the cathodic hydrogen evolution reaction (HER) and the anodic oxygen evolution reaction (OER) leds to the water electrolysis voltage well above the theory value (> 1.23 V), thus increasing the energy consumption.1-2 To lower the electrode polarization and decrease energy consumption, various highly active electrocatalysts for the HER and OER have been prepared.3-4 In addition, to further reduce the cell voltage of H2 production, on the basis of the development of highly active HER electrocatalyst, some pioneer work has explored the replacement of OER with other oxidation reactions with low potential, such as alcohol oxidation reaction,5-6 aldehyde oxidation,7-9 amine oxidation,10 urea oxidation,11-13 and hydrazine oxidation reaction (HzOR).14-16 In fact, even though these oxidation reactions have lower potential relative to the OER, to minimize the energy consumption, developing highly efficient electrocatalyst for these reaction is necessary. Therefore, bifunctional electrocatalyst with highly activity for both HER and the anodic oxidation reaction have became the core in electrolysis H2-producing.1, 17-18 Among various electrocatalysts of the HER, platinum (Pt)based catalysts are still regarded as the state-of-the-art due to their favorable adsorbtion Gibbs free energy for hydrogen $HGH* 0), which causes the fastest HER rate. However, the high price of Pt restricts its practical application, promoting the development of the alternative to Pt-based catalysts. Based on the volcano plot of the free energy of hydrogen adsorption,19 ruthenium (Rh) has a very small HGH* close to that of Pt, and thus can be used as alternative to Pt. In fact, many researchs have demonstrated that Rh-based catalysts have excellent activity for the HER from ammonia borane hydrolysis.20-24 Recently, some work has focused on enhancing the catalysis performance of Rh-based catalysts for the HER in water splitting mainly by three approaches, namely, tailoring the morphology of Rh nanomaterials,25-26 alloying Rh with transition metals,27-29 and loading Rh nanoparticle on various supports. Among them, the widely used is the loading method due to the flexibility of support selection, by which some Rhbased electrocatalyst with high activity for the HER have been prepared. For example, Zhu et al.30 loaded the Rh nanoparticles on silicon nanowire, which showed higher HER activity than :1> I Pt/C catalysts at potential above 6021>"=4 Hu and coworkers immobilized Rh nanoclusters ( 0.77Jnm) on the carbon support through ion-adsorption strategy. This supported catalyst also exhibited better catalytc activity for the HER in in alkaline mediat than that of Pt/C catalyst.31 Kundu et al supported the Rh and Rh2O3 nanoparticles on nitrogendoped carbon, in which the Rh provides active sites for the adsorbed hydrogen, the Rh2O3 supplies adsorption sites for OH?,32 and the substrate of nitrogen-doped carbon with high 2


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reduction of sodium borohydride. The NiRh/NPC-900 can highly efficiently catalyze the chemical decompositon of hydrazine to hydrogen generation.51 It was worth noting that, until now, there are no report on using Rh ultrafine nanoparticles loaded N-doped carbon as electrocatalyst for the HER. Herein, we synthesized successfully the N-doped carbon nanobowls (N-CBs) supported untrafine Rh nanocrystals (Rh/N-CBs). The synthesis route is simple, easy to operate, and thus suitable for large-scale productions. Moreover, the Rh/N-CBs catalyst exhibits superior HER activity and durability than carbon nanobowls supported Rh nanocrystals (Rh/CBs) and commercial Rh/C electrocatalyst. The calculation of density functional theory (DFT) demonstrates the pyrrole N can strongly adsorb Rh atom and thus reduce the reaction activation energy of HER. These experimental and theoretical results confirm the key role of N-CBs in boosting the catalytic activity of Rh nanoparticles. More importantly, the Rh/N-CBs has bifunctional activity for both HER and HzOR. When it was used as electrocatalyst toward the cathode HER and the anode HzOR for electrolysis H2producing, a current density of 20 mA cm-2 can be achieved with a cell voltage as low as 0.2 V. These results suggest its important application value as highly efficient electrocatalyst for energy-saving H2 production.

