Sputter‐Deposited La–Co–Mn–O Nanocolumns as Stable Electrocatalyst for the Oxygen Evolution Reaction

A thin‐film materials library (ML) of the La–Co–Mn–O system is fabricated by hot reactive combinatorial cosputter deposition and screened for its electrocatalytic activity for the oxygen evolution reaction. Within this ML, an area with superior catalytic activity is identified. In‐depth characterization of this region reveals a unique columnar‐grown microstructure showing a large catalytic surface and excellent stability during electrocatalytic measurements. A zoom‐in into these structures shows that the columns are compositionally and structurally not homogeneous but are composed of a mixture of the perovskite phase LaCoMnO3 and Co–Mn–O oxide. Nanoelectrochemistry using the particle on a nanoelectrode approach confirms the high activity as well as stability of the single columns.


Introduction
The increasing demand for energy, together with growing global concern for the environment, is driving the rapid development of sustainable energy sources.To conquer this difficult circumstance, large endeavors have been made to develop electrochemical energy conversion and storage technologies.These include water splitting, which is of particular importance because it can be coupled with both renewable wind and solar energy to produce high-purity hydrogen.Yet, the thermodynamically uphill and sluggish kinetics of the oxygen evolution reaction (OER) deteriorate the overall efficiency of electrochemical water splitting.Hence, developing high-performance OER electrocatalysts is significant for improving the overall water-splitting efficiency.
To date, precious metals are the most common electrocatalyst materials for water electrolysis in acidic and alkaline electrolytes because of their exceptional catalytic activity and stability.State-of-the-art catalysts providing high current densities at low overpotentials for the OER and the hydrogen evolution reaction (HER) are Ir/Ru- [1,2] and Pt-based [3] catalysts, respectively.Due to the scarcity and high price of noble metals, current research strategies involve attempts to replace them with more abundant elements.In the past few years, various transition metal-based compounds have been already used as promising electrocatalysts for water splitting.They include various classes of materials such as oxides, (oxy)hydroxides, sulfides, phosphides, nitrides, borides, alloys, and their composites. [4]wing to their low cost, flexibility, and tailorable properties, perovskite oxides [5,6] and double-perovskite oxides [7][8][9] have emerged as promising catalysts for OER in alkaline water electrolysis.Moreover, composite materials were observed to enhance OER catalysis. [10]There are already known cases of materials that match the activity or surpass the state-of-the-art catalysts. [11,12][15][16] Perovskite oxides have the general formula ABO 3 , where in the case of La-based oxides, La occupies the A-sites and transition metals are located on the B-sites.Substituting the metals in A-and B-sites and/or incorporating additional metals changes the electronic structure of the material by changing the distribution of electronic states, resulting in tailorable properties. [17,18]This characteristic might also be used to overcome a well-known drawback of metal oxides as electrocatalysts, namely their relatively low electrical conductivity.A systematic approach to study the influence of chemical composition on materials properties is combinatorial magnetron sputtering, where so-called materials libraries (MLs) are created.[21] In a previous study, it was demonstrated that reactive combinatorial sputtering is an excellent method to fabricate and study La-based perovskites. [22]It was shown that Mn is beneficial for the conductivity in La-Co-perovskite films and hence crucial for electrocatalytic measurements.25] However, in our previous studies, we did not investigate the electrocatalytic OER activity of the libraries.Since Mn substitution is also reported to increase the catalytic activity of LaCoO 3 perovskites, [26][27][28] the La-Co-Mn-O system was chosen here for further studies.Therefore, a new ML of the La-Co-Mn-O system was fabricated on a platinized Si-Wafer, which is necessary to provide electrical conductivity for electrochemical scanning droplet cell (SDC) measurements.The fabrication, physical characterization (composition, crystal structure, and surface microstructure), and electrochemical activity screening of hundreds of La-Co-Mn-O compositions are presented.This is followed by detailed studies of the performance of unique columnar structures that were grown on the ML and consist of a phase mixture of perovskite and spinel.These structures were showing outstanding catalytic activity for the OER.Single-entity electrochemistry and identical location transmission electron microscopy (TEM) before and after electrochemical tests were carried out on these columnar structures.This approach enables testing of the catalyst at high current densities, similar to those used in industrial applications, providing reliable information about both activity and stability in relevant conditions.

