Metal Doped Carbon Nanofiber for Ratio Tunable Syngas Catalyst By CO2 Reduction
Authors: Lee Gi-Baek , Joo Wonhyo , Lee Jae-Chan , Ahn In-Kyoung , Kim Ji-Yong , Joo Young-Chang 
Published: May 2020

Due to the influence of human beings in the modern society, the concentration of CO2 in the atmosphere is rapidly increasing, which causes various environmental problems such as abnormal climate caused by global warming and destruction of ecosystem due to ocean acidification. Therefore, many institutional, scientific, and engineering efforts are being made to reduce the concentration of CO2 in the atmosphere. From the scientific point of view, to make the fuels and chemicals we need from the atmospheric CO2, electrochemical reduction of CO2 is actively being studied. The electrochemical CO2 reduction reaction can be divided into two categories, one for synthesizing multi-carbon products directly such as ethylene or ethanol and the other for producing syngas mixed with CO and H2. The former is mainly researched on Cu based material and has an advantage in that the desired product can be obtained immediately. In the latter case, by adjusting the ratio of CO and H2 in the synthesis gas, it is possible to synthesize multi-carbon products. The latter method can be said to have an advantage in the purity of the gas phase produced (reduction of separation cost). Research is underway in the form of co-catalysts of CO production catalyst like Au, Ag, Zn, Cu & H2 production catalysts. Because simply increasing the current density is not a good thing, but it must be able to maintain an industrially meaningful range of syngas ratios (1 to 3) while producing a high current density. Therefore, the potential window of studies on the rate-controlled syngas studied so far is typically high below -1.2V Vs RHE and high current density of ~ 30 mA/cm2. The reported lifetime at this level of current density is less than 10 hours, and for papers that report longer lifespans of tens to 100 hours, measured at 1 to 2 mA/cm2 levels. We fabricated Ag nanoparticle embedded in Zn dispersed carbon nanofiber to maintain syngas ratio stably in high potential window. In this catalyst carbon nanofiber produces H2 and Ag produces CO and dispersed Zn helps Ag in high potential.
We annealed electrospun PAN/zinc nitrate&silver nitrate nanofiber at 800°C 3h in N2 ambient after stabilization in air. From this, we obtained Ag nanoparticle embedded in Zn & N doped carbon nanofiber. Ag tends to form nanoparticle and non-particle Zn was dispersed in carbon nanofiber. we obtained almost potential insensitive syngas ratio unlike conventional Ag catalyst in syngas production. Only N-doped carbon nanofiber sample produced increasing H2 with potential. Zn & N doped carbon nanofiber produced ratio of syngas higher than 2. Ag nanoparticle embedded carbon nanofiber produced ratio of syngas lower than 1 and sharply increased in higher potential (< -1.6V Vs. RHE). However, Ag nanoparticle embedded in Zn & N-doped carbon nanofiber maintained its low ratio of around 0.5 even in -2.2 V Vs. RHE and almost potential insensitive. We altered this insensitive syngas ratio with the mass ratio between carbon nanofiber and Ag/Zn-carbon nanofiber. It made syngas ratio increases slowly with potential and overall syngas ratio can be shifted with the mass of carbon nanofiber.