Introduction

Proton exchange membrane (PEM) fuel cells operating at low temperatures and high power densities are considered one of the most promising technologies for clean energy devices capable of producing efficient and environmentally friendly energy, along with the implementation of sustainable development concepts in the energy industry [1,2,3]. Because of their high power density and versatility, PEM fuel cells have received much attention over the last few decades [4,5,6]. However, owing to the use of platinum (Pt) metal and the slow kinetics of the ORR on the cathode side, PEM fuel cells are presently constrained by their expensive cost [7,8,9]. As a result, an acidic electrolyte catalyst with high activity in a non-platinum environment is urgently needed. Because palladium and platinum have similar valence electronic configurations and lattice constants [10, 11], researchers have focused on the development of palladium-based catalyst materials for the oxygen reduction reaction (ORR) in PEM fuel cells and DMFCs (direct methanol fuel cells) [12,13,14,15]. The palladium and palladium-based alloys have shown strong activity in the electro-oxidation of alcohols in alkaline environments, making them viable alternatives to platinum [16,17,18,19,20,21]. Searching for appropriate support materials in the realm of heterogeneous catalysis is another effective technique for minimizing noble metal loading and improving catalyst performance. As electro-catalyst supports, carbon materials with diverse structures and forms, such as active carbon, carbon nanofibers, and carbon nanotubes, have been employed [22,23,24]. Other than carbon, no other material has the necessary attributes for the commercialization of fuel cells, such as corrosion resistance, electronic conductivity, and low cost. Compared to other conductive carbon black grades, Vulcan XC-72R carbon black (VC) may give high conductivity at low loading levels while also being simpler to disseminate throughout the surface [24]. The production of Pd nanoparticles with homogeneous dispersion has taken effort [10, 25,26,27,28]. However, no reports of the Pd–Mo bimetallic catalysts for PEM electro-oxidation have been published to the best of our knowledge. In particular, in this research, less-costly carbon Vulcan XC-72-supported Pd–Mo nanoparticles are synthesized by chemical reduction using sodium borohydride and formic acid methods as electro-catalysts in PEM fuel cells.

The characterization and electrochemical activity measurements of the PdMo2/C catalyst with a 1:2 atomic ratio of Pd: Mo, which showed the best activity in the PEM fuel cell test study, were performed. The structural characteristics of the materials are researched using X-ray diffraction (XRD), a scanning electron microscope (SEM), and the energy dispersion spectrometer (EDX) mapping. Electrochemical activities of the catalyst are determined by electrochemical surface area (ECSA), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) determined by alternating current impedance measurements. Different ratios of the PdMo/C catalyst cathode, including MEA with a 1 cm2 active area, are tested at four different temperatures (40 °C, 50 °C, 60 °C, and 70 °C) in the presence of supplied humidified H2/O2 gases.

Material and Methods

Chemicals

A variety of substances were used in the present investigation. The compounds are described below.

Sodium borohydride (NaBH4, 99%), ethanol (99.9%), formic acid (HCOOH, ≥ 98%), isopropyl alcohol, and Nafion solution (5% dispersion) were purchased from Aldrich; palladium(II) chloride (PdCI4, 98%) and ammonium molybdate tetrahydrate ((NH4)6Mo7O24·4H2O) were bought from Sigma Aldrich and they were used. Vulcan XC-72R carbon powder carbon black is acquired from Cabot. Electrochem Inc. provided the other materials used, including Nafion TM 117 membranes and Teflon TM processed by Toray TM carbon paper, which is purchased from the fuel cell store.

Synthesis of Catalyst

PdMox/C (x:1, 2, 3) catalysts were prepared by the chemical reduction method. The platinum chloride and ammonium molybdate tetrahydrate were used as metal precursors and sodium borohydride and formic acid as reducing agents. All of the catalysts were produced with a metal loading to support 20 wt%. All catalysts synthesized in different ratios of Pd:Mo (1:1, 1:2, and 1:3 molar ratios) were prepared. The proper quantity of carbon (Vulcan XC-72) was then dispersed in 10 ml ethyl alcohol for 1 h using a magnetic mixer. Next, the platinum chloride and ammonium molybdate tetrahydrate were added to the solution and homogenized using an ultrasonic mixer for 30 min. Before the reduction process, to prevent it from reacting with water instead of metal salts of sodium boron hydride added, the pH of the catalyst of the compound was adjusted to pH 11 by adding 0.5 M NaOH, and the solution was heated until the temperature reached 80 °C. Next, the 10 ml NaBH4 and 5 ml of formic acid solutions were dropped slowly, and the reduction process was carried out at a solution temperature of 80 °C. The resulting suspension was filtered, and the obtained sediment was dried in a drying oven at 110 °C in a nitrogen gas medium for 5 h. The Pd/C catalyst was likewise synthesized using the same process as described above.

