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• INTRODUNTION

  The exploration of highly efficient electrocatalyst is an important subject to satisfy the electrochemistry-based sustainable energy requirement. The noble metal rhodium (Rh) nanomaterials are widely utilized in electrocatalysis/catalysis because of unique chemical and physical properties.^[1-6] However, the high cost and scarce availability of Rh metal still remain a cause of concern. Alloying noble metals with transition metals is an efficient way for their wide applications, which not only reduces the noble metal content but also elevates the reactivity due to the synergic and electronic effects.^[7-11] Among various transition metals, nickel (Ni) is commonly used due to its low cost, stability and effective synergistic effect.^[12-14] Specifically, the activation energies and bond enthalpies of adsorption molecules on noble metal can be efficiently tuned by Ni atoms, resulting in improved performance.^[15-19] In addition, the introduction of Ni can improve the intrinsic kinetics of the reactions by controlling and optimizing the electronic and geometrical structure of noble metal.^[20-23] Especially, in alkaline medium, incorporating Ni into noble metal-based nanostructure can promote the dissociation of H₂O to produce OH_ads and accelerate the oxidation of CO_ads at the active sites, which elevates the poisoning tolerance of electrocatalysts. For instance, Wang's group synthesized PdNi/Ni nanotubes, which revealed enhanced durability and CO_ads poisoning resistance than commercial Pd nanoparticles for alkaline ethanol electrooxidation.^[24] Mainly, the introduction of Ni can effectively decrease the adsorption energy of various intermediates by adjusting the electronic structure. Zhang's group successfully prepared PtNi nanowires by a one-step method, which displayed 4.76 times specific activity and 3.02 times mass activity than that of the commercial 20% Pt/C electrocatalyst for methanol oxidation reaction (MOR).^[25] Herein, the optimized chemical composition, electronic structure, and high-index crystal face were responsible for the high electroactivity of PtNi nanowires.

  The performance of direct methanol fuel cells (DMFCs) largely relates to the activity of anodic electrocatalysts for methanol oxidation reaction (MOR). DMFCs pose themselves as a promising candidate for fossil fuels, which is supposed of being a low emission of pollutants, high energy density and easy handling that may find applications for automobiles and power portable electronic devices.^[26-28] Most of current MOR electrocatalysts are noble metals (Pt and Pd) based nanomaterials.^[29-31] Recent reports have indicated that Rh-based nanomaterials can serve as highly efficient electrocatalyst for MOR, which has high resistance CO-poisoning ability, small overpotential and high intrinsic activity for MOR.^[32-34] As an oxyphilic metal, Rh is easy to adsorb OH^-, which plays a key role in the further oxidation of CO_ads inter-mediate.^[35] Meanwhile, recent advanced works suggest that bimetallic Rh nanomaterials usually exhibit enhanced activity and selectivity for some important electrocatalytic reactions (such as ethylene glycol oxidation, glycerol oxidation, and ethanol oxidation reaction), originating from geometric and electronic effects.^[36-38] Although previous studies have demonstrated that noble metal nanoparticles by Ni introduction show enhanced catalytic activity, the application of RhNi alloy nanomaterials in MOR is still vacant position.

  In this work, a series of RhNi alloy nanostructures with different Rh/Ni atomic ratios were synthesized by one-step wet chemical method. During the synthesis, the introduction of Ni^II ion could effectively adjust the morphology of bimetallic RhNi nanostructures. Compared with the commercial Rh nanoparticles (Rh-NPs-C) and monometallic Rh nanodendrites (Rh-NDs), one-dimensional RhNi alloy nanodendrite assemblies (RhNi-NDs-As) with optimized composition revealed enhanced electroactivity for MOR and improved anti-poison ability for CO_ads, indicating that the introduction of Ni and effective morphological control were beneficial to enhance the electroactivity and durability of Rh nanomaterials for MOR.

