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Article

Advances in Liquid-Phase Synthesis: Monitoring of Kinetics for Platinum Nanoparticles Formation, and Pt/C Electrocatalysts with Monodispersive Nanoparticles for Oxygen Reduction

by
Vladimir Guterman
1,*,
Kirill Paperzh
1,
Irina Novomlinskaya
1,2,
Ilya Kantsypa
1,
Alina Khudoley
1,
Yana Astravukh
1,2,
Ilya Pankov
3 and
Alexey Nikulin
1
1
Faculty of Chemistry, Southern Federal University, 7 Zorge St., Rostov-on-Don 344090, Russia
2
Prometheus R&D LLC, 4g/36 Zhmaylova St., Rostov-on-Don 344091, Russia
3
Research Institute of Physical Organic Chemistry, Southern Federal University, 194/2 Stachki St., Rostov-on-Don 344090, Russia
*
Author to whom correspondence should be addressed.
Catalysts 2024, 14(10), 728; https://doi.org/10.3390/catal14100728
Submission received: 31 July 2024 / Revised: 13 September 2024 / Accepted: 14 October 2024 / Published: 17 October 2024
(This article belongs to the Section Catalytic Materials)
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Abstract

:
The growing demand for hydrogen–air fuel cells with a proton-exchange membrane has increased interest in the development of scalable technologies for the synthesis of Pt/C catalysts that will allow us to fine-tune the microstructure of such materials. We have developed a new in situ technique for controlling the kinetics of the transformation of a platinum precursor into its nanoparticles and deposited Pt/C catalysts, which might be applicable during the liquid-phase synthesis in concentrated solutions and carbon suspensions. The technique is based on the analysis of changes in the redox potential and the reaction medium coloring during the synthesis. The application of the developed technique under conditions of scaled production has made it possible to obtain Pt/C catalysts with 20% and 40% platinum loading, containing ultra-small metal nanoparticles with a narrow size distribution. The electrochemically active surface area of platinum and the mass activity of synthesized catalysts in the oxygen electroreduction reaction have proved to be significantly higher than those of commonly used commercial analogs. At the same time, despite the small size of nanoparticles, the catalysts’ degradation rate turned out to be the same as that of commercial analogs.

Graphical Abstract

1. Introduction

Proton-exchange membrane electrolyzers (PEMEs) and proton exchange membrane fuel cells (PEMFCs) are important components of hydrogen power engineering that are finding increasing use. The best electrocatalyst for PEMFCs and PEME cathodes is platinum. The rate of cell reactions on this metal in an acidic medium is significantly higher than on other metallic or non-metallic electrodes [1,2]. Attempts to create platinum-free electrocatalysts as well as their subsequent use in commercially produced devices are still far from successful [3,4]. Therefore, platinum-containing catalysts will hardly lose their monopoly position in the market, at least on the mid-term horizon [4,5]. The prevailing position in sales and commercial application is occupied by deposited Pt/C and PtM/C electrocatalysts, which represent dispersed composite materials consisting of nanoparticles (NPs) of platinum or its alloys deposited on the surface of nano- and microparticles of a carbon support.
Achieving the goal of reducing the amount of precious metal in the catalytic layers of PEMFCs and PEMEs is impossible without enhancing the mass activity and the durability of those catalysts and catalytic layers. The first of these tasks is solved by optimizing the composition and design of catalytic layers as well as improving their deposition methods [6]. The main approaches to increasing the mass activity and the stability of platinum-containing electrocatalysts have been formulated for quite a while and may be described in a simplified form as follows [7,8,9,10,11,12,13,14]: controlling the shape, size, and size distribution of metal NPs; increasing the uniformity of the spatial distribution of metal NPs on the surface of a carbon support, placing the NPs in the mesopores of the support and strengthening the adhesion thereto; grain boundary defect engineering; alloying of platinum with certain d-metals, forming bimetallic NPs with a special architecture (core–shell, onion-like and gradient structures, hollow and nanoframed NPs); and using more stable, heteroatom-doped, composite carbon, or non-carbon supports.
The efforts of a range of studies are focused on the development of scalable technologies for electrocatalyst production which are capable of implementing at least some of the above approaches. It is also important to note that during operation, the composition and microstructure of both the electrocatalysts and the metal particles generally tend to change, and it is the behavior of these transformed structures that determines the functional characteristics of the electrodes. Is there any sense in the ongoing attempts to optimize the composition and microstructure of Pt/C electrocatalysts? In answering this question, the results and conclusions of a recent work [15] appear to be of interest to us, the authors of which have shown that the activity of Pt/C electrocatalysts in the oxygen electroreduction reaction (ORR) in acidic media can be increased fivefold compared to the existing commercial analogs in the case of a combination of certain shapes and sizes of platinum NPs. In turn, this raises the question of whether there are any synthesis technologies that implement fine control of platinum NPs architecture as well as their spatial distribution over the surface of a carbon support.
The methods of obtaining platinum–carbon electrocatalysts are very diverse, and their classification can be based on different principles [16,17]. Nevertheless, in all the highlighted cases, we are discussing about obtaining hierarchically organized nanostructured composites containing billions of metal NPs. The ordering of the composition and microstructure of such systems is certain to be a quite difficult task. The liquid-phase synthesis methods for platinum NPs and platinum–carbon catalysts occupy a special place among the technologies used [17,18]. They are easily implemented in laboratory conditions, with some of them being rather low budget and scalable. The advantage or disadvantage of such technologies is the presence of numerous factors influencing the processes of chemical transformations proceeding in the liquid phase (Figure 1). On the one hand, this makes it possible to affect the composition and microstructure of the product; on the other hand, it makes the results extremely sensitive to the conditions of synthesis.
The transformation of the initial Pt(IV) compound into the Pt(0)x NPs proceeds through the formation of one or another Pt(II) compound, which is further reduced to Pt(0) atoms [18]. Solvated platinum atoms form clusters, some of which disintegrate and some grow, attaching new metal atoms. After the metal nucleus reaches its critical size, the probability of its further growth exceeds the probability of its dissipation [17]. “The smaller the critical size of the nucleus, the smaller the specific surface free energy, which depends, among other things, on the presence of surfactants, and the greater the potential difference between the oxidizer (Pt(II)) and the reducing agent” [17,18]. The stages of formation and growth of platinum nuclei, i.e., the formation of NPs, may proceed sequentially, simultaneously, or with an overlap in time [18]. In the latter two cases, the formation of NPs of different sizes is quite probable; the earlier the critical nucleus is formed, the longer its growth time and, as a result, the bigger the size of the NP. In the event of the successive stages of NP nucleation and growth, the probability of the formation of monodispersive NPsc, i.e., platinum NPs with a narrow size distribution, is high [18]. It is worth noting that in the course of the NPs formation, we may also observe their aggregation.
The substances used for the chemical reduction of platinum precursors in solutions are quite diverse in nature, composition, and redox potential [19,20]. In this regard, the polyol synthesis methods are quite common, with ethylene glycol or other alcohols being used as the reducing agent [21,22,23,24,25]. Alkali metal borohydrides, formaldehyde, formic, ascorbic and citric acids, as well as many other organic and some inorganic reagents may be used as the reducing agent [26,27,28]. For the vast majority of reducing agents, the oxidation process proceeds in stages, with the formation of more or less stable forms [28]. In this case, some intermediate compounds can also act as reductants in relation to metal compounds. As a result, the process of liquid-phase synthesis of platinum NPs proceeds in the presence of various forms of platinum and different reducing agents in the solution, the concentrations of which are interconnected and change over time until the platinum compounds are completely transformed into a metallic form. To optimize the NP sorption process by a support and reduce a negative effect of the NP agglomeration on the microstructure of the Pt/C composite, the liquid-phase synthesis of platinum–carbon catalysts is generally carried out in carbon suspensions. However, it is not yet clear whether the mechanism of phase formation in this case changes from homogeneous to heterogeneous. It is important to note that some reducing agents or their oxidation products, as well as some other components of the reaction medium, may be chemisorbed on the surface of the growing platinum NPs, inhibiting their growth and preventing aggregation [20,29,30,31]. The chemisorption can be size- and structure-sensitive, which creates a large variability in the synthesis results in terms of the characteristics of the Pt/C microstructure [20,31,32].
In consideration of the foregoing, the chemical reactor in which the synthesis takes place may actually be deemed as a “black box”, and the possibilities of observing the mechanism and kinetics of the transformations proceeding in the reactor are strongly limited. The use of one or another method of controlling the phase formation is often possible only in highly diluted media. There is practically no control method that allows for the monitoring of the phenomena occurring in the entire range of transformations—from the beginning to the end of the reaction [18]. The transition to synthesis under technological conditions (at high concentrations of reagents, mixing of a reaction medium, and purging of gases) further limits the possibilities of in situ control over the course and end of the transformation. It is then not surprising that the liquid-phase synthesis still remains a so-called combination of science and art, with the optimization of liquid-phase synthesis methods being implemented by empirical trial and error. By changing the input conditions, researchers study properties of the product obtained at the output of the “black box”, poorly understanding what happens during the transformation process. Meanwhile, the task is to transform the methods of liquid-phase synthesis into a combination of science and technology, which is impossible without monitoring the composition/properties of the reaction medium during the transformation process.
This work describes a new method to control the kinetics of the Pt(IV) → Pt(0)x transformation, which might be applicable to solutions of any concentration, even for the gram-scale synthesis of metal NPs and Pt/C catalysts. The method is based on the simultaneous study of changes in the coloring intensity and the redox potential of the reaction medium. Early attempts to quantify the reaction medium’s color intensity and its change during formic acid and formaldehyde syntheses were made by us in [33,34]. In the paper referred to in [35], for the first time, we carried out a study of changes in the reaction medium redox potential by examining the influence of the atmosphere composition on the phase formation kinetics in the course of the formic acid synthesis of platinum NPs. It should be noted that the analysis of the reaction medium coloring was performed in the above works by photographs of the reaction medium (glass reactor) taken at regular intervals.
This work supplements and develops further the studies previously conducted [33,34]. To assess the kinetics of the platinum NP formation during polyol and citric acid syntheses, we use a refined technique of the continuous in situ measurement of coloring intensity for the solutions, in which the platinum precursors reduction proceeds, coupled with the continuous in situ measurement of the reaction medium redox potential. The solution coloring depends on the presence of various forms of platinum, the concentration of which changes over time. The medium redox potential, which reflects a thermodynamic state of the system, depends on the nature and concentration ratio of these forms, as well as on the concentration of a reducing agent and its oxidation intermediates.
The second part of the work is concerned with the study of the Pt/C catalysts’ microstructure and electrochemical behavior, the conditions for the scaled synthesis of which have been optimized using the developed “kinetic control” technique. The optimization of the method consists in that the nucleation stage acceleration for platinum NPs during the liquid-phase synthesis should contribute to a narrowing in the size distribution of NPs, with the “monodispersive” microstructure of the Pt/C catalyst, in turn, having a beneficial effect on the durability and activity of the catalyst in the ORR.

