Soft Chemical Fabrication of Iron-Based Thin Film Electrocatalyst for

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Soft Chemical Fabrication of Iron-Based Thin Film Electrocatalyst for Water Oxidation under Neutral pH and Structure−Activity Tuning by Cerium Incorporation Jony Saha* and T. P. Radhakrishnan* School of Chemistry, University of Hyderabad, Hyderabad 500 046, India S Supporting Information *

ABSTRACT: Design of electrocatalysts for the fundamentally important oxygen evolution reaction can be greatly aided by systematic structure−activity tuning via composition variation. We have explored the iron−cerium system as they are the most abundant transition and rare earth metals, and also due to the mutualistic impact of their size and electronic attributes that can induce critical changes in the structure and electrochemical activity. Submicrometer thick films of a series of Fe(III)−Ce(III) phosphate(oxyhydroxide) (FeCePH) are fabricated using a soft chemical strategy involving surfactant-aided assembly, spin-coating, and mild thermal annealing. FT-IR, Raman, and Xray photoelectron spectroscopies, chemical analysis, X-ray diffraction, and electron microscopy reveal the systematic structural, electronic, and morphological variation, on tuning the iron−cerium composition. Nitrogen adsorption−desorption studies show the surface area increasing and pore size distribution shrinking with the cerium content, indicating its structure-directing role. The electrocatalysis of water oxidation by FeCePH films on FTO-coated glass is studied in neutral pH conditions. The overpotential and Tafel slope decrease with increasing cerium content, reaching minima at the optimal Fe:Ce ratio of 1:0.5; the turnover frequency shows a corresponding increase and maximum. The trends are explained on the basis of the structural changes in the films, and the coupling of Ce3+/ Ce4+ with Fe3+/Fe4+ that leads to active state regeneration. This study presents a rational strategy to tune the efficiency of easily fabricated transition metal-based electrocatalyst thin films through rare earth metal incorporation; it should prove useful in the design of cost-effective catalysts for water oxidation.

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versely on the catalytic efficiency.9 Requirement of alkaline conditions and dissolution issues has hampered the development of iron-based electrocatalysts.10 The catalytic activity of transition metal-based systems is compromised by rigid ligand environments that impede the redox transitions and coordination changes of the metal centers;3 subtle alteration of the ligands can significantly impact upon the catalytic function.11−13 Incorporation of additional redox couples to modulate or recover the activity of the primary catalyst system could prove beneficial in developing efficient electrocatalysts for OER. In this context, we have carried out a detailed exploration of the electrocatalytic activity of mesoporous FePO4·FeOOH thin film with varying extent of cerium incorporation; in addition to its size and redox characteristics that are very pertinent, cerium (being the most abundant rare earth metal) is a prime choice in terms of cost effectiveness. Some of the earlier reports on FeOOH as a catalyst for OER involved expensive gold or platinum substrates, fabrication by electrodeposition, and/or strongly alkaline conditions for electrolysis.9,10,14 We have

INTRODUCTION The past decade has witnessed enhanced research on the fundamental problem of electrochemical water splitting, within the ambit of alternate energy production.1 Between the basic events of water oxidation at the anode and reduction of protons at the cathode, the former involving the 4H+/4e− system and a proton-coupled electron transfer is more challenging for the development of cost-effective, efficient, and stable catalysts.2 The complex mechanistic pathways involving O−H bond cleavage followed by O−O and OO bond formation lead to sluggish kinetics, making the design and fabrication of an effective electrocatalyst a formidable problem.3 The important issue of material and fabrication cost has driven the search for oxygen evolution reaction (OER) electrocatalysts based on non-noble metals. There has been a burgeoning interest in developing transition metal-based catalysts;4−7 while the cost factor and broad tunability of oxidation states are a significant advantage, low electronic conductivity, poor chemical stability, and lack of systematic design approaches remain critical issues. Iron-based electrocatalysts are cheap, thanks to their high crustal abundance, and are of great practical interest due to their nontoxicity;8 however, low conductivity, high enthalpic costs of modulating the coordination sphere, and formation of hydroxide intermediates during electrocatalysis impact ad© XXXX American Chemical Society

