Shape-control of one-dimensional PtNi nanostructures as efficient electrocatalysts for alcohol electrooxidation

Fei Gao a, Yangping Zhang a, Pingping Song a, Jin Wang a, Bo Yan a, Qiwen Sun a, Lei Li *b, Xing Zhu c and Yukou Du *a
aCollege of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, P.R. China. E-mail: duyk@suda.edu.cn; Fax: +86-512-6588089; Tel: +86-512-65880089
bCollege of Biological, Chemical Sciences and Engineering, Jiaxing University, Jiaxing, Zhejiang 314001, China. E-mail: leili@mail.zjxu.edu.cn
cTesting & Analysis Center, Soochow University, Suzhou 215123, P.R. China

Received 7th December 2018 , Accepted 16th February 2019

First published on 18th February 2019


Abstract

Bimetallic one-dimensional (1D) nanostructures such as nanowires (NWs) and nanorods (NRs), serving as high-efficiency anode electrocatalysts, have attracted extensive attention in the past decade. However, the precise design and synthesis of 1D Pt-based nanocrystals with tunable morphology and size still remain an arduous challenge. Driven by this, we report a facile yet efficient strategy for the first time to prepare PtNi ultrafine NWs (UNWs), sinuous NWs (SNWs) and ultrashort NRs (UNRs) by adjusting the amount of citric acid, ascorbic acid and glucose. Detailed analysis of their electrocatalytic properties has indicated that the as-obtained PtNi SNWs exhibit the most outstanding electrocatalytic activity toward ethylene glycol oxidation reaction (EGOR) and glycerol oxidation (GOR), 4.5 and 4.3 times higher in mass activity as well as 4.3 and 3.9 times higher in specific activity compared with the commercial Pt/C catalyst. The as-prepared PtNi SNWs are also more stable than the commercial Pt/C catalyst after successive durability tests. The proposed method provides insight into more rational designs of bimetallic nanocatalysts with 1D architectures and the as-synthesized PtNi catalysts with improved electrocatalytic performance assist in promoting the further development of direct alcohol fuel cells (DAFCs).


Introduction

The great focus on the rapid consumption of fossil fuels, sharp depletion in energy resources, and increasingly serious environmental problems has prompted researchers to develop efficient sustainable energy devices.1–4 Direct alcohol fuel cells (DAFCs), which have emerged as effective devices, owing to their independence from traditional energy, less carbon emission, and wider supply chains, have increasingly become a research hotspot.5,6 Among those liquid fuels, ethylene glycol (EG) and glycerol, as hopeful fuel substitutes, with properties of high energy density, cost-effectiveness, lower catalyst toxicity, better environmental benignity etc., have shown much more potential to be energy carriers in energy-storage devices including liquid fuel cells.7,8 In fuel cells such as DAFCs, anode catalysts are one highly important part. Noble metal Pt is commonly considered as the most effective electrocatalyst.9–12 However, for Pt, its scarce natural distribution, high cost and easy-to-poison nature by some intermediate species have limited its large-scale application.13,14 In order to reduce the amount and optimize the practical utilization of precious Pt, alloying Pt with some first-row transition metals and tailoring the morphology and size have been the mostly-used approaches.15–19

More recently, bimetallic one-dimensional (1D) nanostructures like nanowires (NWs) and nanorods (NRs) have been developed as a novel series of catalysts with the advantages of large interfacial areas, better electron conductivity, and superior flexibility, which benefit the improvement of electrochemical performance in alcohol oxidation reactions.20–24 Regardless of the huge progress achieved, previously-synthesized 1D nanomaterial electrocatalysts (NWs and NRs) still have drawbacks such as a too smooth surface structure, uneven size, and uncontrollable shapes, limiting further enhancements in electrochemical activity and stability in catalysis to some extent.25–28 Driven by these issues, the rational design of 1D nanocatalysts with large exposed active areas, abundant defects, and uniform structure is highly desirable.29–32

