Electro-Oxidation of Glycerol on Core–Shell M@Pt/C (M = Co, Ni, Sn) Catalysts in Alkaline Medium

Abstract

This study explores the development of core–shell electrocatalysts for efficient glycerol oxidation in alkaline media. Carbon-supported M@Pt/C (M = Co, Ni, Sn) catalysts with a 1:1 atomic ratio of metal (M) to platinum (Pt) were synthesized using a facile sodium borohydride reduction method. The analysis confirmed the formation of the desired core–shell structure, with Pt dominating the surface as evidenced by energy-dispersive X-ray spectroscopy (EDS). X-ray diffraction (XRD) revealed the presence of a face-centered cubic (fcc) Pt structure for Co@Pt/C and Ni@Pt/C. Interestingly, Sn@Pt/C displayed a PtSn alloy formation indicated by shifted Pt peaks and the presence of minor Sn oxide peaks. Notably, no diffraction peaks were observed for the core metals, suggesting their amorphous nature. Electrocatalytic evaluation through cyclic voltammetry (CV) revealed superior glycerol oxidation activity for Co@Pt/C compared to all other catalysts. The maximum current density followed the order Co@Pt/C > Ni@Pt/C > Sn@Pt/C > Pt/C. This highlights the effectiveness of the core–shell design in enhancing activity. Furthermore, Sn@Pt/C displayed remarkable poisoning tolerance attributed to a combined effect: a bifunctional mechanism driven by Sn oxides and an electronic effect within the PtSn alloy. These findings demonstrate the significant potential of core–shell M@Pt/C structures for designing highly active and poisoning-resistant electrocatalysts for glycerol oxidation. The presented approach paves the way for further development of optimized catalysts with enhanced performance and stability aiming at future applications in direct glycerol fuel cells.

1. Introduction

Fuel cells emerge as a promising alternative for obtaining clean energy, as they use renewable sources during their operation, emit practically no toxic pollutants and are highly efficient, and can be applied as sources of electrical energy for both stationary and mobile systems as well as for portable devices [1]. Among the various types of fuel cells, polymer electrolyte membrane fuel cells (PEMFCs) stand out for their versatile application potential. Basically, they operate by oxidizing a fuel at the anode (especially gaseous hydrogen) and reducing oxygen at the cathode. In some cell types, namely direct alcohol fuel cells (DAFCs), H₂ can be replaced by low molecular weight alcohols [2].

The use of glycerol as fuel in DAFCs (direct glycerol fuel cell—DGFC) has aroused the interest of researchers and has become the subject of study in recent years. Glycerol is an alcohol commonly obtained through less costly processes compared to the processes for obtaining H₂. Its low toxicity, high boiling point, high specific energy and the possibility of obtaining it from biomass are very important attractions that tend to make its application viable [3]. It is estimated that the complete oxidation of glycerol can produce 14 free electrons, which enhances its energetic effect. However, the breaking of C–C bonds poses difficulties that must be overcome before its applicability can be made viable. Therefore, several catalysts are tested to facilitate the breaking of bonds and reduce the poisoning effects frequently observed in some of the metal catalysts commonly used in the reaction [4]. Metals such as Pt, Pd, and Au (classified as precious due to their high added value) exhibit notable activity for the electro-oxidation of glycerol. However, their high cost and limited electrochemical stability pose significant challenges. Additionally, Au requires high overpotentials, making it less advantageous for use in DGFCs [5]. Bifunctional catalysts have emerged as crucial for the electro-oxidation of glycerol, as the incorporation of a second metal can enhance reaction performance and influence the mechanisms of by-product formation. Furthermore, the addition of other transition metals to precious metal catalysts can alter selectivity, enabling the targeted production of specific C3 and C2 compounds from alcohol oxidation depending on the catalyst design. Given their lower cost, catalysts composed of non-noble metals (e.g., Ni and Cu) have been investigated for the glycerol oxidation reaction (GOR). These catalysts have demonstrated high selectivity for the production of C2 and C1 compounds, particularly glycolic acid and formic acid, which are commonly obtained in the oxidation of glycerol under basic conditions using non-noble metal catalyst systems [5,6,7].

