Highly active and durable Pd-Cu catalysts for oxygen reduction in alkaline exchange membrane fuel cells

Abstract

A Pd-Cu catalyst, with primary B2-type phase, supported by VulcanXC-7R carbon was synthesized via a solvothermal method. The catalysts were physically and electrochemically characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and both cyclic and linear sweep voltammetry using a rotating disk electrode (RDE). During the RDE testing, the half-wave potential of the Pd-Cu/Vulcan catalyst was 50 mV higher compared to that of commercial Pt/C catalyst for the oxygen reduction reaction (ORR) in alkaline media. The Pd-Cu/Vulcan exhibited a specific activity of 1.27 mA/cm² and a mass activity of 0.59 A/mg_Pd at 0.9 V, which were 4 and 3 times greater than that of the commercial Pt/C catalyst, respectively. The Pd-Cu/Vulcan catalyst also showed higher in-situ alkaline exchange membrane fuel cell (AEMFC) performance, with operating power densities of 1100 MW/cm² operating on H₂/O₂ and 700 MW/cm² operating on H₂/Air (CO₂-free), which were markedly higher than those of the commercial Pt/C. The Pd-Cu/Vulcan catalyst also exhibited high stability during a short-term, in-situ AEMFC durability test, with only around 11% performance loss after 30 hours of operation, an improvement over most AEMFCs reported in the literature to date.

Graphical abstract

Keywords

alkaline exchange membrane (AEM) / fuel cell / Pd-Cu / oxygen reduction / high performance / water

1 Introduction

Fuel cells are one of the most promising alternatives to internal combustion engines as power sources for transportation due to their environmental friendliness and high efficiency [1–2]. Alkaline exchange membrane fuel cells (AEMFCs) have potential superiorities compared to proton exchange membrane fuel cells (PEMFCs) [3–7] because of their less corrosive environment for the catalyst and catalyst support, broadening the choice for catalyst materials as well as more facile oxygen reduction kinetics. However, the development of AEMFCs has been hindered by several obstacles. One of the main issues is that despite significant effort, developing highly active and durable non-Pt catalysts for the oxygen reduction reaction (ORR) at the AEMFC cathode remains a challenge [3,6,8,9]. Unfortunately, many non-Pt catalysts have shown sluggish ORR kinetics in an operating fuel cell [10]. Platinum (Pt) and Pt-based alloy or de-alloyed catalyst have been reported to be highly active for ORR [11–14]. However, their future application in AEMFCs may be limited by high cost [15–16].

Sitting in the same group as Pt in the periodic table, palladium (Pd) is considered as one of the most active pure metal electrocatalysts. Pd has many attractive features including having lower cost than Pt and being more naturally abundant [4,17]. As a potential alternative to Pt, Pd and Pd-based alloy or de-alloyed catalyst have attracted increasing attention due to their relatively high ORR activity [18–22]. In particular, palladium-copper (Pd-Cu) has been mainly reported as a promising ORR electrocatalyst in acid media for PEMFCs [23–27].The improved ORR catalytic activity compared with metallic Pd and Pt mainly comes from a synergistic effect between these two metals, where the alloy has a lower oxygen binding energy, which is closer to the theoretically predicted optimal [27–28] with a 1:1 Pd:Cu ratio expected to be the best. Lowering the oxygen binding energy is thought to make OOH dissociative adsorption easier, which is regarded as the rate determining step for the ORR on Pd and Pd alloys [23,27,29,30].

From a structural perspective, Pd-Cu nanoparticles with an ordered body-centered cubic (B2-type) structure have recently been reported to have excellent catalytic properties for many reactions [31–34]. The unique polymorphism property of Pd-Cu nanoparticles offers a new pathway to explore its effect on ORR activity and AEMFC performance. Although some achievements have been made by applying Pd-Cu bimetallic nanoparticles at the PEMFC cathode, obtaining a high catalytic activity Pd-Cu electrocatalyst for ORR in alkaline media has yet to be shown. In addition, data regarding the ORR performance of Pd-Cu in operating AEMFCs is very limited and its in-situ activity and durability are poorly understood, which are essential for the development of AEMFCs.

