Low-Overpotential High-Activity Mixed Manganese and Ruthenium

Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN) and Advanced Materials and BioEngineering Research (AMBER) Centre, Trinity Coll...
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Low Overpotential High Activity Mixed Manganese and Ruthenium Oxide Electrocatalysts for Oxygen Evolution Reaction in Alkaline Media Michelle Phillippa Browne, Hugo Nolan, Georg S. Duesberg, Paula E. Colavita, and Michael E. G. Lyons ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b02069 • Publication Date (Web): 01 Mar 2016 Downloaded from http://pubs.acs.org on March 4, 2016

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Low Overpotential High Activity Mixed Manganese and Ruthenium Oxide Electrocatalysts for Oxygen Evolution Reaction in Alkaline Media Michelle P. Browne1,2,3*, Hugo Nolan2,3, Georg S. Duesberg2,3, Paula E. Colavita2,3 and Michael E. G. Lyons1,2,3* 1

Trinity Electrochemical Energy Conversion and Electrocatalysis (TEECE) Group, School of Chemistry and AMBER National Centre, Trinity College Dublin, College Green, Dublin 2. 2

School of Chemistry, Trinity College Dublin, College Green, Dublin 2.

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Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN) and Advanced Materials and BioEngineering Research (AMBER) Centre, Trinity College Dublin, Dublin 2, Ireland Keywords: Oxygen Evolution Reaction (OER), Thermally prepared, Manganese oxides, Ruthenium oxides, XPS, XRD, Raman and SEM. *[email protected] and [email protected] Supporting Information Placeholder ABSTRACT: Mixed Mn/Ru oxide thermally prepared electrodes using different compositions of Mn and Ru precursor salts have been fabricated on Ti supports via thermal decomposition at two annealing temperatures. Subsequently, the OER activities of these electrodes were determined. A majority of the mixed Mn/Ru catalysts are highly active for the OER, exhibiting lower overpotential values compared to state of the art RuO2 and IrO2 type -2 materials, when measured at a current density of 10 mA cm . These Mn/Ru oxide materials are also cheaper to produce than the aforementioned platinum group materials therefore rendering the Mn/Ru materials more practical and economical. The Mn/Ru catalysts are also evaluated with respect to their Tafel slopes and Turnover Frequency numbers. Interestingly, Scanning Electron Microscopy (SEM) reveals that the morphology of the electrodes change to be a mud-cracked morphology, similar to the RuO2, with minimal amounts of the Ru precursor salt added to the Mn salt. Fourier Transform Infrared Spectroscopy (FTIR) and X-ray Diffraction (XRD) show that the Mn material fabricated in this study at the two annealing temperatures is largely Mn3O4, while the Ru material is predominately Ruo2. X-Ray Photoelectron Spectroscopy (XPS) was also used to investigate the Mn and Ru composition ratios in each of the films.

INTRODUCTION Hydrogen has been described as the ultimate clean energy 1 source. Molecular hydrogen not only possesses a higher gravimetric energy density when compared with traditional

fossil fuels, but its combustion in energy conversion devices such as fuel cells produces water as the only chemical by1 product. Alkaline water electrolysis produces pure hydrogen gas, which can then be used as a fuel in a H2/O2 fuel cell to produce energy. This is the basis of the so-called hydrogen economy, which consists of the production of molecular hydrogen from non-fossil sources, its distribution and storage, and its cold combustion in a fuel cell to generate 1-3 electricity. In principle, the energy required to drive the water splitting reaction 2H2O + energy  2 H2 + O2 can be derived from any of a number of sources, both renewable and non-renewable, making it a highly versatile energy conversion technology. One of the more attractive options is the coupling of electrochemical water splitting devices – electrolyzers - with grid scale renewable energy harvesting technologies such as wind turbines or photovoltaics. In this way water electrolysis acts as a local energy storage system which could permit the implementation of these intermittent energy sources on a global scale. An alternative method in the context of solar to fuel conversion involves the use of a photoelectrochemical cell. In this approach light harvesting mechanisms, typically involving semiconducting materials, are incorporated directly into the electrode design so that the input solar energy is harvested directly by the electrode materials. Hence, hydrogen generation via water electrolysis or photoelectrolysis, using electricity generated from renewable sources, offers a clean, environmentally friendly and reliable route to the large scale hydrogen production 1 required for a possible hydrogen economy. However, regardless of the electrolysis route taken, although the reaction of interest in an electrolysis cell is the generation of molecular hydrogen at the cathode, it is the generation of molecular oxygen at the anode which is the most energy

