Pt/C catalysts containing CeO2 with different morphologies for the hydrogen evolution reaction

Pt/C catalysts containing four different morphologies of CeO2 as co-catalysts were synthesized in this work, and their electrocatalytic performance for hydrogen evolution reaction (HER) was investigated. As compared with the Pt/C catalyst, these four catalysts containing CeO2 all exhibited improved catalytic activity. Among them, the Pt/C catalyst containing spherical CeO2 with a diameter of 30 ~ 60 nm (Pt/C-CeO2(s2)) possesses the best catalytic activity, displaying an over-potential of 258 mV at 10 mA cm−2 and a Tafel slope of 42 mV dec−1. According to the characterization results of structure, morphology, and elemental valence state, the enhancement of catalytic activity is ascribed to the small particle size and good dispersion degree of Pt, as well as the strong interaction between the exposed (111) crystal plane of small spherical CeO2 and Pt, which leads to a significant increase in metallic Pt content. Moreover, the Pt/C-CeO2(s2) catalyst also demonstrates outstanding long-term stability besides exceptional catalytic activity. The results clearly illustrate that CeO2 with diverse shapes and sizes can remarkably influence the catalytic performance of loaded Pt particles in the HER process.


Introduction
The massive development and utilization of fossil fuels have brought a series of environmental problems, and thus, a sustainable and green energy source is in demand urgently [1,2].As a kind of energy carrier with the advantages of wide sources and pollution-free products, hydrogen has received much attention [3,4].Water electrolysis is believed to be the most promising technology for obtaining high-purity hydrogen, which is safe and efficient and has no pollutant emissions [5][6][7].Overall, it involves two half-cell reactions, namely, the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) [8].However, the slow kinetics of the OER and HER necessitates a large cell voltage, which requires a large electrical input and incurs a high cost.This impedes the commercialization of water electrolysis technology for hydrogen production [9].Therefore, it is necessary and urgent to develop highly active and stable catalysts for the OER and HER.It is well known that the noble metal Pt is considered the most efficient catalyst for HER due to its low over-potential and enhanced electrocatalytic stability [10,11].Nevertheless, the scarcity and high cost of Pt limit its practicality [12].Recently, various non-noble metal electrocatalysts have been extensively studied for HER, such as molybdenum-based [6,9,13,14], iron-based [15,16], cobalt-based [17,18], and nickel-based catalysts [19,20].However, although the performance of these nonnoble metal electrocatalysts has been significantly improved, the catalytic activity of the vast majority of them is still obviously lower than that of Pt/C.Cerium oxide (ceria, CeO 2 ) is one of the most abundant rare-earth oxides, and it plays an important role as a carrier of an active phase or as a co-catalyst in many catalytic processes [21][22][23][24][25][26][27].In our previous work [28], it was found that the addition of CeO 2 into Pd/C catalysts could remarkably improve the catalytic performance for formic acid electrooxidation in terms of current density, peak potential, and kinetics.At present, the majority of research about CeO 2 in the field of water electrolysis has focused on the impact of CeO 2 -loaded metal in an alkaline medium for HER or OER [29][30][31], and there is still a lack of research specifically investigating the use of CeO 2 for HER in acidic medium.In addition, it was reported that strong metal-oxide interactions exist between CeO 2 and Pt, which can modulate the intrinsic electronic structure of Pt and thus enhance the catalytic performance for CO oxidation in a H 2 -rich gas [32].Moreover, different shapes of CeO 2 may have different effects on the interaction between CeO 2 and Pt [33], as different shapes can lead to different exposed crystal planes.This indicates that the co-catalytic effect of CeO 2 is somewhat related to its shape.Additionally, the size of CeO 2 particles is another factor that influences the catalytic activity of Pt particles supported by them.Vayssilov et al. confirmed that the electronic structure of Pt atoms was radically changed after being dispersed on nano-CeO 2 [34].Hence, it can be seen that the morphology (including shape and size) of CeO 2 has an important influence on its interaction with metals and catalytic performance.However, to our knowledge, the application of CeO 2 with different morphologies as co-catalysts to retouch the Pt/C catalyst for HER has not been reported.In this work, the Pt/C catalysts modified with four different morphologies of CeO 2 were synthesized, and their catalytic performance for HER was investigated.These four catalysts all exhibited better catalytic performance than Pt/C, and the effects of CeO 2 with different morphologies were analyzed in detail.The results are helpful in understanding the role of metal-oxide interactions in the electrocatalytic performance of Pt-based catalysts.Furthermore, the improvement of Pt/C catalyst performance also has important economic significance for the commercialization of hydrogen production by water electrolysis.

