Intercalation of Li+ into a Co-Containing Steel-Ceramic Composite

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Intercalation of Li+ into a Co-containing Steel-Ceramic Composite: Substantial Oxygen Evolution at almost Zero Overpotential Helmut Schäfer, Karsten Kuepper, Jonas Koppe, Philipp Selter, Martin Steinhart, Michael Ryan Hansen, and Diemo Daum ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b03566 • Publication Date (Web): 15 Oct 2018 Downloaded from http://pubs.acs.org on October 16, 2018

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Intercalation of Li+ into a Co-containing Steel-Ceramic Composite: Substantial Oxygen Evolution at almost Zero Overpotential Helmut Schäfera*, Karsten Kuepperb, Jonas Koppec, Philipp Selterc, Martin Steinharta, Michael Ryan Hansenc* and Diemo Daumd aInstitute of

Chemistry of New Materials and Center of Physics and Chemistry of New Materials, Universität Osnabrück, Barbarastrasse 7, D-49076 Osnabrück, Germany bDepartment cInstitute dFaculty

of Physics, Universität Osnabrück, Barbarastraße 7, D-49069 Osnabrück, Germany

of Physical Chemistry, Westfälische Wilhelms-Universität Münster, Corrensstr. 28/30, D-48149 Münster, Germany

of Agricultural Science and Landscape Architecture, Laboratory of Plant Nutrition and Chemistry, Osnabrück University

of Applied Sciences, Am Krümpel 31, D-49090 Osnabrück, Germany

*Correspondence to: [email protected], [email protected]

The exploration of promising renewable energy sources for the future is likely the most significant challenge for humanity. Hydrogen is considered to play a major role in the urgently required reorganization of our current energy sector. Water can be split into hydrogen and oxygen and therefore presents an in principle inexhaustible and environmentally friendly hydrogen source. However, electrochemical approaches for the cleavage of H 2O remain challenging, especially considering that the experimentally required potential at which oxygen evolves is substantially higher than the theoretically required potential. This results in significant overpotentials () on the anode side which limits the widespread applicability of this technique. Here, we have applied a two-step activation procedure of a Co-containing steel, which led to a significant reduction of  for the oxygen evolution reaction (OER) down to almost zero. The enhanced electrochemical behavior comes as a result of Li-ion doping, which leads to Li intercalation into the Co3O4 containing surface layer of the steel-ceramic composite material. Thus, our results indicate that additional metal doping and resulting surface modification is a promising strategy for achieving substantial OER at pH-neutral conditions close to the thermodynamic limit. KEYWORDS: oxygen evolution electrocatalysis, steel, surface oxidation, Li + intercalation, zero overpotential behaviour 1 ACS Paragon Plus Environment

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1. Introduction The replacement of limited available fossil fuels by renewable ones presents the urgently required reorganization of our current energy sector and is one of the most significant challenges for scientists and engineers

[1, 2, 3]

. Hydrogen may play an active role as a clean energy source in the future, where in

particular the electro-catalytic splitting of water, utilizing solar energy represents a viable route on the way to the implementation of a hydrogen economy [1, 4, 5, 6, 7, 8, 9, 10, 11,

12, 13, 14, 15, 16 ]

. Although hydrogen is

without any doubt the more valuable product of the catalytic splitting, the half-cell reaction leading to oxygen, known as oxygen evolution reaction (OER), is, due to sluggish kinetics, the critical step of the electrocatalytically initiated cleavage of the bonds in a H 2O molecule. It is the main source for the experimentally encountered overpotentials () occurring during water electrolysis, which is particularly true when the electrochemical cleavage is carried out at pH 7. While non-noble electrocatalysts operate with high efficiency at pH 13- 14 with overpotentials in the range of 200-300 mV at 10 mA cm-2 current density [17, 18, 19, 20], a similar performance has not observed for electrocatalytically initiated OER at pH 7 until very recently [21]. In 2016, we introduced a surface-activated steel-ceramic composite based on a hot work tool steel (X20CoCrWMo10-9/Co3O4) [21], which showed an unusual high OER efficiency at pH 7 (= 298 mV at 10 mA/cm2). A comparison of well-known and very recently developed electrocatalysts is summarized in Figure 1, illustrating the recent advances in OER efficiency at pH-neutral conditions [21, 22, 23, 24, 25, 26, 27, 28]

.


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Figure 1. Advances in OER efficiency using various electrocatalysts in a neutral (pH = 7) environment. (Steel 235 [10]; X20CoCrWMo110-9/Co3O4 [21]; IrO2-RuO2 [21]; Steel 304 [22], Co-Pi [23]; Co3O4 Nanow. [24]; Co-Pi Nano [25]; Co2P [26]; NiBorate [27]; RuO2 [28]; *This work). A part of the Figure was adapted from ref 21 with permission from Royal Society of Chemistry.

The activation of the X20CoCrWMo10-9 steel (resulting in a steel-ceramic composite) was achieved via surface modification and resulted in a monometallic cobalt oxide periphery, suggesting that Co ions act as the catalytically active centers [21]. It is well known that the OER activity of transition metal oxide based OER catalysts can be significantly increased upon doping with additional metal ions [29, 30]. Thus, forming the basis for this new contribution, we decided to take our X20CoCrWMo10-9/Co3O4 steel-ceramic composite material, sample Co-300

[21]

, as the “point of departure” for a further increment of the OER

performance upon doping with additional metal ions. To this end, we decided to introduce Li-ion doping as the second activation scenario for X20CoCrWMo10-9 steel, which is expected to result in an intercalation of Li+ ions into the Co3O4 scaffold (received after first activation) and likely be accompanied by a change of oxidation state for the Co x+ ions. Li+ ions have been chosen as a doping agent for several reasons. It is known that Li+ intercalation can enhance the conductivity of transition metal oxide-based ceramics significantly [31], leading to a reduction of the voltage drop across the catalytic active outer oxide zone. Moreover, the OER activity of a Co-based electrocatalyst is strongly related to the Co oxidation state, e.g. the eg states for the catalytically active Co center plus the covalency of the Co-O bond [32]. Both factors are known to critically depend on Li+ insertion [32]. It is further reasonable to assume that smaller metal 4 ACS Paragon Plus Environment

