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Single-Walled Carbon Nanotubes Induced Optimized Electron Polarization of Rhodium Nanocrystals to Develop Interface Catalyst for Highly Efficient Electrocatalysis Wenqing Zhang, Xin Zhang, Lin Chen, Jianying Dai, Yu Ding, Lifei Ji, Jun Zhao, Ming Yan, Fengchun Yang, Chun-Ran Chang, and Shaojun Guo ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02016 • Publication Date (Web): 19 Jul 2018 Downloaded from http://pubs.acs.org on July 19, 2018

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Single-Walled Carbon Nanotubes Induced Optimized Electron Polarization of Rhodium Nanocrystals to Develop Interface Catalyst for Highly Efficient Electrocatalysis Wenqing Zhang†,#, Xin Zhang †,#, Lin Chen†, Jianying Dai†, Yu Ding†, Lifei Ji†, Jun Zhao†, Ming Yan‡, Fengchun Yang†,*, Chun-Ran Chang‡,* and Shaojun Guo§,∥,⊥ †

Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of

Education, College of Chemistry & Material Science, Northwest University, Xi’an 710127, China ‡

Institute of Industrial Catalysis, School of Chemical Engineering and Technology, Xi’an

Jiaotong University, Xi’an 710049, China §

Department of Materials Science and Engineering, College of Engineering, Peking University,

Beijing 100871, China ∥

BIC-ESAT, College of Engineering, Peking University, Beijing 100871, China

⊥

Key Laboratory of Theory and Technology of Advanced Batteries Materials, College of

Engineering, Peking University, Beijing 100871, China

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ABSTRACT. Carbon nanomaterials have been employed as the crucial support to increase surface loading and electronic conductivity. More than that, there were widely synergistic effects between the metallic species and the carbon supports for accelerated and stable electrocatalysis. In this work, rhodium nanocrystals hybrid with single-walled carbon nanotubes (Rh/SWNTs) was reported as an advanced catalyst for electrocatalytic reactions. SWNTs, as a good electron accepter, could modulate the electronic structure of Rh NPs and produced the optimized electron polarization, which can develop a high performance interface catalyst. More eye-catching is the best hydrogen evolution reaction (HER) properties in acid and alkali achieved by Rh/SWNTs with small overpotential (at 10 mA cm-2) and Tafel slope (25 mV and 20 mV dec-1 in 0.5 M H2SO4, 48 mV and 27 mV dec-1 in 1.0 M KOH respectively). Meanwhile, such the electron polarization could also improve the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) property. The Rh/SWNTs shows high efficient in overall water splitting and integrated zinc-air battery with a low cell voltage of 1.59 V at 10 mA cm-2 and a high opencircuit voltage of 1.42 V. This work highlights an electron polarization strategy on the interface between Rh and SWNTs to develop high-performance multifunctional hydrogen and oxygen catalyst.

KEYWORDS: carbon nanomaterials, rhodium, electron polarization, interface catalyst, electrocatalysis


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INTRODUCTION In electrocatalysis, carbon nanomaterials have garnered significant interests due to their obvious advantage in acting as metal catalyst supports for boosting the energy-revelent

important

electrochemical reactions such as hydrogen evolution reaction (HER), oxygen reduction reaction (ORR) and oxygen evolution reaction (OER).1-3 However, the evidences in regarding the tailorable electrocatalytic property of metal/carbon hybrid show that carbon nanomaterials not only act as a support but also has extraordinary synergistic effect, originating from the chemical bonding effect and charge transfer between the metallic species, carbon support interface, for active and stable electrocatalysis.4-11 And this synergistic effect for enhanced electrocatalytic activity strongly depended on the type of carbon nanomaterials used in the hybrid catalysts. For example, Dai et al. showed the FeCoS nanosheets hybrid with carbon nanotubes exhibited more active HER performance than that with reduced graphene oxide.10 And Guo et al. demonstrated graphene was a better support than carbon black for loading Co/CoO for ORR.11 Accordingly, it may provide an opportunity for efficient electrocatalysts design to obtain better functionality and performance. And optimal coupling effects can make carbon nanomaterials a vital partner of active metal species, which would further improve the electrocatalytic property of metallic catalysts. Compared with other carbon nanomaterials, such as multi-walled carbon nanotubes (MWCNTs),12 graphene,13 etc., single-walled carbon nanotubes (SWNTs) generally has a broader band gap of around 0.7 eV (Figure 1a) associating with the first (S11) van Hove singularity pair,14 which is more liable to be disrupted by ionization potential species, such as ionic liquid, organic molecule and graphene via non-covalent interaction.15-17 Thus, such special electronic structure makes SWNTs as a better electronic accepter than other carbon