  RESULTS AND DISCUSSION

  Herein, Rh₁Ni₁ alloy nanodendrite assemblies (Rh₁Ni₁-NDs-As) were synthesized by wet chemical method (see Experimental for synthesis details in ESI). For comparison, monometallic Rh nanodendrites (Rh-NDs) were synthesized in the absence of NiCl₂ (Figure S1 in ESI). The crystalline structure of Rh₁Ni₁-NDs-As was investigated by powder X-ray diffraction (XRD). Generally, the transition metals are easily alloyed with noble metals by co-reduction method.^{[39, 40]} XRD pattern of Rh₁Ni₁-NDs-As shows the diffraction peaks locate exactly between the standard Rh crystal (JCPDS 05-0685) and Ni crystal (JCPDS 45-1027), suggesting the formation of RhNi alloy (Figure 1A).^[41] The Rh/Ni ratio was texted by energy-dispersive X-ray (EDX) measurement (Figure 1B). EDX data reveal that Rh/Ni ratio is 51:49, close to the result from inductively coupled plasma atomic emission spectroscopy (53:47). The X-ray photoelectron spectroscopy (XPS) measurement also distinctly reveals the co-existence of Rh, Ni and O elements (Figure S2 in ESI). Rh 3d XPS spectrum shows the two pairs of strong Rh 3d_5/2 and Rh 3d_3/2 double peaks (Figure 1C). The peaks at 311.6 and 306.8 eV can be assigned to Rh⁰ species. There are two small peaks at 313.55 and 308.25 eV, which can be attributed to Rh^III species. Compared with the Rh 3d_5/2 binding energy of Rh-NDs (Figure S3 in ESI), the negative shift of Rh 3d_5/2 binding energy at Rh₁Ni₁-NDs-As is ca. 0.2 eV. The negative shift of binding energy is attributed to electron hybridization in d-band between Rh and Ni because of the electron transfer from Ni to Rh.^[42] In Ni 2p XPS spectrum, the peaks at 870.2 and 852.45 eV correspond to metallic Ni⁰ species (Figure 1D).The other peaks as well as relevant satellite peaks suggest the existence of oxidized Ni species, originating from the air oxidation.^{[24, 43]}

  Figure 1

  

  Figure 1.  (A) XRD pattern, (B) EDX spectrum, (C) Rh 3d XPS spectrum, and (D) Ni 2p XPS spectrum of Rh₁Ni₁-NDs-As.

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  To explore the fine structure of Rh₁Ni₁-NDs-As, transmission electron microscopy (TEM) measurement was carried out. TEM image shows that the spherical NDs subunit on chain-like Rh₁Ni₁-NDs-As is composed of small nanoparticles (Figure 2A). Meanwhile, there is abundant channel on the edge (Figure 2B), which is conducive to increasing the contact area between the catalyst and electrolyte.^{[44, 45]} Scan TEM (STEM)-EDX maps reveal that the distribution of Rh and Ni elements on Rh₁Ni₁-NDs-As is similar, confirming the formation of RhNi alloy, again (Figure 2C). Theoretically, the atomic radius of Ni is smaller than that of Rh, which can result in the decrease of lattice spacing after the RhNi alloy formation. High-resolution TEM (HRTEM) image clearly displays the lattice spacing value is 0.215 nm, which is smaller than 0.220 nm for the Rh (111) plane spacing (Figure 2D). In other words, Rh₁Ni₁-NDs-As has alloy characteristic, consistent with the XRD result.

  Figure 2

  

  Figure 2.  (AB) TEM images, (C) STEM-EDX-maps, and (D) HRTEM image of Rh₁Ni₁-NDs-As.

  DownLoad: Full-Size Img PowerPoint

  Since the electrocatalytic performance of catalysts greatly depends on their chemical composition, ^{[46, 47]} bimetallic RhNi catalysts with different Rh/Ni ratios were also prepared by the same wet chemical method. According to Rh/Ni atomic ratio (Figure S4 in ESI) and subsequent morphological characterization (Figure 3), these catalysts are named Rh₁Ni₂ alloy nanodendrite assemblies (Rh₁Ni₂-NDs-As) and Rh₂Ni₁ alloy nanodendrites (Rh₂Ni₁-NDs), respectively. Scanning electron microscopy (SEM) images (Figure 3A–C) and TEM images (Figure 3D–E) clearly show that monodisperse RhNi nanodendrites can evolve into one-dimensional nanodendrite assemblies with the increase of Ni^II feed. This fact indicates that the morphology of bimetallic RhNi catalysis can be selectively controlled by simply adjusting the Rh^III/Ni^II feed ratio.