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ions can be more easily intercalated, which should ensure only minor structural deformation of the host lattice. 2. Methods Synthesis of the samples Co-300 and Co-300/Li. Samples Co-300 samples preparation is described in our previous work (Ref. 21). Unlike in our previous work, the distance between WE and CE was adjusted to 35 mm and the current was set to 8 A. As a consequence, the voltage amounted to ~9.5 V at the beginning of the experiment and was found to be reduced to ~ 5.3 V after 300 min. Freshly prepared samples Co-300 were stored for 48 h under air at room temperature before lithiation. Subsequently, samples of Co-300 were lithiated in an inert gas (Ar) glove box (M. Braun Intertgas Systeme, Garching, Germany) that ensures high gas purity (< 0.1 ppm O2 and < 0.1 ppm H2O). A two-electrode setup based on a steel counter electrode and a representative of sample series Co-300 as cathode was introduced in the glove box. The counter electrode was prepared from X20CoCrWMo 10-9 steel with dimensions of 70x10x1.5 mm. Prior to each usage, the metal used for preparing the counter electrode was cleaned intensively with ethanol and polished with grit 400 SiC sanding paper. Afterwards, the surface was rinsed thoroughly with water and dried under air for 50 min. 25 mL of a 1 M LiPF6 in a 1:1 mixture of ethylencarbonate (EC) and dimethylcarbonate (DMC) purchased from Solvionic (Toulouse, France) were filled into a 50 mL glass beaker. The counter electrode was immersed to a depth of 2.1 cm and the cathode was situated 15 mm deep into the electrolyte. The voltage of the DC power source (Voltcraft VLP 2403) was adjusted to 2.5 V, the current limitation was set to 100 mA. Stirring of the electrolyte was ensured by a magnetic stirrer with a stirrer bar (8x2 mm) at 150 r/min. The cathode and the counter electrode were connected to the negative pole, positive pole of the power source, respectively. The current was controlled by a high-precision power meter (Fluke 28II). At the beginning of the experiment, the current amounted to approximately 10 mA and was found to be reduced to less than 150 µA within ~ 5 min. After 22 min the current dropped down to ~30 µA and the procedure was stopped. Subsequently, after turning off the power source, the specimen was taken out of the 5 ACS Paragon Plus Environment

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electrolyte and the excess LiPF6 solution was removed with a tissue paper. The sample was taken out of the glove box and was washed once with ethanol and once with water. The specimen was dried under air for 24 hours and the weight was determined using a high-precision balance (Sartorius 1712, 0.01 mg accuracy) prior to OER-based electrocatalysis. The lithiation procedure was repeated 126 times, i.e. 127 Co-300/Li samples have been prepared in total (sample list: Table S1). Table S1 summarizes in detail which sample or samples has/have been used for what kind of measurement .Briefly, the powder from a total of 67 samples (obtained upon scratching the oxide layer from the surface) has been used for carrying out the solid-state NMR/EPR- and FTIR spectroscopic investigations (30 samples; sample series Co-300/Li-1 till Co300/Li-30), for performing XPS experiments (20 samples; sample series Co-300/Li-31 till Co-300/Li-50) and for the determination of the composition of the surface for Co-300/Li via ICP OES (17 samples; sample series Co-300/Li-51 till Co-300/Li-67). Fifty-five samples have been used for electrochemical testing (sample series Co-300/Li-71 till Co-300/Li-125, see Table S1 and S2). All samples with the exception of Co300/Li-125 have been used for CV measurements (1 scan) and (afterwards) for chronopotentiometry (CP) measurements (6000 s at j=10 mA/cm2). However, samples Co-300/Li-71 to Co-300/Li-90 have additionally been used for the determination of the Tafel slope carried out between CV and CP measurements. The surface layer for 20 of the electrochemically-pre-tested (6000 s of CP) samples (Co-300/Li-71 to Co-300/Li90) has been removed and the resulting powders have been used for FTIR experiments and ICP OES

investigations (follow-up experiments after electrochemical checking). Electrochemical Measurements A three-electrode set-up was used for all electrochemical measurements. Samples Co-300, Co-300/Li and Ir/Ru (IrO2-RuO2) were used as working electrodes (WE). X20CoCrWMo10-9 steel was purchased from WST Werkzeug Stahl Center GmbH & Co. KG, D-90587 Veitsbronn-Siegelsdorf, Germany. IrO2-RuO2 (sample Ir/Ru) (10 micrometer layer deposited on titanium) with a total geometry of 100x100x1.5 mm was purchased from Baoji Changli Special Metal Co, Baoji, China. To avoid additional contact resistance the 6 ACS Paragon Plus Environment

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plate was electrically connected via a screw. A platinum wire electrode (4x5 cm geometric area) was employed as the counter electrode (CE), a reversible hydrogen reference electrode (RHE, HydroFlex, Gaskatel Gesellschaft für Gassysteme durch Katalyse und Elektrochemie mbH. D-34127 Kassel, Germany) was utilized as the reference standard, therefore all voltages are quoted against this reference electrode (RE). For all measurements the RE was placed between the working electrode and the CE. The measurements were performed in a pH 7 corrected 0.1 M KH2PO4/K2HPO4, solution which was prepared as follows: Aqueous solutions of 0.1 M K2HPO4 and KH2PO4 (VWR, Darmstadt, Germany) were mixed until the resulting solution reached a pH value of 7.0. Measurements were performed at room temperature (295.15 K). The distance between the WE and the RE was adjusted to 1 mm and the distance between the RE and the CE was adjusted to 4-5 mm. All electrochemical data were recorded digitally using a Potentiostat Interface 1000 from Gamry Instruments (Warminster, PA 18974, USA), which was interfaced to a personal computer. All electrochemical measurements were carried out without any correction of the voltage drop unless otherwise stated. Voltage drop compensation (IR compensation) has been applied to the chronopotentiometry (CP) measurements, for which some of the Tafel slope measurements are based on. The IR compensation (50%) is based on a solution resistance of 3.2 Ω derived from the electronic impedance spectroscopy experiments carried out in the lower overpotential region (1.24 V vs. RHE). Cyclic Voltammograms (CV) were recorded in 90 mL of 0.1 M KH2PO4/K2HPO4 in a 100 mL glass beaker under stirring (450 r/min) using a magnetic stirrer (21 mm stirring bar). The scan rate was set to 20 mV/s and the step size was 2 mV for the OER related measurements. Chronopotentiometry scans were conducted at a constant current density of 5 respectively of 10 mA/cm2 in 90 mL of electrolyte for measuring periods < 2000 s, in 800 mL of electrolyte for measuring periods ≥ 20000 s in a 100 mL, respectively 1000 mL glass beaker. The scans were recorded under stirring (450 r/min) using a magnetic stirrer (25 mm stirring bar) for measuring periods < 2000 s, using a magnetic stirrer (40 mm stirring bar) for measuring periods ≥ 20000 s respectively.


