Introduction

Nowadays, under current circumstances, where the energy needs grow daily and the extensive use of fossil fuels leads to severe environmental issues, the matter of alternative energy sources raises rapidly. Therefore, many investigations have focused on the development of fuel cells (FCs), which have several benefits such as high efficiency, silent operating modes, low pollution level, and high durability [1]. Among various types of FCs, the low-temperature proton exchange membrane fuel cells (PEMFC) are the most promising candidates for clean and efficient energy conversion in portable and stationary small devices, as well as electric vehicle applications [2,3,4,5]. The basic structure of a PEMFC consists of a perfluorosulfonate proton exchange membrane in contact with porous electrodes, anode and cathode, coated with a catalyst to let the electrochemical reactions occur on both sides [6]. At the current stage of the technology, the most effective catalysts for the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR) are platinum-based materials [7, 8]. However, the widespread commercialization of PEMFC technology has been greatly hindered mainly due to the high cost and scarcity of Pt materials [9]. Moreover, it has to be emphasized that the Pt loading is usually about 5–10 times higher on the cathode than on the anode due to the much slower kinetics of ORR than the HOR. Another obstacle to the full commercialization of the PEMFCs constitutes their short lifetime, which is commonly caused by catalyst degradation at the cathode side through the development of positive potentials inducing carbon support corrosion. This phenomenon leads to the dissolution of Pt particles, followed by their re-deposition that increases the particle’s size and reduces active electrochemical surface area [10,11,12,13,14,15,16]. In fact, automotive and stationary fuel cell applications are typically limited to about 1700 h and 10,000 h of operations, respectively, while at least 5000 h and 40,000 h of continuous operation are required [17, 18]. Therefore, in recent years, most of the research efforts have been devoted to reducing the cathode Pt loadings, without loss of performance and durability [19,20,21,22,23,24,25,26,27]. The decrease of Pt loading under 0.2 mg cm−2 maintaining the membrane electrode assembly (MEA) performance of 1000 mW cm−2, together with low relative humidity (RH) operation and the enhancement of MEA lifetime are some of the important targets aimed at by the US Department of Energy (DOE) for 2020. Among possible strategies of the cost-reducing and improving durability is designing a carbon co-support that could allow improving metal active phase dispersion and limiting aggregation upon working conditions [28,29,30,31,32].

Heteropolyacids (HPAs) and polyoxometalates (POMs) are inorganic compounds used since before 1900, which are unmatched not only in terms of molecular and electronic structural versatility but also in terms of reactivity and relevance to analytical chemistry, catalysis, biology, medicine, geochemistry, materials science, and topology [33,34,35,36]. Several recent papers have considered the potential use of these materials in fuel cells due to their ability to strongly adsorb metal catalyst inside the micro-meso pores and their high acidity, which can be exploited to improve both durability and performance. In particular, the high acidity enhances the ORR kinetics, while the Pt particle aggregation is limited thanks to voids that can host nanosize Pt particles during operation [37,38,39,40,41]. Additionally, in order to avoid loss of performance due to the solubility of the heteropolyacids, their modification by partially exchanging protons of the parent acid with large cations such as Cs+, K+, and NH4+ is carried out to prepare the insoluble salts. The performed study showed the highest surface area of CsxH(3 + n) − xPMo12 − nVnO40 when the Cs:HPA ratio is in the range of 2.5 to 3.5 [42]. Dsoke et al. [26] also revealed an improved ORR catalytic activity of Pt nanoparticles embedded inside insoluble W-based polyoxometalate (POM) salt, which synthesized from a Keggin-type heteropolyacid. Furthermore, enhancement of catalytic performance of the mixtures of Pt and Pt alloys with such types of co-supports was demonstrated [37,38,39, 43,44,45,46,47]. Especially, the Cs2.5H0.5PW12O40 was reported as excellent mesoporous support for Pt nanoparticles in the preparation of PEM electrodes with low Pt content [26, 43, 48,49,50].

In this work, we describe the synthesis of Cs3H3PMo9V3O40 as co-catalytic material for fuel cell applications. The catalytic activity of commercial carbon-supported PtFe nanoparticles mixed with the obtained mesoporous polyoxometalate salt towards oxygen reduction reaction was evaluated. Preliminary electrochemical characterization was performed using different Pt loading, to estimate the influence of low Pt content on the ORR kinetics. Then, fuel cell tests were carried out with low cathode Pt loading and different relative humidity (RH) values to assess the impact of the use of the proposed catalyst system toward performances in an operating fuel cell. Finally, the durability and stability of prepared materials were gauged by performing an accelerated stress test (AST).

