Hydrogen

Aenert. Research Laboratory news
Hydrogen is an important fuel for many industries. Currently, hydrogen is mainly produced from the fossil fuel methane. In view of the current energy and climate crises, hydrogen has become attractive both as a standalone fuel and as a component in sustainable fuels made with recycled carbon dioxide. However, it would be highly desirable if hydrogen production could be realised using mainly climate-friendly and sustainable methods.

Hydrogen can be burnt cleanly and is an efficient fuel in the right setting. However, there are certain obstacles to producing hydrogen in bulk using eco-friendly technologies which do not have a large carbon footprint. Currently, most hydrogen is produced via gas or steam reforming. It can also be extracted using electrolysers to split water. This is a carbon-neutral method, but it requires a large amount of energy, which can be challenging to deliver employing renewable sources, such as solar, wind or geothermal power. Especially solar power projects are difficult to scale up and are usually limited to systems of less than 100W although there are many projects currently underway dedicated to the production of solar hydrogen. Recently, for example, a team of scientists at the University of Michigan has developed a solar panel which could convert water into hydrogen and oxygen.

Now (2023), a team at the Laboratory of Renewable Energy Science and Engineering, EPFL, Lausanne, Switzerland has managed to perform the successful scaling of a thermally integrated photoelectrochemical device using concentrated solar irradiation to a kW-scale pilot plant which was capable of co-generating hydrogen and heat. The solar-to-hydrogen device efficiency achieved was greater than 20% at an H₂ production rate of >2.0 kW. Also a model-based system optimisation was designed which showed the dominant energetic losses and developed strategies to improve the system-level efficiency of >5.5% towards the device-level efficiency.

In this device, the solar light was concentrated by a dual-axis tracking parabolic dish concentrator and directed to a solar reactor comprising a shield, aperture with flux homogenizer and triple-junction III–V PV module, proton exchange membrane EC stack embedded in the reactor unit and water pump.

A proposed two-pump design helped decouple the conflicting water flow-rate requirements of the PV and EC which enabled efficient heat transfer in the CPV heat exchanger while simultaneously controlling the water temperature increase over the CPV module and water ratio in the EC. However, as the system was a pilot-scale device, the solar dish size was not ideal for the reactor size and therefore an additional water-cooled shield was needed to absorb excess concentrated light. The scientists believe that this shield could be replaced with a larger homogenizer/concentrated PV unit and passively cooled shield in future, which could further reduce the complexity of the installation.

The complete system was operated over a period of more than 13 days in August 2020 and February/March 2021. Operation under different conditions were analysed where the ambient temperatures ranged from about 20°C in August to about 8°C in late February. The meteorological conditions also varied considerably over the time periods of operation.

The H₂ production rate of the device was 0.9 Nm³ hr⁻¹ with a mean of 0.59 N m³ hr⁻¹ over the entire operation period (corresponding to EC current of IEC = 41.3 A). While the fuel efficiency remained approximately constant, the heat efficiency was found to decrease in the winter months, which was probably caused by increased heat loss due to cooler ambient temperatures.

The maximal peak hydrogen production rate within five minutes 14.0 Nl min⁻¹ (1.26 g min⁻¹). During testing, more than 3.2 kg of solar hydrogen was produced. The system was found to produce on average 10.6 kWth of thermal heat at an outlet temperature of 45.1°C. The peak thermal output (during a 5 min period) was 14.9 kWth. A total 679 kWhth was produced during the 13 days operation. Thermal integration was found to reduce the required auxiliary electrical demand by over half due to the removal of an auxiliary heater. Moreover, the thermal integration also removed the associated heater capital costs and simplified the balance of the plant.

Image: Overview of the system. a, Technical illustration of the overall site showing key components such as the solar parabolic concentrator dish, reactor and ancillary hardware and cabinets. b, Close-up of the integrated reactor showing the assembly of the shield, homogenizer, PV and enclosure. c, A simplified process and instrumentation diagram of the system showing material and energy flows. The key input/output/intermediate energy streams are composed of the PV-generated electrical work available for electrolysis, heat output from the heat exchanger and the external work required for water pumping. W and Q stands for work and heat respectively and sensors are denoted by a circle (T = temperature sensor, P = pressure sensor, H₂ = hydrogen concentration sensor). Photographs of the system can be found in Supplementary Fig. 1



Source: Isaac Holmes-Gentle   1,3, Saurabh Tembhurne   1,2,3, Clemens Suter   1  & Sophia Haussener/ Kilowatt-scale solar hydrogen production system using a concentrated integrated photoelectrochemical device/ Nature Energy (2023), 10 April 2023/ doi.org/10.1038/s41560-023-01247-2/ Open Access This is an Open Access article is distributed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0)

In view of the current climate crisis, scientists are trying to find more sustainable means for hydrogen production. In 2017, scientists achieved the solar-driven thermochemical splitting of CO₂ into separate streams of CO and O₂ with 100% selectivity, 83% molar conversion, and 5.25% solar-to-fuel energy efficiency. This was achieved using a 4kW solar reactor with a reticulated porous structure, made of ceria, which was directly exposed to 3000× flux irradiation and undergoing redox cycling via temperature/pressure-swing operation. Because of the dual-scale interconnected porosity (mm and μm-sized pores) of the ceria structure good volumetric radiative absorption and enhanced heat/mass transport for rapid redox kinetics was achieved. 500 consecutive redox cycles validated material stability and structure robustness. A detailed energy balance was compiled which described viable paths for achieving higher efficiencies and for large-scale industrial implementation using an array of modular solar reactors integrated into the established solar concentrating infrastructure.

