Conspectus Photocatalytic solar fuel production, for example, production of hydrogen via water-splitting, is an effective means of chemical storage of solar energy and provides a potential option for ...achieving a zero-emissions energy system. Conveniently, hydrogen can be converted back to electricity either via fuel cells or through combustion in gas turbines, or it can be mixed in low concentrations with natural gas or biogas for combustion in existing power plants. The cornerstone of a practical solar fuel production process is a stable, efficient, and scalable photocatalyst (a semiconductor material that accommodates photon absorption, charge carrier generation and transport, and catalytic reactions). Therefore, the quest for suitable photocatalyst materials is an ongoing process. Recently, carbon nitride (CN) has attracted widespread interest as a metal-free, earth-abundant, and highly stable photocatalyst. However, the catalytic efficiency of CN is not satisfactory because of its poor charge transport attributes. There is a direct relation between the photocatalytic efficiency and charge transport because the basic principle of light-promoted overall photodecomposition of water into H2 and O2 molecules (or, generally speaking, photoredox reactions) relies on separation and subsequent transfer of excited-state electron–hole pairs to relative redox couples. However, the excited states last for a very short time, typically nanoseconds to microseconds in liquids, and unless they are separated within this time frame, the excited-state electron–hole pairs undergo recombination with release of the captured light energy as heat or photon emission. To utilize light in a form other than heat or emitted photons by avoiding the recombination of excited-state electron–hole pairs, charged excitons must be scavenged before the absorption of subsequent photons to sustain a multielectron photoredox reaction. Otherwise, the extraction of charges becomes more difficult. This imposes a potential efficiency-limiting factor. An enhancement in water to hydrogen conversion efficiency in CN therefore requires the use of precious-metal cocatalysts (e.g., Pt) and sacrificial electron donor/acceptors to facilitate multielectron/multiproton transfers to overcome the high kinetic barriers. The use of Pt and sacrificial agents is not consistent with the notion of low-cost and sustainable hydrogen production from water. CN must overcome this dependence to stand out as a truly scalable photocatalyst. To make progress, the foremost requirement is to attain an in-depth understanding of the fundamental charge transport phenomena needed for the rational design of CN-based photocatalysts. In this Account, therefore, our aim is to provide a synopsis of current understanding and progress regarding charge-transport-related phenomena (e.g., recombination, trapping, transfer of charge carriers, etc.) and to discuss the effects of charge transport in enhancing the apparent quantum yield of hydrogen production in CN. This understanding is necessary to broaden the scope of CN for other catalytic applications, for example, efficient CO2 reduction to methanol or methane, fixation of nitrogen to ammonia, and use as an active material in solar cells. We also identify research gaps and issues to be addressed for a more clear elucidation of charge-transport-related phenomena in CN. Thus, this Account may inspire new research opportunities for tuning the extrinsic/intrinsic photophysicochemical properties of CN by rational design to attain the most favorable properties for improved catalytic efficiency.
Abstract
Doping is a well-known strategy to enhance the electrochemical energy storage performance of layered cathode materials. Many studies on various dopants have been reported; however, a general ...relationship between the dopants and their effect on the stability of the positive electrode upon prolonged cell cycling has yet to be established. Here, we explore the impact of the oxidation states of various dopants (i.e., Mg
2+
, Al
3+
, Ti
4+
, Ta
5+
, and Mo
6+
) on the electrochemical, morphological, and structural properties of a Ni-rich cathode material (i.e., LiNi
0.91
Co
0.09
O
2
). Galvanostatic cycling measurements in pouch-type Li-ion full cells show that cathodes featuring dopants with high oxidation states significantly outperform their undoped counterparts and the dopants with low oxidation states. In particular, Li-ion pouch cells with Ta
5+
- and Mo
6+
-doped LiNi
0.91
Co
0.09
O
2
cathodes retain about 81.5% of their initial specific capacity after 3000 cycles at 200 mA g
−1
. Furthermore, physicochemical measurements and analyses suggest substantial differences in the grain geometries and crystal lattice structures of the various cathode materials, which contribute to their widely different battery performances and correlate with the oxidation states of their dopants.
We report hydrothermal synthesis of single crystalline TiO2 nanowire arrays with unprecedented small feature sizes of ∼5 nm and lengths up to 4.4 μm on fluorine-doped tin oxide substrates. A ...substantial amount of nitrogen (up to 1.08 atomic %) can be incorporated into the TiO2 lattice via nitridation in NH3 flow at a relatively low temperature (500 °C) because of the small cross-section of the nanowires. The low-energy threshold of the incident photon to current efficiency (IPCE) spectra of N-modified TiO2 samples is at ∼520 nm, corresponding to 2.4 eV. We also report a simple cobalt treatment for improving the photoelectrochemical (PEC) performance of our N-modified TiO2 nanowire arrays. With the cobalt treatment, the IPCE of N-modified TiO2 samples in the ultraviolet region is restored to equal or higher values than those of the unmodified TiO2 samples, and it remains as high as ∼18% at 450 nm. We propose that the cobalt treatment enhances PEC performance via two mechanisms: passivating surface states on the N-modified TiO2 surface and acting as a water oxidation cocatalyst.