  Figure 3

  

  Figure 3.  SEM images of (A) Rh₁Ni₂-NDs-As, (B) Rh₁Ni₁-NDs-As, and (C) Rh₂Ni₁-NDs; TEM images of (D) Rh₁Ni₂-NDs-As, (E) Rh₁Ni₁-NDs-As, and (F) Rh₂Ni₁-NDs.

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  Cyclic voltammetry (CV) tests were first performed to investigate the electrochemical property of bimetallic RhNi catalysts with different Rh/Ni atomic ratio in N₂-saturated 0.5 M H₂SO₄ electrolyte. CV curves show that the hydrogen region area of three catalysts is very similar (Figure 4A). According to H-desorption peak areas in CV curves, the electrochemical active areas (ECSA) of Rh₂Ni₁-NDs, Rh₁Ni₁-NDs-As, and Rh₁Ni₂-NDs-As are calculated to be 31.80, 39.11 and 29.84 m²·g^-1, respectively (Figure 4B). The MOR electroactivity of bimetallic RhNi catalysts was investigated in 1 M KOH + 0.5 M methanol electrolyte by CV. Rh mass-normalized CV curves reveal that all RhNi catalysts have obvious electroactivity for MOR (Figure 4C). Among them, the MOR peak current at Rh₁Ni₁-NDs-As (174.31 A·g^-1) is higher than that at Rh₂Ni₁-NDs (111.75 A·g^-1) and Rh₁Ni₂-NDs-As (81.98 A·g^-1), which indicates that the introduction of appropriate Ni content will improve the electrocatalytic performance to some extent and overdose can lead to the decrease of electroactivity.^{[17, 48]} Likely, the low active Ni gradually covers the Rh active sites, which decreases the MOR electroactivity when the Ni content is very high. Further ECSA-normalized CV curves also show that Rh₁Ni₁-NDs-As have the highest intrinsic electroactivity for MOR (Figure 4D), indicating the synergistic effect between Rh and Ni plays an important role for MOR electroactivity enhancement. XPS measurement has confirmed the strong interaction between Rh and Ni because Rh atoms with high electronegativity (2.28) pull electrons from neighbouring Ni atoms with low electronegativity (1.98). The previous work has demonstrated that the electron-rich Rh is beneficial for the rate-determining step of MOR.^[49] So far, we reasonably conclude that the formation of RhNi alloy promotes the intrinsic MOR activity.

  Figure 4

  

  Figure 4.  (A) CV curves of Rh₁Ni₂-NDs-As, Rh₁Ni₁-NDs-As, and Rh₂Ni₁-NDs in the N₂-saturated 0.5 M H₂SO₄ electrolyte. (B) ECSA values Rh₁Ni₂-NDs-As, Rh₁Ni₁-NDs-As, and Rh₂Ni₁-NDs. (C) Rh mass-normalized CV curves and (D) ECSA-normalized CV curves of Rh₁Ni₂-NDs-As, Rh₁Ni₁-NDs-As, and Rh₂Ni₁-NDs in 1 M KOH with 0.5 M methanol electrolyte at 50 mV·s^-1.

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  Since Rh₁Ni₁-NDs-As reveals the highest MOR electroactivity, we further compare its activity with commercial Rh nanoparticle (Rh-NPs-C, Figure S6) for potential application prospects. To understand morphology effect, the MOR electroactivity of Rh-NDs was investigated by CV. According to CV curves (Figure S7), ECSA values of Rh₁Ni₁-NDs-As, Rh-NDs and Rh-NPs-C are 39.26, 36.74 and 35.62 m²·g^-1, respectively. Both Rh mass-normalized CV curves (Figure 5A) and ECSA-normalized CV curves (Figure 5B) show that the order of electroactivity is Rh₁Ni₁-NDs-As > Rh-NDs > Rh-NPs-C, indicating dendric morphology, Ni-doping, and high ECSA contribute to high electroactivity of Rh₁Ni₁-NDs-As for MOR.

