Investigation of mesoporous carbon hollow spheres as catalyst support in DMFC cathode

Highlights

• Post-thermal treatment at 850 °C enhanced Pt particle stability.

• Higher catalyst accessibility for HGS-supported Pt than for Vulcan-supported ones.

• Feasibility of MT-DMFC was demonstrated up to 110 °C with Nafion 117.

• Decrease in ohmic resistance by using HGS support.

Abstract

In conventional low temperature PEM fuel cells, catalyst activity and stability rely on adequate structure and morphology of carbon support material, especially at the cathode that is exposed to high potential values. Degree of graphitization as well as pore distribution are decisive parameters for corrosion resistivity, catalyst utilization and mass transport. In this work, hollow graphitized carbon spheres (HGS) with mesoporous structure were tested as catalyst support in middle temperature (90–110 °C) direct methanol fuel cell (DMFC) cathode. Influence of catalyst loading and concentration as well as working temperature on power density was studied in a 5 cm² laboratory cell. Best results in terms of MEA performance were achieved with a catalyst loading of 2 mg_Pt cm⁻² at both electrodes and an extremely dense Pt concentration on carbon support of 50 and 40 wt% at the anode and cathode, respectively at 110 °C. Additionally, thermal pretreatment of carbon-supported catalyst up to 850 °C led to increase in particle size up to 7 nm and as a consequence to substantial higher ECSA retention during accelerated degradation test (ADT) under half-cell conditions.

Introduction

Finding sustainable solutions to face increasing energy demand, reducing our dependence on fossil resources and limiting environmental impacts such as global warming of industrial activities are some urgent challenges to overcome [1], [2]. Hydrogen-based fuel cells are attractive decentralised alternative power sources for automotive [3], portable and stationary applications especially as heat and electricity plant for domestic applications [4], [5] since it is capable of supplying high power densities with high efficiency under rapid load fluctuation. Fuel cells directly convert chemical energy of fossil-based fuels such as H₂ [6]_, CH₃COH [7] and CH₄ [8] into electrical energy.

In case of the low temperature PEMFC, the so-called membrane-electrodes assembly (MEA) is composed of a proton-conducting polymer membrane (e.g. Nafion) and two gas diffusion electrodes (GDE). Depending on the coating strategy and more precisely on desired layer thickness, active material can be applied either directly on the membrane or on the gas diffusion layer (GDL) to form a catalyst coated membrane (CCM) or a catalyst coated electrode (CCE), respectively. In order to improve gas distribution and mass transport of the reaction educts and products, especially at the cathode where water flooding is a concern, a microporous layer (MPL) consisting of carbon black is commonly deposited on macroporous GDL [9]. The latter is generally made of woven or non-woven carbon fibre matrix and is known as carbon cloth or carbon paper, respectively with typically 75–85% void volume.

GDEs are subjected to several partially competitively technical challenges and have permanently to be optimized in terms of fuel and reaction products distribution, water management, electronic conductivity as current collector, heat transfer and mechanical strength compound for polymer electrolyte structure in combination with flow field as well as corrosion resistance [10]. Recently, See et al. [11] investigated effect of the GDL material on thermal behavior along reactant flow channels in PEMFCs by using confocal laser scanning microscopy. They found that particularly in-plane thermal conductivity and morphological structure play an important role with respect to temperature gradient and water distribution in flow channels. Recent developments on single- and dual-layered GDL on reactant diffusion and water management in PEM fuel are reviewed in Park et al. [12].

The reaction layer is commonly made of a mixture of Pt-based nanoparticle dispersed on high surface carbon black support and a binder material such as Nafion/PTFE. One essential prerequisite for high cell performance is related to formation of the so-called triple-phase-boundary that should allow perfect juxtaposition of liquid, gas and solid phases and therefore good accessibility of gas molecules to the active catalyst sites [1], [13]. Numerous works have focused on development of more performing catalyst materials like alloying/dealloying Pt-based catalysts [14], bimetallic core/shell nanoparticles [15] and nanostructured thin-film catalysts for oxygen reduction reaction (ORR). However, up to now, most of them still show challenging instability issues due to coalescence, ripening and dissolution of the active catalyst as well as support corrosion that shorten their practical lifetime [16].