Preparation of the MEA and Fuel Cell Testing

MEAs are traditionally manufactured by two methods, the membrane-based method or the GDL-based method [29]. In the GDL-based method, a catalyst layer is applied onto one side of the GDL forming a catalyst-coated GDL (CCG) [30]. The gas diffusion layer (GDL)-based technique created was employed to create the MEAs (1 cm2 active area) used in this investigation. The concentration of 20% PdMox alloys was utilized as a catalyst in the cathode electrode. In the PdMo/C catalyst, 0.13 mg(Pd) cm−2 was used; in the PdMo2/C catalyst, 0.096 mg(Pd) cm−2 was used; and in the PdMo3/C catalyst, 0.072 mg(Pd) cm−2 was used.

The catalyst and the 5 wt.% Nafion ionomer solution was combined with an isopropanol solution to generate a homogenous catalyst suspension for use as the cathode in the electrochemical cell. The catalyst inks were painted onto the GDLs with a 0.15 mg(Pt) cm−2 metal loading for anode electrodes. For the gas diffusion layer, carbon cloth (Toray TM, no metal) and the solid electrolyte membrane were used on Nafion 117. Anode and cathode diffusion layers were prepared by hot pressing MEAs on Nafion membrane-117 at 135 °C and 65 kgcm−2 pressure for 3 min.

The anode side is tested with 50% humidified H2 gas and the cathode side is tested with 50% O2 gas. H2 and O2 gases had a flow speed of 20 ml dk−1, and the fuel cell had a working temperature of 40, 50, 60, and 70 °C. To get the best performance out of each MEA, polarization curves were taken and activated at 0.6 V until stable results were found. The humidification and cell temperatures were set for 30 min, and then nitrogen was delivered to the anode and cathode sides [31]. The maximum power output was then repeated at that level until no more variation was seen. After the fuel cell had reached the desired temperatures, hydrogen and oxygen were introduced [2, 32].

Characterization of the Catalyst Materials

XRD (Rigaku X-ray diffractometer) and SEM, EDX (JEOL JSM 6610) measurements were carried out to examine the structural and morphological properties of Pd/C and PdMo2/C catalysts.

Electrochemical Measurements

For electrochemical measurements, cyclic voltammetry, chronoamperometry, and electrochemical impedance spectroscopy techniques were used. Measurements were taken in a 0.5 M H2SO4 solution. For electrochemical tests, CHI 660E equipment was employed. The working electrode was a glassy carbon electrode with a diameter of 3 mm, the reference electrode was Ag/AgCl (3 M, KCl), and the counterelectrode was a platinum (Pt) plate. The CV voltammetry measurements were taken in the potential range of − 0.3/1.2 V. Chronoamperometry measurements were performed at − 0.1 V. In this experiment, alternating current impedance measurements were taken at a potential of 0.5 V in the frequency range from 10+5 to 10+2 Hz. An ultrasonic bath was used to sonicate 5 mg of catalyst in a combination of 960 ml of ethanol and 40 µl of Nafion solution (5%), which was then left to stand for 30 min to conduct electrochemical tests. Then, 2 µl of the mixture was applied to the glassy carbon electrode surface using a micropipette, and measurements were performed when the mixture had dried completely.

Results and Discussions

The Results of Physical Characterization

Figure 1 shows the XRD patterns of the PdMo2/C catalyst with the Pd:Mo (1:2) ratio determined as the best Mo ratio and the Pd/C catalyst for comparison. The Pd/C patterns are seen a broad peak at 2θ = 24.96, attributed to the (200) plane of the carbon support, and a succession of peaks at 2θ = 40.18°, 46.03°, and 68.20°, respectively. The single cubic fcc phase of palladium is defined by its peaks, which are attributed to planes (111), (200), and (220). The experimental pattern corresponds to JCPDS card 01–087-0643 [10], containing conventional crystallographic tables. Despite the similarity in the patterns of the Pd/C and Pd/Mo samples, the addition of Mo causes the peaks to be slightly moved to higher values of 2θ, which suggests a contraction of the lattice and the production of alloy [33, 34]. The absence of any Mo element or compound peak in the XRD patterns for PdMo phases indicates that the samples are independent of visible phase segregation [35]. The EDX spectra of the PdMo/C catalyst, on the other hand, unambiguously demonstrate the presence of Mo (Fig. 2). Scherrer’s equation is employed to calculate the average crystallite size for the catalysts based on the (111) diffraction peak seen.