Received: May 19, 2017 Revised: July 29, 2017

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prepared by dissolving the film in 10 vol % aqueous HNO3. Raman spectra of the films were recorded on a WiTec model Alpha 300 R Raman microscope using a 530 nm excitation laser. Grazing incidence X-ray diffraction of the films was recorded at 2θ = 15−90° with an incident angle of 0.3° using a Rigaku Smart Lab diffractometer operating at 45 kV and 200 mA (9 kW); Cu Kα radiation (λ = 1.5405 Å) was used. Nitrogen adsorption−desorption analysis was performed using powder samples obtained by scratching the film; an Autosorb iQ Station 2 operated at 77 K was used after degassing the samples at 120 °C for 12 h under a vacuum of 10−2 Torr. Field emission scanning electron microscope (FESEM) imaging of the films was carried out on a Carl Zeiss model Ultra 55 microscope. For transmission electron microscopy (TEM), the film scratched from the substrate was fixed on a carbon-coated Cu grid and imaged in a FEI TECNAI G2 S-Twin TEM at an accelerating voltage of 200 kV. X-ray photoelectron spectra (XPS) of the films were recorded on a KRATOS AXIS 165 with a DUAL anode (Mg and Al) and Al Kα X-ray source (hν = 1486.6 eV) operating at 225 W in a UHV system with a base pressure of 10−7 Torr. The spectra were deconvoluted specifically to identify the oxidation states of Fe and Ce. Electrochemical Analysis. All electrochemical studies were performed using a Zahner Zennium electrochemical workstation at 25 °C. Cyclic voltammetry (CV) was carried out with 20 mL of 0.1 M sodium phosphate buffer (pH = 6.4) employing a three-electrode configuration in the electrochemical cell; FePH or FeCePHX films on FTO-coated glass substrate (area = 1 cm2), Pt wire, and Hg/Hg2Cl2 were used as the working, counter, and reference electrodes, respectively. The scan rate was maintained at 1 or 5 mV/s. All electrochemical data are corrected for IR compensation, and the electrode potentials are reported with respect to the reversible hydrogen electrode (RHE) [E (vs RHE) = E (vs Hg/Hg2Cl2) + 0.241 V + (0.059 × pH)]. Tafel plots were determined from electrolysis at constant current densities of 0.01, 0.033, and 0.1 mA/cm2, where each step was held for 3 min, and monitoring the final potential at each current density. The turnover frequency (TOF) was calculated as31

fabricated the electrode films through a soft chemical route involving surfactant-aided assembly and spin-coating on an FTO-coated glass substrate, followed by mild thermal annealing; they were deployed for OER under neutral (pH ≈ 7.0) condition. The design and use of this system addresses several critical issues of OER electrocatalysts. The phosphate(oxyhydroxy) group provides an adaptable ligand environment; recent reports show that phosphate stabilizes the local geometry of Co2+ in Na2CoP2O7, while helping Mn2+ oxidation in Mn3(PO4)2·H2O to induce favorable water adsorption and oxidation.11,12 Electrocatalysis under neutral or mild pH conditions15−20 prevent electrode and cell component corrosion and promote cell durability; use of extreme pH conditions (10) to reduce the electrical resistance of the electrolyte not only adds to the cost, but is also environmentally detrimental. Our main motive is to explore the impact of cerium incorporation on tuning the activity of the iron-based catalyst; a detailed understanding of this system can provide insight into general and fundamental design principles for such electrocatalysts. Cerium incorporation, as in the case of several heteroatom additions in metal oxide electrocatalysts,21−24 could enhance the catalyst surface area and hence its efficiency. On the basis of its oxidation potential,25,26 Ce3+/ Ce4+ can be expected to reduce the higher oxidation states of iron, known to form during the OER27−30 and possibly be trapped in the catalyst layer; recovery of the active state of iron would also improve the electrocatalytic efficiency.

