Water oxidation catalysis: an amorphous quaternary Ba-Sr-Co-Fe oxide as a promising electrocatalyst for the oxygen-evolution reaction †

We present an amorphous quaternary Ba-Sr-Co-Fe oxide (a-BSCF) with a specific stoichiometry, readily fabricated via a photochemical decomposition method. a-BSCF demonstrates high catalytic activity towards the oxygen-evolution reaction (OER).

This journal is © The Royal Society of Chemistry 2016 The disappearance of infrared absorption bands from the precursors' coordinated ligand (Fig. S1 †) over a 12 h period indicates that the ligand is photolysed; new bands are attributed to surface carbonate ions † 34 as was documented in our study of amorphous lanthanum-transition metal binary oxides. 30he microstructure of the resultant film on the FTO substrate as seen from scanning electron microscopy (SEM) shows a smooth and featureless film with good coverage of the FTO substrate (Fig. S2 †).Energy-dispersive X-ray spectroscopy elemental mapping of the a-BSCF film (Fig. S3 †) demonstrates that the four metal elements are uniformly distributed within the film, without segregation or local enrichment.Inductively-coupled plasma optical emission spectroscopy (ICP-OES) confirmed an actual film composition of Ba 0.55 Sr 0.45 Co 0.77 Fe 0.23 O x , in excellent agreement with the nominal composition of Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3Àd .These results show that photochemical depostion is a reliable process to fabricate homogeneous quaternary oxides with precisely controlled composition.
The amorphous nature of the film was confirmed by X-ray diffraction (XRD); only Bragg reflections expected from the crystalline FTO substrate are observed, indicating that X-ray amorphous films are obtained (Fig. S4 †).This finding is consistent with previous XRD, 27,30 transmission electron microscopy, 31 extended X-ray absorption fine structure spectroscopy, 33 and atomic pair-distribution function analysis of high-energy X-ray scattering data 35 which indicate that the photochemical deposition process yields amorphous films.
The oxidation states of the constituent metals at the film surface in an as-prepared a-BSCF film were probed using X-ray photoelectron spectroscopy (XPS).7][38] The presence of a carbonate species is expected from the FTIR results presented above.The spectrum collected in the Sr 3d region is consistent with the spin-orbit coupled doublet expected for a Sr 2+ oxide species such as SrO (Sr 3d 5/2 at 133.4 eV; Sr 3d 3/2 at 135.1 eV).The cobalt is primarily attributed to a cobalt(II) oxide species (Co 2p 3/2 at 780.0 eV; Co 2p 1/2 at 795.4 eV; 96.5% relative area of the Co signal) with a minor cobalt(II) hydroxide species accounting for the remainder of the signal (Co 2p 3/2 at 781.9; Co 2p 1/2 797.3 eV). 38The XPS signal for the iron species is rather complex, but appears to be attributable to trivalent iron oxide and oxyhydroxide species. 27hree species adequately fit the envelope of the O 1s spectrum, which are assigned to oxygen in the form of oxide ions O 2-in metal-oxygen bonds (529.4 eV), and adsorbed CO 3 2À (531.1 eV) and OH À (532.4 eV), respectively. 38he metal oxidation states can also be inferred from cyclic voltammetry (CV) measurements.Fig. 1a shows the CVs of a-BSCF after the first, second, and sixth cycles.During the first cycle, an apparent oxidation peak E p,a at 1.08 V vs. RHE is observed.In the second and subsequent cycles, E p,a decreases to 1.01 V with decreased oxidation current density.As the XPS result reveals the presence of Co II O and Co II (OH) 2 , the oxidation peaks at 1.08 and 1.01 V are attributed to the oxidation of CoO and Co(OH) 2 to Co III OOH.The OER catalytic current starts at ca. 1.46 V and increases sharply with increasing potential.Although the coexistence of Co(III) and Co(IV) under anodic conditions (evidenced from in situ Mo ¨ssbauer 39 and X-ray absorption near-edge structure spectroscopies 40 ) has been previously reported for cobalt oxides, no obvious oxidation peak related to Co(III/IV) is observed; this feature is likely masked by the OER catalytic wave.The catalytic current density slightly increased from the first to the second cycle and then remained constant.The reduction peak potential is the same for all the cycles, 0.96 V, attributed to the reduction of Co III OOH to Co II (OH) 2 . 