Effective Platinum-Copper Catalysts for Methanol Oxidation and Oxygen Reduction in Proton-Exchange Membrane Fuel Cell
nanomaterials
Article Effective Platinum-Copper Catalysts for Methanol Oxidation and Oxygen Reduction in Proton-Exchange Membrane Fuel Cell
Vladislav Menshchikov 1, Anastasya Alekseenko 1 , Vladimir Guterman 1,*, Andrey Nechitailov 2, Nadezhda Glebova 2 , Aleksandr Tomasov 2 , Olga Spiridonova 1, Sergey Belenov 1,3, Natalia Zelenina 2 and Olga Safronenko 1 1 Chemistry Faculty, Southern Federal University, Rostov-on-Don 344090, Russia; [email protected] (V.M.); [email protected] (A.A.); [email protected] (O.S.); [email protected] (S.B.); [email protected] (O.S.) 2 Division of Solid State Electronics, Ioffe Institute, St. Petersburg 194021, Russia; [email protected]ffe.ru (A.N.); [email protected]ffe.ru (N.G.); [email protected]ffe.ru (A.T.); [email protected] (N.Z.) 3 PROMETHEUS R&D Ltd., Rostov-on-Don 344091, Russia * Correspondence: [email protected]
Received: 20 February 2020; Accepted: 9 April 2020; Published: 13 April 2020
Abstract: The behavior of supported alloyed and de-alloyed platinum-copper catalysts, which contained 14–27% wt. of Pt, was studied in the reactions of methanol electrooxidation (MOR) and oxygen electroreduction (ORR) in 0.1 M HClO4 solutions. Alloyed PtCux/C catalysts were prepared by a multistage sequential deposition of copper and platinum onto a Vulcan XC72 dispersed carbon support. De-alloyed PtCux y/C catalysts were prepared by PtCux/C materials pretreatment − in acid solutions. The effects of the catalysts initial composition and the acid treatment condition on their composition, structure, and catalytic activity in MOR and ORR were studied. Functional characteristics of platinum-copper catalysts were compared with those of commercial Pt/C catalysts when tested, both in an electrochemical cell and in H2/Air membrane-electrode assembly (MEA). It was shown that the acid pretreatment of platinum-copper catalysts practically does not have negative effect on their catalytic activity, but it reduces the amount of copper passing into the solution during the subsequent electrochemical study. The activity of platinum-copper catalysts in the MOR and the current-voltage characteristics of the H2/Air proton-exchange membrane fuel cell MEAs measured in the process of their life tests were much higher than those of the Pt/C catalysts.
Keywords: platinum electrocatalyst; PtCu/C; oxygen electroreduction; methanol electrooxidation; catalyst activity; durability; fuel cell life tests; de-alloyed catalysts; PEM FC
1. Introduction Low-temperature proton-exchange membrane fuel cells are promising sources of electrical energy that can be used in various types of devices, including automobiles, unmanned aerial vehicles, portable chargers, stationary units with a regular power supply, etc. [1–4]. The study and application of hydrogen-air (PEM FC) and methanol (DMFC) fuel cells are of particular interest [3–7]. During the operation of a hydrogen–air fuel cell with a proton exchange membrane, hydrogen is oxidized at the anode: H 2e = 2H+, while two protons and two electrons are formed from one molecule. In a 2 − − direct methanol fuel cell, an electrooxidation reaction of the simplest monohydric alcohol proceeds at the anode: CH OH + H O CO + 6H+ + 6e . Electrons pass through the external circuit, while 3 2 → 2 − protons migrate through a polymer electrolyte membrane (Nafion) [5–7]. Both electrons and protons
Nanomaterials 2020, 10, 742; doi:10.3390/nano10040742 www.mdpi.com/journal/nanomaterials Nanomaterials 2020, 10, 742 2 of 19
+ reach the cathode, and next they react with the continuously supplied oxygen: O2 + 4H + 4e− = 2H2O. High speed current-forming reactions require the presence of highly efficient electrocatalysts in the cathode and anode layers [8–11]. In an acidic environment, the best electrocatalyst for the above reactions is platinum, which is usually used in the form of nanoparticles deposited on the particles of dispersed carbon carriers [9–12]. It is well known that platinum alloying with the base metals such as Ni, Co, Cu, etc. contributes to an increase in the functional characteristics of catalysts in methanol electrooxidation (MOR) and oxygen electroreduction ORR [6,10–14]. The increase in the activity of bimetallic catalysts compared to Pt/C in the methanol electrooxidation reaction is usually explained by the bifunctional catalysis mechanism, in which OH groups adsorbed on the atoms of the doping component facilitate electrochemical desorption of the intermediate products of methanol oxidation from the Pt surface [6,15–17]. The optimization of catalysts for ORR is associated with an increase in the bond strength of Pt–O, which facilitates the adsorption of oxygen and intermediate oxygen-containing products (OH, HO2−, etc.) formed during its electroreduction. At the same time, an excessively strong adsorption interaction can hinder the desorption of reaction products [18,19]. Platinum doping with copper results in a decrease in the interatomic distance of Pt–Pt in the nanoparticles, as well as the change in the energy of free d-orbitals, which facilitates the adsorption of O2 on the surface of metal nanoparticles [11,18–20]. To reduce the content of the precious metal in the bimetallic catalyst, while increasing its activity in current-forming reactions, attempts have been made to optimize the composition and structure of platinum-containing nanoparticles [10,13,18–24]. Note that the features of the hierarchical structural organization of PtCu/C catalysts are determined not only by the shape and size of the metal nanoparticles, but also by their spatial distribution on the surface of the support, and have a crucial effect on the activity of catalysts in ORR and MOR [20–23]. The evaluation of some PtCu/C catalysts activity and stability in ORR and MOR obtained in electrochemical cells clearly indicates their high functional characteristics [20–22,24]. Unfortunately, the composition of bimetallic nanoparticles changes in the process of their functioning: there is a selective dissolution of atoms of the alloying component. When conducting studies in an electrochemical cell in the presence of a large amount of liquid electrolyte, the concentration of Mz+ cations in the solution is low and it doesn’t have a noticeable effect on the electrode characteristics. A membrane–electrode assembly with a proton-conducting polymer electrolyte is another matter. Under the conditions of the membrane-electrode assembly (MEA) operation, the selective dissolution of the base metal from the nanoparticles is dangerous due to the possible poisoning of both the proton conducting polymer, which is a part of the catalytic layer, and the polymer membrane [24–26]. In our opinion, catalysts based on de-alloyed PtCu nanoparticles containing a relatively small amount of a “strongly bounded” copper could be very promising for use in membrane-electrode assemblies of the fuel cells. The direct synthesis of such catalysts is rather problematic, but one could try to obtain them from the copper rich PtCux/C(x > 1) materials by treating them in acids. It is important to make it sure that, firstly, the predominant dissolution of Cu atoms allows the formation of a secondary Pt shell that protects the internal copper atoms from dissolution [22,27,28]. Secondly, it is necessary to determine, whether such de-alloyed PtCux y/C catalysts retain high stability and activity in ORR and − MOR, which are characteristic of the previously studied alloyed PtCux/C catalysts [27–31]. Note that uneven distribution of the doping component atoms in the “body” of the initial PtM nanoparticles obtained during the synthesis, as well as the changes in conditions of their de-alloying can apparently affect both the composition and activity/stability of the de-alloyed catalysts [21,22,32–35]. Thus, the search for the best PtCu compositions and structures of bimetallic nanoparticles, selection of the optimal conditions for the transformation of alloyed PtCux/C catalysts into a de-alloyed state, as well as the study of the de-alloyed PtCux y/C catalysts behavior, proves to be timely and relevant. − This study is aimed at a comparative analysis of the functional characteristics of de-alloyed PtCux y/C catalysts obtained by the acid pretreatment of PtCux/C materials, as well as their comparison − with the characteristics of commercial Pt/C catalysts. It has been carried out not only in electrochemical Nanomaterials 2020, 10, 742 3 of 19 cells with liquid electrolyte, but also in the MEAs of a hydrogen–air fuel cell with a proton exchange membrane.