Falase et al. [8] investigated the glycerol oxidation on Pt-based binary and ternary alloys: the binary Pt₈₄Ru₁₆ and Pt₉₆Sn₄ catalysts showed a higher GOR activity than that of the ternary Pt₈₈Ru₆Sn₆ catalyst. All these catalysts demonstrated lower adsorption for the intermediates compared to the Pt/C catalyst. When studying different combinations of Pt and Sn in the form of alloys, Dou et al. [9] obtained an effective Pt₉Sn₁/C catalyst for oxidation of glycerol to glyceric acid, with the highest activity among various PtM/C (M = Sn, Mn, Fe, Co, Ni, Cu, Zn, Au) bimetallic catalysts.

When applying catalysts composed of Sn-Pt and Rh-Pt with core–shell structure (Sn@Pt and Rh@Pt, respectively) in the electro-oxidation of glycerol, Pupo et al. [10] observed an increase in catalytic performance of around 2 to 5 times higher when compared to the activity of Pt/C. Castagna et al. [11] tested different combinations of Cu@Pt in the catalysis of GOR. In that study, the researchers concluded that the combination of Pt and Cu in the core–shell format yielded catalysts with greater catalytic activity, 4 to 10 times higher when compared to the commercial catalyst PtRu/C. In addition to these, other relevant works reported core–shell catalysts with an interesting GOR performance [12,13,14,15,16,17,18,19].

On these bases, the aim of this work was to synthesize and apply nanostructured catalysts (Co@Pt/C, Ni@Pt/C, and Sn@Pt/C) for the GOR. The synthesis of these materials was carried out by deposition of Pt on previously synthesized cores. The materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), and cyclic voltammetry (CV). The catalytic behavior for glycerol electro-oxidation was also analyzed by CV, as well as by chronoamperometry and electrochemical impedance spectroscopy (EIS).

2. Materials and Methods

2.1. Catalyst Synthesis

Co@Pt/C, Ni@Pt/C, and Sn@Pt/C were obtained through the sequential chemical reduction of the metal precursors. Briefly, the synthesis occurred in two main stages. The first consisted of obtaining cores formed by nanoparticles of Co, Ni, and Sn. The second involved the coating of these cores with Pt followed by combination with the carbon powder support. The same synthesis sequence was applied in order to obtain catalysts with 20% metal load. The ratio between Pt and the other metals was 1:1 in each synthesis. The precursor salts of Co, Ni, or Sn (CoCl₂∙2H₂O–Sigma-Aldrich–Merck KGaA, Darmstadt, Germany, NiCl₂∙6H₂O–Sigma-Aldrich–Merck KGaA, Darmstadt, Germany, or SnCl₂∙2H₂O–Sigma-Aldrich–Merck KGaA, Darmstadt, Germany) were first added together with sodium citrate (Synth) in 50.0 mL of ultrapure water (18.2 MΩ cm⁻¹ @ 25 °C/Milli-Q–Merck KGaA, Darmstadt, Germany). The citrate/precursor ratio was 1:1. Immediately afterwards, the mixture was heated to 90 °C under stirring while promoting a continuous flow of N₂ into the solution. After reaching the desired temperature, NaBH₄ (Fluka Analytical–Merck KGaA, Darmstadt, Germany) was added in excess to promote the reduction of the precursor salts. The solution was left under stirring for approximately 30 min. Then, the N₂ flow was stopped, and another 25.0 mL of ultrapure water was added to the solution containing the nanoparticles formed by Co, Ni, or Sn. Then, the appropriate amount of an aqueous solution (5%) of H₂PtCl₆.6H₂O (Sigma-Aldrich) was added. Next, ascorbic acid (Chimie Test) was added in excess to promote the reduction of the Pt reducing salt, and the solution was left stirring for 2 h. Vulcan XC-72 (Cabot Corporation, Boston, MA, USA), a carbon powder with a particle size of ~30 nm and a surface area of 250 m² g⁻¹, was added in order to provide supported catalysts with 20% metal loading. Finally, the solution was left stirring at room temperature for 24 h. The catalysts contained in the solution underwent vacuum filtration while being washed with ultrapure water. Next, drying was carried out for 2 h at 70 °C. Finally, the samples were macerated and subjected to heat treatment at 300 °C in a N₂ atmosphere for 1 h in a tubular furnace. For comparison purposes, Pt/C (20%) was also synthesized via chemical reduction with NaBH₄ and applied to GOR.