In this paper, a one-pot, scalable strategy is demonstrated for synthesizing a Pd-Cu alloy supported on Vulcan carbon electrocatalyst for the ORR in alkaline media by using a solvothermal method. The typical synthesis of Pd-Cu supported on Vulcan (Pd-Cu/Vulcan) was based on co-reduction of palladium acetate and copper acetylacetonate by aniline in a solvent mixture of benzyl alcohol and polyvinylpyrrolidone (PVP), where PVP was used as a capping reagent. The Vulcan was added to the mother solution directly to provide adhesion sites for Pd-Cu. The atomic ratio between Pd and Cu of the final product was controlled simply by adjusting the molar ratio of metallic precursors in the solution. The Pd-Cu/Vulcan catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The activity as well as the stability of the prepared catalysts were assessed on a rotating disk electrode (RDE) by cyclic voltammetry and compared with the state-of-art commercial Pt/C. High in-situ activity of Pd-Cu/Vulcan was further demonstrated in an operating 5 cm² AEMFC.

2 Experimental

2.1 Chemicals

Cu (II) acetylacetonate (Cu(acac)₂) (98%, Fisher), palladium(II) acetate (Pd (OAc)₂) (96%, Fisher), benzyl alcohol (Certified, Fisher), aniline (100% w/v, Fisher), polyvinylpyrrolidone (Ave. F, W. 40,000, Fisher), potassium hydroxide (≥85%, Fisher), ethanol (Histological grade, Fisher), 2-propanol (IPA) (Optima, Fisher), acetone (99.5%, Fisher), gas diffusion layer (GDL) with 5% PTFE (Toray TGP-H-060), solid anionomer powder (ETFE-g-poly (VBC) powders), membrane (ETFE-based radiation-grafted alkaline anion exchange membrane, University of Surrey), Pt/TKK catalyst (50%, BASF), Vulcan XC-72R (Cabot), Alfa Aesar HiSPEC 10000 (platinum, nominally 40% (wt.), and ruthenium, nominally 20% (wt.), on Vulcan XC-72R carbon).

2.2 Catalyst preparation

Cu(acac)₂ and Pd(OAc)₂ were used as the metal precusors. Benzyl alcohol was used as the solvent. PVP and aniline were used as the capping reagent and reducing reagent, respectively. In a typical synthesis, 400 mg of PVP was added into 25 mL of benzyl alcohol. The mixture was rigorously sonicated until the PVP was completely dissolved, after which 35 mg of Vulcan XC-72R was added to the liquid mixture, followed by a 10-minute sonication until a homogenous suspension was formed. 70 mg of Cu(acac)₂, 60 mg of Pd acetate, and 0.8 mL of aniline were added into the suspension, followed by intense sonication until a homogenous dark blue solution was formed. The resulting solution was transferred to a 50 mL Teflon-lined autoclave. After a 24-hour heating at 170°C, the autoclave was cooled down to room temperature with cold tap water. The resulting Pd-Cu/Vulcan was washed with an ethanol-acetone (1:1 by volume) mixture three times, separated via centrifugation at 8000 RPM, and dried at 60°C under vacuum overnight. The Pd-Cu/Vulcan particles were then thermally annealed at 200°C in a tube furnace under H₂ atmosphere (H₂ flowrate= 40 sccm) for five hours. One typical synthesis yielded approximately 70 mg of Pd-Cu/Vulcan (40 wt.% metal).

