Modifying Electrode Architectures for Solid Acid Electrochemical Hydrogen Separation Devices

Abundant, inexpensive, high purity molecular hydrogen as a medium for energy distribution is potentially enabling for adoption of alternative electricity generation schemes.  Steam reforming of natural gas remains the dominant method of producing large amounts of hydrogen. However, this process also creates by-products, most notably, CO and CO₂.  Separation to ultra-high purity hydrogen from these syngas reformate streams by traditional methods, such as pressure swing absorption, has its disadvantages including long cycle times, contamination and a large equipment footprint.  Alternative methods of hydrogen separation, such as electrochemical pumping, are a viable alternative to this separation dilemma due to their relative simplicity and potential efficiency.

The solid-state proton conductor cesium dihydrogen phosphate (CsH₂PO₄ or CDP) has shown potential in electrochemical devices operating on reformed hydrocarbons. Solid acid fuel cells operated using methanol and an integrated steam reformer have shown promising results¹.  We have also shown that solid acid electrochemical devices are effective for hydrogen separation from reformate streams containing CO.  Published work has demonstrated devices based on CDP operating in fuel streams containing up to 20% CO².

Syngas can contain 30-60% carbon monoxide (CO), 25-30% hydrogen (H₂), 0-5% methane (CH₄), 5-15% carbon dioxide (CO₂), plus a lesser or greater amount of water vapor, smaller amounts of the sulfur compounds hydrogen sulfide (H₂S), carbonyl sulfide (COS), and finally some ammonia and other trace contaminants³.  At these CO concentrations, further oxidative conversion of CO is desirable to boost hydrogen yield.  Recently, we demonstrated that Ru⁴ and Ni⁵ are viable alternatives to platinum electrocatalysts for hydrogen oxidation and evolution, respectively.  In particular, Ru is capable of enhanced hydrogen production in CO-rich input streams. We attributed this result to a synergistic interaction of the water-gas shift (WGS) reaction and CO electrooxidation.

In this work, we consider separately the heterogeneous WGS and CO electrooxidation properties of Ru and other supported catalysts for optimizing electrode architectures for hydrogen separation from highly CO-enriched simulated syngas streams.  Functionally graded anodes are fabricated to balance CO conversion activity with hydrogen oxidation.  These re-engineered anodes are implemented in conjunction with Ni-based cathodes to demonstrate efficient Pt-free hydrogen separation from syngas-like inputs. 

This work is supported by the National Science Foundation through TN-SCORE (EPS-1004083).

References

¹ T. Uda, D. A. Boysen, C. R. I. Chisholm and S. M. Haile, Electrochem. Solid-State Lett.,

2006, 9(6), A261–A264.

²C. R. I. Chisholm et al., Electrochem. Soc. Interface, 18, 53–59 (2009).

³National Energy Technology Laboratory. Wabash River Coal Gasification Repowering Project: A DOE Assessment. United States: N. p., 2002. Web. doi:10.2172/790376.

⁴A. B. Papandrew, R. W. Atkinson III, R. R. Unocic, and T. A. Zawodzinski, Jr., J. Mater. Chem. A, 3, 3984 (2015).

⁵ A. B. Papandrew, T.A. Zawodzinski Jr., J. Power Sources 245 (2014) 171.