Enhanced Catalysis of the Electrochemical Oxygen Evolution Reaction by Iron(III) Ions Adsorbed on Amorphous Cobalt Oxide

Abstract

The oxygen evolution reaction (OER) is the bottleneck in the efficient production of hydrogen gas fuel via the electrochemical splitting of water. In this work, we present and elucidate the workings of an OER catalytic system which consists of cobalt oxide (CoO x ) with adsorbed Fe 3+ ions. The CoO x was electrodeposited onto glassy-carbon-disk electrodes, while Fe 3+ was added to the 1 M KOH electrolyte. Linear sweep voltammetry and chronopotentiometry were used to assess the system's OER activity. The addition of Fe 3+ significantly lowered the average overpotential (η) required by the cobalt oxide catalyst to produce 10 mA/cm 2 O 2 current from 378 to 309 mV. The Tafel slope of the CoO x + Fe 3+ catalyst also decreased from 59.5 (pure CoO x ) to 27.6 mV/dec, and its stability lasted ∼20 h for 10 mA/cm 2 O 2 evolution. Cyclic voltammetry showed that oxidation of the deposited CoO x , from Co 2+ to Co 3+ occurred at a more positive potential when Fe 3+ was added to the electrolyte. This could be attributed to interactions between the Co and Fe atoms. Comprehensive X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy were conducted. The in situ XANES spectra of Co sites in the CoO x , CoO x + Fe 3+ , and control Fe 48 Co 52 O x catalysts were similar during the OER, which indicates that the improved OER performance of the CoO x + Fe 3+ catalyst could not be directly correlated to changes in the Co sites. The XANES spectra of Fe indicated that Fe 3+ adsorbed on CoO x did not further oxidize under OER conditions. However, Fe's coordination number was notably reduced from 6 in pure FeO x to 3.7 when it was adsorbed on CoO x . No change in the Fe−O bond lengths/strengths was found. The nature and mechanistic role of Fe adsorbed on CoO x are discussed. We propose that Fe sites with oxygen vacancies are responsible for the improved OER activity of CoO x + Fe 3+ catalyst.

Operando X‐Ray Absorption Spectroscopy Shows Iron Oxidation Is Concurrent with Oxygen Evolution in Cobalt–Iron (Oxy)hydroxide Electrocatalysts

“…[14] Co(Fe)O x H y is structurally and electrochemically similar to Ni(Fe)O x H y . [8] Operando XAS suggests oxidation of Co during the OER to as imilar extent for both samples.F e oxidation was not found in either sample,b ut adsorbed Fe 3+ exhibited alower oxygen coordination and was concluded to be responsible for the higher activity of the sample.U ndercoordinated Fe and Co edge-sites have also been hypothesized to be the active sites in Ni(Fe)O x H y [10,[17][18][19] and CoOOH, respectively. Smith et al examined photochemically deposited Fe 100Ày Co y O x films using operando electrochemical XAS and observed oxidation of Co,b ut not Fe during the OER.…”

“…TheF esites in catalyst films here are well-integrated into the CoOOH structure, evidenced by the similarity of the MÀOa nd MÀMs hell distances in the FT-EXAFS (Figure 4). [8] However in that case no concurrent oxidation of Fe was observed and bond length changes due to Co/Feoxidation during the Co 2+/3+ oxidation wave and during the OER were not separated. [6,8,23,24] Interestingly, our potential-dependent Fe À Ob ond lengths are similar to those of Fe 3+ ions adsorbed on CoO x H y ,r eported by Gong et al,which showed adecrease of the Fe À Obond length from 2.01 to 1.94 when changing the potential from open-circuit to OER conditions.…”

“…[3,5,6,8,23,24] This is most likely due to differences in catalyst preparation. [3,5,6,8,23,24] This is most likely due to differences in catalyst preparation.…”

“…[3,5,6,8,23,24] This is most likely due to differences in catalyst preparation. [6,8,23,24] Interestingly, our potential-dependent Fe À Ob ond lengths are similar to those of Fe 3+ ions adsorbed on CoO x H y ,r eported by Gong et al,which showed adecrease of the Fe À Obond length from 2.01 to 1.94 when changing the potential from open-circuit to OER conditions. Since CoOOH is electronically conductive at all potentials studied here,unlike FeOOH, the Fe sites in Co(Fe)O x H y could be expected to undergo potential-dependent structural changes.The samples used for previous studies on Co(Fe)O x H y may include oxide phases or separated Fe phases that reduce the amount of electrochemically accessible Co(Fe)O x H y ,m aking the changes seen here difficult to detect.…”

Metal–Organic Gel‐Derived Multimetal Oxides as Effective Electrocatalysts for the Oxygen Evolution Reaction

“…These electrochemical characterizations suggest that CoFeNi-O-1 is the best OER catalyst, and its performance is comparable with most recently reported catalysts (Table S6). [16,37] Doping too much Fe into the Co and Ni oxidesr esultsi nt he low catalytic activity owing to the lower OER activity of Fe-rich phases or too much positivec hargeo nt he catalytic sites, which adversely affects the separation of product from the catalyst. These results suggest that the derived CoFeNi-O-1 could be employed as ap romising OER electrocatalyst with high catalytic activity,g ood stability, and facile synthesis.…”