Stoichiometry-Dependent Oxygen Evolution Electrocatalysis on Open-Tubular Nitrogen-Doped Carbon Column Supported Transition Metal Oxides

Abstract

Ni-, Co-, and Fe-based transition metal oxides have been extensively studied as heterogeneous electrocatalysts for water oxidation in alkaline media. Here, we investigated how metal composition affects the oxygen evolution electrocatalysis and plotted the intrinsic catalytic response to metal stoichiometries in 21 unary, binary, and ternary transition metal oxides. For the first time, we developed a charge transfer complex [metal-7,7,8,8-tetracyanoquinodimethane (M@TCNQ)]-templated strategy to prepare transition metal oxides supported on open-tubular nitrogen-doped carbon columns, containing Ni, Co, and Fe with excellent stoichiometric controllability as well as element distribution uniformity. We observed strong synergistic effects between different metal components; in particular, a small concentration of Fe significantly improves the electrocatalytic activity and the presence of Co dramatically reduces the Tafel slopes. Through contour mapping of the intrinsic kinetic parameters, the best electrocatalytic parameters of the series were attained with a composition of Ni40Co40Fe20O x , showing a geometric current density of 10 mA cm–2 at 220 mV overpotential, alongside a Tafel slope of ∼60 mV dec–1 and a good stability over 50 h. This work illuminates the importance of synergistic effects between various metal compositions and provides a solid foundation for the broad exploration of other mixed transition metal oxide combinations.

Challenges and Opportunities of Transition Metal Oxides as Electrocatalysts

“…Reproduced with permission from ref. [27]; copyright: 2020, American Chemical Society. g) HRTEM image of a Mn 7.5 O 10 Br 3 surface structure and h) OER performance of different Mn-based oxide catalysts.…”

“…Because of the low‐cost, high abundance, and the anticorrosion properties, substantial efforts have been devoted to OER catalysts including low‐cost TMOs, [24] perovskite, [25] spinel, [23] and the layer structure TMO‐like materials [26] . Most OER catalysts such as Fe‐, Co‐, and Ni‐based TMOs (Figure 2d, e) have been studied as electrocatalysts for water oxidation in base [27] . However, the development of catalysts in acid (especially, in strong acid) is particularly challenging.…”

“…[26] Most OER catalysts such as Fe-, Co-, and Ni-based TMOs (Figure 2d, e) have been studied as electrocatalysts for water oxidation in base. [27] However, the development of catalysts in acid (especially, in strong acid) is particularly challenging. Recently, Pan et al found that a bulk-stable manganese oxybromide (Mn 7.5 O 10 Br 3 ) catalyst had promising OER activity in strong acid and maintained superior stability under OER operating conditions for > 500 h (Figure 2g-i).…”

Understanding the Activity of Carbon-Based Single-Atom Electrocatalysts from Ab Initio Simulations

“…The diffusion of intermediates is feasible when considering traditional catalysts, whose active sites are near-continuous, such as extended surfaces and steps. 12,13 The site-confinement of reaction intermediates in single-atom catalysis could result in unconventional reaction mechanisms, especially for reactions involving the dissociation of reactants or inter-mediates, such as oxygen reduction reaction (ORR) and nitrogen reduction reaction. 14 This has a substantial effect on the design and optimization of catalysts.…”

“…As heterogeneous catalysts, single-atom catalysts should be atomically dispersed and anchored on substrates, such as metal oxide and graphene. , Their active sites are separated from each other; thus, the reaction intermediates normally have large energy barriers to diffuse between active sites. The diffusion of intermediates is feasible when considering traditional catalysts, whose active sites are near-continuous, such as extended surfaces and steps. , The site-confinement of reaction intermediates in single-atom catalysis could result in unconventional reaction mechanisms, especially for reactions involving the dissociation of reactants or intermediates, such as oxygen reduction reaction (ORR) and nitrogen reduction reaction . This has a substantial effect on the design and optimization of catalysts.…”