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EXPERIMENTAL SECTION

Materials. Iron(III) nitrate nonahydrate (Fe(NO3)3·9H2O), cerium(III) acetate, cetyltrimethylammonium bromide (CTAB), and potassium bromide (KBr) were purchased from Sigma-Aldrich. Phosphoric acid (H3PO4, 85%), nitric acid (HNO3, 70%), and ethanol were supplied by Merck Chemicals (India). Disodium hydrogen phosphate dihydrate (Na2HPO4·2H2O) and sodium dihydrogen phosphate dihydrate (NaH2PO4·2H2O) were obtained from SRL Pvt. Ltd. (India), and ammonium hydroxide (NH4OH, 25% aqueous solution) was purchased from Qualigens Chemicals (India). Millipore Milli-Q water (resistivity ∼18.2 MΩ cm) was used in all preparations and electrochemical studies. Fabrication of Films. 0.5 g of CTAB was dissolved in a mixture of 2.5 g of water and 2.5 g of ethanol by stirring. 1.2 g of Fe(NO3)3·9H2O (∼3 mmol) and 0.2 g of H3PO4 (∼1.7 mmol) were added to the surfactant containing solution; the solution turned reddish brown, indicating the formation of FePO4. 0.15, 0.3, 0.5, 0.65, and 0.8 g of cerium acetate (∼0.5, 1, 1.5, 2, and 2.5 mmol, respectively) were dissolved separately in five different batches of the FePO4 solution. The solutions were spin coated (at 2000 rpm for 10 s) on glass substrates; FTO-coated glass (surface resistivity ∼20 Ω/□) was used as the substrate to prepare films for electrochemical studies. Prior to coating, the glass/FTO substrates were cleaned with soap water followed by acetone and dried. The spin-coated films were kept in a closed chamber in the presence of ammonia vapor for 2 h; this was followed by heat-treatment at 150 °C for 2 h in a hot air oven. The films were immersed in ethanol for 12 h and washed with water to obtain the catalyst films. The films without cerium and with the increasing cerium content are labeled as FePH, FeCePH1, FeCePH2, FeCePH3, FeCePH4, and FeCePH5, respectively. Characterization. Thickness of the films was measured using an AMBiOS model XP200 surface profilometer. Fourier transform infrared (FTIR) spectra of the films scratched from the substrate and made into a pellet with KBr were recorded on a Nicolet 380 spectrometer in the 400−4000 cm−1 with a resolution of 4 cm−1 using 200 scans. The composition of Fe, P, and Ce in the catalyst films was determined using an Agilent 700 inductively coupled plasma optical emission spectrometer (ICP-OES); stock solution for the analysis was

TOF =

j × Sgeo 4×F×n

where j, Sgeo, F, and n are the current density (mA/cm2), geometrical surface area of the anode (cm2), Faraday constant, and moles of the catalyst in the film, respectively. Electrochemical double layer capacitance (CDL) was determined by sweeping the potential in the non-Faradaic region (1.12−1.22 V vs RHE) at different scan rates (60, 120, 180, 250, and 300 mV/s) and determining the average slope of the plot of cathodic and anodic currents (at 1.17 V vs RHE) against the scan rate. Electrochemical impedance spectroscopy (EIS) was carried out at 1.9 V vs RHE in the frequency range, 10 mHz to 100 kHz. Estimation of Evolved Oxygen. Prior to electrolysis, the dissolved oxygen concentration of 0.1 M sodium phosphate electrolyte in a four-mouth sealed electrochemical cell was measured using an HI 764080 Digital polarographic probe sensor. The sensor was calibrated against solution as well as air, while argon gas was purged in the electrolyte until the sensor displayed zero concentration of oxygen. The experiment was carried out with the FeCePH4 film as working, Hg/Hg2Cl2 as reference, and Pt wire as counter electrodes, holding the potential at 1.90 V vs RHE for 10 min. Faradaic yield for oxygen evolution was determined from the experimentally observed oxygen evolution and the theoretically expected value estimated from the integration of the current−time curve (chronoamperogram).

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RESULTS AND DISCUSSION The FePH film was synthesized with Fe(NO3)3·9H2O and H3PO4, making use of a CTAB surfactant-aided assembly. The films with different iron−cerium compositions (FeCePHX) were fabricated by incorporating increasing amounts of cerium acetate during the synthesis as described in the Experimental Section. Steps involved in the fabrication of FePH film are B

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Langmuir schematically depicted in Figure 1; similar steps are used for preparing FeCePHX films, the only difference being the addition of the cerium salt along with ferric nitrate. The cationic surfactant CTAB forms micelles in the water/ethanol medium, the hydrophilic and hydrophobic parts orienting appropriately to form the corona and core, respectively.32 Addition of Fe3+ and PO43− ions (in the mol ratio ∼1:0.55) into the surfactant medium (step 1) leads to the formation of FePO4 using one-half of the Fe3+; the inorganic salts attach to the hydrophilic corona of the micelle. The solution is spincoated on a glass substrate (step 2) to produce a film, which is likely to contain the assemblies of micelles shown schematically; the treatment with NH3 vapor generates FeOOH with the free Fe3+, leading to the overall formulation, FePO4· FeOOH. The film is then heated at 150 °C and subsequently washed with ethanol and water (step 3) to remove the surfactants, yielding the final mesoporous structure (FePH). The critical factor in the whole process is to control the stability of the films and protect the ferric ions from hydroxide formation. CTAB acts as a stabilizer, the aliphatic chains forming the core of the micellar ensemble, and the hydrophilic parts at the corona coordinating the metal ions, preventing their agglomeration and effectively slowing the hydrolysis.33 The fabrication process remains similar with the incorporation of cerium; however, the surface area gets enhanced and the nature of porosity changes in the final film, as demonstrated later.