Herein, if we could combine the advantages of tunable morphology, alloy, and 1D structures into the newly-prepared nanomaterials, it may be reasonable and beneficial for the boost of electrochemical behavior. We thus adopted a one-pot solvothermal approach to synthesize a series of ultrathin PtNi NWs and NRs with tunable shape by precisely controlling the introduction of ascorbic acid, citric acid and glucose for the first time. The experimental observations indicated that the as-designed optimized Pt3Ni sinuous NWs could promote the electrochemical properties towards GOR and EGOR with mass activities of 4250.0 mA mg−1 and 4889.5 mA mg−1. As a GOR and EGOR catalyst, the PtNi NWs also exhibited excellent durability with mass loss and still retained 52.2% and 48.5% of their initial activity in electrocatalysis after 250 successive CV cycles, which exceed the performance of commercial Pt/C catalysts and even many previously-reported catalysts to the best of our knowledge.33–35

Results and discussion

The morphology and structure of the as-obtained catalysts were firstly characterized by TEM. In Fig. 1a, the typical TEM image shows Pt3Ni UNWs displaying an ultrafine and elongated structure. The surfaces of the Pt3Ni UNWs are relatively smooth and have few bumps (Fig. 1d). In addition, the Pt3Ni UNWs have a small average width of 1.57 nm and long length of 79.8 nm, forming a high aspect ratio of 50.8 (Fig. S1a and d, ESI). The chemical states of Pt and Ni were measured by XPS analysis (Fig. S2a); zero-valent Pt mostly exists in the Pt3Ni UNWs with small quantities of oxidized Pt (Fig. S2b). For physical characterization of the Pt3Ni SNWs, as shown in Fig. 1b and e, Pt3Ni SNWs are the main products with a high yield. The Pt3Ni SNWs display an uneven sigmate structure and wave border with the average width of 3.74 nm, length of 72.4 nm, and aspect ratio of 19.4 (Fig. S1b and d), which can supply abundant active sites. XPS analysis was also carried out to investigate the chemical valences in the Pt3Ni SNWs. As can be seen in Fig. S2c and S2d, metallic Pt and Ni are the main products. Furthermore, the peak located at 71.6 eV in the Pt 4f spectrum shows a slight shift to a higher binding energy compared with pure Pt, indicating the electron transfer between Ni and Pt in the PtNi alloy phase. In the representative TEM images of the Pt3Ni UNRs (Fig. 2c and f), interestingly, abundant uneven rod-like particles of about 27.4 nm in length and 6.81 nm in width could easily be observed (Fig. S1c and S1f), which have an almost 100% yield and rough surfaces (Fig. 2d). The aspect ratio is 4.0, which can be described as a nanorod structure. In Fig. S2e, the elemental valences in the products were also investigated. The XPS spectra exhibit that Pt is mainly in the zerovalent state on the surface of the nanocatalysts. There is a slight shift to a higher binding energy, implying the formation in a PtNi alloy phase due to the charge transfer from Pt to Ni (Fig. S2f). EDX was employed to quantitatively analyze the content of Pt and Ni in the Pt3Ni UNWs, SNWs, and UNRs and the overall atomic ratio of Pt/Ni is 74.4/25.6, 75.5/24.5, and 74.8/25.2, respectively, consistent with the initial feed ratio (Fig. S3).
image file: c8nr09892a-f1.tif
Fig. 1 Representative TEM images of (a and d) Pt3Ni UNWs, (b and e) Pt3Ni SNWs, and (c and f) Pt3Ni UNRs.

image file: c8nr09892a-f2.tif
Fig. 2 HRTEM images of (a) Pt3Ni UNWs, (b) Pt3Ni SNWs, and (c) Pt3Ni UNRs, EDX elemental mapping images of (d) Pt3Ni UNWs, (e) Pt3Ni SNWs, and (f) Pt3Ni UNRs, line-scan analysis of (g) Pt3Ni UNWs and (h) Pt3Ni SNWs, and (i) XRD patterns of as-prepared catalysts.

To thoroughly investigate the crystal structures of the as-obtained nanocatalysts, high resolution TEM (HRTEM) images of Pt3Ni UNWs, SNWs, and UNRs were obtained and are shown in Fig. 2a–c, where a lattice spacing of 0.22 nm can be ascribed to the (111) facet of face-centered cubic (fcc) Pt3Ni alloy. EDX mapping images were collected as supplementary evidence to reveal the elemental distribution of the as-prepared catalysts. In the Pt3Ni UNW, SNW, and UNR catalysts (Fig. 2d–f), Pt and Ni atoms are dispersed uniformly throughout the selected area, and the results of the line-scan analysis (Fig. 2g and h) further demonstrate the formation of intermetallic PtNi alloy. The powder XRD patterns, displayed in Fig. 2i, were recorded to investigate the alloyed structure and crystal lattice of the Pt3Ni UNWs, SNWs, and UNRs. The 2θ values of the diffraction peaks in the patterns are around 40.4, 47.2, 68.8, and 82.6, which can be assigned to the (111), (200), (220), and (311) planes, respectively. With the introduction of metallic Ni, the 2θ values attributed to Pt (111) shift to a positive degree compared with pure Pt (39.7°, JCPDS No. 04-0802), which indicates lattice contraction in PtNi alloy phase.36