One attractive strategy to get higher cell performance consists on elevating cell temperature. That results in higher reaction and mass transfer rates, and usually in lower cell resistance arising from higher ionic conductivity of the polymer electrolyte. The latter strongly depends on proton mobility within polymer pathway [17]. Another profitable advantage of higher temperature level is related to enhancement of the electro-catalyst CO tolerance in cells running on reformate fuel [18]. An increase in operating pressure generally leads to higher partial pressure of reactant gases, solubility, and mass transfer rates and consequently to higher cell efficiency as noted by Angrist et al. [19].

Mass transport resistance is depending on several inherent parameters such as electrode porosity, capillarity effect, hydrophobicity level, compression force of end plates, and geometry of low field channels and is more pronounced at high current density values where high diffusion rate of reactants and products to/from reaction sites are required to avoid electrode flooding. Bogolowski et al. [20] demonstrated that combination of both high current densities with low methanol concentration can led to methanol yield efficiencies higher than 85%. Ideally, methanol concentration at the membrane/anode interface should tend to zero.

In this context, properties of the catalyst support are of great importance. It is meanwhile well-known, that adequate mesoporous structure with pore size in the range of catalyst particle favors catalyst stability and durability. Combination of ideal pore size distribution in the range of 2 nm and high electrical conductivity of mesoporous CMK-3 support was found to enhance Pt activity for electrochemical methanol oxidation [21]. One distinguishes ordered (OMC) from disordered mesoporous (DOMC) carbon ones, which can be prepared either by using e.g. silica templates e.g. SBA-15 [22] or triblock copolymer template structures [23], [24]. It should be noted that hereby choice of the synthesis route (hard or soft template) may also influence electrochemical cell performance [25]. Nowadays these materials are widely studied for their application in Li-ion batteries and fuel cells as catalyst support materials [26], [27], [28], [29]. Oxygen-functionalized mesoporous carbon surface can improve interaction between metal catalyst and carbon support by promoting uniform dispersion. It is stated that surface chemistry plays an important role in porous materials and predefined monodispersed pore morphology enables easier mass transportation which is much more prominent in case of liquid reaction [30]. It was also observed that ohmic and mass transfer polarization losses were more significant for highly functionalised support samples. This was attributed to decreased electrical conductivity, higher agglomeration and lower specific surface area [31], [32].

Influence of the pore volume and ionomer content in reaction layer on cell performance was addressed in [32]. The influence of mesoporous carbons as support for bi-metallic catalyst systems was investigated in [31], [33]. Because of their high surface area, large pore volume and narrow pore size distribution, more homogeneous dispersion and better accessibility of bi-metallic catalyst was obtained that favoured higher catalyst utilization during the electrochemical reaction in fuel cells and may allow a reduction of both loading and cost. Alloys of Pt with other non-precious metals such as Fe, Co, and FeCo have also been studied on mesoporous carbon support [34]. Therein, polarization tests with cyclic voltammetry revealed that kinetic current density of the bi-metal catalyst on mesoporous support at 0.9 V amounted 1.80 mA cm⁻² and surpassed by 80% that measured at the Pt/Vulcan catalyst (1.00 mA cm⁻²).

Activity of nitrogen-doped mesoporous carbon for ORR was evaluated in [35], [36]. Hereby a very narrow pore-size distribution of ca. 3.8 nm was obtained. By using a aqueous hydrothermal route at 70 °C, mesoporous carbon support with small nitrogen content (<1%) provided a large surface area and a graphitic framework, leading to high electrocatalytic activity, excellent long term stability, and high resistance to methanol crossover effects for the ORR compared to commercial Pt/Vulcan as stated in [37].

The main aim of this work focuses on the decoration of hollow spherical carbon with nano-dispered Pt and their test as cathode material in DMFC.