Synthesis of carbon black-supported Pt-CeO 2 with different morphologies
To synthesize the catalysts, the carbon black (Vulcan XC-72) was firstly dispersed into deionized water (or water-ethanol binary mixtures), and then, the mixture was sonicated for 15 min.Then, cerium nitrate hexahydrate and polyvinylpyrrolidone (PVP) were added into the suspension while stirring thoroughly for 0.5 h.The suspension was subjected to hydrothermal heating at 180 °C for 30 h.Subsequently, a certain amount of potassium chloroplatinate and trisodium citrate was put into the above mixture.After that, an excess of freshly prepared sodium borohydride solution was added dropwise and stirred for 6 h at room temperature.Finally, the resulting products were filtered, washed, and dried in an oven at 60 °C to obtain carbon black-supported Pt catalysts with spherical CeO 2 .By changing PVP in the raw materials into sodium hydroxide and trisodium citrate, carbon black-supported Pt catalysts with cubic CeO 2 (or irregular polyhedral CeO 2 ) were prepared.The theoretical loadings of Pt and CeO 2 in these Pt/C-CeO 2 catalysts are 20 and 10 wt%, respectively.

Material characterization
X-ray diffraction (XRD, Smart Lab (9 kW), Cu Kα) was conducted to analyze the phase composition and crystal structure of the catalysts.The morphology of the materials was analyzed by transmission electron microscopy (TEM, TECNAIG 20) and high-resolution transmission electron microscopy (HRTEM, JEM2100F).X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi) was used to analyze the elemental species as well as valence information on the material surface.Conductivities of catalysts were obtained by a four-probe powder conductivity measuring instrument (ST2722).

Electrochemical measurement
The electrochemical measurements were performed using a three-electrode cell with an electrochemical workstation (CHI760D, Chenhua, Shanghai), and the electrolyte was 0.5 M H 2 SO 4 saturated with N 2 .A platinum electrode was used as the counter electrode, and a saturated calomel electrode was used as the reference electrode.To prepare a catalyst-modified work electrode, 4 mg catalyst was dispersed into 2 mL anhydrous ethanol, and then, 5 μL of the suspension was added to the surface of a glassy carbon electrode after ultrasonic dispersion, and finally, the catalysts were fixed with 2 μL of 0.5% Nafion after the ink was dried.All potentials were calibrated to the reversible hydrogen electrode: Linear sweep voltammetry (LSV) was tested from 0 to − 0.6 V in 0.5 M H 2 SO 4 solution with a scan rate of 50 mV s −1 after the activation process.The potentials obtained from LSV experiments were corrected for iR drop.The cyclic voltammograms (CVs) were measured from − 0.15 to − 0.03 V at different scan rates (from 20 to 200 mV s −1 , interval is 20 mV s −1 ) for obtaining the electrical double-layer capacitances (C dl ).The stability test was carried out by CV scanning for 1000 cycles with a scan rate of 50 mV s −1 .

