Electrochemical Reduction of Carbon Dioxide on Lanthanum Based Transition Metal Oxide Electrocatalysts
Authors: Barman Madhurima , Zhang Jie , Sarkar Arindam 
Published: February 2020

Among the various methods available for reduction of CO2, the electrochemical reduction method stands out due to it being capable of producing industrially important fuels, organic chemicals and pure oxygen. Such process takes place in an electrochemical cell, where carbon dioxide is reduced to at room temperature and atmospheric pressure, using only water, carbon dioxide and electricity at specific electrochemical potentials. Here, electrocatalysts have an important role to play, for enhancing the kinetics of the process and also for rendering the formation of desired products feasible at some selected potentials. In the present research, we are exploring the usage of La-based transition metal (TM) oxides, crystallizing in perovskite structure, as electrocatalysts. The synthesis of these metal oxides is cost effective and tuning the performance via doping is facile. Furthermore, these perovskites are stable in stoichiometric, as well as non-stoichiometric, forms and due to the basicity of La exhibits affinity for CO2. Accordingly, here we have investigated the efficiency and effectiveness of different La-based TM oxides, as electrocatalysts, towards the formation of industrially relevant products upon reduction of CO2. It may be mentioned here that, in order to enhance the efficiency of the electrocatalysts, as well as the mass transport of CO2 to the surface of the electrocatalysts, we have designed a new electrolysis cell where gas diffusion electrode (GDE) containing the electrocatalysts is used as working electrode.
All the La-based TM oxides (i.e., LaCoO3, LaCrO3, LaFeO3, LaMnO3, LaNiO3) were synthesized via citrate solution-combustion route and calcined in air atmosphere at 850 oC for 6 h, with the heating rate being 10 oC/minute. Phase evolution of the TM oxides before and after usage, as electrocatalysts, were checked using X-ray diffraction (XRD; EMPYREAN PANalytical diffractometer) having Cu Kα radiation and scanning at a rate of 1 o/min between 10 to 90o. Inorganic Crystal Structure Database was used for the phase analysis of the powder catalysts. The composition of the perovskite catalysts (in terms of the ratios of the metals) were quantitatively analyzed by energy dispersive spectroscopy (EDS; QUANTA 200). Morphologies of the oxide particles were observed using scanning electron microscopy (SEM) JEOL JSM-7600F. The specific surface area of the perovskite catalysts were measured via BET using Micromeritics 3 FLEX Surface Characterization.
The electrochemical CO2 reduction reaction was performed in an electrolysis cell via chronoamperometry using Biologic potentiostat (Model VSP 300). The reduction reactions were carried out on gas diffusion electrode (GDE) using basic aqueous electrolyte. The applied potentials were -1.1 V, -1.3 V, -1.5 V, -1.7 V, -1.9 V and -2.1 V vs. Hg/HgO, as reference electrode. The chronoamperometry experiments were performed for 3 h at each potential. The current densities obtained during the electrochemical CO2 reduction experiments were obtained for all the La-based TM oxide electrocatalysts used, viz., LaCoO3, LaCrO3, LaFeO3, LaMnO3, LaNiO3. The liquid products present in the electrolyte after the experiments were analyzed by high performance liquid chromatography (HPLC) using Agilent 1260 infinity series model with Aminex HPX-87H column. The gaseous products were analyzed by gas chromatography - thermal conductivity detector (GC-TCD) using Nucon 5700 gas chromatography.
XRD scans recorded with the as-synthesized and calcined La-based TM oxides indicate that the oxides were phase pure (as shown in Fig. 1). The crystal structure was established as perovskite. The composition of the as-synthesized and calcined perovskite electrocatalysts, as determined using EDS, were in good agreement with the target compositions. The current densities were fairly stable during the 3 h long experiments for all the potentials studied; with an example being shown in Fig. 2. All the electrocatalysts produced formate (HCOO-), acetate (CH3COO-) and hydrogen as the products of electrochemical CO2 reduction. Furthermore, ethanol (C2H5OH) and 1-propanol (C3H7OH) were also formed on LaCoO3, with LaMnO3 being able to produce 1-propanol (C3H7OH) as one of the products. Hence, as per the present set of results in the context of the formation of desired products, LaCoO3 and LaMnO3 seem to be more promising among the La-TM-oxide electrocatalysts studied here.

Keywords: CO2 reduction, transition metal oxide, electrocatalyst, chronoamperometry, reaction product.

Figure 1