Alkaline earth metal based single atom catalyst for the highly durable oxygen reduction reaction

Highlights

• An MOF-free approach for the synthesis of Mg-N-C based single atom catalyst.

• Excellent oxygen reduction reaction due to the low charge transfer resistance, high pyridinic N+ pyrrolic N content and degree of graphitization.

• Excellent durability over 10,000 potential cycles.

Abstract

Developing nonprecious group single-atom catalyst (SACs) based on alkaline earth metal has been rarely explored. Mg metal hampered the catalytic activity towards ORR due to the strong bonding of active centers with oxygenated group intermediate, whereas N-bonded Mg atoms have optimal bonding strength with intermediate oxygen species by adjusting the p-band center position. The co-ordination environment of Mg-Nx plays very crucial role in exhibiting the excellent catalytic activity towards ORR. We provide very simple MOF free methodology without any post acid treatment or any subsidiary sacrificial metal like Zn. The Mg-N-C catalyst exhibited a half-wave potential of 0.80 V versus the reversible hydrogen electrode, approaching the recently reported Fe-N-C catalyst. Electrochemical calculation further support the Mg-Nx sites as the origin of ORR via efficient 4-electron transfer pathway in basic medium. Importantly, current density is found to decrease less than 2% in diffusion limited current and loss of activity by only 13% at 0.9 V after 10,000 cycles in alkaline medium which far superior to the durability limit set by the US department of energy and overpasses the state-of-the-art Pt/C catalyst. The charge transfer resistance is the crucial parameter influencing on the ORR activity along with (pyridinic N+ pyrrolic N) content and degree of graphitization. This methodology can be applied to design a variety of other alkaline earth metal based M-N-C electrocatalysts and studied for various applications.

Introduction

Oxygen reduction reaction (ORR) is the key reaction in energy conversion and storage devices such as fuel cells, metal-air batteries, etc. [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. To the date, Pt based catalysts offer excellent ORR activity with low overpotential and high current density. However, the large scale commercialization is hindered due to their scarcity, high cost and poor electrochemical stability [12,13]. Quest is on to find stable and efficient electrocatalysts which can compete with cost offensive precious metal group based catalyst for wide spread applications [14], [15], [16], [17], [18], [19]. Recently, metal-nitrogen moieties integrated in carbon matrix (M-N-C) have evolved as active sites which facilitate the adsorption of O₂ followed by the O ̶ O bond stretching and breaking in alkaline medium [20]. Transition metal such as Fe, Co, Mo, Cr, W etc. based single atom catalysts(SACs) have achieved the tremendous attention even if many of the metals locate to the left of apex of volcano plot having strong binding energy [21], [22], [23], [24], [25], [26].

Researchers have also started exploring alkaline earth metal serving as active catalytic sites for ORR and the study is limited. The few report on Mg based single atom catalysts offer superior catalytic activity towards ORR. In transition metal based SACs, the downshift of d-band centers occur which tune the binding energy that leads to the improved ORR performances. [27] In the contrary, the alkaline earth-metal such Mg and Ca are identified as the inactive for catalytic ORR activity due to the strong bonding of active centers with oxygenated species intermediate [28,29]. In order to decrease the adsorption strength of oxygenated species, the p-band center of Mg shifts upward in the alkaline earth metal based M-N-Cs. Unlike narrow d-band for transition metals, the alkaline earth metal based active sites with broad s band interact with the adsorbate state, leading to the broaden the adsorbate state due to the weak chemisorption [30]. N-bonded Mg atoms have optimal bonding strength with intermediate oxygen species by tuning the p-band center position [29,30]. The co-ordination environment of Mg-Nx plays very crucial role in exhibiting the excellent catalytic activity towards ORR. Finding suggests that strong encapsulation of Mg atom within protective cover of few layered graphene which would be more effective strategy to prevent catalyst corrosion and provides the superb stability in alkaline medium. The bond formation between Mg and N/C provide the strength over the overall catalyst nanostructure which avoid the leaching effect of metal in aqueous electrolyte. Mg based catalyst can have the industrial importance as it is the 7th most abundant materials.

Towards the completion of this work, we came across the alkaline group element Mg based SAC by the Liu et al. [29,31] They have synthesized the Mg-N-C single atom catalyst by pyrolysing of Zn containing ZIF-8 metal organic framework (MOF), Mg salt and melamine as the precursor of carbon and nitrogen at 900 °C under N₂ atmosphere [29]. It is followed by post synthesis treatment of acid to remove the impurities. Overall, it induces not only the additional chemical, time and cost, but also the burden on chemical waste management which require after post synthesis manipulation. Therefore, it is of utmost importance to search for an alternative green, cost effective and very simple methodology that avoids the complexity of the synthesis mechanism and burden on chemical waste management.

Herein, we have demonstrated a very simple, green, cost effective and organic solvent free synthesis protocol to fabricate the Mg-N-C catalysts that uses an Mg precursor and dicyandiamide (DCDA). It is basically a MOF-free synthesis which bypasses the use of subsidiary metal like Zn and also post synthetic treatments making it user friendly, environmentally benign and reduces the burden of chemical waste management facilities which otherwise required in post synthesis techniques. The electrochemistry, high resolution microscopy and spectroscopic investigation confer the heterogeneous catalyst with mononuclear magnesium embedded in nitrogen doped carbon matrix (Mg-N-C) as an efficient ORR. The half-wave potential E_1/2 is found to shift just 16 mV after 10,000 potential cycles with less than 2% reduction in diffusion limited current density, which is much better than that of previously reported state-of-the-art Fe-N-C catalysts, composites and Pt/C catalysts. We have tried the same procedure to get Fe-N-C and compared the ORR performance. Overall, Mg-N-C can be considered as an efficient and durable ORR catalyst.