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 Electrocatalytic Reforming of Biomass


The demand for hydrogen is expected to dramatically increase through:

  • Use to reduce sulfur in diesel through hydrotreating,
  • Development of a hydrogen economy and hydrogen fuel cells, and
  • Upgrading biomass derived biofuels.
Biomass derived compounds have the potential to be a valuable resource for the production of hydrogen, though the complexity of biomass makes finding a feasible process difficult.

One potentially viable route is electrocatalytic reforming of biomass to produce H2 at the cathode and CO2 at the anode.  Electrocatalytic hydrogen production can be an effective means to produce hydrogen suitable for upgrading biomass to the H/C levels of liquid fuels (2.2). After pretreatment, biomass is composed largely of five and six carbon sugars.  Here, we examine the electrocatalytic reforming nature of ethylene glycol at a Pt electrode.  Ethylene glycol is the simplest sugar-like molecule, with OH groups on each carbon atom, and thus is a good starting prototype for studies of electrocatalytic biomass reforming.

Electrocatalytic reforming, illustrated in Fig. 1, consists of oxidation of the sugar-related molecule at the anode, which produces hydrogen ions that migrate through the proton exchange membrane (PEM) to the cathode.  Hydrogen is evolved at the cathode by electroreduction.


Anode: C2H6O2 + 2 H2O  —>  2 CO2 + 10 H+ + 10 e

Cathode: 2 H+ + 2 e —> H2

This process makes pure hydrogen at the cathode.  This hydrogen has a number of uses:

  • As a product itself,
  • Upgrading biomass refining to produce liquid fuels,
  • Reaction in a fuel cell to produce electrical power for the electrocatalytic reformer (ECR), and
  • Excess electrical power. 
Figure 1 shows an ECR/fuel cell arrangement in a biorefinery, in which some of the hydrogen produced by the reformer is reacted in the fuel cell to provide electrical power for the reformer.

The objective of this work is to develop proof-of-concept of electrocatalytic reforming as a viable pathway to hydrogen.  The main challenges are to develop a proton exchange membrane suitable for reactions up to 200°C.  Another aspect is to determine the extent of partial oxidation products, which would reduce the hydrogen yield.  At present this is unknown as electrooxidation of oxygenated hydrocarbons has not been studied at the temperatures of this work.

Selected Results

Cyclic voltammograms as a function of temperature for oxidation of ethylene glycol on a Pt/C catalyst are shown in Fig. 3.  Nafion was used as the PEM for these experiments.  At the lowest temperature of 333 K, the onset of oxidation occurs at 0.38 V, and the main oxidation peak occurs at 0.55 V.  At the highest temperature of 373 K, both the onset and main oxidation peak shift to lower potetentials ending up at 0.3 and 0.48 V, respectively.  This shows that oxidation becomes more facile at higher temperatures, as needed for a technically feasible electrocatalytic reformer.

Sustained current occurs for all cases in Fig. 3.  This means that oxidation is continuous and not a transient processes.  A technically feasible ECR requires sustained current at oxidation potentials of 0.3 V or less in order for the energy needed by the reformer to be less than the energy gained from the product hydrogen.  Work to measure oxidation currents at higher temperatures continues.

Acknowledgement

This work is supported by the National Science Foundation.

Last revised: December 11, 2012





ECR Biomass

Figure 1.  Illustration of the use of fuel cell technology for electrocatalytic reforming of carbohydrates derived from biomass to hydrogen.

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ECR Fuel Cell Network


Figure 2.  Combined electrocatalytic reformer (ECR) and fuel cell for hydrogen production.

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Ethylene Glycol CVs

Figure 3. Positive direction sweep of CVs on a supported Pt/C electrode with Nafion 117 electrolyte at different temperatures: 333, 353, 363, and 373 K. Reaction conditions: CV potential scan rate 25 mv/s; 0.02 M ethylene glycol, 15 psig for the 373 K experimental run.

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