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intensive step in the overall electrolysis process. In practice, the efficiency of water electrolysis is limited by the large anodic overpotential of the Oxygen Evolution Reaction (OER) which is given by: 4OH  2H2O + O2 + 4e . Thus, understanding and optimizing the oxygen evolution process is seen as one of the remaining grand challenges for both physical electrochemistry and energy science. One type of electrocatalyst being studied to overcome these large overpotentials is Dimensionally Stable Anodic ® ® (DSA ) electrodes. DSA electrodes usually consist of an inert support, i.e. titanium, coated with a Platinum Group Metal 4-7 (PGM) to catalyse the OER. For the OER, in alkaline media, the optimum thermally prepared anodic metal oxide/ ® DSA materials are RuO2 and IrO2, due to their ability to produce the lowest known OER overpotential. However, these oxides are expensive and are less stable in alkaline media; which render these materials both impractical and 8 uneconomical. The first row Transition Metals Oxides (TMO), i.e. nickel, iron, cobalt and manganese, show great promise as OER catalysts as they exhibit low overpotentials 2,9-17 and have high stability in alkaline media. Another advantage of these first row TMO’s is their low cost ® compared to the already commercialized DSA type RuO2 and IrO2. Manganese oxides are the least studied metal oxides of the first row transition metals as possible electrocatalysts for the OER in alkaline media. However, the OER catalytic activity is promising, with pure manganese oxide compounds displaying overpotentials between 0.74 -2 9,18 0.49 V at a current density of 10 mA cm . Furthermore, when combined with other compounds this overpotential 19 value further decreases. However, mechanistic studies of ® OER at thermally prepared/ DSA type MnxOy electrodes in alkaline media have been sparse and questions remain about the structure of the catalytic center and reaction steps leading to oxygen evolution. Many studies have explored the OER mechanism of electrodeposited Mn oxide but not of 20-22 thermally prepared Mn oxide. Fernandez et al. studied thermal decomposed catalysts containing RuO2 and β-MnO2 however, this was in acidic media and a possible mechanism 23 was not elucidated. Therefore to our knowledge, our study is the first to investigate the OER in alkaline media using the ® thermally prepared/ DSA type electrodes using the Mn oxide materials Mn3O4 and Mn2O3 combined with RuO2. In this paper we have found that combining Mn and Ru precursor compounds results in mixed oxides in which synergistic effects are observed in OER. Mixed Mn/Ru oxide electrodes were prepared through thermal decomposition on Ti supports and their activity as OER catalysts in alkaline solutions was evaluated. Several of the mixed Mn/Ru electrode materials were found to exhibit significantly improved OER activity and stability when compared with pure RuO2 films, while lowering the cost of producing the 24,25 catalyst. These Mn/Ru materials could therefore offer a competitive low-cost alternative to the already commercially available OER catalysts. EXPERIMENTAL SECTION The materials and reagents used in these experiments were titanium wire (Alfa Aesar- Johnson Matthey company, 99.99% (metals basis), diameter of 1 mm)), sulfuric acid

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(Sigma Aldrich, 95-97%, analytical grade), aluminum powder (Sigma Aldrich), 1200 grit carbimet paper (Buehler), manganese acetate (Sigma Aldrich, ≥99% metal basis, M. 245.09 g/mol), ruthenium chloride (Sigma Aldrich, 99.98% metal basis, M. 207.43 g/mol), butanol (Sigma Aldrich, ≥99%, reagent- grade), mercury-mercuric oxide (Hg/HgO) reference electrode (CH instruments, cat no. 152), sodium hydroxide pellets (Sigma-Aldrich, ≥98%, reagent-grade), carbon tabs (Agar Scientific), copper tape (Agar Scientific), potassium bromide powder (Sigma Aldrich). To prepare the electrocatalysts, titanium wire was partially encapsulated in glass. The electrodes were then dipped in 5 M H2SO4, polished with aluminum powder and 1200 grit carbimet paper, washed with deionized water and set aside to dry. This cleaning process was carried out numerous times until a shiny bright finish was observed. Seven 0.2 M precursor solutions were made by dissolving different ratios of (CH3COO)2Mn.4H2O and RuCl3.xH2O, in butanol, in separate 10 ml conical flasks. The resulting mixtures were evaporated on a hotplate until minimal solvent remained forming the metal oxide pastes used to prepare the working electrodes: each coat was applied followed by drying in an oven at 90 °C for ten minutes and this process was repeated until a homogenous layer covered the titanium wire. The resulting electrode was annealed in air for nine hours at two temperatures, 350 and 450 ᵒC, to ensure the decomposition of the precursor materials. The seven precursor solutions made from various ratios of (CH3COO)2Mn.4H2O and RuCl3.xH2O were used to prepare the electrocatalysts on the Ti wire (Figure 1). Overall, fourteen different OER catalytic materials were fabricated during this study