2.2. Catalyst Characterization

XRD measurements were performed on a SHIMADZU XRD-7000 diffractometer (Shimadzu, Kyoto, Japan) using copper Kα radiation (1.5406 Å—potential of 40 mV and current of 30 mA). The diffraction angles 2θ were varied between 30 and 90° using a step of 0.02° at a rate of 2°/min.

SEM, EDS, and TEM techniques were used to obtain information on morphology, average diameter, particle size distribution, and composition. TEM analyses were performed using an electron microscope manufactured by JEOL—model JEM-1400Flash (JEOL LTD, Tokyo, Japan). SEM and EDS measurements were performed using a VEGA 3—TESCAN electron microscope (TESCAN, Brno, Czech Republic).

2.3. Electrochemical Measurements

The electrochemical characterization by CV and the catalytic activity measurements for GOR were obtained using a potentiostat/galvanostat from Pine Research Instrumentation Inc. (Durham, NC, US), model Wave Driver 40 DC, coupled to a microcomputer. For all measurements, KOH electrolyte (Sigma-Aldrich) with a concentration of 0.5 mol L⁻¹ was used. A platinum plate electrode was used as the counter electrode. A reversible hydrogen electrode (RHE) was used as the reference electrode. A glassy carbon electrode (diameter = 5 mm) was also used as the working electrode. Before all measurements, the working electrode was polished with alumina in different particle sizes: 3.0 μm, 1.0 μm, and 0.3 μm. To deposit the catalysts on the working electrode, paints were prepared with the following proportions: 1 mg of catalyst, 200 μL of isopropyl alcohol, and 10 μL of a 5% (mass/volume) Nafion solution. This suspension remained in ultrasound for 30 min. Finally, 15 μL of this paint were applied to the surface of the glassy carbon electrode seeking its complete coverage. First, the catalysts were characterized by CV. Subsequently, CV measurements were performed in the presence of 1.0 mol L⁻¹ of glycerol (Sigma-Aldrich–Merck KGaA, Darmstadt, Germany). Chronoamperometry analyses were performed by applying 0.6 V for a period of 1800 s. The EIS analyses, in turn, were performed using the potentiostat/galvanostat from the manufacturer Metrohm—model Autolab PGSTAT302N (Utrecht, The Netherlands)—with the following parameters: potential of 0.6 V, frequency range from 10 kHz to 10 mHz, 10 points per decade, and RMS of 10 mV. The results of the electrochemical measurements were normalized by the electroactive surface area (ECSA) of the catalysts, which was determined by the ratio of the charge related to the hydrogen desorption region (measured in μC) with the charge density required to reduce a monolayer of protons on a polycrystalline Pt surface (210 μC cm⁻²).

