[en] Highlights: • The first example of heterogeneous electrocatalysis based on double activation is presented. • Introduction of a secondary organic catalyst to a primary inorganic catalyst improved ORR activity. • The secondary organic catalyst donated its electron to oxygen molecule to form O₂^δ-. • Also, the secondary organic catalyst donated its proton to the single oxygen intermediate (*O). Synergistic effects of dual homogeneous catalysts for chemical reactions have been reported. Double activation (chemical transformation process where both catalysts work in concert to activate reactants or intermediates) was often responsible for the synergistic effects of dual catalyst systems. Herein, we demonstrate the extension of the double activation from chemo-catalysis to electrocatalysis. The activity of low-cost cobalt oxide electrocatalysts for oxygen reduction reaction (ORR) was significantly improved by introducing secondary-amine-conjugated polymers (HN-CPs) as the secondary promoting electrocatalyst (shortly, promoter). It was proposed that HN-CPs activated neutral diatomic oxygen to partially charged species (O₂^δ-) in the initial oxygen adsorption step of ORR. Electron donation number of HN-CP to diatomic oxygen (δ in O₂^δ-) well described the order of activity improvement, i.e., polypyrrole (pPy) > polyaniline (pAni) > polyindole (pInd). The maximum overpotential gain at ~150 mV was achieved by using pPy with the highest δ. Also, it was confirmed that proton of HN-CP was transferred to single oxygen intermediate (*O) of ORR.

[en] Highlights: • Effects of the support structure-activity relationship on ORR were demonstrated. • Pt was deposited on edge-dominant Ti3C2 to basal-dominant Ti3C2. • Edge-dominant Ti3C2 donates more electrons to Pt compared to basal-dominant Ti3C2. • Electron-rich Pt on edge-dominant Ti3C2 shows superior activity and stability. It is demonstrated that the electroactivity of the oxygen reduction reaction (ORR) of Pt depends on the structure of a support. Highly conductive two-dimensional titanium carbide (Ti₃C₂) was selected as the support for Pt because of the expected strong metal-support interaction (SMSI) between Pt and Ti. To control the edge-to-basal ratio, the number of Ti₃C₂ layers was modulated by exfoliation. Pt nanoparticles (4 nm) were loaded on three different Ti₃C₂ supports including multi-, few-, and mono-layered Ti₃C₂ (22L-, 4L-, and 1L-Ti₃C₂, respectively). The edge-to-basal ratio of layered Ti₃C₂ increased as the number of layers increased. The edge-dominant support (22L-Ti₃C₂) donated more electrons to Pt than the basal-dominant supports (4L-Ti₃C₂ and 1L-Ti₃C₂). As a result, electron-rich Pt with less d-band vacancies (e.g., Pt/22L-Ti₃C₂) showed higher ORR activity. In addition, the electron transfer from the support to Pt inducing the strong interaction between Pt and Ti improved the durability of the ORR electroactivity of Pt.

[en] Highlights: • We report a partially hydrous RuO₂ nanocluster embedded in a carbon matrix (0.27-RuO₂@C) as a 4-in-1 electrocatalyst. • 0.27-RuO₂@C shows an excellent water-splitting performance at pH 0, pH 14 and even pH 7. • Also, it drives Pt-level hydrogen oxidation as well as fairly good oxygen reduction. • Moreover, the tetra-functionality of 0.27-RuO₂@C shows the possibility of realizing single-catalyst regenerative fuel cells. -- Abstract: Partially hydrous RuO₂ nanocluster embedded in a carbon matrix (x-RuO₂@C with x = hydration degree = 0.27 or 0.27@C) is presented as a bifunctional catalyst for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) for water splitting. Symmetric water electrolyzers based on 0.27-RuO₂@C for both electrodes showed smaller potential gaps between HER and OER at pH 0, pH 14 and even pH 7 than conventional asymmetric electrolyzers based on two different catalysts (Pt/C || Ir/C) that have been known as the best catalysts for HER and OER respectively. Moreover, 0.27-RuO₂@C showed another bifunctional electroactivity for fuel cell electrochemistry involving hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) that are the backward reactions of HER and OER respectively. Pt-level HOR electroactivity was obtained from 0.27-RuO₂@C, while its ORR activity was inferior to that of Pt with 200 mV higher overpotential required. The tetra-functionality of 0.27-RuO₂@C showed the possibility of realizing single-catalyst regenerative fuel cells.