Figure 2. FTIR spectra of (a) as-prepared and (b) water/ethanol washed FePH and FeCePHX films. Peak assignments are shown: A, P−O asymmetric stretch; B, CH2 bend; C, HOH bend; D, CH stretch; and E, OH stretch.

discussed later. FTIR spectra of the ethanol/water washed films (Figure 2b) are similar to those of the unwashed films, except for the disappearance of the CH2 and CH peaks, B and D; this is clear evidence for the removal of the surfactant in the final films. Henceforth, we discuss only the characterization and catalytic activity of the final meso-/microporous films shown schematically in Figure 1. The final films are submicrometer in thickness as seen from the measurements made using a surface profilometer. Interestingly, the thickness increases from 0.33 ± 0.06 μm in FePH to 0.90 ± 0.02 μm in FeCePH5 (Table 1); this would be relevant in determining the electrochemical performance. FESEM images of the cross-section of representative films (Figure S1) are consistent with these thickness measurements. The empirical formula of the materials in the films was determined using the Fe, P, and Ce composition estimated by ICP-OES analysis (Table 1); the spectroscopy studies described below and microscopy observations discussed later support the following formulations. The composition of FePH is assigned as FePO4·FeOOH. Assuming Ce3+ to be in the form of phosphate and Fe3+ as phosphate and oxyhydroxide,36 the compositions of FeCePHX (X = 1−3) are shown to be FexCe0.5−x(PO4)0.5·FeOOH(1−x); in FeCePH4 and FeCePH5, iron in FePO4 is completely replaced by cerium, with Ce(OH)3 also being formed in the latter. Notably, the estimated ratios of Fe, P, and Ce in the final films are in close agreement with the

Figure 1. Schematic of the fabrication of mesoporous FePO4·FeOOH film by surfactant-aided assembly.

FTIR analysis of the films was carried out to monitor the metal/surfactant interactions and the impact of cerium incorporation at the molecular level. The films heated at 150 °C, prior to washing in step 3 (Figure 1), showed mainly three peaks A, B, and D at ∼1030, 1380−1450, and 2800−2900 cm−1, respectively (Figure 2a); these can be assigned to the P− O stretching vibrations in the phosphate ion, and CH2 and CH bending/stretching vibrations due to the surfactant.34,35 Peaks C and E due to the OH group, found at ∼1630 and 3420 cm−1, are assignable to bending and stretching modes, respectively.35 It may be noted that the OH stretching vibration intensity increases with the cerium loading; the reason for this will be C

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Langmuir Table 1. Thickness, Composition, and Empirical Formula of the FePH and FeCePHX Films Fe:P:Ce atom ratio film FePH FeCePH1 FeCePH2 FeCePH3 FeCePH4 FeCePH5

average thickness [std dev] (μm) 0.33 0.45 0.50 0.59 0.87 0.90

[0.06] [0.09] [0.03] [0.01] [0.06] [0.02]

used in fabrication 1:0.55:0.00 1:0.55:0.17 1:0.55:0.33 1:0.55:0.50 1:0.55:0.66 1:0.55:0.83

from ICP-OES analysis 1:0.51:0.00 1:0.49:0.11 1:0.51:0.23 1:0.51:0.38 1:0.46:0.47 1:0.48:0.59

∼1:0.5:0.0 ∼1:0.5:0.1 ∼1:0.5:0.2 ∼1:0.5:0.4 ∼1:0.5:0.5 ∼1:0.5:0.6

empirical formula Fe0.5(PO4)0.5·(FeOOH)0.5 Fe0.4Ce0.1(PO4)0.5·(FeOOH)0.6 Fe0.3Ce0.2(PO4)0.5·(FeOOH)0.7 Fe0.1Ce0.4(PO4)0.5·(FeOOH)0.9 Ce0.5(PO4)0.5·(FeOOH) Ce0.5(PO4)0.5·(FeOOH)·(Ce(OH)3)0.1

X-ray diffraction of the films was recorded to check the crystallinity of the thin films. As seen in Figure 4, FePH shows a

Figure 3. (a) Raman spectra of FePH and FeCePHX films; and (b) expanded spectra highlighting the peak shifts. Peak assignments are shown: P, Fe−OH bend; Q, Fe−OH symmetric stretch; R, S, Fe−OH asymmetric stretch; T, P−O symmetric stretch; and U, Fe−OH asymmetric stretch.