As shown in Scheme 1, the control of reaction parameters results in diverse morphologies of Pt3Ni nanocrystals. For the investigation of formation mechanism of Pt3Ni SNWs, Pt3Ni UNWs, and Pt3Ni UNRs, the resulting Pt3Ni NWs formed under different reaction conditions were analyzed. The products cannot be reduced under the same reaction conditions as those of the Pt3Ni SNWs and UNRs without the introduction of W(CO)6. Aggregated bulk forms could be observed in the resultant product formed under the same synthetic conditions as those of Pt3Ni UNWs in the absence of W(CO)6 (Fig. S4), which reveals that W(CO)6 is the main reductant and structure-directing agent in the synthesis. In Fig. S5, the products obtained using the same reaction parameters as those of the Pt3Ni SNWs, UNWs, and UNRs in the absence of CTAC are also shown and they formed varisized nanoparticles (Fig. S5a and S5b), rough and accumulated NWs (Fig. S5c and S5d), and big bumps (Fig. S5e and S5f), respectively. The results reveal that CTAC plays an important role in the determination and optimization of the morphologies of Pt3Ni SNWs, UNWs, and UNRs. For a more detailed comparison, the products obtained with the same reaction conditions as those of the Pt3Ni nanocrystals but without the addition of ODE show a partial morphology of the Pt3Ni SNWs, UNWs, and UNRs, but they did not form uniform structures (Fig. S6). The addition of ODE also helps to modulate the uniformity of Pt3Ni nanocrystals (SNWs, UNWs, and UNRs). Considering the effect of glucose (Fig. S7), products obtained with the same reaction conditions as those of the Pt3Ni UNRs but without the addition of glucose hardly form a single uniform morphology while some NPs still exist in a minority, which indicates that glucose plays the key role in stabilizing and guiding the NP self-assembly into NWs and NRs. In addition, AA and CA could serve as structure-directing agents and capping agents for adjusting the fine structure of the as-prepared catalysts.37–40 Moreover, the co-introduction of W(CO)6 and CTAC has a significant impact on the general control of Pt3Ni SNW, Pt3Ni UNW, and Pt3Ni UNR formation.


image file: c8nr09892a-s1.tif
Scheme 1 Schematic illustration of the synthetic route to Pt3Ni nanocrystals with tunable morphologies.