Fig. 1
figure 1

XRD results of Pd/C and PdMo2/C catalysts

Full size image
Fig. 2
figure 2

SEM imagines of a Pd/C and b PdMo2/C

Full size image
$$d=0.9\lambda/(\beta\cos\theta)$$
(1)

β is the width of the peak obtained as a result of x-ray diffraction, d is the crystal size, λ is the wavelength of the x-ray used, and θ is the Bragg angle at which the plane is observed. The average crystallite sizes calculated for the Pd/C and PdMo2/C catalysts are 5.13 and 4.66 nm, respectively.

SEM imaging was used to examine the morphology of Pd/Mo and PdMo2/C catalysts (Fig. 2a and b). The well-controlled and homogeneous nucleation and growth caused by sodium borohydride and formic acid as reducing agents might be ascribed to the Pd/C and PdMo2/C catalyst advantageous morphology. In the pictures, the visible white spots are thought to represent a rough surface covered with agglomerates of well-defined spherical palladium particles deposited in the presence of a Vulcan carbon substrate with a dense and particulate structure. However, energy dispersive X-ray spectroscopy (EDX) analysis on the samples was carried out to collect the requisite experimental proof of the catalyst on the support.

The EDX analysis shown in Fig. 3a, b for the Pd/C and PdMo2/C catalysts clearly shows that bimetallic nanomaterials of Pd and Mo are formed. The mean atomic percentages for Mo and Pd, according to the EDX study, were 6.1% and 9.6%, respectively.

Fig. 3
figure 3

EDX spectrum of catalysts a Pd/C and b PdMo2/C

Full size image

In the molybdenum-rich PdMo2 catalyst, a typical signal of Mo (Lα) at 2.18 keV was observed [36, 37]. In addition, the energy peak of Pd (Lα) was also observed at 2.85 keV [38]. In addition, a low divergence between Pd and Mo atomic ratios is available, indicating a uniform composition. EDX element mapping shows the homogeneous distribution of palladium and molybdenum along the metal line (Fig. 4a, b).

Fig. 4
figure 4

EDX mapping imagines of a Pd/C and b PdMo2/C

Full size image

Electrochemical Measurements

Cyclic Voltammetry

Cyclic voltammograms of the Pd/C and PdMo2/C catalysts were studied in a 0.5 M H2SO4 solution, as shown in Fig. 5. The electrochemical behaviors of Pd-based nanomaterials are shown in these voltammograms. The adsorption/desorption of the H on the catalyst surface was attributed to the observed peaks in the CV potential range of − 0.25 to 0.10 V. The size of these peaks is related to the electrochemically active surface area of Pt. However, because of the hydrogen-absorbing characteristic of Pd, the size of the electrochemically active surface area determined by this approach does not entirely correspond to the absolute values [39, 40], as previously stated. As a result, it is easy to compute Pd’s electrochemically active surface area, which is determined by the charge in the PdO reduction peaks [41], which is the most convenient method. The upper potential limit for the creation of PdO monolayers, on the other hand, should be well established. Finally, the amount of charge passed for the PdO reduction peaks in the CVs obtained by determining different upper potential limits is determined. Charge values displayed as a function of the upper potential limit reveal that the coordinates at the point at which the line changes its slope correspond to a whole PdO monolayer development [42]. According to the results of the measurements, the top potential limit was determined to be 1.4 V.