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Tafel plots Average voltage values for the Tafel plots were derived from 200 second chronopotentiometry scans at constant current densities of 0.5, 1, 2, 2.5, 3, 4, 5, 6, 8 and 10 mA/cm2 (sample Ir/Ru), at 0.66, 1.33, 2, 2.67, 4, 5.33, 6.67, 8, 10, 13.33, 16.67 and 20 mA/cm 2 (sample Co-300) and at 1, 2, 3, 5, 8, 10, 15, 20, 30 and 40 mA/cm2 (sample Co-300/Li) in pH 7 corrected 0.1 M KH2PO4/K2HPO4. The arrangement of RE, WE and CE (taken for recording the chronopontentiometry plots) was identical to the one described above (See paragraph Electrochemical measurements). Determination of Faradaic efficiency (FE) for OER was carried out in close accordance with the procedure described in Ref. 17. Faradaic efficiency of OER was calculated by determining the dependence of the oxygen concentration in the electrolyte during the time of chronopotentiometry at constant current of 10 mA/cm2 in pH 7 corrected 0.1 M K2HPO4/KH2PO4 solution under stirring. The electrode area was 2 cm2. The distance between RE and WE was adjusted to 1 mm and the distance between RE and CE was adjusted to 4-5 mm. The volume of electrolyte was 2210 mL. The working compartment was completely sealed with glass stoppers before starting the chronopotentiometry at 0.08 mg O2/l. Line equation: y=0.00075x + 0.08 with y=Dissolved oxygen (mg/l); x=time (s). Impedance spectroscopy Impedance spectroscopy of samples Co-300 and Co-300/Li was conducted under stirring at pH 7 within frequency range 0.1-50469 Hz with an Autolab PGStat 20 potentiostat, controlled by FRA Windows software (Frequency Response Analysis for Windows version 4.9.007). To ensure more accurate results five measurements were made for each sample at defined potential. The reported results in this paper are average ones of the five derived from each sample. The preparation of the electrolyte as well as the electrode geometry can be taken from below (Electrochemical Measurements). 7

Li MAS-,

59

Co MAS NMR-Spectroscopy and continuous wave (CW) X-band EPR-Spectroscopy.

All solid-state NMR experiments were performed on a Bruker DSX 500 spectrometer operating at a static magnetic field of 11.7 T, corresponding to Larmor frequencies of 118.7 MHz and 195.4 MHz for 59Co and 8 ACS Paragon Plus Environment

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7

Li, respectively. All experiments employed a 2.5 mm triple-resonance HXY magic-angle spinning (MAS)

probe from Bruker using a spinning frequency of 25.0 kHz. It turned out that the very thin oxidic layer which was found to be almost transparent can be removed from the surface modified steel by using a fresh scalpel. Around 100 µg of the oxidic periphery was removed from about 4 cm 2 sample area, i.e. was removed from each sample. Thus, in total 3 mg substance was achieved from 30 samples. Transfer of the oxidic material to the vessel was realized by moving the scalpel on which the layer still sticks directly to the neck of the small glass vessel. To suppress any eddy currents due to the metallic nature of the steel material, the sample was diluted to a ratio of 1:2 by weight using CaSO4 and finally loaded into a 2.5 mm o. d. ZrO2 MAS rotor sealed with Vespel endcaps. The 7Li MAS NMR employed single-pulse excitation using an r.f. field strength of 62.5 kHz. The excitation pulse length was set to 1 µs corresponding to a flip angle of 22.5° for the 1 M LiCl solution. A recycle delay of 1 s was employed and a total of 360 268 transients were accumulated over total of 100 hours. The receiver dead time was set to one full rotor period (40 µs) to avoid distortions of the baseline due to the dead time of the receiver. Referencing and pulse length calibration was performed using 1 M LiCl as an external reference. Processing was done using the Bruker Topspin software package and the analysis of the chemical shift anisotropy (CSA) and quadrupole interaction parameters was performed using the dmFit and SIMPSON simulation packages as described below. The 59Co MAS NMR experiments were performed at the same spectrometer and magnetic-field strength. A 0.1M K3[Co(CN)6] solution was used for 59Co chemical shift referencing and for pulse length calibration. A series of 59Co MAS NMR spectra were acquired in 2000 ppm steps from +60 000 ppm down to -60 000 ppm using a spectral width of 1 MHz for every sub spectrum. A pulse length 1 µs (corresponding to 22.5° flip angle in solution, maximizing the signal of the central transition) and a recycle delay of 1 second was used. The solid-state continues-wave (CW) X-band ((9.80 GHz at 5 K) and (9.66 GHz at 77 K)) EPR experiments were performed using a Bruker E580 EPR and Bruker EMX-Nano. For the 5 K measurements, the sweep width was set to 6000 G with a center field of 3100 G; the sweep time was 9 ACS Paragon Plus Environment

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167.8 s. The microwave power was varied between 0.17 mW and 0.64 mW. A modulation amplitude of 5 G was used in combination with a modulation frequency of 100 kHz. A maximum of 100 scans were accumulated. For the 77 K measurements, the sweep width was set to 4000 G with a center field of 2500 G; the sweep time was 200 s. The microwave power was 0.32 mW. A modulation amplitude of 4 G was used in combination with a modulation frequency of 100 kHz. A single scan was performed. NMR spectral analysis and deconvolution procedures As a first attempt, the 7Li MAS NMR spectrum was fitted by employing the dmFit software [33] using the “quad 1st” model for simultaneous treatment of the first-order quadrupolar interaction and the chemical shift anisotropy (CSA), which is mathematically equivalent to the electron-nuclear interaction, allowing us to employ this model