Results and Discussion
The investigated La-Co-Mn-O ML was obtained by reactive cosputtering onto a heated substrate at a temperature of 500 °C.After deposition, the library was screened for its chemical composition, crystallographic phases, and electrocatalytic activity for the OER using high-throughput characterization methods.Within the library a thorn-shaped, sharply defined and matte region was observed (Figure 1a).This region includes the 12.5% measurement areas (Mas) with the highest catalytic activity of the library (Figure 1b): a maximum value of 2.38 mA cm À2 is reached at a potential of 1.7 V versus reversible hydrogen electrode (RHE) for OER linear sweep voltammogram (LSVs are displayed in Figure S1 and S2, Supporting Information).A parallel coordinate plot of the metal chemical composition and the corresponding OER activity shows that the La-content ranges from 24 to 36 at%, the Co-content ranges from 34 to 49 at%, and the Mn-content ranges from 26 to 36 at% within this region.The most active MA had the chemical composition of La 0.26 Co 0.44 Mn 0.30 O 6 .
The phase analysis performed on the X-ray diffraction (XRD) results, see Figure 2a, indicates a phase mixture of La(Co 0.5 Mn 0.5 )O 3 perovskite and Co 2 MnO 4 spinel in the thorn-shaped area of the library.The detected peaks perfectly fit the used reference pattern.The scanning electron microscope (SEM) analysis of the surface in this region reveals the existence of large columnar structures instead of the to-be-expected dense film (Figure 2b,c).These columnar structures are associated with a blackish appearance due to lower reflectivity of light from these structures.The formation of such columnar structures is unusual for cosputtering.Separated columns can usually only be obtained by special sputtering methods, e.g., glancing angle deposition. [29,30]However, the observed columns resemble more structures typically grown in chemical vapor deposition processes. [31,32]For reactively sputtered La-based oxide systems, there are only a few reports in the literature on similar columnar growth.A comparable structure formation was observed during hot sputter deposition of La-Co-O, [33] whereas nanostructure formation was observed for a Sr-and Ag-substituted La-Co-O system. [34,35]he SEM image in Figure 2b shows the columns in a pristine, as-grown state in top view.The SEM image in Figure 2c is taken from a region where the columns were intentionally broken on the substrate, and Figure 2d shows a TEM image of the columnar structures in cross-section.Taking the scale bars as a reference, the columns have lengths of about 1 μm and widths >100 nm.The expected nominal film thickness of a dense film would have been in the range of 300 to 350 nm.Therefore, the dimensions of these columnar structures are much larger than expected.Moreover, a preferred growth orientation in the direction following the La-particle flow can be observed from the SEM images.
The comparison of SEM images taken from pristine and catalytically treated columns reveals that they are stable during the performed OER.They do not change visually, as shown in Figure 3a,b.This observed stability is also confirmed by XRD (Figure S8, Supporting Information), where the same phase constitution is measured after the electrochemical measurement as before (Figure 2).
A general challenge in the screening of electrocatalytic activity across MLs is that the measured current density is not only determined by the intrinsic activity of the investigated materials but also by the surface roughness.Rough films with pronounced surface microstructures offer a larger surface area for electrocatalytic conversion than smooth films.Therefore, the observed columnar structures, with their large catalytic surface area, make it difficult to assess the intrinsic catalytic activity of the material.The measured activity could either be caused by an intrinsic activity or just by a surface effect.[38] To prepare such a single-particle assembly, columns were scratched from the ML, dispersed in ethanol, and drop-cast on a gold-coated Si wafer.Next, a single, well-defined column was selected and placed with the use of a micromanipulator onto the top of a carbon nanoelectrode (CNE) under SEM control.TEM and energy dispersive X-ray spectroscopy (EDX) were used to confirm the attachment of the La-Co-Mn-O columns at the tips of CNEs.Such nanoassemblies, hereafter denominated as La-Co-Mn-O column@CNE, were investigated in detail with TEM, TEM-EDX, STEM, and high-resolution (HR)-TEM before and after electrochemical experiments (see Figure S3-S7, Supporting Information).