2. Materials and Methods Vulcan XC-72 carbon-supported platinum-copper catalysts were prepared by a four-stage synthesis in the liquid phase, sodium borohydride being used as a reducing agent as described in [22]. Initially, copper nanoparticles were deposited on a carbon support from a CuSO4 solution. At the second and third stages, the calculated amounts of copper (CuSO4) and platinum (H2PtCl6) precursors were added to the suspension. Subsequently, they were reduced with the excess sodium borohydride solution. At the fourth stage, formation of a platinum layer on the surface of nanoparticles was carried out by reducing Pt (IV) from the solution, which did not contain copper ions. The PtCux/C suspension was filtered, and the catalyst was dried over P2O5. Heterogeneity of the components distribution (an increase in the platinum concentration from the center to the surface) in the platinum-copper nanoparticles, formed in accordance with the described synthesis technique, was proved in [22]. The composition of the obtained PtCux/C materials samples, labeled S1–S5 below, corresponded to 1.7 x ≤ 2.9 values (see the “Results and Discussions” section). A portion of each of the synthesized catalysts ≤ was kept in solutions of different acids at 25 ◦C for 2–6 h with constant stirring. After being obtained in the liquid phase or treated in acid solutions, all materials were filtered, washed with the distilled water, dried at room temperature in a desiccator over P2O5. De-alloyed PtCux y/C materials, obtained from − samples S1–S5 as a result of acid treatment, were labeled S1A–S5A, respectively. The more detailed information on the composition of specific materials and conditions of their acid treatment is given in the Results and Discussion section. The ratio of platinum and copper in the platinum-copper catalysts was determined by the method of X-ray fluorescence analysis (XRFA) on a spectrometer with the total external reflection of X-ray radiation RFS-001 (Research Institute of Physics, SfedU, Rostov-on-Don, Russia). The mass fraction of metals was determined by the mass of the residue obtained after 40 min at 800 ◦C in air. For some samples the changes in the mass and the kinetics of high-temperature oxidation were studied by thermogravimetry. For this, the combined TGA/DSC/DTA NETZSCH STA 449 C analyzer was used. Oxidation was carried out in the atmosphere consisting of N2 (80%) and air (20%), in the temperature range from 20 to 750 ◦C at the heating rate of 10 ◦C/min and the gas flow rate of 20 mL/min, using corundum crucibles. When calculating the mass fractions of metals in PtCu/C samples, it was assumed that the non-combustible residue consists of Pt and CuO. The accuracy of mass fractions determination was 0.4%. ± The catalysts phase composition was determined by X-ray phase analysis (the voltage on the X-ray tube was 50 kV, the current was 150 µA, the time for the spectrum recording was 300 s, molybdenum anode was used). Diffractograms were taken on an ARL X‘TRA powder diffractometer with the Bragg-Brentano geometry (θ-θ), CuKα radiation (λ = 0.15405618 nm). Measurements were carried out at room temperature. Samples were thoroughly mixed and placed in a cuvette1.5 mm. in depth. The shooting was carried out in the angles range of 15–55 degrees with the step of 0.02 degrees and the speed of 8 to 0.5 degrees per minute, depending on the task set. The average crystallite size of the metal phase was determined using the Scherrer equation for a more intense reflection (111), as described in [22,36]. Electrochemical behavior of the catalysts in a standard three-electrode cell was studied by cyclic voltammetry (CV) at the solution temperature of 23 ◦C on an AFCBP bipotentiostat (Pine Research Instrumentation, Durham, CA, USA). A 0.1 M HClO4 solution (ChP, manufactured by Vecton, St. Petersburg, Russia) was used as an electrolyte. A saturated silver chloride electrode was used as a reference electrode, a platinum wire—as a counter electrode. All the potentials were given relative to the potential of the reversible hydrogen electrode (RHE). The studied electrode was a catalytic layer formed at the end of the glassy carbon disk electrode a drop of suspension being applied first. Before applying the suspension, the electrode surface was polished and then washed in isopropyl alcohol. Nanomaterials 2020, 10, 742 4 of 19
To obtain a catalyst suspension (catalytic “ink”), 900 µL of isopropyl alcohol and 100 µL of a 0.5% aqueous emulsion of Nafion® polymer were added to 0.0060 g of each sample. Next, the suspension was dispersed by the ultrasound under cooling for 15 min. With continuous stirring, a 6 µL aliquot of “ink” was taken using a micropipette and applied to the end face of a polished and degreased glassy carbon electrode with an area of 0.196 cm2, the exact weight of the drop being recorded. After being dried, 7 µL of a 0.05% Nafion® emulsion were applied to fix the catalytic layer, before the electrode was dried in air for another 15 min. The studied electrode was standardized by means of 100 cycles of potential scanning in the range of values from 0.04 to 1.2 V at the rate of 200 mV/s. Then, 2 cyclic voltammograms (CV) were recorded on the fixed electrode at the potential sweep rate of 20 mV/s. The calculation of the electrochemically active surface area (ECSA) was performed for the second CV. For this purpose, the amount of electricity spent on electrochemical adsorption Qad and desorption Qd of hydrogen was estimated as described in [22]. In some cases, ECSA was additionally measured by the oxidation of a chemisorbed CO monolayer, as described in [22]. In the latter case, the electrode was kept at a potential of 0.1 V in an electrolyte saturated with CO for 20 min. Then, the solution was purged with argon for 20 min, after that, two cyclic voltammograms were recorded, the purge not being stopped, and according to these voltammograms the calculation was performed. ECSA values are given with an accuracy of 10%. ± While studying the catalysts activity in MOR, methanol and perchloric acid were added to the electrochemical cell, thereby obtaining a solution of 0.1 M HClO4 + 0.5 M CH3OH. Cyclic voltammograms were recorded in the potential range of 0.04–1.3 V with a potential sweep rate of 20 mV/s. Chronoamperograms were measured at the potential of 0.70 V. The measurements were carried out in the argon atmosphere. To evaluate the catalysts activity in ORR, a 0.1 M HClO4 solution was saturated with oxygen for 1 h, after that a series of voltammograms was measured in the range from 0.12 to 1.19 V with a linear potential sweep at the speed of 20 mV/s at the electrode rotation speeds of 400, 900, 1600, and 2500 rpm. To take into account the contribution of the ohmic potential drop and non-ORR related processes, the voltammograms, obtained at the potential scanned towards more positive values were normalized according to the generally accepted methods [37–40]. For this, the potential of the electrode under study was refined by the formula: E = Eset – It R where: Eset is the set value of the potential, It × R is the ohmic potential drop equal to the product of the current strength by the resistance (R) of × the solution layer between the reference electrode and the studied electrode, which in this case was 23 ohms. This resistance value is in good agreement with the published data [38]. The contribution of the processes, occurring on the electrode in the oxygen-free solution (Ar atmosphere), was taken into account by subtracting from the voltammogram a similar curve recorded on the same electrode during measurements in the Ar atmosphere: (I(O ) I(Ar)), as described in [39,40]. The catalytic activity of the 2 − catalysts in the ORR (kinetic current) was determined by the normalized voltammograms, taking into account the contribution of mass transfer under the conditions of rotating disk electrode (RDE) [39,40]. The kinetic current was calculated according to the Koutetsky–Levich equation: 1/j = 1/jk + 1/jd, where j is the experimentally measured current, jd is the diffusion current, and jk is the kinetic current. Kinetic currents were calculated for the potential of 0.90 V (RHE). It is well known that the ECSA values and the values of the catalyst activity depend on the composition, structure, and method of the catalytic layer deposition [37,40–42]. Therefore, the electrodes, on which similar catalytic layers were formed by the same method but with commercial Pt/C HiSPEC3000 and HiSPEC4000 (Johnson Matthey, Swindon, UK) electrocatalysts containing 20% and 40% wt. platinum, respectively, were used as the reference samples. The tests of electrocatalysts in the membrane-electrode assembly were carried out as follows. To form the cathode and anode layers, the studied catalyst was applied in the necessary amount to achieve platinum loading of 0.4 mg/cm2. A commercial Nafion solution (DE-2020, 20% by weight) was used. Nanomaterials 2020, 10, 742 5 of 19
Technological operations to prepare the dispersion of the electrode material, required for the subsequent fabrication of the MEA, included two stages: mechanical and ultrasonic dispersion of a mixture of precisely weighed components in an isopropanol-water mixture. The volume ratio of the liquid components of isopropanol to water was ~1: 1. The ratio of the masses in the solid and liquid phases in the final dispersion was in the range 1:40–1:80. Mechanical dispersion was performed on a magnetic mixer of the Milaform MM-5M type for ~0.5 h with an arm rotation speed of ~400 rpm until a visually homogeneous (without visible lumps) mass was obtained. Subsequent ultrasonic dispersion was carried out in a Branson 3510 ultrasonic bath for 40–100 h to obtain a homogeneous dispersion that did not delaminate for one minute. Membrane–electrode assemblies were made by applying a uniform dispersion of components directly to the proton-conducting membrane through a stainless-steel mask. Before applying the electrode material, the membrane was kept in 0.5 M sulfuric acid for 15 min at temperature 70–80 ◦C, followed by five times washing with water. The electrodes were made by smearing the dispersion of components in an isopropanol–water mixture at room temperature onto a 50 µm (46–50 µm) thick Nafion proton-conducting membrane pre-treated at 85 ◦C. The amount of supported catalyst was controlled gravimetrically. Before conducting electrochemical measurements, the MEA was kept in 0.5 M sulfuric acid for 15 min at temperature 70–80 ◦C, followed by five times washing with water. After that, the MEA was placed in a standard measuring cell (FC-05-02, ElectroChem, Inc., Woburn, MA, USA) with graphite collector electrodes. Toray060 standard carbon paper was used as the gas diffusion layer. The change in the MEA characteristics during aging was monitored by the current-voltage characteristics. We used the two-electrode method, potentiostat P-150 (LLC Electrochemical Instruments, Moscow, Russia). Potential sweep rate 10 mV/s. Before starting the main measurements, − the MEA was activated as described in [43]. MEA aging was carried out at room temperature and atmospheric pressure for a given number of cycles (0, 100, 300, 1000, etc.) of a voltage scan in the range 0.6–1.0 V with a potential scan rate of 50 mV/s. Wet ( 100%) N and H were applied to the electrodes. ≈ 2 2 An aging electrode was supplied with N2. The MEA aging regime corresponded to that of the DOE protocols (Table A-1 in [44]).
3. Results and Discussion
3.1. Methanol Electro-Oxidation at the Alloyed and De-Alloyed Platinum-Copper Catalysts The initial alloys and the platinum-copper catalysts obtained from them after two-hour treatment in 1 M HNO3 at 25 ◦C (de-alloyed), which contained from 18.5 to 26.6% wt. of platinum, were studied as catalysts for the methanol electro-oxidation (Table1).
Table 1. Composition and structural characteristics of the obtained PtCu/C and commercial Pt/C materials.
