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Article

Intercalated PtCo Electrocatalyst of Vanadium Metal Oxide Increases Charge Density to Facilitate Hydrogen Evolution

by
Jingjing Zhang
1,
Wei Deng
1,*,
Yun Weng
2,
Jingxian Jiang
1,
Haifang Mao
1,
Wenqian Zhang
1,
Tiandong Lu
1,
Dewu Long
3 and
Fei Jiang
1,*
1
School of Chemical and Environmental Engineering, Shanghai Institute of Technology, Shanghai 201418, China
2
State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Textile, Donghua University, Shanghai 201620, China
3
Key Laboratory in Interfacial Physics and Technology, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China
*
Authors to whom correspondence should be addressed.
Molecules 2024, 29(7), 1518; https://doi.org/10.3390/molecules29071518
Submission received: 27 January 2024 / Revised: 16 March 2024 / Accepted: 18 March 2024 / Published: 28 March 2024
(This article belongs to the Special Issue Recent Progress in Nanomaterials in Electrochemistry)
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Review Reports Versions Notes

Abstract

:
Efforts to develop high-performance electrocatalysts for the hydrogen evolution reaction (HER) are of utmost importance in ensuring sustainable hydrogen production. The controllable fabrication of inexpensive, durable, and high-efficient HER catalysts still remains a great challenge. Herein, we introduce a universal strategy aiming to achieve rapid synthesis of highly active hydrogen evolution catalysts using a controllable hydrogen insertion method and solvothermal process. Hydrogen vanadium bronze HxV2O5 was obtained through controlling the ethanol reaction rate in the oxidization process of hydrogen peroxide. Subsequently, the intermetallic PtCoVO supported on two-dimensional graphitic carbon nitride (g-C3N4) nanosheets was prepared by a solvothermal method at the oil/water interface. In terms of HER performance, PtCoVO/g-C3N4 demonstrates superior characteristics compared to PtCo/g-C3N4 and PtCoV/g-C3N4. This superiority can be attributed to the notable influence of oxygen vacancies in HxV2O5 on the electrical properties of the catalyst. By adjusting the relative proportions of metal atoms in the PtCoVO/g-C3N4 nanomaterials, the PtCoVO/g-C3N4 nanocomposites show significant HER overpotential of η10 = 92 mV, a Tafel slope of 65.21 mV dec−1, and outstanding stability (a continuous test lasting 48 h). The nanoarchitecture of a g-C3N4-supported PtCoVO nanoalloy catalyst exhibits exceptional resistance to nanoparticle migration and corrosion, owing to the strong interaction between the metal nanoparticles and the g-C3N4 support. Pt, Co, and V simultaneous doping has been shown by Density Functional Theory (DFT) calculations to enhance the density of states (DOS) at the Fermi level. This augmentation leads to a higher charge density and a reduction in the adsorption energy of intermediates.

Graphical Abstract

1. Introduction

To resolve the global energy crisis, the desirable search for renewable energy sources to substitute traditional fossil energy has attracted wide attention all over the world, including metal–air batteries, fuel cells, and electrochemical water decomposition [1,2]. The clean, renewable, and abundant nature of hydrogen energy positions it as one of the most promising energy carriers [3,4]. Over the past dozen years, electrocatalysts including oxides and alloys [5,6], phosphide [7,8], nitrides [9,10], selenides [11], and carbides [12,13] have shown remarkable HER activity. However, high-efficient, low-cost electrochemical water splitting development still faces great challenges.
Traditional hydrogen production strategies cannot avoid the emission of carbon dioxide, while electrically driven water splitting technology is a promising technology for hydrogen production [14,15,16]. In the process of water electrolysis, concerning how to obtain high-energy density and pure hydrogen effectively and reduce the overpotential, electrocatalysts play a crucial role. Over the past few decades, platinum (Pt)-based noble metals have been considered benchmark HER electrocatalysts which exhibit extremely low overpotential [17]. In spite of the positive features of Pt-based electrocatalysts, the great number of inconveniences (hard to synthesize) and shortcomings (low abundance and high price) still impede their practical applications [18]. Consequently, it is particularly desirable to seek high-efficient and durable catalysts for water electrolysis to reduce noble Pt content and overpotential, thus improving the energy conversion efficiency [19].
In this study, we present a novel method utilizing hydrogen insertion vanadium powder (V) to manipulate the reaction rate of hydrogen peroxide (H2O2) in the presence of ethanol. By controlling the reaction rate through the presence of ethanol, we can successfully obtain hydrogen metal oxide bronzes (HVOs), such as hydrogen vanadium bronze. Moreover, the HVOs exhibit excellent dispersion and stability in solvents devoid of water, specifically alcoholic-based solvents. This characteristic is highly advantageous for enhancing device stability and facilitating processing. Previous reports have shown that coupling metal nanoparticles with carbon substrates can effectively improve electrocatalytic performance and enhance stability due to the effective surface corrosion inhibition and excellent electron transport properties of carbon materials [20]. Therefore, g-C3N4 is chosen as the substrate in this study. Subsequently, the intermetallic PtCoVO anchored on two-dimensional g-C3N4 nanosheets is prepared by a one-pot solvothermal process at the oil/water interface. The PtCoVO/g-C3N4 catalyst, prepared in the aforementioned manner, demonstrates exceptional performance in the HER, which can be attributed to the substantial impact of oxygen vacancies present in HVOs on the electrical properties of the catalyst, resulting in enhanced HER performance [21]. As anticipated, the PtCoVO/g-C3N4 catalyst demonstrates excellent performance in an alkaline HER, showcasing a low overpotential of η10 = 92 mV and a small Tafel slope of 65.21 mV dec−1. Additionally, the catalyst exhibits remarkable stability, comparable to the near state-of-the-art Pt/C 20% catalyst (90.6 mV, 10 mA cm−2). Detailed structural investigations reveal that the hydrogen insertion strategy modulates the effect of oxygen content in HVOs on electrical properties, which might result in a special V active center that enhances HER activity.