Experimental

POM synthesis

In a typical synthesis of the Cs3H3PMo9V3O40 insoluble salt [51], an aqueous solution of cesium carbonate (Cs2CO3, Sigma-Aldrich, ≥ 99.00%) was added drop-wise to phosphovanadomolibdic acid (H6PMo9V3O40∙8H2O, hereafter labeled as PVM3-9, Nippon Inorganic Colour & Chemical CO., LTD.) solution under vigorous stirring in a water bath at 50 °C, which was continued for another 1 h. Subsequently, the stirring of the fine suspension was kept overnight at room temperature. Next, the mixture was filtered under a vacuum filter system using a hydrophilic filter. The resulting products were dried overnight at 50 °C. Hereafter, the obtained salt will be labeled as POM3-3–9.

Physicochemical characterization

Transmission electron microscopy (TEM) images were obtained with JEOL Model JSM-5400 equipped with a Shimadzu 800HS EDX detector. X-ray diffraction (XRD) spectra were recorded using an automated PHILIPS diffractometer equipped with a CuKα source (λ = 1.541 Å).

Electrode preparation and RRDE measurements

For electrochemical measurements, the ink (PtFe/POM3-3–9) was prepared by mixing equivalent amount of the synthesized salt POM3-3–9 and commercial 20 wt% PtFe on Vulcan XC-72 (Pt-Fe 1:1 atomic ratio, Premetek), and 985 μl of isopropanol. The slurry was sonicated for 30 min to form a homogenous suspension. Immediately afterward, 15 μl of 5 wt% Nafion® solution (Ion-Power, Inc.) was added and the ink was put under vigorous stirring for further 30 min. The POM3-3–9:PtFe:Nafion ratio was equal to 1:1:1. Commercial catalysts 20 wt% Pt on Vulcan XC-72 (Pt/C) and 20 wt% PtFe on Vulcan XC-72 (PtFe/C) were used as references, and their inks were prepared according to the procedure described above but without POM addition. An appropriate amount of the catalysts ink was dropped onto the surface of a glassy carbon disc electrode through a Hamilton 701n syringe with the resultant Pt loadings on the electrode of 15 or 7.5 μg cm−2. The modified electrode surface was left to dry for 30 min in the air. The film thickness was typically in the order of 0.1 μm.

The rotating ring disk electrode (RRDE) method was applied to study the catalytic activity of the obtained materials towards the ORR. Electrochemical tests were conducted in a conventional three-electrode cell (Pine AKCELL3, USA), using a CH Instruments 750A (Austin, TX) bipotentiostat. The solution of 0.1 M HClO4 (Sigma-Aldrich, redistilled, 99.999% metal basis) was used as an electrolyte to avoid strongly adsorbing sulfate/bisulfates on Pt surface [52]. Prior to the electrochemical measurements, the electrolyte was deaerated (utilizing nitrogen) or saturated with oxygen for at least 30 min before each experiment and then during the tests. A K2SO4-saturated Hg/Hg2SO4 (MSE) electrode and a Pt wire were used as reference and counter electrodes, respectively. A glassy carbon disk, concentrically surrounded by a platinum ring (GCE-Pt, surface area GCE = 0.2475 cm2, collection efficiency (N): 37%), was used as the working electrode. In order to obtain an electrochemically clean and stable catalyst surface, a modified working electrode was activated by performing several full voltammetric potential cycles at 50 mV s−1 in an N2-saturated electrolyte until stable results were obtained. The RRDE experiments were carried out at a scan rate of 5 mV s−1 on the disk electrode, while a constant potential of 1.2 V vs. RHE (reversible hydrogen electrode) was applied to the ring electrode, with several rotating speeds ranging between 400 and 2500 rpm. The applied potentials were converted to the RHE scale. The recorded currents were normalized to the geometric area of the electrode and, additionally, corrected by subtracting the background currents measured in the N2 atmosphere.