2. Results and Discussion

2.1. Kinetics of the Pt(IV) Reduction with Citric Acid and the Resulting Pt/C Materials

We studied the kinetics of the Pt NP formation for the citric acid method, without the use of any additives affecting a medium pH, and for the commonly used polyol synthesis method, in which ethylene glycol in an alkaline medium is used as the solvent and the reducing agent.
The reduction of chloroplatinic acid with citric acid is accompanied by a characteristic change in the solution coloring from bright yellow to black. A change in the intensity of three components of the solution coloring during the synthesis at temperatures of 70, 80, and 90 °C is shown in Figure 2.
Minor changes in the intensity of the coloring components at the initial stage of synthesis are mainly due to a change in the concentration ratio of Pt(IV) and Pt(II) compounds. A simultaneous sharp change in the intensity of all three coloring components is associated with the darkening of the reaction medium due to the formation of a colloidal solution of platinum NPs [23,33]. Unfortunately, it is not yet possible to separate the stages of nucleation and growth for platinum NPs by a change in the coloring of the solution. Nevertheless, the onset of a sharp coloring change and the duration of this stage allow us to conclude the following about the rate of phase transformation: With increasing temperature, both the duration of the induction period preceding the beginning of phase formation and the duration of the nucleation/growth stage of platinum NPs are shortened (Figure 3).
As noted earlier, chemical transformations proceeding in the solution are accompanied by a change in the form and concentration of platinum compounds as well as citric acid oxidation products. This causes a regular change in the medium redox potential measured using an indicator electrode (Figure 2, solid black line). The presence of two pronounced potential delays on the E,time curve may be associated with the successive transformations of Pt(IV) → Pt(II) and Pt(II) → Pt(0). Upon comparison with the solution coloring change curves, it can be concluded that the final part of the second section of the potential delay corresponds to the period of a sharp change in the coloring of the solution and, consequently, the stage of NPs nucleation/growth.
It should be pointed out that an increase in the synthesis temperature reduces the length of the characteristic sections without changing a general appearance of the E,time curves (Figure 2 and Figure 3). The potential values corresponding to the characteristic sections of these curves also vary slightly. This allows us to talk about the preservation of the chemical mechanism of a multi-stage reaction in the temperature range of 70–90 °C.
To reduce the degree of NP aggregation in Pt/C, the liquid-phase syntheses are often carried out directly in carbon suspensions rather than in solutions [33]. Therefore, at the next stage of the study, the methodology for controlling the kinetics of platinum phase formation was extrapolated to the synthesis under heterogeneous conditions, i.e., synthesis in an environment initially containing a dispersed carbon support. The black color of this suspension, although it does not allow us to control the solution coloring change during the reaction, does not interfere with the measurement of the reaction medium’s redox potential.
Conducting the citric acid synthesis in a carbon suspension slightly inhibits the decrease in the medium redox potential although, similar to a change in the synthesis temperature, it has no fundamental effect on the general nature of the E,time dependences (Figure 2 and Figure 3). If the nucleation activation energy was reduced as a result of the implementation of a heterogeneous mechanism, the transformation of platinum and corresponding decrease in the potential would accelerate in the presence of carbon. The absence of such an effect indicates a predominantly homogeneous nucleation of NPs and their subsequent sorption by the carbon support. The inhibition of the transformation observed in the carbon suspension is most likely due to a decrease in the volume concentration of citric acid and its oxidation intermediates due to their carbon sorption. In accordance with the laws of chemical kinetics, a decrease in the concentration of the reagent in the volume of the solution leads to a decrease in the rate of the volumetric redox reaction.
The product yield as per the estimated value and the mass fraction of platinum in the Pt/C materials obtained upon completion of the synthesis in suspensions are quite high (Table 1). The mass fraction of platinum in Pt/C grows slightly with an increase in the synthesis temperature from 70 to 80–90 °C.
The X-ray diffraction patterns for the Pt/C materials obtained demonstrate the presence of intense peaks near the values of 2θ 39.9° and 46°, corresponding to the platinum phase (Figure 4). The average size of platinum crystallites calculated by the Scherrer equation for the half-width of peak 111 is 2.2 ± 0.2 nm in the case of the citric acid synthesis (Table 1). No reliable effect of the synthesis temperature on the DAv value was revealed.

