Electrochemical Reduction of Carbon Dioxide on Lanthanum Based Transition Metal Oxide Electrocatalysts

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 (T_M) 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 T_M 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 T_M 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 T_M oxides before and after usage, as electrocatalysts, were checked using X-ray diffraction (XRD; EMPYREAN PANalytical diffractometer) having Cu K_α radiation and scanning at a rate of 1 ^o/min between 10 to 90^o. 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 T_M 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 T_M 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-T_M-oxide electrocatalysts studied here.

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

Figure 1