Palladium–Ceria Catalysts with Enhanced Alkaline Hydrogen Oxidation Activity for Anion Exchange Membrane Fuel Cells

Abstract

Anion exchange membrane fuel cells (AEMFCs) offer several important advantages with respect to proton exchange membrane fuel cells, including the possibility of avoiding the use of platinum catalysts to help overcome the high cost of fuel cell systems. Despite such potential benefits, the slow kinetics of the hydrogen oxidation reaction (HOR) in alkaline media and limitations in performance stability (because of the degradation of the anion conducting polymer electrolyte components) have generally impeded AEMFC development. Replacing Pt with an active but more sustainable HOR catalyst is a key objective. Herein, we report the synthesis of a Pd−CeO 2 /C catalyst with engineered Pd-to-CeO 2 interfacial contact. The optimized Pd−CeO 2 interfacial contact affords an increased HOR activity leading to >1.4 W cm −2 peak power densities in AEMFC tests. This is the only Pt-free HOR catalyst yet reported that matches state-of-the-art AEMFC power performances (>1 W cm −2 ). Density functional theory calculations suggest that the exceptional HOR activity is attributable to a weakening of the hydrogen binding energy through the interaction of Pd atoms with the oxygen atoms of CeO 2 . This interaction is facilitated by a structure that consists of oxidized Pd atoms coordinated by four CeO 2 oxygen atoms, confirmed by X-ray absorption spectroscopy.

Synthesis of CeOx‐Decorated Pd/C Catalysts by Controlled Surface Reactions for Hydrogen Oxidation in Anion Exchange Membrane Fuel Cells

“…The sharp H des peak of 0.38 CeO x -Pd/C starts at slightly lower potential (0.32 V) in comparison to Pd/C (0.33 V). This sharp H des peak with CeO x -Pd/C catalysts is believed to be the weaker Pd-H interaction of CeO x -Pd catalysts compared to Pd/C, which is consistent with the decrease in the HBE recently calculated by DFT by Bellini et al [37] The well-defined oxide formation and reduction features were not observed in some of the previously reported Pd/C-CeO x catalysts, [35][36][37] which might be due to the high CeO x content (above 50 at%). It is important to mention that all the CeO x -Pd/C catalysts are stable during cycling (see Figure S11 in the Supporting Information).…”

“…The decrease in activity is mainly attributed to the deposition of CeO x onto carbon rather than Pd and the formation of large CeO x nanoislands, as confirmed by XPS and STEM analyses. , compared to the previously published data ( [34][35][36][37]39] ). The bulk at% of CeO x is calculated with respect to the sum of Pd and CeO x .…”

“…[20,22,56] The decrease in HBE by the addition of CeO 2 to Pd was posteriorly demonstrated by DFT calculations. [37] In another work of the authors, the Pd metal loading was optimized in the Pd/C-CeO 2 catalysts, showing the best HOR activity for the case of 10 wt% Pd. [35] More recently, Pd-CeO 2 catalysts have been prepared by using a spray flame-based process.…”

“…[32,33] In order to increase the HOR kinetics of Pd, previous studies focused their efforts in the development of Pd-CeO 2 composites, as the most promising Pt-free catalyst for HOR. [34][35][36][37][38][39] The use of ceria (CeO 2 ) is justified because i) it is an oxygen-deficient compound with a fast OH − saturation [34,40] and ii) it has a high oxophilic character. [41] The metal-metal oxide interaction on Pd-CeO 2 systems has been thoroughly investigated for several chemical reactions, such as CO oxidation, CH 4 oxidation, and NO reduction.…”

Bifunctional Pd‐CeO2 Nanorods/C Nanocatalyst with High Electrochemical Stability and Catalytic Activity for the ORR and EOR in Alkaline Media

“…[14] Moreover, some Pd nanoparticles may tend to deposit on CeO 2-NR , promoting an enhanced dispersion of nanoparticles. [15] The chemical composition of the Pd/C and Pd-CeO 2-NR /C is shown in Table 1. In both cases, the C content is fairly close to the theoretically expected value of 80 wt.…”