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nanomaterials, which could give rise to larger electronic polarization of ionization potential species (See Figure S1 in supporting Information for more discussion). Here, we firstly investigate whether this electron interaction also exists between SWNTs and appropriate metal. The metallic rhodium (Rh) has high catalytic property for many reactions,18-20 and recent studies show the Rh-based electrocatalysts maybe more active than Pt for HER and ORR.21-24 Therefore, with attempts to explored novel catalyst for enhanced electrocatalysis, the electronic structures and interface interaction between Rh and SWNTs were intensively evaluated. In the combination of Rh and SWNTs, SWNTs would draw the electrons to deviate from Rh under the noncovalent effect. The higher electronic density in the interface could promote the reduction reactions (like HER and ORR) easily.25-28 Meanwhile, the partially electropositive Rh nanoparticles (NPs) could absorb hydroxyl easily and benefit for the OER process.29 Therefore, this electron polarization between Rh NPs and SWNTs may provide a prospective way to further improve electrocatalysis. With this purpose, we employ a simple and surfactant-free sonochemical method to synthesize well-dispersed Rh NPs on SWNTs with ultrasmall size of ~2 nm. The Rh/SWNTs interface catalyst with the optimized electron polarization display excellent HER activity in both acidic and alkaline conditions. Specially, the Rh/SWNTs yield the highest activity in the acidic condition with a current density of 10 mA cm-2 at an overpotential of 25 mV and Tafel slope of 20 mV dec-1, representing the best electrocatalysts towards acidic HER, to the best of our knowledge. The density functional theory (DFT) calculations reveal Gibbs free energy of hydrogen adsorption (∆GH*) in the interface of Rh/SWNTs is –0.09 eV. Meanwhile, as expected, the Rh/SWNTs shows a comparable OER activity to RuO2, which is the first Rh based OER catalyst, and also shows impressive ORR activity. The excellent catalytic performances offered by Rh/SWNTs interface catalyst highlights the importance of choosing combination of metal


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catalyst and carbon support in boosting multifunctional hydrogen and oxygen catalysis for the development of high-performance water splitting and zinc–air battery devices. EXPERIMENTAL SECTION Synthesis of Rh/SWNTs catalysts: To obtain the composites, the purified SWNTs (20 mg) were dispersed in 20 mL ethanol. Then the different volume of 0.1 M RhCl3 aqueous solution and corresponding Borane morpholine complex (C4H12BNO) were added (see Table S1 for detailed dosages), the system was sonicated for another 20 minutes to form Rh/SWNTs with different contents of Rh. The obtained black mixture was processed by centrifugation at 7000 rpm for 10 min. The sediment was washed with ethanol three times and dried under vacuum overnight at room temperature to obtain the solid. The pure Rh nanoparticles were obtained with the same procedure as above without adding SWNTs. The exact mass ratio of Rh in the composites was identified by ICP-OES. Synthesis of Rh/C (C=carbon black, multi-walled carbon nanotubes and graphene) catalysts: 20 mg of carbon black (CB), MWCNTs or G was sonicated in 20 mL ethanol for 1 h. After that 240 µL of 0.1 M RhCl3 and 24 mg of C4H12BNO were added with ultrasonic process for 20 min to form Rh/carbon materials catalysts. The exact mass ratio of Rh in the composites was identified by ICP-OES (Table S2). Characterization: The Rh/SWNTs catalysts crystallographic structure was obtained by X-ray diffraction (XRD, RigakuD/max-2400, USA) with high-intensity Cu Kα radiation (λ=1.5406A). Morphologies of the composites were determined using scanning electron microscopy (SEM, Hitachi S-4800 Japan) and Transmission electron microscopy (TEM, Tecnai G2 F30, FEI, USA). X-ray photoelectron spectroscopy (XPS, ESCALAB250xi, USA) was used to analyze the