2.2. Kinetics of the Pt(IV) Reduction with Ethylene Glycol and the Resulting Pt/C Materials

The study of the sensitivity of new methods in examining the kinetics of platinum NP formation was subsequently conducted for polyol synthesis, with ethylene glycol as the reducing agent. The synthesis in an alkaline medium did not affect the characteristic coloring transition from yellow to black which accompanies the formation of a colloidal solution of platinum NPs. In this case, the effect of the solution pH value, or rather the ratio of the amounts of alkali and chloroplatinic acid introduced into the chemical reactor: s = [NaOH]/[H2PtCl6], on the kinetics of the platinum reduction has been studied. The changes in the indicator electrode potential and the intensity of three components of the solution coloring during the syntheses carried out at the s values equal to 5, 6.4, and 8 are shown in Figure 5.
The observed slight changes in the solution coloring intensity at the initial stage of the synthesis are presumably associated with a change in the ratio of Pt(IV) and Pt(II) concentrations, as was the case with the citric acid synthesis. An abrupt change in the intensity observed for the three coloring components is due to the formation of a colloidal solution of platinum NPs. At the same time, the duration of the phase formation induction period grows in the following order: s = 5 < s = 8 < s = 6.4 (Figure 6a); the fastest stage of nucleation/growth is observed at s = 6.4 (Figure 6b).
An increase in the alkali content of the solution (an increase in s) leads to a regular change in the E,time curve, i.e., the potential range in which the redox reaction proceeds narrows and the curve itself shifts towards more negative values (Figure 5). The above changes may be associated with an increase in the concentration of the reducing agent, with an increase in s, or with a change in the nature of the product(s) into which ethylene glycol is transformed within the solution. It is obvious that part of the added alkali is neutralized with the introduction of H2PtCl6 into the reaction medium. The rest of the alkali can interact with ethylene glycol to form glycolate anions by the following reactions:
HO–CH2–CH2–OH + NaOH ↔ -OCH2–CH2–OH + H–OH + Na+
HO–CH2–CH2–OH + 2NaOH ↔ -OCH2–CH2–O− + 2H–OH + 2Na+
Although the equilibrium in these reactions, especially in the presence of water, is shifted towards ethylene glycol, the formation of small amounts of glycolate anions capable of reacting with oxidizing agents cannot be excluded.
It is noteworthy that under the conditions of an excess of hydroxyl anions in the reaction medium, part of the Cl− anions in the coordination spheres of Pt(IV) and Pt(II) cations may be replaced by OH− anions. Given the possibility of the transformation of ethylene glycol into various products [36], the influence of the medium pH on the route and mechanism of transformation, as well as the composition of the ethylene glycol oxidation intermediate and resulting products, cannot also be excluded. Changes in the composition (form) of the oxidizer and the reductant transformation products should affect the value of the mixed redox potential of the reaction medium, which we observed during the measurements.
The transition from synthesis in the solution to synthesis in the carbon suspension weakly (within the experimental error) affects the nature of a change in the electrode potential during the synthesis in a highly alkaline solution (s = 8). In the solution with an average value of s = 6.4, the medium redox potential stabilizes more slowly in the suspension (and at s = 5-faster) than in the solutions of similar composition (Figure 5). It should be noted that under all the conditions of the polyol synthesis, the NP nucleation/growth stage determined by a change in the solution coloring is accompanied by a more or less pronounced jump in the medium redox potential (Figure 5).
We have already noted that the changes in the solution coloring in course of the citric acid and polyol syntheses are of a similar nature. At the same time, the potential ranges and the nature of the E,t dependences describing the transformation differ significantly (Figure 2 and Figure 5). Ethylene glycol in an alkaline medium is a much stronger reducing agent than citric acid in a slightly acidic medium, i.e., the stationary values of the indicator electrode redox potentials in the solutions of ethylene glycol and citric acid before introducing the oxidizer are close, respectively, to −0.58 V and −0.05 V relative to the silver chloride electrode. The transformation of Pt(IV) to Pt(0)x NPs at the same temperature also proceeds faster under the polyol synthesis conditions (compare Figure 2 and Figure 5). At the same time, the duration of the NP nucleation/growth stage under the conditions of polyol synthesis is significantly lower than under the conditions of citric acid synthesis (Figure 3 and Figure 6).
The yield of the reaction product in polyol synthesis decreases slightly with an increase in s, with the platinum mass fraction in the Pt/C materials for all the syntheses being about 40 wt.% (Table 1). It should be pointed out that the higher values of the product yield and, possibly, the platinum mass fraction in the “polyol” materials are largely associated with the introduction of excess sulfuric acid into the resulting Pt/C suspension (see Section 3). The efficiency of separating Pt/C directly from the alkaline suspensions would be significantly lower.
The X-ray diffraction patterns for the Pt/C materials obtained in the polyol synthesis are shown in Figure 7. The calculation made according to the Scherrer equation for the most intense platinum reflection <111> presents almost the same values of the average crystallite diameter of 2.2 ± 0.2 nm, as in the materials obtained by the citric acid synthesis. At first glance, this result seems extraordinary since the critical size of platinum nuclei should be smaller in the redox reaction where the potential difference between the oxidizer and the reducing agent is greater. However, citric acid and its transformation products, as well as ethylene glycol transformation products in the alkaline polyol synthesis, are known to be surface-active adsorbates with respect to platinum NPs [28,37]. It is not possible to compare the adsorption activity of different adsorbates, the composition of which is also questionable. Nevertheless, it is clear that the chemisorption of such adsorbates, which prevents the growth of NPs, has a great influence on their size in the Pt/C materials formed in both polyol and citric acid syntheses.