Metal Component Composition of the Platinum Catalysts (According to XRFA) Average Loading, Sample After 2θ (111), grad Crystallite In “as-Prepared” ω (Pt), Electrochemical Size, DAv *, nm State % wt. Standardization S1 Pt Cu Pt Cu 19.9 0.5 41.2 3.0 0.2 37 63 67 33 ± ± S2 Pt Cu Pt Cu 23.4 0.5 41.3 3.0 0.2 31 69 63 38 ± ± S3 Pt Cu Pt Cu 26.6 0.5 41.2 2.8 0.2 26 74 67 33 ± ± S1A Pt Cu Pt Cu 18.5 0.5 40.9 2.7 0.2 53 47 63 38 ± ± S2A Pt Cu Pt Cu 22.4 0.5 41.0 3.0 0.2 53 47 67 33 ± ± S3A Pt Cu Pt Cu 26.4 0.5 40.8 2.7 0.2 56 44 67 33 ± ± JM20 (Pt/C) Pt Pt 20.0 0.5 39.9 2.0 0.2 ± ± * DAv, nm—average crystallite size, calculated by the Scherrer formula. Nanomaterials 2020, 10, x FOR PEER REVIEW 6 of 19 starting materials (S1, S2, S3) to the de-alloyed catalysts obtained from them (Table 1, Figure 1). This indicates a decrease of the copper content in the catalysts metal component and correlates with the obtained results, when their composition was determined (Table 1). The average crystallite diameters in the studied bimetallic materials, calculated by the Scherrer formula, are from 2.7 to 3.0 nm (Table 1). Given the accuracy of the measurements, this indicates the absence of a significant effect of the initial composition and acid treatment on their size. At the same time,Nanomaterials it should2020 ,be10 ,noted 742 that the application of the Scherrer equation to calculate the average size6 of of 19 bimetallic nanoparticles may provide results which are not quite accurate, since such calculation does not take into account the contribution of the inhomogeneous structure of nanoparticles to the broadeningThe X-ray of reflection diffraction 111 patterns [45]. of platinum-copper materials contain reflections corresponding to the phases of carbon and platinum (Figure1). In this case, platinum characteristic reflections are shifted Acid treatment of all the initial PtCux/C samples leads to the dissolution of a significant amount oftoward copper large (Table values 1). ofA thedecrease 2 theta in angles, the concentration this being a consequenceof copper in ofthe a Pt–Cusamples solid cannot solution be associated formation. onlyA significant with the di ffdissolutionerence in the of compositionits amorphized of the oxides, initial S1–S3since samplesjudging hadby the practically data of no the eff ectX-ray on diffractometry,the position of the characteristiccontent of metallic reflections copper of in the the X-ray solid disolutionffraction based pattern, on platinum, in particular, decreases. on 2 thetaThis leadsangles to of the the observed characteristic decrease maximum in the 1112-theta (Table angl1,e Figure for the1b). maximum Apparently, (111) a largerduring or the a smaller transition fraction from theof copper initial to in the these acid-treated samples is samples contained (Table in X-ray 1, Figure amorphous 1). Note oxidesthat despite [45], whilethe significant the composition difference of inthe the metal initial component compositions of various of the catalystsplatinum-copper is indeed catalysts, similar. Atthe the compositions same time, theof the 2-theta three angle de-alloyed of the maximum 111 in the diffraction patterns decreases with the transition from the starting materials (S1, samples are similar to each other: Pt53Cu47–Pt56Cu44. Thus, the amount of “firmly bound” copper, whoseS2, S3) selective to the de-alloyed dissolution catalysts from obtainedplatinum–copper from them nanoparticles (Table1, Figure does1). not This occur indicates at the a stage decrease of acid of treatment,the copper slightly content depends in the catalysts on the metal initial component composition and of correlates the materials, with theobtained obtained by results,the synthesis when methodtheir composition described wasabove. determined (Table1).
Figure 1. (a) X-ray diffraction patterns of the “as-prepared” S1, S2, S3 PtCux/C and de-alloyed S1A, Figure 1. (a) X-ray diffraction patterns of the “as-prepared” S1, S2, S3 PtCux/C and de-alloyed S1A, S2A, S3A PtCux y/C catalysts. (b) In the inset (on the right)—an enlarged fragment of the diffractogram S2A, S3A PtCux−−y/C catalysts. (b) In the inset (on the right)—an enlarged fragment of the diffractogram area, highlighted by the dotted line in Figure (a). area, highlighted by the dotted line in Figure (a). The average crystallite diameters in the studied bimetallic materials, calculated by the Scherrer The behavior of the initial and de-alloyed catalysts in the process of electrochemical formula, are from 2.7 to 3.0 nm (Table1). Given the accuracy of the measurements, this indicates the standardization has significant differences. CVs of S1–S3 samples during the first few cycles show absence of a significant effect of the initial composition and acid treatment on their size. At the same anodic maxima in the potential range 0.25–0.35 V, associated with the dissolution of copper from the time, it should be noted that the application of the Scherrer equation to calculate the average size intrinsic phase (Figure 2) [17,24]. For the samples of de-alloyed catalysts S1A–S3A, similar maxima of bimetallic nanoparticles may provide results which are not quite accurate, since such calculation on CVs were not observed. It should be noted that the materials surface development in the “as does not take into account the contribution of the inhomogeneous structure of nanoparticles to the prepared” state continues during a greater number of cycles than for catalysts treated in acid (Figure broadening of reflection 111 [45]. 2). This indirectly indicates a long-term reorganization of the nanoparticles surface, due to a more Acid treatment of all the initial PtCux/C samples leads to the dissolution of a significant amount intensive anodic dissolution of copper atoms. According to the published data [17,21], the anodic of copper (Table1). A decrease in the concentration of copper in the samples cannot be associated only with the dissolution of its amorphized oxides, since judging by the data of the X-ray diffractometry, the content of metallic copper in the solid solution based on platinum, decreases. This leads to the observed decrease in the 2-theta angle for the maximum (111) during the transition from the initial to the acid-treated samples (Table1, Figure1). Note that despite the significant di fference in the initial compositions of the platinum-copper catalysts, the compositions of the three de-alloyed samples are similar to each other: Pt53Cu47–Pt56Cu44. Thus, the amount of “firmly bound” copper, whose selective Nanomaterials 2020, 10, x FOR PEER REVIEW 7 of 19 dissolution of copper from the Pt–Cu solid solution mainly occurs in the potential range 0.60–0.85 V and is reflected by a gradually decreasing local maximum on the CVs at the above potentials (Figure 2, samples S1–S3). It was found that during the standardization stage the copper content in all studied catalysts (alloyed and de-alloyed) decreased due to its selective dissolution, the composition of the materials, asNanomaterials a result, being2020, 10 approximately, 742 the same Pt63Cu37–Pt67Cu33 (Table 1). Moreover, 70% to 83% and7 of 33 19 to 45% of the copper content are dissolved from the alloyed and de-alloyed catalysts, respectively. Given the lower content of the alloying component in de-alloyed catalysts (Table 1), we can assume dissolution from platinum–copper nanoparticles does not occur at the stage of acid treatment, slightly a significant decrease in the concentration of copper (II) cations in the electrolyte and, therefore, a depends on the initial composition of the materials, obtained by the synthesis method described above. decrease in their possible negative effect on Nafion. Nevertheless, in order to exclude the influence The behavior of the initial and de-alloyed catalysts in the process of electrochemical standardization of copper (II) cations, which were present in the solution, on the behavior of the catalysts, prior to has significant differences. CVs of S1–S3 samples during the first few cycles show anodic maxima further electrochemical studies, the electrolyte was replaced with a freshly prepared 0.1 M HClO4 in the potential range 0.25–0.35 V, associated with the dissolution of copper from the intrinsic phase solution, which was then saturated with argon for 20 min. (Figure2)[ 17,24]. For the samples of de-alloyed catalysts S1A–S3A, similar maxima on CVs were Figure 3a,c shows cyclic voltammograms of the standardized catalysts. They have the form not observed. It should be noted that the materials surface development in the “as prepared” state characteristic of Pt/C and PtM/C electrocatalysts [17,21,24]. The ECSA values of PtCu/C materials, continues during a greater number of cycles than for catalysts treated in acid (Figure2). This indirectly determined by the areas of electrochemical adsorption/desorption of atomic hydrogen and the indicates a long-term reorganization of the nanoparticles surface, due to a more intensive anodic oxidation peaks of the monolayer of chemisorbed CO, are in the range from 30 to 40 m2/g (Pt) (Table dissolution of copper atoms. According to the published data [17,21], the anodic dissolution of copper 2), which is much lower than that of the commercial Pt/C sample. The lower ECSA values of the from the Pt–Cu solid solution mainly occurs in the potential range 0.60–0.85 V and is reflected by a platinum–copper catalysts, compared to the commercial Pt/C sample, are largely due to the larger gradually decreasing local maximum on the CVs at the above potentials (Figure2, samples S1–S3). average crystallite size (Table 1), as well as the agglomeration (aggregation) of the nanoparticles.
Figure 2. Cyclic voltammograms of catalysts in the standardization process (100 cycles). The electrolyte Figure 2. Cyclic voltammograms of catalysts in the standardization process (100 cycles). The solution is 0.1 M HClO4 saturated with argon at atmospheric pressure. The sweep rate of potential is electrolyte200 mV/s. solution is 0.1 M HClO4 saturated with argon at atmospheric pressure. The sweep rate of potential is 200 mV/s. It was found that during the standardization stage the copper content in all studied catalysts
(alloyed and de-alloyed) decreased due to its selective dissolution, the composition of the materials, as a result, being approximately the same Pt63Cu37–Pt67Cu33 (Table1). Moreover, 70% to 83% and 33 to 45% of the copper content are dissolved from the alloyed and de-alloyed catalysts, respectively. Given the lower content of the alloying component in de-alloyed catalysts (Table1), we can assume a significant decrease in the concentration of copper (II) cations in the electrolyte and, therefore, a decrease in their Nanomaterials 2020, 10, x FOR PEER REVIEW 8 of 19
The acid treatment of S1–S3 catalysts does not lead to a considerable change in the platinum ECSA (Table 2). Apparently, the stage of electrochemical standardization, leading to the formation of catalysts with approximately the same composition, eliminates the differences in ECSA values that could occur in the initial and de-alloyed samples. The oxidation of chemisorbed CO on all platinum-copper catalysts begins at the potentials lower than those on Pt/C, and ends in a narrower range of potentials, approximately 0.63–0.88 V (Figure 3b,d). Bimodal maxima of CO oxidation (Figure 3b,d) can be related to heterogeneity of the composition and structure of bimetallic nanoparticles, as well as their size dispersion [30]. Electrocatalysts activity in the MOR was determined by the methods of cyclic voltammetry and chronopotentiometry. Using the CVs measured in 0.5 M solutions of CH3OH, we calculated the specific amount of electricity QCH3OH (Cl/g(Pt)) spent on methanol oxidation in the direct course of potential sweep (Figure 4a,c, Table 2). Despite the smaller ECSA values for all platinum-copper catalysts, QCH3OH is greater than that for Pt/C (Table 2).