  Figure 5

  

  Figure 5.  (A) Rh mass-normalized CV curves and (B) ECSA-normalized CV curves of Rh₁Ni₁-NDs-As, Rh-NDs and Rh-NPs-C in 1 M KOH + 0.5 M methanol solution at 50 mV·s^-1. (C) Chronoamperometric curves of Rh₁Ni₁-NDs-As, Rh-NDs and Rh-NPs-C in 1 M KOH + 0.5 M methanol solution at 0.6 V potential. (D) CO-stripping CV curves of Rh₁Ni₁-NDs-As, Rh-NDs and Rh-NPs-C.

  DownLoad: Full-Size Img PowerPoint

  The durability of Rh₁Ni₁-NDs-As, Rh-NDs and Rh-NPs-C is evaluated by chronoamperometry tests (Figure 5C). During running, MOR current at Rh₁Ni₁-NDs-As is higher than that at Rh-NDs and Rh-NPs-C, also suggesting that Rh₁Ni₁-NDs-As has the highest MOR activity. At 6000 s, MOR currents at Rh₁Ni₁-NDs-As, Rh-NDs and Rh-NPs-C decrease to 24.7%, 12.6% and 12.4% of their initial MOR current values, respectively, which demonstrates that Rh₁Ni₁-NDs-As has the best durability for MOR. The anti-poisoning ability of electrocatalyst is always an important factor for the durability. The onset and peak oxidation potential of adsorbed CO (CO_ads) can be used to evaluate the anti-poisoning ability. The lower potential represents the better CO_ads oxidation ability and the higher anti-poisoning ability. The anti-poison capability of three catalysts was investigated by CO-stripping test (Figure 5D). The oxidation peak potentials of CO at Rh₁Ni₁-NDs-As is more negative than that of Rh-NDs and Rh-NPs-C (Rh₁Ni₁-NDs-As: 0.57 V; Rh-NDs: 0.60 V; Rh-NPs-C: 0.62 V). Meanwhile, the onset oxidation potential of CO at Rh₁Ni₁-NDs-As is lower than that at Rh-NDs and Rh-NPs-C (Rh₁Ni₁-NDs-As: 0.38 V; Rh-NDs: 0.47 V; Rh-NPs-C: 0.46 V). This fact indicates that Rh₁Ni₁-NDs-As has the best anti-poison capability due to the introduction of Ni, which is beneficial to improve the durability of catalyst for MOR. The high electrocatalytic durability of Rh₁Ni₁-NDs-As can also be ascribed to their high self-stability. On one hand, Rh₁Ni₁-NDs-As is inserted in the alkaline solution, resulting in high stability in the KOH solution. On the other hand, the three-dimensional structure formed by crossed two-dimensional nanosheets can effectively suppresses Ostwald ripening, which endows the Rh₁Ni₁-NDs-As enhanced structure stability. After durability test, SEM image and EDX data show that the one-dimensional structure and chemical composition still basically maintain (Figure S8). Thus, the struc-tural stability of Rh₁Ni₁-NDs-As also contributes to its excellent durability for MOR.

  CONCLUSION

  In summary, a series of bimetallic RhNi nanomaterials with different Rh/Ni atomic ratios were synthesized by a simple wet chemical method. The monodisperse RhNi nanodendrites could evolve into one-dimensional nanodendrite assemblies with the increase of Ni^II feed, indicating that the morphology of bimetallic RhNi nanostructures could be selectively controlled by simply adjusting the Rh^III/Ni^II feed ratio. Electrochemical measurements revealed that Rh₁Ni₁-NDs-As behaved the highest electroactivity for MOR, originating form dendric morphology, Ni-doping, and high ECSA. In addition, Rh₁Ni₁-NDs-As also have excellent durability for MOR, originating form high anti-poison ability for CO_ads due to bifunctional mechanism. This work revealed a new synthesis strategy for one-dimensional Rh-based bimetallic nanostructures and indicated that one-dimensional RhNi nanodendrite assemblies could serve as active and stable anodic catalyst for DMFCs.

