No abstract

The low formation overpotential and strong adsorption ability of CO on palladium surface sites constrain the lower potential limit and current density of the electrochemical reduction of CO2 (CO2ER) to formate on Pd although this reaction has been considered one of the most effective methods for CO2 recycling. Among various factors, the participation of adsorbed hydrogen atoms seems to be a key factor affecting the selectivity of CO2ER on Pd. This article discusses the relationship between the selectivity and hydrogen adsorption on the Pd nanoparticles/carbon (Pd/XC72) catalyst through two kinds of systems: gas diffusion electrode (GDE) and rotating ring disk electrode (RRDE). The main product of the CO2ER on Pd/XC72 is changed from formate to CO when both the terrace and step sites of Pd particles are under a low coverage of adsorbed H atoms. In addition, the progress of CO self-poisoning on the Pd/XC72 catalyst and the peak corresponding to the oxidation of COOH* on the Pt ring electrode are clearly observed by the electrochemical analysis methods. The high selectivity of CO generation from the CO2ER on the Pd/XC72 catalyst is attributed to the progressive adsorption of CO which inhibits the participation of Hads during the CO2ER.

The electrochemical reduction of CO 2 (CO 2 ER) to formate on Pd has attracted the broad interest in the chemical energy storage/ conversion systems. However, this reaction is commonly constrained to a narrow potential range (positive to −0.25 V (vs RHE)) and low reactivity due to CO poisoning on the Pd surface. Here, the Pd/carbon catalyst was refluxed in glacial acid (A-Pd/XC72) to enhance the CO tolerance and the formate forming activity of Pd. The results demonstrate that the faradaic efficiency (F.E.) and charges for formate formation are over 95% and 60 coulombs over 1 h of electrolysis at −0.3 V (vs RHE) in the CO 2 -saturated 0.5 M KHCO 3 solution, which are 4 times as high as original Pd/XC72. The F.E. of formate formation remains around 80% when the applied potential is shifted to −0.5 V (vs RHE). Both i-t and cyclic voltammetric (CV) analyses indicate the low rate of CO self-poisoning on A-Pd/XC72. The UV-vis spectra recording the surface change of Pd NPs in the glacial acetic acid suggest the adsorption of acetate species onto the surface of Pd, which are decomposed into carbonaceous species after heating, enhancing the CO 2 ER selectivity toward the formate formation.