3D-Graphene supports for palladium nanoparticles: Effect of micro/macropores on oxygen electroreduction in Anion Exchange Membrane Fuel Cells

Highlights

• Morphologically modified 3D-Graphenes fabricated using sacrificial silica templates.

• Oxygen electroreduction kinetics depended on pore size distribution.

• Absence of templated pores in 2D-Graphenes lead to diffusion limitations.

• Higher degree of micropores increased peroxide generation at higher potentials.

• Macropores improved power densities & reduced concentration polarization losses.

Abstract

Hierarchically structured 3D-Graphene nanosheets as supports for palladium nanoparticles (Pd/3D-GNS) were fabricated using the Sacrificial Support Method. The pore size distribution of the 3D-GNS supports were tuned by utilizing smaller and larger sized sacrificial silica templates, EH5 and L90. Using a combination of Scanning Electron Microscopy (SEM), N₂ sorption and Rotating Ring Disc Electrode (RRDE) technique, it was demonstrated that the EH5 and L90 modified 3D-GNS supports had higher percentage of micro- (<2 nm) and macropores (>50 nm), respectively. The templated pores also played a role in enhancing the oxygen reduction reaction (ORR) as well as membrane electrode assembly (MEA) performance of the Pd nanoparticles in comparison to non-porous 2D-GNS supports. Particularly, incorporation of micropores increased peroxide generation at higher potentials whereas presence of macropores increased both limiting current densities and reduce peroxide yields. Integration of the Pd/GNS nanocomposites into a H_2/O₂ fed Anion Exchange Membrane Fuel Cell (AEMFC) operating at 60$°$C also demonstrated the effect of modified porosity on concentration polarization or transport losses at high current densities. This strategy for the tunable synthesis of hierarchically 3D porous graphitized supports offers a platform for developing morphologically modified nanomaterials for energy conversion.

Introduction

Fuel cells are considered to be one of the most promising sustainable energy conversion technologies for various transportation and industrial applications. Currently, state-of the-art catalysts based on platinum and its alloys such at PtCo are utilized in fuel cell vehicles operating with Proton Exchange Membrane Fuel Cells (PEMFCs) due to its high electrocatalytic activity [1], [2], [3]. However, the wide scale implementation of PEMFCs is still hindered by several limitations such as the utilization of costly platinum (or platinum-based) electrocatalysts at the electrodes, and instability of the Pt-based cathode materials under harsh acidic conditions [4]. However, with recent progress made in developing high performance anion exchange membranes (AEMs) [5], [6], [7], Anion Exchange Membrane Fuel Cells (AEMFCs) have recently emerged as an alternative technology and are now also being considered to be one of the most promising alternate sources of energy [8].

AEMFCs operate by electrochemically reducing O₂ at the cathode to produce OH−, which gets transferred to the anode compartment through the anion exchange membrane where it reacts with H₂ to produce water [9], [10]. The kinetics of these cathodic oxygen reduction reaction (ORR) is what largely determines the overall efficiency of the fuel cell. With studies demonstrating the high instability of platinum-based catalysts in alkaline media [11], it has become imperative to design non-platinum based electrocatalysts that are not only durable, but also show efficient catalytic activity in AEMFCs. Various cathode catalysts such as Ru [12], Ag [13], and Co [14] have been used instead of Pt for studying ORR in alkaline electrolytes. There has also been an increased research activity in ORR electrocatalysis on Pd-based catalysts during the last decade [15], [16], [17], and the performance of Pd-based catalysts in fuel cell catalysis has been reviewed by many [18], [19], [20]. While the differences in cost-effectiveness between Pt and Pd are not that significant, studies have demonstrated the superior electrochemical performance of Pd-based catalysts towards the electroreduction of oxygen [21] in both acidic and alkaline media [22], [23], [24], [25]. Moreover, Pd nanoparticles were also shown to be significantly more durable than Pt under constant load cycling in alkaline media, making Pd-based electrocatalysts promising cathode materials for AEMFCs in comparison to Pt [26].