ratios used in the fabrication, confirming insignificant loss of the metals or the phosphate during the surfactant removal. Raman spectra of the films are presented in Figure 3. FePH shows a weak peak in the 400−800 cm−1 region and at ∼1090 cm−1; the latter (T) can be assigned to a P−O symmetric stretching vibration.37 Ce3+ incorporation leads to stronger Raman signals; the characteristic Fe−OH bending and symmetric stretching vibrations at ∼272 and 384 cm−1 (P, Q) and the Fe−OH asymmetric stretching vibrations at ∼470, 580, and 1300 cm−1 (R, S, U) are indicative of the formation of FeOOH.38−40 The P−O symmetric stretching vibration (T) shifts to lower frequencies with increasing Ce3+ incorporation in FeCePHX films (Figure 3b, Table 2). This can be related to

Figure 4. X-ray diffraction pattern of FePH and FeCePHX films. Expected peak positions are shown: CePO4 [U (011), V (120), W (012)]; FePO4 [X (100), Y (012)]; FeOOH [Z (110)].

broad peak at 20−30°; this is likely to be due to the amorphous glass substrate. However, this broad background diminished with the cerium substitution and nearly disappeared in the diffractograms of the FeCePHX films with X > 1; this may be attributed to the increasing thickness of the films (Table 1), which reduces the interaction of the X-rays with the glass substrate. Crystallinity of the films appears to improve starting from FeCePH4; two peaks discernible at ∼28.8° and 31.2° can be indexed to the (120) and (012) planes for CePO4 (PCPDF no. 84-0247; monoclinic P21/n, No. 14). The peaks expected for FePO4 (PCPDF no. 84-0876) and FeOOH (PCPDF no. 89-6096) are also indicated in Figure 4; these are likely to be buried in the background in the case of FePH; their absence in the FeCePHX films is suggestive of the amorphous nature of these films. Morphology of the samples was examined by FESEM imaging (Figure S2). FePH showed a mesoporous structure; cerium loading led to the formation of flaky sheets on the mesoporous structure, which increased with the extent of loading. EDX-based area mapping (Figure S3) showed that the Fe, P, O, Ce composition is uniform across the structures with different morphologies, ruling out any segregation of the chemical components in the film. TEM images of FePH and FeCePH4 (which showed the best electrochemical activity in the studies discussed later) are provided in Figure 5. Partially ordered mesoporous structure is seen in FePH (Figure 5a); the EDX spectrum indicated clearly the presence of Fe, P, and O. The diffuse ring pattern in the electron diffraction and the absence of well-defined lattice structure (Figure 5b) point to the amorphous nature of FePO4· FeOOH, consistent with the XRD pattern shown in Figure 4. FeCePH4 film exhibited a different morphology (Figure 5c);

Table 2. Raman Shift of the P−O Stretch Vibration and Estimated P−O Bond Length in the Phosphate Ions Present in the FePH and FeCePHX Films film

Raman shift (cm−1)

bond length (pm)

FePH FeCePH1 FeCePH2 FeCePH3 FeCePH4 FeCePH5

1093 1015 993 962 958 954

151.0 153.1 153.7 154.6 154.7 154.8

an increase in the P−O bond length resulting from the replacement of Fe3+ by Ce3+. Table 2 lists the P−O bond lengths determined using the empirical relation:41 υ = 224500 × e−R/28.35

where υ and R are the Raman shift and bond distance, respectively. It is likely that Ce3+ with the reduced shielding effect of the 4f electrons induces stronger charge transfer from PO43− leading to decreasing P−O bond order, and increasing P−O bond length. D

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favorably oriented crystallites allowed the visualization of these lattice planes in the TEM image. Following the basic characterization of the composition and structure of the FePH and FeCePHX thin films, we have estimated their surface area as this has a direct bearing on their electrocatalytic performance. These were determined by N2 gas sorption study at 77 K; the results are presented in Figure 6a. All samples showed type IV (IUPAC nomenclature) adsorption−desorption isotherms with an enhanced slope at higher pressure; this is characteristic of a mesoporous structure.42 Gradual shrinking of the hysteresis loop in the relative pressure range 0.40−0.85 along the series is indicative of a decrease of capillary condensation of mesopores; the overlap of adsorption and desorption isotherms at relative pressures below 0.4 suggests a monolayer adsorption on the micropores in the FeCePHX films.43 The overlap region is found to expand with higher amount of gas uptake and release (during gas filling and evacuation) with increasing cerium loading; this could be associated with increasing population of micropores in the thin films. The pore size plots (Figure 6b) reveal that the range of distribution of the mesopores shrinks with increasing cerium content; this is also consistent with the morphological variation on cerium incorporation seen in the TEM images. The multipoint Brunauer−Emmett−Teller (BET) surface area estimated for the thin films increased steadily from 97 to 311 m2/g in FePH to FeCePH5. It is noteworthy that bulk ferrihydrite formed by agglomeration of smaller units via condensation of the surface hydroxyl groups generated through coordinately unsaturated sites has been shown to result in reduced surface area.21−24 In our case, we believe that the Ce3+ ions inhibited such condensations at the FeCePHX surface;21 plausible mechanistic pathways for the development of different structures in the presence and absence of cerium are outlined in Figure 7. With Fe3+ and PO43−, the FePO4·FeOOH formed on the CTAB micelle (the micelle/ metal phosphate(oxyhydroxide) structure) would have hydroxyl groups exposed on the surface. Condensation of the hydroxyl groups of neighbors in the micellar ensembles would lead to a