Figure 1. Fabrication route for Mn/Ru electrocatalysts A rough estimate of the mass loading of the materials is in the range of 3-5 mg. This was roughly the mass difference between the bare electrodes and those loaded with catalyst. However, we feel the double-layer capacitance, Figure S1, is a truer indication of the active material present on the surface of each electrode for the OER given that this accounts for the actual amount of electrode material which interacts with the electrolyte. The materials will be labelled by their relative mole percentage of the initial precursor salts added to make the paste i.e. a Mn 75 or a Mn/Ru 75:25 will describe an electrode fabricated from the paste which is made from 75 %

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(CH3COO)2Mn.4H2O and 25 % RuCl3.xH2O; see Table S4 in the Supporting Information for a full list of all materials. The materials fabricated were characterised by various analytical techniques. The morphology of the electrocatalysts on the titanium wire were determined using a Karl Zeiss Ultra Field Emission Scanning Electron Microscope (SEM) at an accelerating voltage between 2-15 kV at a working distance between 1-5 mm; elemental analysis was carried out using an Energy Dispersive X-ray (EDX) spectrometry detector from Oxford Instruments. X-ray Photoelectron Spectroscopy (XPS) measurements were carried out using a VG Scientific ESCALab MKII system using Al Kα X-rays source (1486.7 eV). Pass energy was set at 200 and 20 eV for survey and high resolution scans, respectively. Mn 2p and Ru 3p1/2 core level peaks were fitted with Casa XPS software and used to determine Mn and Ru atomic ratios. The Ru 3p1/2 peak was chosen because it is not affected by interference from C 1s and Ti 2p peaks. Powder X-Ray Diffraction (XRD) was carried out using a Bruker Duo with a Cu Kα source (λ = 1.5418 Å). Fourier Transform Infrared (FTIR) spectroscopy was carried out using a Spectrum 100 FTIR spectrometer; spectra were recorded from KBr pellets prepared with a sample ratio of 100:1 by weight. All Raman spectroscopy measurements were performed using a Witec alpha 300 R -1 confocal Raman microscope using a 600 line mm grating and a 532 nm diode laser at an incident power of ~ 10 mW. All electrochemical experiments were undertaken in a standard three-electrode cell. The working electrodes consisted of the titanium film substrate with a metal salt paste annealed at different temperatures, as described in the section above. A graphite rod was employed as a counterelectrode and a mercury-mercuric oxide (Hg/HgO) reference electrode was used as a reference standard. The various electrolyte solutions used for the electrochemical behavior studies of the different electrocatalysts produced, including polarisation and Tafel analysis were prepared from NaOH pellets. Electrochemical measurements were taken at a constant temperature of 25 °C, using a thermal bath with the temperature maintained by a thermostat. All solutions were degassed with N2 for 15 minutes before commencing any analysis, to eliminate any dissolved oxygen present in the electrolyte. Cyclic Voltammetry (CV) experiments were -1 conducted in aqueous 1.0 M NaOH at 100 mV s between the limits of -0.6 V and +0.8 V vs. Hg/HgO. The CV measurements were performed using a high performance digital potentiostat (CH model 1760 D Bi-potentiostat system monitored using CH1760D electrochemical workstation beta software). Polarisation and Tafel plot measurements were -1 performed at a sweep rate of 1 mV s in the forward oxidation direction. The uncompensated solution resistance was usually determined at a 90% compensation level. All plotted data are presented in iR compensated form; the iR compensation was calculated from a built-in iR compensation module in the CH- instrument software. These measurements were also performed using a CH model 1760 D Bi-potentiostat system monitored by CH1760D electrochemical workstation beta software. An average value and Standard Deviation (SD) for a material was obtained from three different electrodes of the same material prepared during different fabrication runs. The Standard Deviation (SD) is calculated by:

