Optimizing the oxygen evolution reaction: Role of crystallographic phases in support materials under acidic conditions

Proton exchange membrane water electrolysis (PEMWE) emerges as a formidable contender in renewable energy conversion, distinguished by its swift response times, high current densities, reduced resistance losses, and enhanced hydrogen purity [1], [2], [3], [4], [5]. These attributes position PEMWE as a critical component in the sustainable energy landscape, facilitating the efficient production of green hydrogen. However, the technology’s future is hindered by significant challenges, notably the corrosive acidic and oxidative conditions prevalent at the anode, which necessitate the development of robust and efficient PEMWE catalysts [2].

Iridium-based materials are often favored for acidic OER due to their high effectiveness and sustainability; however, their scarcity and high cost limit large-scale application in PEMWE [6], [7]. In contrast, ruthenium-based electrocatalysts offer a more cost-effective alternative, combining high catalytic activity with greater economic feasibility [8], [9], [10]. Nevertheless, the application of Ru catalysts in acidic OER environments is complicated by over-oxidation, leading to structural instability and a loss of active sites [11]. Recent research has focused on modulating the electronic structure of RuO₂-based catalysts as a strategy to mitigate these issues. Approaches such as heteroatom doping, alloying, defect engineering, and surface reconstruction have been explored to enhance catalytic performance and durability under harsh conditions [12], [13], [14], [15]. Notably, developing strong oxide-support interactions (SOSI) has shown promise in significantly improving the intrinsic catalytic activity and stability of electrocatalysts [16], [17]. However, the acidic OER environment demands substrates that form strong electronic interactions with RuO₂ and exhibit high corrosion resistance and electron mobility, making the selection of suitable carriers based on the SOSI strategy particularly challenging.

Manganese dioxide (MnO₂) has garnered considerable interest in OER catalysis due to its diverse valence states, varied spatial configurations, and non-toxic nature. Its porous structure enhances the specific surface area, promoting OER activity, while the rich valence states allow for fine-tuning of the electronic density, facilitating optimal Ru-Mn synergy [18], [19]. The structural diversity of MnO₂ encompasses various polymorphs, including hollandite (α), pyrolusite (β), intergrowth (γ), birnessite (δ), and defect spinel (λ), resulting from different bonding configurations of Mn(IV)O₆ octahedra [20]. The heterogeneity among these configurations is due to single or multiple linkages of Mn(VI)O₆ octahedral chains along the a-axis. Edge-sharing Mn(VI)O₆ octahedra form duplicated chains (as seen in α- and γ-MnO₂, and the planar layers in δ-MnO₂), while corner-sharing structures create tunnel formations. Distinct bonding environments for oxygen are evident in these structures: sp³-hybridized pyramidal oxygen (O_pyr) is associated with edge-sharing octahedra, whereas sp²-hybridized planar oxygen (O_pla) is linked to corner-sharing octahedra [20], [21], [22]. As illustrated in Fig. 1, β-MnO₂ consists exclusively of O_pla, δ-MnO₂ contains only O_pyr, and γ-MnO₂ features a variable ratio of both types. This structural versatility has spurred numerous studies into the Ru-Mn synergistic effect on enhancing OER performance. For instance, Qin et al. [23] examined the structural impact and OER activity of Ru integrated into β-, α-, and τ-MnO₂, observing significant lattice and valence state alterations in β-MnO₂ that optimized OER activity, requiring an overpotential of only 278 mV to achieve a current density of 10 mA·cm⁻². Li et al. [24] developed a RuO₂/MnO_2−γ heterojunction catalyst demonstrating excellent OER activity under acidic conditions (η = 220 mV @ 10 mA·cm⁻²) and stable operation for 80 h. Our previous work synthesized Ru/α-MnO₂, achieving a current density of 10 mA·cm⁻² at an overpotential of just 167 mV, with sustained activity for over 200 h [25]. These studies highlight the significant influence of MnO₂ phases on enhancing Ru-based OER catalysts, yet further research is necessary to elucidate the specific effects of different MnO₂ phases on Ru-Mn synergy and OER performance.

Building on foundational studies, this work presents a crystal phase engineering approach to synthesizing Ru/MnO₂ electrocatalysts, focusing on various MnO₂ crystallographic phases, specifically β-, γ-, and δ-MnO₂. The observed trend in intrinsic oxygen evolution reaction (OER) activity follows the order: Ru/δ-MnO₂ > Ru/γ-MnO₂ > Ru/β-MnO₂, indicating a significant correlation between MnO₂ crystal structure and OER efficiency. Notably, Ru/δ-MnO₂, which features a predominance of sp³-hybridized pyramidal oxygen (O_pyr), demonstrates the highest intrinsic activity. In contrast, Ru/β-MnO₂, comprised entirely of sp²-hybridized planar oxygen (O_pla), shows the greatest durability under operational conditions. This study underscores the critical role of oxygen coordination in Ru-based catalysts and offers a promising strategy for optimizing electrocatalyst performance in the OER.