Table 2. ECSA values and parameters characterizing the catalysts behavior in the methanol electrooxidation reaction. Nanomaterials 2020, 10, 742 8 of 19 Chronoamperometry ECSA ECSA Imax oxidation results at E = 0.70 V QCH3OH*102, possibleSample negative ( eHffadsect/des on), Nafion.(CO), Nevertheless, in order to excludeCH3OH, the influenceIinitial of, copper (II) cations, Cl/g(Pt) Ifinal, A/g(Pt) which were presentm2/g(Pt) in the solution,m2/g(Pt) on the behavior of the catalysts,A/g(Pt) prior toA/g(Pt) further electrochemical
studies, the electrolyte was replaced with a freshly prepared 0.1 M HClO4 solution, which was then saturatedS1 with argon 39 ± for4 20 min. 31 ± 3 127.8 970 492.3 230.6 S2Figure 3a,c shows 31 ± 3 cyclic voltammograms 33 ± 3 116.6 of the standardized 816 catalysts. 464.4 They have 239.0the form characteristicS3 of Pt 37/C ± and4 PtM/ 29C electrocatalysts± 3 70.1 [17,21,24]. The 600 ECSA values 432.3 of PtCu/C materials, 82.8 S1A 38 ± 4 31 ± 3 96.0 868 529.5 248.1 determined by the areas of electrochemical adsorption/desorption of atomic hydrogen and the oxidation S2A 43 ± 4 37 ± 4 107.8 997 2 578.7 269.9 peaksS3A of the monolayer 33 ± 3 of chemisorbed 32 ± 3 CO, are 72.3 in the range from 647 30 to 40 m /g 411.7 (Pt) (Table2), 117.7 which is muchJM20 lower than that 81 ± of8 the commercial 78 ± 8 Pt/C sample. 32.9 The lower 350 ECSA values 291.6of the platinum–copper 126.5 catalysts,*When comparedcalculating tothe the specific commercial values of Pt the/C electric sample, current are largely and the due amount to the of larger electricity, average ECSA crystallite values size (Tablewere1), used, as well calculated as the agglomerationby the adsorption/desorption (aggregation) ofof atomic the nanoparticles. hydrogen.
Nanomaterials 2020, 10, x FOR PEER REVIEW 9 of 19 (a) (b)
(c) (d)
FigureFigure 3. 3. CyclicCyclic voltammograms voltammograms of of electrocatalysts electrocatalysts in in the the argon argon atmosphere atmosphere ( (aa,c,c)) after after completion completion of of standardizationstandardization and and after after CO CO purging purging ( (bb,d,d).). Commercial Commercial Pt/C Pt/C (JM20) (JM20) and and initial initial PtCu PtCux/Cx/C materials— materials— ((aa,b,b);); commercial commercial Pt/C Pt/C (JM20) (JM20) and and de-alloyed de-alloyed PtCu PtCuxx−y/Cy/C materials. materials. − TheThe same acid is treatment true for the of maximum S1–S3 catalysts currents does of notmeth leadanol to oxidation a considerable on CVs: change for bimetallic in the platinumcatalysts theyECSA are (Table essentially2). Apparently, higher than the stage for Pt/C of electrochemical (Figure 4 and standardization, Table 2). The potentials leading to for the the formation onset of methanolcatalysts withoxidation, approximately determined the for same platinum-copper composition, eliminates catalysts theby ditheff erencescorresponding in ECSA CVs values of thatthe forwardcould occur scan, in are the approximately initial and de-alloyed 150–200 samples. mV less than for commercial Pt/C. Despite a considerably higherThe methanol oxidation oxidation of chemisorbed rate and, CO onpossibly, all platinum-copper a higher concentration catalysts begins of intermediate at the potentials reaction lower productsthan those on on the Pt/ C,electrode and ends surface, in a narrower platinum-coppe range of potentials,r catalysts, approximately as a whole, 0.63–0.88showed higher V (Figure current3b,d). valuesBimodal when maxima chronoamperograms of CO oxidation (Figure were 3recordedb,d) can beat relatedthe potential to heterogeneity of 0.7 V of(Table the composition 2, Figure 4b,d) and comparedstructure ofto bimetallic Pt/C. Together nanoparticles, with a lower as well oxidation as their sizepotential dispersion of a [monolayer30]. of chemisorbed CO (Figure 3b,d), this indicates a higher tolerance of bimetallic catalysts to intermediate products of methanol oxidation. The least activity in MOR was demonstrated by S3 and S3A catalysts.
(a) (b)
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Table 2. ECSA values and parameters characterizing the catalysts behavior in the methanol electrooxidation reaction.
Chronoamperometry ECSA QCH3OH * Imax oxidation ECSA (CO), 2 Results at E = 0.70 V Sample (Hads/des), 2 10 , CH3OH, 2 m /g(Pt) m /g(Pt) Cl/g(Pt) A/g(Pt) Iinitial, Ifinal, A/g(Pt) A/g(Pt)
S1 39 4 31 3 127.8 970 492.3 230.6 ± (c) ± (d) S2 31 3 33 3 116.6 816 464.4 239.0 ± ± S3Figure 3. Cyclic 37 voltammograms4 29 of3 electrocatalysts 70.1 in the argon atmosphere 600 (a,c) after 432.3 completion of 82.8 ± ± S1A 38 4 31 3 96.0 868 529.5 248.1 standardization± and after CO purging± (b,d). Commercial Pt/C (JM20) and initial PtCux/C materials— S2A 43 4 37 4 107.8 997 578.7 269.9 (a,b); commercial± Pt/C (JM20) and± de-alloyed PtCux−y/C materials. S3A 33 3 32 3 72.3 647 411.7 117.7 ± ± JM20 81 8 78 8 32.9 350 291.6 126.5 The same is true± for the maximum± currents of methanol oxidation on CVs: for bimetallic catalysts *they When are calculating essentially the specifichigher valuesthan offor the Pt/C electric (Figur currente 4 andand the Table amount 2). ofThe electricity, potentials ECSA for values the wereonset used, of calculatedmethanol byoxidation, the adsorption determined/desorption for of atomicplatinum-copper hydrogen. catalysts by the corresponding CVs of the forward scan, are approximately 150–200 mV less than for commercial Pt/C. Despite a considerably Electrocatalystshigher methanol activityoxidation in rate the MORand, possibly, was determined a higher byconcentration the methods of ofintermediate cyclic voltammetry reaction and products on the electrode surface, platinum-copper catalysts, as a whole, showed higher current chronopotentiometry. Using the CVs measured in 0.5 M solutions of CH3OH, we calculated the specific values when chronoamperograms were recorded at the potential of 0.7 V (Table 2, Figure 4b,d) amount of electricity QCH3OH (Cl/g(Pt)) spent on methanol oxidation in the direct course of potential compared to Pt/C. Together with a lower oxidation potential of a monolayer of chemisorbed CO sweep (Figure4a,c, Table2). Despite the smaller ECSA values for all platinum-copper catalysts, (Figure 3b,d), this indicates a higher tolerance of bimetallic catalysts to intermediate products of Q is greater than that for Pt/C (Table2). CH3OHmethanol oxidation. The least activity in MOR was demonstrated by S3 and S3A catalysts.
Nanomaterials 2020, 10, x FOR PEER REVIEW 10 of 19 (a) (b)
(c) (d)
FigureFigure 4. (a 4.,c )(a Cyclic,c) Cyclic voltammograms voltammograms and and ((b,d) chronoamperograms chronoamperograms of commercial of commercial Pt/C Pt (JM20)/C (JM20) and and the studiedthe studied (a,b ()a alloyed,b) alloyed and and (c (,dc,)d de-alloyed) de-alloyed PtCuPtCu/C/C catalysts. catalysts. The The currents currents are arenormalized normalized to ECSA. to ECSA. The potentialThe potential sweep sweep rate rate during during CVs CVs registration registration is is 20 20 mV/s. mV/s. The The electrolyte electrolyte is a issolution a solution of 0.1 of M 0.1 M HClO4 + 0.5 M CH3OH saturated with Ar at atmospheric pressure. HClO4 + 0.5 M CH3OH saturated with Ar at atmospheric pressure.
