determined as 3. The results presented here indicate that liquid
ammonia is a promising energy medium for hydrogen storage
and generation. However, the experimental electrolysis voltage
of more than 1 V is still high for practical use. To decrease
these over potential of ammonia electrolysis, the effective
electrode should be developed with the investigation of electrode
reaction mechanism in the future.
This work has been partially supported by New Energy and
Industrial Technology Development Organization (NEDO)
under ‘‘Advanced Fundamental Research Project on Hydrogen
Storage Materials’’. The authors gratefully acknowledge
Mr. Takeharu Hamade in Sophia University for designing
the electrochemical cell.
Fig. 3 Chronoamperometry profile for liquid ammonia with 1 M
KNH2 at 2.0 V of cell voltage for 10 h with cell pressure monitoring
line. Inset of the figure is GC profile for the partial gas in the
electrochemical cell after the chronoamperometry measurement.
Notes and references
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observed as shown in the inset of Fig. 3. The average mol ratio
of H2 to N2 is estimated to be 2.960 ꢂ 0.004 by using the area
of GC peaks (Table S1, ESIz). This ratio corresponds to the
stoichiometry of ammonia decomposition according to reaction
(4). Moreover, the amount of released hydrogen is evaluated
to 4.62 ꢁ 10ꢀ3 mol by using the cell pressure increase and gas
ratio obtained by GC measurement. Since an ideal amount of
released hydrogen calculated from the value of experimental
total electric charge is 5.46 ꢁ 10ꢀ3 mol, the current efficiency
of the electrolysis is estimated to be 85% (the details with
original data in ESIz). The remaining current efficiency could
be explained by the reverse reaction of reaction (4) for ammonia
generation. Because hydrogen and nitrogen gases are certainly
soluble in liquid ammonia,9 generated hydrogen and nitrogen
molecules partially remain as adsorbed states on the electrode
and the reverse reaction would proceed.
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In conclusion, the electrolysis of liquid ammonia has been
confirmed by adding the metal amides as the supporting
electrolytes. The voltage efficiency according to the current
density depends on the solubility and concentration of the
metal amides in liq. NH3. By the potentiostatic measurement
for liq. NH3 with 1 M KNH2, the high current efficiency of
85% is obtained at 2 V of cell voltage with the release of
hydrogen and nitrogen gases. And, the ratio of H2/N2 is
c
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 7775–7777 7777