MEA preparation and fuel cell tests

The PtFe/POM3-3–9 catalyst performance as ORR cathodic catalyst was evaluated in a 5 cm2 PEMFC. A Nafion 112 membrane of 60 μm thickness (Ion-Power, Inc.) was used as the electrolyte, after pre-treating with H2O2 and H2SO4 to remove organic and inorganic impurities [53, 54]. The ink was prepared by following the same procedure reported in the “Electrode preparation and RRDE measurements” section but with a mass ratio PtFe/C:POM3-3–9:pure Nafion of 2:1:1. The electrodes were prepared by brushing the catalyst ink onto a gas diffusion layer (GDL, BASF), as uniformly as possible, to obtain a homogenous thickness throughout the entire surface. A commercial 20 wt% Pt/C (E-TEK) was used as an anodic catalyst. The Pt loading on the cathode was 0.2 mg cm−2, while on the anode was 0.5 mg cm−2, which allowed to avoid any possible limitations during the HOR. The reference MEA with both commercial anode and cathode (named MEA Comm.), with Pt loading of 0.5 mg cm−2, was also characterized. The MEA was obtained by hot pressing at 120 °C and 50 bar for 3 min. All the chemicals were used as obtained without further purification. Solutions were prepared using Millipore Milli-Q water of resistivity ≈ 17 MΩ cm.

For the PEMFC test, the MEA was mounted into a 5-cm2 triple-serpentine cell (model 05–02 from Electrochem. Inc). A fuel cell test bench (Scribner Associated 890CL) was used for the evaluation. The anodic and cathodic compartments were fed with pure H2 (100 ml min−1) and pure O2 (200 ml min−1), respectively. The gas pressure was maintained constant at 2 bar. The fuel cell performances were characterized by holding the cell temperature at 70 °C, while the temperatures of the gas humidifiers were changed in order to obtain different inlet gas humidity values, which result in different final RHs. The temperatures of cell and feed gases are hereafter specified as TA/TM/TC, where TA and TC are anode and cathode feed gas temperatures, respectively, while TM is cell temperature (°C). Before starting tests, the activation cell process was performed feeding fully humidified (100% RH) oxygen and hydrogen at cathode and anode, respectively, until stable polarization curves were obtained. All the potentials are reported versus the reversible hydrogen electrode (RHE).

Results and discussion

Structural and morphological characterization

The prepared catalytic materials were characterized for their phase structure and crystallinity by X-ray powder diffraction (XRD). The typical XRD patterns of PtFe/C, POM3-3–9, and POM3-3–9-modified PtFe/C are displayed in Fig. 1. The results revealed the presence of the crystalline POM3-3–9 with a characteristic cubic (Pn3m) Keggin-type structure commonly associated with the pure alkaline heteropolysalts [55]. Due to the introduction of larger Cs+ cations into the heteropolyacid lattice, which greatly influences the tertiary structure, the shifting in the diffraction peaks towards the lower 2θ angels is observed in comparison to the pure PVM3-9 acid [56]. The XRD patterns of modified and unmodified PtFe/C samples show the diffraction peaks at 2θ of 39.3°, 45.3°, 66.7°, and 79.9° assigned to the Pt planes (111), (200), (220), and (311), respectively. The positions of these peaks are shifted towards lower 2θ angels compared to pristine Pt (JCPDS# 04–0802), which is related to the effect of alloying. Furthermore, due to the presence of the carbon support, a wide peak at ~ 25° that corresponds to the graphite plane (002) can also be distinguished [57, 58].

Fig. 1
figure 1

X-ray diffraction patterns of the PtFe/C, POM3-3–9, and POM3-3–9-modifed PtFe/C samples

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The morphology of the catalytic materials was examined by transmission electron microscopy, and the obtained results are presented in Fig. 2a, b. The TEM images show a uniform distribution of spherical or quasi-spherical PtFe nanoparticles on unmodified and POM-modified carbon support. Moreover, microscopic analysis confirmed the nanometric size of the alloy nanocrystals. In the case of the unmodified samples, the average size of the PtFe particles in the range of 6–12 nm was observed, while for the POM3-3–9, modified catalysts were varied between 8 and 14 nm. Additionally, the EDX maps (Fig. 2c) were obtained to get more information about the distribution of PtFe/C nanoparticles in the presence and absence of POM3-3–9.

