High energy conversion systems are required to satisfy global consumption demand. Fossil fuel usage is causing gradual environmental deterioration due to CO
2 emission into the atmosphere [
1], and consequently, the search for novel substitute sources is vital. Polymer electrolyte membrane fuel cell (PEMFC) technology is an innovative alternative to efficiently produce cleaner energy. Among these, direct methanol fuel cells (DMFCs) are supplied with methanol solutions as fuel at the anode. In contrast to other fuels derived from petroleum and organic sources, methanol has the largest oxidation electro-activity [
1,
2]. Commonly, DMFCs are used in portable systems due to their versatility and easy re-fueling and because they are very appealing from economic and environmental points of view [
2,
3,
4]. However, a few technical barriers restrict DMFC commercialization; the main concerns are (i) the slow electro-kinetics of methanol oxidation and oxygen reduction at the anode and cathode, respectively, at low temperatures, forcing the use of platinum-based catalysts [
3]; (ii) membrane degradation; (iii) performance loss due to methanol crossover caused by the low tolerance to permeated methanol of the cathodic catalysts commonly used (Pt) [
4,
5]. Nevertheless, all the other components, such as the cell housing, bipolar plates, gaskets and stack auxiliaries, also contribute to DMFC durability issues [
3,
5]. The methanol crossover above refers to the permeation of methanol through the electrolyte from the anode to the cathode, which causes a substantial performance decrease due to the formation of a mixed potential at the cathode [
6,
7,
8,
9]. Great efforts have been made to overcome these problems, particularly to replace the platinum based catalysts with non-noble metals [
10] or other cheaper noble metals to be used as methanol tolerant cathodes, such as palladium [
11]. It is well known that palladium-based catalysts present a good methanol tolerance as fuel cell cathodes [
12] since the methanol oxidation process is negligible on Pd in acid media [
13]. In this context, there are some recent investigations explaining the methanol tolerance of Pd-based cathodes; e.g., DFT studies of methanol adsorption on a Pt and Pd single layer composed of thirteen atoms showed how platinum distorts the molecular structure of the adsorbed methanol and favors its deprotonation in the first step of methanol oxidation process [
14]. However, the deprotonation of methanol on palladium is kinetically and thermodynamically not favored, which is attributed to the Pd and Pt d-orbital extension difference. It is also known that Pd alloys with several 3d transition metals present higher oxygen reduction reaction (ORR) electro-activity than pure Pd [
15]. Metals like Cu, Fe [
13,
15], Ni, Cr, Co [
16,
17], have been deeply studied for improving ORR activity and methanol tolerance and, therefore, minimizing the crossover effects [
13,
15,
16,
17]. On the other hand, core-shell palladium nanoparticles represent a good alternative as a cathodic material [
18]. In this sense, Jia X. Wang et al. showed how a Pt monolayer growth (shell) on Pd with PdCo as the core exhibited prominent activity enhancement compared to Pt nanoparticles, due to strain and surface contraction effects. Likewise,
[email protected] compared to
[email protected] core shell structures, with low metal loading ~0.3 mg·cm
−2 at the PEMFCs cathode side, exhibited a high performance attributed to Ru core electronic interaction with the Pd-Pt alloy shell [
19]. It is often reported that Pd-d electron filling from another transition metal promotes a decrease of the density of states (DOS) due to the hybridization of the d-band of Pd by the incorporation of a second electropositive metal; thus, the adsorbed oxygen bonds are weakened and the dissociation mechanism is more feasible [
15,
16].
Previously, we proposed a trimetallic PdFeIr/C catalyst as a novel DMFC cathode due to its high tolerance toward methanol crossover effects, as demonstrated both in rotating disk electrode (RDE) experiments in half-cell configuration and at the cathode of a DMFC in single-cell tests. This was attributed to the surface composition rich in iron and iridium oxides. Nevertheless, a PdFe/C (without iridium) exhibited a similar behavior than PdFeIr/C in the kinetic region of ORR polarization curves in RDE experiments [
13]. Despite PdFeIr/C’s low Pd content, the presence of iridium results in a cost increase compared to bimetallic PdFe catalyst for equivalent Pd loadings in the electrode. From that point of view, this modification represents inherent cost savings and, for this reason, we have decided to study the DMFC performance of the bimetallic catalyst in depth in this work.