   ̅ Where  = mean, ̅ = variance and = population.   Σ

The Electrochemical Surface Area (ECSA) was estimated for all electrodes by measuring the double-layer capacitance in a non-faradaic region of CV scans. ECSA experiments were conducted in 1 M NaOH in a potential window of 100 mV at -1 the scan rates of 1, 2, 5, 10, 20 and 50 mV/sec . The anodic and cathodic charging current in the middle of the potential window of each CV was plotted versus the scan rate yielding a linear plot whose slope equaled the double-layer capacitance (Figure S1). ECSA values reported were -2 calculated by using a specific capacitance of 40 µF cm , 13,27 corresponding to that of Jaramillo et al.. Geometric area is used to normalize LSV curves in the main body of the text. However, the LSV’s are also normalized by ECSA for comparison in the SI. It should be noted that ECSA assumes a fixed specific capacitance value based on an average across various transition metals, where this value is known to differ 13,27 between oxide materials. The voltammetric charge (Q) obtained for use in the normalisation of the Tafel plots and for the calculation of the Turnover Frequency (TOF) was determined by integrating the area under the oxidation sweep of the relevant voltammogram in 1 M NaOH, Figure S13. It is assumed the redox active sites for a material are the same active sites involved in OER. The TOF was calculated at an overpotential of 0.4 V as:    /4. In this expression  is the current density at the potential E, Q is the charge associated with the redox processes occurring on the electrode and the multiplication factor 4 is used, as the OER is a four electron transfer reaction. RESULTS AND DISCUSSION Structural Characterization of OER Electrocatalysts Thermal decomposition in air of the precursor Mn and Ru pastes chosen for the experiments is known to lead to the 28,29 To identify the structure of formation of metal oxides. Mn and Ru compounds formed at 350 and 450 °C the films were characterized by FTIR Spectroscopy, Figure S2, XPS, Figure S3-5, XRD, Figure S6, Raman Spectroscopy, Figure S7, and SEM Figure 2 and Figure S8-9.

Figure 2. SEM images of (a) Mn 100 (b) Mn 90 (c) Mn 50 (d) Mn 10 and (e) Mn 0 films fabricated at the annealing temperatures of 350 ᵒC; scalebar = 10 μm

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The SEM analysis reveals that the morphology of the electrode surface varies significantly with the composition of the precursor paste: the flower like morphology of Mn 100 electrodes transforms to the mud-cracked morphology, that is well established for RuO2 electrodes, after only 10 mol % of 30 the latter is added to the former, Figure 2. This mudcracked morphology has been used to explain the low OER 12 overpotential exhibited by RuO2. Interestingly, at higher magnifications on the mud cracked structure of the Mn 0 electrode, particles can be observed, Figure S8. These particles could provide higher surface area allowing for more sites for OER reactions to take place. XPS analysis was performed on all materials to determine the surface composition, Figure S3-5. Survey scans show characteristic peaks for O, Mn, Ru and Ti; a peak associated to C 1s is also present, Figure S3. The surface percentage content of Mn, with respect to total Mn and Ru, was found to closely match the precursor content in all samples, thus suggesting that no surface enrichment takes place under the sample preparation conditions, Figure S5. FTIR analysis of the Mn 100 and Mn 50 materials reveals that, at 350 ᵒC, the MnxOy formed is predominately Mn3O4 28,31,32 with minimal amounts of Mn2O3, Figure S2. For the oxide films formed at 450 ᵒC, the vibrational peak at ca. -1 451 cm observed in the FTIR spectrum is marginally enhanced, thereby indicating that the amount of Mn2O3 present in the surface film increases, but the layer is largely Mn3O4. Also, FTIR reveals that RuO2 is formed in both Mn 50 and Mn 0 electrodes at both annealing temperatures 33 examined. Powder XRD patterns of pure and mixed Mn/Ru materials are in good agreement with the FTIR results. The XRD analysis shows that the predominant MnxOy compound at both annealing temperatures of 350 ᵒC and 450 ᵒC is Mn3O4, Figure S6. For electrodes prepared with Ru salt precursors, XRD patterns show the presence of RuO2 at both 350 and 450 °C. A small amount of metallic Ru was also detected after annealing at 450 ᵒC; this is likely due to the reduction of RuO2 by carbon residues from the solvent or the organic anions of the Mn precursor, as previously observed after 34 thermal decomposition of Ru salts at >400 °C. Finally, Raman spectra of the mixed oxide materials show that peaks characteristic of Mn3O4 are prevalent in the materials prepared at both annealing temperatures, Figure 35-37 S7, with RuO2 present at all annealing temperatures. Raman findings are therefore consistent with those of FTIR and XRD. All material characterization is discussed in greater detail in the accompanying supplementary information. The physical characterization results are in good agreement with previous results reported by Nohman et al. 28 on Mn oxides thermally derived from manganese acetate. Nohman’s work clearly shows that the nature of the manganese salt precursor determines the type of Mn oxide formed via thermal annealing using thermogravimetry and differential thermal analysis. For example it was shown that Mn(NO3)2.4H2O undergoes stepwise dehydration up to 175 °C and decomposes above 200 °C via an unstable oxynitrate intermediate yielding MnO2, which subsequently decomposes at ca. 550°C to the α-Mn2O3 phase. In contrast and very importantly for this work, Mn(CH3COO)2.4H2O dehydrates in three steps up to 130 °C, and then decomposes

