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The selectivity of carbon dioxide (CO2) electroreduction to formate on lead (Pb) catalysts can be improved by using oxide-derived (OD) materials. However, the activity on these materials remains low. In this work, we unexpectedly discovered that the formate-producing activity of Pb materials is correlated to their morphologies. We have prepared sulfide-derived (SD)-Pb and OD-Pb electrodes by respectively electroreducing PbS and PbOx precursors. SD-Pb had a morphology consisting of wafer structures oriented edge-on, while OD-Pb had more contiguous, rounded features. The SD-Pb cathodes reduced CO2 to formate (HCOO−) with good selectivity and activity in 0.1 M KHCO3 electrolyte (Faradaic efficiency = 88% and geometric partial current density, jHCOO−,geom = −12 mA cm−2 at −1.08 V vs. RHE). When normalized to the electrochemically active surface area (ECSA), the jHCOO−,ECSA of the SD-Pb was distinctly larger than those of OD-Pb and polished Pb discs. X-ray photoelectron spectroscopy (XPS) showed no detectable trace of sulfur on the SD-Pb, and Tafel analysis did not suggest any mechanistic differences between the reactivity of SD-Pb, OD-Pb and Pb. These analyses ruled out chemical and mechanistic explanations for the increased activity of SD-Pb, and indicated that morphological differences are the reason for the variation in activity. Extensive control experiments with Pb surfaces prepared in different ways were performed. We provide compelling evidence showing that only Pb samples with the wafer structures exhibited enhanced selectivity and jHCOO−,ECSA, even when no sulfur was used in the preparation method. Our work demonstrates, for the first time, that the morphology of the Pb cathodes is a critical parameter that influences their catalysis of CO2 reduction to formate.

The complexities of the physical and analytical chemistry surrounding the CO2 reduction reaction introduce significant challenges to how appropriate catalysts can be designed. This perspective describes the important factors to consider when analyzing the performance of catalysts and reaction mechanisms for the CO2 reduction reaction in aqueous electrolytes. It focuses on the practical aspects of experimental design and instrumentation that are required for the accurate study of these complex reactions. We discuss the common figures-of-merit and suggest appropriate methods for analyzing and reporting experimental data, such that comparisons between research reports can be more easily drawn. In addition, we highlight notable instances of applications of our suggested practices that already exist in the literature in order to emphasize the excellent work that is being performed in the field.

A facile electropolymerization process was utilized to prepare electrodes modified with thin films of cobalt protoporphyrin IX. These thin films exhibited a high Faradaic efficiency (84±2 %) for the reduction of CO2 to CO in aqueous solutions near neutral pH with 450 mV of overpotential and a turnover frequency at zero overpotential (log(TOF0)) of −5.9. The production of CO was stable over several hours at these modest potentials. The use of a 13CO2 reactant led exclusively to 13CO as the product. Polymeric films of the unmetalated porphyrin did not demonstrate catalysis for CO2 reduction. UV/Vis spectroelectrochemical experiments indicate that the parent CoII complex is reduced to CoI at the electrode surface before interaction with CO2. It is proposed that the rate-determining step in the reduction of CO2 is the initial reduction of the CoII moiety to CoI, which binds CO2 and then undergoes a proton-coupled electron transfer and a loss of water to form CO. Additionally, a new metric for the evaluation of electrocatalysts, the catalytic efficiency, is proposed. The catalytic efficiency is the ratio of the power stored to power consumed for a given electrochemical reaction and can be used to describe both the kinetics and overpotential considerations of a given system.

The role of metastable surface oxides in the reduction of CO2 on lead, bismuth, tin, and indium electrodes was probed using in situ attenuated total reflectance infrared (ATR-IR) spectroelectrochemistry. The effect of the surface oxide on the Faradaic efficiency of CO2 reduction to formic acid was studied by etching and anodizing the electrodes, and the results were correlated with respect to the observed spectroscopic behavior of the catalysts. A metastable oxide is observed on lead, tin, and indium cathodes under the electrochemical conditions necessary for CO2 reduction. Spectroscopic evidence suggests that bismuth electrodes are fully reduced to the metal under the same conditions. The dynamics of the electroreduction of CO2 at lead and bismuth electrodes appears to be different from that on on tin and indium electrodes, which suggests that these catalysts act through different mechanistic pathways. The post-transition-metal block can be divided into three classes of materials: oxide-active materials,...

The reduction of CO2 on tin cathodes was studied using in situ attenuated total reflectance infrared spectroscopy (ATR-IR). Thin films of a mixed Sn/SnOx species were deposited onto a single-crystal ZnSe ATR crystal. Peaks centered at about 1500, 1385, and 1100 cm–1, attributed to a surface-bound monodentate tin carbonate species, were consistently present under conditions at which CO2 reduction takes place. It was shown that these peaks are only present at potentials where CO2 reduction is observed. Moreover, these peaks disappear if the pH of the reaction is too low or if the tin surface is chemically etched to remove surface oxide. Sn6O4(OH)4 and SnO2 nanoparticles were shown to be catalytically active for CO2 reduction, and insights into the oxidation state of the catalytically active species are gained from a comparison of the catalytic behavior of the two nanoparticle species. From these experiments, a mechanism governing the reduction of CO2 on tin electrodes is proposed.

Electrodeposited Fe(III) meso-Tetra(4-carboxyphenyl) porphyrin with cyanide ligands coordinated to the central iron ion is being investigated as a possible material capable of selectively oxidizing H2S in the presence of CO and NO in physiologically relevant solutions. Preliminary results suggest this to be a very attractive material as it oxidizes H2S at low potentials while exhibiting minimal activity towards both CO and NO. However certain results suggest that the Fe may not be completely coordinated with cyanide ligands. This may explain the limitations of the material at elevated H2S concentrations and imperfect selectivity over the interfering gases. Additionally, a cathodic-post run treatment is suggested in order to maintain the electrochemical activity of the porphyrin-modified electrode surface by stripping any S0 that deposits on the electrode surface during the H2S oxidation.

A vanadium pentoxide (V 2 O 5 ) film was electrodeposited onto the end of a PtIr wire ( d = 125 μm) via potential cycling in order to preliminarily assess its viability as a prospective material for in vivo H 2 S monitoring. The deposited oxide film exhibited catalytic activity towards the oxidation of H 2 S in a physiologically relevant pH buffer solution, which resulted in a large increase in peak current compared to that exhibited by the bare PtIr surface. The modified electrode exhibited a linear response over a total sulfide concentration range of 0.5–15 μM. The catalytic activity of the V 2 O 5 -modified wire towards the oxidation of upper physiological levels of NO and CO, two gaseous interferences found in vivo , was investigated using amperometry. While the electrode demonstrated some activity towards NO, it exhibited significant activity towards CO in the potential region of interest. However, the response towards NO and CO was much slower than the response towards H 2 S and the linear range remained unchanged despite the presence of the interfering gases.

A simple approach for expanding metalloporphyrins as electrocatalytic materials is introduced. This new method involves the direct electropolymerization of an electrically conducting layer of ferriprotoporphyrin followed by a metal-substitution reaction executed by soaking the electrode in an aqueous metal-salt solution. This process exchanges the iron center of the surface-bound protoporphyrin with the metal ion of the salt solution. X-ray photoelectron spectroscopy and electrochemical methods confirm that this new metal is incorporated into the surface-bound porphyrin structure and alters the electrochemical properties of the electrode. This simple electrode modification process may expand the use of metalloporphyrins for electrocatalytic applications. © 2013 The Electrochemical Society. [DOI: 10.1149/2.005310eel] All rights reserved.