Research 2012

Metal Nitrogen Carbon (MNC) oxygen reduction catalysts for Proton Exchange Membrane Fuel Cells (PEMFC)

1. Schematic of Catalyst showing important elements of morphology and function.

One major challenge for fuel cell commercialization involves high cost, low availability and poor durability of state-of-the-art platinum based electrocatalysts. Our work focuses on developing new processes for synthesizing inexpensive Metal-Nitrogen-Carbon (MNC) catalysts for oxygen reduction cathodes.  Through a high-pressure pyrolysis approach, active MNC catalysts can be obtained from transition metals (iron or cobalt) and nitrogen precursors (pyridine, melamine) combined with high surface area carbon materials in a closed, constant volume reactor. High pressure generated due to precursor evaporation increases the nitrogen activity during pyrolysis, the surface nitrogen content of the post-pyrolysis catalysts, and contributes to increased activity towards oxygen reduction [1].

2. Pore Size Distribution of various carbon precursors and the associated catalysts modeled from DFT calculations based on nitrogen adsorption at 77K.

We are currently exploring the catalyst synthesis process to build understanding of the structure and function of the catalyst and to increase activity. For example, the amount of mesopores in the carbon component (Fig. 1) has been found to impact both nitrogen loading and activity [2]. The activity is measured as a kinetically-limited current density using a rotating disk electrode. The porosity of the carbon materials and catalysts is measured by nitrogen physisorption and the data (Fig. 2) is analyzed using nonlinear density functional theory (NLDFT) using density distributions defined specifically for disordered carbon [4].

3. Nitrogen precursors with varying nitrogen:carbon ratios

Similarly the effect of nitrogen precursor composition has been explored in detail, by varying the nitrogen:carbon ratio of the nitrogen precursor (Fig. 3). Activity increased at higher N:C, which we attribute to reduced carbon deposition in the pores of the carbon support during pyrolysis [3]. Current efforts focus on exploring carbon-free, ammonia-generating nitrogen precursors to completely eliminate carbon deposition in the catalysts.

We gratefully acknowledge support from the United States Dept. of Energy Hydrogen and Fuel Cell Program via Northeastern University.

1. R. Kothandaraman, V. Nallathambi, K. Artyushkova, S. C. Barton, Applied Catalysis B: Environmental, Vol. 92, 1-2, 2008, 209-216. link
2. Leonard, N. D., Nallathambi, V., & Calabrese Barton, S. (2011). “Carbon Supports for Non-Precious Metal Proton Exchange Membrane Fuel Cells”, ECS Transactions (pp. 1175-1181). link
3. V. Nallathambi, N. Leonard, R. Kothandaraman, S. C. Barton, Electrochem. Solid-State Lett., 92:1-2,, 209-216 (2008). link
4. E. A. Ustinov, D. D. Do and V. B. Fenelonov. Carbon, 44(4), 653-663 (2006). link

Multiscale Porous Materials for Electrocatalysis

High surface area porous electrodes provide large active sites for heterogenerous reactions, which is essential for electrochemical power generation. We study porous electrodes for fuel cells, including hydrogen fuel cells, biofuel cells, and solid oxide fuel cells.

1. Carbon nanotube coated carbon fiber microelectrode. Insets showing CNT instrinsic porosity and bare carbon fiber under SEM.

For example, we have applied high surface area carbon nanotubes to carbon fiber microelectrode as support for bioelectrode applications [1]. Application of a nanotube coating to a single carbon fiber allowed detailed study via electrochemical techniques and electron microscopy (Fig. 1). Further improvements were achieved by modifying the nanotube matrix with 500 nm diameter polystyrene beads as templates, introducing macro-pores in addition to the ~50 nm natural pores between nanotubes [2]. These structures were characterized using focused ion beam (FIB) microscopy (Fig. 2)

2. Electron micrograph of nanotube layer on a carbon fiber showing macropores introduced using sacrificial polystyrene beads

1. Wen, H., Nallathambi, V., Chakraborty, D., & Calabrese Barton, S. (2011). Carbon fiber microelectrodes modified with carbon nanotubes as a new support for immobilization of glucose oxidase. Microchimica Acta, 283-289. link
2. Hao Wen, H. M. Bambhania, & S. C. Barton “Carbon Nanotube Modified Biocatalytic Microelectrodes with Multiscale Porosity”, accepted by Journal of Applied Electrochemistry, (2011). link

High-rate cofactor regeneration at nanostructured interfaces for bioelectrocatalysis

By transferring electrons between the substrate and the cofactor, dehydrogenase enzymes are able to catalyze oxidation/reduction reactions. For example, glycerol dehydrogenase (TmglyDH) can catalyze the oxidation of glycerol to produce dihydroxyacetone (DHA) involving NADH/NAD⁺ as the cofactor (Fig. 1). However, NADH oxidation occurs at very high overpotentials, which lead to serious energy inefficiencies and increase the possibility of side reactions.[1]