3. Results and Discussion

3.1. Structural Characterization

The XRD patterns of the Co@Pt/C, Ni@Pt/C, and Sn@Pt/C catalysts are presented in Figure 1a and compared with the Pt/C catalyst. It can be observed that all samples presented the diffraction peaks characteristic of the face-centered cubic (fcc) structure of Pt, with 2θ values of approximately 40.0°, 46.4°, 67.8°, 81.5°, and 86.0° corresponding respectively to the planes 111, 200, 220, 311, and 222. In the XRD pattern of Sn@Pt/C, two other small peaks are observed at 2θ values of 34.1° and 51.9°, ascribed to the 101 and 211 planes of SnO₂, respectively (JCPDS No. 41-1445). For the Co@Pt/C and Ni@Pt/C samples, no other diffraction peaks are observed besides those attributed to Pt. No diffraction peaks were observed for core metals, indicating their amorphous nature. Figure 1b,c shows the diffraction peaks of the 200 and 220 planes, respectively, in which no displacement of the Pt peaks is evident, despite the presence of other metals. The 2θ values for the Co@Pt/C and Ni@Pt/C catalysts were very similar to those presented by Pt/C, indicating no alloy formation with the non-precious metals. A shift of the 200 and 220 peaks to lower angles, instead, was observed for the Sn@Pt/C catalyst, indicating the formation of a PtSn alloy. The lattice parameters of Co@Pt/C, Ni@Pt/C, Sn@Pt/C, and Pt/C catalysts are reported in Table 1. The lattice parameter of the Co@Pt/C and Ni@Pt/C was the same as Pt, indicating no alloy formation, while that of Sn@Pt/C was lower, indicating the partial formation of a PtSn solid solution. Pt peak shifts were similarly observed in the study by Cantane et al. [20], where the authors attributed this phenomenon to the partial formation of a solid solution between Pt and the secondary metal at the interface between the core and the Pt surface layer. The fcc Pt peaks in Sn@Pt/C and Pt/C catalysts were sharper than those in Co@Pt/C and Ni@Pt/C, indicating that the crystallite size of Co@Pt/C and Ni@Pt/C is larger than that of Sn@Pt/C and Pt/C. To calculate the crystallite size, the Scherrer equation was applied to the 220 peak of Pt. A proportionality constant “K” of 0.9 was used, based on the assumption of predominantly spherical particle shapes. The parameter “β” represents the full width at half maximum (FWHM) of the diffraction peak, while the wavelength “λ” was set to 0.15406 nm [11,15,21], corresponding to the Kα radiation of Cu utilized by the equipment. The crystallite size values obtained are presented in Table 1.

Figure 2a–c shows the SEM micrographs of the Co@Pt/C, Ni@Pt/C, and Sn@Pt/C catalysts. These images reveal that the catalysts exhibit predominantly globular morphological structures with a seemingly uniform and homogeneous particle size distribution. The analysis of the surface chemical composition, performed by means of EDS, shows the largely majority presence of Pt on the surface of the materials. A similar result was found for Ni@Pt by Habibi and Ghaderi [22].

Figure 3a–c presents the TEM analysis results for the Co@Pt/C, Ni@Pt/C, and Sn@Pt/C catalysts. The particles exhibit a predominantly spherical shape and satisfactory dispersion on the carbon support. However, regions with particle agglomeration were also identified. The particle size distribution histograms provide a quantitative assessment of the particle sizes, with average values listed in Table 1. A notable congruence is observed between the particle sizes determined via TEM and the crystallite sizes calculated from XRD measurements.

3.2. Electrochemical Characterization

The Co@Pt/C, Ni@Pt/C, and Sn@Pt/C catalysts were subjected to cyclic voltammetry analysis in 0.5 M KOH electrolyte saturated with N₂ at room temperature and compared with the Pt/C catalyst. Figure 4 shows the profiles obtained for each sample using a scan rate of 50 mV s⁻¹ in a potential range from 0.05 to 1.1 V vs. RHE. The voltammograms obtained for the Co@Pt/C, Ni@Pt/C, and Sn@Pt/C catalysts showed the characteristic profiles for Pt-based materials. Three well-defined regions can be observed: (i) 0.05 to 0.4 V vs. RHE, known as the hydrogen adsorption/desorption region; (ii) 0.4 to 0.8 V vs. RHE, corresponding to non-faradaic processes, which originate from the charging of the electric double layer; and (iii) 0.8 to 1.1 V vs. RHE, referred to as the oxygen region, associated with the OH⁻ adsorption process. In the cathodic scan, it is possible to observe peaks around 0.75 V vs. RHE, which is related to the reduction of oxides formed in the anodic scan [23]. The absence of peak shifts in the X-ray diffraction (XRD) patterns of Co@Pt/C, Ni@Pt/C, and Sn@Pt/C compared to Pt/C suggests that the surface of these catalysts is primarily composed of Pt. This is further supported by the presence of hydrogen adsorption/desorption regions in their voltammograms, similar to those observed for polycrystalline Pt. However, the incorporation of Co, Ni, and Sn within the core likely influences the formation of the electrical double layer at the electrode surface, as evidenced by the changes observed in the voltammetric profiles [24,25,26,27,28].

