A cobalt phthalocyanine having an electron-poor CoN4 (+δ) in its phthalocyanine moiety was presented as an electrocatalyst for hydrogen peroxide oxidation reaction (HPOR). We suggested that hydrogen peroxide as an electrolysis medium for hydrogen production and therefore as a hydrogen carrier, demonstrating that the electrocatalyst guaranteed high hydrogen production rate by hydrogen peroxide splitting. The electron deficiency of cobalt allows CoN4 to have the highly HPOR-active monovalent oxidation state and facilitates HPOR at small overpotentials range around the onset potential. The strong interaction between the electron-deficient cobalt and oxygen of peroxide adsorbates in Co─OOH− encourages an axially coordinated cobalt oxo complex (O═CoN4) to form, the O═CoN4 facilitating the HPOR efficiently at high overpotentials. Low-voltage oxygen evolution reaction guaranteeing low-voltage hydrogen production is successfully demonstrated in the presence of the metal–oxo complex having electron-deficient CoN4. Hydrogen production by 391 mA cm−2 at 1 V and 870 mA cm−2 at 1.5 V is obtained. Also, the techno-economic benefit of hydrogen peroxide as a hydrogen carrier is evaluated by comparing hydrogen peroxide with other hydrogen carriers such as ammonia and liquid organic hydrogen carriers. 

Presented herein is the possibility of activating water molecules associatively by cations in electrolytes, as well as active sites of electrocatalysts. Cation–water interaction (CW), weakening the intramolecular O–H bonds of water molecules, significantly affected dissociation of water molecules in proton-deficient media, resulting in hydrogen evolution reaction (HER) based on water reduction reaction (WRR). Both the quantitative and qualitative hydration nature of cations (i.e., hydration strength and number) determined the strength of CW and therefore the WRR activity. The cationic dependency of CW on electrolytes was confirmed by bulk and surface-specific spectroscopic techniques. After the cation–water complexes based on strong CW (cation(water)n) were defined as the main reactants for WRR, the intermediate adsorbate on the catalyst surface was suggested to be water molecules doubly coordinated to cations as well as active sites (cation-water-catalyst). The double activation picture of the tricomponent intermediate was strongly supported by the surface-specific Raman spectra, confirming the polarization-induced weakening of the O–H bonds of water molecules near the Pt catalyst surface in addition to the CW-induced O–H weakening found in the electrolyte as well as on the catalyst surface. The smallest divalent Be2+ among a series of test cations, including the monovalent, divalent, and trivalent ones, showed the most remarkable WRR kinetic gain, the superiority of which was expected from its high charge density nature guaranteeing strong hydration strength and cationic acidity. The beryllium anomaly to eminently weaken the O–H bonds accelerated WRR at pH2 with 600 mV overpotential gain for 150 mA cm–2 hydrogen production (c.f., 28 mA cm–2 with Na+). 

In order to realize electrochemically efficient hydrogen production, various endeavors have been devoted to developing hydrogen evolution reaction (HER) electrocatalysts having zero hydrogen binding energy (ΔGH* = 0) for balancing between adsorption and desorption. This work demonstrated that nitrogen doping improved the HER activity of ruthenium oxide by letting its ΔGH* approach zero or facilitating hydrogen desorption process. A highly nitrogen-doped ruthenium oxide catalyst guaranteeing the ruthenium-nitrogen intimacy was prepared by employing a polymer whose nitrogen-containing moiety (pyrrolidone) was strongly coordinated to ruthenium ion in the precursor solution prior to calcination. The less electronegative nature of nitrogen (when compared with oxygen) decreased the free energy uphill required for desorption of hydrogen intermediate species sitting on the nitrogen (H-*N to 1/2 H2 + *N) to make the desorption process more favored. Also, the nitrogen dopant facilitated OH- desorption from its neighboring ruthenium site (HO-*Ru + e- to HO- + *Ru) since the less electronegative nitrogen withdrew less electrons from the ruthenium site. The ruthenium-nitrogen intimacy of the catalyst more than doubled the electrocatalytic HER current from 33 mA cm-2 for an undoped RuO2 to 79 mA cm-2 for the nitrogen-doped RuO2 at -50 mVRHE. 

Catalyst-support interaction triggering biased electron flows between catalyst and reactant has been studied for electrocatalysis. The interaction was limited to the interfacial region between catalyst and support when nanoparticular catalysts, which are bulky from the viewpoint of atomic dimension, were employed. To clarify and maximize the effects of supports, herein, we investigated the catalyst-support interaction of a molecular catalyst loaded on a support. Iron phthalocyanine (FePc) as the molecular catalyst for oxygen reduction reaction (ORR) was loaded on two-dimensional monolayer leaf of titanium carbide (1L-Ti3C2). The strong interaction between Fe of FePc and Ti of 1L-Ti3C2 developed via FeTi2 coordination encouraged the square planar structure of FePc to be concavely distorted. The electron-rich Fe active site having extra electrons given by less electronegative Ti of Ti3C2 allowed the single oxygen intermediate species (*O) readily protonated to be *OH, moving the RDS to the desorption step having a lower free energy uphill or kinetic barrier. Resultantly, the strong FePc-Ti3C2 interaction decreased the potential required for reducing oxygen and moreover completed ORR via four-electron (4e) process rather than 2e ORR. The catalyst durability was also improved due to the absence of peroxide generated from the 2e process. 