  EXPERIMENTAL

  Reagents and Chemicals. Rhodium acetate (Rh(OAc)₂, ≥98%), polyvinyl pyrrolidone (PVP) with 58000 molecular weight (≥95%), nickel chloride hexahydrate (NiCl₂·6H₂O, ≥98%), potassium hydroxide (KOH, ≥85%), hydrazine hydrate (N₂H₄·H₂O, ≥85%), sodium borohydride (NaBH₄, ≥85%), and methanol (CH₃OH, ≥99.5%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).

  Preparation of Bimetallic RhNi Catalysts. In a typical synthesis, 2.0 mL of 0.05 M NiCl₂·6H₂O aqueous solution, 2.0 mL of 0.05 M Rh(OAc)₂ aqueous solution, and 20 mL of 0.1 M PVP aqueous solution were mixed and stirred for 30 min. After heating mixture to 60 ℃, 5.0 mL of 0.2 M N₂H₄·H₂O aqueous solution were added into mixture and stirred for 8 h. Then, the obtained Rh₁Ni₁ alloy nanodendrite assemblies (Rh₁Ni₁-NDs-As) were separated by centrifugation and washed with 0.1 M acetic acid solution and deionized water for three times, alternately. Finally, Rh₁Ni₁-NDs-As was dried at 50 ℃ for 8 h. Using same synthetic method, RhNi nanostructures with different metal ratios were also synthesized by simply adjusting the Rh^III/Ni^II feed ratio.

  Physical Characterization and Electrochemical Measurements. The experimental details about physical characterization and electrochemical measurements were provided in Supplementary information.

  
  ACKNOWLEDGEMENTS: This research was sponsored by the National Natural Science Foundation of China (21875133), Natural Science Foundation of Shaanxi Province (2020JZ-23), and Key Research and Develop-ment Program of Shaanxi (2020SF-355). COMPETING INTERESTS
  The authors declare no competing interests.
  ADDITIONAL INFORMATION
  For submission: https://mc03.manuscriptcentral.com/cjsc
  Supplementary information is available for this paper at http://manu30.magtech.com.cn/jghx/EN/10.14102/j.cnki.0254-5861.2022-0019
  
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  Figure 1  (A) XRD pattern, (B) EDX spectrum, (C) Rh 3d XPS spectrum, and (D) Ni 2p XPS spectrum of Rh₁Ni₁-NDs-As.

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  Figure 2  (AB) TEM images, (C) STEM-EDX-maps, and (D) HRTEM image of Rh₁Ni₁-NDs-As.

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  Figure 3  SEM images of (A) Rh₁Ni₂-NDs-As, (B) Rh₁Ni₁-NDs-As, and (C) Rh₂Ni₁-NDs; TEM images of (D) Rh₁Ni₂-NDs-As, (E) Rh₁Ni₁-NDs-As, and (F) Rh₂Ni₁-NDs.

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  Figure 4  (A) CV curves of Rh₁Ni₂-NDs-As, Rh₁Ni₁-NDs-As, and Rh₂Ni₁-NDs in the N₂-saturated 0.5 M H₂SO₄ electrolyte. (B) ECSA values Rh₁Ni₂-NDs-As, Rh₁Ni₁-NDs-As, and Rh₂Ni₁-NDs. (C) Rh mass-normalized CV curves and (D) ECSA-normalized CV curves of Rh₁Ni₂-NDs-As, Rh₁Ni₁-NDs-As, and Rh₂Ni₁-NDs in 1 M KOH with 0.5 M methanol electrolyte at 50 mV·s^-1.

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  Figure 5  (A) Rh mass-normalized CV curves and (B) ECSA-normalized CV curves of Rh₁Ni₁-NDs-As, Rh-NDs and Rh-NPs-C in 1 M KOH + 0.5 M methanol solution at 50 mV·s^-1. (C) Chronoamperometric curves of Rh₁Ni₁-NDs-As, Rh-NDs and Rh-NPs-C in 1 M KOH + 0.5 M methanol solution at 0.6 V potential. (D) CO-stripping CV curves of Rh₁Ni₁-NDs-As, Rh-NDs and Rh-NPs-C.

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