Fig. 2
figure 2

TEM images of the a PtFe/C and b POM3-3–9-modified PtFe/C samples. Additionally, EDX maps of the POM3-3–9-modified PtFe/C catalyst are presented (c)

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Electrochemical characterization

The cyclic voltammetric curves of the PtFe/POM3-3–9 and PtFe/C samples with different amounts of Pt loadings on the electrode surface, equal to 15 μg cm−2 and 7.5 μg cm−2, are shown in Fig. 3a, b, respectively. The well-known voltammetric pattern of the carbon supported Pt (Pt/C, loading 15 μg cm−2) with the characteristic hydrogen adsorption/desorption peaks and the broader peak reflecting formation of Pt-oxo species is provided in Supplementary Information (Fig. S1). In all cases, within the potential range from 0 to 0.3 V, typical phenomena, such as hydrogen adsorption/desorption (below \(\sim\) 0.3 V) together with the Pt/PtO redox couple (in the potential region above \(\sim\) 0.6 V), can be observed. Furthermore, the well-defined redox peaks for the PtFe/POM3-3–9 are noticed in the potential range, \(\sim\) 0.3 – 0.45 V. Based on previously reported voltammograms for Pt-free POMs [59], the observed redox couple should be assigned to oxidation/reduction reactions of the molybdate component of POM3-3–9. However, due to the overlapping with hydrogen adsorption/desorption peaks and formation Pt-oxo process, clear distinction and interpretation of all POM3-3–9 redox transitions are not possible. For the same reasons, determination of the electrochemical surface area (ECSA) and the roughness factor has not been done here.

Fig. 3
figure 3

Cyclic voltammograms of POM3-3–9-modified PtFe/C catalyst (blue solid line) compared with POM-free 20% PtFe/C reference sample (red dash line), with different content of Pt loadings: a 15 µg cm−2 and b 7.5 µg cm−2. Electrolyte: deoxygenated HClO4 (0.1 M). Scan rate: 50 mV s−1

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In order to examine the ORR activity, the voltammetric measurements of the investigated catalysts using the RRDE method were performed in an O2-saturated electrolyte solution. The ORR curves for the PtFe/POM3-3–9 sample compared to benchmark 20% Pt/C and 20% PtFe/C catalysts recorded at 1600 rpm are demonstrated in Fig. 4. The recorded potentiodynamic curves for all studied catalysts are characterized by similar profiles over the entire examined potential window, with the limiting current density of \(\sim\) 5.75 mA cm−2. As it can be seen in Fig. 4a, the half-wave potentials (E1/2) for catalysts with PtFe alloys are positively shifted, when compared to the Pt/C, ca. 71 mV for the PtFe/C and 54 mV for the PtFe/POM3-3–9, despite of the same Pt loadings (15 μg cm−2). The higher potential values indicate lower activation energy due to more favorable kinetics of the ORR process. The slightly lower E1/2 in the case of POM-modified catalyst can be related to the higher thickness of the layer after modification, with respect to the reference samples. The enhancement catalytic activity in comparison to Pt/C was also confirmed in the case of measurements with lower Pt content (7.5 μg cm−2), with a half-wave potential shift towards a positive value of 19 mV for PtFe/C and 29 mV for PtFe/POM3-3–9 (Fig. 4c).

Fig. 4
figure 4

a, c The ORR curves for the POM3-3–9-modified PtFe/C sample (red line) compared to benchmark 20% Pt/C (black dash line) and 20% PtFe/C (blue line) catalysts recorded in O2-saturated 0.1 M HClO4 solution at 1600 rpm and the scan rate of 5 mV s−1, with corresponding b, d percentage of H2O2 calculated from the RRDE measurements with Ering = 1.2 V. The tests were performed with different content of Pt loadings: a, b 15 µg cm−2 and c, d 15 μgPt cm−2 for Pt/C, and 7.5 μgPt cm−2 for PtFe/C and POM3-3–9-modified PtFe/C

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To delineate the pathways of the oxygen reduction reaction on the obtained catalysts, from the recorded RRDE profiles, the percentage of the produced hydrogen peroxide species with respect to the total ORR products was calculated according to Eq. (1) [60]:

$${\mathrm{\%H}}_{2}{\mathrm{O}}_{2}=\frac{200\frac{{I}_{r}}{N}}{{I}_{d}+\frac{{I}_{r}}{N}}$$
(1)

where Id is the current measured at the GC disk, Ir is the current measured at the Pt ring, and N is the collection efficiency.