Results and discussion
TEM and HRTEM images of Pt/C-CeO 2 with different morphologies are shown in Fig. 1, and it can be seen that these CeO 2 particles exhibit clear but various features.In Fig. 1(a 1 ), (b 1 ), CeO 2 is nearly spherical, while CeO 2 in Fig. 1(c 1 ), (d 1 ) has a cubic and irregular polyhedral shape (although a few particles present a rhombic hexahedron shape), respectively.This shape difference mainly stems from the difference in the raw materials of the synthesis process.Furthermore, it is obvious that the CeO 2 particle sizes in Fig. 1(a 1 ), (c 1 ) are markedly larger than those in Fig. 1(b 1 ), (d 1 ).Specifically, the diameters of CeO 2 spheres in Fig. 1(a 1 ), (b 1 ) are 225 ± 25 nm and 30 ~ 60 nm, respectively.Similarly, the edge lengths of CeO 2 cubes and irregular polyhedrons in Fig. 1(c 1 ), (d 1 ) are approximately 200 nm and 80 ~ 180 nm, respectively.Under the same raw materials and thus the similar shape, the differences of particle sizes result from different synthesis media.The large and small particles were synthesized in the deionized water and water-ethanol binary mixtures, respectively.In other words, with the increase of ethanol, the particle size of CeO 2 becomes smaller.It is inferred that when the volume ratio of ethanol-water rises, the solubility of the reactants decreases, resulting in a higher supersaturation.This elevated supersaturation affects the nucleus growth period, namely, causing it to be shortened.As a result, the size of CeO 2 becomes smaller [35].For the convenience of the statement, these catalysts in Fig. 1 As reported previously [33,35] and revealed by the HRTEM images (Fig. 1 220) planes (Fig. 1(d 2 )), which may be due to various shapes of CeO 2 in this sample.The decoration of Pt nanoparticles can be observed on carbon black and/or CeO 2 surfaces with a typical interplanar spacing of 0.23 nm, which is attributed to the (111) plane of Pt.
The XRD patterns in Fig. 2 demonstrate the successful synthesis of Pt/C-CeO 2 with different morphologies.According to our previous work [36], the Pt/C catalyst was also prepared for comparison, and its XRD pattern is displayed in Fig. 2, too.The peaks at 25° in all the The dispersion degree and the particle size distribution of Pt in the four catalysts containing CeO 2 can be clearly observed in Fig. 3. Specifically, Pt/C-CeO 2 (s 1 ), Pt/C-CeO 2 (s 2 ), and Pt/C-CeO 2 (c) exhibit more uniform dispersions (Fig. 3(a 1 )-(c 1 )), which are superior than Pt/C compared to our previous work [36], while Pt/C-CeO 2 (p) (Fig. 3(d 1 )) shows more severe agglomeration.It is well known that the dispersion of catalytic active sites plays a crucial role in improving the electrochemically active surface area and catalyst utilization.In addition, it is evident that the particle size distribution of Pt in these four catalysts (Fig. 3(a 2 )-(d 2 )) almost corresponds to the Pt particle sizes calculated from Scherrer's equation in the XRD patterns (Fig. 2).
Figure 4 displays the high-resolution XPS spectra of Pt 4f for Pt/C-CeO 2 with different morphologies and Pt/C.Taking the Pt/C-CeO 2 (s 2 ) as an example (Fig. 4a), on the fitting curves, the most intense peaks (70.60 and 73.87 eV) are the characteristics of Pt 0 , and the second and weaker doublet (71.25 and 74.68 eV) can be assigned to Pt 2+ [37][38][39][40].Furthermore, the Pt binding energy of all four catalysts containing CeO 2 demonstrates a negative shift compared to the Pt/C catalyst in Fig. 4a, b, suggesting an increase in the content of low-valence Pt species.The binding energy (B.E.) and relative intensity of Pt species in the five catalysts are exhibited in Table 1 by fitting calculation, which shows that the order of Pt 0 content is Pt/C-CeO 2 (s 2 ) > Pt/C-CeO 2 (s 1 ) > Pt/C-CeO 2 (c) > Pt/C-CeO 2 (p) > Pt/C. Figure 4c reveals the high-resolution XPS spectra of Ce 3d for Pt/C-CeO 2 with different morphologies.The Ce 3d spectra are divided into eight peaks [41].Among them, the peaks u and u′ located at 906.64 eV and 885.86 eV are attributed to Ce 3+ , and the other peaks are assigned to Ce 4+ .CeO 2 has a fluorite structure, where the cation valence state can be switched between Ce 3+ and Ce 4+ [42,43].Additionally, the relative intensity of Ce species in the four catalysts is listed in Table 2, which displays that the order of Ce 4+ content is Pt/C-CeO 2 (s 2 ) > Pt/C-CeO 2 (s 1 ) > Pt/C-CeO 2 (c) > Pt/C-CeO 2 (p).It is interestingly found that the order of Ce 4+ content is exactly the same as that of Pt 0 content.In other words, the more Ce 4+ , the more Pt 0 .For Pt/C, the content of Pt 0 is the lowest since it does not contain CeO 2 .This indicates the "metallization" effect of CeO 2 on Pt, which may be ascribed to the electrostatic interaction between CeO 2 and Pt.It is well known that Pt has a valence electron configuration of 4f 14 5d 9 6s 1 , which contains unoccupied orbits, but the valence electron configuration of Ce 3+ is 4f 1 , namely, betatopic configuration.Thus, the electrons of Ce 3+ may have a trend to transfer to Pt, leading to an increase of Pt 0 and Ce 4+ .This reveals the reason for the consistent order of Pt 0 and Ce 4+ content in these four catalysts containing CeO 2 .In addition, it has been reported that the (111) crystal plane is more likely to lose electrons than the ( 200) and ( 220) crystal planes in CeO 2 owing to different O-vacancy concentrations in different crystal planes [44].Therefore, Pt/C-CeO 2 (s 1 ) and Pt/C-CeO 2 (s 2 ) contain more Pt 0 than Pt/C-CeO 2 (c) and Pt/C-CeO 2 (p) because the CeO 2 in the former two is nearly spherical, mainly exposing the (111) crystal plane, according to TEM and HRTEM images (Fig. 1).Furthermore, the large discrepancy of CeO 2 particle size between Pt/C-CeO 2 (s 1 ) and Pt/C-CeO 2 (s 2 ) (Fig. 1(a 1 ), (b 1 )) results in significant differences in the Pt 0 content between them.For Pt/C-CeO 2 (s 2 ), a much smaller particle size of CeO 2 inevitably leads to a significant increase in the contact surface between Pt and CeO 2 , thereby strengthening their interaction and increasing the Pt 0 content.However, this phenomenon does not exist between Pt/C-CeO 2 (c) and Pt/C-CeO 2 (p) despite a smaller particle size of CeO 2 in the latter one because CeO 2 (200) plane is easier to lose electrons than CeO 2 (220) plane [44]   Pt/C are calculated, and the values are 52.9, 42.0, 59.5, 70.7, and 83.3 mV dec −1 , respectively (Fig. 5b).Undoubtedly, these four catalysts containing CeO 2 all demonstrate remarkable catalytic activity and intrinsic kinetics superior to Pt/C, and the Volmer-Heyrovsky mechanism indicates the rate-determining step [45].Besides, the catalytic activity of these five homemade catalysts is compared with that of other advanced HER electrocatalysts (containing rare earth substances or Pt/C) reported in the literature in a similar medium in Table 3.It is found that the catalytic activity of the Pt/C-CeO 2 (s 2 ) catalyst exceeds that of most catalysts to some extent.In addition, the intrinsic electrical conductivity of an electrocatalyst is also    Under the same Pt loadings of these catalysts, it is reasonable to attribute the differences in electrocatalytic activity to various CeO 2 morphologies.The excellent catalytic activity of Pt/C-CeO 2 (s 2 ) is mainly ascribed to the following three reasons.Firstly, the conductivity of Pt/C-CeO 2 (s 2 ) is the highest as previously described.Secondly, according to the results of TEM, XRD, and the Pt particle size distribution characterization, Pt/C-CeO 2 (s 2 ) possesses the smallest particle size and a better dispersion degree of Pt, which can provide more active sites and electrochemically active surface area.Moreover, the smaller particle size of CeO 2 also has more surfaces to contact with Pt.Thirdly, the catalytic activity of Pt for HER is related to the content of Pt 0 .It has been proven that the catalyst with a higher content of the metallic state exhibits superior catalytic performance [46].Metallic Pt contributes to the high catalytic activity for HER because the 5d orbital of Pt 0 appears to hybridize with the H 1 s orbital to form weak Pt-H valence bonds, leading to a ∆G H* (relative free energy) value of approximately zero eV for H* absorption based on the results of density functional theory calculations [47].As mentioned by XPS analyses, the interaction between Pt and CeO 2 in Pt/C-CeO 2 (s 2 ) is the strongest because the (111) crystal plane is exposed on the surface of small spherical CeO 2 particles, which results in a highest content of Pt 0 .In summary, the catalytic activity order of the four catalysts containing CeO 2 is Pt/C-CeO 2 (s 2 ) > Pt/C-CeO 2 (s 1 ) > Pt/C-CeO 2 (c) > Pt/C-CeO 2 (p), which is almost positively correlated with the dispersion degree and particle size distribution of Pt and the Pt 0 content mentioned above.
In addition to catalytic activity, stability is another crucial indicator of the catalyst performance.Figure 6 illustrates the LSV curves of Pt/C-CeO 2 (s 2 ), Pt/C-CeO 2 (p), and Pt/C before and after 1000 CV cycles.It is evident that the two LSV curves of Pt/C-CeO 2 (s 2 ) show minimal difference, indicating that this catalyst not only exhibits excellent catalytic activity but also possesses good long-term stability.However, the over-potential of Pt/C-CeO 2 (p) changes from 270 to 280 mV after 1000 CV cycles when the cathode current density is 10 mA cm −2 , which suggests a decline in stability.It is worth noting that the stability of both catalysts (Pt/C-CeO 2 (s 2 ) and Pt/C-CeO 2 (p)) is superior to that of Pt/C.This finding further emphasizes that the addition of CeO 2 also contributes to the improvement of the catalyst stability to some extent.