2. Results and Discussion

2.1. Morphologies and Structures

The tunable polymetallic nanocomposites PtCoVO/g-C3N4 were successfully synthesized through a solvothermal method, as illustrated in Figure 1, which utilized three kinds of metal precursors, cobalt acetylacetonate (Co(acac)3), H2PtCl6·6H2O, and vanadium bronze (here, we use HxV2O5 to represent HVOs), grown on two-dimensional g-C3N4 nanosheets. In particular, the unique low-temperature solution-processed hydrogen vanadium bronze HxV2O5 was synthesized through a regulable hydrogen insertion method. Under ambient temperature conditions, vanadium powder was gradually oxidized by H2O2 in the existence of ethanol. Through the carefully controlled, sluggish reaction rate facilitated by ethanol, the solution-processed hydrogen metal oxide bronze HxV2O5 was successfully obtained [22]. The HxV2O5 color changed from yellow to brown during the preparation process, as can be seen in Figure S1. Subsequently, the polymetallic PtCoVO supported on two-dimensional g-C3N4 nanosheets was synthesized by using a one-pot solvothermal method at the oil/water interface. The enlarged PtCoVO surface structure was selected as the computational model for the illustration of as-prepared PtCoVO/g-C3N4 electrocatalysts. For the purpose of comparison, our focus was directed toward three distinct materials with varying chemical compositions that were prepared in advance: PtCo/g-C3N4, PtCoV/g-C3N4, and PtCoVO/g-C3N4, respectively. We utilized V powder to directly prepare PtCoV/g-C3N4 to compare the different influences between V powder and HxV2O5 bronzes. (Here, we denote PtCo/g-C3N4 as prepared by adding no vanadium, PtCoV/g-C3N4 as prepared by adding V powder, and PtCoVO/g-C3N4 as prepared by HxV2O5) By adjusting the metal atoms, Pt, Co, and V, to appropriate molar ratio, the composite materials were meticulously optimized to achieve desired tunability in the elemental components. A detailed description of the synthesis process for composites with varying proportions is provided in the experimental section.