2.3. Cyclic Voltammetry and Electrochemically Active Surface Area of Pt/C Electrocatalysts

Despite the close size of platinum crystallites, the cyclic voltammograms (CVs) of the obtained Pt/C materials differ significantly (Figure 8). The calculation carried out for the hydrogen region of the CVs shows that, among the samples synthesized by the citric acid method, the electrochemically active surface area (ESA) grows with an increase in the synthesis temperature to 80–90 °C; CA_70 < CA_80 ≈ CA_90 (Table 1). Among the samples synthesized by the polyol method, the ESA grows in the following order: EG_6.4 < EG_8 < EG_5 (Table 1). At the same time, the highest ESA value of ~85 m2/g (Pt) is exhibited by a sample synthesized by the polyol method at s = 5. It is worth noting that the above ESA value is noticeably higher than that of the commercial Pt/C catalysts with a 40% platinum loading, e.g., HiSPEC 4000 (Johnson Matthey, London, UK).
As a rule, the ESA value of platinum–carbon catalysts correlates well with the average size of NPs and crystallites. However, in this case, the crystallite size for all the samples obtained is practically the same. The reason for the observed differences in the ESA may be a difference in the NP agglomeration degree and the uniformity of their spatial distribution over the surface of the support. Apparently, the “polyol” material synthesized at s = 5 contains platinum NPs with the most uniform spatial distribution and lowest agglomeration. By analyzing the features of the kinetics for synthesizing the Pt/C materials with the lowest ESA values (CA_70 and EG_6.4), it can be pointed out that, in the event of the citric acid synthesis, the nucleation/growth stage is longest at 70 °C (Figure 3). The synthesis of the EG_6.4 sample is characterized by a combination of a long induction period and a short NP nucleation/growth stage. The reasons for the relatively high degree of NP aggregation, which may cause a relatively low ESA value, are not yet clear in this case.
The study of the changes in the coloring intensity of the platinum precursor solutions as well as in the redox potential of the solutions and carbon suspensions in course of NP synthesis has allowed us to obtain new information on the kinetics of the complex transformation process, demonstrating the possibility of determining the duration of the induction period and the nucleation/growth stage under different synthesis conditions. The results obtained indirectly indicate the invariance of the nature of chemical transformations under the conditions of citric acid synthesis at different temperatures. The measurement results are reproducible, and the techniques themselves demonstrate high sensitivity to the influence of various factors on the kinetics of phase formation. The accumulation and analysis of the experimental data obtained using the developed methods for controlling the transformation kinetics are believed to allow for progress to be made in controlling the structural characteristics of nanoscale platinum-containing materials.

2.4. Advantages of Pt/C Electrocatalysts Containing Nearly Monodispersive Platinum Nanoparticles

The next part of our work demonstrates the positive results of using the methods developed for controlling the kinetics of NP nucleation/growth in the gram-scale production of Pt/C catalysts (more than 10 g of the catalyst per one production cycle) implemented by PROMETHEUS R&D LLC (Rostov-on-Don, Russia). Knowledge about the stabilization time of the mixture redox potential under conditions of the liquid-phase synthesis has made it possible to shorten its duration. At the same time, the main task of optimizing the know-how technique was to search for conditions under which it is possible to obtain platinum NPs with a narrow size distribution. The study of the influence of the temperature and composition of the reaction mixture allowed us to find the conditions under which the platinum NPs nucleation/growth proceeds at the highest rate. As a result, the PMO20 and PMO40 Pt/C catalysts were obtained, containing, respectively, 20% and 40% platinum loading. The X-ray patterns for the synthesized catalysts and the commercial analogs HiSPEC 3000 (20% wt. Pt) and HiSPEC 4000 (40% wt. Pt) are shown in Figure 9.
The calculated platinum crystallite size in PMO20 and PMO40 was 1.2 and 1.4 nm, respectively, and the NP size determined by the results of transmission electron microscopy (TEM) (Figure 10) was 2.0 and 2.6 nm. For the HiSPEC 3000 and HiSPEC 4000 catalysts studied as the conventional samples, the crystallite sizes were 2.5 and 3.5 nm, respectively, and the NP sizes were 2.7 and 3.7 nm. The analysis of histograms of the NP size distribution in the Pt/C materials studied also indicates their narrower size distribution in the PMO series catalysts (Figure 10).
The uniformity of the NP spatial distribution (Figure 10) was assessed using TEM micrographs of the materials studied. For this purpose, the proportions of NPs that did not come into contact with their “neighbors” or overlap over one, two, or three neighboring NPs were calculated. Obviously, the smaller the proportion of overlapping NPs, the more uniform the particle distribution over the surface of the carbon support. In the PMO20 material, about 60% of the NPs proved to exhibit no intersections, while in HiSPEC 3000, about 60% of the particles overlapped over neighboring ones, which indicates a more uniform distribution of nanoparticles over the surface of the Vulcan XC72 carbon in the sample synthesized by PROMETHEUS R&D LLC.
A similar situation was observed for the materials with a platinum mass fraction of 40%, i.e., PMO40 (more than 40% of the particles with no intersections) and HiSPEC 4000 (less than 30% of the particles with no intersections). At the same time, despite the higher mass fraction of platinum in PMO40 compared to HiSPEC 3000, the former was characterized by a higher uniformity of the NP distribution.
Therefore, the Pt/C catalysts synthesized contained NPs with a close size, uniformly distributed over the surface of the carbon support.
Figure 11a shows cyclic voltammograms for the materials studied. The PMO20 material is characterized by the highest currents in the hydrogen (0.04–0.35 V) and oxygen (~0.60–1.20 V) regions of the CVs, with HiSPEC 4000 exhibiting the lowest ones. The ESA values of the catalysts calculated from the hydrogen region of the CVs decrease in the follwoing order: PMO20 ≫ PMO40 ≥ HiSPEC 3000 > HiSPEC 4000 (Figure 11b). The above ESA values and the order of their changes correlate well with the aforementioned NP sizes and the higher (in the PMO materials) or lower (in the HiSPEC catalysts) uniformity of the NP distribution over the surface of the support.
The activity in the ORR was studied using linear sweep voltammetry (see Section 3). The half-wave potential values calculated from linear sweep voltammograms (LSVs) at a disk rotation speed of 1600 rpm increased from 0.90 to 0.92 V in the following order: HiSPEC 4000 < HiSPEC 3000 < PMO40 < PMO20 (Figure 11c). The ORR on the catalysts under study proceeded according to a four-electron mechanism, as indicated by the slopes of the dependence in the Koutetsky–Levich coordinates (Figure 11d). The ORR mass activity of the catalysts increased from 230 to 430 A gPt−1 in the following order: HiSPEC 4000 < HiSPEC 3000 < PMO40 < PMO20 (Figure 11b). Obviously, the ESA was a key factor determining the ORR mass activity of the PMO series catalysts.
A decrease in the size of platinum NPs and an increase in the ESA of Pt/C catalysts are generally known to lead to a decrease in their stability [38]. However, the accelerated stress testing (AST) of the four catalysts under study showed that the degradation rate for the ESA values decreased with an increase in the mass fraction of platinum in the samples, although it was practically the same for the catalysts with significantly different microstructures, i.e., in PMO20 and HiSPEC 3000, PMO40, and HiSPEC 4000, respectively (Figure 12). At the same time, the PMO series materials were characterized by higher ESA values compared to their analogs of the HiSPEC series both before and after the stress testing. It should be noted that the PMO20 catalyst after the AST was characterized by an ESA value of 103 m2 gPt−1, which exceeds even the initial ESA values of the HiSPEC samples.
The catalysts’ mass activity value after the stress testing was 67 to 71% of the initial one, which had no effect on their position in the following mass activity increase order: HiSPEC4000 (165 A gPt−1) ≤ HiSPEC3000 (171 A gPt−1) < PM040 (275 A gPt−1) ≤ PM020 (297 A gPt−1). The absence of the negative effects of the small size of NPs and the high ESA of the PMO series catalysts on their stability may be due to the inhibition of degradation by Ostwald ripening, the redeposition of platinum from smaller NPs to larger ones, being practically impossible in this case since most NPs are of a similar size. The high uniformity of the platinum NPs’ spatial distribution also complicates the process of their aggregation. All these reduce the rate of ESA and Imass degradation during the stress testing, bringing the degradation rate closer to that of the HiSPEC catalysts that contain larger NPs and have a lower ESA.