To the best of our knowledge, as-prepared catalysts are usually loaded on carbon species for further electrochemical tests.41–43 In this work, carbon black (Vulcan XC72R carbon, C), with the properties of better electrical conductivity and high surface area, could make the as-tested catalysts disperse well and prevent nanoparticles from conglomerating, which is conducive to boosting electrocatalytic behavior.44–46 For comparison, the catalytic activity of the not C loaded Pt3Ni SNWs, Pt3Ni UNWs, Pt3Ni UNRs and commercial Pt/C catalysts was investigated for GOR and EGOR (Fig. S8). The no C loaded catalysts showed an obviously lower mass activity than that of the C loaded catalysts, and the corresponding mass activity values can be seen in Table S1. The effect of the KOH concentration on the catalytic performances was also investigated (Fig. S9). The as-prepared carbon loaded Pt3Ni SNWs, Pt3Ni UNWs, and Pt3Ni UNRs were evaluated as electrocatalysts towards GOR and EGOR (Fig. S10). The CVs, as shown in Fig. 3a, of the PtNi NWs and NRs were recorded in 1 M KOH with a scanning rate of 50 mV s−1. The electrochemically active surface areas (ECSAs) of the Pt3Ni SNW, Pt3Ni UNW, Pt3Ni UNR, and commercial Pt/C catalysts were measured using the charge formed in the process of hydrogen adsorption and desorption, which can be calculated by the following equation: ECSA = Q/(0.21 × m), where m represents the mass of Pt loading on the surface of the GCE, and 0.21 mC cm−2 is related to the monolayer adsorption of hydrogen on Pt.47 For the Pt3Ni SNW, Pt3Ni UNW, Pt3Ni UNR, and commercial Pt/C catalysts, their ECSA values are 58.3 m2 g−1, 52.5 m2 g−1, 45.7 m2 g−1, and 49.7 m2 g−1, respectively (Fig. 3b). Moreover, the mass activity of the Pt3Ni SNWs, Pt3Ni UNWs, Pt3Ni UNRs, and Pt/C for GOR was studied by CV in the corresponding solution. The forward peaks at −0.15 V correspond to the oxidation of glycerol molecules in the positive scan while the reverse ones were associated with the intermediate species formed during the GOR process in Fig. 3c.48,49 The mass activity of the Pt3Ni SNWs is 4250.0 mA mg−1, which is 1.14 times higher than that of the Pt3Ni UNWs (3721.5 mA mg−1), 1.41 times higher than that of the Pt3Ni UNRs (3022.5 mA mg−1), and 4.32 times higher than that of Pt/C (983.5 mA mg−1), respectively. From Fig. 3d, the specific activity of the series of PtNi catalysts can also be observed and it obeys the following order: Pt3Ni SNWs (7.3 mA cm−2) > Pt3Ni UNWs (7.1 mA cm−2) > Pt3Ni UNRs (6.6 mA cm−2) > Pt/C (2.0 mA cm−2), further confirming that the Pt3Ni SNWs exhibit the highest mass activity and specific activity, even higher than those of formerly-reported Pt-based catalysts (Table S2). For the mechanism of GOR, CO32− is the main product.50,51 The possible reaction evolution processes are shown in the following equations:

 
CH2OH–CHOH–CH2OH → (CH2OH–CHOH–CH2OH)ads(1)
 
image file: c8nr09892a-t1.tif(2)
 
4OH → 4OHads + e(3)
 
(CHO)CHOH(CHO) + 16OH → 3CO32− + 10H2O + 10e(4)


image file: c8nr09892a-f3.tif
Fig. 3 (a) CV curves of Pt3Ni SNWs, Pt3Ni UNWs, Pt3Ni UNRs and commercial Pt/C catalysts conducted in 1 M KOH with a scanning rate of 50 mV s−1. (b) The calculated ECSA values of the four catalysts. (c) CV curves of the four catalysts operated in 1 M KOH and 1 M glycerol with a scanning rate of 50 mV s−1. (d) The calculated mass activities of the four catalysts. (e) Durability comparison of the four catalysts towards GOR for 250 successive CV cycles. (f) The normalized current density and retained mass activity of the as-tested catalysts after 250 cycles.

Additionally, stability is another key parameter to estimate the electrochemical behaviors of catalysts.52 As can be seen in Fig. 3e and f, 250 successive CV cycles were conducted to calculate the normalized current density and retained mass activity of the Pt3Ni SNW, Pt3Ni UNW, Pt3Ni UNR, and Pt/C electrocatalysts. After 250 cycles, the Pt3Ni SNWs still retain 52.2% of their initial mass activity, while the normalized current densities of the Pt3Ni UNWs, Pt3Ni UNRs and Pt/C are 49.6%, 42.7%, and 13.1%, respectively, which is in step with the trend of retained mass activity. The retained mass activity of the Pt3Ni SNWs, Pt3Ni UNWs, Pt3Ni UNRs, and Pt/C is 2218.5 mA mg−1, 1846.0 mA mg−1, 1290.5 mA mg−1, and 129.0 mA mg−1. In addition, the results of the CA curves recorded in 1.0 M KOH + 1.0 M glycerol solution manifest the least decay of the Pt3Ni SNWs with a retained mass activity of 424.0 mA mg−1, which is higher than that of the Pt3Ni UNW (218.2 mA mg−1), Pt3Ni UNR (88.0 mA mg−1), and Pt/C (20.9 mA mg−1) catalysts (Fig. S11), implying the excellent durability of Pt3Ni SNWs towards GOR.