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at 350 ᵒC to form a mixture of manganese oxides Mn3O4 (major) and Mn2O3 (minor) through the intermediates Mn(OH)CH3COO and/or MnOCH3COO and MnCO3. OER Evaluation The catalytic activity of the thermally prepared oxide materials was evaluated via linear potential sweep -1 voltammetry in aqueous base at a sweep rate of 1 mV dec . The pertinent Linear Sweep Voltammetry (LSV) is presented in Figure 3 where the rising current indicates the region in which oxygen evolution is observed. The onset potential for pure RuO2 electrodes is 0.2 V, in good agreement with 29 literature values. The onset potential recorded for Mn 100 electrodes was found to be more anodic than that observed for pure RuO2 electrodes. In contrast, the LSV curves obtained for the mixed oxides with Mn contents of 10%, 25% and 50% prepared thermally at 350 ᵒC and 450 ᵒC demonstrate a cathodic shift in OER overpotential when compared with the pure RuO2 film, Figure 3a-b, This observation carries significant economic consequences for alkaline electrolysis reactor usage in an industrial setting, since diluting the more expensive catalytically active Ru component with various amounts of Mn results in a decrease in OER onset potential and an increase in OER activity. The OER catalysts fabricated were evaluated and compared with reference to the following Key Performance Indicators -2 (KPIs): overpotential at a current density of 10 mA cm , (ii) the current density measured at an overpotential of 300 mV, (iii) the initial overpotential value (iv) Tafel slope value, (v) TOF Numbers and (vi) film stability under prolonged polarization. The current density is quoted with respect to geometric area with LSV’s normalized to the ECSA in the SI.

Figure 3. Polarization curves for all the electrocatalysts generated via thermal decomposition of the metal salt precursor at annealing temperatures of (a) 350 ᵒC (b) 450 ᵒC (c) Oxygen overpotential measured at a current density of 10 mA cm-2.

Previous work has indicated that RuO2 electrodes made from commercially available pure oxide exhibit an overpotential of 0.366 V at a current density of 10 mA -2 24,25 cm ; Pure RuO2 electrocatalysts fabricated in the present -2 work, Figure 3c, exhibited overpotential values at 10 mA cm

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of 0.275 ± 0.038 V and 0.351 ± 0.011 V at 350 ᵒC and 450 ᵒC, respectively; these values are all lower than the above literature value. Results for mixed oxide systems with Mn content of 25-10% and 90% are extremely encouraging compared to pure Ru electrodes due to their enhanced OER activity and decrease in cost. For instance the Mn 10 and Mn 25 electrodes annealed at both 350 ̊C and 450 ᵒC exhibits lower overpotentials compared to the pure Ru at the reference current density respectively. Other interesting materials which exhibits similar activity to the pure RuO2 with regard to this KPI is Mn 90 prepared at 350 ᵒC and 450 ᵒC, which exhibit overpotential values of 0.312 ± 0.03 and 0.339 ± 0.04, respectively. Previous findings on precipitated Mn1-nRunOx materials also show that by adding low amounts of Ru oxide (20%) into a Mn oxide and low amounts of Mn oxide (20%) into the Ru enhances the OER activity when 38 compared to pure RuO2. These results indicate that mixed electrodes can lead to a reduction in energy consumption per unit catalyst cost, when scaled up in industrial practice. Furthermore, normalization via ECSA as opposed to geometric area also indicates that Mn/Ru mixed electrodes provide an advantage over pure RuO2, Figure S10. ECSAnormalized results suggest further that Mn 10, 25 and 90 electrodes at the annealing temperature of 350ᵒC and the Mn 90 catalyst at 450ᵒC might be viable alternatives to pure RuO2 electrocatalysts. This shows that some of the mixed Mn/Ru oxides are a better catalyst for OER over RuO2 regardless of the normalization standard. This enhancement in OER activity, compared to the RuO2, for these materials at the two annealing temperatures may be due to the change in the electronic state of the Mn oxide due to the insertion of Ru oxide atoms into the Mn lattice, which can be observed from the XPS surveys, Figure S3. It is also interesting to note that part of the improvement might arise from the ability of MnxOy to disrupt crystallinity in the RuO2 phase. This is suggested by XRD patterns, Figure S6, of mixed MnxOy and RuOx materials, which show broader RuO2 reflections than in the pure RuO2 samples. This less structured phase may indicate defects in the lattice which may enhance the OER activity of this material. Nonetheless, morphology also can play a role in enhancing OER activity. Zeradjanin et al. demonstrated that there is a correlation between the structure of a material and it’s OER 39 performance. Zeradjanin et al. studied RuO2 as an OER catalyst and fabricated a ‘cracked’ and ‘crack-free’ structured 39 film. The ‘cracked’ RuO2 film exhibited superior OER activity compared with the ‘crack-free’ film. This enhancement of the ‘cracked’ RuO2 film was attributed to the cracked film promoting increased nucleation of O2 gas bubbles with defined growth of O2 in confined regions (cracks) and had a higher O2 gas bubble detachment 39 frequency. In this study, as observed from Figure 2, the morphology of the films containing as little as 10 % Ru content has a ‘cracked’ structure, while the Mn 100 has a flower like morphology. In Figure 3c, the 90, 50, 25 and 10 Mn show OER activity equal within error for overpotential to the Mn 0, while lowering the cost This may be due to the formation of the ‘cracked’ structure enabling the mixed Mn/Ru oxide to have optimum gas evolution and detachment. Additionally, the formation of the terraces in all films, excluding Mn 100, may help facilitate the OER. These