The electrochemical activity for the glycerol oxidation of Co@Pt/C, Ni@Pt/C, Sn@Pt/C, and Pt/C catalysts was investigated by CV measurements in KOH solution + 1.0 M glycerol at room temperature, and the results are shown in Figure 5 and Table 2. As shown in Figure 5 and Table 2, the onset potential for glycerol oxidation (GOR) on Co@Pt/C is lower than that of Pt/C. This suggests that the reaction on Co@Pt/C requires less activation energy. In contrast, Ni@Pt/C and Sn@Pt/C do not exhibit a decrease in the onset potential compared to Pt/C. Interestingly, all three modified catalysts (Co@Pt/C, Ni@Pt/C, and Sn@Pt/C) show higher current density peaks in both the anodic and cathodic scans compared to Pt/C. The maximum current density follows the order Co@Pt/C > Ni@Pt/C > Sn@Pt/C > Pt/C. Since GOR produces C3, C2, and C1 products, the higher current densities observed in Figure 5 and Table 2 likely correspond to a greater degree of oxidation [3,5].

The improvement in catalytic activity observed for the oxidation reactions in this study can be clearly attributed to the unique core–shell structure formed. The combination of Pt with the core metals induces electronic effects by modifying the Pt d-band energy sublevels, enhancing its catalytic properties. Additionally, geometric effects resulting from the rearrangement of the Pt crystal structure create more active catalytic sites for the glycerol oxidation reaction (GOR) [29,30].

The superior performance of the Co@Pt/C catalyst, as observed in the CV analyses with glycerol, can be attributed to its lower tendency to form Pt oxides on the surface. This behavior is evident in the CV graphs presented in Figure 4, where Co@Pt/C exhibits a delayed onset of oxide formation during the anodic scan compared to the other catalysts. Moreover, during the cathodic scan, Co@Pt/C shows earlier reduction of oxides. The ability to control the formation and reduction of oxides on the Pt surface is critical for achieving optimal performance in alcohol electrocatalysis.

The addition of Co, Ni, and Sn contributes to a reduced interaction energy between the catalytic sites and adsorbed species, further enhancing catalytic efficiency [31]. This synergistic interaction between the Pt shell and the core metals highlights the significance of the core–shell design in optimizing the activity and stability of the catalysts for GOR.

Chronoamperometric experiments were conducted at 0.6 V in a 0.5 M KOH solution containing glycerol over a period of 1800 s for the Co@Pt/C, Ni@Pt/C, Sn@Pt/C, and Pt/C catalysts. These tests aimed to evaluate the electrocatalytic activity under steady-state conditions as well as the extent of poisoning of the active catalytic surface. The current–time curves for Co@Pt/C, Ni@Pt/C, Sn@Pt/C, and Pt/C catalysts are shown in Figure 6a. In all CA curves the current drops with elapsed time; this occurs rapidly at the beginning and then becomes relatively stable. Between approximately 0 and 100 s, a sharp decline in current densities is observed for all samples. This behavior is primarily attributed to the consumption and subsequent decrease in the concentration of chemical species in the diffusion-limited layer near the electrode surface. After 100 s, a more gradual but continuous decay is observed until reaching a stable zone. This can be explained by the occupation of active sites by by-products formed during the glycerol oxidation reaction [32]. The continuous decay after stabilization at 100 s is likely due to the poisoning of surface active sites. Initially, the active sites are free from adsorbed glycerol molecules, but as the glycerol molecules adsorb again, their re-adsorption is hindered by intermediate species formed during the initial period, which leads to the poisoning of the catalytic sites. As can be seen in Figure 6b and Table 2, the order of GOR activity at the steady state was Co@Pt/C > Sn@Pt/C > Ni@Pt/C≈Pt/C.