2.3 Characterization

XRD patterns for the Pd-Cu/Vulcan were collected, using a Bruker D8 Advance X-ray diffractometer with a Cu Ka1 ceramic X-ray tube (λ=0.1540562 nm) and a LynxEye Super Speed detector, to investigate the crystallinity and structure. Data was collected from 25° to 90° at a scan rate of 0.0285 °/s. XPS was conducted using a Physical Electronic multiprobe with a Perkin-Elmer dual anode X-ray source and a Kratos AXIS-165 surface analysis system to determine the oxidation states of Pd and Cu, as well as the Pd:Cu atomic ratio in the Pd-Cu/Vulcan catalyst. The particle size and morphology of Pd-Cu/Vulcan was determined by TEM using a FEI Tecnai T12. Gas diffusion electrode (GDE) surface morphology was imaged by a FEI Quanta FEG 250 scanning electron microscope (SEM). Energy-dispersive X-ray spectroscopy (EDS) (TEAM^TM EDS Analysis System) and EDS mapping were used to estimate and visualize the Pd and Cu distribution. The actual mass ratio between metal and Vulcan was determined by thermal gravimetric analysis (TGA) with a NETZSCH STA 449, which confirmed that the true mass loading of Pd-Cu on Vulcan was almost exactly 40 wt.% (Fig. S2 in Electronic Supplementary Material), as intended. The atomic ratio between Pd and Cu was examined using inductively coupled plasma optical emission spectrometry (ICP-OES).

2.4 Electrochemical measurements

Electrochemical measurements were conducted on a thin film rotating disk electrode (RDE) in a custom three-electrode glass cell (Adams & Chittenden Scientific Glass) using a platinum mesh as the counter electrode and a double junction Ag/AgCl reference electrode (Pine Research Instrumentation). Cyclic voltammograms and ORR polarization curves were recorded using an Autolab PGSTA302N potentiostat. The working electrode was prepared on a glassy carbon disk electrode (geometric area: 0.2826 cm²; Pine Research Instrumentation) by dropping 13 mL of a catalyst ink with the following composition onto the electrode: 7.6 mg of catalyst, 7.6 mL of IPA, 2.4 mL of DI water, and 20 mL of 5% Nafion ionomer dispersion (DuPont). The film was dried on a leveled, inverted rotator at 700 RPM at room temperature. Prior to thin-film deposition, the glassy carbon electrodes were polished with a 0.05 mm alumina suspension and carefully washed with 18.2 MW Millipore DI water and dried in air for 20 min. The Pt and Pd loading on the glassy carbon were approximately 19 mg_Pt/cm² and 12 mg_Pd/cm², respectively. The three-electrode cell was washed using DI water and 0.1 mol/L KOH five times each before use.

2.5 GDE fabrication and fuel cell testing

Prior to formulation of the catalyst ink, a ETFE-g-poly (VBTMAC) powderanionomer [35] was first ground with a well-cleaned mortar and pestle for 10 min to reduce the number of aggregated particles. Next, the catalyst and 1 mL of DI water was added to the ground aniomer and ground for an additional 10 min until a visually homogeneous catalyst slurry was formed. The ETFE powder mass comprised of 20% of the total solid mass of all of the gas diffusion electrodes (GDEs) in this paper. After the slurry was homogenized, 1.5 mL of isopropanol was added into the mortar followed by another 5 min grinding. A final 5 mL of isopropanol was added to the mortar and the final ink mixture was transferred to a PTFE-lined vial and sonicated for 1 hour, changing the sonicator water every 15 min to avoid heating. The catalyst ink was sprayed onto the GDL using an air-assisted sprayer (Iwata) to fabricate GDEs. Since the particle size of the ETFE powder was around 20–30 mm [35], the catalyst layer was maintained at around a thickness of 30 mm in order to have a uniform thickness. Both the cathode and anode GDEs were hydrated in DI water for 20 min and then soaked twice in 1 mol/L KOH for 20 min to remove impurities and ion exchange the quaternary ammonium groups [36]. The membrane and GDEs were washed with DI water to remove excess KOH before cell assembly. A Pt-Ru catalyst was used at the anode. The noble metal loading for all electrodes was 0.60±0.03 mg/cm².

