Enhancing the water splitting performance of cryptomelane-type α-(K)MnO2
Graphical abstract
Introduction
One of the major drawbacks of renewable power sources is their intermittent operation, because they depend on sunlight or wind intensity [1]. Owing to this, efficient storage of surplus harvested energy and its release for on-demand usage is necessary. Energy storage in hydrogen bonds through the electrocatalytic splitting of water is a promising option, as hydrogen possesses a very high gravimetric energy density [2]. However, electrochemical water splitting is inherently energy intensive and particularly necessitates high overpotentials to drive the oxygen evolution reaction (OER) at a meaningful rate [3], [4]. Designing novel low-cost and highly active catalysts for the OER is therefore mandatory for a large scale application. In nature, the oxygen evolution takes place at a CaMn4O5 cluster of the photosystem II protein complex in chloroplasts [5]. Therefore, mimicking the active cluster of photosystem II (PSII) to develop functional manganese oxide-based OER catalysts is of great interest, considering their high abundance, low toxicity, and unique redox properties [6]. However, reproducing the structure and activity of the cubane-like CaMn4O5 clusters in PSII in manganese oxides is only possible to a limited extent, since the biological environment around the CaMn4O5 cluster plays a decisive role in the mechanism of water oxidation [7]. For example, calcium in the CaMn4O5 cluster is suggested to act as a Lewis acid site and is responsible for the activation of water molecules for eventual nucleophilic attack to the Mn ion [8]. Incorporating calcium ions into a manganese oxide indeed led to an improvement of the efficiency of water oxidation [9].
Importantly, some structural guidelines can be derived from the CaMn4O5 structure to design MnOx-based OER catalysts. CaMn4O5 is composed of a smaller CaMn3O4 cubane unit with one Mn ion positioned outside. The Mn ions within the smaller unit are interconnected through di-µ-oxo-bridges, while the dangling Mn ion and the Ca ion are connected through a mono-µ-oxo bridge [10]. This structural arrangement weakens the Mn-O bonds, leading to their elongation and hence higher flexibility. The dioxygen formation takes place at the dangling Mn ion, while the remaining Mn ions promote water oxidation and maintain the structure [11]. Shevchenko et al. [12] demonstrated the necessity of the presence of di-µ-oxo/hydroxo bridges in MnOx-based OER catalysts and claimed that they satisfy two similar motifs as in the CaMnO4O5 cluster. Regarding the latter, the potential during the required successive oxidation of the four Mn ions (Kok-cycle) ought to increase after each step [13]. However, deprotonation of the µ-hydroxo group in each of the steps diminishes the potential for the subsequent manganese oxidation step. Furthermore, the di-µ-oxo bridges may operate as internal Brønsted basic sites, which may facilitate proton-coupled electron transfer (PCET) [14]. The PCET mechanism is proposed to be favorable under alkaline conditions, while under acidic conditions Mn3+ disproportionation occurs [15]. Takashima et al. [16], [17] showed that the formation of Mn3+ is necessary for water oxidation, which indeed makes alkaline media inevitable to prevent its disproportionation. Mn3+ possesses a Jahn-Teller distorted high-spin t2g3eg1 configuration which leads to elongated Mn-O bonds and higher flexibility [18]. According to this hypothesis, a low average oxidation state of OER-active MnOx between +3 and +4 is proposed to be beneficial for water oxidation [19], [20]. However, MnOx materials with di-µ-oxo bridges, e.g., layered birnessite-type structures, show only moderate activity for alkaline electrocatalytic OER [21], and only moderate activities were achieved for thin films of bulk MnOx deposited on glassy carbon electrodes [22]. However, in situ EXAFS studies demonstrated structural rearrangements at an applied potential of 1.8 V vs. RHE (reversible hydrogen electrode), leading to the formation of birnessite-type MnOx [23]. Among the different MnOx polymorphs tested to serve as OER catalysts, tunneled structures showed superior activities in comparison to layered structures [14], [24], [25]. Particularly, cryptomelane-type α-(K)MnOx with a 2 × 2 tunnel structure and an average oxidation state of 3.8, which provides accessibility for water molecules to be oxidized within the bulk, exhibited superior performance in the OER. The beneficial effect of tunnel structures is related to the presence of mono-µ-oxo bridges. Whereas layered structures are composed of vertically aligned edge-sharing octahedra leading to preferential formation of di-µ-oxo bridges, tunneled structures are composed of vertically arranged edge-sharing MnO6 octahedra interconnected by corner-sharing MnO6 octahedra leading to mono-µ-oxo bridges. Smith et al. [26] provided strong evidence that mono-µ-oxo bridges of corner-sharing octahedra are favorable sites for water oxidation. Compared with the CaMn4O5 cluster, the mono-µ-oxo bridges mimic the structural motif of the binding situation between the calcium and the dangling Mn ion [26], [27]. Additionally, heat treatment has a beneficial impact on the electrochemistry of various manganese oxides according to previous works [28], [29], [30], [31].