Image: (a) Schematic of the solar reactor configuration for splitting CO₂ into separate streams of CO₂ and O₂via a 2-step thermochemical redox cycle. It comprises a windowed cavity-receiver containing a reticulated porous ceramic (RPC) foam-type structure made of ceria directly exposed to high-flux solar irradiation. The redox cycle is carried out under a combined temperature/pressure-swing operational mode. Red arrow: endothermic reduction generating O₂, eqn (2), is performed at high temperatures (Treduction = 1450–1500°C) and vacuum pressures (ptotal = 10–1000 mbar) using concentrated solar energy (Psolar = 2.4–4.1 kW). Blue arrow: exothermic oxidation with CO₂ generating CO, eqn (3), is performed at lower temperatures (Toxidation = 700–1000°C) and ambient pressure (ptotal = 1 bar) without input of solar energy (Psolar = 0). Inset: Infiltrated ceria RPC with dual-scale porosities in the mm and μm ranges. (b) Photographs of the solar reactor, showing the front face of the solar reactor with the windowed aperture and its interior containing the octagonal RPC structure lined with alumina thermal insulation



Source: Daniel Marxer, Philipp Furler, Michael Takacsa and Aldo Steinfeld/ Solar thermochemical splitting of CO₂ into separate streams of CO and O₂ with high selectivity, stability, conversion, and efficiency/ Energy & Environmental Science Issue 5, 2017, 21 Feb 2017/ DOI: 10.1039/c6ee03776c/ Open Access This is an Open Access article is distributed under the terms of the Attribution-NonCommercial 3.0 Unported (CC BY-NC 3.0)

In 2019, scientists designed a large area photoelectrochemical–photovoltaic (PEC–PV) solar water splitting device with a metal oxide-based top absorber. The stand-alone 50 cm² device consisted of cobalt phosphate-coated tungsten-doped BiVO₄ (CoPi/W:BiVO₄) photoanodes combined with series-connected silicon heterojunction (SHJ) solar cells. The specific challenges encountered comprised: (i) the high resistivity of the fluorine-doped tin oxide substrate, (ii) non-uniform CoPi deposition, and (iii) limited ionic conductivity of the 0.1 M phosphate buffer electrolyte typically used for small area BiVO₄ devices. The former two problems were solved by applying Ni lines to the fluorine-doped tin oxide substrate, and the latter to some extent by increasing the electrolyte concentration to 2.0 M. Despite the high buffer concentration, the overall performance of the large area photoelectrodes was found to be limited by H+/OH− transport in this near-neutral pH electrolyte. This limitation caused H+/OH− depletion towards the center of the large area electrode and significant potential drop. However, this could be solved by using a cell design with a small electrode-area-to-electrolyte-volume ratio. The optimised photoanodes were then integrated into tandem PEC–PV devices in either a single or dual photoanode configuration and demonstrated solar to hydrogen (STH) efficiencies of 1.9%. Optimised small area (0.24 cm²) PEC–PV devices based on a similar configuration showed a STH efficiency of up to 5.5%.

Image: (a) Colour gradient plots of the simulated potential distribution across a 5 × 10 cm² photoanode (i) without and (ii) with Ni lines for a photocurrent density of 3 mA cm⁻², and (b) shows the graphical plots of the potential drop at different points across the photoanodes (dashed lines in Fig. 3a). (c) Photographs of two 50 cm2 area W:BiVO₄ photo-anodes, (i) without and (ii) with Ni lines. (d) J–V curves of the corresponding photoanodes in an electrolyte containing 0.5 M Na₂SO₃ and 0.1 M KPi buffer of pH₇, with backside AM1.5G illumination (BSI)



Source: Ibbi Y. Ahmet, Yimeng Ma, Ji-Wook Jang, Tobias Henschel, Bernd Stannowski, Tânia Lopes, António Vilanova, Adélio Mendes, Fatwa F. Abdi and Roel van de Krol/ Demonstration of a 50 cm² BiVO₄ tandem photoelectrochemical-photovoltaic water splitting device/ Sustainable Fuels & Energy, Issue 9, 2019, 04 Jul 2019/ DOI doi.org/10.1039/C9SE00246D/ Open Access This is an Open Access article is distributed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0)

The photothermal hydrogen production device has many advantages: The workflow of the system during operation is fast. The startup and shutdown sequences can each be completed in approximately 5 minutes. The system is quick in recovering from perturbations, such as controlled variable set points and during periods of unstable DNI. Furthermore, the synergistic effect of thermal integration is shown as beneficial in both the operating current and voltage when the system returns to steady operating temperatures. Furthermore, the system performance was sensitive to the dynamics of the two-axis optical tracking mechanism, highlighting the importance of accurate solar tracking. The flow-rate control could be employed to stabilise system outputs and device operating temperature in response to changes in input irradiance.

In the study, a large-scale and efficient co-generating hydrogen and heat system has been demonstrated on sun. Important operational challenges were overcome, such as the complex process control and judicious management of water flow rates to realise the synergistic effect of thermal integration. If the research goes as planned, there is hope that this system will soon make a valuable contribution to mitigating CO₂ emissions and reducing pollution on this planet.

By the Editorial Board