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composition and chemical valence. Raman spectra were recorded using a Hormonal HR800 Raman microscope with the 633 nm excitation source. Electrochemical Measurements: All the electrochemical measurements were carried out using a CHI 760E electrochemical workstation (CH Instruments). A conventional three-electrode system, consisting of a modified glassy carbon electrode (GCE) with a diameter of 3 mm (working electrode area: 0.071 cm2) as the working electrode for HER and OER and a rotating disk electrode (RDE) with a diameter of 5 mm (working electrode area: 0.1963 cm2), a saturated Ag/AgCl electrode and a carbon rod or Pt wire were applied as the reference electrode and the counter electrode (carbon rod was employed for HER while Pt wire were used for OER), respectively. The potential values of the Ag/AgCl reference electrode were calibrated with respect to RHE in all measurements in different electrolyte solution, Evs.RHE=Evs.Ag/AgCl + 0.198 V + 0.059 pH. The catalyst ink was prepared by added 1 mg prepared catalysts into 1 ml of the mixed solvent containing DMF and 5% Nafion (v/v = 915:5) for sonication to obtain homogeneous ink dispersion liquid. After that, a certain amount of suspensions were dropped onto the GCE or RDE surface (loading ~ 0.141 mg cm-2) and dried in air. Linear Sweep Voltammetry (LSV) for HER was recorded in N2-saturated 0.5 M H2SO4 and 1.0 M KOH solutions at a scan rate of 5 mV s-1 to obtain the polarization curves, while LSV curves for OER was collected at a scan rate of 5 mV s-1 in 1.0 M KOH at room temperature. For ORR, the LSV curves were collected in O2-saturated 0.1 M KOH with a scan rate of 5 mV s-1. Electrochemical impedance spectroscopy (EIS) was performed in 0.5 M H2SO4 and 1.0 M KOH solution at -0.05 V vs. RHE with frequency ranging from 0.01 Hz to 100 KHz. RRDE measurements were also carried out to determine the reaction mechanism for ORR with a scan


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rate of 5 mV s-1 and rotation rate of 1600 rpm. The ring-electrode potential was set to 1.5 V vs. RHE. The overall water splitting was performed in a two-electrode system in 1.0 M KOH, the catalyst was loaded on two 1×0.5 cm CFP by drop coating (loading amount: ~ 1 mg cm-2). Conventional zinc-air battery consisting of the carbon fiber paper (CFP) supported catalysts as the air cathode (0.5 mg cm-2) and the Zn plate as anode is tested in 6.0 M KOH solution. The galvanostatic charge and discharge test was performed on LAND testing system in 6.0 M KOH with 0.2 M zinc acetate. The homemade water splitting device was assembled with two interconnected electrolyzers separated by a proton membrane. The reversible Zn-air battery is fabricated by using the Zn plate as anode and the carbon fiber paper with the pre-coated Rh/SWNTs catalysts as the cathode. The electrolyte filling between the anode and cathode is the hydrogel polymer electrolyte solution. The hydrogel polymer is synthesized by mixing 10 mL PVA aqueous solution (contain 1 g PVA powder) and 18 M KOH aqueous solution at 95 °C for 10 min. After that, the hydrogel polymer is obtained by transferring the colloidal solution to refrigerator for 4 h. RESULTS AND DISCUSSION The Rh precursor is added to the SWNTs suspension and reduced by borane morpholine complex (C4H12BNO) to synthesize the Rh/SWNTs composites with various Rh contents of 1.8, 3.2, 6.1, 12.5, 25.8, 60.1 wt%, respectively (identified by ICP-OES). The structures and morphologies of the as-prepared Rh/SWNTs and neat SWNTs samples are presented in Figure S2 to S6. Their electrochemical activities indicate the content of Rh as low as 6.1 wt% is enough to show the outstanding electrocatalytic performance (Figure S7). Thus the 6.1 wt% Rh/SWNTs is chosen in this work for investigation and noted as Rh/SWNTs.


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Figure 1. (a) The density of state of graphene (G), MWCNTs and SWNTs. (b) TEM and (c) HRTEM image of Rh/SWNTs, the insert in is SAED patterns. (d) Element mapping showing the C (red) and Rh (green) of Rh/SWNTs catalysts. The transmission electron microscopy (TEM) images of the Rh/SWNTs (Figure 1b, S8 and S9) clearly shows the Rh NPs were loaded in the SWNTs successfully with diameters around 2 nm. High-resolution TEM (HRTEM) image of Rh/SWNTs (Figure 1c) shows the Rh NPs with the lattice spacing of 0.22 nm, corresponding to the (111) interplanar distance of Rh with cubic structure observed by selected-area electron diffraction (SAED) (inserted in Figure 1c).30 Energy dispersive spectroscopy (EDS) mapping shows the elemental distribution of carbon (red) and rhodium (green), respectively (Figure 1d). For comparison, Rh NPs supported on other carbon


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materials (carbon black, multi-walled carbon nanotubes and graphene) were synthesized with similar loading contents of Rh (Figure S10).