Herein, we modified the chemical and structural environment of α-(K)MnO2 by increasing the number of structural defects, Mn3+ centers, and µ-oxo bridges to enhance the OER performance. The modification was performed by mild heat treatment of reflux-precipitated α-(K)MnO2 in He, H2O/He and O2/N2 atmospheres. A temperature of 300 °C was chosen for the treatment to effectively remove oxygen from the lattice while preventing the α-MnO2 structure from collapsing and being further reduced to Mn2O3. EXAFS studies demonstrated that thermal treatment of α-MnO2 leads to an increased density of Mn3+ sites at the corner-sharing positions, which are the most likely sites for electrocatalysis of the OER.
Section snippets
Synthesis and treatment of α-(K)MnO2
Cryptomelane-type α-(K)MnO2 was synthesized by a method analogous to the work of DeGuzmann et al. [32] by dropwise addition of a solution of 1.909 g of MnSO4·H2O in 30 mL water to a solution containing 1.185 g KMnO4 in 100 mL ultra-pure water and 3 mL conc. HNO3. The mixture was stirred for 24 h at 100 °C. Afterwards, the precipitate was filtered, washed several times until neutral pH and dried at 80 °C overnight. The thermal treatments were performed by loading 150 mg of the dried product into
X-ray diffraction
Powder X-ray diffraction was applied to determine the MnOx phases present in the samples. The diffraction peaks at 12.7°, 18.0°, 28.8°, 37.6°, 41.9°, 49.8° and 60.2° 2Θ confirm that cryptomelane-type α-MnO2 with K+ located in the (2x2) tunnels is present in the as-prepared sample after precipitation and after the thermal treatments (Fig. 1a).
The sharp reflections indicate that the samples are of high crystallinity, and they remained essentially unchanged by thermal, air and water/He treatment.
Discussion
Many factors certainly influence the properties of manganese oxides when applied as electrocatalysts for water oxidation, among which three factors seem to have a more significant impact on their performance, namely: the number of Mn3+ sites, structural defects and the nature of the µ-oxo bridges. Heat treatment of reflux precipitated α-(K)MnO2 in different atmospheres at 300 °C had a direct impact on these parameters and was thus used to tune its OER activity. He and air atmospheres were
Conclusions
It was possible to lower the overpotential of reflux precipitated α-(K)MnO2 in the OER by thermal treatments at 300 °C in He, H2O/He and synthetic air. The thermal treatments caused a decrease of the average oxidation state of Mn on the surface and in the bulk due to partial reduction of Mn4+ to Mn3+, as evidenced by EXAFS, XPS, XRD, and EIS. In addition, structural defects and the number of di-µ-oxo bridges increased on the surface as indicated by XPS, TEM and EDLC. Therefore, we conclude that
Acknowledgments
Financial support from the German Federal Ministry of Education and Research (BMBF) (MANGAN: 03SF0500) is gratefully acknowledged. D. M. Morales acknowledges the financial support from Deutscher Akademischer Austauschdienst (DAAD) and from Consejo Nacional de Ciencia y Tecnología (CONACyT).
Declaration of interest
None.
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