Devices For Hydrogen Generation Research Articles

Graphene Enhanced Photo‐Electrochemical Water Splitting by Diminishing Pt/p‐Si Photocathode Interfacial Barrier

AbstractConverting solar energy into hydrogen by photo‐electrochemical cells is believed to be one of the most promising strategies to acquire clean energy. However, one barricade for further improving the performance of the photocathode system is flattening of the Schottky barrier formed at the interface between the p‐type photo‐absorber and metal co‐catalyst (e. g. Pt/p‐Si) due to the mismatch of their Fermi levels. Here, we present a facile way of making photocathodes by transferring high‐quality chemical vapor deposition (CVD) graphene onto p‐type Si as a buffer layer, which can effectively diminish the Pt/p‐Si interfacial barrier and promote charge separation efficiency, hence, improving the photo‐electrochemical performance of such a device for hydrogen generation. The measured photocurrent density is 3.9 times higher than that achieved without graphene buffer. This approach offers a novel route to decrease the Pt/p‐Si interfacial barrier for developing highly efficient solar conversion systems in the future. It can easily be extended to many other photocathode systems.

MOF-Derived In2 O3 /CuO p-n Heterojunction Photoanode Incorporating Graphene Nanoribbons for Solar Hydrogen Generation.

Solar-driven photoelectrochemical (PEC) water splitting is a promising approach toward sustainable hydrogen (H2 ) generation. However, the design and synthesis of efficient semiconductor photocatalysts via a facile method remains a significant challenge, especially p-n heterojunctions based on composite metal oxides. Herein, a MOF-on-MOF (metal-organic framework) template is employed as the precursor to synthesize In2 O3 /CuO p-n heterojunction composite. After incorporation of small amounts of graphene nanoribbons (GNRs), the optimized PEC devices exhibited a maximum current density of 1.51mA cm-2 (at 1.6V vs RHE) under one sun illumination (AM 1.5G, 100mW cm-2 ), which is approximately four times higher than that of the reference device based on only In2 O3 photoanodes. The improvement in the performance of these hybrid anodes is attributed to the presence of a p-n heterojunction that enhances the separation efficiency of photogenerated electron-hole pairs and suppresses charge recombination, as well as the presence of GNRs that can increase the conductivity by offering better path for electron transport, thus reducing the charge transfer resistance. The proposed MOF-derived In2 O3 /CuO p-n heterojunction composite is used to demonstrate a high-performance PEC device for hydrogen generation.

Modeling the Photostability of Solar Water-Splitting Devices and Stabilization Strategies.

The practical implementation of photoelectrochemical devices for hydrogen generation is limited by their short lifetimes. Understanding the factors affecting the stability of the heterogeneous photoelectrodes is required to formulate degradation mitigation strategies. We developed a multiscale and multiphysics model to investigate and quantify the photostability of photoelectrodes. The model considers the photophysical processes in an electrocatalyst-coated semiconductor immersed in electrolyte, and the kinetics of the competing water-splitting and photocorrosion reactions. When applied to 12 promising compound semiconductors for use as photoanodes (GaAs, GaP, InP, GaN, SiC, AlP, AlAs, CdTe, CdS, CdSe, ZnS, and ZnSe), the semiconductor-electrocatalyst interfacial charge transfer rate constant was found to be the most significant parameter for stabilizing the photoelectrodes. Its increase induced a sigmoid-like increase in photostability, and its optimization made it possible to stabilize nine semiconductors. The semiconductor surface back-bond energy also increased the photostability in a sigmoid-like response, and its optimization allowed to stabilize five semiconductors. The increase of irradiance induced a logarithmic drop in photostability, and limiting it to 100 W/m2 could stabilize three semiconductors. We further observed that the photostability in a device of centimeter-scale can vary by more than 50%, showing stable zones and photocorrosion hotspots. The photoelectrode's length as well as the electrocatalyst and electrolyte conductivities were found to be relevant parameters to impact this heterogeneity in the photostability. Thus, the photostability is not only defined by material properties but also by the specific combination of materials and by the device architecture. The model presented in this study can also be applied to photocathodes and other PEC designs, and can serve as a design tool for understanding, quantifying, and improving the stability.