The identical location of TEM of a single La-Co-Mn-O column was enabled by the use of a specific CNE-TEM holder. [36]The TEM-EDX measurements show a chemical inhomogeneity of the columns.HR-TEM images and corresponding fast Fourier transform (FFT) images of La-Co-Mn-O column@CNE reveal the presence of multiple crystal lattices before and after the cyclic voltammograms (CVs) sequence, see Figure 4, indicating their high stability.This is consistent with the XRD results presented in Figure 2.During identical location, HR-TEM lattice fringes with an interplanar spacing of 0.272 and 0.269 nm (before) and 0.273, 0.275, and 0.398 nm (after) were identified.Those values correspond well to the d-spacing of planes ( 111) and (110) of La(Co 0.5 Mn 0.5 )O 3 : 0.275 and 0.398 nm, respectively (COD ID: 1 532 786).
OER activity of La-Co-Mn-O column@CNE was investigated in a two-electrode configuration using a Ag|AgCl|3 M KCl as reference/counter electrode.The electrocatalytic activity was initially evaluated by CV in purified 1 M KOH at a scan rate    of 200 mV s À1 .[40] Figure 5 presents CV curves measured on a single column and bare CNE.The measured absolute currents for La-Co-Mn-O column@CNE reach 20 nA at 1.8 V versus RHE.The geometric surface area of the studied column was determined by means of TEM by assuming a cylinder shape with 550 nm height and 170 nm diameter.Using these values, the calculated current density is ≈5 mA cm À2 at 1.7 V and 57 mA cm À2 at 1.8 V, showing good OER activity in alkaline electrolyte.Notably, the current does not change significantly during 5 CV cycles suggesting good stability of the electrocatalyst at high current densities (only 10Â lower than industrial conditions, 0.5 A cm À2 ). [41]This result was confirmed by the postelectrocatalytic TEM-EDX and HR-TEM images (lower panel of Figure 4) demonstrate no substantial morphology, composition, and structure change after the electrochemical measurements.
[44] Co 2 MnO 4 was investigated either for OER [45] or as a bifunctional electrocatalyst for OER/oxygen reduction reaction (ORR). [46]Its activity was evaluated to be rather poor compared to other spinel oxides and stability was not tested in detail.For the ORR, it was observed that the electrocatalytic activity decreases in the order: CoMn 2 > CoMn > Co 2 Mn and for the OER in the order of ZnCo La(Co 0.55 Mn 0.45 ) 0.99 O 3Àδ nanorod/graphene hybrid was reported to have high intrinsic activity and good stability for ORR and OER. [42]The overpotential for OER at 10 mV cm À2 was determined to be 0.450 V and the potential increase during 160 min of chronopotentiometry at a current density of 5 mV cm À2 was determined to be 22.5 mV h À1 .
To complement the previous analyses regarding composition and phase constitution of the columnar structures before and after electrochemical characterization, TEM lamellae were prepared from the pristine and the OER-measured columns (see Figure 3).The characterization of these columns regarding their crystallographic phases and local chemical composition were performed on an aberration probe-corrected TEM and are presented in Figure 6, 7, and the corresponding Table 1.Bright field (BF) TEM images and elemental distribution maps reveal that the microstructures of the columnar structures are very similar and consist of three distinctive components.By measuring the interplanar distances (Table 1), it was possible to identify three phases: LaCo 0.5 Mn 0.5 O 3 (blue on EDS maps), Co 2 MnO 4 (yellow on EDS maps), and Co 3 O 4 (red on EDS maps).In each case, the majority of the La-rich phase was located at the substrate interface and then extended to the columnar structures.
The phase identification was carried out using the interplanar distances measured from the FFTs acquired based on the correlative STEM-EDS element distribution maps and HR-TEM data.The following phases were considered as possible candidates: LaCo 0.5 Mn 0.5 O 3 ICSD-5981, Co 0.5 Mn