terraces have been shown to be areas of high activity could 39 be obtained. The geometric current density measured at the overpotential of 300 mV presents a peaked behaviour with increasing amounts of Ru added to the MnxOy film, Figure 4a. For annealing at both 350 and 450 °C, small amounts of Mn (10-25%) in the precursor mixture lead to improved current densities with respect to the pure RuO2 electrodes. After annealing at 450 ᵒC the Mn 25 material results in the highest current density; interestingly, this is the same material which exhibits the lowest onset OER overpotential over the whole data set and the lowest Tafel slope for oxygen evolution (see below) compared with the other mixed oxide materials. The Mn 10 material exhibit the highest current -2 density value at η=300 mV of 32 ± 1.6 mA cm . The ECSA current densities at the same overpotential can be observed in Figure S11. Interestingly, Mn 90 has the highest current -2 density of 42 ± 2 mA cm .

Figure 4. (a) Current density at an overpotential of 300 mV (b) Initial overpotential values for OER (c) Tafel slope values and (d) Numerical TOF values for all electrocatalysts studied.

Figure 4b shows the initial overpotential value, taken from Tafel plots, for active oxygen evolution exhibited by all the electrode materials tested. The onset potential of the pure manganese oxide electrode increases with increasing annealing temperature, Figure 4b. This behavior can be attributed to changes in composition of the MnxOy material as the annealing temperature is increased, as discussed in the previous section. The addition of Ru to the precursor mixture, even in small amounts, results in a significant decrease of the overpotential. Of the fourteen electrocatalysts studied, eight exhibited lower initial OER overpotentials than pure RuO2; this is a significant result given that this material is one of the best performing OER catalysts to date. The Mn 25 electrocatalyst annealed at 450 ᵒC has the lowest overpotential 0.157 ± 0.005 V, a value which is approximately 20% lower than that of RuO2. Tafel slope values also vary both with oxide composition and annealing temperature. Figure 4c shows a summary of Tafel slope values obtained from Tafel plots of the materials tested, Figure S12 and Table S5. The Tafel slope (defined as   2.303  / , where T is temperature, α is the

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symmetry factor and R and F are the gas and Faraday constants, respectively) is a measure of the sensitivity of the OER rate with respect to change in OER driving force (the applied potential). Its numerical value can be correlated with the nature of the rate determining step for a multistep electron transfer reaction. The lower the value of the Tafel slope the more efficient is the electrocatalyst. As shown in Figure 4c the Tafel slope decreases in a regular manner as the oxide composition is varied from pure Mn to pure Ru. Furthermore, the general trend in Tafel slope values vs. metal oxide composition is similar for the two annealing temperatures examined. Tafel slopes of pure MnxOy -1 electrodes increase from 96 to 110 mV dec when increasing the annealing temperature from 350 to 450 °C. This significant variation is likely due to changes in the conductivity of the oxide as a function of annealing temperatures, as previously observed in the literature and 40 attributed to changes in the oxidation state of Mn atoms. -1 The Tafel slope value drops to ca. 60 mV dec over the Mn 90-10 composition range. For materials prepared from -1 pure Ru precursors at 350 and 450 °C, it falls to 40 mV dec -1 and 48 mV dec , respectively; as expected based on literature 26 values for the Tafel slope of RuO2. The slight increase in Tafel slope between the two annealing temperatures could be due to the formation of trace amounts of metallic Ru, which can be observed in the XRD, Figure S6. It is interesting to note that the addition of only 10% RuOx to the MnxOy film reduces the observed Tafel slope values while also leading to a mud-cracked electrode morphology and with surface particles for the Mn 0 sample, as shown in Figure 2 and Figure S8. The mud-cracked/ surface particle morphology has previously been suggested to contribute to the excellent OER catalytic activity exhibited by RuO2 29 because it enhances the availability of electroactive sites. This morphology is therefore also likely to contribute to increased OER activity in mixed Mn/Ru electrodes. This enhancement of the ‘cracked’ RuO2–containing films compared to the Mn oxide flower morphology may be attributed to the cracked film promoting increased nucleation of O2 gas bubbles with defined growth of O2 in confined regions (cracks) and had a higher O2 gas bubble detachment frequency leading to a higher frequency of free 39 OER active sites on the surface. The TOF of an OER electrocatalyst provides a metric of the material’s ability to produce oxygen per active site per unit time, and is defined via the expression    /4, where  denotes the current density measured at a fixed overpotential and Q represents the voltammetric charge 8 density, Figure S13-14. One can readily rationalise the form of this expression by considering the current density as a rate and relating the charge density to the quantity of electrochemically active material in the oxide film. Hence, the TOF is a rate per unit active material. The larger the TOF number, the better the catalyst for OER. The TOF values obtained for the pure Mn and Ru oxide electrocatalysts decrease with increasing temperature, Figure 4d. Literature derived TOF values for mixed Mn/Ru oxide materials are, to the authors’ knowledge, not available for reference. We note from Figure 4d that the TOF data is somewhat scattered so extracting general trends of genuine significance is difficult and unwarranted. We also note that we are assuming the