AEMFCs with a 5 cm² active area were assembled in single cell hardware with a single channel serpentine flow field. A benzyl-quaternary ammonium radiation-grafted membrane was used as anion exchange membrane in the AEMFC [37]. The average thickness of the GDE was 200 mm, and the total thickness of the Teflon gasket on each side was 152 mm, which led to a total pinch of 96 mm, corresponding to 24% of the total GDE thickness. The AEMFCs were tested on a Scribner 850e fuel cell test station at a cell temperature of 60°C under H₂/O₂ flow at 0.2 L/min and 0.7 L/min, respectively, at atmosphere pressure (no back pressure). The cell was pre-operated at a voltage of 0.5 V for break-in and the relative humidity (RH) of both the cathode and anode was adjusted to help the cell to be operated at optimal conditions [38].

3 Results and discussion

3.1 Physical characterization

The XRD pattern for the synthesized Pd-Cu/Vulcan catalyst is shown in Fig. 1(a). There were no evident peaks that could be attributed to either elemental Pd or Cu, suggesting the formation of a Pd-Cu alloy. The XRD pattern with Rietveld refinement confirms that Pd-Cu/Vulcan bimetallic nanoparticles are comprised of a primary Pd-Cu B2-type phase and a small amount of primitive cubic phase (cp) and face-centered cubic (fcc) solid solution. The peaks at 29.45°, 41.29°, 42.96°, 62.26° and 78.75° were assigned to the Pd-Cu cp (100), fcc (111), B2-type (110), B2-type(200) and B2-type (211), respectively. A peak corresponding to the Vulcan carbon support was also observed at 26.38° [39].

Since Pd²⁺ is generally considered to be more easily reduced than Cu²⁺, an uneven reduction rate of Pd²⁺ and Cu²⁺is possible during synthesis, which might result in a phase-segregated structure due to the initial formation of small Pd(0) clusters [40]. Thus, the formation of a B2-type Pd-Cu nanoalloy in this study can likely be attributed to the usage of PVP as a capping reagent, which balanced the reduction rate of Pd and Cu precursors as well as applying aniline as a mild reducing reagent to slow down the reaction rate. In addition, thermal annealing under H₂ atmosphere at relatively low temperature for an extended time helps facilitate the chemical ordering of Pd and Cu atoms through interaction of the transition metal atoms with dissociated H₂ [40–41].

TEM images of Pd-Cu/Vulcan at different scales are shown in Fig. 1(b–d). As shown in Fig. 1(b) and 1(c), the particles were well attached to, and widely dispersed on, the Vulcan surface, forming a single layer of coverage with an even particle distribution, which can decrease resistance between the catalyst and catalyst support and increase catalyst utilization during AEMFC operation. High resolution TEM (Fig. 1(d)) of the sample shows that the lattice fringes align clearly parallel to each other. The d-spacing of 0.210 nm measured by TEM corresponds to the (110) facets in B2-type Pd-Cu (0.211 nm), calculated from the Rietveld analysis (Maud). The particle size distribution inset in Fig. 1(c) was determined using Image J software over an area containing around 100 particles, showing that the major particle diameter is around 7–8 nm.

The elemental composition and chemical state of the Pd-Cu/Vulcan electrocatalyst was examined by high-resolution XPS. The broad survey was collected within binding energies from 1100 to 0 eV at 100 eV pass energy with all of the expected elements observed (Fig. 2(a)). The high resolution C 1s spectra could be deconvoluted into 3 bands, corresponding to (binding energy from low to high): graphitic carbon at 284.56 eV, carbon singly bound to oxygen (C-O) at 285.85 eV and carbon doubly bound to oxygen (C=O) at 287.44 eV [42–43].

