Electrocatalysis Research for Fuel Cells and Hydrogen Production

Available online at www.sciencedirect.com

Energy Procedia 29 (2012) 401 – 408

World Hydrogen Energy Conference 2012

Electrocatalysis research for fuel cells and hydrogen production Mkhulu K Mathe *, Tumaini Mkwizu and Mmalewane Modibedi HySA Infrastructure Center of Competence, Materials Science and Manufacturing, Council for Scientific and Industrial Research (CSIR), PO Box 395; Pretoria 0001; South Africa

Abstract

The CSIR undertakes research in the Electrocatalysis of fuel cells and for hydrogen production. The Hydrogen South Africa (HySA) strategy supports research on electrocatalysts due to their importance to the national beneficiation strategy. The work reported here presents choice methods for the production of Platinum Group Metals (PGM) electrocatalysts, which are characterized for their performance. Investigations on the commercial feasibility of such electrocatalysts in the fuel cells including hydrogen production continue to be subject of global interest, to ensure energy security of supply. The paper aims to present possible synthesis routes for PGM electrocatalysts for commercial gains. © Selection and/or peer-review under responsibility of Canadian Hydrogen and © 2012 2012Published PublishedbybyElsevier ElsevierLtd. Ltd. Fuel Cell Association Selection and/or peer-review under responsibility of Canadian Hydrogen and Fuel Cell Association Keywords: Electrocatalysts; Fuel Cells; Hydrogen production; PGM; Electrochemical Atomic Layer Deposition; Energy Security

1. Introduction Fuel cells (FCs) are an attractive option for energy conversion as they offer high efficiency with little or no pollution. Research activities worldwide are focussed on improving the performance of fuel cell catalysts, reducing the amount of Pt required in FCs and in developing catalysts that are tolerant to poisoning. These activities are aimed at lowering the cost, increasing the efficiency, and improving the durability of fuel cells hence accelerating the commercialisation of fuel cells. Pt-based catalysts have proved to be the best electrocatalysts in FC [1-3]. In catalysis, it is well known that the surface reactions are controlled by the atomic level details of the catalytic surface and hence the catalyst preparation

* Mkhulu K Mathe. Tel.: +27 12 841 3665; fax: +27 12 841 2135 E-mail address:[email protected]

1876-6102 © 2012 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of Canadian Hydrogen and Fuel Cell Association. doi:10.1016/j.egypro.2012.09.047

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method is important in tuning the morphology and composition of the catalyst. Various methods such as conventional physical and chemical, electrodeposition as well as sputtering methods are used in the preparation of catalysts. The electrochemical deposition method is well known for the fabrication of nanostructured catalysts for energy materials. Platinum cost was until recently higher than gold. The fuel cell commercialization challenges include the high cost of platinum. A report [4] stated that: ‘With current platinum loadings of about 0.65 mg/cm2 and a current platinum price of $1500 per troy-ounce (about $48/g) (the first 3-month average in 2010), a 50kW fuel cell with a power density of 700 mW/cm2 has approximately 46 g of platinum, costing $2240.’ Further analysis on the platinum cost implication on the fuel cell vehicles (FCVs) was positive indicating a projected cost reduction due to a likely reduction in Pt price coupled with decreased platinum loadings due to set technical goals of 0.1 – 0.9 mg/cm2. The US DoE 2009 report [5] on Hydrogen and Fuel Cell Activities, Progress and Plans to Congress, identified hydrogen as a key component for powering vehicles. Subsequent plans on Hydrogen Production & Delivery had development of low cost hydrogen production methods as one of their objectives. On the Fuel Cells development, the aim was cost reduction with improved performance durability. Research efforts classified into Basic Science Research and Manufacturing R&D respectively had focused on nanostructured catalysts for fuel cells and manufacturing cost reduction in PEM fuel cell system including hydrogen production. The Electrocatalysis work was motivated by the accelerated developments in the field known as, “the hydrogen economy”. This economy amongst others is described as: “The hydrogen economy is a vision of where the world can move to through use of hydrogen as an energy carrier in place of oil and fossil fuels. Here, hydrogen would be used to heat homes and power vehicles (whether with fuel cells or internal combustion engines). Eventually, this hydrogen would be generated from renewable energy, leading to no emissions even of carbon dioxide”. [6] Nomenclature Anode Negative electrode Cathode Positive electrode CL

Catalyst layer

EAS

Electrochemical active surface

FF

Flow field

GDL

Gas diffusion layer

MEA

Membrane electrode assembly

PEM

Proton exchange membrane

PGM

Platinum Group Metals

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1.1. Electrocatalysis and hydrogen economy Figure 1 presents a view on the supply and demand of hydrogen [7]. The electrocatalysis work reported here is for mostly renewables hydrogen production. The hydrogen demand landscape presented below is in transport, buildings and industry applications. The electrocatalysis work reported here is for mostly renewables hydrogen production.