Conspectus The enhanced catalytic activity of Pd–Au catalysts originates from ensemble effects related to the local composition of Pd and Au. The study of Pd–Au planar model catalysts in an ultrahigh ...vacuum (UHV) environment allows the observation of molecular level catalytic reactions between the Pd–Au surface and target molecules. Recently, there has been progress in understanding the behavior of simple molecules (H2, O2, CO, etc.) employing UHV surface science techniques, the results of which can be applied not only to heterogeneous catalysis but also to electro- and photochemical catalysis. Employing UHV methods in the investigation of Pd–Au model catalysts has shown that single Pd atoms can dissociatively adsorb H2 molecules. The recombinative desorption temperature of H2 varies with Pd ensemble size, which allows the use of H2 as a probe molecule for quantifying surface composition. In particular, H2 desorption from Pd–Au interface sites (or small Pd ensembles) is observed from 150–300 K, which is between the H2 desorption temperature from pure Au (∼110 K) and Pd (∼350 K) surfaces. When the Pd ensembles are large enough to form Pd(111)-like islands, H2 desorption occurs from 300–400 K, as with pure Pd surfaces. The different H2 desorption behavior, which depends on Pd ensemble size, has also been applied to the analysis of dehydrogenation mechanisms for potential liquid storage mediums for H2, namely formic acid and ethanol. In both cases, the Pd–Au interface is the main reaction site for generating H2 from formic acid and ethanol with less overall decomposition of the two molecules (compared to pure Pd). The chemistry behind O2 activation has also been informed through the control of Pd ensembles on a gold model catalyst for acetaldehyde and ethanol oxidation reactions under UHV conditions. O2 molecules molecularly adsorbed on continuous Pd clusters can be dissociated into O adatoms above 180 K. This O2 activation process is improved by coadsorbed H2O molecules. It is also possible to directly (through a precursor mechanism) introduce O adatoms on the Pd–Au surface by exposure to O2 at 300 K. The quantity of dissociatively adsorbed O adatoms is proportional to the Pd coverage. However, the O adatoms are more reactive on a less Pd covered surface, especially at the Pd–Au interface sites, which can initiate CO oxidation at temperatures as low as 140 K. Acetaldehyde molecules can be selectively oxidized to acetic acid on the Pd–Au surface with O adatoms, in which the selectivity toward acetic acid originates from preventing the decarboxylation of acetate species. Moreover, the O adatoms on the Pd–Au surface accelerate ethanol dehydrogenation, which causes the increase in acetaldehyde production. Hydrogen is continuously abstracted from the formed acetaldehyde and remaining ethanol molecules, and they ultimately combine as ethyl acetate on the Pd–Au surface. Using Pd–Au model catalysts under UHV conditions allows the discovery of molecular level mechanistic details regarding the catalytic behavior of H and O adatoms with other molecules. We also expect that these findings will be applicable regarding other chemistry on Pd–Au catalysts.
Forty years after the failed introduction of rechargeable lithium-metal batteries and 30 years after the successful commercialization of the lower capacity, graphite-anode-based lithium-ion battery ...by Sony, demand for higher energy density batteries is leading to reinvestigation of the problem of dendrite growth that makes the metallic lithium anodes unsafe and prevented commercialization to begin with. One strategy to mitigate dendrite growth is to deposit thin, tailored, corrosion-passivating coatings on the metallic lithium, instead of allowing the metal to spontaneously react with the organic electrolyte solution to form its passivating solid electrolyte interface (SEI). The challenge is to find and to deposit a coating that is electronically insulating yet allows uniform permeation of Li+ at a high cycling rate, such that Li-metal is electrodeposited uniformly on the nanoscale below the tailored coating. Recently, a number of studies have examined multicomponent films, taking advantage of the properties of two different materials, which can be tuned separately or chosen for their complementary properties. Use of these multicomponent coatings will likely enable future researchers to create rationally designed SEIs capable of effectively suppressing the growth of Li dendrites. This review discusses recent developments in micro- and nanoscale tailored coatings to meet that need.