The relative Pd/Cu atomic ratio in the Pd-Cu/Vulcan catalyst estimated from the broad survey was 0.93, which also corresponded well with ICP-OES (Pd/Cu ratio= 0.99). The high-resolution Pd3d spectra (Fig. 2(c) could be deconvoluted into two doublets with around 90% of Pd in the metallic state. The four peaks at 335.32 eV, 336.40 eV, 340.48 eV and 342.62 eV corresponded to the binding energies for Pd 3d_5/2, Pd²⁺3d_5/2, Pd 3d_3/2 and Pd²⁺3d_3/2, respectively. The high-resolution Cu 2p spectra (Fig. 2(d) could be deconvoluted into two doublets, Cu 2p_3/2 and Cu 2p_1/2, with a Cu 2p_3/2, satellite peak. It was found that the Cu was comprised of approximately 69% of its atoms in the metallic state.

3.2 Electrochemical measurements

The cyclic voltammograms (CVs) for Pd-Cu/Vulcan (Fig. 3(a)) and Pt/C (Fig. S3 in Electronic Supplementary Material) were recorded at room temperature in N₂-saturated 0.1 mol/L KOH electrolyte at a sweep rate of 20 mV/s. All potentials reported and discussed in this work are relative to RHE. The CV exhibited two distinct potential regions related to H_upd adsorption/desorption processes between 0.05 V<E<0.35 V and the Pd oxidation/reduction processes between 0.6 V<E<0.9 V. Due to the fact that Pd can absorb large quantities of hydrogen in its lattice [44], the real electrochemically active surface area (ECSA) of Pd-Cu/Vulcan was estimated using Eq.(1),

(1)\mathrm{ECSA}={\displaystyle \frac{Q}{\mathrm{0.405}\times \mathrm{Pd}}},

where Q is the charge corresponding to the reduction of a PdO monolayer, in mC, which was found by integrating the cathodic charge in the voltammogram between 0.6–0.85 V. The result was divided by the product of the Pd mass loading (mg_Pd/cm²) and the theoretical charge needed to reduce a PdO monolayer (0.405 mC/cm²) [9,21,45] in order to determine the ECSA. The as-prepared Pd-Cu/Vulcan had a reasonably high ECSA of 50.0 m²/g_Pd.

The ORR polarization curves for Pd-Cu/Vulcan and Pt/C thin-film rotating-disk electrodes in O₂-saturated 0.1 mol/L KOH are shown in Fig. 3(b). The polarization curves exhibited two distinct potential regions: diffusion-limiting currents were obtained in the potential region below 0.6 V whereas the mixed kinetic-diffusion control region existed between 0.6 V and 1.03 V. The half-wave potential of Pd-Cu/Vulcan at 1600 RPM was 0.90 V, which was 50 mV higher than that of Pt/C (0.85 V), indicating improved ORR kinetics [46]. RDE experiments were further conducted from 400 to 2500 RPM to examine the reaction kinetics of the Pd-Cu/Vulcan catalyst. As shown in the inset of Fig. 3(c), the limiting current densities increased with increasing electrode rotation speed [47]. The linearity of the Koutechy-Levich plots and near parallelism of the fitting lines suggest first-order reaction kinetics and similar electron transfer numbers for ORR at different potentials [48]. The number of electrons (n) transferred during the ORR was calculated to be 3.94 (0.65–0.875 V) from the slopes of Koutechy-Levich plots (Fig. 3(c)), suggesting that Pd-Cu/Vulcan overwhelmingly prefers a 4e⁻ oxygen reduction process, similar to the commercial Pt/C catalyst in the same 0.1 mol/L KOH electrolyte [15].