Fig. 5
figure 5

Cyclic voltammograms of Pd/C and PdMo2/C catalysts in N2 sat. 0.5 M H2SO4 at a scan rate of 50 mVs.−1 (vs. Ag/AgCl)

Full size image

In cathodic scanning, the PdO reduction peaks are around 0.44 and 0.41 V for Pd/C and PdMo2/C, respectively. It was observed that the peak for PdMo2/C is the largest of these peaks. This shows that the PdMo2/C catalyst has a greater surface area that is electrochemically active. The downshift of the Pd d-band center, which weakens the binding energy of atoms, may contribute to the reduction of the active surface area during alloy formation [43]. Based on the results of Fig. 5, we can conclude that the alloying of Pd metal with Mo lower d-band position of the top layer of Pd metal may have a significant impact on activity by producing straining and electron transfer between the substrate and over layer [44]. Electrochemically active surface area (ESA) determination is a reliable normalization technique that considers the active sites accessible for reaction. The theoretical charge required for this process (424 µC/cm2) is divided by the experimental charge utilized to decrease a whole monolayer of Pd oxide in this method [45, 46]. For Pd/C and PdMo2/C nanocubes calculated, the ESA values were 6.3 m2/gPd and 37.2 m2/gPd, respectively.

Chronoamperometric Measurements

Chronoamperometric measurements are valuable methods for investigating the electrochemical activity and stability of catalysts in fuel cells [47, 48]. According to the chronoamperometry measurements performed on Pd/C and PdMo2/C catalysts, as shown in Fig. 6, the change in current values is very constant following a quick current decrease at the start. This indicates that the catalysts are stable. It is observed that the current values obtained for the PdMo2/C catalyst are much greater than the current values obtained for the Pd/C catalyst. A greater current density is a sign of increased electroactivity. This indicates that PdMo2/C is a more electroactive catalyst.

Fig. 6
figure 6

Chronoamperometric curves of Pd/C and PdMo2/C catalysts in O2 sat. 0.5 M H2SO4

Full size image

Impedance Measurements

The electrochemical impedance measurements were carried out using MEAs for the catalysts Pd/C and PdMo2/C. Figure 7 shows the spectra of the single cells that used the two catalysts as cathode catalysts. Electrochemical impedance spectroscopy measurements show a partial semicircle in the high-frequency range of the Nyquist curves. Furthermore, it is shown the charge transfer resistance, and its diameter is based on the size of the charge transfer resistance. Since the electrochemical reaction rate is inversely proportional to the charge transfer resistance [49, 50], a low charge transfer resistance indicates high electrochemical activity. In Fig. 7, it is shown that since the diameter of the partial semicircle in the high-frequency zone produced for the electrode with the PdMo2/C catalyst is smaller than the diameter obtained for the Pd/C, the charge transfer resistance of the electrode with the PdMo2/C catalyst is lower. These suggest that the PdMo2/C catalyst has more significant electrochemical activity than the Pd/C catalyst, consistent with previous findings. The results of cyclic voltammetry and chronoamperometry are also consistent with this hypothesis. It may be attributed to the fact that increasing current density in the low polarization zone might accelerate the electrochemical kinetics of ORR [51].

Fig. 7
figure 7

Nyquist curves obtained from electrochemical impedance spectroscopy measurements of Pd/C and PdMo2/C catalysts in O2 sat. 0.5 M H2SO4 solution

Full size image

PEM Single Cell the ORR Activity Evaluation

Pd/C and atomic ratios PdMox/C electrocatalysts (1:x, x = 1, 2, and 3) were utilized as the cathode, while Pt/C was used as the anode membrane electrode assembly (MEA) was evaluated in a single PEM fuel cell test station. In addition, several atomic ratios of PdMo/C with comparable cathode loading were polarized to assess the performance and make comparisons. Figure 8 shows the polarization curves of Pd/C and different atomic ratios of PdMox/C cathodes at temperatures ranging from 40 to 70 °C. The performance of a single cell increased when the temperature of the cell was increased higher.

Fig. 8
figure 8

Polarization curves a Pd/C, b PdMo/C, c PdMo2/C, and d PdMo3/C electrocatalysts used as cathode with Pt/C as the anode. At ambient pressure, the cell was operated with humidified H2/O2 gas flow rate 20 mldk.−1 at different temperatures (40 °C, 50 °C, 60 °C, and 70 °C)

Full size image

The open-circuit voltage decreased slightly as the Pd:Mo atom ratios increased. The reduction in OCV may be due to the effect of pore size on the electrode layer. That is, at a low Pd:Mo atomic ratio (high Mo content), it can cause a high degree of catalyst agglomeration with a low level of dispersion. However, it is clear that the different Pd:Mo atom ratios in the PdMo2/C electrocatalyst significantly affected cell performance in the activation region at high cell voltage and in the Ohmic region at medium to low cell voltages. This agglomeration causes a greater pore size to grow on the electrode layer, resulting in a more significant fuel crossover from the anode to the cathode and hence a lower OCV [52, 53]. Then, the oxygen reduction reaction (ORR) activity of the binary electrocatalyst may therefore be expressed in terms of the geometric surface area of the electrode by taking into account the influence of Pd:Mo atom ratios on cell performance in the activation-controlled region (the values of the current density at 0.83 V (j0.83 V) and the potential at 7 mA/cm2).