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same active sites in the redox reactions on the electrode are the same which are used in the OER. This may not be the case and error may be involved in the determination of the TOF values. When comparing the mixed Mn/Ru catalysts, the Mn 25 material prepared at 350 °C exhibits the highest TOF value. This material also had a low Tafel slope and a characteristic overpotential value of 0.296 ±0.029 V at a -2 current density of 10 mA cm , which is lower than previously 12,19 reported values for RuO2 and IrO2 films in aqueous base. This, therefore, represents a promising candidate material for electrocatalysis of OER. Finally, stability tests were conducted galvanostatically on the Mn 90 and the pure Ru oxide electrodes polarized at a -2 current density of 10 mA cm for 14.5 h. RuO2 is known to be 41 unstable in alkaline media, however, the results show, Figure S15, that by increasing the amount of MnxOy material added to the RuO2 we can observe an increase of stability over a long period of time under OER conditions, which is extremely important for industrial relevance. OER Mechanism To elucidate a possible OER mechanism it is pertinent to examine relevant kinetic data, including Tafel slopes. Tafel -1 slopes of 120, 60 and 40 mV dec correspond to a first electron transfer step, a chemical step and the second electron transfer step being rate determining in the 26 consecutive OER reaction sequence. We have previously suggested that the OER mechanism at thermally prepared RuO2 electrodes involves the participation of octahedral coordinated surfaquo groups located in a thin hydrated layer at the oxide/solution interface. The latter groups may well be anionic. It is well established that the Ru oxidation state of IV exist within oxide films over a broad potential range prior to active oxygen evolution in aqueous base. We suggest that the following mechanistic schemes are chemically reasonable for OER at RuO2 in aqueous base: -

-

-(

(Surf-O-)2RuO (OH)3 + OH  (-O-)2RuO OH)2 + H2O + e (A1) -

(Surf-O-)2RuO (OH)2  (-O-)2RuO2(OH)2 + e-

-

slow rds (A2)

-

(Surf-O-)2RuO2(OH)2 + 2 OH  (-O-)2RuO2 + 2 H2O + O2 + 2e (A3) -

-

(Surf-O-)2RuO2 + OH + H2O  (-O-)2RuO (OH)3 (A4) Here the first step A1 is fast and involves a Ru(VI)/Ru(VII) transition. The second step A2 is slow and rate determining and involves the generation of an unstable Ru(VIII) surfaquo species which subsequently decomposes to molecular oxygen 42 regenerating Ru(VI). Pourbaix diagrams for Mn oxide indicate that in OH 20 media at 1.69 V (vs RHE) MnO4 is formed from MnO2. Electrodeposited Mn oxide for OER has been evaluated by Gao et al. in various alkaline electrolytes including LiOH, 20 CsOH, KOH and NaOH. The Mn oxide was fabricated onto a gold substrate from a manganese sulfate deposition solution. The results have shown that depending on the cations and anions in the electrolyte the formation of MnO4

