Edge-Enhanced Oxygen Evolution Reactivity at Ultrathin, Au-Supported Fe₂O₃ Electrocatalysts

Abstract

Transition metal oxides have gained attention as promising oxygen evolution reaction (OER) electrocatalysts in alkaline electrolytes, but heterogeneities in typical catalyst samples often obscure key structure–property relationships that are essential for developing higher performance materials. Here, we have combined ultrahigh vacuum surface science techniques, electrochemical measurements, and density functional theory (DFT) to quantify structure-dependent OER activity in a series of well-defined electrocatalysts. We describe a direct correlation between the population of Fe edge-site atoms and the OER activity of ultrathin Fe2O3 nanostructures (∼0.5 nm apparent height) grown on Au(111) substrates. Hydroxylated Fe atoms residing at edge-sites along the catalyst/support interface were spectroscopically identified as key reaction centers, and these Fe edge-site atoms were estimated to produce OER turnover frequencies approximately 150 times higher than that of Fe atoms on the catalyst surface at an applied potential of 1.8 V vs the reversible hydrogen electrode. Impressively, ultrathin Fe2O3/Au nanostructures with a high density of catalytically active Fe edge-site atoms outperformed an ultrathin IrO x /Au catalyst at moderate overpotentials. DFT calculations revealed more favorable OER at edge sites along the Fe2O3/Au interface, with lower predicted overpotentials due to beneficial modification of intermediate binding. Our results demonstrate how a combination of surface science, electrochemistry, and computational modeling can be used to identify key structure–property relationships in a well-defined electrocatalytic system.

Effects of Metal Electrode Support on the Catalytic Activity of Fe(oxy)hydroxide for the Oxygen Evolution Reaction in Alkaline Media

“…The metallic Au surface post‐electrochemistry was verified by XPS peak fitting of the Au 4 f peak in Figure a. Perhaps it is the continual reformation of the AuO x −FeO x H y interface that results in highly‐active sites for the OER, as suggested by both our previous work and other reports that showed high activity for Fe atoms substituted at the surface of Au 2 O 3 and edges sites at the Fe 2 O 3 −Au interface . Redox features are also observed for all other substrates except for C (Figures , 2, and S5).…”

“…Recently, Kauffman et al. found that edge sites at the interface of Au and Fe 2 O 3 showed high activity for the OER …”

Discontinuity‐Enhanced Thin Film Electrocatalytic Oxygen Evolution

“…The continuous CMOH (1 × 1 cm 2 ) electrodes and discontinuous ones (25 isolated pieces with a total coverage area of 1 cm 2 identical to the coverage of continuous ones) were tested for electrocatalytic OER comparison ( Figure a,b). The linear sweep voltammetry (LSV) results show a similar trend that the discontinuous CMOH on both the substrates exhibits greater OER performance as the overpotentials lowered than the continuous ones (Figure a,b), indicating that the edge enhancement may generally occur independent on specific substrate materials, although some specific case on gold has been reported . In addition, the varied distance between each isolated CMOH pieces on the both substrates shows no appreciable influence on the LSV results (Figure S1, Supporting Information).…”

“…As shown in Figure b, at the beginning of the conditioning, all of the samples exhibit the increasing of current density in different magnitude, which roughly saturated at the first 5 h conditioning and reached the values of 75.02 mA cm −2 (CMOH‐py‐S), 29.03 mA cm −2 (CMOH‐py‐M), 10.70 mA cm −2 (CMOH‐py‐L), and 5.45 mA cm −2 (CMOH‐py‐XL), corresponding to the current enhancement of 1500%, 580%, 214%, and 109%, respectively, by referencing the initial current density (5 mA cm −2 ). Such the unusually high increase of 1500% OER current density in electrochemical conditioning has not been reported in the other similar systems during the conditioning . The first 5 h stage of current climbing may reflect the intrinsic capacity of Co 3+ /Co 4+ and phase activation; the upper limits of current density saturation may suggest that certain quantity of idle sites is not allowed to be further activated regardless of extension of the conditioning time.…”

“…With the example of photolithography‐patterned electrocatalysts toward improved OER, correlation of OER performance to systematic edge exposure normalizing to unit area is then becoming the most desired information. Recent study on the edge‐site atom population of discrete, atom‐scale deposition of iron oxides shows a proportional relationship to OER activities . Despite the emerging of edge‐dependent OER, reliable deposition over large area with versatile coverage/continuity to achieve a wide‐scale edge exposure poses another difficulty in sample preparation.…”