2.2. Electrocatalytic Enhancement Mechanism

In order to thoroughly investigate the mechanism behind the hydrogen evolution reaction (HER) enhancement and the improved properties of PtCoVO/g-C3N4, the crystal structures of PtCo/g-C3N4 and PtCoVO/g-C3N4 (illustration of Figure 2a) were constructed [23]. Owing to having the identical supporter on two-dimensional g-C3N4 nanosheets, we focused on PtCo and PtCoVO for simplified models. DFT studies were executed to survey the effect of PtCo and PtCoVO on the atomic-level electronic structure [24]. A potential electrocatalyst is characterized by favorable hydrogen adsorption free energy (ΔGH*), where a ΔGH* value close to zero can lead to enhanced hydrogen evolution activity due to an optimal equilibrium between the absorption and desorption of hydrogen atoms from the active site. The PtCo free energy change (ΔGH*) in the hydrogen adsorption value deviates significantly from the optimal value (≈0 eV) [25], that is, the PtCo surface is inactive in Figure 2a. However, the ΔGH* value of PtCoVO combined with an H atom with HMOs (hydrogen metal oxide bronzes) is much smaller, 0.18 eV, which is conducive to the hydrogen evolution reaction [26]. The structure diagram of PtCo (H*) and PtCoVO (H*) is shown in Figures S2 and S3. Likewise, the density of states (DOS) analysis demonstrates that the PtCoVO catalyst exhibits a significantly higher density of states in close proximity to the Fermi level compared to the other substrates [27], when compared with PtCo/g-C3N4 (Figure 2b,c). When HxV2O5 (HMOs) is added to frame, the DOS of PtCoVO is much higher than that PtCo, which indicates that the implanting of HxV2O5 significantly enhances the DOS. This indicates that V is the active site in PtCoVO. The high DOS indicates increased electron transfer, thereby accelerating the electrocatalytic reaction and resulting in a significant improvement in HER performance [28]. Figures S4–S9 also verify this conclusion. Additionally, the intensity of the DOS is strongly correlated with the conductivity [29]. The enhanced charge transfer dynamics of PtCoVO can be attributed to its higher DOS intensity, which aligns with the reduced charge transfer resistance. In addition, we can derive data from Figure S10 showing that the DOS of V in PtCoVO at the Fermi level is 2.511 eV, which is much higher than the DOS of PtCo, but close to that of PtCoVO. Therefore, we can ascertain that the introduction of V improves the DOS. The catalytic activity is closely related to the catalyst surface free energy of the hydrogen adsorption [30]. Moreover, the charge density difference model of PtCo and PtCoVO is depicted in Figure 2d,e. The charge density distribution of PtCo and PtCoVO indicate that doping with V atoms exacerbates the inhomogeneity of the distribution of electric charge. The HER activity of the catalyst is thus increased, so doping with V can introduce a large number of active catalytic sites, which has a crucial advantage for the successful release of hydrogen [31,32]. In addition, we also plot the molecular models of PtCoV and PtCoV (*), the band structure, and the PDOS (partial density of states), which can be shown in Figures S11–S14.
Figure 3a illustrates the scanning electron microscopy (SEM) image of the as-synthesized trimetallic PtCoVO nanoparticles decorated onto the g-C3N4 nanosheets. The corresponding energy dispersive X-ray (EDX) spectrum confirmed the uniform distribution of Co, Pt, V, C, N, and O atoms throughout the material (Figures S15 and S16). It appeared that Pt, Co, V, C, N, and O existed in PtCoVO/g-C3N4. Weight (Wt) ratios of PtCoVO/g-C3N4 elements were measured by EDX analysis. The Wt ratio of each element in the PtCoVO/g-C3N4 was 26.80:13.86:21.50:9.08:9.56:20.20 (C:N:O:Pt:Co:V). The precise mass ratio of each element in PtCoVO/g-C3N4 was determined using inductively coupled plasma emission spectrometry (ICP-AES), which was 6.43:7.02:15.95 (Pt:Co:V), consistent with the mass ratio data measured by EDX (Table S1). The incorporation of the PtCoV nanoalloy into the g-C3N4 nanosheets was further confirmed by the high-resolution transmission electron microscopy (HRTEM) image shown in Figure 3b, which exhibits a lattice fringe spacing of 0.226 nm. This lattice was identified as the (111) crystallographic plane of Pt, according to the Powder Diffraction File (PDF#04-0802) [33]. Meanwhile, structural information could be extracted by analyzing the fast Fourier transform (FFT) and inverse fast Fourier transform (IFFT) modes of the selected region for details of the structure [34] (Figure 3c,d). The lattice stripes observed in Figure 3e, Selected Area Electron Diffraction (SAED) images, indicate that they originated from the (111) and (200) crystallographic planes of Pt, as identified by the (PDF#04-0802). The FFT images shown in Figure 3f provide additional confirmation of the lattice spacing of approximately 2.265 nm (equivalent to 10 lattice distances) between the atomic layers of the exposed (111) facet. The use of High Angle Annular Dark Field Scanning Transmission Electron Microscopy (HAADF-STEM) confirmed the spatial distribution of various elements within the nanocomposite. The representative TEM EDS elemental mapping analysis depicted in Figure 3g demonstrates the uniform distribution of Pt, Co, V, C, N, and O elements throughout the structure [35], respectively. TEM image characterization of PtCoV/g-C3N4 is shown in Figures S17 and S18. The crystal structure of the materials was examined using X-ray diffraction (XRD) analysis, as represented in Figure 3h. The peaks of 39.76°, 46.24°, and 67.46° belong to the (111), (200), and (220) crystal faces of Pt (PDF#04-0802), respectively [36]. For PtCoVO/g-C3N4, the diffraction peak of g-C3N4 can be observed, which fully indicates that PtCoVO and g-C3N4 are formed between chemical bonds. In the XRD pattern, there were fewer diffraction peaks associated with Pt, which might be caused by the low content of Pt. Electron paramagnetic resonance (EPR) is a shortcut and highly sensitive technique for monitoring the presence of oxygen vacancies [37]. PtCoVO/g-C3N4 exhibited a greater EPR signal strength at g = 1.97 (g = hv/βH) than PtCo-C3N4 (Figure 3i). The superior value of g can be attributed to oxygen vacancies on the catalytic material surfaces [38], which indicates the existence of surface oxygen vacancies on PtCoVO/g-C3N4.
In order to investigate the alterations in the electronic structure and elemental valence of PtCoVO/g-C3N4 and PtCoV/g-C3N4, X-ray photoelectron spectroscopy (XPS) analysis was conducted. The findings are presented in Figure 4a–d. The full XPS spectrum indicates the presence of Co, Pt, V, O, C, and N elements in the PtCoVO/g-C3N4 and PtCoV/g-C3N4 samples, corroborating the results obtained from the EDS analysis [39]. Figure 4a illustrates the XPS analysis of PtCoVO/g-C3N4, revealing two prominent peaks at 71.4 eV and 74.88 eV, which correspond to metallic Pt 4f7/2 and 4f5/2, respectively. Additionally, the presence of other peaks at 71.9 eV and 75.3 eV in the PtCoVO/g-C3N4 spectrum confirms the existence of oxidized states of Pt [40]. Compared to PtCoV/g-C3N4, the binding energy of PtCoVO/g-C3N4 displays a minor negative shift, which is related to the charge density change in the core metal ions [41]. In the case of V 2p, the peaks observed at binding energies of 521.23 eV and 525.18 eV in the PtCoVO/g-C3N4 spectrum (Figure 4b) can be assigned to V 2p1/2 and V 2p3/2 peaks, while a peak at 517.48 eV corresponds to V 2p3/2 in PtCoVO/g-C3N4, respectively [42]. In Figure 4c, the Co 2p spectra of PtCoVO/g-C3N4 exhibit two prominent peaks at 781.7 eV and 796.8 eV, corresponding to Co 2p3/2 and Co 2p1/2, respectively. Additionally, two shakeup satellite peaks (labeled as Sat.) are observed in the Co 2p spectra of PtCoVO/g-C3N4, located at 786.9 eV and 803.1 eV, indicating characteristic peaks associated with Co2+ species [43]. It can be observed that there is a peak at 781.7 eV in PtCoVO/g-C3N4 that can be designated as a Co-N bond. To further explain the existence of this Co-N bond, we tested the N 1s XPS spectra of PtCoVO/g-C3N4 (Figure S19), which were divided into 399.6 and 400.7eV for g-C3N4, corresponding to Co-N and graphite N, respectively [44]. Figure 4d presents the high-resolution spectrum of O 1s. In the PtCoVO/g-C3N4 spectrum, the binding energy of the M-OH bond is measured to be 530.9 eV [45]. In comparison to PtCoV/g-C3N4, PtCoVO/g-C3N4 exhibits a minor shift in the O 1s peak, with a forward shift of 0.5 eV for one sample and a negative shift of 0.6 eV for another sample. This observation suggests that the variation in vanadium species in PtCoVO/g-C3N4 resulted in an increased number of active sites, potentially altering electron transfer processes and enhancing the catalytic performance of PtCoVO/g-C3N4.
Ultraviolet photoelectron spectroscopy (UPS) was employed to examine the energy band structures of the synthesized HxV2O5, PtCoVO/g-C3N4, and PtCoV/g-C3N4 samples [46]. As depicted in Figure 4e, the UPS results reveal a strong correlation between the doping of Pt and Co metals and the secondary electronic cutoff edge of the catalytic materials [47]. The work function (WF) of a semiconductor is the difference between the Fermi level (EF) and the energy level E0 of the electron at rest in a vacuum [48,49]. The WF of a material is typically around half the ionization energy of its corresponding metal-free atom. The WF can be computed using the formula provided in Note S1 [50]. The magnitude of the WF reflects the strength of electron binding within the metal. A smaller WF indicates that electrons are more likely to be released from the metal [51]. Pt possesses the highest WF of 5.65 eV. This high work function is advantageous in demonstrating the ease with which catalytic materials can be prepared by electronic means, resulting in superior electrochemical catalytic performance. As calculated from the UPS results, the PtCoVO/g-C3N4 presents an EF of 9.37 eV [52,53]. Compared with PtCo/g-C3N4 (WF = 5.2 eV) and PtCoV/g-C3N4 (WF = 4.83 eV), PtCoVO/g-C3N4 has the lowest WF = 4.42 eV. By examining the slope of the VB-XPS curve displayed in Figure 4f, we can observe the valence band maximum (VBM), denoted as EVB, of different materials. Specifically, HxV2O5, PtCo/g-C3N4, and PtCoVO/g-C3N4 have EVB values of 1.48 eV, −0.48 eV, and −1.20 eV, respectively. These data suggest that PtCoVO/g-C3N4 exhibits the most negative EVB value among the three materials, resulting in a higher Schottky barrier at the metal/interface for charge transport. Figures S20 and S21 display the UPS spectra of PtCoVO/g-C3N4 at various levels of g-C3N4 added, alongside the VB-XPS spectrum.

