High Efficiency Anodes for Hydrogen Separation Based on Solid Acid Electrolytes

Abstract

Abundant, inexpensive, high purity molecular hydrogen as a medium for energy distribution is potentially enabling for adoption of alternative electricity generation schemes. Steam reformation of natural gas remains the most economical way to produce large amounts of hydrogen. In contrast with methane gas, derivatives may exhibit advantages in volumetric energy density, ease of transport and versatility. However, any methane or methane derivative steam reformation process also creates by-products, most notably, CO and CO2 (syngas). Syngas can contain 10-60% 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 contaminants1. Separation to produce 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 fuels such as reformed NG or methanol. CDP undergoes a solid-solid phase transition at 228 °C and an associated increase in proton conductivity of more than three orders of magnitude (8.5 x 10-5 S cm-1 at 223 °C to 2.5 x 10-2 S cm-1 at 250 °C). These systems have been shown to tolerate reformate streams in H2-air cells containing CO, H2S, NH3, CH3OH, C3H8 and CH4 of 20%, 100ppm, 100 ppm, 5%, 3% and 5% respectively, retaining 90-95% of pure H2 performance2. Solid acid fuel cells operated using methanol and an integrated steam reformer have also shown similar results3. In the majority of these earlier studies, platinum was used as the catalyst for both the hydrogen reduction and oxidation reactions. 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, but mostly direct CO electrooxidation. In this work, functionally graded anodes are fabricated to balance CO conversion activity with hydrogen oxidation. Although Ru has shown potential as a hydrogen oxidation catalyst, Pt is still superior. A dual-phase Pt and Ru electrode has been fabricated in order to increase the energy efficiency over a single-metal anode. Various carbon supports have be used in order to increase the triple phase boundary leading to drastically reduced metal loadings. These re-engineered anodes are implemented in conjunction with Ni-based cathodes to demonstrate efficient hydrogen separation using ultra low loadings of Pt from syngas-like inputs. We gratefully acknowledge the support of this work by the TN-SCORE (EPS-1004083) and the ARPA-E REBELS project. References 1 National Energy Technology Laboratory. Wabash River Coal Gasification Repowering Project: A DOE Assessment. United States: N. p., 2002. Web. doi:10.2172/790376. 2C. R. I. Chisholm et al., Electrochem. Soc. Interface, 18, 53–59 (2009). 3 T. Uda, D. A. Boysen, C. R. I. Chisholm and S. M. Haile, Electrochem. Solid-State Lett., 2006, 9(6), A261–A264. 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. Figure 1