Figure 4b, d illustrate the percent content of H2O2 oxidized at the ring electrode as a function of the potential applied to the disk electrode. The amount of produced hydrogen peroxide species at 0.6 V (the potential relevant to fuel cell cathodes) is around 3% for the commercial 20% Pt/C layer. Independently from the metal loadings, a value of formed H2O2 less than 1% for unmodified and POM-modified PtFe samples was observed, which implies that the reduction of oxygen proceeds along a favorable four-electron pathway. Furthermore, in all cases, a significant increase in the ring current was detected when the disk potential approached the H-underpotential deposition region (0.05–0.3 V). This phenomenon is generally related to hydrogen adsorption on Pt surface defects which leads to decreases in the number of neighboring Pt atoms, thereby reducing the ORR selectivity [60, 61].

The data were also analyzed by considering the Koutecky–Levich (K–L) model, assuming that only transport of oxygen in solution or the chemical (catalytic) reaction was the rate-determining steps. For this purpose, the dependency of the reciprocal values of the experimental current density on the square root of the rotation rate can be demonstrated on the K–L plots and expressed by the following equations [62,63,64]:

$$\frac{1}{j}=\frac{1}{{j}_{\mathrm{k}}}+\frac{1}{{j}_{\mathrm{L}}}=\frac{1}{n\mathrm{F}{k}_{\mathrm{het}}{C}_{{\mathrm{O}}_{2}}^{o}}+ \frac{1}{{j}_{L}}$$
(2)

where j is the measured current density, \({j}_{\mathrm{k}}\) is the kinetic current density, \({j}_{\mathrm{L}}\) is the diffusion limited current density (given by the Levich equation), n is the number of transferred electrons, F is the Faraday constant (96.485 C mol−1), khet is the electron-transfer heterogeneous rate constant (cm s−1), \({C}_{{\mathrm{O}}_{2}}^{o}\) is the bulk concentration of oxygen (in 0.1 M HClO4, \({C}_{{\mathrm{O}}_{2}}^{o}\) = 1.38 × 10−6 mol cm−3 [59]).

The K–L plots (for simplicity not shown here) that were prepared for the investigated samples at 0.75 V are characterized by non-zero intercepts clearly indicating kinetic limitation associated with the catalytic film. Based on the intercepts of the reciprocal plots, the kinetic parameter, khet, which reflects intrinsic rates of heterogeneous charge transfer was estimated. The calculated values (Table 1) are as follows: 0.22 cm s−1 for Pt/C, and they lie in the range from 0.1 to 0.3 cm s−1 for the other systems, reaching the highest value for PtFe/POM3-3–9 (15 μgPt cm−2).

Table 1 Comparison among the calculated values of khet and Eonset for investigated samples
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Also, onset potentials for ORR under RRDE conditions are summarized in Table 1. To make comparison accurate, we consider here the onset potentials as equivalent to the potentials at which the measured currents reach the 5% level of the respective convectional-diffusional limiting (maximum) currents. Such parameters were postulated to have the kinetic meaning because they are related to the purely kinetic performance of the ORR electrocatalytic systems [65]. On the whole, it can be concluded that the POM-salt admixied or supported PtFe systems seem to be characterized by the most positive potential values at which ORR becomes operative.

Additional kinetic analysis for all the investigated catalysts was addressed using Tafel plots, which were obtained at the low overpotential region (Fig. 5) and prepared after correction for mass transport limitation [62]:

$${j}_{k}=\frac{{j}_{L}\times j}{{j}_{L}-j}$$
(3)

where \({j}_{\mathrm{k}}\) is the mass transport corrected kinetic current density, \({j}_{\mathrm{L}}\) is the diffusion limited current density, and \(j\) is the measured current density.

