Summary The application of nonprecious metal catalysts, such as iron (Fe) and cobalt (Co) catalyst, to direct liquid fuel cells (DLFCs), especially in direct methanol fuel cells, has been widely investigated. However, the application of such non‐Pt catalysts as cathode catalysts in direct formic acid fuel cell (DFAFC) operations has not yet been investigated. This study intends to evaluate the formic acid tolerance of such catalysts in case of oxygen reduction reaction. In addition, we investigate their performances in DFAFC using the Fe‐ and Co‐nitrogen‐doped carbon nanotubes (Fe‐NCNT and Co‐NCNT) as the cathode catalysts and compare these performances with the commercial Pt/C catalyst. Herein, Fe‐NCNT and Co‐NCNT were synthesized using the conventional method by the pyrolysis of the multiwalled carbon nanotubes, dicyandiamide, and metal salt under the flow of N2 at 800°C. Both the Fe‐NCNT and Co‐NCNT catalysts exhibit higher formic acid tolerance when compared with that exhibited by the Pt/C catalyst. Further, single‐cell tests with hydrogen‐fed polymer electrolyte fuel cell (PEFC) and DFAFC operations were conducted under various operating conditions to compare the performances of the cells while using the prepared catalysts and the conventional Pt/C catalyst. The PEFC performances in both the Fe‐NCNT and Co‐NCNT catalysts were significantly low (94.9mW cm−2 for Fe‐NCNT and 164.0 mW cm−2 for Co‐NCNT at 60°C). Regardless, the Co‐NCNT catalyst exhibited a maximum power density of 160.7 mW cm−2 in DFAFC operated at 60°C and7‐M formic acid. This value is comparable with that for DFAFC with a Pt/C catalyst (128.9mW cm−2) and is considerably higher than that obtained for other DLFCs while using a non‐Pt catalyst. Therefore, the usage of a non‐Pt metal catalyst as the cathode catalyst is preferable in case of DFAFC.
This study reports on direct formate fuel cell (DFFC) operation using non-precious metal (NPM) as the cathode catalyst. Iron-and cobalt-nitrogen-doped carbon nanotube (Fe-NCNT and Co-NCNT) were synthesized by pyrolysis of multiwalled carbon nanotubes, metal precursor and nitrogen precursor. These NPM catalysts showed high fuel tolerance in acidic medium, but the fuel tolerance in alkaline medium remains unclear. Herein, we determine the formate tolerance on the NPM catalysts and commercial Pt/C catalyst in alkaline medium. The DFFC performance test was conducted and the results compared with the direct formic acid fuel cell (DFAFC) reported in our previous work. The oxygen reduction reaction (ORR) activity and the formate tolerance on the NPM catalysts were evaluated by rotating disk electrode (RDE) in alkaline medium. Both NPM catalysts showed lower ORR activity than the Pt/C catalyst, but they exhibited higher formate tolerance than the Pt/C catalyst. Comparing the single-cell performance under various HCOONa concentrations, DFFC with Co-NCNT catalyst showed higher maximum power density than with Pt/C catalyst with 2 M KOH containing 4 M and 6 M HCOONa due to its higher formate tolerance. Therefore, it is considered that the NPM cathode is available for high concentration operation of DFFC. However, DFFC with Fe-NCNT and Co-NCNT cathode catalyst exhibited the highest maximum power density of 39.7 mW cm 2 and 89.8 mW cm 2 , respectively, at 60°C with optimal fuel concentration, i.e. 2 M KOH containing 4 M HCOONa. These performances were lower than that of DFFC with Pt/C cathode catalyst at optimal fuel concentration, i.e. 2 M KOH containing 2M HCOONa. Considering the fact that the power density of DFAFC (acidic condition) using NPM catalyst was higher than that with Pt/C catalyst at optimum fuel concentration, the NPM catalysts are more preferable for the acidic condition than the alkaline condition.
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