Pd-Catalyzed Electrohydrogenation of Carbon Dioxide to Formate: High Mass Activity at Low Overpotential and Identification of the Deactivation Pathway

Abstract

Electrochemical reduction of CO2 to formate (HCO2(-)) powered by renewable electricity is a possible carbon-negative alternative to synthesizing formate from fossil fuels. This process is energetically inefficient because >1 V of overpotential is required for CO2 reduction to HCO2(-) on the metals currently used as cathodic catalysts. Pd reduces CO2 to HCO2(-) with no overpotential, but this activity has previously been limited to low synthesis rates and plagued by an unidentified deactivation pathway. Here we show that Pd nanoparticles dispersed on a carbon support reach high mass activities (50-80 mA HCO2(-) synthesis per mg Pd) when driven by less than 200 mV of overpotential in aqueous bicarbonate solutions. Electrokinetic measurements are consistent with a mechanism in which the rate-determining step is the addition of electrochemically generated surface adsorbed hydrogen to CO2 (i.e., electrohydrogenation). The electrodes deactivate over the course of several hours because of a minor pathway that forms CO. Activity is recovered, however, by removing CO with brief air exposure.

CO2 Reduction: From the Electrochemical to Photochemical Approach

“…One possible remedy to this poor selectivity is to control its particle size below 5 nm for optimal edge and corner site density 81. Occasionally, formate is identified as the main reduction product on Pd as probably mediated by the surface PdH x 82. There are also a handful of reports about Zn‐, In‐, or Bi‐based materials, mostly prepared from electrodeposition, for selectively reducing CO 2 to CO 83, 84, 85, 86, 87, 88, 89.…”

Cu–Sn Bimetallic Catalyst for Selective Aqueous Electroreduction of CO2 to CO

“…10 However, their implementation at an industrial scale is unsustainable and has limitations owing to the scarcity of noble metals. [7][8][9][10] Previous studies by Hori and coworkers have demonstrated that Cu is unique compared with other metals in its ability to produce hydrocarbons at potentials more negative than −1 V vs RHE. 11 Nevertheless, the large overpotential renders the process inefficient.…”

Performance Limits of Photoelectrochemical CO2 Reduction Based on Known Electrocatalysts and the Case for Two-Electron Reduction Products

“…This drop in photocurrent is compensated by the increasing energy content (per electron) of the formate or CO products so the overall result is a similar STF efficiency. In fact, if a CO 2 reduction catalyst existed, which could make either CO or formate at overpotentials similar to HER on Pt (in 2015 such a catalyst was reported for formate 41 and very recently it showed great performance in PEC application 43 ), CO 2 reduction could reach higher STF efficiency than water splitting. This is illustrated in Figure 3, which shows the ideal performance under conditions of zero electrochemical losses, but otherwise standard conditions (Table 1).…”

“…In 2015 Min and Kanan reported efficient reduction of CO 2 to formate over Pd nanoparticles 41 , and in early 2016 Gao et al…”