Figure 5. TEM images of (a,b) FePH and (c,d) FeCePH4 films; SAED and EDX spectra are provided in the insets of (a) and (c); (b) and (d) are the high-resolution images; lattice of CePO4 in FeCePH4 film is visible in the latter.

sheet-like structures possibly related to the flaky morphology observed in the FESEM are observed, and the EDX spectrum demonstrated the presence of Ce in addition to Fe, P, and O. Even though the electron diffraction does not reveal a significant improvement in the crystallinity, the high-resolution image (Figure 5d) showed a lattice with spacings in agreement with the (011) and (120) planes of CePO4; this is consistent with the formulation in Table 1. It may be noted that even though the relatively weak (011) peak of CePO4 could not be identified in the XRD pattern (Figure 4), the presence of

Figure 6. (a) N2 adsorption−desorption isotherms of FePH and FeCePHX films; the estimated surface area is indicated in each case. (b) Pore size distribution determined for each film. E

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Figure 7. Pathways leading to the mesoporous structure in the FePH and the microporous sheets in the FeCePHX films (the final structures represent materials obtained by the thermal treatment and washing as shown in Figure 1).

Figure 8. (a) Cyclic voltammograms, (b) plots of overpotential and TOF, (c) Tafel plots obtained from electrolysis at 0.01, 0.033, and 0.1 mA/cm2 current densities (Tafel slopes are also indicated), and (d) chronoamperogram at 1.9 V vs RHE, for the OER at the FePH and FeCePHX (X = 1−5) catalyst thin films, at pH = 6.4 and a scan rate of 1 mV/s.

mesoporous structure. As the process is likely to be efficient and well-ordered, the surface area of the resultant materials would be low. Introduction of Ce3+ with an ionic radius and hardness very different from that of Fe3+ causes mismatch and disorder in the aggregation process,23,24 making the hydroxyl condensation process inefficient leading to enrichment of micropores in the film. The enhanced intensity of the OH stretch vibration peak in the FTIR spectra of the FeCePHX films as compared to the FePH film (Figure 2) is consistent with this scenario. Introduction of increasing amount of Ce3+

leads to a collapse of micelle assembly and development of sheet-like structures; the net result is the higher than 3-fold increase of surface area from FePH to FeCePH5. The simple strategy that we have developed for the fabrication of the FePH and FeCePHX thin films allowed a systematic variation of the materials’ characteristics including the electronic structure, crystallinity, morphology, and surface area, through the heteroatom incorporation; this provides a novel approach to tuning the electrocatalytic activity. We have explored the impact of cerium incorporation on the electroF

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Table 3. Overpotential and TOF (Also Normalized by the Surface Area) of the FePH and FeCePHX Catalyst Films in Water Oxidationa scan rate = 1 mV/s

scan rate = 5 mV/s

catalyst film

η (mV) (j = 1 mA/cm2)

TOF (h ) at 707 mV

normalized TOF (h−1 g m−2)

η (mV) (j = 1 mA/cm2)

TOF (h−1) at 690 mV

normalized TOF (h−1 g m−2)

FePH FeCePH1 FeCePH2 FeCePH3 FeCePH4 FeCePH5

880 812 755 735 707 727

6.1 12.6 18.0 26.3 32.7 29.9

0.063 0.108 0.113 0.099 0.107 0.096

806 755 721 711 689 691

4.6 12.2 18.7 25.2 33.8 34.5

0.047 0.104 0.116 0.094 0.110 0.110

a

−1

Data for different scan rates are provided.

