Energy and Earth Science Vol. 3, No. 2, 2020 www.scholink.org/ojs/index.php/ees ISSN 2578-1359 (Print) ISSN 2578-1367 (Online) 49 Original Paper From Renewable Energy to Renewable Fuel: A Sustainable Hydrogen Production Rafiq Mulla 1 & Charles W. Dunnill 1* 1 Energy Safety Research Institute, Swansea University, Bay Campus, Fabian Way, SA1 8EN, UK * Charles W. Dunnill, E-mail: c.dunnill@swansea.ac.uk Received: July 17, 2020 Accepted: July 31, 2020 Online Published: September 16, 2020 doi:10.22158/ees.v3n2p49 URL: http://dx.doi.org/10.22158/ees.v3n2p49 Abstract Hydrogen, a zero-emission fuel and the universal energy vector, can be easily produced from many different energy sources. It is a storable, transportable product that can be used on demand to overcome supply and demand imbalances. As of today, most of the hydrogen produced comes from natural gas; the production process itself is in fact not so pollution free. As the world is looking for a low carbon future, researchers have therefore been looking for more sustainable, environmentally friendly pathways of hydrogen production by using renewable energy sources such as solar and wind. Among the different methods, water electrolysis is a conventional and promising method of hydrogen production if renewable energy sources are to be employed in the process. Lots of progress has been made over the past few years in extending the use of hydrogen in different sectors. This perspective article briefly covers the recent developments in the hydrogen fuel-based projects and technologies and provides a description of the advantages of employing renewable energy sources for sustainable hydrogen production. Keywords Hydrogen, renewable energy, electrolysis, thermoelectric, zero emission, sustainable production 1. Introduction Renewable energy sources (solar and wind etc.) will be the fastest-growing power sources for electricity generation in the near future. These intermittent sources, however, suffer from supply and demand imbalances (Jones, Al-Masry et al., 2018; Quarton, Tlili et al., 2020), as there is no correlation between say the strength of the wind or the position of the sun in the sky and the consumers desire for electricity. At low levels this is not too much of an issue, but as the percentage mix increases this will become an increasingly important topic. Balancing this supply and demand is developing into one of www.scholink.org/ojs/index.php/ees Energy and Earth Science Vol. 3, No. 2, 2020 50 Published by SCHOLINK INC. the biggest challenges facing energy scientists around the globe. Much work is currently underway, correlating the supply with demand producing “Smart systems” however there is an alternative which is to decouple supply from demand using a university energy vector. The storage of energy in the form of hydrogen is just such a possible energy vector. All forms of energy can easily and relatively efficiently be converted into hydrogen and all forms of power can be utilized from the hydrogen; electricity and heat via fuel cells or heat from combustion, either 100% pure or as a enrichment to natural gas (Jones, Al-Masry et al., 2018). Once produced, hydrogen can be stored for unimaginable lengths of time thus decoupling supply from demand. As long as there is storage capacity and the demand does not exceed the supply, the inherently “Smart” system will balance itself out, even with a 100% renewable intermittent input. Figure 1. The Number of Micro-CHP Systems Installed to Date (Solid Lines) and Near-Term Projections (Dotted Lines) for Residential Use (YoY Stands for Year-Over-Year). Reproduced from (Staffell, Scamman et al., 2019) Published by the Royal Society of Chemistry The implementation of hydrogen-fuel based technologies is growing rapidly all over the world to cut the burning of fossil fuels wherever possible. Japan has been working towards a hydrogen society to reduce environmental burdens and revitalize regional economies by extending its use by (i) promoting Fuel Cell Electric Vehicles (FCEVs), (ii) increasing hydrogen production, and (iii) introducing fuel cells for residential applications (Energy 2014; Staffell, Scamman et al., 2019). It is also envisioning to establish a carbon-free hydrogen supply by 2040 (METD 2017; Staffell, Scamman et al., 2019). Japan has deployed a large number of residential fuel cell systems (micro-CHP) and is one of the leading countries in the world in implementing hydrogen technologies for daily uses (micro-CHP technology: Micro combined heat and power, is a technology that generates heat and electricity from the same www.scholink.org/ojs/index.php/ees Energy and Earth Science Vol. 3, No. 2, 2020 51 Published by SCHOLINK INC. energy source, and the fuel cell is a device that converts the chemical energy of a fuel (hydrogen) into electricity). Recently, many European (EU) countries are beginning to promote hydrogen as a fuel in different fields; it is also being introduced for automobile industries as new techniques to store hydrogen in liquefied form have been developed and found to be successful on primary testing which is crucial to implement the technology for small vehicles such as scooters (FCW 2020). Figure 1 illustrates the contribution of major countries in implementing micro-CHP technology for residential use. The extension of hydrogen usage to automobile industry will improve air quality as vehicles’ contribution to the CO2 emission is very high and ever increasing. Similarly, France has launched a hydrogen project; hydrogen is produced from the renewable energy and will be stored and transported in the natural gas grid which flows in pipes under the streets of a village, aiming to energize hundreds of homes (ENGIE 2016). This year, Scotland is also planning for its first ever hydrogen-powered homes; as a part of a pilot scheme (Sampson, 2020), a few homes will be incorporated with micro-CHP fuel cell technology. Interestingly, hydrogen can also be burnt as a flame or catalytically (du Preez, Jones et al., 2019; du Preez, Jones et al., 2020). When hydrogen burns, it produces no toxic/greenhouse gases but water (Figure 2), therefore, a 100% clean energy pathways can be created using production methods powered by renewable energy sources. At present most of the hydrogen is produced from steam reformation of fossil fuels, so called Brown Hydrogen and hence even hydrogen is also indirectly responsible for carbon emissions. This can be avoided or minimized by sequestering the carbon dioxide (Blue hydrogen, or by electrolyzing water using renewable energy such as excess wind; implementation of these clean methods is already under progress that releases virtually zero greenhouse gas to the environment. www.scholink.org/ojs/index.php/ees Energy and Earth Science Vol. 3, No. 2, 2020 52 Published by SCHOLINK INC. Figure 2. An Illustration Showing H2 Burning Produces No Greenhouse Gases 2. Hydrogen Production from Renewable Sources A direct and promising method to generate green hydrogen is found to be water electrolysis: a process of splitting water into hydrogen and oxygen using electricity (Gannon, Warwick et al., 2020). A simple two electrode (anode and cathode) unit called an electrolyzer (usually, it consists of three parts: electrodes, separators/membranes, and electrolyte) is used for the purpose (Mulla & Dunnill, 2019). There are three types of electrolyzer designs such as (i) Alkaline Electrolysis Cells (AEC), (ii) Proton Exchange Membrane Electrolysis Cells (PEMEC), and (iii) Solid Oxide Electrolysis Cells (SOEC) of which AECs have been in use in various industries as they have simple designs and can be built at a relatively low capital cost and are durable (Passas & Dunnill, 2015; Phillips & Dunnill, 2016; Phillips, Edwards et al., 2017; Mulla & Dunnill, 2019). To start electrolysis process, a DC electric supply is passed through an electrolytic solution, splitting water (H2O) into its component parts (hydrogen (H2) & oxygen (O2)) where H2 being generated on the cathode and O2 on the anode (Figure 3). The decomposition of H2O into H2 and O2 happens due to the application of electric voltage which forces ions to undergo either oxidation or reduction at the electrodes, the mechanism can be expressed as follows: .................. (1) www.scholink.org/ojs/index.php/ees Energy and Earth Science Vol. 3, No. 2, 2020 53 Published by SCHOLINK INC. Figure 3. A Schematic of a Simple Electrolyzer for Hydrogen Production through Electrolysis In general, the efficiency of water splitting is determined by the amount of electrical energy used to generate an amount of hydrogen, can be defined as follows (Phillips & Dunnill, 2016; Phillips, Edwards et al., 2017; Mulla & Dunnill, 2019): ....................... (2) where the term “Energy Output” refers to the amount of hydrogen produced and “Total Energy Input” is in general refers to the total electrical energy consumed in the production. www.scholink.org/ojs/index.php/ees Energy and Earth Science Vol. 3, No. 2, 2020 54 Published by SCHOLINK INC. Figure 4. Water Electrolysis Powered by Renewable Energy for Hydrogen Production As illustrated by Figure 4, the required electrical energy can be provided by different renewable energy sources such as wind turbine, photovoltaic (solar panels), and also from solar thermoelectric generators (STEG). Wind turbine and photovoltaic systems are already well-known, successfully working on a large scale in different sectors, and are also recently being linked to hydrogen production units. Both the energy sources appear to be promising for water splitting according to recent developments; alongside, research on coupling STEGs to the electrolysis is also under progress (Baranowski, Snyder et al., 2012). Industrial waste heat is a fantastic source of cheap energy that can be utilized for hydrogen production (Mulla & Dunnill, 2019). Thermoelectric generators work on the principle of Seebeck effect, in which a temperature gradient applied across the device produces useful electric potential. STEGs generate electricity from concentrated sunlight; solar radiation is used as a source of heat to develop temperature gradient across the device. Larger temperature gradients across the generators can be achieved by using solar concentrators/lenses which result in better output power www.scholink.org/ojs/index.php/ees Energy and Earth Science Vol. 3, No. 2, 2020 55 Published by SCHOLINK INC. density from the generator. The conversion efficiency (η) of the thermoelectric generators can be estimated using the following standard equation: ..................... (3) where ZT is the thermoelectric figure of merit of the materials used in the generator, Tc and Th are the cold and hot side temperatures of the generators. Therefore, a large temperature difference across the generator designed with good thermoelectric materials (i.e., materials with high ZT value) is important to generate a decent electric power. Up to 15-30% efficiency is predicted from these STEGs when thermoelectric materials of ZT > 2 are used in the generators (Baranowski, Snyder et al., 2012). 3. Challenges and Opportunities Currently, most of the hydrogen is generated from fossil fuels as they provide more economical methods over electrolytic water splitting. This production method, however, is unsuitable for a low carbon future. Presently, the largest consumers of hydrogen (produced by fossil fuels) are chemical industries where nearly half of hydrogen is used in ammonia production; it alone causes about 1% of total global greenhouse gases emission (Boerner, 2019). In the meantime, as the world is looking for a low carbon future, there is an increasing demand for renewable sources, shortly; they may become available at competitive prices. By utilizing clean nature of hydrogen, blending it with biogas (Mehr, Moharramian et al., 2020), implementing hydrogen injection in coal-dominated regions (Wang, Klemeš et al., 2020), etc can help reduction of CO2 emission. Hydrogen can be safely mixed in small quantities with natural gas depending on the end-use appliances. Countries like Germany and the Netherlands have 10-12% (by volume) blending levels (Staffell, Scamman et al., 2019). The advantage of hydrogen blending can be seen from the data in Figure 5 that shows the relationship between energy content and carbon savings with respect to hydrogen injection mixtures (Staffell, Scamman et al., 2019). So, about 20% hydrogen (by volume) blend would give 13 gCO2 per kWh carbon emissions savings. However, at present, higher blending is not possible due to technical and administrative constraints (Staffell, Scamman et al., 2019). www.scholink.org/ojs/index.php/ees Energy and Earth Science Vol. 3, No. 2, 2020 56 Published by SCHOLINK INC. Figure 5. The Relationship between Energy Content and Carbon Savings with Respect Hydrogen Injection Mixtures. Reproduced from (Staffell, Scamman et al., 2019) Published by the Royal Society of Chemistry The low efficiency of the thermoelectric generators is a major drawback to be used independently in the production of hydrogen, and therefore they can be used as a supplementary source by coupling with photovoltaic to improve overall efficiency or production rate (Mulla & Dunnill, 2019). At present, the commercial thermoelectric generators are mainly contained materials like Bi2Te3 and PbTe which makes them quite expensive, and also the elements like tellurium (Te) are highly toxic and scarce hence, a large-scale implementation facing difficulties (Mulla & Dunnill, 2019; Mulla & Rabinal, 2019). Nevertheless, rigorous scientific investigations are under progress which produced many new, low-cost and non-toxic materials and improvements can be realized in the near future (Mulla & Rabinal, 2018; Mulla & Dunnill, 2019). The efficiency argument however does need to be taken in context as the use of otherwise wasted heat yields green value from waste (Mulla & Dunnill, 2019) and is economically viable even with low efficiency, especially if the TEG can be produced cheaply (Mulla & Rabinal, 2018; Mulla & Dunnill, 2020; Mulla, Jones et al., 2020). There are also other methods to generate hydrogen from solar energy such as photoelectrochemical (PEC) water splitting which also offers a promising approach for hydrogen production; however, requires scalable PEC cells in addition to the stable and efficient photoelectrodes (Landman, Halabi et al., 2020). Another drawback of this method is associated with the collection of hydrogen gas from www.scholink.org/ojs/index.php/ees Energy and