Topics in Current Chemistry (2018) 376:41
1 3
1 Introduction
Over the past half century, atmospheric CO 2 concentration has increased dramatically. The current CO 2 concentration is the highest ever recorded since consistent measurements began in the 1950s. The high level of CO 2 concentration
has resulted in global climate change and severe environmental damage [1–3].
Recently, much effort has been devoted to identifying potential methods to tackle
CO 2 emission issues. A potential solution is to use CO 2 as the carbon source to
produce value-added chemicals; however, reduction of CO 2 requires additional
energy input to overcome activation barriers. If the required energy is obtained
from fossil sources, the process does not necessarily have a negative (or even carbon neutral) CO 2 emission. Therefore, developing an effective catalyst that can
lower activation barriers of CO 2 reduction, as well as utilize renewable energy is
an important task in the field of carbon utilization.
In nature, green plants capture CO 2 directly from air and convert it to reduced
carbon species through sunlight-driven photosynthesis processes. However,
nature’s photosynthesis processes are usually slow and they alone are not sufficient to offset the extremely large quantity of CO 2 emitted from human activities [4]. A variety of alternative CO 2 utilization approaches, including biological, thermochemical, photochemical, and electrochemical methods, are currently
under investigation [5, 6]. Among them, electrochemical conversion of CO 2 has
attracted much attention recently because this approach has several advantages,
including fine control of production rates, wide scalability of modular electrolyzer designs, and the potential to produce a variety of high-value products [5, 7].
More importantly, the electrolysis system can be readily powered by carbon-free
energy sources, such as wind, solar, and nuclear, providing a zero-CO 2 emission
(or even negative) pathway for commodity chemical production. A recent scientific study on photovoltaics (PV) clearly showed a decrease of PV electricity price
over time with a projected PV electricity price as low as $0.03 per kWh in the
near future [8]. A similar trend also holds for wind energy with the wind electricity price already at ~ $0.02 per kWh [9]. The low electricity price makes electrochemically driven CO 2 utilization technologies potentially profitable for commercial applications.
In a typical CO 2 electrolysis cell, there are three key components, a cathode
for CO 2 reduction reaction, an anode for water oxidation reaction, and a membrane separator for physical separation of two electrode chambers. Because CO 2
reduction is a thermodynamically uphill reaction, external bias, often with a high
overpotential, is required to drive the reaction [10, 11]. Additionally, hydrogen
evolution reaction (the competing reaction) can occur under the CO 2 electrolysis
conditions, which may substantially decrease selectivity and Faradaic efficiency
(FE) of CO 2 RR. Therefore, an efficient CO 2 reduction electrocatalyst is essential
to achieve highly selective and energy efficient CO 2 electrolysis systems. In the
past few decades, tremendous effort has been devoted to the investigation of CO 2
electrocatalysts. Systematic investigations of monometallic catalysts have been
conducted [12–16]. Among all the monometallic CO 2 electrocatalysts, Cu is the
Reprinted from the journal
106
1 3
1 Introduction
Over the past half century, atmospheric CO 2 concentration has increased dramatically. The current CO 2 concentration is the highest ever recorded since consistent measurements began in the 1950s. The high level of CO 2 concentration
has resulted in global climate change and severe environmental damage [1–3].
Recently, much effort has been devoted to identifying potential methods to tackle
CO 2 emission issues. A potential solution is to use CO 2 as the carbon source to
produce value-added chemicals; however, reduction of CO 2 requires additional
energy input to overcome activation barriers. If the required energy is obtained
from fossil sources, the process does not necessarily have a negative (or even carbon neutral) CO 2 emission. Therefore, developing an effective catalyst that can
lower activation barriers of CO 2 reduction, as well as utilize renewable energy is
an important task in the field of carbon utilization.
In nature, green plants capture CO 2 directly from air and convert it to reduced
carbon species through sunlight-driven photosynthesis processes. However,
nature’s photosynthesis processes are usually slow and they alone are not sufficient to offset the extremely large quantity of CO 2 emitted from human activities [4]. A variety of alternative CO 2 utilization approaches, including biological, thermochemical, photochemical, and electrochemical methods, are currently
under investigation [5, 6]. Among them, electrochemical conversion of CO 2 has
attracted much attention recently because this approach has several advantages,
including fine control of production rates, wide scalability of modular electrolyzer designs, and the potential to produce a variety of high-value products [5, 7].
More importantly, the electrolysis system can be readily powered by carbon-free
energy sources, such as wind, solar, and nuclear, providing a zero-CO 2 emission
(or even negative) pathway for commodity chemical production. A recent scientific study on photovoltaics (PV) clearly showed a decrease of PV electricity price
over time with a projected PV electricity price as low as $0.03 per kWh in the
near future [8]. A similar trend also holds for wind energy with the wind electricity price already at ~ $0.02 per kWh [9]. The low electricity price makes electrochemically driven CO 2 utilization technologies potentially profitable for commercial applications.
In a typical CO 2 electrolysis cell, there are three key components, a cathode
for CO 2 reduction reaction, an anode for water oxidation reaction, and a membrane separator for physical separation of two electrode chambers. Because CO 2
reduction is a thermodynamically uphill reaction, external bias, often with a high
overpotential, is required to drive the reaction [10, 11]. Additionally, hydrogen
evolution reaction (the competing reaction) can occur under the CO 2 electrolysis
conditions, which may substantially decrease selectivity and Faradaic efficiency
(FE) of CO 2 RR. Therefore, an efficient CO 2 reduction electrocatalyst is essential
to achieve highly selective and energy efficient CO 2 electrolysis systems. In the
past few decades, tremendous effort has been devoted to the investigation of CO 2
electrocatalysts. Systematic investigations of monometallic catalysts have been
conducted [12–16]. Among all the monometallic CO 2 electrocatalysts, Cu is the
Reprinted from the journal
106