Conclusions
The study has successfully demonstrated a promising strategy of utilizing CeO 2 with a special morphology as a co-catalyst to enhance the electrocatalytic performance of Pt-based catalysts for the HER process.All four catalysts containing CeO 2 prepared in this study have exhibited superior performance compared to the Pt/C catalyst.XPS analyses revealed that the Pt 0 content in all four CeO 2 -containing catalysts was higher than that in Pt/C because CeO 2 has a "metallization" effect on Pt, and then, a high Pt 0 content results in ∆G H* value close to 0 eV, leading to the excellent catalytic activity for the HER.Among the catalysts investigated, the Pt/C catalyst containing spherical CeO 2 with a diameter of 30 ~ 60 nm (Pt/C-CeO 2 (s 2 )) possesses the best catalytic activity and outstanding long-term stability.This improvement of catalytic activity is attributed to the high conductivity, the small particle size, and the good dispersion degree of Pt, along with the strong interaction between the exposed (111) crystal plane of small spherical CeO 2 and Pt, which effectively modulates the electron structure of the electrocatalyst.This work not only highlights the significance of controlling the shape and size of CeO 2 to adjust the morphology, electronic structure, and thus catalytic performance of the catalysts but also contributes to a deeper understanding of Ptbased catalysts containing metal oxides as co-catalysts toward the HER process.
(a 2 )-(d 2 )), the (111) crystal plane is exposed by the spherical CeO 2 selectively, and the (200) crystal plane is exposed by the cubic CeO 2 selectively.However, Pt/C-CeO 2 (p) has different exposed crystal planes, such as (200) and (

