however, in many cases, the conversion of CO 2 involves inefficient CO 2
ÁÀ formation. On the other hand, homogeneous catalysts, such as ruthenium-, rhodium-, and
iridium-based complexes, often form metal hydrides as intermediates [11, 12]. This
circumvents the CO 2
ÁÀ formation, and hence, more efficient electrochemical conversions are expected. For example, [Rh(bpy 2 (TFMS) 2 )]
+ (bpy, 2,2
0 -bipyridine;
TFMS, trifluoromethanesulfonate anion) exhibited 80% Faradaic efficiency
(FE) for HCO 2
À production in CH 3 CN at À0.96 V. Moreover, electroreduction of
CO 2 in aqueous solution was achieved with high FE at a very low overpotential
using iridium catalysts. This chapter, therefore, describes the development of
iridium-based homogeneous catalysts for the electroreduction of CO 2 to formate
(HCO 2
À ), carbon monoxide (CO), and oxalate (C 2 O 4
2À
).
2 Electroreduction of Carbon Dioxide to Formate
2.1 Homogeneous Catalysts
HCO 2
À (HCO 2 H), a two-electron reduction product of CO 2 , has recently attracted
significant attention as a liquid organic hydrogen (H 2 ) carrier, in which H 2 is
established as a new energy vector [13, 14]. Through HCO 2
À (HCO 2 H) dehydrogenation (Eqs. 1 and 2), H 2 can be released with a lower energy consumption than those
required for other H 2 storage media, such as methylcyclohexane or ammonia.
Furthermore, in contrast to other chemical H 2 carriers, HCO 2 H can produce highpressure H 2 using only thermal reactions [15]. The potential for the CO 2 to HCO 2
À
transformation is À0.49 V, which indicates a slightly higher energy consumption than
H 2 generation (À0.41 V). If the electroreduction of CO 2 to HCO 2
À can be realized with
a high FE (Eq. 3, where X is a product such as CO, HCO 2
À , C 2 O 4
2À or H 2 , F is the
Faraday constant, and n is the number of reaction electrons) and a low overpotential, it
becomes an energy storage method comparable to water electrolysis. Additionally,
C-H bond formation is one of the most essential elementary reactions in synthetic
chemistry. The conversion of CO 2 to HCO 2
À (HCO 2 H) involves C-H bond formation,
which can be considered as an electrochemical method of C-H bond formation.
HCO 2
À
þ H 2 O ! H 2 þ CO 2 þ OH
À
ð1Þ
HCO 2 H ! H 2 þ CO 2
ð2Þ
FE X
ð Þ %
½ ¼ F C mol
À1
Â
à  n X mol
½
=total Q C
½ Â 100
ð3Þ
In 1996, [Ir 2 (dimen) 4 ]
2+ (dimen, 1,8-diisocyano-n-menthane) was studied using
infrared spectro-electrochemistry, whereby HCO 2
À and bicarbonate were detected,
although the products were not quantified [16]. In addition, the obtained results
indicated that [Ir 2 (dimen) 4 ]
2+ accepted 2 electrons to form [Ir 2 (dimen) 4 ]
0 , which then
reacted with CO 2 and H 2 O. In addition, the electroreduction of CO 2 to HCO 2
À was
reported using [Cp*Ir(bpy)Cl]
+ (Cp*, pentamethylcyclopentadienyl) in CH 3 CN
Electroreduction of Carbon Dioxide by Homogeneous Iridium Catalysts
327
ÁÀ formation. On the other hand, homogeneous catalysts, such as ruthenium-, rhodium-, and
iridium-based complexes, often form metal hydrides as intermediates [11, 12]. This
circumvents the CO 2
ÁÀ formation, and hence, more efficient electrochemical conversions are expected. For example, [Rh(bpy 2 (TFMS) 2 )]
+ (bpy, 2,2
0 -bipyridine;
TFMS, trifluoromethanesulfonate anion) exhibited 80% Faradaic efficiency
(FE) for HCO 2
À production in CH 3 CN at À0.96 V. Moreover, electroreduction of
CO 2 in aqueous solution was achieved with high FE at a very low overpotential
using iridium catalysts. This chapter, therefore, describes the development of
iridium-based homogeneous catalysts for the electroreduction of CO 2 to formate
(HCO 2
À ), carbon monoxide (CO), and oxalate (C 2 O 4
2À
).
2 Electroreduction of Carbon Dioxide to Formate
2.1 Homogeneous Catalysts
HCO 2
À (HCO 2 H), a two-electron reduction product of CO 2 , has recently attracted
significant attention as a liquid organic hydrogen (H 2 ) carrier, in which H 2 is
established as a new energy vector [13, 14]. Through HCO 2
À (HCO 2 H) dehydrogenation (Eqs. 1 and 2), H 2 can be released with a lower energy consumption than those
required for other H 2 storage media, such as methylcyclohexane or ammonia.
Furthermore, in contrast to other chemical H 2 carriers, HCO 2 H can produce highpressure H 2 using only thermal reactions [15]. The potential for the CO 2 to HCO 2
À
transformation is À0.49 V, which indicates a slightly higher energy consumption than
H 2 generation (À0.41 V). If the electroreduction of CO 2 to HCO 2
À can be realized with
a high FE (Eq. 3, where X is a product such as CO, HCO 2
À , C 2 O 4
2À or H 2 , F is the
Faraday constant, and n is the number of reaction electrons) and a low overpotential, it
becomes an energy storage method comparable to water electrolysis. Additionally,
C-H bond formation is one of the most essential elementary reactions in synthetic
chemistry. The conversion of CO 2 to HCO 2
À (HCO 2 H) involves C-H bond formation,
which can be considered as an electrochemical method of C-H bond formation.
HCO 2
À
þ H 2 O ! H 2 þ CO 2 þ OH
À
ð1Þ
HCO 2 H ! H 2 þ CO 2
ð2Þ
FE X
ð Þ %
½ ¼ F C mol
À1
Â
à  n X mol
½
=total Q C
½ Â 100
ð3Þ
In 1996, [Ir 2 (dimen) 4 ]
2+ (dimen, 1,8-diisocyano-n-menthane) was studied using
infrared spectro-electrochemistry, whereby HCO 2
À and bicarbonate were detected,
although the products were not quantified [16]. In addition, the obtained results
indicated that [Ir 2 (dimen) 4 ]
2+ accepted 2 electrons to form [Ir 2 (dimen) 4 ]
0 , which then
reacted with CO 2 and H 2 O. In addition, the electroreduction of CO 2 to HCO 2
À was
reported using [Cp*Ir(bpy)Cl]
+ (Cp*, pentamethylcyclopentadienyl) in CH 3 CN
Electroreduction of Carbon Dioxide by Homogeneous Iridium Catalysts
327