28teady-state measurements in the Tafel regime were performed to further evaluate the electrode kinetics of a-BSCF towards the OER (Fig. 1b).The attained Tafel parameters in terms of Tafel slope, the onset overpotential of catalysis, and the overpotentials required to achieve current densities of 0.05 and 1 mA cm À2 are listed in Fig. 2. The Tafel parameters for the films annealed at 500 1C for 1 h and the reported results for crystalline Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3Àd 23 are also listed for comparison.The a-BSCF film annealed at lower temperature outperforms both the sample annealed at 500 1C and the reported crystalline material with the same composition. 23Compared with the latter crystalline material, the Tafel slope and Z @ 0.05 mA cm À2 of a-BSCF are lower by ca.20 and 40 mV, respectively.The current density at the same overpotential (B0.25 V) increases by a factor of ca.20 for the present a-BSCF film, taking caution to specify that these current densities are related to geometric, not real, surface areas.Furthermore, the activity of a-BSCF (30 AE 2 mV dec À1 , 0.252 AE 0.004 V @ 1 mA cm À2 ) is remarkably higher than that of  the state-of-the-art crystalline IrO x (49 AE 1 mV dec À1 , 378 AE 4 mV @ 1 mA cm À2 ). 12 In addition, a-BSCF demonstrates significantly higher electrocatalytic performance towards the OER than the reported typical perovskite oxides, e.g.LaNiO 3 (42 mV dec À1 , 16 0.32 V @ 0.05 mA cm À2 ), 23 LaCoO 3 (70 mV dec À1 , 13 0.32 V @ 0.05 mA cm À2 ), 23 and La 0.6 Ca 0.4 CoO 3 (55-60 mV dec À1 ). 18ollectively, these results demonstrate the superior electrocatalytic activity of a-BSCF towards the OER.We ascribe this performance to the presence of a larger number of coordinately unsaturated surface metal sites available for reaction, and the isotropic and single-phase nature of amorphous materials. 41e also hypothesize that similar trends correlated with e g occupancy may be present; 23 verifying this assertion will be the focus of future work.
To gain further insight into the processes occurring during the OER, electrochemical impedance spectroscopy was carried out on the films after Tafel measurements.The typical impedance spectra of a-BSCF at various applied potentials are presented in Fig. 3a and b.All spectra are composed of two semi-circles, which are well fit with the equivalent circuit presented in Fig. 3c, where R s is the solution resistance (i.e., the intercept of the highest frequency with the abscissa axis); Q film and R film are related to the dielectric properties and resistivity of the film; Q dl and R ct are the double-layer capacitance and the interfacial charge-transfer resistance, respectively. 42R s (not shown) and R film (Fig. 3d) remain constant over the applied potential range studied here, whilst R ct decreased with increasing applied potential, consistent with the rapid increase of current density after the onset potential (Fig. 1).The resistance of the film is higher than the charge-transfer resistance at applied potentials greater than 1.50 V vs. RHE, and thus becomes the dominating resistance; this result infers that increasing the electrical conductivity of this material would further enhance the electrocatalytic activity of a-BSCF towards the OER.
Steady-state measurements at different KOH concentrations (i.e., different activities, a OH À) were carried out to gain further insight into the reaction mechanism.The resultant log J vs. E plots are presented in Fig. 4a; the current density J increases with OH À activity.The reaction order with respect to OH À was determined to be 4.0 AE 0.7 by fitting the data of Fig. 4b.