Fig. 5
figure 5

Tafel plots based on the data reported in Fig. 4a, c for the POM3-3–9-modified PtFe/C sample (red points) compared to benchmark 20% Pt/C (black points) and 20% PtFe/C (blue points) catalysts. The content of Pt loadings on the electrode surface was a 15 μgPt cm−2 for all sample, and b 15 μgPt cm−2 for Pt/C, and 7.5 μgPt cm−2 for PtFe/C and POM3-3–9-modified PtFe/C

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The relevant values of mass activities, 0.343 mA μgPt−1 for PtFe/C and 0.356 mA μgPt−1 for POM3-3–9-modified PtFe/C, have been determined from Tafel plots E vs logjk measured at 0.9 V where the influence of mass transport is negligible [59]. The results are consistent with reasonably high catalytic activity of the Pt-based catalysts.

For the benchmark platinum catalyst, the Tafel slope changes from ∼60 mV dec−1 at lower overpotentials to ∼120 mV dec−1 at higher overpotentials indicating that the only rate-determining step (RDS) over the entire potential range is the initial electron transfer [66]. Based on the microkinetic modeling, this phenomenon was explained in terms of the influence of oxygen-containing reaction intermediates on the rate at low overpotentials through partially surface site-blocking, even if their removal takes place by fast quasi-equilibrated steps. The coupling between the removing processes of intermediates from the catalyst surface and the RDS leads to a changeable Tafel slope, which is dependant on the coverage and potential [66].

In the cases of unmodified and POM-modified PtFe/C catalysts, shifting of log(jL × j)/(jL − j) toward positive values has been observed, which implies better kinetics when compared to the reference Pt/C layer. In particular, the layer of low Pt loading present higher performance with respect to Pt/C, probably due to the activation effect of the combination of both Fe and POM components. Some uncertainty may arise from the site-blocking by OHads intermediates and the related electronic effects [67]. It is reasonable to expect that, in the presence of POM and Fe sites, formation of OHads intermediates is suppressed. This statement can be rationalized in terms of the catalytic activity of additives (particularly POM) toward decomposition of hydrogen peroxide intermediate or mutual activating interactions between Pt and co-catalytic additives. But the performance improvement is likely to be somewhat hindered by the catalytic site dilution effect due to the presence of additional materials on the electrode surface and higher resistance to charge (electron, ion) transfers. On the other hand, the most promising results have been observed during ORR at the low Pt loading system utilizing the PtFe/POM3-3–9 modified layer. While the catalyst’s morphology is likely to be the crucial issue (further research is needed along this line), the co-catalytic effect originating from the combination of both POM and Fe additives can be postulated. The obtained results imply potential utility of the low Pt loading Pt-Fe/POM-type catalysts in fuel cell research.

Application in fuel cell

The polarization and power density curves of MEA PtFe/POM3-3–9 at different relative hymidities (RHs), namely 100%, 62%, and 17% RHs, are presented in Fig. 6, in comparison to the commercial membrane electrode assembly (MEA). These results were recorded after 3 h of the activation process when the open-circuit voltage (OCV) reached steady values close to 0.96 V [48]. In the case of application of 100% and 62% RH, the POM-modified MEA shows comparable performance in terms of the current density in the region of the activation loss (0.65 V) with respect to commercial reference MEA, despite reducing of Pt loading at the cathode. The modified MEA is characterized by the current density values, 228 mA cm−2 at 100% RH and 167 mA cm−2 at 62% RH, which are lower than that of the reference MEA. But in the case of the lower relative humidity, RH = 17%, the MEA with PtFe/POM3-3–9 has exhibited the current density 79 mA cm−2 higher than that characteristic of commercial MEA. It is noteworthy that the H+ excess in the cathode region faciltates the activation process and improves the ORR kinetics by maintaining hydration and counteracting the performance loss visible for the commercial MEA (MEA Comm) at higher RH values [59, 68]. Moreover, at low RH conditions, any possible flooding is likely to be absent at the cathode and results in the performance enhancement. In addition, it is believed that the mesoporous POM can act as water reservoir to maintain good hydration of the Nafion membrane despite the low RH [69]. Also, in the low polarization region (at 0.3 V), the representative polarization and power curves characteristic of the MEA with PtFe/POM3-3–9 show higher current density values. Having in mind the low Pt cathode loading of the modified MEA, it is reasonable to expect some contribution of POM toward the gas transport properties. In the presence of the Fe additive, the Pt loading can be further lowered, in comparison to that described recently [68], thus maintaining the performance close to that characteristic of commercial MEA at high RHs, and yielding better performance at the low RH. The relevant data are summarized in Table 2.