In the polarization curve, it was observed that the performance of the PEM fuel cell with MEA containing PdMo/C (Pd:Mo = 1:2 ratio) was higher than the performance of the cell with Pd/C and various Pd:Mo atomic ratios under similar test circumstances (Fig. 9). The ORR activity of the Pd/C electrocatalyst appears to increase with increasing Mo content in the PdMo/C electrocatalyst. However, the best result is seen at Pd:Mo (1:2). Furthermore, when the polarization curve is examined, it is seen that the Ohmic losses and the concentration losses decrease when the atomic ratio of Mo in Pd increases by up to 2 mol. Therefore, it can be stated that increasing the Pd/C electrocatalyst property of Mo metal increases the ORR activity of MEA.

Fig. 9
figure 9

Polarization curves of a single PEM fuel cell for ORR with Pd/C and different Pd:Mo atomic ratios, evaluated at 70 °C and ambient pressure. PdMox/C represents the Pd:Mo atomic ratio of 1:x (x = 1,2,3). Anode: 20 wt.% Pt/C, anode Pt loading: 0.15 mg(Pt) cm−2. Cathode Pd loading: 0.15 mg(Pd) cm−2, 0.13 mg(Pd) cm−2, 0.096 mg(Pd) cm−2 versus 0.072 mg(Pd) cm−2. H2/O2 flow rate: 20 mldk−1. Membrane: Nafion 117

Full size image

The efficiency of the as-synthesized catalysts was evaluated and compared at 70 °C cell temperature. Maximum current densities of synthesized catalysts at 70 °C cell temperature were measured as 61, 116, 188, and 168 mA cm−2 for Pd/C, PdMo/C, PdMo2/C, and PdMo3/C catalysts, respectively. The efficiency according to the current of the PdMo2/C cathode catalyst at 70 °C was 52%, and Pd/C, PdMo/C, and PdMo3/C cathode catalysts at the same temperature were calculated as 26%, 38%, and 50%, respectively. Under the same conditions, the maximum power density was 107 mW cm−2 obtained for the PdMo2/C catalyst, while the Pd/C, PdMo/C, and PdMo3/C cathodes provided a maximum power density of 14.50, 33.60, and 88.56 mWcm−2, respectively. The ORR catalytic activity of as-synthesized electrocatalysts could be ordered as PdMo2/C > PdMo3/C > PdMo/C > Pd/C. The results of single-cell polarization reveal that Pd-decorated spinal have enhanced performance in the fuel cell by promoting the ORR process.

Conclusion

In this study, less costly carbon Vulcan XC-72-supported Pd and PdMo nanoparticles are synthesized by chemical reduction using sodium borohydride and formic acid methods as electro-catalysts in PEM fuel cells. Electrocatalysts made with different atomic ratios were used as cathode catalysts in PEM fuel cell applications at four different temperatures. The PdMo2 electrocatalyst was found to be the most active cathode catalyst. The Pd/C and PdMo2/C catalysts exhibit diffraction peaks consistent with the face-centered cubic Pd structure at the appropriate locations in the X-ray patterns. The electrochemical results indicate that the PdMo2/C catalyst has a lower starting potential, a greater peak current density, and superior electrocatalytic activity than the Pd/C catalyst. A fuel cell test with a single cell also shows that the activity of the ORR increases with the temperature of the cell. The single-cell test calculated a maximum power density of 107 mWcm−2 with PdMo2/C as the cathode catalyst. The PdMo2/C catalyst has higher activity than Pd/C and other catalysts due to its smaller size and higher Mo content. Since the PdMo/C catalysts presented in this paper have good properties and are easy to make, we think it will lead to more research into cheaper electrocatalysts for PEM fuel cells under development.