3. Materials and Methods

3.1. Reagents and Synthesis Methods

3.1.1. Reagents Used

The following chemicals and materials were used in the experimental work: ethylene glycol (top grade, not less than 99.8%, Rehacor, LLC (Moscow, Russia)), H2PtCl6*6H2O (TU 2612-034-00205067-2003, mass fraction of Pt 37.6%, JSC Aurat (Moscow, Russia)), sodium hydroxide (Rehacor, LLC (Moscow, Russia)), citric acid (JSC Vekton (St. Petersburg, Russia)), sulfuric acid (JSC Vekton), carbon support Vulcan XC-72 (Cabot Corporation (Boston, MA, USA)), deionized water (conductivity < 5 μS/cm, GOST 58144–2018) [39], perchloric acid HClO4 (extra pure, Sigma-Aldrich (Merck KGaA, Darmstadt, Germany).

3.1.2. Synthesis of Pt NPs and Pt/C Using Citric Acid as the Reducing Agent

Citric acid monohydrate weighing 0.82 g was added to the mixture consisting of 45 mL of ethylene glycol and 55 mL of distilled H2O. The mixture was transferred to a 250 mL chemical reactor and heated to a given temperature (70, 80, or 90 °C) with constant stirring. While heating to a given temperature, the reaction mixture was saturated with Ar for at least 30 min. Then, with constant stirring and purging with Ar, an aqueous solution of H2PtCl6·6H2O with a volume of 20 mL and a concentration of 0.035 M was introduced. During the synthesis, we measured the solution coloring intensity using the Digital Microscope camera and the platinum indicator electrode potential (redox potential of the reaction medium) using the potentiostat P-40X, SN 30-30-280 (Elins LTD, (Chernogolovka, Russia)). A silver chloride electrode was used as the reference electrode.
Synthesis in the carbon suspension was carried out similarly to the above-mentioned method, except that the support was introduced into the reaction mixture before heating, in the amount necessary to obtain Pt/C with a 40% platinum loading. After 3 h of the synthesis, the heating was ceased, and the mixture was naturally cooled with stirring. After 24 h, the product was filtered and repeatedly rinsed with isopropanol and then with distilled water. Pt/C was dried at 80 °C and then in a desiccator over P2O5 for 24 h. The Pt/C materials obtained in this way are referred to as CA_70, CA_80, and CA_90, where the numbers correspond to the temperature at which the synthesis was carried out.

3.1.3. Synthesis of Pt NPs and Pt/C Using Ethylene Glycol as the Reducing Agent

Ethylene glycol with a volume of 107 mL in the chemical reactor of 250 mL was heated to 90 °C with constant stirring. Then, 4.4 mL of an NaOH aqueous solution of various concentrations (0.75 M, 1 M, and 1.25 M) was introduced, corresponding to the molar ratios with platinum of 5, 6.4, and 8, respectively. The reaction mixture was saturated with Ar for 30 min with constant heating and stirring. Then, a 20 mL H2PtCl6·6H2O aqueous solution with a concentration of 0.035 M was added. During the synthesis, the coloring intensity of the solution and the redox potential of the reaction medium systems were measured.
Synthesis in the carbon suspension was carried out similarly to the above-described method, except that the support was introduced into the reaction mixture before heating in the amount necessary to obtain a 40% platinum loading in the product. The mixture was allowed to stand at a given temperature for 3 h, after which it was naturally cooled with stirring. An hour after the heating was ceased, 20 mL of the 1 M H2SO4 aqueous solution were added to the suspension, after which it was allowed to stand for 24 h with stirring. The resulting Pt/C material was filtered using a Büchner funnel and repeatedly rinsed, first with isopropanol and then with deionized water. Pt/C was dried in the same way as in the citric acid synthesis. The materials obtained in this way are referred to as EG_5, EG_6.4, and EG_8, where the numbers correspond to the molar ratio of alkali to platinum in the initial solution.