2.3. HER Electrocatalytic Performance Analyses

To assess the electrocatalytic characteristics of the prepared materials, the HER properties were examined using a conventional three-electrode system in a 1 M potassium hydroxide solution. This evaluation aimed to gain insights into the electrocatalytic performance of the polymetallic-doped materials. To facilitate comparison, a commercially available Pt/C 20% catalyst was utilized as a reference. To ensure consistency, all potentials were calibrated with respect to the reversible hydrogen electrode (RHE). This standardization allows for accurate and meaningful comparisons between the different catalysts. According to the polarization curves plotted in Figure 5a, Pt/C 20% has the lowest overpotential of η10 = 90.6 mV to drive a current density of 10 mA cm−2, as expected [54,55]; contrarily, the bimetallic material (PtCo/g-C3N4) demonstrates a higher overpotential, specifically η10 = 386 mV, which indicates poor activity in comparison. In our this work, all trimetallic catalysts performed better than bimetallic materials. The catalytic performance of PtCoVO/g-C3N4 was found to be exceptional, as evidenced by its significantly lower overpotential of η10 = 92 mV when compared to that of PtCoV/g-C3N4 (η10 = 346 mV). This remarkable improvement of 254 mV highlights the superior activity of PtCoVO/g-C3N4 as a catalyst.
However, this difference in activity between PtCoVO/g-C3N4 and PtCoV/g-C3N4 could be a result of the higher catalyst activity due to the insertion of HxV2O5 (Figure 5a). This result was further confirmed by the optimization of the PtCoVO/g-C3N4 element ratio as the HER activity exhibited an upward trend with increasing HxV2O5 content. Notably, the HER activity reached its maximum when the ratio of Pt:Co:V was 0.1:0.4:0.4 (Figure S22). In addition, we also explored the polarization curves of Pt prepared by adding vanadium powder in different proportions (Figure S23), which also confirmed that the overpotential is the lowest at a ratio from 0.1 to 0.4. Thus, the PtCo/g-C3N4 and PtCoV/g-C3N4 used for comparison were prepared in this ratio. The catalytic mechanism of the HER can also be revealed by Tafel plots [56,57]. In general, there are three classical reactions for H2 evolution: electrochemical hydrogen ion adsorption (Volmer reaction), chemical desorption (Tafel reaction), and electrochemical desorption (Heyrovsky reaction) [58,59]. The first step in the HER is always the Volmer reaction, but the next step is either the Heyrovsky reaction or the Tafel reaction [60]. The calculated Tafel slopes of all samples are given in Figure 5b. The PtCoVO/g-C3N4 material displays the smallest Tafel slope (65.21 mV dec−1) among all tested materials, excluding Pt/C 20%. This result confirms the faster kinetics of the PtCoVO/g-C3N4 catalyst, indicating its superior performance in facilitating the hydrogen evolution reaction. However, PtCoVO/g-C3N4 exhibits a lower Tafel slope, thus demonstrating a much faster and efficient process of the HER compared to other compositions [61]. The verification of this result is supported by the comparison of performance, Tafel slopes, and overpotentials of the PtCoVO/g-C3N4 catalyst with some previously reported electrocatalytic materials, as shown in Table S2. This comparison further confirms the superior performance and efficiency of PtCoVO/g-C3N4 in catalyzing the desired electrochemical reactions. Zhang et al. reported the good synergy between NiCo2O4 and C3N4, as many electrons tend to transfer from the hydrophobic region (NiCo2O4) to the electrophilic region (C3N4) and then to water, resulting in electron accumulation in water. DFT calculations shows that the EH2O* and ΔGH* of NiCo2O4@C3N4 are decreased [62]. In the study of Lu, the formed C3N4 layer with a large degree of graphitization can not only improve the conductivity of the catalyst, but can also prevent the corrosion of the active tungsten dioxide nanorods during the HER, exerting a protective effect [63]. Zhao et al. reported that CuSCN/C3N4 exhibited an overpotential of 85 mV, which is much higher than that of pure CuSCN. An interfacial N-Cu-S coordination mode is formed, which can promote electron penetration from the CuSCN crystal into the N atoms of the C3N4 shell, thereby improving the HER reactivity [64]. This indicates that these materials have a good synergistic effect with C3N4 (Table S3). By adjusting the surface structure of transition metal atoms, reasonable design can obtain low-cost and efficient electrocatalysts, which is the key to developing an electrocatalytic HER.
The Nyquist diagram, depicted in Figure 5c, was obtained through electrochemical impedance spectroscopy (EIS) measurements. The measured data align closely with the equivalent circuit model comprising charge transfer resistance (Rct), solution resistance (Rs), and a constant phase element (CPE), as illustrated in the insert of Figure 5c. The EIS results revealed that the Rct of the PtCoVO/g-C3N4 catalyst was significantly lower in the lower frequency region compared to that of PtCo/g-C3N4 and PtCoV/g-C3N4. This indicates that PtCoVO/g-C3N4 exhibited the highest electronic conductivity for the HER process, facilitating the fastest charge transfer rate and displaying the most sensitive electrocatalytic kinetics. Therefore, the resistance obtained from the EIS spectra of various materials shows that the PtCoVO/g-C3N4 catalyst has obvious lower impedance, which explains that the PtCoVO/g-C3N4 catalyst has faster reaction kinetics than the PtCo/g-C3N4 and PtCoV/g-C3N4 electrodes. In addition, the EIS spectra of different HxV2O5 contents proved the minimum impedance at a millimolar ratio of 0.1:0.4:0.4, as shown in Figure S24. Moreover, the EIS measurements showed that PtCoVO/g-C3N4 possess the smallest electron transfer resistance (the charge transfer resistance decreases as the arc size decreases). The trend observed in EIS for solid solutions is generally consistent with the trend of HER activity [65,66].
The double-layer capacitance (Cdl) of PtCo/g-C3N4, PtCoVO/g-C3N4, and PtCoV/g-C3N4 was evaluated through cyclic voltammetry (CV) measurements performed at different scan rates ranging from 0.02 to 0.1 V [67]. Figures S25–S28 provide the electrochemical double-layer capacitance of various materials. Based on the resulting Cdl values depicted in Figure 5d, PtCoVO/g-C3N4 has the highest value of all samples; this finding further validates the enhanced performance of PtCoVO/g-C3N4. The electrochemically active surface area (ECSA) of PtCo/g-C3N4 (28.95 cm2), PtCoVO/g-C3N4 (81.05 cm2), and PtCoV/g-C3N4 (42.88 cm2) was estimated by the Cdl.
The XPS spectra shown in Figure 6a–d provide clear evidence that the PtCoVO/g-C3N4 material maintains excellent stability after undergoing electrocatalysis, providing a possible solution to the problem of durability in industrial applications. This statement can also be proved by the information in Figure S29. The chronoamperometry curve for PtCoVO/g-C3N4 demonstrates that the current density of PtCoVO/g-C3N4 is maintained at a constant value, with a minimal decrease observed only after 48 h of continuous operation, indicating the exceptional long-term electrochemical stability of PtCoVO/g-C3N4 (Figure 6e) [68]. The TEM image, as shown in the inset of Figure 6e, confirms that the morphology of PtCoVO/g-C3N4 remained intact, with minimal changes. Furthermore, the long-term durability test of PtCoVO/g-C3N4 was evaluated by potential cycling in 1 M KOH. As shown in Figure 6f, no significant loss of activity occurred after 5000 cycles and the value remained overlapped with the previous data, highlighting the PtCoVO/g-C3N4 catalyst’s excellent electrocatalytic stability [69]. The stability of the HER electrocatalyst properties of PtCoVO/g-C3N4 can be attributed to the notable influence of oxygen vacancies in HxV2O5 on the electrical properties and optimized structural properties.