A plausible reaction mechanism for the OER 13 on a-BSCF is shown in Fig. 5.While this mechanism involves only cobalt surface sites, iron sites also likely participate in the OER to a lesser extent.Considering the high concentration of coordinately unsaturated surface metal sites expected on the surface of a highly defective amorphous material, high surface coverage y (0.2 o y o 0.8) by hydroxide ions is likely.Accordingly, under the conditions of the Temkin isotherm, a Tafel slope of 30 mV dec À1 and reaction order with respect to OH À of 4 are expected, assuming Step 3 of Fig. 5 is the rate-determining step (RDS). 13The present experimental results (Tafel slope of 30 AE 2 mV dec À1 , Fig. 1b; and reaction order of 4.0 AE 0.7, Fig. 4b) are in excellent agreement with this proposed mechanism. 13This would identify desorption of the hydroxide ion from the surface of a-BSCF film as the RDS.Considering the antibonding character of the d z 2 orbital of the 3d metal in M-OH suggested by Bockris and Otagawa, 14 populating the M d z 2 orbital would result in weakening of the M-OH bond strength; conversely, the absence of electrons in the d z 2 orbital for Co 4+ suggests strong Co IV -OH binding, hindering the release of OH À and liberation of oxygen gas.
Finally, the short-term stability of a-BSCF at a constant current density of 1 mA cm À2 was evaluated in 0.1 M KOH by chronopotentiometric means.The potential increased by 5.5% at a current density of 1 mA cm À2 after 24 h of testing (Fig. S6a, ESI †); meanwhile the Tafel slope increased from   30.8 to 37.2 mV dec À1 , and Z @ 1 increased by 13% (Fig. S6b, ESI †).Comparing surface SEM images of the film before and after testing (Fig. S2, ESI †) indicates cracks formed during testing, and a thinning of the catalyst film as the surface roughness of the underlying FTO substrate becomes more apparent.The metal molar ratio of the film after the stability test is found to be 0.80 : 0.20 : 0.75 : 0.25 Ba : Sr : Co : Fe by ICP-OES, indicating preferential dissolution of Sr during testing.Ba, Sr, and Fe were also detected in the electrolyte solution after testing.Accordingly, there is still room to improve the stability of the a-BSCF.
In summary, the amorphous quaternary Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3Àd film is first prepared by photochemical deposition and was investigated as an electrocatalyst for the OER.The breakage of the Co IV -OH bond is suggested to be the RDS under Temkin isotherm conditions.The superior activity towards the OER renders the a-BSCF materials a competitive OER catalyst.Importantly, this study demonstrates heeding guidance from the rational design of crystalline high-performance OER catalysts which may be a fruitful approach to the identification of high-performance amorphous OER catalysts.Photochemical deposition is, to the extent of our knowledge, the only method amenable to the controlled formation of such quaternary amorphous metal-oxide OER catalysts.
MITACS and FireWater Fuel Corp. sponsored this project.This research used facilities funded by the University of Calgary and the CFI's John R. Evans Leaders Fund.Dr Shu Yao is thanked for her assistance with XPS data collection.

Fig. 1
Fig. 1 (a) CVs and (b) Tafel plot for a-BSCF in 0.1 M KOH at a scan rate of 10 mV s À1 .The Tafel plot of a film annealed at 500 1C is also shown for comparison.Lines in (b) are a linear fit to the data.

Fig. 3
Fig. 3 (a and b) Impedance spectra of a-BSCF film at different potentials (vs.RHE) in 0.1 M KOH.The lines in (a and b) are fits using the equivalent circuit shown in (c).(d) Variation of R film and R ct with applied potential.

Fig. 4
Fig. 4 (a) Tafel plots in KOH solution of different activities.The lines are the linear fit to the data; (b) log J vs. log a OH À at the same potential of 0.636 V vs. NHE.

Fig. 5
Fig.5Proposed OER mechanism on a-BSCF consistent with experimental data.