A rapid screening process for electrocatalytic materials generally consists of determining the electrochemical activity of the catalyst with and without the reactant present. Typically a cyclic voltammogram (CV) would be conducted in the electrolyte with an atmosphere of inert gas such as Ar and compared to a CV with the reactant present. However, due to the high concentration of H+ and relatively low kinetic barriers to the HER compared to CO2R, H2 production may dominate the current under either atmosphere. In order to develop a rapid screening method of numerous CO2R catalysts a facile product analysis setup is necessary to distinguish between current towards HER, and current towards CO2R. While typical gas chromatography (GC) experiments are viable, the time needed for the separation portion of the analysis is lengthy and somewhat unnecessary. For rigorous individual product quantification separation and detection would be ideal; however, for a rapid screening process this step is extraneous. For these catalysts it is possible to leverage the fact that the figure of merit is the ratio of the current towards CO2R relative to HER. It is then necessary to only know the current and the amount of H2 produced to determine the current towards CO2R. The Faradaic efficiency (FE), or percentage of the current towards a given process, may then be determined for
HER and by the remainder of the current can be assumed to go towards CO2R. Materials with low HER activity and high CO2R activity may then be further characterized using more rigorous product detection methods to determine which products were made and at what potentials.
An ideal technique for this type of screening process is online electrochemical mass spectroscopy (OLEMS). This technique uses a typical quadrupole mass spectrometer (MS) with a heated quartz inert capillary (QIC) inlet connected to the headspace of an electrochemical cell. The headspace is then sampled at a constant rate and directly injected into the ionization source. The m/z for each mass fragment from the products are then detected as a function of time and plotted every 5 sec. This rapid product detection method allows for real-time detection of gaseous products such as H2 or CH4. An
electrode may then be polarized negatively and the products detected. The potential may then be made more negative (or positive), and the products detected again. Using this technique the FE towards H2 or various gaseous hydrocarbons such as CH4 may then be determined rapidly at numerous different potentials.
Several downsides exist for OLEMS used to screen CO2R catalysts. The main disadvantage of this technique is the lack of product separation. Because the analytes are injected directly into the MS any products which fraction at the same mass:charge ratio will give rise to overlapping signals. This becomes problematic when trying to detect a CO2R product such as CO in an atmosphere of CO2 or N2. Because both CO2 and N2
fraction partially at m/z = 28 both gases give large signals at this m/z and the small
amount of CO produced cannot be distinguished above these background signals. Several products such as CH4 or C2H4 do not give overlapping signals with reactants and may be
detected using this technique. The other downside with OLEMS is the ability to rigorously quantify products. This technique is extremely sensitive to experimental variables such as carrier gas flow rate, pressure inside headspace of the electrochemical cell, and even residual water inside the QIC. For these reasons it is necessary to calibrate the instrument each time before use. However, a rapid calibration protocol was
developed, and by using the signal at m/z = 2 corresponding to H2, new catalysts were evaluated under a variety of operating conditions.
There are several hypothesized reaction mechanisms to reach highly reduced products, defined here as any product requiring ≥ 6 e-/H+ transfers to one molecule of CO2. Based on these mechanisms and what is known about the reactivity of certain metals, predications regarding which materials might possess the proper adsorption energetics to produce the desired products were made.19,28 For example, nickel (Ni) has been shown to be a poor CO2 reducing metal because of its strong CO binding energy.19 Tin (Sn) has been shown to be a good HCOO- producing metal because it has very weak interactions with CO2 and CO, and likely forms a weak metal hydride which could be transferred to CO2 producing HCOO-.29 By combining Ni and Sn it was hypothesized a material with intermediate CO binding strength would be generated. These NiSn bimetallics could transfer hydrides from Sn to an adsorbed CO molecule on Ni, producing a product such as CH3OH or CH4. Additionally, alloy phase diagrams were consulted to determine if the selected metals had a room temperature stable phase. Alloys with multiple stable phases such as NiPd were synthesized and their stoichiometry was controlled by tuning the power of the sputter target. To further test the reactivity of certain intermetallics, nanoparticles were synthesized via solvothermal methods. This
allows for the creation of high surface area catalysts that expose crystal faces not present in the sputtered thin films. Lastly, inspiration was gathered from the CO hydrogenation literature which uses Cu/ZnO catalysts industrially. Unfortunately, because these catalysts are on oxide supports they are ill-suited as reductive electrocatalysts;
fortunately, several bimetallic catalysts such as nickel-gallium do not require an oxide support and were tested here as electrocatalysts.