A general strategy for synthesizing hierarchical architectures assembled by dendritic Pt-based nanoalloys for electrochemical hydrogen evolution

The development of multimetallic Pt-based nanostructures as high-performance cathodic electrocatalysts to be used in water-splitting devices for hydrogen generation is the focus of increasing attention. In this study, a family of hierarchical architectures constructed from dendritic quaternary PtFeRuRh, ternary PtFeRh or PtFeRu and binary PtFe nanoalloys are achieved via a general liquid-phase strategy for hydrogen evolution in alkaline electrolyte. Among them, the PtFeRuRh nanoalloys exhibit the lowest overpotential (20.0 mV at 10 mA cm−2) and Tafel slope (21.1 mV dec−1). At a potential of −0.07 V, the mass activity of the PtFeRuRh nanoalloy is 7.04 A mgPt−1, it is 6.97 times that of commercial Pt/C. The dendritic PtFeRuRh nanoalloys exhibit a negligible decrease in activity after 20 h of continuous testing at 10 mA cm−2/100 mA cm−2 and 3000 cyclic voltammetry cycles. In a practical application, the cell voltage of a PtFeRhRu (−) || IrO2 (+) couple is 1.568 V at 10 mA cm−2 with almost 100% faradaic efficiency. The turnover frequency of the PtFeRhRu electrocatalyst at 70 mV was 78.5 s−1, which is 11.71 times as large as that of commercial Pt/C (6.7 s−1).

In Operando Generation and Storage of Hydrogen by Coupling Monolithically Integrated Photoelectrochemical Cell with Clathrate Hydrates Molecular Storage

Hydrogen production from water has been considered as one of the potential clean and futuristic alternates to fossil fuel. However, the development of a complete efficient system for hydrogen fuel coming from water lacks not only efficient, stable, and cost-effective generation but also off-line efficient storage. Herein, first, we present a newly designed monolithically integrated photoelectrochemical (PEC) device for hydrogen generation with a semitransparent MoOx/MoS2/FTO heterostructure as a photoanode and black 2D-MoS2/Si as a photocathode. This integrated cell eliminates the use of expensive metal (like Pt) as a counter electrode and enhances the overall stable H2 and O2 evolution, with a stable measured rate of 61 μmol/h and 28 μmol/h, respectively. The applied bias photon-to-current efficiency (ABPE) was measured to be ∼3.5% at −0.3 V vs reversible hydrogen electrode (RHE). Second, we demonstrate the storage of molecular H2 as solid hydrates, where the release of hydrogen from the hydrate requires no additional energy as it is not chemically bound to water. Operating at a moderate pressure of 10 MPa, we achieved hydrates of hydrogen with a maximum storage capacity of 1.3 wt %, which could be enhanced to 3 wt % with further fine-tuning of the process. The theoretical capacity of hydrogen storage in clathrate hydrates are ∼5 wt %. This concept of monolithic generation and molecular storage of hydrogen provides a complete long-demanding efficient and stable solution for large-scale commercial applications.

Samarium oxide modified Ni-Co nanosheets based three-dimensional honeycomb film on nickel foam: A highly efficient electrocatalyst for hydrogen evolution reaction

The development of highly efficient and stable noble-metal-free electrocatalysts towards HER is of great significance. Herein, an electrodeposition method for the preparation of samarium oxide modified Ni-Co vertical nanosheets assembled three-dimensional honeycomb film on porous Ni foam (Sm2O3-Ni-Co/NF) is proposed. As-prepared Sm2O3-Ni-Co vertical nanosheets interweave together into a honeycomb morphology and uniformly cover the entire surface of Ni foam. The influence of content of Sm2O3 in electroplating bath on the electrocatalytic performances of Sm2O3-Ni-Co/NF electrode towards HER is studied in 1.0 M KOH electrolyte. Polarization curve indicates that the self-supported Sm2O3-Ni-Co/NF electrode exhibits remarkably enhanced electrocatalytic activity towards HER. To attain a current density of 10 mA cm−2, Sm2O3-Ni-Co/NF electrode needs lower overpotential (276 mV) than Ni-Co/NF electrode (301 mV). In addition, Sm2O3-Ni-Co/NF electrode exhibits a lower Tafel slope, demonstrating more favorable HER kinetics. Sm2O3-Ni-Co/NF electrode has smaller charge transfer resistance than Ni-Co/NF electrode, indicating a faster Faradic process and better HER kinetics. Besides, Sm2O3-Ni-Co/NF electrode features satisfactory long-term durability in alkaline media. The results indicate that Sm2O3-Ni-Co/NF electrode has potential application to water splitting devices for hydrogen generation.

