Reports: DNI555009-DNI5: Identification of Electrocatalysts and Reaction Mechanisms for Electrochemical Conversion of Methane to Methanol Using Density Functional Theory
printer friendlyNatural gas flaring causes enormous damage to the environment, in addition to the wasted energy. The
importance of this problem is highlighted by an initiative by the United Nations to end flaring by 2030. There exists an immediate need for identifying routes for converting flare into usable energy. In this study, we propose a scheme that at the core utilizes an electrochemical cell to convert methane into
methanol, an easily transportable fuel. The electrochemical cell uses electricity provided by solar photovoltaics to power the electrochemical cell. We carry out a detailed techno-economic analysis of the entire system and analyze the merits and demerits of the proposed approach as compared with other flare gas recovery systems, gas-to-liquid (GTL), electricity generation with gas turbine, gas compression system, and electricity generation by solid oxide fuel cell (SOFC). The developed model shows that the current state-of-the-art materials available for different system components, proton conductor, and electrocatalysts are inadequate to make the scheme practical. We outline the minimum performance metrics, i.e., input voltage at the cell level of ∼0.5 V that corresponds to an overpotential of ∼1 V and a current density of 0.5 A/cm2 that requires a proton conductor that can conduct from 10−1 to 10−2 S/cm in the temperature range of 100−250 °C, required for the system to become financially competitive. Of note, improvements in the conductivity of proton conductors at intermediate temperatures and identification of active and selective electrocatalysts for the conversion of methane to methanol are the key parameters that determine the overall viability of the proposed scheme. We discuss the environmental impacts of the proposed scheme and provide an outlook on directions required in materials research that could meet the outlined performance metrics.
The proposed technical concept attempts to marry the benefits of the GTL system that produces the transportable liquid fuel and a SOFC, which requires low capital investment. The fundamental challenges in developing a solid oxide fuel cell that is capable of converting methane into methanol are 2-fold: (i) high temperature operation using methane as a fuel typically results in complete oxidation to CO2, while at (ii) room temperatures, it is not possible to activate methane on catalyst surfaces. Hence, intermediate temperature fuel cells (ITFC) have been touted to be attractive candidates for enabling partial oxidation of methane to methanol. The electrochemical cycle used here borrows the core scheme from the recent experimental work of Hibino and co-workers. A schematic of the cell concept is shown in TOC figure. The anode is fed a stream of methane and steam. At the cathode, oxygen from air is reduced. The protons generated at the anode are conducted using a proton-conducting membrane to the cathode. The overall cell reaction is the oxidation of methane to methanol. A benefit of this scheme is that it uses readily available water vapor as the active oxygen source. The best-identified system thus far is based on V2O5/SnO2 as the catalyst/support for the anode and carbon-supported platinum for the cathode with Sn0.9In0.1P2O7 as the proton conductor at a temperature of about 100− 200 °C. This system was able to achieve a selectivity of 61% with a continuous operation of 6 h at this conversion rate. The system was run at a current density of ∼2 mA/cm2 with an overall single-pass product yield of about 0.3%. It is worth highlighting that the thermodynamics of methane to methanol yielded a power-producing device. However, experiments show that a device only works in power-consuming mode, requiring nearly >1 V of applied potential.
The main aim of this work is to identify the technical targets at the material and device levels required for the proposed scheme to become economically feasible, and the identified targets can be used to guide material discovery. Our analysis identified the following required operating parameters that can satisfy the zero NPV over the system’s lifetime. This shows that if the conversion efficiency is at 60% with methanol’s price at 2.6 $/gallon and there is no enhancement in current density from the assumed base case of 0.1 A/cm2, the input voltage needs to be reduced by ∼0.8 V to obtain a system with zero NPV. On the other hand, if the overall current density can be improved 4-fold, which requires dramatic improvements in conductivity of proton conductors, the input voltage needs to only be reduced by ∼0.3 V. It also is shown that at the same current density cells with higher conversion efficiency or with a higher methanol price could have higher voltage (overpotential) and still be financially competitive. This analysis provides a comprehensive understanding of the interplay between the different factors. Additionally, methanol price plays a significant role in determining NPV, and an increase in methanol price can accelerate adoptation of GTL technology.
The proposed technical concept attempts to marry the benefits of the GTL system that produces the transportable liquid fuel and a SOFC, which requires low capital investment. The fundamental challenges in developing a solid oxide fuel cell that is capable of converting methane into methanol are 2-fold: (i) high temperature operation using methane as a fuel typically results in complete oxidation to CO2, while at (ii) room temperatures, it is not possible to activate methane on catalyst surfaces. Hence, intermediate temperature fuel cells (ITFC) have been touted to be attractive candidates for enabling partial oxidation of methane to methanol. The electrochemical cycle used here borrows the core scheme from the recent experimental work of Hibino and co-workers. A schematic of the cell concept is shown in TOC figure. The anode is fed a stream of methane and steam. At the cathode, oxygen from air is reduced. The protons generated at the anode are conducted using a proton-conducting membrane to the cathode. The overall cell reaction is the oxidation of methane to methanol. A benefit of this scheme is that it uses readily available water vapor as the active oxygen source. The best-identified system thus far is based on V2O5/SnO2 as the catalyst/support for the anode and carbon-supported platinum for the cathode with Sn0.9In0.1P2O7 as the proton conductor at a temperature of about 100− 200 °C. This system was able to achieve a selectivity of 61% with a continuous operation of 6 h at this conversion rate. The system was run at a current density of ∼2 mA/cm2 with an overall single-pass product yield of about 0.3%. It is worth highlighting that the thermodynamics of methane to methanol yielded a power-producing device. However, experiments show that a device only works in power-consuming mode, requiring nearly >1 V of applied potential.
The main aim of this work is to identify the technical targets at the material and device levels required for the proposed scheme to become economically feasible, and the identified targets can be used to guide material discovery. Our analysis identified the following required operating parameters that can satisfy the zero NPV over the system’s lifetime. This shows that if the conversion efficiency is at 60% with methanol’s price at 2.6 $/gallon and there is no enhancement in current density from the assumed base case of 0.1 A/cm2, the input voltage needs to be reduced by ∼0.8 V to obtain a system with zero NPV. On the other hand, if the overall current density can be improved 4-fold, which requires dramatic improvements in conductivity of proton conductors, the input voltage needs to only be reduced by ∼0.3 V. It also is shown that at the same current density cells with higher conversion efficiency or with a higher methanol price could have higher voltage (overpotential) and still be financially competitive. This analysis provides a comprehensive understanding of the interplay between the different factors. Additionally, methanol price plays a significant role in determining NPV, and an increase in methanol price can accelerate adoptation of GTL technology.
