Electrocatalytic activity and surface oxide reconstruction of bimetallic iron–cobalt nanocarbide electrocatalysts for the oxygen evolution reaction

For renewable energy technology to become ubiquitous, it is imperative to develop efficient oxygen evolution reaction (OER) electrocatalysts, which is challenging due to the kinetically and thermodynamically unfavorable OER mechanism. Transition metal carbides (TMCs) have recently been investigated as desirable OER pre-catalysts, but the ability to tune electrocatalytic performance of bimetallic catalysts and understand their transformation under electrochemical oxidation requires further study. In an effort to understand the tunable TMC material properties for enhancing electrocatalytic activity, we synthesized bimetallic FeCo nanocarbides with a complex mixture of FeCo carbide crystal phases. The synthesized FeCo nanocarbides were tuned by percent proportion Fe (i.e. % Fe), and analysis revealed a non-linear dependence of OER electrocatalytic activity on % Fe, with a minimum overpotential of 0.42 V (15–20% Fe) in alkaline conditions. In an effort to understand the effects of Fe composition on electrocatalytic performance of FeCo nanocarbides, we assessed the structural phase and electronic state of the carbides. Although we did not identify a single activity descriptor for tuning activity for FeCo nanocarbides, we found that surface reconstruction of the carbide surface to oxide during water oxidation plays a pivotal role in defining electrocatalytic activity over time. We observed that a rapid increase of the (FexCo1−x)2O4 phase on the carbide surface correlated with lower electrocatalytic activity (i.e. higher overpotential). We have demonstrated that the electrochemical performance of carbides under harsh alkaline conditions has the potential to be fine-tuned via Fe incorporation and with control, or suppression, of the growth of the oxide phase.


Figure S8 .Section 9 .
Figure S8.Representative double-layer capacitance measurements for determining electrochemically active surface area for a 45% Fe containing FeCo nanocarbide.a) Cyclic voltammograms (CVs) were measured in a non-Faradaic current region, using scan rates of 0.01, 0.02, 0.05, and 0.10 V s -1 .The cathodic and anodic charging currents were both measured at a fixed potential of 0.9 V vs. RHE (shown by dashed line), and are shown as a plot of current vs. scan rate in b) with the resulting slopes representing the double layer capacitance used to calculate electrochemical surface area.

Figure S10 .
Figure S10.Three linear sweep voltammograms of monometallic Fe carbide samples drop cast on glassy carbon electrodes in 1 M KOH.Samples did not achieve a current density of 10 mA cm -2 .

Figure
Figure S11.a) Linear sweep voltammograms in 1 M KOH were collected of FeCo nanocarbide (15% Fe) drop cast on a glassy carbon electrode at varying mass loadings of 0.1 mg cm -2 (red), of 0.4 mg cm -2 (blue), and 0.8 mg cm -2 (black).Current densities extracted at 1.9 V and overpotentials extracted at a current density of 10 mA cm -2 were plotted as a bar graph in b) at varying mass loadings.

Figure
Figure S12.a) XPS quantification of oxide present in as synthesized Fe x Co 1-x C y samples before (light gray) and after (dark gray) Ar ion sputtering at 5 keV/1 μA for 15 minutes.Note that carbon atomic percentages were not included as samples were run on a carbon puck, therefore the relative amount of only metal and oxygen can be accurately observed.b) Percent difference of oxide in various Fe x Co 1-x C y samples.Interestingly, 15% shows the smallest oxide percent difference suggesting that minor oxide surface layers may enhance electrocatalysis.

FigureSection 15 .
Figure S13.a) CVs of the 1 st and 100 th cycle of RuO 2 at a scan rate of 5 mV s -1 , and b) the decay of current density at a maximum potential of 1.8 V vs. RHE over the 100 CV cycles.

Figure S14 .
Figure S14.Stability measurement of FeCo nanocarbide (15% Fe) with overpotentials extracted from a current density of 10 mA cm -2 , collected at a scan rate of 50 mV s -1 in 1 M KOH with a) representing the first measurement and b) the second measurement collected.

Table S1 .
X-ray fluorescence (XRF) elemental composition for FeCo PBA precursors and FeCo carbides.Results show ratio of metals are maintained from precursor to resultant carbide.

Table S - 2 .
Overpotential comparisons of bimetallic FeNi, FeCo, and other bimetallic catalyst systems in alkaline conditions for OER electrocatalytic activity.