Reaching the goal of economical photoelectrochemical (PEC) water splitting will likely require the combination of efficient solar absorbers with high activity electrocatalysts for the hydrogen and ...oxygen evolution reactions (HER and OER). Toward this goal, we synthesized an amorphous FeOOH (a-FeOOH) phase that has not previously been studied as an OER catalyst. The a-FeOOH films show activity comparable to that of another OER cocatalyst, Co-borate (Co–Bi), in 1 M Na2CO3, reaching 10 mA/cm2 at an overpotential of ∼550 mV for 10 nm thick films. Additionally, the a-FeOOH thin films absorb less than 3% of the solar photons (AM1.5G) with energy greater than 1.9 eV, are homogeneous over large areas, and act as a protective layer separating the solution from the solar absorber. The utility of a-FeOOH in a realistic system is tested by depositing on amorphous Si triple junction solar cells with a photovoltaic efficiency of 6.8%. The resulting a-FeOOH/a-Si devices achieve a total water splitting efficiency of 4.3% at 0 V vs RHE in a three-electrode configuration and show no decrease in efficiency over the course of 4 h.
Deposition of materials as a thin film is important for various applications, such as sensors, microelectronic devices, and membranes. There have been breakthroughs in gas‐phase metal‐organic ...framework (MOF) thin‐film growth, which is more applicable to micro‐ and nanofabrication processes and also less harmful to the environment than solvent‐based methods. Three different types of gas‐phase MOF thin film deposition methods have been developed using chemical vapor deposition (CVD), atomic layer deposition (ALD), and physical vapor deposition (PVD)‐CVD combined techniques. The CVD‐based method basically converts metal oxide layers into MOF thin films by exposing the surface to ligand vapor. The ALD‐based method allows growing MOF thin films following layer‐by‐layer (LBL) growth by sequentially exposing gas‐phase metal and ligand precursors. The PVD‐CVD method uses PVD for metal deposition and CVD for ligand deposition, which is similar to LBL growth. These gas‐phase growth methods can broaden the use of MOFs in diverse areas. Herein, the current progress of gas‐phase MOF thin film growth is discussed and future directions suggested.
It′s a gas gas gas: Gas‐phase metal and organic ligand depositions are newly introduced for the growth of metal‐organic framework (MOF) thin films. Various gas‐phase deposition methods are employed for the growth of MOF thin films, which are environmentally friendly and also compatible to micro‐ and nanofabrication processes. The gas‐phase MOF thin‐film growth is expected to broaden the use of MOFs in sensors, microelectronic devices, membranes, and catalysts.
Pd–Au catalysts have shown exceptional performance for selective hydrogen production via HCOOH decomposition, a promising alternative to solve issues associated with hydrogen storage and ...distribution. In this study, we utilized temperature-programmed desorption (TPD) and reactive molecular beam scattering (RMBS) in an attempt to unravel the factors governing the catalytic properties of Pd–Au bimetallic surfaces for HCOOH decomposition. Our results show that Pd atoms at the Pd–Au surface are responsible for activating HCOOH molecules; however, the selectivity of the reaction is dictated by the identity of the surface metal atoms adjacent to the Pd atoms. Pd atoms that reside at Pd–Au interface sites tend to favor dehydrogenation of HCOOH, whereas Pd atoms in Pd(111)-like sites, which lack neighboring Au atoms, favor dehydration of HCOOH. These observations suggest that the reactivity and selectivity of HCOOH decomposition on Pd–Au catalysts can be tailored by controlling the arrangement of surface Pd and Au atoms. The findings in this study may prove informative for rational design of Pd–Au catalysts for associated reactions including selective HCOOH decomposition for hydrogen production and electro-oxidation of HCOOH in the direct formic acid fuel cell.
Nanoarchitecture of bismuth vanadate (BiVO4) photoanodes for effectively increasing light harvesting efficiency and simultaneously achieving high charge separation efficiency is the key to ...approaching their theoretic performance of solar-driven water splitting. Here, we developed hierarchical BiVO4 nanoporous sphere arrays, which are composed of small nanoparticles and sufficient voids for offering high capability of charge separation. Significantly, multiple light scattering in the sphere arrays and voids along with the large effective thickness of the BiVO4 photoanode induce efficient light harvesting. In addition, attributed to ultrathin two-dimensional Bi2WO6 nanosheets as the precursor, the synergy of various enhancement strategies including WO3/BiVO4 nanojunction formation, W-doping, and oxygen vacancy creation can be directly incorporated into such a unique hierarchical architecture during the one-step synthesis of BiVO4 without complex pre- or post-treatment. The as-obtained photoanode exhibits a water splitting photocurrent of 5.5 mA cm–2 at 1.23 V versus RHE under 1-sun illumination, among the best values reported up-to-date in the field.
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