Fig 1. Schematic of bioreactor based on NAD/NADH dependent dehydrogenase

To solve this problem, we have successfully fabricated nanostructured interfaces by electropolymerizing azines on a carbon nanotube (CNT) modified electrode with high-surface area, uniform, controllable properties, and excellent electrocatalytic activity towards NADH oxidation (Fig 2). [2]

Fig 2. Surface characterization of CNT and PMG-CNT. Top: Morphology characterization via scanning electron microscopy (SEM); Bottom: Elemental analysis via Energy-dispersive X-ray spectroscopy

We have extended the incorporation of poly(azine) and CNT to carbon paper support and verified the bioactivity of NAD⁺ electrogenerated by such high-surface area composite anode spectroscopically. A mathematical model calibrated by measurements of NADH oxidation at PMG-CNT-modified glassy carbon electrodes was applied to predict transient NADH consumption. The model shows good agreement with the experimental data. (Fig. 3)

Fig. 3 NADH electrocatalysis on PMG-CNT. Left: Kinetics for concentration study; Right: Conversions in bulk oxidation Bulk

We are collaborating Mark Worden‘s group in Chemical Engineering and Claire Vieille’s group in Microbiology to study the dehydrogenase-NADH/NAD+ binding properties and to explore effective approaches to immobilize cofactors and dehydrogenases, to create multi-step electrodes which can be used as economical bioreactors.

1. S. Calabrese Barton, J. Gallaway, and P. Atanassov, Enzymatic biofuel cells for Implantable and microscale devices. Chemical Reviews, 2004. 104(10): p. 4867-4886. link
2. Li, H., H. Wen, and S. Calabrese Barton, NADH Oxidation Catalyzed by Electropolymerized Azines on Carbon Nanotube Modified Electrodes. Electroanalysis, 2012. 24(2): p. 398-406. link

Synthesis and characterization of redox polymer mediated bioanodes

1. Immobilized redox polymer mediated enzyme system

Biofuel cells hold significant scientific and technological promise for sustainable energy generation by taking advantage of the electrocatalysis of enzymes. In mediated enzyme electrodes (Fig. 1) electrons are transferred between the enzyme and electrode surface via a mediator species.  The rate of electron transfer can be controlled by  by tailoring the potential at which the mediator oxidizes and reduces, compared to that of the chosen enzyme. Our previous work describes the optimization of mediator redox potential for a laccase-catalyzed oxygen reduction electrode [1].

Ligand electrochemical parameter series for Os(III)/Os(II) couple in water

Based on these results, our current research focuses on design of mediators for a glucose oxidase-catalyzed glucose bioanode. We are addressing the impact of mediator structure and electrode composition using a numerical model that predicts electrode activity as a function of composition. This research includes the synthesis and characterization of osmium redox polymers.

To identify suitable redox polymers with pertinent redox potentials for glucose oxidase mediation, lever analysis [2,3] was performed and range of redox mediators are being synthesized.

1. S. Calabrese Barton, J. Gallaway and P. Atanassov, “Enzymatic biofuel cells for Implantable and microscale devices,” Chemical Reviews, 104(10), 4867-4886 (2004). link
2. J. W. Gallaway and S. Calabrese Barton, “Kinetics of redox polymer-mediated enzyme electrodes,” Journal of the American Chemical Society, 130(26), 8527-8536 (2008). link
3. A. B. P. Lever, “Electrochemical Parametrization of Metal-Complex Redox Potentials, Using the Ruthenium(Iii) Ruthenium(Ii) Couple to Generate a Ligand Electrochemical Series,” Inorganic Chemistry, 29(6), 1271-1285 (1990). link

Simulation of multistep enzyme kinetics in biofuel cells

1. Schematic of multistep enzyme catalyzed methanol oxidation and mediated electron transfer

Mathematical modeling of biofuel cell processes (Fig. 1) can be a complex combination of mass transport and chemical-electrochemical reaction kinetics. Mathematical modeling of these systems is challenging as well as rewarding. In our research we have resolved the multistep enzymatic oxidation of methanol and co-factor mediated electron transfer into a series of simple ordinary differential equations. Solving these using COMSOL and MATLAB, we obtain concentration profiles of reactants and products, which can be used to evaluate biofuel cell performance. In our model, we evaluated biofuel performance subjected to variation of pH, enzyme concentration, and electrode area (see references 1 and 2, below). The model-based analysis can be used to optimize electrode designs and the study of methanol oxidation can be extended to a variety of enzymatic biofuel cells and biosensors.

1. C. Hettige, S. Minteer and S. Calabrese Barton, “Simulation of Multi-Step Enzyme Electrodes,” ECS Transactions, 13(21), 99-109 (2008). link
2. P. Kar, H. Wen, H. Li, S. D. Minteer and S. C. Barton, “Simulation of Multistep Enzyme-Catalyzed Methanol Oxidation in Biofuel Cells,” Journal of the Electrochemical Society, 158(5), B580-B586 (2011). link