were synthesized according to our previous work without melamine.35 2.3. Synthesis of the Rh /N-CBs. 1 mL of RhCl3 (5 mM), 10 mg of the dried N-CBs and 20 mL distilled water were uniformly mixed by ultrasonic treatment for 2 h, followed by adjusting pH value to 9.3 with 1 M NaOH solution. And then, the solution was heated at 40 °C for 6 h, into which 0.38 g NaBH4 was added. After reaction for 60 min, the Rh/N-CBs were obtained by centrifugation, water wash and dryness. As control sample, the CBs supported Rh particles (Rh/CBs) was also synthesized with same processes. 2.4. Electrochemical Measurement. Electrochemical experiments were carried out at room temperature on electrochemical workstation (CHI 760E) equipped with RDE apparatus. The reference and counter electrodes were saturated calomel electrode (SCE) and carbon rod, respectively. The working electrode was a glassy carbon disk electrode coated with the electrocatalyst. All the values of potential presented in this work were relative to reversible hydrogen electrode (RHE). The potential was calibrated to E(RHE) from E(SCE) by following the formula E(RHE) = E(SCE)+0.2412+0.0591 × pH. The preparation of working electrode was as follows. Firstly, electrocatalyst (10 mg) and of water/Nafion solution (5 mL) were mixed by ultrasound for 60 min, to obtain a homogeneous electrocatalyst ink. Then, 10 L of electrocatalyst ink was drop-cast onto the polished glassy carbon electrode surface, and dried at room temperature. In the measurements of cyclic voltammetric (CV) and linear sweep voltammetry (LSV), a standard three-electrode system was used, and the electrolyte (1 M KOH, or 1 M KOH containing N2H4) was saturated by N2. 2.5. Quantum Mechanical Calculations. The interaction of Rh atom and N atom was calculated with density functional theory (DFT).52 All the calculations were performed using the Materials Studio 7.0 Dmol3 program from Accelrys Software Inc.. The exchange-correlation energy calculations were carried out with the generalized gradient approximation (GGA) within PW91. In computational procedure, the DNP basis set and All-electron-core treatment were applied. The structure was fully optimized until the convergence criteria were as follows: the maximal force on the atoms was 0.004 Ha/Å, the maximal atomic displacement was 0.001 Å, the maximal energy change per atom was 1.0 e-5 eV, and the SCF convergence criteria was 1.0 e-6. The adsorbed energy (Eads) was defined as Eads (Rh-N or Rh-C) = E Rh-N or Rh-C - ERh - EN or C, where E Rh-N or Rh-C, ERh and EN or C refer to the energy of the optimized adsorption structure, the isolated Rh, the doped N or the framework C atom, respectively. DFT calculations of HER at Rh/N-CBs and Rh/CBs were performed by Material studio within the local density approximation. Brillouin zone was controlled within a 3 × 3 × 1 Monkhorst-Pack grid. The optimized structure was obtained until the force on per atom is less than 10-4 eV/Å. To avoid periodic interaction, a vacuum layer of 30 Å was added into the plate. The free energy (G) was computed by G = E + ZPE - TS, in which E, ZPE, T and S were the total energy, the zero-point

2. EXPERIMENTAL SECTION 2.1. Reagents and Chemicals. Sodium borohydrde (NaBH4) and potassium hydroxide (KOH) were obtained from Shanghai Chemical Regent Ltd.. RhCl3 was bought from Shanghai Jiuling Chemical Regent Ltd. Rh/C (5 wt% of Rh) was purchased from Premetek Co. Ltd.. The suppliers of resorcinol (C6H6O2), NH3·H2O (25 wt%), melamine, formaldehyde (HCHO, 37 wt%), ethanol, NaOH, cetyltrimethyl ammonium bromide (CTAB), and tetraethyl orthosilicate (TEOS) were Aladdin Reagent Co. Ltd. and Sinopharm Chemical Reagent Co. Ltd., China, respectively. 2.2. Synthesis of the N-CBs. The typical synthesis processes of the N-CBs as follows. Into a uniformly mixed solution containing NH3·H2O (0.45 mL), C6H6O2 (0.30 g), CTAB (0.45 g) and ethanol (30 mL), 2.25 mL of TEOS was poured, and mixed constantly for 0.5 h, followed by addition of 0.42 mL of HCHO. The resultant mixture was names as A mixture. To introduce the nitrogen source, 0.17 g of melamine was dissolved into a solution with 21 mL ethanol and 15 mL H2O at 80 oC, followed by the addition of 0.21 mL of HCHO. After stirring for 0.5 h at 80 oC, a pale yellow solution was achieved, which was named as B solution. B solution was mixed with A mixture for 24 h at oC, 40 then poured into an autoclave with polytetrafluoroethylene inner, and heated at 100 oC for 24 h. The solid product was recovered by centrifugation, followed by drying and carbonization at 800 oC for 2 h. To etch the silica inserted in the products, the carbonized products were subjected to ultrasonic treatment for 2 h in 3 M NaOH solution. After washing and drying, the N-CBs are obtained. The CBs 3




