Orthorhombic
iron niobate (FeNbO4) has a band structure to form an effective
heterojunction with hematite to make an efficient photoanode for photoelectrochemical
water splitting. However, this high temperature phase is difficult
to synthesize by conventional thermal annealing (CTA) without damaging
F:SnO2 (FTO) substrate. In contrast, hybrid microwave annealing
(HMA) selectively forms a few atomic overlayers of highly crystalline
orthorhombic FeNbO4 phase covering hematite nanorods in
an extremely short time (2 min) without any FTO damage forming a core–shell-type
Fe2O3@FeNbO4 nanorod heterojunction
on FTO. At the same time, hematite is codoped naturally with Nb and
Sn during the HMA synthesis by diffusion from FeNbO4 and
FTO, respectively. The optimized Nb,Sn:Fe2O3@FeNbO4/FTO electrode loaded with NiFeO
x
cocatalyst achieves a stable photocurrent density of 2.71
mA cm–2 at 1.23 VRHE under simulated
sunlight (100 mW cm–2) with ∼100% faradaic
efficiency of hydrogen production, which is ∼3.4 times higher
than that of bare hematite prepared by CTA (0.8 mA cm–2).
We report here a
facile, one-step precipitating metal nitrate deposition
(PMND) method to prepare amorphous metal oxyhydroxide films containing
Fe, Co, and Ni as efficient electrocatalysts for water oxidation.
The unique synthesis technique allows easy control of the metal composition
over a wide range on various substrates. A series of unary and binary
metal oxyhydroxides of 30 compositions are synthesized by PMND on
fluorine-doped tin oxide (FTO) substrate as water oxidation electrocatalysts.
The activity of the metal oxyhydroxide films is represented by a volcano
plot as a function of a single experimental descriptor, i.e., the
fraction of hydroxide in the surface oxygen species. The optimum compositions
for binary metal oxyhydroxide (NiFe, NiCo, and CoFe) are determined
on conductive substrates of FTO, nickel foam (NF), nickel mesh (NM),
and carbon felt (CF), and the best NiFe (2:8) electrocatalyst on NF
exhibits a water oxidation current density of 100 mA/cm2 with only 280 mV of overvoltage, which outperforms conventional
noble metal catalysts like IrO
x
and RuO
x
in an alkaline medium. Finally, we demonstrate
a tandem PV–electrolysis system by using a c-Si PV module with
a power conversion efficiency of 13.71% and an electrochemical cell
composed of NiFe (2:8)/NF anode and a bare NF cathode with a conversion
efficiency of 71.8%, which records a solar-to-hydrogen conversion
efficiency of 9.84%.
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.
Benchmark solar to hydrogen conversion efficiency of 18.7% was achieved via photovoltaic cell-electrolyzer system without using well-known expensive III-V-based light absorber. Comparison with previous studies indicates that this efficiency bench is impressive for using less junction (2jn instead of 34jn for III/V), relatively cheap perovskite and silicone solar cell, and high solar electricity to chemical efficiency (74.2%).
Highly efficient electrocatalysts for the oxygen evolution
reaction
(OER) in neutral electrolytes are indispensable for practical electrochemical
and photoelectrochemical water splitting technologies. However, there
is a lack of good, neutral OER electrocatalysts because of the poor
stability when H+ accumulates during the OER and slow OER
kinetics at neutral pH. Herein, we report Ir species nanocluster-anchored,
Co/Fe-layered double hydroxide (LDH) nanostructures in which the crystalline
nature of LDH-restrained corrosion associated with H+ and
the Ir species dramatically enhanced the OEC kinetics at neutral pH.
The optimized OER electrocatalyst demonstrated a low overpotential
of 323 mV (at 10 mA cm–2) and a record low Tafel
slope of 42.8 mV dec–1. When it was integrated with
an organic semiconductor-based photoanode, we obtained a photocurrent
density of 15.2 mA cm–2 at 1.23 V versus reversible
hydrogen in neutral electrolyte, which is the highest among all reported
photoanodes to our knowledge.
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