[34]

. The fitting allowed peak position, amplitude, width, as well as the CSA and

quadrupolar parameters to be varied. Additionally, the Euler angle beta was included as a free parameter to account for the differences in orientation of the two interaction tensors. The angles alpha and gamma were fixed in order to speed up calculations. Both a single-component model and a two-species model were tested with the two-component fit giving the best overall results. The final values for all of the above-mentioned parameters from dmFit were then used as starting points for the SIMPSON simulations [34, 35]. These simulations took into account the quadrupolar interaction (CQ and Q), shift anisotropy (aniso and aniso), intensity, linewidth, and the Euler angle beta (angle between the main axis of the shift and quadrupolar tensor) for each component. In addition, the excitation profile of the finite rectangular pulse as well as the probe bandwidth (Q factor) were accounted for in the simulations. Furthermore, to increase the accuracy of the fit, only the spinning sidebands (intensity and lineshape corresponding to the integral over each spinning sideband) were included in the fit and the center band (isotropic signal) was excluded from the RMS calculations. The optimization was performed via the “opt” Tcl package for SIMPSON using the simplex algorithm and the 95% confidence intervals were estimated by varying one parameter, while optimizing the others and observing the influence on the lowest possible RMS value. This was done for the quadrupolar coupling parameters as well as the shift 10 ACS Paragon Plus Environment

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anisotropy parameters. The SIMPSON input files are available upon request from the authors via e-mail. The two-component model resulted in the best fit of the 7Li MAS NMR spectrum, i.e., the lowest residual mean square, RMS, as summarized in Table S3 and Figure S1. The two components appear to be present in a 1:2 ratio although with large errors determined on the basis of 95% confidence intervals, see Table S3). We note that the errors determined on the basis 95% confidence intervals are in general fairly large, which is a result of the fact that the quadrupolar couplings and shift anisotropies are of similar magnitude. 59

Co MAS NMR

Although we searched carefully over a large 59Co chemical shift range (+60 000 ppm to -60 000 ppm) we could not detect any resonance attributable to 59Co. The only signal detected was a broad 13C resonance from the rotor endcaps (Vespel) at the expected frequency, corresponding to a +60 000 ppm shift for 59Co. The fact that no 59Co NMR signal could be detect is likely due to the strong paramagnetic influence of the unpaired electrons, leading to an undetectable and broad NMR signal due to extremely short T2 relaxation. Alternatively, the Fermi contact shift and Knight shift contributions could be beyond 60 000 ppm, which is very unlikely when compared to for example Co3O4 [36, 37]. XPS spectroscopy measurements XPS measurements were performed using a PHI 5600ci multitechnique spectrometer equipped with a monochromatic Al Kα source with 0.3 eV full width at half-maximum. The overall resolution of the spectrometer is 1.5% of the pass energy of the analyser, 0.45 eV in the present case. The measurements were recorded with the sample at room temperature. No argon etching was applied to the samples. Around 100 µg of the oxidic periphery was removed from about 4 cm2 sample area, i.e. was removed from each sample. Thus, in total 2 mg substance was achieved from 20 samples (Co-300/Li-31 to Co300/Li-50) and was found to be sufficient for conducting the XPS experiments. 3. Results and Discussion We utilized untreated X20CoCrWMo10-9 steel instead of Li-metal as the anode, because Li + intercalation into the X20CoCrWMo10-9/Co3O4 cathode based on utilizing of LiPF6 as both conducting 11 ACS Paragon Plus Environment

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electrolyte and Li+ source turned out to be easier controllable. This has resulted in an optimized standard protocol (see supplementary information), where our “point of departure” material X20CoCrWMo109/Co3O4 (Co-300) was converted into an outstandingly active OER electrocatalyst, henceforth referred to as Co-300/Li. Electrochemical properties The new Co-300/Li material substantially outperforms all other materials compared to what we and other groups have reported in earlier contributions with respect to the voltage-current behavior as demonstrated in Figures 2a-2e. The outstanding activity of sample Co-300/Li can be taken from the very close distance of the corresponding chronopotentiometry curve (Figure 2a, blue curve) to the standard Nernst potential line (Figure 2a, red dotted line) at 1.228 V vs. a reference hydrogen electrode (RHE, = 0 mV) during the first 4000 s of the measurement exhibiting an overpotential between 0 and 75 mV. Intensive and stable oxygen formation, clearly visible on the surface of representatives for the sample series Co-300/Li was obtained e.g. during CP experiments (Figure 2a). The potential determined for sample Co-300/Li required to ensure 10 mA/cm2 current density was found to be reduced by around 200 mV throughout the chronopotentiometry measurement when compared to sample Co-300 (Figure 2a: black and blue curve). Noteworthy is also that anodic waves in cyclic voltammetry (CV) diagrams that are typically assigned to the quasi reversible redox couple CoO2/CoO(OH) [38], does not exist in the case of Co300/Li (Figure 2d: blue curve). The existing current seen in the CV scan at zero overpotential (Figure 2d, blue curve) likely has its origin in charging effects or pre-oxidation of the catalyst itself as it disappears in the corresponding chronoamperometry scan when the potential is hold at 1.228 V vs. RHE (Figure S2). In addition to steady-state electrocatalysis at a current density of 10 mA/cm2 (Figure 2a), we applied also OER-based chronopotentiometry at 5 mA/cm2 to a series of freshly prepared Co-300/Li samples (Figure 2e). This series constituted 125 specimens of Co-300/Li (see Experimental section) and about 50% have been solely used for electrochemical purposes. The electrochemically tested samples basically manifested the unusual voltage-current characteristics shown in Figures 2a-2d (See Table S2 for details). A selection 12 ACS Paragon Plus Environment