Developing titanium micro/nano porous layers on planar thin/tunable LGDLs for high-efficiency hydrogen production

Proton exchange membrane electrolyzer cells (PEMECs) have been considered one of the most promising devices for hydrogen generation and energy storage from water splitting, especially when coupled with sustainable energy resources. Microporous layers (MPLs), which have been widely used in fuel cells for better catalyst access and product/reactant removal, have limited investigations in PEMECs due to harsh environments and carbon corrosion. In this study, the MPLs with both irregular micro (∼5 μm) and spherical nano (30–50 nm) titanium particles are developed on novel thin/tunable liquid/gas diffusion layers (TT-LGDLs) and are investigated comprehensively both in-situ and ex-situ for the first time. The MPLs change the wettability of the TT-LGDLs and show super hydrophobic property. The results reveal that micro particle MPLs exhibit improved catalytic activity but increased ohmic resistances, and that nano particle MPLs do not impact catalytic activity meaningfully but exhibit even greater increases in ohmic resistance. The effects of the thickness of the MPLs are also investigated and the typical MPL is also studied by in-situ visualization in a transparent PEMEC with a high-speed and micro-scale visualization system (HMVS). The results indicate the strong feasibility of the TT-LGDLs with small pore size and large porosity for high-efficiency and low-cost PEMEC practical applications.

Low-Voltage Electrolytic Hydrogen Production Derived from Efficient Water and Ethanol Oxidation on Fluorine-Modified FeOOH Anode

Highly active, earth-abundant anode catalysts are urgently required for the development of electrolytic devices for hydrogen generation. However, the reaction efficiencies of most developed electrocatalysts have been intrinsically limited due to their insufficient adsorption of reactants leading to high energy intermediates. Here, we establish that electronegative fluorine can moderate the binding energy between the Fe sites (FeOOH) and reactants (OH– or C2H5O–), resulting in more optimized adsorption, and can enhance the positive charge densities on the Fe sites to facilitate oxygen evolution reaction (OER) and ethanol oxidation. Consequently, a low electrolytic voltage (1.43 V to achieve 10 mA cm–2) for H2 production was obtained by integrating the efficiently anodic behaviors of OER and ethanol oxidation. The results reported herein point to fluorine moderation as a promising pathway for developing optimal electrocatalysts and contribute to ongoing efforts of mimicking water splitting.

Energy Level Engineering of MoS2 by Transition-Metal Doping for Accelerating Hydrogen Evolution Reaction.

Water-splitting devices for hydrogen generation through electrolysis (hydrogen evolution reaction, HER) hold great promise for clean energy. However, their practical application relies on the development of inexpensive and efficient catalysts to replace precious platinum catalysts. We previously reported that HER can be largely enhanced through finely tuning the energy level of molybdenum sulfide (MoS2) by hot electron injection from plasmonic gold nanoparticles. Under this inspiration, herein, we propose a strategy to improve the HER performance of MoS2 by engineering its energy level via direct transition-metal doping. We find that zinc-doped MoS2 (Zn-MoS2) exhibits superior electrochemical activity toward HER as evidenced by the positively shifted onset potential to -0.13 V vs RHE. A turnover of 15.44 s-1 at 300 mV overpotential is achieved, which by far exceeds the activity of MoS2 catalysts reported. The large enhancement can be attributed to the synergistic effect of electronic effect (energy level matching) and morphological effect (rich active sites) via thermodynamic and kinetic acceleration, respectively. This design opens up further opportunities for improving electrocatalysts by incorporating promoters, which broadens the understanding toward the optimization of electrocatalytic activity of these unique materials.

Enhanced InGaN/GaN photoelectrodes for visible‐light‐driven hydrogen generation by surface roughening

III‐Nitride semiconductor materials are considered as promising candidates for photoelectrodes (PEs) due to their adjustable direct band gap covering a very broad spectral range. In this study, InGaN/GaN based p–i–n photoelectrodes have been fabricated and nano‐sized surface roughening process has been applied. The photocurrent gets 2.5 times enhancement and the incident photon conversion efficiency (IPCE) is improved to be ∼30% at the wavelength of 400 nm. In addition, the turn‐on voltages of InGaN/GaN‐based PEs have been effectively reduced from +0.8 V down to about −0.4 V (vs. reversible hydrogen potential, RHE). Such improvements are mainly attributed to reduced dislocation density, increased surface area, and optimized build‐in electrical field. These findings give innovative insight to improve photoelectrochemical devices for hydrogen generation.