Thus, catalysts are crucial components in many chemical processes, but their performance can be hampered by the presence of intermediate species. These intermediate species can act as poisons, hindering the catalyst’s ability to drive the desired reaction efficiently. To assess a catalyst’s tolerance to such poisoning, two key electrochemical techniques are employed: cyclic voltammetry (CV) and chronoamperometry (CA).

CV provides valuable insights into a catalyst’s electrochemical behavior. By scanning the applied potential across a set range and monitoring the resulting current response, CV offers a snapshot of the catalyst’s activity. The ratio between the forward peak current (I_f) and the backward peak current (I_b), known as the I_f/I_b ratio, is a valuable parameter for evaluating the catalyst’s resistance to poisoning by adsorbed intermediates. A higher I_f/I_b ratio indicates greater tolerance. This suggests that the catalyst is less susceptible to deactivation by intermediate species during the reaction cycle. On the other hand, CA takes a different approach. It measures the current response of a catalyst under a constant applied voltage over time. By analyzing the steady-state current (I_ss), it is possible to gain insights into the balance between glycerol adsorption and the oxidation of adsorbed intermediates. A higher I_ss suggests more active sites available for glycerol conversion, implying a lower degree of poisoning. The percentage of current retention at the steady state (R = I_ss/I_o, where I_o is the initial current density) is another useful parameter. A higher R value indicates that the catalyst retains a greater proportion of its initial activity over time, suggesting enhanced resistance to poisoning.

In this context, Sn@Pt/C, Ni@Pt/C, and Co@Pt/C core–shell catalysts have shown promising results. As shown in Figure 7, the Sn@Pt/C catalyst exhibits the highest I_f/I_b ratio among the three, followed by Ni@Pt/C and Co@Pt/C. This superior tolerance in Sn@Pt/C can be attributed to two factors: a bifunctional mechanism involving Sn oxides and an electronic effect between platinum and tin in the PtSn alloy [33].

Interestingly, while the R values for all core–shell catalysts were lower than that of Pt/C (similar to observations for Pt/C and Pt-based alloys in Ref. [34]), the order of the R values mirrored the I_f/I_b ratios. This further confirms the superior poisoning tolerance of the Sn@Pt/C catalyst. Therefore, the results presented here showcase the promising performance of Sn@Pt/C as a catalyst for glycerol oxidation, demonstrating its enhanced resistance to poisoning compared to Ni@Pt/C, Co@Pt/C, and Pt/C. This work paves the way for further development of high-performance catalysts with improved stability and long-term operation.

Figure 8a presents Nyquist plots at a potential of 0.6 V, which reveal the resistance to the electrochemical reaction processes of the electrocatalysts. The charge transfer resistance (R_ct) at the electrode/electrolyte interface is reflected by the diameter of the small semicircle (quasi-semicircle) in the plots. A smaller diameter indicates lower R_ct and better electrical conductivity. The order of the quasi-semicircle diameter is Co@Pt/C < Ni@Pt/C < Sn@Pt/C < Pt/C. This aligns well with the results from CV and CA, suggesting that Co@Pt/C exhibits the lowest electron transfer resistance and the best electrical conductivity.

Curiously, the Nyquist diagrams reveal additional features beyond the electrolyte resistance (R_e) and the primary charge transfer resistance (R_ct). Two additional charge transfer resistances (R_ct1 and R_ct2) are identified for all catalysts except Pt/C (Table 3). Notably, the Sn@Pt/C catalyst presents a unique third semicircle, indicating a third transfer resistance value (R_ct3).

