Short communicationElectrochemical oxidation of hydrazine derivatives by carbon-supported metalloporphyrins
Graphical abstract
Highlights
► We investigated electro-oxidation of hydrazine derivatives by metalloporphyrins. ► The hydrazine derivatives are difficult to be oxidized by conventional catalysts. ► A Co porphyrin exhibited the strongest activity toward carbohydrazide. ► Co porphyrins have higher activity toward carbohydrazide than hydrazine.
Introduction
Hydrazine hydrate is a strong reducing agent that has high energy density and high reactivity with appropriate catalysts. Direct hydrazine fuel cells have been studied as potential future polymer electrolyte fuel cells that may deliver high power density [1], [2], [3], [4], [5], [6].
An anion exchange membrane (AEM) is generally used as an electrolyte for hydrazine fuel cells. In an AEM-FC, the environment is not acidic. Hence, the problem of corrosion is overcome, and base metals can act as electrocatalysts. Actually, hydrazine fuel cells that use an AEM and non-noble metal catalysts have recently been reported to have good performance [6]. To date, a wide variety of anode catalysts that use a base metal for the oxidation of hydrazine have been developed. Co, Ni and Co–Ni alloy have been studied as novel anode catalysts [6], [7], [8], [9].
On the other hand, not only hydrazine hydrate but also hydrazine derivatives (Chart 1) such as carbohydrazide (carbodihydrazide or 1,3-diaminourea) and methyl carbazate have been studied as fuels to overcome the toxicity of hydrazine. Unfortunately, on metal catalysts, the rate of the oxidation of these derivatives is much lower than that of hydrazine [9]. If we wish to use hydrazine derivatives as fuels, we must first develop a catalyst that can oxidize them at a high rate.
Against this background, we focused on metallomacrocycle complexes. We have already studied Rh porphyrins adsorbed on a carbon black as anode catalysts [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. These electrocatalysts can oxidize CO [10], [11], [12], [13], [20], BH4− [14], [15], [16], oxalic acid [17], [18], and glucose [19] at low overpotentials. A membrane electrode assembly (MEA) that contains Rh porphyrins generates electricity (44 mW cm−2) when CO is supplied to the anode [10]. This result indicates that porphyrins adsorbed on carbon deliver high power in polymer electrolyte fuel cells, and suggests that such metalloporphyrin-based electrocatalysts may work in an AEM-FC using hydrazine (and its derivatives).
To date, the electro-oxidation of hydrazine by Co macrocycles [21], [22], [24], [26], [27], [28], [29], [30] and Fe-macrocycles [23], [25] has been studied. This reaction was investigated in terms of its analytical application and mechanistic viewpoints. However, little is known about the application of this reaction for catalysts. Furthermore, the oxidation of hydrazine derivatives by metallocomplexes has not yet been examined. Since metallomacrocycles have electronic properties that are completely different than those of bulk metal catalysts, these catalysts may have activity in the oxidation of hydrazine derivatives, which can hardly be oxidized by bulk metal catalysts. We try to adsorb metallomacrocycles on a carbon black to use them as catalysts in fuel cells. Because fuel-cell applications require high current, metalloporphyrins should be adsorbed on a conductive support with a large surface area, such as a carbon black.
In this study, we report that Ru-, Rh-, Co-, and Fe-porphyrins adsorbed on carbon black acted as electrocatalysts that oxidize hydrazine and its derivatives. A Co-porphyrin catalyst gave a high current density for the electro-oxidation of carbohydrazide as well as hydrazine, in contrast to conventional electrocatalysts. The current per 1 g of metal is extremely high. The dependence of the activity on the ligand structure was examined. The current density of carbohydrazide oxidation with the best catalyst reached 40 mA cm−2.
Section snippets
Materials
Carbohydrazide and methyl carbazate were kindly donated by Otsuka Chemicals. [CoII(OEP)] (OEP = 2,3,7,8,12,13,17,18-octaethylporphinate), [FeIII(OEP)(Cl)], [RuII(OEP)(CO)], [PdII(OEP)], [CuII(OEP)], [MnIII(OEP)(Cl)], [PtII(OEP)], [CoII(TPP)] (TPP = 5, 10, 15, 20-tetraphenylporphinate), and [CoII(T(OCH3)PP)] (T(OCH3)PP = 5, 10, 15, 20-tetrakis(4-methoxyphenyl)porphinate) were purchased from Aldrich. [CoIII(TPPS)(Cl)] (TPPS = 5, 10, 15, 20-tetrakis(4-sulfonatophenyl)porphinate) and [CoIII(TCPP)(Cl)] (TCPP
Electrocatalytic oxidation of hydrazine by metalloporphyrins on carbon black
Fig. 1 shows cyclic voltammograms of hydrazine (0.95 M) by several carbon-supported metalloporphyrins. Among the porphyrins tested, only Co-OEP/C, Fe-OEP/C, Rh-OEP/C, and Ru-OEP/C exhibited clear hydrazine-oxidation activity. Other carbon-supported porphyrins ([PdII(OEP)], [CuII(OEP)], [MnIII(OEP)(Cl)], and [PtII(OEP)]) exhibited only slight activity (see the inset in Fig. 1). The activity was similar to that with a carbon black (Vulcan XC-72R) support (line i). Hence, these metalloporphyrins
Conclusions
We prepared hydrazine electro-oxidation catalysts using Co-, Fe-, Ru-, and Rh-porphyrins. We also demonstrated that carbon-supported Co porphyrins catalyzed the electro-oxidation of carbohydrazide at a high rate. The rate of the oxidation of carbohydrazide is comparable to (or higher than) that of hydrazine on Co porphyrins, while the oxidation of carbohydrazide is much weaker than that of hydrazine with bulk metal catalysts. The concentration-dependence of this reaction indicates that
Acknowledgments
The authors are grateful to Tokuyama Corporation for providing an ionomer (AS-4). The authors thank Otsuka Chemical Co. Ltd., for providing hydrazine derivatives.
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