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can be altered. For example, in CsOH at a potential of 1.8 V the predominant Mn oxide is Mn(III) with a minor contribution to Mn(IV) and no Mn(II) can be observed using XANES. However, in LiOH at the same potential the dominant peak can be assigned to Mn(II) while small contributions can be given to Mn(III) and Mn(IV), using the same technique. Unfortunately, Gao et al. did not use XANES to investigate the Mn oxide species occurring during OER for the KOH and NaOH. Additionally, these authors were able to conclude that the NaOH electrolyte was not stable against Mn oxide dissolution in this electrolyte, while all other electrolytes are, which may explain the fact that the highest 20 Mn oxidation state in the other electrolytes being Mn(IV). This could potentially mean that in NaOH, MnO4 could 20 exist as a bound surface species to the material, see SI. However, there are indeed other possible OER mechanism 43 discussed in the literature for Mn oxides. Interestingly, Takashima et al. uses in-situ Spectroelectrochemistry to detect electrodeposited Mn oxide, made form reducing KMnO4, intermediates prior to oxygen evolution on FTO. In solutions with a pH of 9 or greater, these authors findings speculate that the symproportionation of Mn(II) and Mn(IV) during the OER results in the formation of Mn(III) prior to 33 OER. This step also results in lower overpotentials compared to the same material in lower pH. This result is indeed different to Gao et al’s result in a CsOH electrolyte, as 20 no Mn(II) species were detected at OER potetnials. These studies indicate that the OER mechanism on Mn oxide electrodes may be dependent on the nature of the electrolyte, substrate and/or the preparation, which is clearly significant. Further in-situ experiments will be conducted on these materials presented in this study to investigate the exact OER mechanism at these thermally prepared Mn oxide materials. -1

The Tafel slopes of approximately 40 and 120 mV dec for the pure Ru and Mn oxide materials, Figure S12, correspond to a second electron transfer step and a first electron transfer step, respectively, as the rate determining step in the OER sequence. Subsequently, the Tafel slopes observed for the mixed Mn/Ru oxides suggest a chemical step before the OER. This chemical step may be linked to the work previously mentioned by Takashima et al. Perhaps in this work, the oxides in the mixed Mn/Ru react together with their relevant counterparts i.e. Mn with Mn, to form a different oxidation state. The FTIR and XRD studies in this work reveal that Mn3O4 (Mn II.III) and Mn2O3 (Mn III) are present in the materials therefore with increasing potentials the oxidation states of these materials could undergo similar symproportionation reaction, which could lead to the decrease in overpotential of some of these mixed Mn/Ru 43 materials. Therefore, the reduction in the overpotential may be induced by adding the Ru to the Mn to produce Tafel slopes causing a chemical step needed for OER, at which point the overpotential is decreased even further by a symproportionation reaction between the Mn oxides.

of oxide composition, oxide annealing temperature, electrode potential and hydroxide ion activity. These mixed oxides exhibited excellent activity for oxygen evolution at very low overpotential which bodes well for utilization of the latter materials in water electrolysis cells. The catalysts containing a Mn precursor concentration of 10, 25 and 90 % all exhibit similar, or better, OER activity than Ru02 which represents an important progression in research into metal oxide OER catalysts. Subsequently, the RuO2 in this paper evolves oxygen at a lower overpotential than current literature values, also represents an important outcome in this field. This enhancement in OER activity could be due to the change in the electronic state of the Mn oxide due to the insertion of Ru oxide atoms into the Mn lattice or the lower crystallinity in the RuO2 phase due to the addition of the Mn atoms is likely to lead to improved OER activity compared to the pure RuOx. OER enhancement could also be due to the cracked morphology, observed by the Ru containing films, which may promote increased access to active sites. Subsequently, all of the mixed materials were cheaper to produce than pure RuO2. This process of adding a first row TMO to a platinum group oxide, while achieving high activity, could be a new design of OER catalyst which provides an economical and sustainable route for producing hydrogen.

ASSOCIATED CONTENT Supporting Information Further electrochemical measurements and the Characterization data including the XPS, FTIR, SEM-EDX, XRD and Raman can be found in the SI. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors [email protected] and [email protected]

Author Contributions The manuscript was written through contributions of M.P.B, H.N, P.E.C and M.E.G.L. All authors have given approval to the final version of the manuscript. M.P.B did the electrochemical analysis, XPS, SEM-EDX, XRD and FTIR. H.N did the Raman analysis.

Funding Sources This publication has emanated in part from research conducted with the financial support of Science Foundation Ireland (SFI) under Grant Number SFI/10/IN.1/I2969 and SFI 12/RC/2278.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT

CONCLUSIONS In this paper we have fabricated a series of MnxOy/RuO2 /Ti mixed oxide electrodes and examined their anodic water oxidation behavior in aqueous alkaline solution as a function

We would like to thank the staff in the AML, CRANN for help with the SEM analysis, especially Dermot Daly. We

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would also like to thank Dr. Brendan Twamley for help with the XRD analysis.

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