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of additional chronopotentiometry plots can be taken from Figure S3. Notably, an OER characteristic shared by all tested samples is the decrease in performance after around 6000 s of chronopotentiometry performed at 10 mA/cm2 current density-, reaching almost the OER efficiency of non-lithiated samples Co300. To shed more light on the OER performance of representatives of the sample series Co-300/Li, we have performed CV measurements carried out after intensive usage as an OER catalyst (6000 s at 10 mA/cm2, see Figure S4). Indeed, also the non-steady state OER performance dropped down to (almost) Co-300 level (Figure S4). The averaged chronopotentiometry curve course based on 53 representatives of the sample series Co-300/Li can be taken from Figure 2b (black curve; standard error bars in red) exhibiting an (averaged) overpotential of around 40 mV. Figure S5 shows all chronopotentiometry based data points derived from the 53 samples. Tafel slope values have been derived from chronopotentiometry data of twenty (as-prepared) representatives of the sample series Co-300/Li. The average Tafel slope amounted to 33 mV/dec (17 mV/dec with IR compensation) in the lower overpotential region and 181 mV (17 mV/dec with IR compensation) in the higher overpotential region (Table S2, Figure S6), which is significantly lower than that of both Co-300 (140.8 mV/dec without IR compensation) and IrO2/RuO2 (sample Ir/Ru:225.8 mV/dec without IR compensation; Figure 2c). Dual Tafel behavior was found for Co-300, Ir/Ru and Co300/Li samples when no IR correction was applied (Figure 2c). However, even weak compensation of the voltage drop (see experimental part) resulted in single Tafel behavior (Figure S7) of sample Co-300/Li. The onset of oxygen evolution near the surface of Co-300/Li can already be seen at the lower limit of the voltage range, whilst recording CVs (≥ 1.22 V vs. RHE). Moreover, an adequate charge to oxygen conversion rate (88.7% after 1200 s) whilst chronopotentiometry measurement (j= 10 mA/cm2) was quantitatively determined by direct fluorescence-based sensing of the evolved oxygen (Figure 2f, black dotted curve). To further confirm that the detected oxygen has indeed been produced by the electrochemical water-splitting half reaction, we additionally monitored the dissolved oxygen in the electrolyte upon immersing a freshly synthesized Co-300/Li sample under current less condition (Figure 2f, blue dotted curve). No significant increase of the content of dissolved oxygen was detected within a measuring period of 1200 s (Figure 2f, 13 ACS Paragon Plus Environment

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blue dotted curve), suggesting that neither decomposition of Co-300/Li with release of oxygen takes place, nor does oxygen flow from the external environment into the measuring cell. We would like to emphasize that a FE lower than 100% does not question the low overpotential derived from chronopotentiometry measurements (around 40 mV through the curve course seen in Figure 2a). In fact, a FE lower than 100% is as such not unusual. For instance, Chemelewski et al. noticed that the FE for the OER upon electrodeposited FeOOH even after a relatively short measuring period (200 s) showed a substantial deviation from 100%

[39]

. This measurement (like ours) is based on a chronopotentiometry

experiment carried out at relatively high current density (j= 10 mA/cm2). Reduced FEs at higher current densities might to some extent be ascribed to a slight measurement inaccuracy due to intensive O2 bubble formation. An example of this behavior was recently reported by Qiu et al. who obtained a significant reduction of the FE by increasing current densities (from 97% at 1 mA/cm2 to 43% at 10 mA/cm2) [40]. Generally, charges that “go into the electrolysis cell” without resulting in oxygen evolution can in principle lead to dissolution of the ingredients, e.g. upon oxidation of metallic fractions like Fe, Co or Li or it can lead to an oxidation of the catalyst, which does not lead to partly dissolution of the catalyst itself, but will lead to a growth of the oxide zone. Considering the very mild electrolysis conditions (j= 10 mA/cm2, T= 20°C) employed in our work we can exclude a further growth of the oxide zone, occurring during long term chronopotentiometry measurements. The very harsh conditions during sample preparation (j= 2000 mA/cm2, T= 50°C) makes a further growth of the oxidic layer at substantially milder conditions unlikely. In addition, the amount of oxidic material that could be scratched form the electrodes was for the as prepared ones even higher (0.131 mg) than for the used ones (0.106 mg), see Table S5. Regarding a possible dissolution of the catalyst whilst OER as the reason for a FE < 100%: Firstly, we did not find any evidence that Li atoms have been intercalated into the Co3O4 scaffold during the preparation of the Co-300/Li samples; however solid state NMR did reveal that Li+ ions are indeed intercalated. Notably: For Li+ deintercalation out of samples Co-300/Li no current is required (current-less wash out of Li+ out of the scaffold). Secondly, the electrolyte used for long-term chronopotentiometry 14 ACS Paragon Plus Environment

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measurements carried out for the samples Co-300/Li-71, Co-300/Li-72, and Co-300/Li-73 have been investigated (Table S4) using ICP OES and the averaged (total) amount of detected ions in solution was only 0.07 mg corresponding to ~ 0.1% of the total charge passed over the 20000 s.


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Figure 2. OER properties for Co-300/Li, Co-300, IrO2/RuO2 (Ir/Ru) in pH 7 corrected buffer solution. All CVs were recorded with a scan rate of 20 mV/s and the electrode area for all samples was 2 cm2. Stirring of the electrolyte was performed for all measurements. (a) Chronopotentiometry results of samples Co-300/Li and Co-300 at 10 mA/cm2 current density (blue and black curve) compared to the Nernst potential ( = 0 mV, red dashed line). (b) Averaged chronopotentiometry curve based on 53 samples of the sample series Co-300/Li (black squares) with standard error bars (magenta). (c) Tafel plots of samples Co-300, Ir/Ru and Co-300/Li based on 200 s chronopotentiometry scans. (d) CVs of samples Co-300/Li and Ir/Ru. (e) Chronopotentiometry result for Co-300/Li at 5 mA/cm2 current density (blue curve) compared to the Nernst potential (red dashed line). (f) Correlation of oxygen evolution for Co-300/Li-70 at 10 mA/cm2 (black dotted curve) and 0 mA/cm2 (Co-300/Li-68; blue dotted curve) with the charge passed through


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the electrode system (red line corresponds to 100% Faradaic efficiency). The Faradaic efficiency of sample Co-300/Li126 amounted to 88.7%.