3.2. Methods for Studying the Kinetics of Pt NPs Phase Formation

3.2.1. Measuring the Solution Coloring Intensity During the Synthesis

The formation of colloidal solutions of Pt NPs that proceeded during the reaction was accompanied by a change in the coloring of the solution from light yellow to black. To monitor the course of the reaction, the coloring change was monitored with the Celestron Handheld Digital USB Microscope (Celestron, LLC, Torrance, CA, USA). To quantify the solution coloring intensity, we used a program developed by us that converts the coloring of the solution into three components of the additive RGB color model [37], in which each shade of the continuously recorded image was determined by the contributions of three primary colors—red, green, and blue. This made it possible to graphically interpret the processes proceeding in the reaction medium, displaying a change in the quantitative content of each component over time.

3.2.2. Measuring the Redox Potential of the Reaction Medium During the Synthesis

During the synthesis, the potential of the platinum indicator electrode immersed in the reaction medium was measured. For this purpose, the reactor was equipped with a salt bridge, one tube of which was lowered into the reaction mixture and filled (before adding a solution of chloroplatinic acid). The second tube was filled with a 3.5 M potassium chloride solution and lowered into a beaker with a solution of the same composition in which the silver chloride electrode (reference electrode) was located. The electrodes were connected to the P-40X potentiostat (Elins LTD, (Chernogolovka, Russia)), which was used to record the potential difference of the electrodes that was identical to the redox potential of the medium.

3.3. Assessment of Structural and Morphological Characteristics of the Pt/C Materials

3.3.1. Determination of the Pt Mass Fraction

The mass fraction of platinum in the materials was determined by gravimetry, with the mass of the unburned residue left after heating Pt/C to 850 °C. The error margin was ±1%.

3.3.2. Determination of the Average Pt Crystallite Size

The average crystallite size (DAv) was calculated based on the results of X-ray powder diffraction, using the same technique as in [40]. After the fitting of the X-ray pattern using the SciDavis 2.3.0 free software, we determined a width of the 111 reflection at half maximum and calculated DAv according to the Scherrer equation, as described in [40,41,42]. The X-ray patterns were recorded in the angle range of 2θ from 20° to 55° using the ARL X’TRA powder diffractometer with Bragg–Brentano geometry (θ-θ) and CuKα radiation (λ = 0.154056 nm). The error in determining DAv did not exceed ±8%.

3.3.3. Determination of the Average Size and the Size Dispersion of Pt NPs

The determination of the average size of the NPs as well as the analysis of their size dispersion and uniformity of their spatial distribution over the surface of the carbon support were carried out based on the results of TEM. To obtain the catalysts’ micrographs, we used the JEM-2100 transmission electron microscope (Jeol Ltd., Tokyo, Japan) at a voltage of 200 kW and a resolution of up to 0.2 nm. To plot the histograms of the NP size distribution, we took into account no less than 400 randomly selected individual NPs localized at different sections of the sample. Detailed descriptions of the processing technique for TEM micrographs as well as the method to assess uniformity of the NP distribution over the support surface are given in [11]. The error in determining the average size of NPs was ±5%.

3.4. Study of the Electrocatalysts Electrochemical Behavior

3.4.1. Formation of the Catalytic Layer

The formation of a catalytic layer at the end face of the disk electrode was conducted by applying, to its surface, the calculated amount of a catalyst suspension (so-called “catalytic ink”). The catalytic ink was prepared by mixing 0.006 g for 20% wt. Pt and 0.0040 g for 40% wt. Pt of the catalysts with 2 mL of the solution consisting of 1500 µL of extra-pure isopropanol, 400 µL of deionized H2O, and 100 µL of the 1% Nafion aqueous emulsion. The resulting suspension was dispersed for 25 min in an ultrasonic bath, the water temperature in which did not exceed 20 °C. Further, 6 µL of the catalytic ink for the catalysts containing 40% wt. Pt or 8 µL of the catalytic ink for the catalysts containing 20% wt. Pt were applied to the polished and degreased end face of the glass–carbon disk electrode. After the catalytic layer was dried at the electrode, rotating at a speed of 700 rpm, the platinum content in the catalytic layer amounted to 20 μgPt cm−2.

3.4.2. Surface Activation and Determination of the Pt/C ESA

The surface activation and a study of the catalysts’ electrochemical behavior were carried out in a three-electrode electrochemical cell using the VersaSTAT 3 potentiostat (AMETEK, Berwyn, PA, USA). The electrolyte was 0.1 M HClO4 with a temperature of 25 °C. The electrode under study was a catalytic layer at the end face of the rotating or stationary disk electrode. A platinum wire was used as the auxiliary electrode. The silver chloride reference electrode was immersed in a glass shaft into the 3.5 M KCl solution, which was membrane-separated from the electrolyte in the cell. All the potentials in Section 2.4 were provided as per the reversible hydrogen electrode (RHE) scale. The potentials were adjusted to the RHE scale using the following equation: ERHE = Emeasured + E0Ag/AgCl + 0.059 pH (pH = 1), where E0Ag/AgCl = 0.208 V.
Prior to studying the electrochemical behavior, the electrode under study had been activated with 100 cycles of the potential sweep in the range of 0.04–1.2 V, at a scanning rate of 200 mV/s (in an Ar atmosphere). The potential scanning rate was then changed to 20 mV/s with the registration of two CVs in the range of potentials from 0.04 to 1.2 V. The ESA value was calculated for the second CV as per the charge consumed for the adsorption/desorption of atomic hydrogen, as described in [43,44]. The error in determining the ESA was ±10%.

3.4.3. Determination of the ORR Activity

The ORR activity of the catalysts was determined by voltammetry. After the electrolyte was purged with oxygen for 1 h, we measured the LSVs in the potential window from 0.1 to 1.1 V at a potential scanning rate of 20 mV/s at the rotating disk electrode. The measurements were repeated at disk electrode rotation speeds of 400, 900, 1600, and 2500 rpm. The measured LSVs were adjusted as per the solution ohmic resistance, and the LSVs were measured under the same conditions in an argon atmosphere. By the current values at a potential of 0.90 V, we plotted straight-line dependences in the Koutetsky–Levich coordinates, by the extrapolation of which to the y-axis, we determined the values of the kinetic current, as described in [44]. The catalysts ORR mass activity (Imass) was calculated by the kinetic current values, taking into account a platinum loading at the electrode. The accuracy of determining Imass was ±10%. A detailed description of the method is given in [44].

3.4.4. Determination of the Pt/C Catalysts Resistance to Degradation

To study the catalysts durability, we used the stress testing protocol approved in [45], i.e., the sawtooth cycles of a potential change, in the range from 0.6 to 1.0 V at a sweep rate of 100 mV/s, were repeated at the stationary electrode 5000 times while purging the cell with argon. At the same time, every 500 cycles, we registered two CVs in the potential range of 0.04–1.2 V (RHE) at a scanning rate of 20 mV/s. By the second one of these CVs, the ESA was calculated, as described in Section 3.4.2. The durability was assessed by ratios of the ESA values measured before and after the stress testing.

