A Direct Urine Fuel Cell Operated at Intermediate Temperatures

Abstract

Direct utilization of urine in fuel cells is a promising technology for conversion into electricity. Here, we report the design of anode materials for high-temperature direct urine fuel cells and the performance of a fuel cell with an optimized anode at 300°C. The resultant peak power densities reached 16.7 mW cm ¹2 for urine and 26.5 mW cm ¹2 for urea.Human urine can be regarded as an alternative energy source to fossil fuels because it is an abundant waste product that includes urea, creatinine, uric acid, and ammonia as hydrogen carrier compounds. In particular, urea contains a gravimetric hydrogen content of 6.7 wt %, 1 which constitutes a high energy density when used as a fuel. In addition, this substance is nonflammable and nontoxic in contrast to other hydrogen carrier chemicals. On the other hand, a large amount of digestion gas is produced by anaerobic digestion in sewerage disposal plants; however, ca. 30% of the mass of digestion gas is incinerated without being used.In this study, we have developed a direct urine fuel cell using the unused digestion gas as a heat source. Operation of a fuel cell at elevated temperatures makes the anode catalyst more active to reductants in the urine, which allows for the low polarization resistance of the anode. Furthermore, there is the possibility that such reductants are internally reformed to more reactive compounds over the anode, which further reduces the polarization resistance. Two types of fuel cells have been proposed for direct urine utilization in fuel cells: alkaline membrane 2,3 and microbial fuel cells. 4 However, they are both operated at low temperatures with respective power densities of 4 mW cm ¹2 (alkaline membrane fuel cell) and 5 mW m ¹2(microbial fuel cell), which are much lower than those of hydrogen-powered fuel cells. As the production cost of human urine is substantially close to zero, high power density may not necessarily be required for this type of fuel cell. Nevertheless, further increase in the power density would enhance the position of direct urine fuel cells as the preferred energy conversion devices for practical applications.Hydroxide ion conductors and cathodes were important substances in this study. The hydroxide ion conductor used was Sn 0.92 Sb 0.08 P 2 O 7 because this material possesses a hydroxide ion conductivity of ca. 0.05 S cm ¹1 in the temperature range above 0.01 S cm ¹1 between 100 and 300°C. 5 Sn 0.92 Sb 0.08 P 2 O 7 was prepared in a similar manner to that reported previously. 6 A total of 0.04 g of poly(tetrafluoroethylene) (PTFE) powder was added to 1.00 g of Sn 0.92 Sb 0.08 P 2 O 7 powder, followed by kneading with a mortar and pestle, and then cold-rolling to a thickness of 150¯m using a laboratory rolling mill. The cathode used was Pt/nitrogen-doped graphene, which exhibited higher activity for the oxygen reduction reaction and better thermal stability, compared to conventional Pt/C cathodes. This electrode powder was synthesized according to a previously reported procedure.7 Pt/C, Ru/C, Ni/C, Pd/C, and Rh/...

Urea‐Based Fuel Cells and Electrocatalysts for Urea Oxidation

“…Nagao et al. developed a DUFC with Sn 0.920 Sb 0.08 P 2 O 7 ‐PTFE composite electrolyte as hydroxide ion conductors, Pt/N‐doped graphene as cathode, and Ru/C as anode . Argon passed through a urea solution or urine bubbler was heated up to 300 °C and supplied to the anode.…”

“…Nagao et al developedaDUFCw ith Sn 0.920 Sb 0.08 P 2 O 7 -PTFE composite electrolyte as hydroxide ion conductors,P t/Ndoped graphene as cathode,a nd Ru/C as anode. [16] Argon passed through au rea solution or urine bubbler was heated up to 300 8Ca nd supplied to the anode.T he maximum power density increased froml ess than 1mWcm À2 at 100 8Ct o 16.7 mW cm À2 at 300 8Cu sing human urine.T he peak power density for 20 wt %u rea solution at 300 8Cr eached 26.5 mW cm À2 .…”

Metal–organic framework–derived Ni@C and NiO@C as anode catalysts for urea fuel cells

“…Under optimum conditions studied (0.33 M urea/1.0 M KOH at 50 °C), the MPD of the DUFC with Ni@C anode equaled 13.8 mW cm −2 with an open circuit of 0.93 V, which was higher than those reported previously (Suppl. Table S2) [4][5][6]11,15,19,29,[43][44][45][46][47] . The excellent performance of the Ni@C anode was mainly attributed to its high specific surface area and mesoporous structure, which provided a high number of active Ni-catalyst sites and allowed fast mass transfer of urea and products in the anode.…”

Urea removal from aqueous solutions—a review

“…The latter cell having twin Pt/C electrodes or Ni/MWCNTs-Pt/C electrodes exhibited maximum power densities of 0.03 mW cm -2 (at OCV = 0.4 V) and 0.05 mW cm -2 (at OCV = 0.25 V), respectively. Nagao et al [109] investigated a wide variety of materials (Pt/C, Ru/C, Ni/C, Pd/C, and Rh/C) in DUFCs at 300°C. The ruthenium catalyst was identified as the most active Table 3 Comparison of properties of Ni-based electro-catalysts extracted from cyclic voltammograms in alkaline media [ 84] among the tested materials, with a maximum power density of 26.5 mW cm -2 .…”