Figure 1: Hydrogen: primary energy sources, energy converters and applications parts of fuel cells

1.2. Comparison of energy densities of storage methods Hydrogen is best understood as a powerful alternative to your main stream fuels due to its comparatively high energy density. As presented in table 1 below [8]: Table 1: Comparison of energy densities of storage methods Storage Method

Energy density/kWh kg-1

Hydrogen

38

Gasoline

14

Lead acid battery

0.04

Flywheel, fused silica

0.09

(Methanol)

(6)

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1.3. An outline diagram of a typical fuel cell [9]

Figure 2: Cross-section of a fuel cell

The prepared electrocatalysts will be part of the CL (catalyst layer) segment above. Other parts of the fuel cell will not be part of this paper, with exceptions of statements on how they affect the percentage activity of the electrocatalyst. Motivation for the developments of the electrocatalysts includes: load reduction, improved engineering with respect to good coherent structural strength in addition to better activity at a higher efficiency. Fuel cell parts that are affected and influence the electrocatalysts are: anode, cathode and electrolyte bonded together into a Membrane Electrode Assembly (MEA) known as the heart of the fuel cell. The PGM electrocatalysts synthesized for fuel cells should facilitate reaction of oxygen and hydrogen. Properties of the electrocatalysts are discussed further with the synthesis methods below. 1.4. Synthesis options Part of the cost reduction with manufacturing of electrocatalysts will be simplification of methods through improvements on process efficiency. Conventional methods used to date such as impregnation have been excellently reviewed [10] with the ‘in situ electrocatalyst methods’. The main disadvantage of the conventional methods multiple steps thus affecting the quality of the MEA to be prepared. The ‘in situ methods’ are reportedly advantageous in that they can prepare electrocatalysts onto the MEA. The Chemical synthesis and Sequential electrodeposition are choice methods reported previously [11] in our research. Several studies were and continue to investigate the competitive advantages of other synthesis methods. Some of the electrocatalysts prepared using the Pechini method are Pd/C and Pd-Ni/C. Doping of such systems was reported to improve the catalytic activity with ternary systems. Small portable power applications are candidates for catalytic hydrogen production. The developed electrocatalyst will find use with fuel processors. Efforts towards this area are in early stages. [12,13]. A two steps sonochemical synthesis of Pd electrocalysts [Haitao] for oxygen electro-reduction reaction (ORR) suggested this method is a plausible future synthesis route for fuel cell electrocatalysts. The character of sequential electrodeposition is established in other fields of applications such as nanostructured semiconductors [14], solar cells [15] and magnetic materials [16]. The flexibility inherent in electrodeposition has made it a preferred method for work reported here. The US Geological Survey report on PGM World Supply and Demand [17] reported on the importance of ‘Catalysis on the nanoscale’ for mobile and stationary applications. Electrocatalysts synthesized at the nanoscale are predicted to provide efficient power to vehicles and improve hydrogen production efficiency for distributed power generation. Some of the efforts reported investigating alternatives to

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PGM based electrocatalysts were on the use of nickel powder cathode catalysts in hydrogen production. [10] Tuning electrocatalysis: Sequential Electrodeposition Recent work in our labs continues the development of an automated electrodeposition system described in Langmuir [11] and presented in figures 3 and 4 below. This method is an ‘in situ’ method which has been investigated in industry for its commercial potential. Of interest for this paper is the morphological characterisation presented below. The relevance of morphology to commercialization of electrocatalysts will be described in the paper.

Figure 3: Schematic diagram of the instrumental setup developed for automated electrodeposition and electrochemical characterization using a flow-cell.

Bare substrate Special chamber (Electrodeposition Cell)

Applications (e.g. Fuel Cells)

Precursor solutions

Catalyst substrate decorated with nanoparticles

Characterisation

Figure 4: Schematic presentation of setup for sequential electrodeposition

The formation of any electrocatalyst is achieved following the above sequence of Figure 4 using the equipment in Figure 3. The substrate is critical for the successful synthesis because it presents a start for the possible building of an MEA. The precursor solutions are at very low concentrations in the millimolar regime. This is an advantage of electrodeposition against vacuum technologies in addition to being a room temperature process. Using the above scheme, several [11] systems were investigated with interesting results. Morphological studies as presented below clearly show the differences in particles sizes including their surface distribution, both not conclusive of the electrocatalyst activity.

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Figure 5: SEM micrographs of nanoparticles obtained after eight sequential deposition cycles and corresponding EDX spectra (insets) of (a) Pt, (b) Ru|Pt, and (c) codeposited Pt-Ru, all obtained with SLRR cycles involving Cu.

Figure 6: Schematic representation of the preparation routes of Pd- Ru - Sn/C nanocatalyst Modibedi [18] described the synthesis of supported nanostructured Pd based electrocatalysts as presented in Figure 6 above. The Electrochemical active surface area (EAS) comparative table presented below indicated the better performing catalyst as: Pd-Ru-Sn/C. Further work comparing EAS values of different electrocatalysts synthesised using different methods would in future be used to establish a generally superior method.

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Figure 7: Current-potential curve for H2-air fuel cell at 80qC

Activation polarization, Ohmic polarization and Mass-transport polarization are electrochemical areas of interest in understanding the performance of any electrocatalyst in a system. The interpretation of polarization results could suggest possible mechanisms. The conversion efficiency as computed was ~ 65% (> 2 times that of heat engine)

1.5. Conclusion The synthesis of Pd and Pt based nanostructured electrocatalysts were synthesized successfully and fully characterized. The use of these materials in a commercial demonstration unit would be the next steps. It can thus be concluded for any method to be considered successful; it must be evaluated with regards to the overall system performance. Any electrocatalysts that match the ideal properties of cost, performance and reliability results will make it a preferred one.

Acknowledgements The authors would like to acknowledge useful discussions including continued support from the following direct and indirect collaborators: CSIR Executive and Energy Materials, HySA – Infrastructure Director Dr Dmitri Bessarabov, Dr Hongze Luo, Dr Haitao Zheng, Prof. Ignacy Cukrowski, University of Pretoria, South Africa, Prof. John Stickney, University of Georgia, USA and National Centre for NanoStructured Materials, MSM, CSIR, South Africa

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