The electrochemical properties of the as-prepared catalysts were also investigated towards ethylene glycol oxidation. The CVs of the above four catalysts recorded in 1 M KOH and 1 M EG solution at a sweep rate of 50 mV s−1 are shown in Fig. 4a. For one thing, the peaks appearing in the forward scan of all tested catalysts can be assigned to the oxidation of EG molecules, and for another, the much weaker peaks located in the reverse scan can be attributed to the removal of some partially oxidized carbonaceous species formed in the forward scan.53,54 As can be seen in Fig. 4b, the Pt3Ni SNWs display the best mass activity of 4889.5 mA mg−1, which is around 1.15, 1.43, and 4.42-fold higher than that of the Pt3Ni UNWs (4256.0 mA mg−1), Pt3Ni UNRs (3429.5 mA mg−1), and commercial Pt/C (1106.0 mA mg−1), respectively. The specific activities also obey the same order as mass activity: Pt3Ni SNWs (8.6 mA cm−2) > Pt3Ni UNWs (8.0 mA cm−2) > Pt3Ni UNRs (7.5 mA cm−2) > Pt/C (2.3 mA cm−2). The above results demonstrate that the as-synthesized Pt3Ni SNWs exhibit improved electrochemical activity for EGOR that is even much better than that of previously-synthesized Pt-based catalysts (Table S3).


image file: c8nr09892a-f4.tif
Fig. 4 (a) CV curves of Pt3Ni SNW, Pt3Ni UNW, Pt3Ni UNR and commercial Pt/C catalysts recorded in 1 M KOH and 1 M EG with a scanning rate of 50 mV s−1. (b) The calculated mass activity and specific activity of the four catalysts. (c) Current–time (It) curves for EGOR at −0.15 V for 3600 s. (d) The normalized current density and retained mass activity of these four catalysts after 3600 s. (e) Durability comparison of the four catalysts towards EGOR for 250 successive CV cycles. (f) The normalized current density and retained mass activity of these four catalysts after 250 cycles.

Similar to the mechanism for GOR, the EG molecule can be oxidized to nontoxic C2O42−, and the probable reaction process could be concluded as follows:55,56

 
(CH2–OH)2 → (CH2–OH)2ads(5)
 
(CH2–OH)2ads → CH2OH(CHO)(6)
 
CH2OH(CHO) → CHO(COO)(7)
 
CHO(COO) → (COO)2(8)

The EGOR durability of the different catalysts was evaluated as well. As Fig. 4c shows, the CA measurements of the Pt3Ni SNW, Pt3Ni UNW, Pt3Ni UNR, and Pt/C catalysts were carried out at the potential of −0.15 V for 3600 s. The four curves all exhibit obvious decay at the beginning stage during the EGOR. After 1 h, the Pt3Ni SNWs show the best durability with a retained mass activity of 605.2 mA mg−1, which is higher than that of the Pt3Ni UNWs (408.3 mA mg−1), Pt3Ni UNRs (205.4 mA mg−1), and Pt/C (28.9 mA mg−1). The results of the normalized current also show the same trend as the retained mass activity (Fig. 4d); the Pt3Ni SNWs still exhibit the least decay for EGOR, 12.1% of the initial catalytic activity, making them much more stable than the Pt3Ni UNWs (9.6%), Pt3Ni UNRs (6.0%), and Pt/C (2.6%). The durability of the as-prepared catalysts towards EGOR was also analyzed by recording 250 successive CV scans. From Fig. 4e, it can be seen that the Pt3Ni SNWs can retain 48.5% of their initial catalytic activity, which is much higher than that of the Pt3Ni UNWs (41.6%), Pt3Ni UNRs (31.8%), and Pt/C (11.3%). Moreover, as shown in Fig. 4f, the retained mass activity of the four tested catalysts obeys the following order: Pt3Ni SNWs (2419.5 mA mg−1) > Pt3Ni UNWs (1770.5 mA mg−1) > Pt3Ni UNRs (1090.5 mA mg−1) > Pt/C (125.0 mA mg−1). As can be seen in Fig. S12, a slight change of the morphology and structure occurred in some of the Pt3Ni SNW, Pt3Ni UNW, and Pt3Ni UNR catalysts, while the most well-defined features were well retained after the electrochemical measurements, indicating their superior durability. For a general investigation of alcohol oxidation reactions, electrochemical measurements of the Pt3Ni SNW, Pt3Ni UNW, Pt3Ni UNR and commercial Pt/C catalysts for methanol oxidation reaction (MOR) and ethanol oxidation reaction (EOR) were also carried out (Fig. S13). The mass activity order of the as-prepared catalysts for MOR/EOR was as follows: Pt3Ni SNWs (3404.0/3096.5 mA mg−1) > Pt3Ni UNWs (2621.0/2378.5 mA mg−1) > Pt3Ni UNRs (2058.5/1986.5 mA mg−1) > Pt/C (871.5/942.5 mA mg−1). Through the above-mentioned results, conclusions can be reached that the as-synthesized Pt3Ni SNWs display both enhanced electrochemical activity and stability towards alcohol oxidation reactions.