Fig. 4 a
Fig. 4 a, b High-resolution XPS spectra of Pt 4f for Pt/C-CeO 2 with different morphologies and Pt/C.c High-resolution XPS spectra of Ce 3d for Pt/C-CeO 2 with different morphologies one of the crucial factors influencing the performance.The electrical conductivity of the Pt/C-CeO 2 (s 1 ), Pt/C-CeO 2 (s 2 ), Pt/C-CeO 2 (c), Pt/C-CeO 2 (p), and Pt/C catalysts was measured to be 1396, 1678, 1277, 1111, and 754 S m −1 , respectively.Obviously, Pt/C-CeO 2 (s 2 ) has the highest conductivity, which should contribute to its excellent catalytic activity.Additionally, the number of active sites on the catalyst was evaluated through C dl , which is illustrated in Fig.5c.Based on the fitting results, Pt/C-CeO 2 (s 2 ) has the highest C dl value, which is 5.03 mF cm −2 , indicating that this catalyst possesses the highest number of active sites and the largest electrochemical active surface area.However, the most serious agglomeration of Pt particles in Pt/C-CeO 2 (p) (Fig.3(d 4)) may be the reason why it has the lowest C dl value.

Fig. 5 a
Fig. 5 a LSV curves, b Tafel plots and c the C dl of Pt/C-CeO 2 with different morphologies and Pt/C

Fig. 6
Fig. 6 LSV curves before and after 1000 cycles of CVs for a Pt/C-CeO 2 (s 2 ), b Pt/C-CeO 2 (p) and c Pt/C Author contribution All authors contributed to the study conception and design.P.R. Qi, J. You, and Y. Wang wrote the main manuscript text.T. Qi also revised the main manuscript text.L.L. Tian prepared Figs.1 and 2.FundingThe authors are grateful for the financial support from the National Key Research and Development Program of China (2020YFC1909001) and the cooperation project between universities in Chongqing and institutes affiliated to the Chinese Academy of Sciences (Grant No. HZ2021013).

Table 1
Binding energy (B.E.) and relative intensity of Pt species in four Pt/C-CeO 2 with different morphologies as well as Pt/C

Table 2
Relative intensity of Ce species in four Pt/C-CeO 2 with dif-

Table 3
Comparison of over-potential and Tafel slope of five homemade catalysts in this work with those of other HER electrocatalysts reported in the literature in a similar medium