The kinetic current was calculated by mass-transport correcting the ORR polarization curves at 0.9 V (Eq. S3 in Electronic Supplementary Material) [49]. Pd-Cu/Vulcan exhibited a specific activity of 1.27 mA/cm², which was over 4 times greater than Pt/C (0.28 mA/cm²). After being normalized by the Pd loading, the mass activity of Pd-Cu/Vulcan was calculated to be 0.59 A/mg_Pd, which was around 3 times higher than that of Pt/C (0.21 A/mg_Pt). Figure 3(d) presents the mass-transport corrected Tafel plots of the logarithm of kinetic current density, log J_k (mA/cm²), versus the electrode potential for the ORR on Pd-Cu/Vulcan and Pt/C catalysts. The Tafel plots for both materials have a linear region with slopes of ca. 120 mV/decade at high overpotential and ca. 60 mV/decade at low overpotential, which are consistent with the expected reaction mechanism on the catalyst surface [50]. The increased activity for ORR of Pd-Cu/Vulcan likely comes from an optimized d-band center to facilitate the OOH dissociative adsorption as stated above. Second, a mixed phase consisting of B2-type and disordered fcc structure can also contribute to enhancement of ORR activity due to a stable intermetallic Pd and Cu arrangement [33].

In addition to the high specific and mass activities, the Pd-Cu/Vulcan also exhibited excellent electrochemical stability. The catalyst was cycled between 0.6 and 1.0 V for 5000 cycles in N₂-saturated 0.1 mol/L KOH at a scan rate of 50 mV/s to mimic fuel cell operation where catalysts often suffer from poor stability [51]. After 5000 cycles, the Pd-Cu/Vulcan largely retained its activity, exhibiting only an 8 mV shift in its half-wave potential (Fig. 3(f)) post-cycling. The enhanced durability is ascribed to the synergic effect between Pd and Cu, resulting in a lower coverage of oxygenated intermediates because of a weaker oxygen binding energy, which reduces the possibility for Pd dissolution.

3.3 GDE morphology and AEMFC performance

Figure 4(a) and (b) show that the catalyst particles are uniformly distributed onto the carbon paper mesh and that there are no obvious cracks or mud-like clusters formed during ink spraying. Fig. 4(c) and (d) are higher magnification images that show a porous architecture and good contact between the ETFE ionomer powder and the supported catalyst.

The surface morphology of the GDEs prepared using Pd-Cu/Vulcan as the cathode catalyst was examined by SEM. As shown in Fig. 4, the GDE possessed a uniform agglomeration of catalyst particles and very porous architecture, which can be beneficial for reactant and product mass transfer, and even water back diffusion from the anode to cathode [38]. Figure 4(c) shows the ETFE ionomer power with a diameter around 30 mm. Figure 4(d) shows that the Pd-Cu/Vulcan catalyst is in intimate contact with the surface of the ETFE powder ionomer, which is thought to help form the triple-phase boundary and decrease charge transfer resistance. EDS and elemental mapping results are shown in Fig. S1 in Electronic Supplementary Material, where it also confirms the existence ETFE power and a homogeneous distribution of catalyst on its surface.

Since water management in the catalyst layer plays an important role in electrode flooding as well as the ionic conductivity of the membrane and ionomer, choosing an appropriate operating RH for both the cathode and anode is critical to achieving the best possible AEMFC performance [52]. In order to optimize the cell operating conditions, single-cell AEMFCs with Pt-Ru/C applied as the anode catalyst and the synthesized Pd-Cu/Vulcan as the cathode catalyst were assembled and tested. As shown in Fig. S4 in Electronic Supplementary Material, these AEMFCs with the ETFE membrane and ionomer powder do not prefer to be operated at full humidity and cell [38] performance is actually increased as the anode RH decreased. The cell reached the highest power density at 87% anode RH. After choosing the optimized anode RH, cathode RH was also optimized at the settled anode RH. As shown in Fig. S5 in Electronic Supplementary Material, the cell performance increased as cathode RH decreased and reached peak performance at 75% cathode RH. Therefore, an anode RH of 87% (dew point 57°C) and a cathode RH of 75% (dew point 54°C) were selected as the optimized cell operation condition in this work. It should be noted that even at this low RH for both cathode and anode, the AEMFC high frequency resistance (HFR) was still lower than 70 MW∙cm². The ETFE membrane used in this work has unusually high ionic conductivity [37], which is key descriptor for H₂O transport [52,53]. Therefore, other membrane materials may not show identical behavior if they have limited ionic conductivity.