Figure 2. (a, b) LSV curves of various Rh/carbon materials and commercial Pt/C in 0.5 M H2SO4 (a) and 1.0 M KOH (b) for HER. (c, d) Corresponding Tafel plots and (e) mass activity at


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-100 mV vs. RHE. (f) The mass activity of Rh/SWNTs and Pt/C before and after stability test in acidic and alkaline condition. The HER tests for the synthesized Rh/carbon materials hybrid catalysts are performed in both acidic and alkaline electrolyte. Figure 2a shows the LSV curves of Rh/carbon materials and commercial Pt/C in acid. The Rh/SWNTs hybrid catalyst exhibit the best HER activity with the smallest onset overpotential (η) of 2 mV and η10 of 25 mV, even surpass that of the Pt/C. More than that, the Rh/SWNTs showed the equally outstanding HER activity in alkali electrolyte with the η10 of 48 mV (Figure 2b), also superior to Pt/C. The Tafel slope of Rh/SWNTs was as low as 20 and 27 mV dec-1 in acid and alkali (Figure 2c and 2d) respectively. To the best of our knowledge, they are lower than all the reported HER catalysts (Table S3 & S4), which could be ascribed to the nearly neutral ∆GH*, effective electron deviation and optimized electron polarization, all about that will be explored in more details later. Besides, the superior performance of the Rh/SWNTs hybrid over other Rh/carbon materials hybrids was also reflected from the highest mass activity of Rh/SWNTs in both acidic and alkaline solution at -100 mV vs RHE (Figure 2e) and its lowest charge transfer resistance, indicating the fastest HER reaction kinetics of Rh/SWNTs (Figure S11).31,32 Furthermore, the Rh/SWNTs catalyst shows good stability with only loss of 2.08% mass activity and 4.02% in acid and alkali of electrolytes respectively after the 10000 cycles. Under the same condition, the Pt/C shows the 63.6% and 47.2% loss of mass activity (Figure 2f). The polarization curves measured for Rh/SWNTs and Pt/C before and after 10000 voltammetric cycles between -0.2 and 0.2 V (vs. RHE) in 0.5 M H2SO4 and 1.0 M KOH at a scan rate of 100 mV s-1 are shown in Figure S12. The Rh/SWNTs exhibits negligible losses in current density compare to the initial curve, and there is a significant change on neither the morphology and crystal lattice of the Rh catalyst (Figure S13), nor the


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chemical structure of SWNTs after stability tests (Figure S14), implying the excellent HER stability of Rh/SWNTs in both acidic and alkaline media. Such an outstanding electrocatalytic performance in both acidic and alkaline media indicates the excellent partnership between Rh NPs and SWNTs.

Figure 3. (a) OER polarization curves and (b) Tafel plots of various Rh/carbon materials in 1.0 M KOH. (c) ORR polarization curves (rotating rate: 1600 rpm) of Rh/carbon materials and commercial Pt/C in 0.1 M O2-saturated KOH. (d) The calculated electron transferred numbers and kinetic current densities of Rh/carbon materials. The OER activity of the synthesized catalyst is investigated in 1.0 M KOH. It can be seen from Figure 3a that the Rh/SWNTs still showed the best OER performance with the onset