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ACS Applied Materials & Interfaces 20

A

the N-CBs is chemisorbed on the N-CBs. These theoretic results support the XPS analysis. -0.374 0.192

0.092 -0.096

0.141 0.091 -0.103

0.131 -0.157

0.084 -0.079

0.028

0.107

0.094

0.050

-0.102 -0.119

-0.124

0.091

-0.099

0.090 0.107

0.084

0.09 -0.163

-0.093 0.005 0.005 -0.093 0.091 0.091 -0.102 -0.102 -0.099

0.09 -0.163

0.086

0.086

(a’)

Eads=-1.71 eV

(b’)

2.112

2.018

0.103

5

0.268

0

0.06 0.04

46.2 mA dec-1

0.02 0.00

0.00

0.103

30

C

0.091

0.03 0.06 0.09 Potential / V (vs. RHE)

0.12

0.15

-0.9 6

-0.6

-0.3 0.0 0.3 log[-i (mA cm-2)]

10mM N2H4

(c’) 2.033

20

50mM N2H4 70mM N2H4 90mM N2H4 110mM N2H4

10

0.6

0.9

D

0mM N2H4 30mM N2H4

Eads=-2.28 eV

38.28 mA dec-1

82.71 mA dec-1

-0.222

-0.059 -0.137

0.086 0.090

Eads=-2.30 eV

-0.137

0.268 -0.222 0.094 0.058 -0.125 0.242 -0.396

-0.471 -0.085 -0.085 0.091 0.091 -0.104 0.197 -0.104 0.197

0.094 0.080

-0.059

10

0.091

i / mA cm-2

0.098

Commercial Rh/C Rh/CB Rh/N-CB

0.08

E (V vs. RHE)

(c)

(b)

i / mA cm-2

(a)

B

Commercial Rh/C Rh/CBs Rh/N-CBs

15

i / mA cm-2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Rh/N-CBs Rh/CBs

4

2

0 0.00

Figure 3. Top (a, b, and c) and side (a’, b’, and c’) views of the optimized configurations with Mulliken charges assigned for the Rh atom-adsorbing pyrrole-N, Pyridinic-N, and graphitic-N system. Atomic color code: gray, carbon; blue, nitrogen; white, hydrogen; and blue-green, rhodium.

0.15

0 0

2000

4000

6000

Time / s

Figure 4. (A) LSV curves of the Rh/N-CBs, Rh/CBs and commercial Rh/C in N2-saturated 1 M KOH electrolyte with 50 mM N2H4 at 5 mV s?1. (B) Tafel curves of the Rh/N-CBs, Rh/CBs and commercial Rh/C. (C) HzOR polarization curves of the Rh/N-CBs electrocatalysts in N2-saturated 1 M KOH solution with variable hydrazine concentrations. (D) Chronoamperometry curves of the Rh/N-CBs, Rh/CBs in N2saturated 1 M KOH electrolyte with 50 mM N2H4 at 0.03 V potential.