Figure 3. Results from atomic force microscopy (AFM) and electronic impedance spectroscopy (EIS) for Co-300 and (as-prepared) Co-300/Li-70. (a) AFM image (left) and photograph (right) of Co-300 where the area marked by B on the photograph represents the oxidized area. (b) AFM image (left) and photograph (right) of sample Co-300/Li-70 where the area marked by B on the photograph represents the lithiated area. (c) and (d) Nyquist plots for Co-300 and Co-300/Li-70, respectively. The frequency response behavior was recorded in a pH 7 corrected 0.1 M phosphate buffer solution.

The origin of the unusual OER activity An increase in the active surface area, a possible effect caused by the lithiation procedure as obtained by Wang et al. [41, could be a simple reason for the higher current densities at given potentials as seen in Figure 2c, 2d. Therefore, we have performed a roughness analysis using AFM experiments prior to 17 ACS Paragon Plus Environment

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and after the electrochemical tuning (lithiation) step as summarized in Figure 3a (dark black area B) and Figure 3b (brownish area B), respectively. No significant increase in the surface area between the samples Co-300 (1211.310 µm2) and as prepared Co-300/Li (1143.466 µm2) can be observed (projected area: 900 µm2), suggesting that the significant improvement of the electrochemical properties of sample Co-300 has an “electronic origin” based on an intercalation of Li + into the Co3O4 phase. These results were manifested by BET measurements carried with 20 representatives of the sample series Co-300 (surface area:0.345 m2/g) and Co-300/Li (surface area:0.348 m2/g); Figure S8a, b). Moreover, intensive oxygen evolution for the Co-300/Li samples was found to have no substantial influence on the roughness (surface area:0.362 m2/g; Figure S8c). First hints that Li intercalation into the oxidized surface has indeed taken place can be derived from the ICP-OES analysis of the electrolyte (supplementary materials, Table S4) used for the long term chronopotentiometry measurement (20000 s at 5 mA cm-2) carried out with representatives of the sample series Co-300/Li. Three test runs (samples Co-300/Li-71, Co-300/Li-72 and Co-300/Li-73) have been evaluated and the Li concentration determined in the aqueous electrolyte solution was 0.03-0.09 mg/L (Table S4). In addition, the surface layer of as prepared samples Co-300/Li (Co-300/Li-51 to Co-300/Li-67; referred as as-prepared Co-300/Li) as well as the layer of samples used for extensive OER testing (6000 s of CP at 10 mA/cm2 (Co-300/Li-71-Co-300/Li-90, referred to as Co-300/Li, after OER) has been mechanically removed and investigated via ICP OES (Table S5). Whereas as-prepared Co-300/Li definitely contains Li, the concentration of Li in the solution derived from dissolution of the surface layer of 20 pre-tested Co300/Li samples was below the detection limit of ICP-OES method (Table S5). Two conceivable mechanism for the increase in OER activity due to Li+ insertion shall be mentioned at this point. Firstly, literature has shown that Li+ intercalation will reduce the resistivity of the outer oxide layers and therefore reduce the voltage drop across the catalytic active outer oxide zone, leading to lower values for  [31]. To this end, we have carried out EIS spectroscopy on samples Co-300 and Co-300/Li at offset potentials that ensure oxygen evolution (Co-300:1.45 V vs. RHE; Co-300/Li: 1.27 V vs. RHE). The EIS data impressively underpin the drastic reduction of the charge-transfer-based resistance upon lithiation. (Figures 3c, d). Notably, a much smaller 18 ACS Paragon Plus Environment

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characteristic semicircle is obtained at defined offset potential for the lithium-doped electrode (cf. Figures 3c, d). The EIS findings agree perfectly with earlier investigations of lithiated metal oxides [31, 42]. The second mechanism is based on the electronic influence of Li intercalation. After we physically proofed the existence of Li in the surface of (as-prepared) Co-300/Li samples (Table S5), we attempted to find further evidence using other experimental techniques for Li+ intercalation into the Co3O4 scaffold and, reclining on this, tried to find evidence for an increased electron withdrawing effect on the catalytic active centers that should go hand in hand with the improved ability of the material to function as a water oxidation catalyst. To verify these assumptions, we have initially characterized representatives of samples Co-300 and Co-300/Li using FTIR and XRD as summarized in Figure 4a and S9-S11. Significant differences between the samples Co-300 and as-prepared representatives from the sample series Co-300/Li can be seen from FTIR experiments (Figure 4a, black and blue curve). IR bands that are typically assigned to Co-O stretching for instance in Co3O4 located at 560 cm-1 (OB3 vibration with B=Co2+ in octahedral holes) and 650 cm-1 (ABO3 vibration with A=Co2+ in tetrahedral holes) [43] as well as in Co(OH)2 and CoO(OH), located at 760- and 860 cm-1, [44] can be clearly seen in the FTIR spectrum of sample Co-300 (Figure 4a, black curve). However, with the exception of the band located at 560 cm-1, these bands are completely absent in the corresponding FTIR spectrum of (as prepared) Co-300/Li (Figure 4a, blue curve). Moreover, similarities between the FTIR spectrum of (as-prepared) Co-300/Li with those of known lithiated cobalt-oxide species were not obtained [45, 46]. LiCoO2 has been intensively investigated in the past and the FTIR spectra basically show two absorption bands located at 522 and 610 cm-1 [45]. In addition, earlier investigations have shown that Li+ embedded into spinel structured Co3O4 did not influence the FTIR spectrum significantly at low doping concentration

[46]

. We also derived FTIR data from Co-300/Li

specimen which have been used for extensive OER testing (6000 s of CP at 10 mA/cm2; Figure 4a red curve). Although the (FTIR) curve course achieved with these OER tested Co-300/Li samples differs from the one derived from Co-300 (Figure 4a, black curve) the IR bands discussed before that can typically be assigned to O-H and Co-O stretching vibrations in Co3O4 (between 500 and 860 cm-1) can also be seen in the FTIR 19 ACS Paragon Plus Environment