The Bode diagram (Figure 8b) shows the charge transfer phenomena that confirm the R_cts observed in Nyquist. Furthermore, it is observed that Co@Pt/C and Ni@Pt/C present a greater capacitive character, indicating a greater capacity of this catalyst to adsorb glycerol on its sites [35]. The greater capacitive character of these materials can also be observed in the voltammograms of Figure 4, in which a more prominent electric double layer region is noted in comparison to Pt/C. The Sn@Pt/C catalyst, in turn, presented greater resistance than Co@Pt/C and Ni@Pt/C, as well as a lower capacitive character. The emergence of the third R_ct for Sn@Pt/C can probably be associated with the SnO₂ phase, also observed in the XRD results. Thus, the phenomena observed in Bode corroborate the results presented in the initial phase of the chronoamperometry tests (Figure 6).

Overall, the EIS analysis demonstrates lower R_ct values for Co@Pt/C, Ni@Pt/C, and Sn@Pt/C compared to Pt/C (Table 3). This signifies faster electron transfer processes occurring on the surfaces of these modified catalysts.

4. Conclusions

This study successfully synthesized core–shell Co@Pt/C, Ni@Pt/C, and Sn@Pt/C catalysts using a NaBH₄ reduction method. The electrocatalytic performance of these catalysts for glycerol oxidation significantly surpassed that of Pt/C, a benchmark catalyst. Notably, Co@Pt/C exhibited the highest activity, achieving a lower onset potential and a higher maximum current density for glycerol oxidation compared to all other catalysts. Furthermore, Sn@Pt/C demonstrated superior poisoning tolerance due to a combined effect of Sn oxide-mediated bifunctional mechanism and electronic interactions within the PtSn alloy. These findings highlight the potential of core–shell structures for designing highly active and poisoning-resistant electrocatalysts for glycerol oxidation. Future work could explore optimizing the core composition and shell thickness to further enhance catalytic performance and stability.

Author Contributions

Conceptualization, R.N.S. and R.R.P.; methodology, R.N.S.; validation, L.A.P., E.A. and R.R.P.; formal analysis, R.N.S.; investigation, R.N.S. and R.R.P.; resources, L.A.P. and R.R.P.; data curation, R.N.S.; writing—original draft preparation, R.N.S.; writing—review and editing, E.A., L.A.P. and R.R.P.; visualization, E.A., L.A.P. and R.R.P.; supervision, R.R.P.; project administration, R.R.P.; funding acquisition, L.A.P. and R.R.P. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors upon request.

Acknowledgments

The authors would like to thank the Graduate Program in Chemistry of the Federal University of Amazonas (PPGQ-UFAM); the Central Analytical Laboratory of the Federal Institute of Education, Science and Technology of Amazonas (IFAM) for XRD analysis; the Thematic Laboratory of Optical and Electron Microscopy of the National Institute for Research in the Amazon (LMTOE/INPA) for SEM/EDS analysis; the Multiuser Center for Analysis of Biomedical Phenomena of the State University of Amazonas (CMABio/UEA) for TEM analysis; the Amazonas State Research Support Foundation (FAPEAM) for their support (Grants # 062.01163/2019-PAMEQ/2019; # 01.02.016301.03391/2021-90-EDITAL N. 010/2021—CT&I ÁREAS PRIORITÁRIAS); Coordenação de Aperfeiçoamento de Pessoal de Nível Superior—Brasil (CAPES)—Finance Code 001, and Conselho Nacional de Desenvolvimento Científico e Tecnológico—CNPq.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

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Figures and Tables

Figure 1. (a) XRD patterns of Co@Pt/C, Ni@Pt/C, Sn@Pt/C, and Pt/C; (b) comparison of the peaks of the Pt 200 fcc plane of Pt; (c) comparison of the peaks of the Pt 220 fcc plane. (NOTE: * identifies peaks attributed to SnO2).