4. Conclusions

The study has demonstrated the possibility of controlling the kinetics of the transformation of Pt(IV) into Pt(0)x NPs in the liquid phase using facile methods for measuring the solution coloring intensity and the indicator electrode potential, which is identical to the redox potential of the reaction mixture. The onset and duration of the Pt NP nucleation/growth stages accompanied by a darkening of the solution can be determined by the characteristic sections of a change in the solution coloring. Changes in the concentrations of different forms of the oxidizer (Pt(IV), Pt(II), and Pt(0)x) as well as successive forms of the reducing agent cause a change in the potential of the system, which is characteristic of each method with its synthesis conditions.
In case of the citric acid synthesis, the change in the reaction mixture redox potential was established to be similar in the synthesis temperature range of 70–90 °C; however, with increasing temperature, both the transformation as a whole and the growth/nucleation stage, in particular, accelerate. The average size of platinum crystallites in the obtained Pt/C materials turned out to be almost the same; however, the ESA of the samples synthesized at higher temperatures (80 and 90 °C) proved to be slightly higher. It can be assumed that this is due to both the narrower NP size distribution and their more ordered spatial distribution. In fact, an increase in the temperature is conducive to a decrease in the nanoseed nucleation/growth stage duration. Ultimately, this may lead to a narrowing of the NP size dispersion.
In the case of polyol synthesis at different pH values, the change in the reaction mixture redox potential depends on the initial ratio of the concentrations of alkali and chloroplatinic acid in the mixture (s = [NaOH]/[H2PtCl6]). With an increase in s from 5 to 8, the transformation proceeds in a more negative and, at the same time, narrower range of potentials. Revealing the reasons for this effect requires additional research. The rate of Pt NPs formation under the polyol synthesis conditions turned out to be higher than in the citric acid synthesis conditions. Interestingly, despite the significant difference in the potentials of the oxidizer (Pt(IV) and Pt(II)) and the reducing agents in the citric acid and polyol syntheses, the size of the platinum crystallites formed proved to be the same. Apparently, this is due to a stronger decline in the formation of the platinum nuclei surface in the presence of citric acid molecules than in the event of ethylene glycol and its oxidation products. At the same time, the Pt/C sample obtained by the polyol synthesis in the solution with a molar concentration ratio of [NaOH]/[H2PtCl6] = 5 was characterized by the highest ESA (85 m2 gPt−1).
The use of the developed “kinetics control” techniques has made it possible, within the framework of scaled liquid-phase synthesis, to obtain nearly monodispersive Pt/C catalysts (PMO20 and PMO40) containing ultra-small platinum NPs of 2 and 2.6 nm, respectively. The ESA and the ORR activity of the obtained catalysts turned out to be significantly higher than those of the commonly used analogs (HiSPEC 3000 and HiSPEC 4000 (Johnson Matthey)). At the same time, the degradation rate of the synthesized catalysts under conditions of the accelerated stress testing proved to be almost the same as that of the analogs by Johnson Matthey. We believe that the unexpectedly high stability of the catalysts characterized by higher ESA and Imass values is due to the positive effect of monodispersive NPs and their uniform spatial distribution on the processes of the catalyst degradation.
The facile methods proposed for controlling the kinetics of the transformations proceeding in the liquid-phase synthesis may provide additional useful information on the behavior of a wide range of systems. They are applicable for solutions of high concentrations and carbon suspensions under mixing conditions or gas purging. At the same time, it is obvious that studying the effect of the phase formation kinetics on the microstructure of the resulting Pt/C materials requires further accumulation, generalization, and analysis of the experimental data.

Author Contributions

V.G. worked out and developed the general idea and methodology of the project, analyzed the results obtained, and wrote and edited the article; K.P. took part in the results analysis and the article design, developed the methodology of the electrochemical performance study, and edited the article; I.N., I.K. and A.K. performed the kinetics and materials synthesis studies; Y.A. performed the electrochemical performance study; I.P. performed the TEM study of the Pt/C materials; and A.N. performed the XRD study of the Pt/C materials. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Strategic Academic Leadership Program of the Southern Federal University (Priority 2030).

Data Availability Statement

Data are contained in the article. Any additional data and explanations to them are available on request from the corresponding author.

Acknowledgments

The authors are grateful to Maltsev, A.V., for his support in the translation and editing processes and assistance in the communications with the Editorial board. The authors are also grateful to Volik, K.K., for his assistance in the XRD pattern registration and to the Shared Use Center “High-Resolution Transmission Electron Microscopy” (SFedU) for conducting the TEM and EDX studies.