3.2 Hydrazine Oxidation Catalysis. In alkaline solution, the products of HzOR

0.03 0.06 0.09 0.12 Potential / V (vs. RHE)

on the anode are

clean N2 and H2O, N2H4 + 4OH? N2 + 4H2O + 4e (E° = ?14DD V vs. RHE, pH 14). Efficient HzOR anode catalyst is the key point in direct hydrazine fuel cells.59 To decrease the reaction overpotential, various electrocatalysts have been developed.6063 Here, the catalytic activity of Rh/N-CBs toward the HzOR was evaluated in a typical three-electrode alkaline electrolyzer with 50mM N2H4. Figure 4A displays the LSV curves of Rh/N-CBs, Rh/CBs and commercial Rh/C (5 wt.%). It can be seen that, the Rh/N-CBs has high HzOR activity, and only needs a potential of 72 mV to achieve a current density of 10 mA cm?3, which is much lower than that of Rh/CBs (E10 mA cm?3 = 118 mV) and Rh/C (E10 mA cm?3 = 145 mV) catalysts. To better understand the HzOR kinetics, the Tafel plots of these electrocaralysts were compared. As observed in Figure 4B, Rh/N-CBs shows a quite low Tafel slope of 38.28 mV dec?0, while 46.2 mV dec?0 is required for Rh/CBs, 82.71 mV dec?0 for commercial Rh/C, demonstrating the favorable catalytic kinetics and fast electron transfer of Rh/N-CBs toward HzOR. Moreover, the Rh/N-CBs has a larger electrochemically active surface areas (46.8 m2 g Rh) than that of Rh/CBs (42.5 m2 g Rh)(Figure S4), further proving the high dispersion and improved reactivity of ultrafine Rh nanoparticles in the sample of Rh/N-CBs. Figure 4C shows the HzOR performances of Rh/N-CBs with variable hydrazine concentrations. Markedly, there is no signficant voltammetric response in the working potential window in the absence of hydrazine. In contrast, a significant anodic response current arises in 10 mM hydrazine alkaline electrolyte, and it increases with the increase of hydrazine concentration, suggesting the effectiveness of Rh/N-CBs as electrocatalyst for the HzOR. In addition, the Rh/N-CBs also has an excellent stability for HzOR, which is verified by a chronoamperometry experiment (Figure 4D). After run for 6000 s at a current density of 0.03 V in 50 mM hydrazine alkaline electrolyte, the current density at the Rh/N-CBs electrode decreased only 12%, but 42% at the Rh/CBs electrode.

3.3 HER performance and stability of the Rh /N-CBs The catalysis performance of the Rh/N-CBs for the HER was evaluated in 1 M KOH with a scan rate of 5 mV s?1. The resultant polarization curve was presented in Figure 5A. At the current density of 10 mA cm?2, the overpotential at the Rh/N-CBs is 77 mV, which is lower than that at Rh/CBs (114 mV) and commercial Rh/C (112 mV), demonstrating its superior HER activity. Further, the catalytic reaction kinetics of HER was investigated by Tafel equation, and the linear part of the Tafel curves was extracted to calculate the Tafel slope. As observed in Figure 5B, the Rh/N-CBs exhibits the smallest Tafel slope of 74.16 mV dec?1, compared to the Rh/CBs (98.86 mV dec?1) and commercial Rh/C (79.29 mV dec?1), suggesting the fast HER kinetics in Rh/N-CBs. Moreover, the mass-normalized LSV curves show that the Rh/N-CBs displays higher current relative to the commercial Rh/C and Rh/CBs at same potential (Figure S5), indicating its great advantage as Rh-saving electrocatalyst. The long-term durability is another important parameter of electrocatalyst, which is usually measured with chronoamperometry and cyclic voltammetry. The chronoamperometric curves in Figure 5C show that, at 1.7 V potential in a N2-saturated 1 M KOH solution, the current attenuation of Rh/N-CBs is slow and 71% current can be remained even after working for 6000 s, which is 2.5 times higher than that of the commercial Rh/CB (28%). In addition, the excellent durability of Rh/N-CBs is also proved by the result of cyclic voltammetry. As shown in Figure 5D, even running for 5000 CV cycles in the potential range of 01.0 V, the Rh/N-CBs only has a small change of 15 mV in the overpotential at 10 mA cm-2, which is much less than that of Rh/CBs (HE= 40 mV). To investigate the stability of ultrafine Rh nanoparticles in 5000 CV cycles, the TEM image of the Rh/N-CBs after 5000s cycles was characterized, and present in Figure 5E. Compared with TEM image before 5000s cycles 6


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Center in Shenzhen for providing computing services with the Materials Studio 7.0 software.

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