In order to facilitate the better utilization of dispersed Pd nanoparticles for electrochemical reactions, Pd nanoparticles are generally supported on porous carbonaceous materials such as Vulcan XC 72R or Ketjen Black [27]. One of the most attractive features of utilizing porous materials is that they enable the nanoparticles to interact with the reactants not only at the surfaces but also through their porous frameworks. Among carbonaceous materials, porous materials based on graphene [28] – such as graphene nanosheets [29], graphene oxide, nitrogen doped carbon nanotubes [30] - have attracted the most interest due to several obvious advantages over other porous carbon materials due to their extraordinary physical and chemical properties such as high surface areas (up to 2630 m² g⁻¹ arising from its 2D morphology estimated theoretically), mechanical and chemical stability as well as excellent electrical conductivity [30], [31], [32], [33], [34].

However, most graphene-like supports are typically synthesized using methods such as (i) chemical vapor deposition (CVD) [35], microwave assisted [36] or electrochemical exfoliation techniques [37]. Although these techniques produce high quality graphene sheets – they also yield materials in low or un-scalable quantities with two dimensional morphologies. On the other hand, highly graphitic materials usually have a low surface areas mainly attributable to the strong aggregation tendency of graphene or restacking of graphene sheets, which not only hinder the dispersion of nanoparticles deposited on them, but also its mass transport capabilities [38], [39]. Hence, the task of rationally designing graphitized materials with tunable pore structures and controlled morphologies has become critical from the point of view in preparation of materials with high level of graphitization, high surface area and controllable morphology.

Although there has been multiple studies that have investigated the intrinsic properties of graphene itself, there is still little understanding of the influence of hierarchical porosity of 3D graphene materials on catalytic performances, and only recently exploited in the field of catalysis [40], [41], [42]. The controlled synthesis of 3D graphene nanostructures remains a challenging task. Furthermore, the incorporation of Pd-based electrocatalysts in H₂/O₂ fed Anion Exchange Membrane Fuel Cells has been quite limited to date. The Pd-based catalysts that were utilized in the electrodes required relatively high loadings and alloying with bifunctional metals such as Ni or other modifications [43], [44], [45]. Additionally, the best of our knowledge, there is almost no reported literature on integrating porous graphene-like supports into a membrane electrode assembly for fuel cell testing, as most of the studies on have focused on optimizing the performance of AEMFCs using Pt and Pt alloys in one or both electrodes [46], [47], [48]. As a result, in spite of the extensive amount of literature that have investigated the electrochemical performances of Pd/Graphene nanocomposites [21], [22], [29], the knowledge related to understanding the influence of the graphitic supports morphology on ORR performances, and the structure-to-property correlation between the porosity of the three dimensional graphene supports and ORR kinetics in alkaline media still requires further investigating. Accordingly, it has become increasingly challenging but also necessary to rationally design non-Pt based graphene supported electrocatalysts with porous structures for fuel cell applications.

In this paper, we demonstrate a highly scalable and cost effective synthetic procedure for fabricating spatially arranged graphene nanosheets (GNS) with a three-dimensional morphology (3D) with controlled yet varying levels of tunable micro-, and macro-porosities by templating, acid etching and thermal pyrolysis. In particular, this study focused on tailoring the morphology of the 3D Graphene nanosheets (3D-GNS) by using the Sacrificial Support Method (SSM) [49], [50], [51], where amorphous silica particles were utilized as sacrificial templates for etching size-controlled pores into the graphene matrix, hence giving it the desired 3D morphology [52], [53], [54], [55].The volume of the incorporated micro/macro-pores was tailored by adjusting the size of the hard template. The 3D-GNS materials were then utilized as supports for Pd nanoparticles and comprehensively characterized for their physicochemical properties and electrochemical performance towards ORR using various surface analysis and potentiometric techniques. It was observed that the ORR performance of the electrocatalysts strongly depended on the pore structures under the same operating conditions, and that the porosity of the 3D-Graphene nanosheets definitely played an important role in its electrocatalytic performance for electroreduction of oxygen in terms of limiting current and hydrogen peroxide yields in alkaline media, as well as mass transfer kinetics in the membrane electrode assembly. Our study provides a rational design strategy for manipulating the morphology of graphitized supports to have an interconnected 3D structure with pore channels to counter the stacking of the sheets, which facilitates ORR kinetics and enhances AEMFC performance.