Conclusions

In conclusion, a facile yet effective method has been developed as the first example to precisely synthesize a set of Pt3Ni nanocrystals with controllable structure (SNWs, UNWs, and UNRs). This approach sheds light on the synthesis of many other 1D nanomaterials. The as-prepared Pt3Ni nanocatalysts combine a unique 1D structure, electronic effects, and abundant active sites, which enable them to show enhanced electrochemical activity and stability in liquid fuel electrooxidation. The structure-optimized Pt3Ni SNWs exhibit values of 4250.0 mA mg−1 and 4889.5 mA mg−1 in mass activity, and 7.3 mA cm−2 and 8.6 mA cm−2 in specific activity for GOR and EGOR, respectively. Furthermore, the Pt3Ni SNW catalyst is more stable than the Pt3Ni UNW, Pt3Ni UNR, and commercial Pt/C catalysts. In view of this, the series of Pt3Ni nanocatalysts provides promising candidates among high-efficiency electrocatalysts in alcohol oxidation reactions.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 51873136 and 21503092), Natural Science Foundation of Zhejiang Province (LY19B030005), National Natural Science Foundation of Jiangsu Province (BK20181428), the Suzhou Industry (SYG201636), the Project of Scientific and Technologic Infrastructure of Suzhou (SZS201708), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

