Ruthenium oxide (RuO2) is the best oxygen evolution reaction (OER) electrocatalyst. Herein, we demonstrated that RuO2 can be also efficiently used as an oxygen reduction reaction (ORR) electrocatalyst, thereby serving as a bifunctional material for rechargeable Zn–air batteries. We found two forms of RuO2 (i.e. hydrous and anhydrous, respectively h-RuO2 and ah-RuO2) to show different ORR and OER electrocatalytic characteristics. Thus, h-RuO2 required large ORR overpotentials, although it completed the ORR via a 4e process. In contrast, h-RuO2 triggered the OER at lower overpotentials at the expense of showing very unstable electrocatalytic activity. To capitalize on the advantages of h-RuO2 while improving its drawbacks, we designed a unique structure (RuO2@C) where h-RuO2 nanoparticles were embedded in a carbon matrix. A double hydrophilic block copolymer-templated ruthenium precursor was transformed into RuO2 nanoparticles upon formation of the carbon matrix via annealing. The carbon matrix allowed overcoming the limitations of h-RuO2 by improving its poor conductivity and protecting the catalyst from dissolution during OER. The bifunctional RuO2@C catalyst demonstrated a very low potential gap (ΔE
OER-ORR = ca. 1.0 V) at 20 mA cm−2. The Zn||RuO2@C cell showed an excellent stability (i.e. no overpotential was observed after more than 40 h).
Silicon nanowires (SiNWs) were prepared by chemically etching silicon wafers with silver nanoparticles. Their electrical conductivities and porosities were tuned by adjusting the doping concentration of silicon wafers from which the SiNWs were prepared. Porosity of the SiNWs were proportional to doping concentrations of the mother wafer because the dopant population provides nucleation sites for etching. The electrical conductivities of the doped SiNWs were 100 times higher than those of the intrinsic SiNW. However, there was no difference in the conductivity between two different doping level SiNWs (Na = 2.7 × 1015 and 5.7 × 1019) due to the trade-off between porosity and the intrinsic conductivity of the solid backbone. The doping-dependent properties of SiNWs determined the capacity, stability and kinetics of the lithium alloying reaction of the SiNWs. The medium-level doping SiNWs, characterized by a mechanically obust porous structure, showed the most improved electrochemical performances in a full cell of a lithium manganese oxide || SiNW battery as a result of the balanced trade-off between coulombic efficiency and capacity retention.
A powerful
synthetic protocol based on a molecularly engineered
anchoring carbon platform (ACP) is reported to stabilize concentrated
edge-hosted single-atom catalytic sites of M–N (M = Fe, Co,
Ni, Cu) on carbon supports. Polymerization with l-cysteine
as an additional organic precursor produces an ACP sheath around the
carbon nanotube (CNT)–graphene (GR) hybrid support made of
a small domain size with abundant edge sites and doped with sulfur.
A few-minute-long microwave pyrolysis anchors strongly the single-atomic
M–N moiety on the ACP while suppressing its agglomeration during
the high-temperature synthesis and makes the ACP highly graphitized.
As a typical example, the edge-hosted single-atomic catalytic sites
in Fe–N/S-CNT–GR provide superior pH-independent oxygen
reduction reaction (ORR) activity to previously reported Fe–N–C
catalysts and commercial Pt/C while demonstrating oxygen evolution
reaction (OER) activity in basic conditions similar to known state-of-the-art
catalysts. In particular, the Fe–N/S-CNT–GR catalyst
is much more stable than commercial Pt/C and Ir/C catalysts during
ORR and OER in both base and acid solutions. Inferior stability is
a common problem of this type of single-atom heterogeneous catalyst
(SAC). An aqueous Zn–air battery with our Fe–N/S-CNT–GR
catalyst operates as effectively as the device with the commercial
Pt/C–Ir/C catalysts. We believe that our protocol based on
the molecularly engineered ACP and microwave pyrolysis can provide
a new concept to synthesize a new generation of durable SACs, which
could have broad applications in electrochemical energy conversion
and storage.
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