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overpotential of 265 mV and η10 of 320 mV, as well as its Tafel slope (89 mV dec-1), much lower than other Rh/carbon materials (Figure 3b). Additionally, the Rh/SWNTs shows the highest mass activity of OER about 3.9 A mg-1metal at 400 mV among Rh/carbon materials and commercial RuO2 (Figure S15), and the polarization curve of Rh/SWNTs shows negligible loss in current density after cycling experiment and very slight degradation of cathodic current during the timedependent tests (Figure S16). The Rh/SWNTs catalyst shows only loss of 10.8% in mass activity after the durability tests, while the RuO2 shows the 38.6% loss (Figure S17), displaying the Rh/SWNTs is quite stable for OER in alkaline media. After the durability tests, the Rh/SWNTs shows the maintained particles size (Figure S18), and the Raman spectrum of SWNTs barely changed. (Figure S19). Notably, the XPS spectrum of Rh 3d reveal the presence of high valent Rh (Figure S20), leading to the further effective OER process of Rh/SWNTs. Furthermore, in addition to the observed HER/OER bifunctional electrochemical activity, the Rh/SWNTs also have impressive ORR catalytic activities, which is investigated by the rotating disk electrode (RDE) in O2-saturated 0.1 M KOH. As shown in Figure 3c, the half-wave potential of Rh/SWNTs is 0.82 V, higher than other Rh/carbon materials and comparable with Pt/C (0.85 V) under the same conditions. The corresponding Tafel slope of Rh/SWNTs is 55 mV dec−1, close to that of Pt/C (49 mV dec−1). This means that the rate-determining step in ORR for Rh/SWNTs is probably the transfer of the first electron (Figure S21).33 The Koutechy–Levich (K–L) plots of Rh/carbon materials (Figure S22 and S23) show the outstanding ORR activity of Rh/SWNTs with the electron transferred number (n) of 3.92 and kinetic current density (jk) of 22.73 mA cm−2, better than those of Rh/MWCNTs (3.65 and 16.31 mA cm−2), Rh/G (3.41 and 14.68 mA cm−2) and Rh/C (3.26 and 12.76 mA cm−2) (Figure 3d). And the RRDE results of Rh/SWNTs display 3.9 for the number of electrons transferred and 6% for H2O2 yield


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respectively, lower than other Rh/carbon materials (Figure S24). Besides, the Rh/SWNTs present excellent stability (Figure S25) and immunity towards methanol crossover. After the addition of 1.0 M methanol, the Rh/SWNTs show negligible change of the current, while the Pt/C catalyst suffers a sharp decrease (Figure S26).

Figure 4. (a) The Rh 3d and C 1s XPS and (b) Raman spectrums of the Rh/carbon materials. (c) Schematic illustration of electronic skewing in Rh/SWNTs interface and the plausible reaction mechanism. (d) Calculated free energy diagram for hydrogen evolution on Rh18/SWNTs at 298.15 K. The free energy change for H+ + e- → 1/2 H2 is defined as zero energy line and the free energies are plotted with respect to the zero reference. (i), (ii) and (iii) are the optimized adsorption structures of H atom on Rh18 NPs, at the interface of Rh18 and SWNT and on SWNT, respectively. Rh atom: cyan, C atom: grey, H atom: yellow. To understand the electrocatalytic activity of Rh/SWNTs, XPS spectrums are employed to investigate the electronic state in Rh/carbon materials. Figure 4a shows the positive shift of


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binding energy of Rh 3d at all the Rh/carbon materials, indicating the decreased screening of outer electrons in Rh.34 However, the G-band of carbon materials in the Raman spectra (Figure 4b) and the electronic absorption spectrum of SWNTs (Figure S27) exhibited no shift after loading of the Rh NPs, implying the electronic structures of the carbon materials were unchanged.35,36 The results suggest the electrons of Rh NPs did not transfer to the carbon materials, but to the interface between Rh and carbon materials, namely, electrons deviated from Rh under the noncovalent effect of the carbon materials. As diagramed in Figure 4c, deviated electrons enhance the electronic density in the interface of Rh/SWNTs, which would facilitate the reduction reaction like HER and ORR.37,38 Meanwhile, positive charged Rh would contribute to the adsorption of OH-, which would be favorable in the OER. In addition, the positive shifts of Rh 3d for all the Rh/carbon materials in the XPS results were proportional to their electrocatalytic performance. Moreover, our DFT calculation display the adsorption of hydrogen at the interface of Rh NP and SWNT is nearly neutral with a ∆GH* of –0.09 eV (Figure 4d and Figure S28), fitting well with the descriptor (∆GH* ≈ 0 eV) of a good HER catalyst with low Tafel slope. In the acidic solution, the adsorbed proton in Rh NPs of Rh/SWNTs could easily receive the electron from the interface and form an adsorbed H atom. Then the adsorbed H atom could combine directly with another adsorbed H atom to form hydrogen by the fast Tafel step. However, the Volmer step in alkali is limited due to the extra ∆G(H2O), which is related to the dissociation of water molecule.39,40 Such process causes the sluggish Volmer step and leads to the higher Tafel slope and slower HER kinetics in alkali. Simultaneously, the effect of oxygen functional group in SWNTs has been further investigated. The oxygen species-rich SWNTs (o-SWNTs), was fabricated by concentrated nitric acid to be


