Nanoscale Active Sites for the Hydrogen Evolution Reaction on Low Carbon Steel

To fully elucidate the structural controls on corrosion-related processes at metal surfaces, experimental measurements should correlate and compare directly structure and activity at the scale of surface heterogeneities (e.g., individual grains, grain boundaries, inclusions, etc.). For example, the hydrogen evolution reaction (HER), which usually serves as the cathodic counterpart to anodic metal dissolution in acidic media, may be highly sensitive to surface microstructure, highlighting the need for nanoscale-resolution electrochemical techniques. In this study, we employ scanning electrochemical cell microscopy (SECCM) in conjunction with colocated scanning electron microscopy, electron backscatter diffraction, and energy-dispersive X-ray spectroscopy to elucidate the relationship between surface structure/composition and HER activity on low carbon steel in aqueous sulfuric acid (pH ≈ 2.3). Through this correlative electrochemical multimicroscopy approach, we show that the HER activity of the low index grains (slightly) decreases in the order (100) > (111) > (101), with grain-dependent free energy of hydrogen adsorption (calculated for the low index planes of iron by using density functional theory, DFT) proposed as a tentative explanation for this subtle structural dependence. More significantly, we show that the HER is greatly facilitated by submicrometer surface defects, specifically grain boundaries, and MnS inclusions, directly identifying these heterogeneities as potential "cathodic sites" during (atmospheric) corrosion. This study demonstrates the considerable attributes of correlative SECCM for identifying nanoscale active sites on surfaces, greatly aiding the understanding of corrosion and electrocatalytic processes.