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spectrum of Co-300/Li samples determined after extensive OER testing (Figure 4a, re curve). Thus, the outcome from the electrochemical testing (Figures 2, 3, S3, S4, S6, S7, and Table S2), the ICP OES investigation (Table S4 and S5), and the FTIR experiments (Figure 4a) clearly suggests: 1. Li+ intercalation into the Co3O4 scaffold causes the outstanding OER performance of Co-300/Li samples, and- 2. Long term usage of Co-300/Li samples as electrodes in water electrolysis causes Li de-intercalation resulting in the formation of Co3O4 accompanied by a decrease of OER performance. X-ray diffraction experiments carried out at different temperatures (see supplementary material for details) did not show any sign of crystallinity related to the outer layers of both samples (Co-300 and as prepared Co-300/Li). We note that the Co-Pi catalyst introduced by Nocera and coworkers also exhibited an amorphous nature [47]. The diffractograms (Figures S9a, b, S10 a-d, S11 a-e) showed either a reflection caused by the steel substrate (hkl = 110 at 44.3°, Fe, Figures S9a, b) or a reflection originating from the complete oxidation of iron on the surface at high temperature (hkl = 311, Fe 3O4, Figure S11f). The results from FTIR and XRD suggest that both samples are of amorphous nature and that the local neighborhood of the Co ions in sample Co-300 agrees well with the one of the corresponding ions in spinel-type Co3O4 due to similar FTIR spectra; however, the FTIR spectrum for as prepared Co-300/Li deviates substantially. Here, the Co

x+

ions have quite different atomic environments, which likely has its origin from the

embedding of Li ions. To shed further light onto the structural changes occurring in the amorphous Cocontaining outer layers of the steel-ceramic composite material upon lithiation we have utilized x-ray photoelectron (XPS) and solid-state NMR spectroscopies, enabling a direct surface characterization and an element-selective view of the intercalation of Li, respectively.


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Surface spectroscopic characterization Spinel Co3O4 [48], LiCoO2 [49] and sample Co-300 exhibit almost the same Co-2p core level XPS spectra as is evident from Figure 4b, whereas the XPS spectrum for (as-prepared) sample Co-300/Li showed significant differences (cf. Figure 4b, c). While the peak maximum of the XPS spectra for Co3O4, LiCoO2 and Co-300 are situated at binding energies below 780 eV, the XPS spectrum of (as-prepared) Co-300/Li is substantially shifted to a higher binding energy of 782 eV. Moreover, the curve profile derived from the Co-2p spectrum of (as prepared) Co-300/Li can be seen to approach that of Co(OH)2 (Figure 4b). This suggests that the content of Co(II) species located near the surface of (as-prepared) Co-300/Li is increased as compared to Co-300, underpinning a change of the electronic structure for the Co atoms in the material upon lithiation. Furthermore, the rather broad Co 2p3/2 and Co 2p1/2 peaks of (as prepared) sample Co-300/Li indicate a mixed valence state for Co. From deconvolution via iterative fitting, the Co(III) and/or Co(IV) ions contributions to the overall Co 2p core level spectrum yields contributions at the low binding energy site at around 779 eV (Co2p3/2) and 794 eV (Co2p1/2). These values are in excellent agreement with Favaro et al. [50] who very recently found novel contributions to the Co2p3/2 spectrum of CoOx also at exactly 779 eV, employing operando XPS under OER potentials, which they associated with Co(IV) ions. Such Co(IV) sites on surfaces are believed to be key intermediates in the pathway leading to oxygen evolution [51]. The same group also observed a reduced intensity in the XPS valence band around 2 eV at the position where the Co(II) and Co(III) 3d bands of most Co oxides are localized. A similar situation can also be observed for the valence band of (as-prepared) Co-300/Li (Figure 4d). The valence band maximum is located slightly higher around 4 eV, indicating that a large amount of the Co2+ ions are in tetrahedral coordination. Unfortunately, XPS is not suitable for the precise detection of light elements in low concentrations, since the photoelectric cross sections of light elements and in particular that for Li is very small. This explains the nonexistence of a distinct Li 1s peak in the XPS spectrum of as-prepared Co-300/Li (see Figure S12). Thus, the XPS experiments strongly suggest a valence mixture of Co ions consisting of Co(IV) and Co(II) as the basic origin for the significant increase in OER activity. We are aware that such a mixed valence cobalt species 21 ACS Paragon Plus Environment

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consisting of Co(IV) and Co(II) is very unusual, which for as-prepared samples Co-300/Li further demonstrates that there is a substantial gap between the oxidation state of the weakest oxidized and strongest oxidized cobalt ion. This gap seems to be widened when compared to Co-300 [13]. In this respect, it is worthwhile to keep in mind that the preparation of Co-300/Li occurs through an electrochemical oxidation and an electrochemical reduction step as the electrochemically initiated intercalation of Li+ into the pre-oxidized steel was realized upon attaching a negative bypass potential to the steel electrode.


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Figure 4. (a) FTIR spectra of samples Co-300, Co-300/Li (as prepared) and Co-300/Li after usage for electrochemical testing. Measurements have been carried in ATR (attenuated total reflection) mode with powders mechanically removed from the surface of the samples Co-300/Li-1-Co-300/Li-30 (as-prepared Co-300/Li), Co-300/Li-71-Co300/Li-90 (Co-300/Li, after OER), respectively. (b) High-resolution XPS spectra (Co-2p) of Co-300 and (as prepared) Co-300/Li (sample series Co-300/Li-31-Co-300/Li-50) together with the reference compounds Co(OH)2, CoO, LiCoO2, and Co3O4. The binding energies of reference compounds are taken from literature [48, 49, 52] and are indicated by vertical dashed lines. (c) Deconvoluted high-resolution XPS spectrum (Co-2p) for (as prepared) Co-300/Li (black points) and optimized fit (red line). The deconvolution resulted in peak positions at 779.05 eV (Co(IV)), 781.55 eV (Co(II)), and 785.95/789.15 eV (charge-transfer satellites) for Co 2p3/2 and 794.1 eV (Co(IV)), 796.65 eV (Co(II), and 803/805.3 eV (charge-transfer satellites) for Co 2p1/2. (d) Comparison of valence-band XPS spectra for (as prepared) Co-300/Li ((sample series Co-300/Li-31-Co-300/Li-50)), CoO, Co-300, Co3O4, and LiCoO2. The binding energies for the reference compounds [49, 49, 53, 54] are indicated by vertical dashed lines. A part of the Figure was adapted from ref 21 with permission from Royal Society of Chemistry


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It should be noted that a reduction of oxide species when used as cathodes in the charging procedure in lithium battery setups is known and was mentioned by Tarascon [55]. Thus, the specific preparation procedure could be the origin of the unusual mixture of oxidation states found for cobalt ions at the surface of freshly prepared Co-300/Li, resulting in the unique electrochemical behaviour.