Notes and references

  1. R. Rizo, R. M. Aran-Ais, E. Padgett, D. A. Muller, M. J. Lazaro, J. Solla-Gullon, J. M. Feliu, E. Pastor and H. D. Abruna, J. Am. Chem. Soc., 2018, 140, 3791–3797 CrossRef CAS PubMed.
  2. L.-X. Dai, X.-Y. Wang, S.-S. Yang, T. Zhang, P.-J. Ren, J.-Y. Ye, B. Nan, X.-D. Wen, Z.-Y. Zhou, R. Si, C.-H. Yan and Y.-W. Zhang, J. Mater. Chem. A, 2018, 6, 11270–11280 RSC.
  3. S. Y. Ma, H. H. Li, B. C. Hu, X. Cheng, Q. Q. Fu and S. H. Yu, J. Am. Chem. Soc., 2017, 139, 5890–5895 CrossRef CAS PubMed.
  4. X. L. Chen, L. Zhang, J. J. Feng, W. Wang, P. X. Yuan, D. M. Han and A. J. Wang, J. Colloid Interface Sci., 2018, 530, 394–402 CrossRef CAS PubMed.
  5. J. Bai, X. Xiao, Y. Y. Xue, J. X. Jiang, J. H. Zeng, X. F. Li and Y. Chen, ACS Appl. Mater. Interfaces, 2018, 10, 19755–19763 CrossRef CAS PubMed.
  6. J.-X. Tang, Q.-S. Chen, L.-X. You, H.-G. Liao, S.-G. Sun, S.-G. Zhou, Z.-N. Xu, Y.-M. Chen and G.-C. Guo, J. Mater. Chem. A, 2018, 6, 2327–2336 RSC.
  7. W. Du, G. Yang, E. Wong, N. A. Deskins, A. I. Frenkel, D. Su and X. Teng, J. Am. Chem. Soc., 2014, 136, 10862–10865 CrossRef CAS PubMed.
  8. C. Zhu, S. Guo and S. Dong, Adv. Mater., 2012, 24, 2326–2331 CrossRef CAS PubMed.
  9. K. Jiang, Q. Shao, D. Zhao, L. Bu, J. Guo and X. Huang, Adv. Funct. Mater., 2017, 27, 1700830 CrossRef.
  10. Y. Qin, M. Luo, Y. Sun, C. Li, B. Huang, Y. Yang, Y. Li, L. Wang and S. Guo, ACS Catal., 2018, 5581–5590,  DOI:10.1021/acscatal.7b04406.
  11. J. Pei, J. Mao, X. Liang, Z. Zhuang, C. Chen, Q. Peng, D. Wang and Y. Li, ACS Sustainable Chem. Eng., 2017, 6, 77–81 CrossRef.
  12. M. Roca-Ayats, O. Guillén-Villafuerte, G. García, M. Soler-Vicedo, E. Pastor and M. V. Martínez-Huerta, Appl. Catal., B, 2018, 237, 382–391 CrossRef CAS.
  13. H. J. Kim, S. M. Choi, S. Green, G. A. Tompsett, S. H. Lee, G. W. Huber and W. B. Kim, Appl. Catal., B, 2011, 101, 366–375 CrossRef CAS.
  14. J. K. Yoo, M. Choi, S. Yang, B. Shong, H.-S. Chung, Y. Sohn and C. K. Rhee, Electrochim. Acta, 2018, 273, 307–317 CrossRef CAS.
  15. W. Zhou, M. Li, L. Zhang and S. H. Chan, Electrochim. Acta, 2014, 123, 233–239 CrossRef CAS.
  16. P. Song, H. Xu, B. Yan, J. Wang, F. Gao, Y. Zhang, Y. Shiraishi and Y. Du, Inorg. Chem. Front., 2018, 5, 1174–1179 RSC.
  17. W. Zhang, Q. Dong, H. Lu, B. Hu, Y. Xie and G. Yu, J. Alloys Compd., 2017, 727, 475–483 CrossRef CAS.
  18. V. K. Ocampo-Restrepo, A. Calderón-Cárdenas and W. H. Lizcano-Valbuena, Electrochim. Acta, 2017, 246, 475–483 CrossRef CAS.
  19. Y. Hao, X. Wang, Y. Zheng, J. Shen, J. Yuan, A.-j. Wang, L. Niu and S. Huang, Int. J. Hydrogen Energy, 2016, 41, 9303–9311 CrossRef CAS.
  20. X. Zhang, J. Zhang, H. Huang, Q. Jiang and Y. Wu, Electrochim. Acta, 2017, 258, 919–926 CrossRef CAS.
  21. C. Alegre, M. Gálvez, R. Moliner and M. Lázaro, Catalysts, 2015, 5, 392–405 CrossRef CAS.
  22. Z. Qi, C. Xiao, C. Liu, T. W. Goh, L. Zhou, R. Maligal-Ganesh, Y. Pei, X. Li, L. A. Curtiss and W. Huang, J. Am. Chem. Soc., 2017, 139, 4762–4768 CrossRef CAS PubMed.
  23. S. Zhao, H. Yin, L. Du, G. Yin, Z. Tang and S. Liu, J. Mater. Chem. A, 2014, 2, 3719 RSC.
  24. N. Zhang, L. Bu, S. Guo, J. Guo and X. Huang, Nano Lett., 2016, 16, 5037–5043 CrossRef CAS PubMed.
  25. L. Huang, X. Zhang, Q. Wang, Y. Han, Y. Fang and S. Dong, J. Am. Chem. Soc., 2018, 140, 1142–1147 CrossRef CAS PubMed.
  26. F. Wu, J. Lai, L. Zhang, W. Niu, B. Lou, R. Luque and G. Xu, Nanoscale, 2018, 10, 9369–9375 RSC.