It should be noted that de-alloyed PtCux−y/C catalysts are not inferior in activity to MOR electrocatalysts, which were deposited on a glass-graphite electrode in the “as-prepared” state and initially contained a significantly larger amount of copper. Obviously, the reason for their similar behavior is the proximity of the composition/structure of the platinum-copper catalysts, formed after a portion of copper being selectively dissolved due to a chemical and/or electrochemical treatment of the initial materials (Table 1) [45]. A comparative analysis of the electrochemical behavior of the platinum-copper catalysts in MOR (Figure 4, Table 2) suggests a noticeably higher activity of samples S1, S1A, S2 and S2A compared to S3 and S3A: initial S1 and S2, and de-alloyed S1A, S2A catalysts exhibit higher values of QCH3OH (Table 2), electric current of “methanol maximum” on CVs (Figure 4a,c) and currents on chronoamperograms of methanol oxidation (Figure 4b,d). This difference in the catalysts behavior can be partially due to the difference in the composition of the metal component after electrochemical standardization: for example, S3A sample contains slightly less copper than S1A and S2A (see Table 1). In this case, as was noted above, reliable differences in the ECSA values of different platinum-copper catalysts were not established (Table 2). In our opinion, one should take into account possible differences in the character of atoms localization of a “firmly bound” copper in the standardized nanoparticles, their surface being enriched in platinum. The smaller the thickness of the secondary platinum shell, the stronger the promoting effect of the alloying component atoms on the activity of platinum in electrochemical reactions can be expressed. This study has shown that platinum-copper catalysts exhibit much higher mass activity (A/g(Pt)) and specific activity (A/m2) in MOR compared to the commercial Pt/C material, despite a significantly lower ECSA. The pretreatment of catalysts in the nitric acid promotes leaching of a larger copper fraction from the nanoparticles, but does not lead to a decrease in the activity of catalysts in MOR. Therefore, such a procedure can be used for the pretreatment of catalysts to reduce the subsequent pollution of MEAs with copper cations.
3.2. Electroreduction of Oxygen on Alloyed and De-Alloyed Platinum-Copper Catalysts The next step in our research was to test the hypothesis that changes in the composition and structure of the platinum-copper nanoparticles, and, as a result, their activity in the electroreduction of oxygen may depend on the processing conditions in acids, in particular, on the nature and concentration of the acid. At this stage, the Pt31Cu69/C (S4) catalyst with a low platinum content (about 14 wt%) was used as the initial sample. Equal portions of this material were subjected to a 6-h treatment in solutions of different acids at room temperature ~23 °C (Table 3). Then the composition,
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The same is true for the maximum currents of methanol oxidation on CVs: for bimetallic catalysts they are essentially higher than for Pt/C (Figure4 and Table2). The potentials for the onset of methanol oxidation, determined for platinum-copper catalysts by the corresponding CVs of the forward scan, are approximately 150–200 mV less than for commercial Pt/C. Despite a considerably higher methanol oxidation rate and, possibly, a higher concentration of intermediate reaction products on the electrode surface, platinum-copper catalysts, as a whole, showed higher current values when chronoamperograms were recorded at the potential of 0.7 V (Table2, Figure4b,d) compared to Pt/C. Together with a lower oxidation potential of a monolayer of chemisorbed CO (Figure3b,d), this indicates a higher tolerance of bimetallic catalysts to intermediate products of methanol oxidation. The least activity in MOR was demonstrated by S3 and S3A catalysts. It should be noted that de-alloyed PtCux y/C catalysts are not inferior in activity to MOR − electrocatalysts, which were deposited on a glass-graphite electrode in the “as-prepared” state and initially contained a significantly larger amount of copper. Obviously, the reason for their similar behavior is the proximity of the composition/structure of the platinum-copper catalysts, formed after a portion of copper being selectively dissolved due to a chemical and/or electrochemical treatment of the initial materials (Table1)[ 45]. A comparative analysis of the electrochemical behavior of the platinum-copper catalysts in MOR (Figure4, Table2) suggests a noticeably higher activity of samples S1, S1A, S2 and S2A compared to S3 and S3A: initial S1 and S2, and de-alloyed S1A, S2A catalysts exhibit higher values of QCH3OH (Table2), electric current of “methanol maximum” on CVs (Figure4a,c) and currents on chronoamperograms of methanol oxidation (Figure4b,d). This di fference in the catalysts behavior can be partially due to the difference in the composition of the metal component after electrochemical standardization: for example, S3A sample contains slightly less copper than S1A and S2A (see Table1). In this case, as was noted above , reliable differences in the ECSA values of different platinum-copper catalysts were not established (Table2). In our opinion, one should take into account possible differences in the character of atoms localization of a “firmly bound” copper in the standardized nanoparticles, their surface being enriched in platinum. The smaller the thickness of the secondary platinum shell, the stronger the promoting effect of the alloying component atoms on the activity of platinum in electrochemical reactions can be expressed. This study has shown that platinum-copper catalysts exhibit much higher mass activity (A/g(Pt)) and specific activity (A/m2) in MOR compared to the commercial Pt/C material, despite a significantly lower ECSA. The pretreatment of catalysts in the nitric acid promotes leaching of a larger copper fraction from the nanoparticles, but does not lead to a decrease in the activity of catalysts in MOR. Therefore, such a procedure can be used for the pretreatment of catalysts to reduce the subsequent pollution of MEAs with copper cations.
3.2. Electroreduction of Oxygen on Alloyed and De-Alloyed Platinum-Copper Catalysts The next step in our research was to test the hypothesis that changes in the composition and structure of the platinum-copper nanoparticles, and, as a result, their activity in the electroreduction of oxygen may depend on the processing conditions in acids, in particular, on the nature and concentration of the acid. At this stage, the Pt31Cu69/C (S4) catalyst with a low platinum content (about 14 wt%) was used as the initial sample. Equal portions of this material were subjected to a 6-h treatment in solutions of different acids at room temperature ~23 ◦C (Table3). Then the composition, structural characteristics (Table3) and the electrochemical behavior of the obtained de-alloyed catalysts were studied. Nanomaterials 2020, 10, x FOR PEER REVIEW 11 of 19 structural characteristics (Table 3) and the electrochemical behavior of the obtained de-alloyed Nanomaterialscatalysts were2020 studied., 10, 742 11 of 19 The initial S4 sample treatment in acids leads to a selective dissolution of a greater amount of copper and, apparently, to a small amount of platinum. As a result, the composition of the metal Table 3. Composition and structural characteristics of alloyed PtCu/C catalyst (S4) and samples component varies from Pt31Cu69 (S4) to Pt57Cu43–Pt58Cu42 for most of the de-alloyed samples (Table 3). obtained after its treatment in various acids. The total mass fraction of metals also changes accordingly: from 23.4% mass (S4) up to 16.6–17.8% Composition of Metals Loading, % wt. Average size of massSample for pre-treatedComposition samples of the (Table 3). Note that the sample treated in a 5 M solution of the nitric Solution for Treatment Metallic Component Crystallites (XRD), nm acid is characterized by the lowest residual copper contentMetals (Pt63Cu Platinum37) in comparison with other de- alloyedS4 materials (Table - 3). This is due to Pt the31Cu high69 concentration23.4 0.2 and 13.6 aggressiveness0.2 of 2.8 the0.2 acid, used ± ± ± S4-1N 1M HNO Pt Cu 17.4 0.2 13.9 0.2 2.4 0.2 for pretreatment. It is also 3impossible to exclude57 43 completely± the possibility± of corrosion± of the carbon S4-1Cl 1M HClO Pt Cu 17.2 0.2 13.8 0.2 2.6 0.2 4 57 43 ± ± ± carrier,S4-1S which occurs 1M under H SO the conditionsPt ofCu acid treatment,17.8 0.2especially 14.4 when0.2 S4-5N 2.5is obtained.0.2 2 4 58 42 ± ± ± S4-5N 5M HNO Pt Cu 16.6 0.2 13.9 0.2 1.8 0.2 3 63 37 ± ± ± Table 3. Composition and structural characteristics of alloyed PtCu/C catalyst (S4) and samples Theobtained initial after S4 its sample treatment treatment in various in acids. acids leads to a selective dissolution of a greater amount of copper and,Composition apparently, of to a small amount of platinum.Metals As a loading, result, the% wt. composition Average of the size metal of Composition of componentSample the varies solution from for Pt31 Cu69 (S4) to Pt57Cu43–Pt58Cu42 for most of the de-alloyed samplescrystallites (Table3 ). metallic component Metals Platinum The total masstreatment fraction of metals also changes accordingly: from 23.4% mass (S4) up to(XRD), 16.6–17.8% nm massS4 for pre-treated- samples (TablePt3).31Cu Note69 that the sample23.4 ± treated 0.2 in a 513.6 M solution ± 0.2 of the2.8 nitric ± 0.2 acid isS4-1N characterized 1M by HNO the3 lowest residualPt57Cu copper43 content (Pt17.463 Cu± 0.237 ) in comparison13.9 ± 0.2 with other de-alloyed2.4 ± 0.2 materialsS4-1Cl (Table1M3 HClO). This4 is due toPt the57Cu high43 concentration17.2 and± 0.2 aggressiveness13.8 ± 0.2 of the acid,2.6 used ± 0.2 for pretreatment.S4-1S 1M It isH2 alsoSO4 impossiblePt to58Cu exclude42 completely17.8 ± the 0.2 possibility14.4 of ± corrosion 0.2 of the2.5 ± carbon 0.2 S4-5N 5M HNO3 Pt63Cu37 16.6 ± 0.2 13.9 ± 0.2 1.8 ± 0.2 carrier, which occurs under the conditions of acid treatment, especially when S4-5N is obtained. The thermograms of high-temperature oxidation of PtCu/C materials (Figure5) have the form The thermograms of high-temperature oxidation of PtCu/C materials (Figure 5) have the form characteristic of platinum-carbon catalysts [46,47]. In this case, the most rapid oxidation of the carbon characteristic of platinum-carbon catalysts [46,47]. In this case, the most rapid oxidation of the carbon carrier is observed for the initial Pt31Cu69/C (A4) sample and the Pt57Cu43/C (A4-1N) material subjected carrier is observed for the initial Pt31Cu69/C (A4) sample and the Pt57Cu43/C (A4-1N) material subjected to treatment in 1 M nitric acid. Taking into account the data on the composition and size of the to treatment in 1 M nitric acid. Taking into account the data on the composition and size of the nanoparticles given in Table3, as well as the results of Ref. [ 46], one can make an assumption that a nanoparticles given in Table 3, as well as the results of Ref. [46], one can make an assumption that a longer or shorter duration (temperature range) of intense combustion of the studied materials may be longer or shorter duration (temperature range) of intense combustion of the studied materials may due to a more (rapid oxidation) or less (slow oxidation) ordered distribution of the nanoparticles over be due to a more (rapid oxidation) or less (slow oxidation) ordered distribution of the nanoparticles the surface of the carbon carrier microparticles. over the surface of the carbon carrier microparticles.