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composed with Rh NPs (Rh/o-SWNTs). As shown in Figure S29, the Rh/o-SWNTs displays the much inferior electrocatalytic performance than Rh/SWNTs, indicating the increase of oxygen or other functional groups on SWNTs would decrease the performance of the Rh/SWNTs.

Figure 5. (a) HER and OER polarization curves of various Rh/carbon materials and Pt/C in 1.0 M KOH. (b) Overall water splitting curves of Rh/SWNTs||Rh/SWNTs couples with a scan rate of 2 mV s−1 in 1.0 M KOH. The inset is an optical photograph during the measurements. (c) The demonstration of Rh/SWNTs in a homemade two-electrode water splitting device by chronopotentiometry. (d) Oxygen and (e) hydrogen collection process with displacement of water. (f) Bubbles generated from the electrode (e.g., O2 electrode) during the water splitting process.


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Figure 5a shows the HER and OER polarization curves of various Rh/carbon materials and Pt/C for in alkaline solution. The difference of voltage (∆E) between HER and OER of Rh/SWNTs is 1.59 V, much lower than the commercial Pt/C (1.805 V). Based on the outstanding HER and OER performances of Rh/SWNTs, we constructed a two-electrode electrolyzer using Rh/SWNTs as both anode and cathode (loaded on two 1 × 0.5 cm carbon cloth with loaded amount of ~ 1 mg cm-2) for overall water splitting in 1.0 M KOH. As present in Figure 5b, Rh/SWNTs||Rh/SWNTs couples display a cell voltage of 1.59 V at 10 mA cm-2, corresponding with the calculated voltage difference (∆E) between overpotential of HER and OER at 10 mA cm-2 in alkaline. The long-term stability of Rh/SWNTs is then evaluated in 1.0 M KOH, where the negligible decrease in potential after 10 h shows its excellent long-term stability, while Pt/CRuO2 couples show extremely poor performance after the same measurement (Figure S30). Furthermore, the homemade water splitting device based on two carbon cloth electrodes (3 × 2 cm) with the pre-coated Rh/SWNTs catalyst to collected H2 and O2 by displacement of water methods (Figure 5c and Figure S31). This device can steadily collect the H2 and O2 under the constand voltage, and the volume ratio of H2 and O2 approaching to the theoretical value of 2:1 during the electrolysis, indicate nearly 100% Faradaic efficiency of the two-electrode setup for overall water splitting (Figure 5d to 5f and Movie S1 to S3).


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Figure 6. (a) Bifunctional catalytic activities of Rh/SWNTs, Pt/C and RuO2 toward both ORR and OER. (b) A photograph showing the voltage and an LED bulb powered by two zinc–air batteries. (c) Rechargeable tests of the zinc–air battery at a current density of 5 mA cm−2. Then based on the electrocatalytic activity of Rh/SWNTs observed in ORR and OER (Figure 6a), the two-electrode zinc-air battery assembled is consisted of the Rh/SWNTs as the aircathode and the zinc slice as the anode, and maintain an open-circuit of 1.42 V for more than 20h (Figure S32). Furthermore, the two serial batteries can easily power up a light-emitting diode (LED) bulb (Figure 6b) with the voltage of 2.55 V. Meanwhile, the rechargeable tests exhibit stable cycling performance with a charge–discharge voltage gap of only 0.7 V (Figure 6c) at 5 mA cm−2. After 12 h (10 min per charge and discharge period), the rechargeable zinc–air battery