Fig. 6
figure 6

Polarization and power density curves obtained using the POM3-3–9-modified PtFe/C catalyst (red points) and a commercial Pt/C catalyst (black points) at the cathode at different relative humidity: a 100% RH (T = 70/70/70 °C); b 62% RH (T = 60/70/60 °C); c 17% RH (T = 40/70/40 °C). Feed gases: H2 100 ml min−1, O2 200 ml min−1, P = 2 bar

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Table 2 Performance parameters for fuel cell tests in Fig. 6 for the POM3-3–9-modified PtFe/C catalyst and the commercial Pt/C catalyst
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In a case of the low cell humidity, the presence of POM and Fe tended to enhance the interfacial charge transfer, thus promoting the ORR and compensating for the low Pt content at the cathode. These results can be explained in terms of the high surface POM acidity which enhances proton availability and consequently ORR kinetics. Furthermore, the Fe component seems to promote — in the presence of POM3-3–9 — the four-electron oxygen reduction. Finally, the high mesoporosity and zeolitic nature of POM facilitates formation of the humidity reservoir capable of maintaining three-boundary contact between polymer electrolyte, electrode, and gaseous fuel.

Because the stability of Pt-based bimetallic alloy catalysts is far from meeting the requirement for practical long-term operations, the accelerated stress test (AST) based on the procedure used in the recent work [68] has been performed here (Fig. 7) to evaluate durability of the performance of the electrocatalyst as well as the persistence of its activity during long-term experiments at conditions of the fuel cell operation. Under such circumstances, the particles of metal catalysts are subject to the continuous growth and dissolution, especially upon application of high electrode potentials. Indeed, serious problems related to the dissolution of Fe, its growth, or loss were reported for bimetallic PtFe catalysts [70,71,72]. The parasite processes of dissolution and sintering are typically accelerated under repetitive potential cycling conditions. Our present results are, generally, consistent with good stability and persistence of performance after 96 h operation. The decrease in the current density has only been on the level 85 mA cm−2. It is reasonable to attribute the improvement in stability to the presence of polyoxometallates as co-support materials at the electrocatalytic interface. They are likely to interact with both Pt and Fe components, undergo chemisorption, form ultra-thin films on their surfaces, and exhibit stabilizing effects during electrocatalytic applications [73]. The strategy reported in this work can be of practical importance when it comes to designing or upgrading electrocatalytic systems for fuel cell research and other catalytic applications.

Fig. 7
figure 7

a Accelerated stress test (AST) for the MEA with the POM3-3–9-modified PtFe/C at RH 100%. b The changes of current density at 0.65 V during durability tests

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Conclusions

The utility of Cs3H3PMo9V3O40 polyoxometallate as co-catalyst, or co-support, for PtFe during electroreduction of oxygen acid medium has been demonstated here, and the resulting hybrid has been characterized by different physicochemical and electrochemical techniques. We have alo addressed the feasibility of applications in PEMFC. The MEA assembled with use of PtFe/POM modified cathode with low Pt loading has showed a comparable performance to the analogou commercial fuel cell at high RH, despite the reduction of Pt mass content. Moreover, the improved performance of the modified cell has been found at low RH. It has been concluded that the enhanced activity is related to the high surface acidity of POM, thus increasing the proton availability and, consequently, improving the ORR kinetics. Furthermore, the high polyoxometalate mesoporosity can provide a humidity reservoir capable of maintaining the three-boundary contact between polymer electrolyte, electrode, and gaseous fuel. In addition, the presence of POM seems to improve the interfacial charge (electron, proton) transfer dynamics, which leads to promoting the ORR and compensating for the low Pt content at the cathode. The performed accelerated stress test (AST), utilizing the potential square wave between 0.4 and 0.8 V, has been performed to evaluate MEA stability for ca. 100 h. Following the initial insufficient performance for the first 48 h, the system has exhibited promising stability until at least 96 h. The initial decay can be explained in terms of the aging efects concerning first the particles outside the POM pores. The other particles, which are embedded inside the pores, have not been subject to degradation and, therefore, the current density has reached steady-state values after 48 h of testing. In other words, introduction of the polyoxometalate-cesium-salt co-support/co-catalytic material results in hindering of the Pt particles aggregation and, consequently, in stable long-term operation.