Since
severe global warming and related climate issues have been
caused by the extensive utilization of fossil fuels, the vigorous
development of renewable resources is needed, and transformation into
stable chemical energy is required to overcome the detriment of their
fluctuations as energy sources. As an environmentally friendly and
efficient energy carrier, hydrogen can be employed in various industries
and produced directly by renewable energy (called green hydrogen).
Nevertheless, large-scale green hydrogen production by water electrolysis
is prohibited by its uncompetitive cost caused by a high specific
energy demand and electricity expenses, which can be overcome by enhancing
the corresponding thermodynamics and kinetics at elevated working
temperatures. In the present review, the effects of temperature variation
are primarily introduced from the perspective of electrolysis cells.
Following an increasing order of working temperature, multidimensional
evaluations considering materials and structures, performance, degradation
mechanisms and mitigation strategies as well as electrolysis in stacks
and systems are presented based on elevated temperature alkaline electrolysis
cells and polymer electrolyte membrane electrolysis cells (ET-AECs
and ET-PEMECs), elevated temperature ionic conductors (ET-ICs), protonic
ceramic electrolysis cells (PCECs) and solid oxide electrolysis cells
(SOECs).
The faradaic process of transition-metal-based oxides
or/and hydroxides
is widely utilized to increase the capacity of electrochemical energy
storage significantly. However, poor electrical conductivity and low
stability of these materials are two crucial factors hindering the
maximization of their potential application. To address this issue,
carbon–transition-metal oxide composites are strategized to
combine the high electrical conductivity of carbon materials and the
capacity of transition metals. Here, inspired by the self-assembly
process of metal–organic frameworks, a hollow carbon nanosphere
(HCNS)@Ni precursor/rGO is synthesized by a one-pot hydrothermal method.
HCNS@Ni nanoparticles/rGO (HCNS@Ni NP/rGO) is derived in subsequent
calcination. The synthesized HCNS@Ni NP/rGO was assembled as an electrode
and evaluated with outstanding electrochemical properties and performances.
A high specific capacity of 1589 F g–1 at a current
density of 0.3 A g–1 and even 733.33 F g–1 at 50 A g–1 are obtained, with 46.2% of capacity
remaining. After 1600 charge–discharge cycles at 10 A g–1, the assembled electrode exhibits the desirable capacity
retention of 77.9%.
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