Conclusion
The fabrication and analysis of a La-Co-Mn-O ML were presented.Within the ML, an area with a unique columnar microstructure was identified, which consists of a mixture of three phases, the perovskite phase LaCo 0.5 Mn 0.5 O 3 , and the two spinel phases Co 2 MnO 4 and Co 3 O 4 .The discovery itself of such columnar structures, that grow by normal cosputtering, is already a terrific experimental result.Moreover, the discovered columnar structures show outstanding electrocatalytic activity as well as stability for the OER.Analyses of the columns in pristine state and after electrocatalytic characterization show that there is no change of structure or composition.Single-entity electrochemistry with the particle on a nanoelectrode confirms the high activity and stability of the columns.This makes these structures a new promising class of OER catalysts in alkaline media and subject of recent catalyst research.[49][50][51] Due to their stability and high activity, the discovered La-Co-Mn-O columns could be a new functional coating for OER-electrodes, when the sputter process could be scaled up to an industrial level.In further studies, the chemical composition of the columns could be adjusted by doping or substitution with other elements, which might improve activity and stability of the structures.

Experimental Section
Synthesis of La-Co-Mn-O ML: The La-Co-Mn-O ML was fabricated by hot reactive combinatorial magnetron cosputtering in a commercial fourcathode sputter system (AJA International, ATC 2200).All cathodes were confocally aligned to the substrate.La was sputtered with a radio frequency power of 200 W from an oxidic La 2 O 3 target (4-inch diameter, 99.99%, Evochem Advanced Materials), Co was sputtered with direct current (DC) power of 70 W from an elemental Co target (4-inch diameter, 99.99%, Sindlhauser Materials), and Mn was sputtered with pulsed DC power of 130 W (frequency 10 kHz, pulse length 5 μs) from an elemental Mn target (4-inch diameter, 99.95%, Sindlhauser Materials).The La 2 O 3 and Co cathodes were positioned 180°from each other and the Mn cathode was positioned with 90°from the other ones, respectively.The sputter process and the cathode position are schematically shown in Figure 8.
The base pressure of the system was about 10 À5 Pa.The deposition of the ML was carried out for 14 400 s in reactive O 2 /Ar atmosphere with a constant O 2 /Ar flow ratio of 40 sccm/80 sccm at a fixed pressure of 0.4 Pa.The used process gas had 6 N purity (99.9999%).The Ar gas line had an additional getter: SAES pure gas type MC1-903T purifier that typically removes O 2 , H 2 O, CO, CO 2 , and organic/NMHC compounds <100 ppt and a 0.003 μm particle filter.The O 2 gas line has no getter purifier.As substrate for the ML, a platinized 4-inch (100)-Si wafer (with 500 nm thermal SiO 2 ) was used.The Pt-layer is needed as bottom electrode for electrochemical measurements and has a thickness of ≈50 nm.For increasing the adhesion of Pt on the SiO 2 , 10 nm of Ta adhesion layer underneath the Pt-electrode was used.During deposition, four points at the edge of the platinized wafer were kept covered to obtain an uncoated Pt-surface for the electrochemical measurements.Using the in-built heater of the sputter system, the substrate was heated to 500 °C and kept at this temperature during deposition.After finishing the deposition, all cathodes were instantly powered off, the gas flux was switched off and the heater was shut down.This way, the ML cooled down under vacuum and did not experience any further intentional oxidation or heat treatment.
High-Throughput Characterization: High-throughput characterization means the fast, automated, and frequently subsequent characterization of the ML with automated methods.The characterization follows a

Figure 1 .
Figure 1.a) Photographic image of the La-Co-Mn-O ML.A thorn-shaped blackish region is highlighted in magenta; b) OER activity of the whole ML (342 MAs) at 1.7 V versus RHE.The highlighted region includes the 12.5% best-performing chemical compositions; and c) the parallel coordinates plot shows the chemical composition of all MAs and their corresponding OER activity, wherein the 12.5% best catalysts are colored and all other MAs are grayed out.