Li-intercalation environments. Solid state NMR spectroscopy has emerged a powerful technique to identify catalytic active species on electrodes used for energy applications [56, 57, 58, 59]. Utilizing solid-state 7Li MAS NMR spectroscopy it possible to verify the presence of Li within Co-300/Li in addition to characterize its chemical and electronic environments [57, 58, 59]. The 7Li MAS NMR spectrum for a freshly prepared Co-300/Li powder (Figure 5) can be seen to be split into a broad spinning sideband manifold with an asymmetric intensity distribution and a total spectral width close to 300 kHz. We note that such broad 7Li MAS NMR spectra have previously only been reported for Li in transition-metal-doped oxide-based cathode materials

[60, 61, 62, 63]

. The isotropic 7Li resonance is located at ca. -3 ppm,

demonstrating that lithium is present as Li+ in Co-300/Li with a negligible Fermi contact shift [64]. While an asymmetric spinning manifold could indicate the presence of a significant 7Li chemical shift anisotropy (CSA), it is well known that 7Li in diamagnetic solids has a very narrow range of isotropic chemical shifts [65]

. Therefore, the 7Li CSA is negligible and we can attribute the large shift anisotropy observed for 7Li in

Co-300/Li to the interaction between 7Li and a paramagnetic center, resulting from the electron-nuclear dipolar interaction

[57, 66]

. The magnitude of this interaction in combination with the 7Li quadrupolar

interaction can be determined from the 7Li MAS NMR spectrum (Figure 5, S1, Table S3). The 7Li MAS NMR spectrum in Figure 5 is best described using two Li+ components (species Li-1 and Li-2), most likely in a 1:2 ratio, characterized by a large (CQ = ~415 kHz) and a small (CQ = ~170 kHz) 7Li quadrupolar coupling, respectively, see Figure S1 for further details. The large values for the quadrupolar coupling constants show that both Li+ components are fixed at their atomic positions. Interestingly, the electron-nuclear interaction in terms of the shift anisotropy caused by the interaction between the two 7Li components and 24 ACS Paragon Plus Environment

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the paramagnetic center in Co-300/Li is exactly opposite in magnitude as compared to the quadrupolar coupling, i.e. the smaller shift anisotropy is associated with Li-1 and the larger shift anisotropy with Li-2. This result can also be rationalized in terms of the 7Li linewidth (insert of Figure 5), where the narrow and broader 7Li linewidth for Li-1 and Li-2, respectively, reflects the difference in T2 relaxation induced by the different nuclear-electron distances between the Li+ components and the paramagnetic center. The XPS results suggest Co(II) or Co(IV) as the paramagnetic center; however, no 59Co NMR signal was detected and continues-wave solid-state EPR at 5k and 77 K performed at X-band were also inconclusive (see Supporting Information for details). These observations suggest that the electron-Co(II)/Co(IV) hyperfine coupling is particular large, resulting in severely broadened and featureless solid-state NMR and EPR spectra.

Figure 5. 7Li MAS NMR spectrum of (as-prepared) Co-300/Li recorded at 11.7 T and a MAS frequency of 25.0 kHz, showing a broad, asymmetric spinning sideband pattern characteristic of a strong electron-Li dipolar interaction. The asymmetric isotropic 7Li signal suggests the presence of two Li sites with different linewidths and electron-nuclear interactions, see inserts and text for details.

4. Conclusions


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In summary, we have shown that a two-step electrochemical activation procedure applied to a Cobalt containing tool steel led to a material (Co-300/Li) that raises hope for the feasibility of water splitting with nearly zero overpotential at reasonable OER based current densities. XPS investigation carried out for the non-lithiated (Co-300) and lithiated samples (Co-300/Li) revealed an energy gap between the oxidation state of the weakest and strongest oxidized cobalt ion that becomes significantly more pronounced upon lithiation. This suggests that the lithiation step causes Li intercalation into the Cobalt containing layers of the steel-ceramic composite material, resulting in a valence mixture of Co(IV) and Co(II). It is reasonable to assume that this finding has its origin in the particular synthesis protocol. The preparation of the highly active compound Co-300/Li occurs through an oxidation step (harsh electrochemical oxidation of the steel starting compound) followed by an electrochemical reduction, implemented by attaching a negative bypass potential to the steel-ceramic composite in LiPF6 electrolyte. The reduction step is accompanied by intercalation of Li. The entry of Li in the Co-containing steel-ceramic composite was characterized using solid-state NMR and EPR spectroscopies. These methods showed that the lithiated Co-300/Li material includes two distinct Li+ sites located at fixed positions within the Co-containing steel-ceramic framework due to their different interaction with the paramagnetic Co(II) or Co(IV) centers.

Acknowledgements Carsten Doerenkamp and Prof. Claudio J. Magon at the Instituto de Física de São Carlos, Universidade de São Paulo, Brazil are acknowledged for recording the solid-state X-band EPR spectrum of Co-300/Li sample at 5 K. M. Schmidt is acknowledged for carrying out AFM experiments. Supporting Information Supporting Information is available free of charge on the ACS Publications website at DOI: Description of materials and methods; 7Li MAS NMR spectrum; amperometry measurement; additional steady state and non-steady state OER measurements; sample table; overview of the OER properties of lithiated samples; table with optimized parameters for the 7Li MAS NMR experiment; results from BET and XRD measurements; additional high resolution XPS spectra; two tables with results from ICP-OES analysis. 26 ACS Paragon Plus Environment

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Conflict of Interest The authors declare no conflict of interest.

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Intercalation of Li+ into a Co-Containing Steel-Ceramic Composite

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