Theoretical computational studies have claimed that the catalytic activity of a family of heterogeneous catalysts (e.g., metal catalysts) is governed by a linear scaling relationship (LSR) between adsorption energy levels of intermediates on active sites of catalysts. The volcano shape of the activity versus the adsorption energy of one of the intermediates was obtained from the LSR and the Brønsted–Evans–Polanyi relationship. An improved activity can be achieved using a catalyst having optimized adsorption energy of the volcano or alternatively by circumventing or breaking the LSR. Herein, we demonstrated that the LSR of a series of transition metal terephthalates (MTPs; M = Fe, Co, Ni, Cu, or Zn) as electrocatalysts for the oxygen reduction reaction (ORR) was broken in the presence of polypyrrole (pPy) as a proton donor. The reason for the LSR breakage was that the intermediate to which the proton of pPy was delivered was different depending on the metal of MTP. Also, pPy affected the adsorption energy of the specific intermediate (the target of the proton transfer) more strongly while the other intermediates were less affected by pPy. Experimentally as well as theoretically, pPy significantly improved the ORR activity of MTPs, altering the activity volcano plot. The most significant improvement was found on CoTP: the onset potential of ORR on CoTP was shifted toward the more easy-to-be reduced direction from 0.7 to 0.85 VRHE at 1 mA cm–2. 

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 (O2δ-) in the initial oxygen adsorption step of ORR. Electron donation number of HN-CP to diatomic oxygen (δ in O2δ-) 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. 

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. 

This study demonstrates the impact outer membrane permeability has on the power densities generated by E. coli-based microbial fuel cells with neutral red as the mediator, and how increasing the permeability improves the current generation. Experiments performed with several lipopolysaccharide (LPS) mutants (ΔwaaC, ΔwaaF and ΔwaaG) of E. coli BW25113 that increase the outer membrane permeability found the power generated by two of the truncated LPS mutants, i.e., ΔwaaC and ΔwaaF, to be significantly higher (5.6 and 6.9 mW/m2, respectively) than that of the wild-type E. coli BW25113 (2.6 mW/m2). Branched polyethyleneimine (BPEI, 400 mg/L), a strong chemical permeabilizer, was more effective, however, increasing the power output from E. coli BW25113 cultures to as much as 29.7 mW/m2, or approximately 11-fold higher than the control MFC. BPEI also increased the activities of the mutant strains (to between 10.6 and 16.3 mW/m2), as well as when benzyl viologen was the mediator. Additional tests found BPEI not only enhanced membrane permeability but also increased the zeta potential of the bacterial cells from a value of −43.4 mV to −21.0 mV. This led to a significant increase in auto-aggregation of the bacterial cells and, consequently, better adherence of the cells to the anode electrode, as was demonstrated using scanning electron microscopy. In conclusion, our study demonstrates the importance of outer membrane permeabilities on MFC performances and defines two benefits that BPEI offers when used within MFCs as an outer membrane permeabilizer. 

Nitrogen-containing electrocatalysts such as metal-nitrogen-carbon (M-N-C) composites and nitrogen-doped carbons are known to exhibit high activities for oxygen reduction reaction (ORR). Even if the mechanism by which nitrogen improve the activities is not completely understood, strong electronic interaction between nitrogen and active sites has been found in these composites. Herein, we demonstrate a case in which nitrogen improves electroactivity, but in the absence of strong interaction with other components. The overpotentials of ORR and oxygen evolution reaction (OER) on perovskite oxide catalysts were significantly reduced simply by mixing the catalyst particles with polypyrrole/carbon composites (pPy/C). Any strong interactions between pPy (a nitrogen-containing compound) and active sites of the catalysts were not confirmed. A scenario based on the sequential role allocation between pPy and the oxide catalysts for ORR was proposed: (1) molecular oxygen is incorporated into pPy as a form of superoxide (pPy+O2-); (2) the superoxide is transferred to the active sites of perovskite catalysts; and (3) the superoxide is completely reduced along 4e ORR process. 

Understanding Li-O2 electrochemistry without ambiguities is the basis required for achieving high energy density with efficient cycling for lithi-um-air batteries. Oxygen reduction reaction (ORR) on carbon is supposed to proceed via a three step mechanism: oxygen (O20) to superoxide (O2- or LiO2) to peroxide (O22- or Li2O2) to oxide (O2- or Li2O). In this work, we provide clear evidences relevant to three controversial issues: (1) whether the superoxide intermediate is really formed; (2) whether the superoxide exists for a significant time period or they are immediately con-verted to the peroxide species; and (3) whether conversion of peroxide to oxide is feasible or the final discharge product of ORR is not oxide but peroxide. ORR on carbon electrode with LiClO4 or LiPF6 in dimethyl sulfoxide (DMSO) as an electrolyte was used as a model system. In addi-tion to conventional voltammetry and Raman spectroscopy, the staircase cyclic voltammetry combined with Fourier transform electrochemical impedance spectroscopy (SCV-FTEIS) was used to investigate the Li-O2 electrochemistry in situ during potential scans. Molecular oxygen was quasi-reversibly reduced to superoxide in the first step. The superoxide was stable enough to be detected in the electrolytes. The superoxide was reduced to peroxide that existed as a surface-adsorbed species. Reduction proceeded further to produce oxide as its final product. Also, Li2CO3 formation resulting from electrolyte decomposition/electrode corrosion was observed only with LiClO4. In addition, we identified a novel chemical route for the oxide formation occurring even at not-enough overpotential: peroxide is further reduced to oxide by the help of superoxide as an in situ formed one-electron reducing agent. This report gives a clear picture of the ORR mechanism on carbon electrode in Li+-containing non-aqueous electrolyte by providing evidences for the formation of superoxide intermediate and oxide for the first time. 

Conductivity makes the difference: Conductive environments surrounding active sites, achieved by more conductive perovskite catalysts (BSCFO, NBSCO) or higher carbon contents, result in a higher number of electrons transferred during complete four-electron (4e) reduction of oxygen, changing the rate-determining step from a two-step 2e process to a single-step 1e process.