This counterintuitive strategy of operating at low relative humidity, especially at the cathode, might be explained as follows. Since oxygen has limited diffusion capability through the catalyst layer and very limited solubility in water, it is necessary to keep the cathode at a relatively low RH to keep cathode catalyst layer un-flooded, which requires that the anode RH should be higher than cathode to provide enough water for the cathode to react with oxygen through back diffusion, not just the water supplied from the gas phase. Robust water back diffusion from anode to cathode also helps keep the membrane hydrated, which in return sustains high ionic conductivity, and explains the very low HFR reported here.

As shown in the polarization curves (Figs. 5(a) and 5(c)), the Pd-Cu/Vulcan was able to achieve a superior performance than Pt/C, not only in the Ohmic and mass transport controlled regions, but also in kinetic controlled region (inset of Fig. 5(a) and 5(c)). This shows that the Pd-Cu Vulcan had a higher ORR activity than Pt/C in-situ, which is also in agreement with the ex-situ RDE measurements. The AEMFCs with a Pt/C cathode reached peak power densities of 827 mW/cm² when operating with H₂/O₂ feeds and 642 mW/cm² under H₂/Air (Fig. 5(d)), comparable to the state-of-the-art [54]. The AEMFC with the Pd-Cu/Vulcan cathode was superior, approaching a peak power density of 1100 and 700 mW/cm² with H₂/O₂ and H₂/Air feeds, respectively (Fig. 5(c)). The significant improvement in cell performance was mainly due to a combination of faster ORR kinetics of Pd-Cu/Vulcan than Pt/C as well as improved mass transfer, which might have been due to a larger percentage of carbon (and hence porosity) in the catalyst layer.

The AEMFC with the Pd-Cu cathode catalyst in this work is more extensively compared to other AEMFC studies from the literature in Table S1 in Electronic Supplementary Material [36,54–60]. In general, the Pd-Cu catalyst in this work offers superior performance in terms of both current density at 0.7 V and peak power density. The Pd-Cu catalyst also shows especially high performance with regard to peak power when normalized to precious metal loading (1.83 W/mg). Though there are three reports of higher specific power densitites [54,59,60], it is important to note that all of those studies accomplished this feat with significant back pressurization of the gas feeds, which was not done in the work reported here.

3.4 Pd-Cu/Vulcan in-situ AEMFC stability

AEMFC stability has long been a serious concern, with many studies focused on identifying new membrane and catalyst materials [61]. In this paper, in order to demonstrate the feasibility of applying a stable platinum-free cathode, the Pd-Cu/Vulcan catalyst was subjected to continuous operation in an AEMFC set at 0.6 V for 30 h. 0.6 V was selected to ensure that the current density was maintained high enough to limit membrane dryout from the lower RH gases and carbonation. As shown in Fig. 5(e), the Pd-Cu/Vulcan AEMFC only showed a performance loss of around 11% over 30 h. The performance loss mainly resulted from decreased membrane water, shown by a 9% increase in the high frequency resistance (HFR) in Fig. 5(f).

4 Conclusions

In this paper, Pd-Cu bimetallic nanoparticles supported on Vulcan XC-72R carbon was synthesized by a solvothermal method. The Pd-Cu/Vulcan was characterized both physically and electrochemically. TEM revealed a uniform coverage of Pd-Cu nanoparticles on the Vulcan, while a combination of XRD and XPS showed primarily a B2-type highly metallic alloy. Ex-situ RDE and in-situ AEMFC experiments showed that Pd-Cu/Vulcan has a higher ORR activity than Pt/C with impressive stability. A 5 cm² active area of AEMFC with the Pd-Cu/Vulcan catalyst at the cathode and a commercial Pt-Ru/C at the anode was able to achieve a peak power density of 1100 mW/cm² under H₂/O₂ flow and 700 mW/cm² under H₂/Air flow. This paper demonstrates the feasibility of developing Pt-free cathode catalysts for real AEMFC operations.

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