3. Experimental Methods

3.1. Materials

Cobalt acetylacetonate (C15H21CoO6, 98%), chloroplatinic acid hexahydrate (H2PtCl6·6H2O, 99%), oil amine (C18H37N, 90%), vanadium powder (V, 200 mesh, 99.999%), potassium hydroxide (KOH, 90%), absolute ethanol (EtOH, 99.7%), hydrogen peroxide (H2O2, 30%), and urea (CH4N2O, 99%) were used. The materials mentioned above were acquired from Adamas Reagent. The Nafion solution (5wt.%, containing 15–20% water) was purchased from Sigma-Aldrich (Burlington, MA, USA). The aforementioned materials were utilized directly without undergoing any purification. The deionized water employed in the experiments was obtained by passing it through an ultrapure purification device.

3.2. Methods

3.2.1. Fabrication Process for g-C3N4 Nanosheets

Bulk g-C3N4 was prepared by the thermal treatment of urea. In the typical fabrication process, 5.0 g of urea was ground into a powder and subjected to calcination at 550 °C for 3 h, and the heating rate during this process was maintained at 5 °C/min. After cooling, a yellow solid was obtained and ground into powder for 30 min until the yellow color of g-C3N4 changed into a light-yellow color.

3.2.2. Synthesis of HxV2O5 HMOs

Material Oxides Synthesis: A total of 0.5 g of vanadium powder was dispersed in 50 mL of ethanol; during the dispersion process, continuous stirring was maintained for several minutes. Next, the vanadium metal suspension solution was supplemented with 2.5 mL of H2O2 solution. Following a reaction period of 3 h, the vanadium oxide solution underwent a color change from orange to eventually brown. The solution was subsequently dried using a vacuum chamber and then ground for subsequent utilization.