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only shows a small voltage gap increase by ≈ 0.1 V, indicating the excellent stability of our catalyst in practical air electrodes. CONCLUSION In summary, we demonstrate electronic polarization ability of different carbon nanomaterials to metallic Rh follows the ranking: SWNTs > MWCNTs > graphene > carbon black, which may relate to their density of state. Owing to the optimized electron polarization, Rh/SWNTs exhibited more remarkable multifunctional performances than other Rh/carbon materials. In particular, the Rh/SWNTs with the rather low loading of noble metal yield the highest activity in the acidic condition, showing a current density of 10 mA cm-2 at an overpotential of 25 mV and Tafel slope of 20 mV dec-1, far exceeding the other reported electrocatalysts towards acidic HER. Meanwhile, the Rh/SWNTs also shows an impressive OER and ORR activity, which could made it utilized in the overall water-splitting and rechargeable zinc–air battery. Thus, the electron polarization strategy in this work opens up diverse avenues to construct metal/carbon interface catalyst for much improved electrocatalyst performance. ASSOCIATED CONTENT Supporting Information. Details of the experiments and Tables S1−S4, Figures S1−S32 and Movie S1-S3 as described in the text (PDF) AUTHOR INFORMATION Corresponding Author *Email: [email protected].


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*Email: [email protected]. Author Contributions #

Wenqing Zhang and Xin Zhang contributed equally

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We gratefully acknowledge the support from the National Natural Scientific Foundation of China (No. 21405120, 21603170, 91645203), the Shaanxi Provincial Science and Technology Development Funds (No. 2016KW-061), and Graduate Student Independent Innovation Project Foundation of Northwest University (No. YZZ15043). This work was also supported by the “Top-rated Discipline” construction Scheme of Shaanxi higher education. The DFT calculations were performed by using supercomputers at the Shen-Zhen Cloud Computing Center. REFERENCES (1)

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Figure 1. (a) The density of state of graphene (G), MWCNTs and SWNTs. (b) TEM and (c) HRTEM image of Rh/SWNTs, the insert in is SAED patterns. (d) Element mapping showing the C (red) and Rh (green) of Rh/SWNTs catalysts. 448x335mm (72 x 72 DPI)


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Figure 2. (a, b) LSV curves of various Rh/carbon materials and commercial Pt/C in 0.5 M H2SO4 (a) and 1.0 M KOH (b) for HER. (c, d) Corresponding Tafel plots and (e) mass activity at -100 mV vs. RHE. (f) The mass activity of Rh/SWNTs and Pt/C before and after stability test in acidic and alkaline condition. 518x564mm (299 x 299 DPI)


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Figure 3. (a) OER polarization curves and (b) Tafel plots of various Rh/carbon materials in 1.0 M KOH. (c) ORR polarization curves (rotating rate: 1600 rpm) of Rh/carbon materials and commercial Pt/C in 0.1 M O2saturated KOH. (d) The calculated electron transferred numbers and kinetic current densities of Rh/carbon materials. 521x386mm (299 x 299 DPI)


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Figure 4. (a) The Rh 3d and C 1s XPS and (b) Raman spectrums of the Rh/carbon materials. (c) Schematic illustration of electronic skewing in Rh/SWNTs interface and the plausible reaction mechanism. (d) Calculated free energy diagram for hydrogen evolution on Rh18/SWNTs at 298.15 K. The free energy change for H+ + e- → 1/2 H2 is defined as zero energy line and the free energies are plotted with respect to the zero reference. (i), (ii) and (iii) are the optimized adsorption structures of H atom on Rh18 NPs, at the interface of Rh18 and SWNT and on SWNT, respectively. Rh atom: cyan, C atom: grey, H atom: yellow. 120x89mm (300 x 300 DPI)


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Figure 5. (a) Polarization curves of various Rh/carbon materials and the Pt/C for HER and OER in 1.0 M KOH. (b) Polarization curves for overall water splitting obtained in a two-electrode configuration (Rh/SWNTs||Rh/SWNTs) with a scan rate of 2 mV s−1. The inset is an optical photograph during the measurements. (c) The demonstration of Rh/SWNTs in a homemade two-electrode water splitting device by chronopotentiometry. The carbon clothe (3 × 2 cm) was used as an all-carbon electrode for deposition the Rh/SWNTs catalyst. (d) Oxygen and (e) hydrogen collection process with displacement of water. (f) Bubbles generated from the electrode (e.g., O2 electrode) during the water splitting process. 553x395mm (299 x 299 DPI)


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Figure 6. (a) Bifunctional catalytic activities of Rh/SWNTs, Pt/C and RuO2 toward both ORR and OER. (b) A photograph showing the voltage and an LED bulb powered by two zinc–air batteries. (c) Cycling performance of the zinc–air battery at a current density of 5 mA cm−2. 521x356mm (299 x 299 DPI)


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