Figure 2 .
Figure 2. Exemplary XRD pattern, a) of the area of the La-Co-Mn-O ML fabricated at 500 °C, which features columnar structures; b) SEM top view of the grown columns, c) SEM image of mechanically damaged columns, and d) TEM image of the columnar structures in cross-section, grown on the platinized Si-substrate.The Pt-layer appears black underneath the film and columns.The columns have a length of about 1 μm.Indication of peaks using ICSD: La(Co 0.5 Mn 0.5 )O 3 ID5981, Co 2 MnO 4 ID201314, Pt ID243678.

Figure 3 .
Figure 3. SEM images of the columnar structures with an approximate chemical composition of La 0.26 Co 0.44 Mn 0.30 O 6 in a) pristine state and b) after the OER measurement.SEM images were taken with 30 k magnification.

Figure 4 .
Figure 4. Comparison of a single La-Co-Mn-O column before (upper panel) and after electrochemical measurements (lower panel).a) TEM images of La-Co-Mn-O column@CNE; b) HR-TEM of La-Co-Mn-O column@CNE with marked determined lattice distances; c) EDX elemental mapping of the single La-Co-Mn-O column@CNE; and d) EDX line scans of a single La-Co-Mn-O column@CNE.

Figure 5 .
Figure 5. a) CVs of 5 cycles of single La-Co-Mn-O column@CNE, b) CVs of single La-Co-Mn-O column@CNE and bare CNE, c) CVs of single La-Co-Mn-O column@CNE and bare CNE in current density obtained by dividing the measured current by geometrical surface area of CNE and column, respectively.For the column, a cylinder with 550 nm height and 170 nm in diameter was used.Measurements were conducted in purified 1 M KOH with a scan rate of 200 mV s À1 .

Figure 6 .
Figure 6.Results of TEM analysis of the as-deposited pristine electrocatalyst: a) BF TEM image revealing the structure of the pristine area of the thin film; b) STEM-EDS elemental distribution map with highlighted rectangles where HR-TEM images were acquired.c,f,i) HR-TEM images of the three distinct compounds revealed in the elemental map b).The highlights represent the regions from which corresponding FFTs d,g,j) were calculated.e,h,k) show filtered inverse FFTs, where the interfering atomic planes are visible.

Figure 7 .
Figure 7. Results of TEM analysis of the electrocatalyst after electrochemical measurements: a) BF TEM image revealing details of the columnar structure; b) corresponding STEM-EDS elemental distribution map with highlighted rectangles where the HR-TEM images were acquired; c,f,i) HR-TEM images of the three distinctive constituents with highlighted rectangles where the corresponding FFTs d,g,j) were calculated; e,h,k) filtered inverse FFTs, where the interfering atomic planes are visible.

Figure 8 .
Figure 8. a) Schematic illustration of the reactive magnetron codeposition process to fabricate the La-Co-Mn-O ML. b) A photo of the ML deposited on a 4-inch diameter wafer is shown and the sputter target positions are indicated by arrows and the respective elements.

Table 1 .
Measured d-spacing from the diffraction data and corresponding crystallographic (hkl) of the identified phases.The given deviation represents the difference between the measured and the crystallographic reference values.T.H.P. and O.A.K. contributed equally to this work.This work was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)project number 388390466-TRR 247, project C04, and A02.Additionally, SFB TR 103, project B05 (project number 190389738-TRR 103) is acknowledged for support.ZGH at Ruhr University Bochum is acknowledged for XRD, SEM, and TEM measurements.The identicallocation TEM measurements of the nanoassemblies were supported by the Center for Solvation Science ZEMOS, funded by the German Federal Ministry of Education and Research BMBF and by the Ministry of Culture and Research of Nord Rhine-Westphalia.Dr. Jian Zhang