Figure 5. Thermograms of high-temperature oxidation of metal-carbon materials (marking corresponds to Table2).
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Figure 5. Thermograms of high-temperature oxidation of metal-carbon materials (marking Nanomaterialscorresponds2020, to10 ,Table 742 2). 12 of 19
A change in the composition of the metal component of the catalysts obtained after acid A change in the composition of the metal component of the catalysts obtained after acid treatment treatment is also confirmed by the results of X-ray diffractometry (Figure 6a,b): the reflection is also confirmed by the results of X-ray diffractometry (Figure6a,b): the reflection maximum 111 for maximum 111 for all post-treated samples (Figure 6b) shifts to lower 2 theta angles compared to the all post-treated samples (Figure6b) shifts to lower 2 theta angles compared to the maximum for the maximum for the starting material S4, which is due to a decrease of copper concentration in the starting material S4, which is due to a decrease of copper concentration in the nanoparticles. nanoparticles.
FigureFigure 6. 6. (a(a)X-ray)X-ray diffraction diffraction patterns patterns of of the the initial initial meta metal-carbonl-carbon material material and and the the material material obtained obtained afterafter its its treatment treatment in in acids. acids. (b (b) )In In the the inset inset (on (on the the right)—an right)—an enla enlargedrged fragment fragment of of the the diffractogram diffractogram area,area, highlighted highlighted by by the the dotted dotted line line in in Figure Figure (a (a).).
AllAll the the catalysts catalysts obtained obtained after after the the acid acid treatmen treatmentt are are characterized characterized by by a adecrease decrease in in the the average average crystallitecrystallite sizes sizes from from the the initial initial 2.8 2.8 to to 2.6–1.8 2.6–1.8 nm nm (Table (Table 33).). The The most most significant significant decrease decrease in in this parameterparameter compared compared to to the the initial initial sample sample is is also also ob observedserved for for sample sample S4-5N S4-5N obtained obtained after after processing processing thethe starting starting material material in in the the most most aggressive aggressive medi medium.um. Probably Probably under under conditions conditions of of rapid rapid structural structural restructuringrestructuring caused caused by by the the intense intense corrosion corrosion of of coppe copper,r, the the crystallites crystallites of of this this material material acquire acquire the the highesthighest defectiveness, defectiveness, which which makes makes an an additional additional co contributionntribution to to the the peak peak broadening broadening in in the the X-ray X-ray diffractiondiffraction pattern (Figure(Figure6 b)6b) and and underestimates underestimates their their average average size, calculatedsize, calculated by the Scherrerby the Scherrer equation. equation.CVs of the initial (S4) and acid-treated electrocatalysts recorded during standardization have the formCVscharacteristic of the initial of platinum(S4) and nanoparticlesacid-treated electrocatalysts or its alloys deposited recorded on a during carbon supportstandardization (Figure7) have [ 17,21 the,24 ]. formIn the characteristic hydrogen region of platinum of CV (0.03–0.3 nanoparticles V), a gradual or its increasealloys deposited in currents on is a observed carbon support due to the (Figure cleaning 7) [17,21,24].and development In the hydrogen of the surface region ofof theCV PtCu(0.03–0.3/C materials, V), a gradual as was increase noted in above currents (Figure is observed7). Moreover, due tothere the cleaning are no peaks and indevelopment the potential of range the surface 0.25–0.35 of Vthe associated PtCu/C materials, with the dissolution as was noted of copper above from(Figure the 7).intrinsic Moreover, phase there [17, 24are] onno thepeaks anode in the CV potential branches ra ofnge the 0.25–0.35 initial (S4) V andassociated all de-alloyed with the PtCu dissolution/C materials of copper(Figure from7). This the indirectlyintrinsic phase indicates [17,24] the absenceon the anode of direct CV contact branches of theof the copper initial phase (S4) withand all the de-alloyed electrolyte. PtCu/CNote that materials the development (Figure 7). ofThis the indirectly surface of indicate samples S4 the (“as absence prepared” of direct state) contact is much of the more copper pronounced phase with(continues the electrolyte. for a larger Note number that the of cycles) development than for of PtCu the/ Csurface catalysts of treatedsample inS4 acid (“as (Figure prepared”7). Obviously, state) is muchthis is more largely pronounced due to a more (continues intense anodicfor a larger dissolution number of copperof cycles) atoms. than The for diPtCu/Cfferences catalysts in the charactertreated inof acid changes (Figure in the7). Obviously, anode part this of CV is largely observed due for to S4,a more as compared intense anodic to de-alloyed dissolution catalysts of copper at potentials atoms. Theof about differences 1 V (Figure in the7 ),character are also of apparently changes in due the to an aode significant part of diCVff erenceobserved in thefor surfaceS4, as compared composition to de-alloyedof nanoparticles. catalysts at potentials of about 1 V (Figure 7), are also apparently due to a significant difference in the surface composition of nanoparticles.
Nanomaterials 2020, 10, 742 13 of 19 Nanomaterials 2020, 10, x FOR PEER REVIEW 13 of 19
Figure 7. Cyclic voltammograms of PtCu/C electrocatalysts corresponding to 1, 25, 50, 75 and Figure 7. Cyclic voltammograms of PtCu/C electrocatalysts corresponding to 1, 25, 50, 75 and 100 d 100 d standardization cycles. The arrows indicate the direction of CV displacement in the standardization cycles. The arrows indicate the direction of CV displacement in the standardization standardizationprocess. process.
DeterminationDetermination ofof thethe catalysts ECSA ECSA was was carried carried out outby the by amount the amount of electricity of electricity spent on spent the on the electrochemicalelectrochemical adsorption adsorption and desorption desorption of ofatom atomicic hydrogen hydrogen during during CV registration. CV registration. The highest The highest ECSAECSA value value among among platinum-copper platinum-copper catalysts was was observed observed for for the the sample sample S4-1N—41.2 S4-1N—41.2 m2/g m (Pt),2/g (Pt), the lowest,the lowest, 35.7 and 35.7 35.2 and m35.22/g m (Pt)2/g was(Pt) was for S4-1Clfor S4-1Cl and and S4-1S, S4-1S, respectively respectively (Table (Table4 ).4). Unfortunately, Unfortunately, taking intotaking account into theaccount accuracy the accuracy of the ECSAof the ECSA values values determination determination (Table (Table4), the4), the measurements measurements result result in itself doesin notitself allow does usnot to allow speak us of to a speak reliable of dependencea reliable dependence of its values of its on values the conditions on the conditions for processing for the processing the S4 catalyst in acids. At the same time, the ECSA of the commercial Pt/C reference S4 catalyst in acids. At the same time, the ECSA of the commercial Pt/C reference sample JM20 is sample JM20 is approximately 2 times higher than that for platinum-copper samples of the S4 series approximately 2 times higher than that for platinum-copper samples of the S4 series (Table4), being (Table 4), being similar to what was previously established for the samples of the S1–S3 series (Table similar2). to what was previously established for the samples of the S1–S3 series (Table2).
TableTable 4. 4.Composition Composition and and parameters parameters characterizing characterizing electrocatalysts electrocatalysts electrochemical electrochemical behavior. behavior.
PtPt andand Cu Cu Ratio ratio in in the the Sample after KineticKinetic current, Current, jкj к ECSA, E1/2,V (at E = 0.90 V) Sample sampleCompletion after completion of Electrochemical ECSA, 2 E1/2, V 1 (at E = 0.90 V) n Sample m /g(Pt) (1600 min− ) n of electrochemicalMeasurements m2/g(Pt) (1600 min−1) A/g (Pt) A/m2 (Pt) A/g (Pt) A/m2 (Pt) S4measurements Pt67Cu33 39.4 0.90 161 4.1 3.8 S4S4-1N Pt67Cu Pt3365 Cu35 39.4 41.20.90 0.90161 2004.1 4.9 4.43.8 S4-1Cl Pt Cu 35.7 0.89 130 3.6 4.0 S4-1N Pt65Cu3569 31 41.2 0.90 200 4.9 4.4 S4-1S Pt68Cu32 35.2 0.90 117 3.3 4.2 S4-1Cl Pt69Cu31 35.7 0.89 130 3.6 4.0 S4-5N Pt72Cu28 39.2 0.90 140 3.6 3.5 S4-1SJM20 Pt68Cu32 Pt /C35.2 81.80.90 0.89117 1563.3 1.9 4.04.2 S4-5N Pt72Cu28 39.2 0.90 140 3.6 3.5 JM20 Pt/C 81.8 0.89 156 1.9 4.0 From the comparison of the metal component composition of the platinum-copper catalysts before (Table3From) and the after comparison measurement of the completionmetal component (Table composition4), it follows of thatthe platinum-copper in the process ofcatalysts conducting electrochemicalbefore (Table 3) measurements and after measurement (standardization completion of electrodes, (Table 4), ECSAit follows determination, that in the process evaluation of of the catalystsconducting activity electrochemical in ORR at di measurfferentements speeds (standardization of the disk electrode of electrodes, rotation) allECSA the determination, studied samples lose someevaluation amount of of the copper. catalysts The activity loss is in from ORR 77% at diffe (catalystrent speeds S4) to of 28–39% the disk (acid electrode pretreated rotation) materials) all the of the studied samples lose some amount of copper. The loss is from 77% (catalyst S4) to 28–39% (acid initial copper concentration in the catalysts deposited on the electrode. In this case, the lowest degree pretreated materials) of the initial copper concentration in the catalysts deposited on the electrode. In of copper recovery is characteristic of catalysts S4-1N and S4-5N, treated in the nitric acid solutions this case, the lowest degree of copper recovery is characteristic of catalysts S4-1N and S4-5N, treated (Tablesin the3 nitricand4 acid). solutions (Tables 3,4). ElectrocatalystsElectrocatalysts activity activity in in the the reaction reaction of oxygenof oxygen electroreduction electroreduction (ORR) (ORR) was was studied studied by by recording voltammogramsrecording voltammograms with a linear with scan a linear of potential scan of for potential RDE in for an RDE oxygen in an atmosphere. oxygen atmosphere. Normalized, as describedNormalized, in the as described Experimental in the section,Experimental linear section, sweep linear voltammograms sweep voltammograms (LSVs) re(LSVs shown) re shown in Figure 8a. Theinrelationship Figure 8a. The between relationship the current between strength the current and rotationstrength speedand rotation was analyzed speed was at potentialanalyzed at 0.90 V in thepotential Koutetsky-Levich 0.90 V in the coordinates Koutetsky-Levich (Figure coordinates8b). (Figure 8b).