Figure 2. SEM micrographs and EDS mapping of (a) Co@Pt/C, (b) Ni@Pt/C, and (c) Sn@Pt/C catalysts.

Figure 3. TEM micrographs and corresponding particle size distribution histograms for the Co@Pt/C, Ni@Pt/C, and Sn@Pt/C catalysts.

Figure 4. Cyclic voltammograms of the Co@Pt/C, Ni@Pt/C, and Sn@Pt/C catalysts. Electrolyte: 0.5M KOH saturated with N2; scan rate: 50 mV s−1.

Figure 5. Voltammograms of the Co@Pt/C, Ni@Pt/C, and Sn@Pt/C catalysts compared with Pt/C. Electrolyte: 0.5 M KOH + 1.0 M glycerol. Scan rate: 50 mV s−1.

Figure 6. (a) Chronoamperometry at 0.6 V vs. RHE for 1800 s for the Co@Pt/C, Ni@Pt/C, Sn@Pt/C, and Pt/C catalysts. Electrolyte: 0.5 M KOH + 1.0 M glycerol; (b) current density values after 1800 s.

Figure 7. Histograms of If/Ib e R (Iss/Io) values for the Co@Pt/C, Ni@Pt/C, Sn@Pt/C, and Pt/C catalysts.

Figure 8. Nyquist diagram (a) and Bode diagram (b) of Co@Pt/C, Ni@Pt/C, Sn@Pt/C, and Pt/C catalysts. Electrolyte: 0.5 M KOH + 1.0 M glycerol. Frequency range: 10,000–0.01 Hz. Potential: 0.6 V vs. RHE.

Table 1

Crystallite size (determined by XRD), lattice parameter (calculated from XRD), and particle size (measured by TEM) of the Co@Pt/C, Ni@Pt/C, Sn@Pt/C, and Pt/C catalysts.

Catalyst	XRD		TEM
	Crystallite Size (nm) a	Lattice Parameter (nm) b	Particle Size (nm)
Co@Pt/C	12.0	0.391	11.2
Ni@Pt/C	12.0	0.391	13.9
Sn@Pt/C	8.0	0.392	8.7
Pt/C	8.0	0.391	n.d. c

^a Parameter determined by the Scherrer Equation: d = (K.λ)/(β.cos θ). ^b Parameter determined by combining the equations of Bragg’s Law and the equation that relates particle size and lattice parameter for a cubic crystal structure: a_c = d(h² + k² + l²) ½. ^c TEM measurements were not performed for the sample.

Table 2

ECSA, glycerol oxidation onset potential (E_onset), peak current density in the anodic scan (I_f) and peak current density in the cathodic scan (I_b) by CV; steady state current density (I_ss) by CA for the Co@Pt/C, Ni@Pt/C, and Sn@Pt/C catalysts.

Catalyst	ECSA (m2 g−1)	Eonset (V)	If (CV) (mA cm−2)	Ib (CV) (mA cm−2)	Iss (CA) (mA cm−2)
Co@Pt/C	3.17	0.38	3.619	2.109	0.135
Ni@Pt/C	3.47	0.44	1.993	1.042	0.107
Sn@Pt/C	6.42	0.42	1.413	0.611	0.121
Pt/C	17.89	0.42	0.732	0.539	0.106

Table 3

EIS parameters for Co@Pt/C, Ni@Pt/C, Sn@Pt/C, and Pt/C. Electrolyte: 0.5 M KOH + 1.0 M glycerol. Frequency range: 10,000–0.01 Hz. Potential: 0.6 V vs. RHE.

Catalyst	Re(Ohm)	Rct1(Ohm cm2)	Rct2(Ohm cm2)	Rct3(Ohm cm2)
Co@Pt/C	19.2	6.5	1911.5
Ni@Pt/C	21.5	19.9	4113.9
Sn@Pt/C	10.4	133	2762.5	2411.5
Pt/C	23.5	5.9	5615.1

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