Visible light harvesting Pt/CdS/Co-doped ZnO nanorods molecular device for hydrogen generation

Co-doped ZnO nanorods (Co–ZnO NRs) were synthesized by hydrothermal method using cationic surfactant cetyltrimethylammonium bromide (CTAB). Their physical growth mechanism was traced in the light of FESEM and HRTEM studies. A strong correlation between their electronic structural arrangement and photocatalytic activity was established. Broad and uniform peaks were found between 525 and 700 nm in UV–Vis. diffuse reflectance spectra, which were attributed to the d–d transition of Co2+ ion. Successful loading of the nano-sized sensitizer CdS onto the Co–ZnO NRs' surface, contributes to the band gap reduction in CdS/Co–ZnO NRs (Eg = 2.25 eV) sample. X-ray absorption spectroscopy confirms the presence of the lower degree structural disorders in CdS/Co–ZnO NRs with respect to the pristine ZnO and Co–ZnO NRs. Gradual modification in pristine ZnO NRs enhances the photocatalytic activity. Hetero-assembly of 1.5% Pt/CdS/Co–ZnO NRs, exhibited excellent photocatalytic responses in terms of quantum efficiency (1.98%) and hydrogen generation capacity (67.20 mmol/H2 g) under 1 Sun (1.5AM G) light exposure.

2D materials. Graphene, related two-dimensional crystals, and hybrid systems for energy conversion and storage.

Graphene and related two-dimensional crystals and hybrid systems showcase several key properties that can address emerging energy needs, in particular for the ever growing market of portable and wearable energy conversion and storage devices. Graphene's flexibility, large surface area, and chemical stability, combined with its excellent electrical and thermal conductivity, make it promising as a catalyst in fuel and dye-sensitized solar cells. Chemically functionalized graphene can also improve storage and diffusion of ionic species and electric charge in batteries and supercapacitors. Two-dimensional crystals provide optoelectronic and photocatalytic properties complementing those of graphene, enabling the realization of ultrathin-film photovoltaic devices or systems for hydrogen production. Here, we review the use of graphene and related materials for energy conversion and storage, outlining the roadmap for future applications.

Artificial leaf device for hydrogen generation from immobilised C. reinhardtii microalgae

We have engineered a hydrogen generating device replicating the function of a leaf undergoing photosynthesis based on fabric-immobilized microalgae culture with controlled perfusion with nutrients and collection of hydrogen through parallel microchannel networks.

Self-Biased Solar-Microbial Device for Sustainable Hydrogen Generation

Here we demonstrate the feasibility of continuous, self-sustained hydrogen gas production based solely on solar light and biomass (wastewater) recycling, by coupling solar water splitting and microbial electrohydrogenesis in a photoelectrochemical cell-microbial fuel cell (PEC-MFC) hybrid device. The PEC device is composed of a TiO2 nanowire-arrayed photoanode and a Pt cathode. The MFC is an air cathode dual-chamber device, inoculated with either Shewanella oneidensis MR-1 (batch-fed on artificial growth medium) or natural microbial communities (batch-fed on local municipal wastewater). Under light illumination, the TiO2 photoanode provided a photovoltage of ~0.7 V that shifted the potential of the MFC bioanode to overcome the potential barrier for microbial electrohydrogenesis. As a result, under light illumination (AM 1.5G, 100 mW/cm(2)) without external bias, and using wastewater as the energy source, we observed pronounced current generation as well as continuous production of hydrogen gas. The successful demonstration of such a self-biased, sustainable microbial device for hydrogen generation could provide a new solution that can simultaneously address the need of wastewater treatment and the increasing demand for clean energy.

Development and Improvement of Devices for Hydrogen Generation and Oxidation in Water Detritiation Facility Based on CECE Technology

Water detritiation facility based on CECE (Combined Electrolysis and Catalytic Exchange) technology needs an electrolyser for water conversion to hydrogen. Use of a conventional alkali electrolyser requires a very deep purification of hydrogen stream from alkali prior to injection to LPCE (Liquid Phase Catalytic Exchange) column. In some applications conversion of detritiated hydrogen back into water is required. This is usually performed via hydrogen catalytic oxidation in a recombiner. This paper presents results of study to improve hydrogen and oxygen purification for alkali electrolysers and develop a hydrogen recombiner based on use of hydrophobic catalyst.