Figure 9. (a) Experimental (■) and theoretically expected (red line) oxygen evolution using FeCePH4 thin film catalyst; (b) plot of cathodic (black) and anodic (red) charging currents at 1.17 V vs RHE versus scan rate in the non-Faradaic potential region (1.12−1.22 V RHE), and (c) the Nyquist plot for the FePH and FeCePHX catalyst films in water oxidation.

catalytic water oxidation (specifically the OER) activity of iron phosphate(oxyhydroxide). Keeping in view the practical relevance, the electrocatalysis was conducted in near neutral conditions. Figure 8a shows the cyclic voltammograms for water oxidation at the FePH and FeCePHX catalyst thin film anodes formed on FTO-coated glass. As the cerium content is increased, the onset potential is found to decrease, the values changing from ∼1.80 to 1.65 V vs RHE with FePH to FeCePH5. Variation of the catalyst efficiency across the series, expressed in terms of the overpotential (η) and TOF, is shown in Figure 8b; a control experiment with CePO4 thin film (fabricated following protocol similar to that used for the catalyst films) showed negligible OER (Figure S5). The complete electrochemical results for the thin film catalysts are

summarized in Table 3 (data for two different scan rates are provided). Significant decrease of η and increase of TOF along the series illustrate clearly the importance of compositional tuning in the thin films in improving the electrocatalytic performance. η and TOF are found to change from 880 to 707 mV and 6.1 to 32.7 h−1, respectively, from FePH to FeCePH4 thin films; a minor increase in η and decrease in TOF are observed with FeCePH5. TOF value normalized by the respective surface areas increases from FePH to FeCePH1 as expected, but remains nearly similar in the rest (Table 3); the decline at FeCePH5 is less prominent. It is clear that the surface area plays a dominant role in enhancing the catalytic activity; it may also be noted that thickness variation of the films has little impact. Tafel plots for the OER activity of the G

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Figure 10. XPS of (a) FePH and (b,c) FeCePH4 films before (1) and after (2) electrocatalysis of water oxidation; and (d) schematic of the transformation of Fe4+ to Fe3+ in the presence of Ce3+.

catalyst films (Figure 8c) determined from electrolysis experiments (Figure S6) show a gradual decrease of the slope with increasing cerium content; the lowest value of FeCePH4 once again confirms it to be the best catalyst in the series. Stability of the thin film catalysts was checked by recording the current density at 1.9 V vs RHE against time; the chronoamperometry curves in Figure 8d show that even though the current density declines in the initial period, it shows fairly good stability afterward. Oxygen evolution as a function of time is shown in Figure 9a; on the basis of the experimental values, and the theoretical estimates, the Faradaic yield is estimated to be 94%. The maximum electrocatalytic efficiency of the FeCePHX films obtained at an optimum cerium content suggests competing influences of the heteroatoms. Cerium promotes the surface area of the films as shown earlier. Further, a consideration of the oxidation potentials of Ce3+/Ce4+ (1.81 V vs RHE)25,26 relative to that of Fe3+/Fe4+ (1.0−1.44 V vs RHE)44 suggests that Ce3+ may also help in the recovery of the active state of the iron during the catalytic cycle. These factors would improve the electrocatalytic performance of the thin films. On the other hand, increasing cerium incorporation is accompanied by a steady decrease in the Fe3+ concentration that eventually leads to a situation wherein the catalytic activity is adversely affected; our data suggest that this happens when the Fe:Ce ratio falls below 1:0.5 as in FeCePH5. To rationalize the improved activity with cerium loading, double layer capacitance (CDL) measurements and EIS spectroscopy (Nyquist plot) were carried out using the thin film catalysts (Figure 9b,c) (see the Experimental Section for details). The CDL is found to increase with increasing cerium content; the

Nyquist plots indicate concomitant decrease of charge transfer resistance. These factors support increasing catalytic activity from FePH to FeCePH5; the slight decrease observed with FeCePH5 possibly arises due to the overwhelming effect of CDL that results in strong attachment of the reactant species impeding mass transfer. To probe the electrochemical impact of the cerium incorporation, we have carried out an XPS analysis of FePH and FeCePH4 films before and after electrocatalysis; the respective spectra are designated as 1 and 2 in Figure 10. Deconvolutions of the spectra are indicated; the peaks due to iron have been fitted using multiplets and surface peaks following the protocols reported earlier.45−48 In the case of fresh FePH film, the mean peak positions can be assigned to Fe3+ at 711.9 eV (2p3/2) and 725.5 eV (2p1/2); in the film after catalysis, presence of Fe4+ is clearly indicated by the peaks at 712.9 eV (2p3/2) and 726.9 eV (2p1/2).49−52 As noted in earlier reports, the metal in the higher oxidation state is indicative of the catalyst trapped in the film matrix in an inactive state, unable to complete the water oxidation process,27−29 resulting in a significant decrease in the catalytic efficiency. Addition of cerium appears to change this situation for the better. Figure 10b shows that the mean position of the peaks due to Fe3+ occurs at 711.4 eV (2p3/2) and 724.5 eV (2p1/2) in the fresh FeCePH4 film and only a very small shift is observed after catalysis, the peaks being found at 711.8 eV (2p3/2) and 725.1 eV (2p1/2). This suggests that in the presence of cerium, iron is found mostly in the 3+ (or 3 + x, where x < 1) state even after the OER. XPS spectra of FeCePH4 exhibit peaks of Ce3+ at 881.8, 884.6, and 886.5 eV (3d5/2) and 900.7, 903.0, and 905.0 eV (3d3/2);53,54 following catalysis, several of these peaks H