Conflicts of Interest

Authors Irina Novomlinskaya and Yana Astravukh were employed by the company Prometheus R&D LLC (Rostov-on-Don, Russia). The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. Schematic representation of the factors influencing the processes of chemical transformations proceeding in the liquid phase.
Figure 1. Schematic representation of the factors influencing the processes of chemical transformations proceeding in the liquid phase.
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Figure 2. Change in the indicator electrode potential (black lines) and the intensity of red, green, and blue components of the reaction medium coloring during the reduction of H2PtCl6 with citric acid at temperatures of 70 °C (a), 80 °C (b), and 90 °C (c). Solid lines represent synthesis in the solution, while dotted lines represent synthesis in the carbon suspension.
Figure 2. Change in the indicator electrode potential (black lines) and the intensity of red, green, and blue components of the reaction medium coloring during the reduction of H2PtCl6 with citric acid at temperatures of 70 °C (a), 80 °C (b), and 90 °C (c). Solid lines represent synthesis in the solution, while dotted lines represent synthesis in the carbon suspension.
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Figure 3. Effect of the synthesis temperature on the duration of (a) the induction period and (b) the nucleation/growth stage of platinum NPs during the reduction of H2PtCl6 with citric acid.
Figure 3. Effect of the synthesis temperature on the duration of (a) the induction period and (b) the nucleation/growth stage of platinum NPs during the reduction of H2PtCl6 with citric acid.
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Figure 4. X-ray diffraction patterns for the Pt/C materials obtained during the citric acid synthesis in carbon suspensions.
Figure 4. X-ray diffraction patterns for the Pt/C materials obtained during the citric acid synthesis in carbon suspensions.
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Figure 5. Change in the indicator electrode potential (black lines) and the intensity of red, green and blue components of the reaction medium coloring during the reduction of H2PtCl6 with ethylene glycol at the s values equal to 5 (a), 6.4 (b), and 8 (c). Solid lines represent synthesis in the solution, while dotted lines represent synthesis in the carbon suspension.
Figure 5. Change in the indicator electrode potential (black lines) and the intensity of red, green and blue components of the reaction medium coloring during the reduction of H2PtCl6 with ethylene glycol at the s values equal to 5 (a), 6.4 (b), and 8 (c). Solid lines represent synthesis in the solution, while dotted lines represent synthesis in the carbon suspension.
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Figure 6. Effect of the ratio of initial concentrations of [NaOH]/[H2PtCl6] in the reaction medium on the duration of (a) the induction period and (b) the nucleation/growth stage of platinum NPs during the reduction of H2PtCl6 with ethylene glycol.
Figure 6. Effect of the ratio of initial concentrations of [NaOH]/[H2PtCl6] in the reaction medium on the duration of (a) the induction period and (b) the nucleation/growth stage of platinum NPs during the reduction of H2PtCl6 with ethylene glycol.
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Figure 7. X-ray diffraction patterns for the Pt/C materials obtained during the polyol synthesis in carbon suspensions.
Figure 7. X-ray diffraction patterns for the Pt/C materials obtained during the polyol synthesis in carbon suspensions.
Catalysts 14 00728 g007
Catalysts 14 00728 g008
Figure 8. Cyclic voltammograms for the Pt/C materials synthesized by the (a) citric acid and (b) polyol methods.
Figure 8. Cyclic voltammograms for the Pt/C materials synthesized by the (a) citric acid and (b) polyol methods.
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Catalysts 14 00728 g009
Figure 9. X-ray diffraction patterns for the PMO20, PMO40 materials and the HiSPEC 3000 and HiSPEC 4000 reference samples.
Figure 9. X-ray diffraction patterns for the PMO20, PMO40 materials and the HiSPEC 3000 and HiSPEC 4000 reference samples.
Catalysts 14 00728 g009
Catalysts 14 00728 g010
Figure 10. Results of the TEM study. Microphotographs of (a,b) PMO20; (c,d) HiSPEC 3000; (e,f) PMO40; (g,h) HiSPEC 4000 catalysts and corresponding histograms of the NPs size distribution. The lower figure shows what proportion of platinum nanoparticles overlaps with three, two and one “neighbors”, respectively, as well as individual NPs.
Figure 10. Results of the TEM study. Microphotographs of (a,b) PMO20; (c,d) HiSPEC 3000; (e,f) PMO40; (g,h) HiSPEC 4000 catalysts and corresponding histograms of the NPs size distribution. The lower figure shows what proportion of platinum nanoparticles overlaps with three, two and one “neighbors”, respectively, as well as individual NPs.
Catalysts 14 00728 g010
Catalysts 14 00728 g011
Figure 11. (a) Cyclic voltammograms for the Pt/C samples; (b) ESA values (blue columns) and Imass (line with red markers) of the materials studied; (c) linear sweep voltammograms of the ORR. The disk rotation speed is 1600 rpm; (d) 1/J − ω−1/2 dependence at 0.90 V. The potential sweep rate is 20 mV/s, the 0.1 M HClO4 solution saturated with Ar (a) or O2 (c,d) at atmospheric pressure.
Figure 11. (a) Cyclic voltammograms for the Pt/C samples; (b) ESA values (blue columns) and Imass (line with red markers) of the materials studied; (c) linear sweep voltammograms of the ORR. The disk rotation speed is 1600 rpm; (d) 1/J − ω−1/2 dependence at 0.90 V. The potential sweep rate is 20 mV/s, the 0.1 M HClO4 solution saturated with Ar (a) or O2 (c,d) at atmospheric pressure.
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Catalysts 14 00728 g012
Figure 12. Dependence of the Pt/C catalysts ESA on the number of cycles of the accelerated stress testing. The slope ratios of the straight lines are presented in the form of the degradation rates for the catalyst ESA (k), the values of which are given next to the corresponding straight line.
Figure 12. Dependence of the Pt/C catalysts ESA on the number of cycles of the accelerated stress testing. The slope ratios of the straight lines are presented in the form of the degradation rates for the catalyst ESA (k), the values of which are given next to the corresponding straight line.
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Table 1. Parameters characterizing the synthesis efficiency and some characteristics of the obtained Pt/C materials.
Table 1. Parameters characterizing the synthesis efficiency and some characteristics of the obtained Pt/C materials.
SampleProduct Yield, %Pt Mass Fraction, wt.%DAv (Pt), nmESA, m2/g (Pt)
CA_7086 ± 234 ± 12.1 ± 0.260 ± 6
CA_8083 ± 238 ± 12.3 ± 0.271 ± 7
CA_9087 ± 238 ± 12.4 ± 0.272 ± 7
EG_586 ± 239 ± 12.1 ± 0.285 ± 8
EG_6.480 ± 241 ± 12.2 ± 0.266 ± 7
EG_880 ± 240 ± 12.1 ± 0.271 ± 7
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Guterman, V.; Paperzh, K.; Novomlinskaya, I.; Kantsypa, I.; Khudoley, A.; Astravukh, Y.; Pankov, I.; Nikulin, A. Advances in Liquid-Phase Synthesis: Monitoring of Kinetics for Platinum Nanoparticles Formation, and Pt/C Electrocatalysts with Monodispersive Nanoparticles for Oxygen Reduction. Catalysts 2024, 14, 728. https://doi.org/10.3390/catal14100728

AMA Style

Guterman V, Paperzh K, Novomlinskaya I, Kantsypa I, Khudoley A, Astravukh Y, Pankov I, Nikulin A. Advances in Liquid-Phase Synthesis: Monitoring of Kinetics for Platinum Nanoparticles Formation, and Pt/C Electrocatalysts with Monodispersive Nanoparticles for Oxygen Reduction. Catalysts. 2024; 14(10):728. https://doi.org/10.3390/catal14100728

Chicago/Turabian Style

Guterman, Vladimir, Kirill Paperzh, Irina Novomlinskaya, Ilya Kantsypa, Alina Khudoley, Yana Astravukh, Ilya Pankov, and Alexey Nikulin. 2024. "Advances in Liquid-Phase Synthesis: Monitoring of Kinetics for Platinum Nanoparticles Formation, and Pt/C Electrocatalysts with Monodispersive Nanoparticles for Oxygen Reduction" Catalysts 14, no. 10: 728. https://doi.org/10.3390/catal14100728

APA Style

Guterman, V., Paperzh, K., Novomlinskaya, I., Kantsypa, I., Khudoley, A., Astravukh, Y., Pankov, I., & Nikulin, A. (2024). Advances in Liquid-Phase Synthesis: Monitoring of Kinetics for Platinum Nanoparticles Formation, and Pt/C Electrocatalysts with Monodispersive Nanoparticles for Oxygen Reduction. Catalysts, 14(10), 728. https://doi.org/10.3390/catal14100728

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