Miguel A. Modestino

Design and cost considerations for practical solar-hydrogen generators

Solar-hydrogen generation represents a promising alternative to fossil fuels for the large-scale implementation of a clean-fuel transportation infrastructure. A significant amount of research resources has been allocated to the development of photoelectrochemical components (i.e. photovoltaic and water splitting catalysts) that are able to spontaneously split water in the presence of solar irradiation, which has led to major advances in the solar-fuels field. At the same time, only limited attention has been given to understanding the key aspects that drive economically viable solar-fuel generators. This study presents a generalized approach to understand the economic factors behind the design of solar-hydrogen generators composed of photovoltaic components integrated with water electrolyzers. It evaluates the underpinning effects of the material selection for the light absorption and water splitting components on the cost of the generated fuel (

Robust production of purified H2 in a stable, self-regulating, and continuously operating solar fuel generator

per Kg of H2). The results presented in this work provide insights into important engineering aspects related to the sizing of devices and the use of light concentration components that, when optimized, can lead to costs below

Hollow Mesoporous Plasmonic Nanoshells for Enhanced Solar Vapor Generation.

2.90 per kilogram of hydrogen after compression and distribution. Most significantly, the analysis demonstrates that the cost of hydrogen is defined primarily by the light-absorbing component (up to 97% of the cost) while the material selection for the electrolysis components has, to a large extent, minor effects. The findings presented here can help direct research and development efforts towards the fabrication of deployable solar-hydrogen generators that are cost competitive with commercial energy sources.

Modeling, Simulation, and Implementation of Solar-Driven Water-Splitting Devices.

The development of practical solar-driven electrochemical fuel generators requires the integration of light absorbing and electrochemical components into an architecture that must also provide easy separation of the product fuels. Unfortunately, many of these components are not stable under the extreme pH conditions necessary to facilitate ionic transport between redox reaction sites. By using a controlled recirculating stream across reaction sites, this work demonstrates a stable, self-regulating and continuous purified solar-hydrogen generation from near neutral pH electrolytes that yield continuous nearly pure H2 streams with solar-fuel efficiencies above 6.2%.

A membrane-less electrolyzer for hydrogen production across the pH scale

In the past decade, nanomaterials have made their way into a variety of technologies in solar energy, enhancing the performance by taking advantage of the phenomena inherent to the nanoscale. Recent examples exploit plasmonic core/shell nanoparticles to achieve efficient direct steam generation, showing great promise of such nanoparticles as a useful material for solar applications. In this paper, we demonstrate a novel technique for fabricating bimetallic hollow mesoporous plasmonic nanoshells that yield a higher solar vapor generation rate compared with their solid-core counterparts. On the basis of a combination of nanomasking and incomplete galvanic replacement, the hollow plasmonic nanoshells can be fabricated with tunable absorption and minimized scattering. When exposed to sun light, each hollow nanoshell generates vapor bubbles simultaneously from the interior and exterior. The vapor nucleating from the interior expands and diffuses through the pores and combines with the bubbles formed on the outer wall. The lack of a solid core significantly accelerates the initial vapor nucleation and the overall steam generation dynamics. More importantly, because the density of the hollow porous nanoshells is essentially equal to the surrounding host medium these particles are much less prone to sedimentation, a problem that greatly limits the performance and implementation of standard nanoparticle dispersions.

Mass transport aspects of electrochemical solar-hydrogen generation

An integrated cell for the solar-driven splitting of water consists of multiple functional components and couples various photoelectrochemical (PEC) processes at different length and time scales. The overall solar-to-hydrogen (STH) conversion efficiency of such a system depends on the performance and materials properties of the individual components as well as on the component integration, overall device architecture, and system operating conditions. This Review focuses on the modeling- and simulation-guided development and implementation of solar-driven water-splitting prototypes from a holistic viewpoint that explores the various interplays between the components. The underlying physics and interactions at the cell level is are reviewed and discussed, followed by an overview of the use of the cell model to provide target properties of materials and guide the design of a range of traditional and unique device architectures.

An Integrated Device View on Photo-Electrochemical Solar-Hydrogen Generation

The development of deployable water-splitting devices is hindered by the cost of electricity and the lack of stable ion conducting membranes that can operate across the pH scale, impose low ionic resistances and avoid product mixing. The membrane-less approach developed in this work breaks this paradigm and demonstrates for the first time an electrolyzer capable of operating with lower ionic resistance than benchmark membrane-based electrolyzers using virtually any electrolyte. Our method separates product gases by controlling the delicate balance between fluid mechanic forces in the device. The devices presented here are able to split water at current densities over 300 mA cm−2, with more than 42% power conversion efficiency, and crossover of hydrogen gas into the oxidation side as low as 0.4%, leading to a non-flammable and continuous hydrogen fuel stream. Furthermore, ability to use buffered electrolytes allows for the incorporation of earth-abundant catalysts that can only operate at moderate to high pH values.

The potential for microfluidics in electrochemical energy systems

The conception of practical solar-hydrogen generators requires the implementation of engineering design principles that allow photo-electrochemical material systems to operate efficiently, continuously and stably over their lifetime. At the heart of these engineering aspects lie the mass transport of reactants, intermediates and products throughout the device. This review comprehensively covers these aspects and ties together all of the processes required for the efficient production of pure streams of solar-hydrogen. In order to do so, the article describes the fundamental physical processes that occur at different locations of a generalized device topology and presents the state-of-the-art advances in materials and engineering approaches to mitigate mass-transport challenges. Processes that take place in the light absorber and electrocatalyst components are only briefly described, while the main focus is given to mass transport processes in the boundary-layer and bulk liquid or solid electrolytes. Lastly, a perspective on how engineering approaches can enable more efficient solar-fuel generators is presented.

Structure determination of Pt-coated Au dumbbells via fluctuation X-ray scattering

Devices that directly capture and store solar energy have the potential to significantly increase the share of energy from intermittent renewable sources. Photo-electrochemical solar-hydrogen generators could become an important contributor, as these devices can convert solar energy into fuels that can be used throughout all sectors of energy. Rather than focusing on scientific achievement on the component level, this article reviews aspects of overall component integration in photo-electrochemical water-splitting devices that ultimately can lead to deployable devices. Throughout the article, three generalized categories of devices are considered with different levels of integration and spanning the range of complete integration by one-material photo-electrochemical approaches to complete decoupling by photovoltaics and electrolyzer devices. By using this generalized framework, we describe the physical aspects, device requirements, and practical implications involved with developing practical photo-electrochemical water-splitting devices. Aspects reviewed include macroscopic coupled multiphysics device models, physical device demonstrations, and economic and life cycle assessments, providing the grounds to draw conclusions on the overall technological outlook.

Integrated microfluidic test-bed for energy conversion devices

Flow based electrochemical energy conversion devices have the potential to become a prominent energy storage technology in a world driven by renewable energy sources. The optimal design of these devices depends strongly on the tradeoffs between the losses associated with multiple transport processes: convection and diffusion of reactants and products, migration of ionic species, and electrical charge transport. In this article we provide a balanced assessment of the compromise between these losses and demonstrate that for a broad range of electrochemical reactors, the use of microfluidics can enhance the energy conversion efficiency. Moreover, we propose proven scale-up strategies of microelectrochemical reactors which could pave the way to the large scale implementation of energy microfluidic systems.