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Langmuir disappear, and the remaining peaks at 885.1 eV (3d5/2) and 903.0, 906.5 eV (3d3/2) are assignable to Ce4+ (Figure 10c).54,55 Table S1 summarizes the relevant XPS peak positions reported in earlier studies for comparison with the current findings. It may be noted that XPS analysis of the thin film after electrolysis at 1.9 V vs RHE (Figure S8) shows changes consistent with those in Figure 10. The XPS data point to the likely scenario of the Ce3+/Ce4+ couple with the higher oxidation potential reducing the Fe4+ trapped during the water oxidation and recovering the active state, Fe3+ (Figure 10d), facilitating more efficient catalytic water oxidation. Representative samples of FePH and FeCePH4 films after a catalytic run were imaged using TEM (with EDX spectral analysis) (Figure S9). Slight morphological changes are observed; the relative composition of Fe, P, and Ce remains nearly the same as in the original samples (Table 1), indicating the stability of the thin film catalysts. The decline in the initial electrocatalytic activity seen in the chronoamperometry curves in Figure 8d may be related to the morphological changes in the thin films. A comparison of our thin film catalyst, FeCePH4, with the transition metal-based water oxidation catalysts operating in the near neutral range for which electrochemical data are available in the literature is provided in Table S2. Even though a few excellent electrocatalysts for OER based on cobalt have been reported,56,57 the overpotential and TOF values of our catalyst are better than many in the literature. However, it is relatively poorer than some of the iron-based ones such as those reported by Cao et al.45,58 The latter are prepared by electrodeposition, and are only a few atomic layers thick; this is an important factor that contributes to their efficiency. Soft chemical methods like what we have used lead to films that are typically micrometers in thickness, and rarely attain such activities.10,38,59 However, the main message of our current work is that incorporation of heteroatoms could prove to be a viable and systematic strategy to improve the material characteristics and electrocatalytic performance of these materials, and the simple fabrication methodology involving a micelle-mediated assembly and spin-coating with mild thermal treatment could prove to be a valuable alternative for electrodeposition and similar approaches.

transition metal catalysts by incorporating heteroatoms such as rare earth metals.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b01647. Microscopy images, electrochemical and XPS data, and comparison with data from earlier reports (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Phone: 91-40-2313-4827. Fax: 91-40-2301-2460. E-mail: [email protected]. *Phone: 91-40-2313-4827. Fax: 91-40-2301-2460. E-mail: tpr@ uohyd.ac.in. ORCID

T. P. Radhakrishnan: 0000-0002-0318-4461 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.S. thanks the UGC, New Delhi, for a D. S. Kothari postdoctoral fellowship. Financial support from the DST, New Delhi (J. C. Bose Fellowship), and infrastructure support from the DST, New Delhi (under the PURSE and FIST programs), the UGC, New Delhi (under the CAS and UPE programs), as well as the Central Facility for Nanotechnology at the University of Hyderabad, are acknowledged with gratitude. We thank the Centre of Excellence on Surface Science at CSIR-National Chemical Laboratory, Pune, for the XPS data. We thank also Mr. Priyajit Jash and Dr. Amit Paul (Indian Institute of Science Education and Research − Bhopal) for the oxygen quantification experiment.

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REFERENCES

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CONCLUSION A facile, soft chemical approach to fabricate thin film electrocatalysts for the OER is illustrated; surfactant-mediated assembly, spin-coating, and mild thermal annealing form the key steps. The method is employed to prepare FePO4·FeOOH thin film with increasing content of Ce3+; the specific combination of iron and cerium is chosen not only due to their status as the most abundant among the transition and rare earth metals, but also the unique combination of the characteristics of their ions. The impact of tuning the Fe:Ce composition on the electronic, electrochemical, and morphological characteristics has a direct bearing on the electrocatalytic efficiency of the thin films; this is explained by a combination of factors including the systematic variation in the effective surface area and the coupling of the metal redox systems. It is argued that the cerium incorporation helps in recovering the active state of the iron catalyst, which otherwise gets trapped in a higher oxidation state during the catalytic process. The current work, while presenting a simple methodology for fabricating electrocatalysts for the OER, proposes a systematic design principle for tuning the catalytic efficiency of cost-effective I

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