  27. M. Farsadrooh, J. Torrero, L. Pascual, M. A. Peña, M. Retuerto and S. Rojas, Appl. Catal., B, 2018, 237, 866–875 CrossRef CAS.
  28. K. Wang, H. Du, R. Sriphathoorat and P. K. Shen, Adv. Mater., 2018, e1804074,  DOI:10.1002/adma.201804074.
  29. Y. Zhou and Y. Shen, Electrochem. Commun., 2018, 90, 106–110 CrossRef CAS.
  30. N. Zhang, S. Guo, X. Zhu, J. Guo and X. Huang, Chem. Mater., 2016, 28, 4447–4452 CrossRef CAS.
  31. L. Huang, Y. Han, X. Zhang, Y. Fang and S. Dong, Nanoscale, 2017, 9, 201–207 RSC.
  32. S. Bai, B. Huang, Q. Shao and X. Huang, ACS Appl. Mater. Interfaces, 2018, 10, 22257–22263 CrossRef CAS PubMed.
  33. X. Weng, Q. Liu, A. J. Wang, J. Yuan and J. J. Feng, J. Colloid Interface Sci., 2017, 494, 15–21 CrossRef CAS PubMed.
  34. C. Du, X. Gao, Z. Zhuang, C. Cheng, F. Zheng, X. Li and W. Chen, Electrochim. Acta, 2017, 238, 263–268 CrossRef CAS.
  35. W. Hong, C. Shang, J. Wang and E. Wang, Energy Environ. Sci., 2015, 8, 2910–2915 RSC.
  36. N. Zhang, Y. Zhu, Q. Shao, X. Zhu and X. Huang, J. Mater. Chem. A, 2017, 5, 18977–18983 RSC.
  37. L. Wang, C. Hu, Y. Nemoto, Y. Tateyama and Y. Yamauchi, Cryst. Growth Des., 2010, 10, 3454–3460 CrossRef CAS.
  38. M. Liu, S. He, W. Chen and K. Jiang, Electrochim. Acta, 2016, 199, 218–226 CrossRef CAS.
  39. Y. Gong, X. Liu, Y. Gong, D. Wu, B. Xu, L. Bi, L. Y. Zhang and X. S. Zhao, J. Colloid Interface Sci., 2018, 530, 189–195 CrossRef CAS PubMed.
  40. Y. Zhang, H. Xu, F. Gao, P. Song, B. Yan, J. Wang, C. Wang and Y. Du, J. Taiwan Inst. Chem. Eng., 2018, 91, 405–412 CrossRef CAS.
  41. Z. Wang, X. Ren, Y. Luo, L. Wang, G. Cui, F. Xie, H. Wang, Y. Xie and X. Sun, Nanoscale, 2018, 10, 12302–12307 RSC.
  42. Y. Ji, L. Yang, X. Ren, G. Cui, X. Xiong and X. Sun, ACS Sustainable Chem. Eng., 2018, 6, 9555–9559 CrossRef CAS.
  43. Y. Zhang, F. Gao, P. Song, J. Wang, J. Guo, Y. Shiraishi and Y. Du, ACS Sustainable Chem. Eng., 2019, 7, 3176–3184 CrossRef CAS.
  44. J. Zhao, X. Li, G. Cui and X. Sun, Chem. Commun., 2018, 54, 5462–5465 RSC.
  45. W. Wang, W. Jing, F. Wang, S. Liu, X. Liu and Z. Lei, J. Power Sources, 2018, 399, 357–362 CrossRef CAS.
  46. Y. Feng, L. Bu, S. Guo, J. Guo and X. Huang, Small, 2016, 12, 4464–4470 CrossRef CAS PubMed.
  47. F. Gao, H. Xu, Y. Zhang, J. Wang, C. Wang and Y. Du, Int. J. Hydrogen Energy, 2018, 43, 9644–9651 CrossRef CAS.
  48. Y. Kang, W. Wang, Y. Pu, J. Li, D. Chai and Z. Lei, Chem. Eng. J., 2017, 308, 419–427 CrossRef CAS.
  49. F. Gao, Y. Zhang, P. Song, J. Wang, C. Wang, J. Guo and Y. Du, J. Power Sources, 2019, 418, 186–192 CrossRef CAS.
  50. H. Xu, P. Song, C. Fernandez, J. Wang, M. Zhu, Y. Shiraishi and Y. Du, ACS Appl. Mater. Interfaces, 2018, 10, 12659–12665 CrossRef CAS PubMed.
  51. J. Qi, N. Benipal, C. Liang and W. Li, Appl. Catal., B, 2016, 199, 494–503 CrossRef CAS.
  52. G. L. Caneppele, T. S. Almeida, C. R. Zanata, É. Teixeira-Neto, P. S. Fernández, G. A. Camara and C. A. Martins, Appl. Catal., B, 2017, 200, 114–120 CrossRef CAS.
  53. Z.-Z. Yang, L. Liu, A.-J. Wang, J. Yuan, J.-J. Feng and Q.-Q. Xu, Int. J. Hydrogen Energy, 2017, 42, 2034–2044 CrossRef CAS.
  54. P. Song, X. Cui, Q. Shao, Y. Feng, X. Zhu and X. Huang, J. Mater. Chem. A, 2017, 5, 24626–24630 RSC.
  55. S. S. Chen, Z. Z. Yang, A. J. Wang, K. M. Fang and J. J. Feng, J. Colloid Interface Sci., 2018, 509, 10–17 CrossRef CAS PubMed.
  56. L. An and R. Chen, J. Power Sources, 2016, 329, 484–501 CrossRef CAS.

Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c8nr09892a

This journal is © The Royal Society of Chemistry 2019