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(a) (b)
FigureFigure 8. 8. (a(a) )LSV LSV curves curves of of electroreduction electroreduction of of oxygen oxygen on on different different catalysts. catalysts. Disk Disk rotation rotation speed speed of of −1/21/2 16001600 rpm. rpm. (b (b) )Dependencies Dependencies 1/ 1I/I onon ωω− at ata potential a potential ofof 0.90 0.90 V. V. Samples: Samples: 1—S4, 1—S4, 2—S4-1N, 2—S4-1N, 3—S4-1Cl, 3—S4-1Cl, 4—S4-1S,4—S4-1S, 5—S4-5N, 5—S4-5N, 6—JM20. 6—JM20.
TheThe voltammograms voltammograms obtained obtained for for all all studied studied PtCu/C PtCu/C catalysts catalysts (Figure (Figure 8a)8a) are are characterized by by closeclose values values of of a a half-wave half-wave potential potential of of the the oxygen oxygen reduction reduction reaction reaction (Table (Table 4).4). The The analysis of of the 0.5 straight-linestraight-line dependences dependences in in the the coordinates coordinates 1/ 1/I—I—ωω−0.5− (Figure(Figure 8b)8b) made made it it possible possible to to calculate calculate the the kinetickinetic currents ofof ORR ORR and and confirmed confirmed the four-electronthe four-electron reaction reaction mechanism mechanism characteristic characteristic of platinum of platinum(the number (the of number electrons of nelectrons ~4) (Table n 4~4)). The(Table specific 4). The activity specific of platinum-copperactivity of platinum-copper catalysts in ORRcatalysts was inapproximately ORR was approximately 1.7–2.6 times 1.7–2.6 higher times than higher that of than JM20 th (Tableat of JM204). The (Table order 4). in The the order increase in the of catalystsincrease ofmass–activity catalysts mass–activity can be presented can be as presented follows: S4-1S as follows:< S4-1Cl S4-1SS4-5N < S4-1Cl< JM20 ≤ S4-5NS4 << S4-1NJM20 ≈ (Table S4 < 4S4-1N). The ≤ ≈ (Tablecombination 4). The ofcombination the minimum of the specific minimum and relatively specific highand relatively mass activity high observed mass activity for JM20 observed is due for to a JM20higher is ECSAdue to ofa higher this catalyst ECSA compared of this catalyst to platinum-copper compared to platinum-copper materials. Note materials. that the highest Note that residual the highestconcentration residual of concentration the copper atoms, of the mainly copper localized atoms, in mainly the inner localized layers ofin PtCu the inner nanoparticles, layers of may PtCu be nanoparticles,one of the reasons may forbe one the higherof the reasons activity for of S4-1Nthe higher and S4activity samples of S4-1N in ORR. and S4 samples in ORR. ItIt is is important important to to note note that that the the selective selective disso dissolutionlution of of copper copper during during standardization standardization of of the the electrodeselectrodes and and electrochemical electrochemical measurements measurements that that followed, followed, takes takes place place on on all all PtCu/C PtCu/C catalysts. catalysts. However,However, for for the the samples samples previously previously treated treated in in acid, acid, as as was was previously previously established established for for samples samples of of the the S1–S3S1–S3 series, series, the the amount amount of of copper copper passing passing into into th thee solution, solution, and and the the change change in in the the composition composition for for allall de-alloyed de-alloyed catalysts catalysts obtained obtained from from S4, S4, is is much much less less pronounced pronounced than than for for the the initialPt initialPt3131CuCu6969/C/C (S4) (S4) catalystcatalyst (compare (compare the the composition composition of of the the catalysts catalysts before before (Table (Table 3)3) and and after after (Table (Table 44)) electrochemical measurements).measurements). This This confirmsconfirms ourour assumption assumption that that an “acidan “acid treatment” treatment” can becan used be toused wash to out wash “loosely out “looselybound” bound” copper fromcopper the from platinum the platinum catalysts, ascatalysts, its presence as its makes presence their makes use in their the MEA use in problematic. the MEA problematic.It is known that the catalysts exhibiting high ORR activity in the electrochemical cell do not always demonstrateIt is known similar that resultsthe catalysts when testedexhibiting in the high MEA ORR [1– 3activity]. In the in case the ofelec platinum-coppertrochemical cell catalysts, do not alwaysthe negative demonstrate effect can similar also beresults associated when withtested the in poisoning the MEA of[1–3]. Nafion In the by coppercase of cations,platinum-copper formed as catalysts,a result of the its negative selective effect dissolution can also from be associated the bimetallic with nanoparticles the poisoning [24 of– 26Nafion]. Based by oncopper the resultscations, of formedelectrochemical as a result behavior of its selective of alloyed dissolution/de-alloy from PtCu the/Cmaterials, bimetallic we nanoparticles synthesized [24–26]. the platinum–copper Based on the resultscatalyst of S5, electrochemical an analogue of behavior S4, which of contained alloyed/ ade-alloy greater PtCu/C loading ofmaterials, metals. Throughwe synthesized a three-hour the platinum–coppertreatment of portions catalyst of S5, this an material analogue in of 1 S4, M sulfuricwhich contained and nitric a greater acids at loading 25 ◦C, de-alloyedof metals. Through samples aof three-hour similar composition—S5N treatment of portions and of S5S, this respectively,material in 1 wereM sulfuric obtained. and nitric The massacids fractionat 25 °C, ofde-alloyed platinum samplesin these of catalysts similar wascomposition—S5N 18.3 0.5% wt., and and S5S, the respectively, ratio of platinum were andobtained. copper The corresponded mass fraction to theof ± platinumformula Ptin53 theseCu47 .catalysts These catalysts was 18.3 were ± 0.5% used wt., to and form the the ratio cathodic of platinum and anodic and catalyticcopper corresponded layers during tothe the assembly formula of Pt H532Cu/air47. PEMFC These catalysts MEAs. The were MEAs used were to form assembled the cathodic using a and similar anodic technology catalytic based layers on duringthe commercial the assembly Pt/C of PM40 H2/air catalyst PEMFC (40% MEAs. wt. Pt, The PROMETHEUS MEAs were assembled R&D, Rostov-on-Don, using a similar Russia), technology which basedpreviously on the demonstrated commercial highPt/C functional PM40 catalyst characteristics (40% wt. [ 48Pt,]. PROMETHEUS The conditions for R&D, conducting Rostov-on-Don, MEAs life Russia), which previously demonstrated high functional characteristics [48]. The conditions for conducting MEAs life tests, which included 5000 cycles of potential changes, are described in the Experimental section, and some test results are presented in Figures 9 and 10.
Nanomaterials 2020, 10, x FOR PEER REVIEW 15 of 19
Initially, all three MEAs showed similar current-voltage and watt-ampere characteristics (Figure 9a).Nanomaterials However, 2020 as, 10 the, x FOR cycling PEER REVIEWprogressed, a decrease in the characteristics of the Pt/C catalyst 15 of 19 based MEAs was observed, while both MEAs with PtCu/C catalysts during 2000 cycles even slightly increasedInitially, their allcharacteristics three MEAs showed (Figure similar 9b,c). current-voltage After 2800 sweep and watt-ampere cycles, the characteristicsPt/C-based MEA (Figure tests had 9a). However, as the cycling progressed, a decrease in the characteristics of the Pt/C catalyst based to be stopped due to electrode degradation. At the same time, for the MEA based on A5S after 5000 MEAs was observed, while both MEAs with PtCu/C catalysts during 2000 cycles even slightly cycles of potential scanning, only a small decrease in power was recorded, while for the MEA increased their characteristics (Figure 9b,c). After 2800 sweep cycles, the Pt/C-based MEA tests had containingto be stopped A5N, due the to current-voltage electrode degradation. characteristics At the same even time, slightly for the increased MEA based (Figure on A5S 10). after Thus, 5000 the test results of electrocatalysts in the MEAs (Figures 9 and 10) correlate well with the results obtained by Nanomaterialscycles of 2020potential, 10, 742 scanning, only a small decrease in power was recorded, while for the MEA 15 of 19 studyingcontaining their A5N, activity the current-voltage in ORR (Tables characteristics 2 and 4) and even stability, slightly increasedwhen tested (Figure in 10).an electrochemicalThus, the test cell [49].results The ofnegative electrocatalysts influence in theof CuMEAs2+ cations (Figures formed 9 and 10) during correlate the we operationll with the of results the de-alloyed obtained by PtCu/C electrocatalyststests,studying which their included onactivity proton 5000 in ORRconductivity cycles (Tables of potential 2 (memand 4)brane changes,and stability, resistance) are describedwhen of tested Nafion in in the anwas Experimentalelectrochemical not recorded. section, cell and 2+ some[49]. test The results negative are influence presented of Cu in Figures cations9 formedand 10 .during the operation of the de-alloyed PtCu/C electrocatalysts on proton conductivity (membrane resistance) of Nafion was not recorded.