3.2.3. Fabrication of PtCo/g-C3N4, PtCoV/g-C3N4, and PtCoVO/g-C3N4 Catalysts

(1)
Preparation process for PtCo/g-C3N4 materials
In a conical flask, 10 mL of water, 5 mL of EtOH, and 2 mL of oil amine were added. Then, 40 mg of g-C3N4, 142.5 mg of cobalt acetylacetonate, and 51.79 mg of chloroplatinic acid hexahydrate were weighed and introduced into the aforementioned solution. After subjecting the mixture to ultrasonic treatment for 60 min, it was transferred to a 25 mL reactor and maintained at 160 degrees Celsius for 16 h. The resulting solution was then allowed to cool naturally. When the temperature dropped down to room temperature, the product was obtained by centrifugation, washing, and drying. (Note: This material is named PtCo/g-C3N4 not because there is no oxygen element in the synthetic material, but in contrast with PtCoVO/g-C3N4, the original material of this catalyst is vanadium powder, whereas the original material of PtCoVO/g-C3N4 is HxV2O5, so the name distinguishes it. The same is true for PtCoV/g-C3N4).
(2)
Preparation of PtCoVO/g-C3N4 materials
The preparation method for this material is similar to that of PtCo/g-C3N4, with the addition of HxV2O5.
(3)
Preparation of PtCoV/g-C3N4 materials
The preparation method for this material is similar to that of PtCoVO/g-C3N4, with the difference being that vanadium powder is used instead of HxV2O5.

4. Conclusions

By controlling the reaction rate, hydrogen metal oxides bronzes (HMOs), namely, the hydrogen vanadium bronze (HxV2O5), were obtained. Its oxygen content has a significant effect on its electrical properties. Subsequently, a novel hybrid nanocomposite consisting of g-C3N4 doped with a multi-metal was synthesized using a solvothermal method. Compared with other catalysts, the obtained PtCoVO/g-C3N4 catalyst possesses the characteristics of close contact between non-metals and metals, remarkable activity, fast electron transport, easy release of bubbles generated, and exceptional electrocatalytic durability. This study of the design of high-efficiency catalysts for the HER provides a novel concept, detailing the theoretical analysis to guide the experiment and the experimental results to support the theoretical calculation, with the theory and experiment complementing each other. The present study introduces an efficient and robust catalyst for integrated catalysts, along with a scalable insertion preparation technique and design concept. These advancements open up possibilities for broader applications in the fields of electrocatalysis and material science.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/molecules29071518/s1. Refs. [63,64,65,70,71,72,73,74,75,76,77,78,79,80,81,82,83] are cited in Supplementary Materials.

Author Contributions

F.J. conceived the project and revised the manuscript. J.Z. synthesized the materials and analyzed the electrocatalytic performances. W.D. conducted the TEM results. Y.W. conducted the SEM results. J.J. analyzed the XPS data and the UPS data. H.M. conducted the electrocatalytic results. T.L. analyzed EPR data. D.L. conducted DFT calculations. W.Z. revised the XRD results. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article and the Supplementary Materials.

Acknowledgments

F.J. and W.D. thank the computing resources staff in the super-computational center in the Shanghai Institute of Applied Physics, Chinese Academy of Sciences. The authors would like to thank Lingyu Wan from Shiyanjia Lab (www.shiyanjia.com) (accessed on 18 March 2024) for the TEM analysis.

Conflicts of Interest

The authors declare no conflicts of interest.