(a) (b) (c) (a) (b) (c) Figure 9. Polarization and power density curves of MEAs after 100 (a), 300 (b) and 2000 (c) stress Figure 9. Polarization and power density curves of MEAs after 100 (a), 300 (b) and 2000 (c) stress testingFigure cycles. 9. Polarization Catalyst andlayers power were density prepared curves using of PM40, MEAs S5N, after and 100 (S5Sa), 300catalysts. (b) and The 2000 loading (c) stress of testing cycles. Catalyst layers were prepared using PM40, S5N, and S5S catalysts. The loading of platinumtesting cycles. in each Catalyst catalytic layers layer werewas of prepared 0.4 mg/cm using2. PM40, S5N, and S5S catalysts. The loading of platinumplatinum in in each each catalyticcatalytic layer was was of of 0.4 0.4 mg/cm mg/cm2. 2.
Figure 10. The maximum power of MEAs based on the studied PtCu/C and Pt/C catalysts after 100, 300, 2000 and 5000 cycles of stress testing in the membrane-electrode assembly. FigureFigure 10. TheThe maximum maximum power power of of MEAs MEAs based based on on the the st studiedudied PtCu/C PtCu/C and and Pt/C Pt/C catalysts catalysts after after 100, 100, 300, 2000 and 5000 cycles of stress testing in the membrane-electrode assembly. 4. 300,Conclusions 2000 and 5000 cycles of stress testing in the membrane-electrode assembly. ≤ ≤ 4. ConclusionsInitially,PtCux/C all catalysts three MEAs of various showed compositions similar current-voltage (1.7 x 2.9), andcontaining watt-ampere bimetallic characteristics nanoparticles (Figure in 9a). However,which the as platinum the cycling concentration progressed, increases a decrease from in th thee center characteristics to the surface of the of Pt the/C catalystparticles basedwere MEAs PtCux/C catalysts of various compositions (1.7 ≤ x ≤ 2.9), containing bimetallic nanoparticles in wassynthesized observed, by while sequential both MEAs chemical with reduction PtCu/C of catalysts copper, and during next 2000, of copper cycles and even platinum slightly from increased the their whichsolutions the platinumof their precursors. concentration By treating increases portions from of PtCuthe xcenter/C materials to the in surfacethe solutions of the of particlesacids of were characteristics (Figure9b,c). After 2800 sweep cycles, the Pt /C-based MEA tests had to be stopped synthesizeddifferent nature by sequential and concentr chemicalation, reductionthe de-alloyed of co PtCupper,x−y /Cand catalysts next , of (13.8–26.4% copper and wt. platinum Pt) with froma the due to electrode degradation. At the same time, for the MEA based on A5S after 5000 cycles of solutionsreduced of copper their contentprecursors. (x − y By= 0.58–0.90) treating were portions obtained. of PtCu x/C materials in the solutions of acids of potentialBoth scanning, alloyed and only de-alloyed a small catalysts decrease are in characterized power was by recorded, a decrease while in the for copper the MEAcontent containing in different nature and concentration, the de-alloyed PtCux−y/C catalysts (13.8–26.4% wt. Pt) with a A5N, the current-voltage characteristics even slightly increased (Figure 10). Thus, the test results of reducedtheir composition copper content as a result (x − yof =el 0.58–0.90)ectrochemical were standardization obtained. of their surface. Nevertheless, for de- electrocatalystsalloyed materials, in the the MEAs amount (Figures of copper9 and 10transfe) correlaterred to well the withelectrolyte the results is considerably obtained by less. studying The their Both alloyed and de-alloyed catalysts are characterized by a decrease in the copper content in activity in ORR (Tables2 and4) and stability, when tested in an electrochemical cell [ 49]. The negative their composition as a result of electrochemical standardization of their surface. Nevertheless, for de- influence of Cu2+ cations formed during the operation of the de-alloyed PtCu/C electrocatalysts on alloyed materials, the amount of copper transferred to the electrolyte is considerably less. The proton conductivity (membrane resistance) of Nafion was not recorded.
4. Conclusions
PtCux/C catalysts of various compositions (1.7 x 2.9), containing bimetallic nanoparticles ≤ ≤ in which the platinum concentration increases from the center to the surface of the particles were synthesized by sequential chemical reduction of copper, and next, of copper and platinum from the solutions of their precursors. By treating portions of PtCux/C materials in the solutions of acids of Nanomaterials 2020, 10, 742 16 of 19
different nature and concentration, the de-alloyed PtCux y/C catalysts (13.8–26.4% wt. Pt) with a reduced − copper content (x y = 0.58–0.90) were obtained. − Both alloyed and de-alloyed catalysts are characterized by a decrease in the copper content in their composition as a result of electrochemical standardization of their surface. Nevertheless, for de-alloyed materials, the amount of copper transferred to the electrolyte is considerably less. The composition of the post-treated PtCux y/C catalysts weakly depends on their initial composition and, to a greater extent, − on the acid treatment conditions: the higher the acid concentration and the processing time, the lower the residual copper content is. We have not established a significant effect of the initial composition and pretreatment conditions of the catalysts on their ECSA, measured after standardization. The ECSA values of the alloyed and de-alloyed electrocatalysts measured after standardization of the electrodes ranged from 31 to 43 m2/g (Pt), while the ECSA values of the commercial Pt/C catalyst (40% wt. Pt) used for comparison, were about 80 m2/g (Pt). Despite the lower values of ECSA, the supported platinum-copper catalysts exhibit significantly higher mass activity and specific activity in MOR compared to the commercial Pt/C electrocatalyst. The difference in mass activity of the platinum-copper and platinum catalysts in ORR is not that crucial, however, some samples of PtCu/C catalysts were also more active than Pt/C. Pretreatment of PtCu/C catalysts in acids leads to the leaching of a significant fraction of copper from the nanoparticles, but it does not cause a decrease in their activity in MOR and ORR. The fact is that the electrochemical standardization of the catalysts causes an even higher degree of extraction of a “weakly bound” copper from the nanoparticles, therefore their initial composition does not play the leading role. At the same time, the conditions of acid pretreatment of the initial PtCux/C catalysts can have some effect on the change in the composition/structure of the nanoparticles and, as a consequence, on their electrochemical behavior. For example, the highest activity in ORR was demonstrated by a de-alloyed catalyst pretreated in 1 M HNO3. The influence of acid treatment conditions on the structural and electrochemical characteristics of platinum-copper catalysts was also noted by Hodnik et al. [28,50]. Therefore, acid treatment conditions can be optimized. The combination of high activity in ORR and satisfactory resistance to the selective dissolution of copper made it possible to conduct life tests of de-alloyed PtCu/C electrocatalysts in the H2/Air MEA. At the initial stage of stress testing, the current-voltage characteristics of MEAs containing bimetallic catalysts and MEA, based on commercial Pt/C, were close but during stress testing the platinum-copper catalysts showed much higher stability. As a result, the life tests of the MEA with a Pt/C catalyst were completed after 2800 cycles of potential change; when de-alloyed PtCu/C catalysts were used, the MEAs successfully withstood 5000 cycles, and one of them even slightly increased its power. These results correlate well with the data on the increased stability of PtCu/C catalysts, which were obtained in [49] from stress testing of PtCu/C and Pt/C catalysts in an electrochemical cell. Note that we did not detect the negative effect of copper cations formed during the stress testing of the de-alloyed catalysts on the current-voltage characteristics of the MEA. This fact confirms the assumption that the acid pretreatment of PtCux/C materials allows one to obtain the de-alloyed PtCux y/C catalysts that possess not only high durability, but also stability, which is sufficient from the − point of the copper selective dissolution. The results of the study are in good agreement with the data, that we have previously obtained [22,29,49] and with those published in literature [11,27,28]. At the same time, they demonstrate new opportunities to control the composition, structure, and functional characteristics of the de-alloyed PtCux y/C catalysts. − Pretreatment of the platinum-copper catalysts in acids does not practically reduce their activity in ORR and MOR, but when the “excess” of copper is removed, resistance to its selective dissolution significantly increases. As a result, the de-alloyed PtCux y/C catalysts exhibit high activity and stability − during the life testing of MEAs and do not cause proton-conducting polymer poisoning. In our opinion, the results obtained also confirm the feasibility of further de-alloyed PtCux y/C catalysts testing in the − fuel cell MEAs. Nanomaterials 2020, 10, 742 17 of 19
Author Contributions: V.M.—studied electrochemical behavior of the catalysts in the methanol electrooxidation reaction, took part in the results analysis and the article design; V.G. and A.A.—worked out and developed the general idea and methodology of the project, analyzed the results obtained (with A.N. participation); O.S. (Olga Spiridonova)—prepared materials, studied their electrochemical behavior in ORR; O.S. (Olga Safronenko), A.A., V.G.—wrote the script, reviewed and edited the article; S.B.—analyzed the composition and studied of the structural characteristics of the materials obtained, participated in the results discussion; A.N.—developed methodology of the MEAs performance study; A.T., N.G. and N.Z.—studied the catalysts behavior in MEAs, analyzed and described the results. All authors have read and agreed to the published version of the manuscript. Funding: This research was financially supported by the Ministry of Science and Higher Education of the Russian Federation (State assignment in the field of scientific activity, Southern Federal University, 2020). Conflicts of Interest: The authors declare no conflict of interest.
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