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Molecules 29 01518 g001
Figure 1. Schematic illustration of the synthesis process of PtCoVO/g-C3N4 nanocomposites at the oil/water interface.
Figure 1. Schematic illustration of the synthesis process of PtCoVO/g-C3N4 nanocomposites at the oil/water interface.
Molecules 29 01518 g001
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Figure 2. (a) Free energy diagrams for the HER of PtCo and PtCoVO. The diagram depicts the molecular structure of PtCo and PtCoVO. (b,c) The calculated DOS of PtCoVO and PtCo. (d,e) The charge density distribution of PtCoVO and PtCo.
Figure 2. (a) Free energy diagrams for the HER of PtCo and PtCoVO. The diagram depicts the molecular structure of PtCo and PtCoVO. (b,c) The calculated DOS of PtCoVO and PtCo. (d,e) The charge density distribution of PtCoVO and PtCo.
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Figure 3. (a) SEM images of PtCoVO/g-C3N4. (b) TEM images of PtCoVO/g-C3N4. (c,d) FFT and IFFT pattern of PtCoVO/g-C3N4. (e) SAED images of PtCoVO/g-C3N4. (f) The PtCoVO/g-C3N4 spacing was 0.2265 nm. (g) HAADF of PtCoVO/g-C3N4 and elemental mapping of C, N, Pt, V, Co, and O, respectively. (h) XRD of PtCoVO/g-C3N4 and PtCo/g-C3N4, standard card (PDF#04-0802). (i) EPR spectra of PtCo/g-C3N4 and PtCoVO/g-C3N4 at 103 K in liquid N2.
Figure 3. (a) SEM images of PtCoVO/g-C3N4. (b) TEM images of PtCoVO/g-C3N4. (c,d) FFT and IFFT pattern of PtCoVO/g-C3N4. (e) SAED images of PtCoVO/g-C3N4. (f) The PtCoVO/g-C3N4 spacing was 0.2265 nm. (g) HAADF of PtCoVO/g-C3N4 and elemental mapping of C, N, Pt, V, Co, and O, respectively. (h) XRD of PtCoVO/g-C3N4 and PtCo/g-C3N4, standard card (PDF#04-0802). (i) EPR spectra of PtCo/g-C3N4 and PtCoVO/g-C3N4 at 103 K in liquid N2.
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Figure 4. XPS spectra of PtCoVO/g-C3N4 and PtCoV/g-C3N4. (a) Pt 4f. (b) V 2p. (c) Co 2p. (d) O 1s. (e) UPS spectra of HxV2O5, PtCoVO/g-C3N4, and PtCoV/g-C3N4. (f) VB-XPS valence band spectra of HxV2O5, PtCoVO/g-C3N4, and PtCoV/g-C3N4.
Figure 4. XPS spectra of PtCoVO/g-C3N4 and PtCoV/g-C3N4. (a) Pt 4f. (b) V 2p. (c) Co 2p. (d) O 1s. (e) UPS spectra of HxV2O5, PtCoVO/g-C3N4, and PtCoV/g-C3N4. (f) VB-XPS valence band spectra of HxV2O5, PtCoVO/g-C3N4, and PtCoV/g-C3N4.
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Figure 5. (a) HER polarization curves. (b) Tafel plots of PtCo/g-C3N4, Pt/C 20%, PtCoVO/g-C3N4, and PtCoV/g-C3N4. (c) EIS Nyquist plots of PtCoVO/g-C3N4, PtCo/g-C3N4, and PtCoV/g-C3N4. (d) Cdl values of current density differences plotted against scan rates of Pt/C 20%, PtCoVO/g-C3N4, PtCo/g-C3N4, and PtCoV/g-C3N4.
Figure 5. (a) HER polarization curves. (b) Tafel plots of PtCo/g-C3N4, Pt/C 20%, PtCoVO/g-C3N4, and PtCoV/g-C3N4. (c) EIS Nyquist plots of PtCoVO/g-C3N4, PtCo/g-C3N4, and PtCoV/g-C3N4. (d) Cdl values of current density differences plotted against scan rates of Pt/C 20%, PtCoVO/g-C3N4, PtCo/g-C3N4, and PtCoV/g-C3N4.
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Figure 6. (a–d) XPS spectra of survey, Pt 4f, Co 2p, and V 2p for PtCoVO/g-C3N4 before and after electrochemical testing. (e) The PtCoVO/g-C3N4 catalyst was subjected to a long-term stability measurement at current densities of 10 mA cm−2 for 48 h; the inset displays the TEM images before and after the stability measurement. (f) Continuous CV scanning after 5000 circles at 100 mV s−1 of PtCoVO/g-C3N4.
Figure 6. (a–d) XPS spectra of survey, Pt 4f, Co 2p, and V 2p for PtCoVO/g-C3N4 before and after electrochemical testing. (e) The PtCoVO/g-C3N4 catalyst was subjected to a long-term stability measurement at current densities of 10 mA cm−2 for 48 h; the inset displays the TEM images before and after the stability measurement. (f) Continuous CV scanning after 5000 circles at 100 mV s−1 of PtCoVO/g-C3N4.
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MDPI and ACS Style

Zhang, J.; Deng, W.; Weng, Y.; Jiang, J.; Mao, H.; Zhang, W.; Lu, T.; Long, D.; Jiang, F. Intercalated PtCo Electrocatalyst of Vanadium Metal Oxide Increases Charge Density to Facilitate Hydrogen Evolution. Molecules 2024, 29, 1518. https://doi.org/10.3390/molecules29071518

AMA Style

Zhang J, Deng W, Weng Y, Jiang J, Mao H, Zhang W, Lu T, Long D, Jiang F. Intercalated PtCo Electrocatalyst of Vanadium Metal Oxide Increases Charge Density to Facilitate Hydrogen Evolution. Molecules. 2024; 29(7):1518. https://doi.org/10.3390/molecules29071518

Chicago/Turabian Style

Zhang, Jingjing, Wei Deng, Yun Weng, Jingxian Jiang, Haifang Mao, Wenqian Zhang, Tiandong Lu, Dewu Long, and Fei Jiang. 2024. "Intercalated PtCo Electrocatalyst of Vanadium Metal Oxide Increases Charge Density to Facilitate Hydrogen Evolution" Molecules 29, no. 7: 1518. https://doi.org/10.3390/molecules29071518

APA Style

Zhang, J., Deng, W., Weng, Y., Jiang, J., Mao, H., Zhang, W., Lu, T., Long, D., & Jiang, F. (2024). Intercalated PtCo Electrocatalyst of Vanadium Metal Oxide Increases Charge Density to Facilitate Hydrogen Evolution. Molecules, 29(7), 1518. https://doi.org/10.3390/molecules29071518

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