Earth Science Vol. 3, No. 2, 2020 57 Published by SCHOLINK INC. millions of PEC cells (Landman, Halabi et al., 2020). Therefore, large-scale PEC water splitting under sunlight is yet to be demonstrated. Similarly, studies are under progress on improving photocatalytic hydrogen production using different and new photocatalyst materials and also on the use of photocathode materials (Warwick, Barreca et al., 2015; Crespo-Quesada, Pazos-Outón et al., 2016; Kuehnel & Reisner, 2018; Ghosh, Nakada et al., 2020). Although producing hydrogen from electrolysis is currently dominated by the cost of electricity, the capital cost of electrolyzer will become important in the future when electricity generated from renewable energy becomes abundant and cheap (Esposito, 2017). The recent cost reductions of wind and solar power enable new opportunities for a competitive hydrogen production. In such scenario, the cost of electricity could be around $ 30/MWh (as estimated by wind farms in Morocco and solar plants in Chile and Dubai) and it is predicted that hydrogen production cost would not exceed $ 2/kg (Philibert, 2017). Research priorities with regard to electrolysers include reducing the capital cost of the electrolyzer units, operation life, improving their designs, and efficiency (Phillips & Dunnill, 2016). Typically, in basic design of an electrolyzer, the two electrodes are separated by a liquid electrolyte, which hinders the charge flow and results in low current densities. So, efforts on improving electrolyzer cell designs are going on and new design concepts like “zero gap cell” are under investigation where the charges can move much easier than in the basic cell design (Phillips & Dunnill, 2016). Similarly, research interest in membraneless electrolyzers is increasing to reduce the cost of the units (Esposito, 2017). Further, when there is excess electricity available from windmills, instead of curtailing, it can be used to generate hydrogen which can be stored for the future and it could be integrated with electric power at a wind farm for better supply-demand flexibility (Perni, Liu et al., 2008; Cloete & Hirth, 2020). In addition, offshore renewable energy resources also have a great potential to contribute to the sustainable hydrogen production (Gondal, 2019). Overall, there are numerous opportunities to firmly establish a clean production technology for hydrogen production from renewable sources. 4. Conclusions Hydrogen can be used in a wide range of applications and the hydrogen-powered fuel cells could power our vehicles, replacing the fossil fuels used in most vehicles today. In order to achieve a clean future, the use of hydrogen must be extended. Clean production of hydrogen, therefore, is essential to reduce greenhouse gas emissions. With the help of wind and solar powers, sustainable production of hydrogen can be achieved. Acknowledgments Authors are thankful to the Welsh Government (EU European Regional Development Fund) for funding the RICE (Reducing Industrial Carbon Emission) project (Grant Number: 81435). www.scholink.org/ojs/index.php/ees Energy and Earth Science Vol. 3, No. 2, 2020 58 Published by SCHOLINK INC. Conflicts of interest There are no conflicts of interest to declare. References Baranowski, L. L., Snyder, G. J., & Toberer, E. S. (2012). Concentrated solar thermoelectric generators. Energy & Environmental Science, 5(10), 9055-9067. https://doi.org/10.1039/c2ee22248e Boerner, L. K. (2019). Industrial ammonia production emits more CO2 than any other chemical-making reaction. Chemists want to change that. C&EN, ACS. 97. Cloete, S., & Hirth, L. (2020). Flexible power and hydrogen production: Finding synergy between CCS and variable renewables. Energy, 192, 116671. https://doi.org/10.1016/j.energy.2019.116671 Crespo-Quesada, M., Pazos-Outón, L. M., Warnan, J., Kuehnel, M. F., Friend, R. H., & Reisner, E. (2016). Metal-encapsulated organolead halide perovskite photocathode for solar-driven hydrogen evolution in water. Nature Communications, 7(1), 12555. https://doi.org/10.1038/ncomms12555 du Preez, S. P., Jones, D. R., Bessarabov, D. G., Falch, A., Mota das Neves Quaresma, C., & Dunnill, C. W. (2019). Development of a Pt/stainless steel mesh catalyst and its application in catalytic hydrogen combustion. International Journal of Hydrogen Energy, 44(49), 27094-27106. https://doi.org/10.1016/j.ijhydene.2019.08.168 du Preez, S. P., Jones, D. R., Warwick, M. E. A., Falch, A., Sekoai, P. T., Mota das Neves Quaresma, C., Bessarabov, D. G., & Dunnill, C. W. (2020). Thermally stable Pt/Ti mesh catalyst for catalytic hydrogen combustion. International Journal of Hydrogen Energy. https://doi.org/10.1016/j.ijhydene.2020.04.112 Energy, A. F. N. R. A. (2014). Summary of the Strategic Road Map for Hydrogen and Fuel Cells, Ministry of Economy, Trade and Industry. https://doi.org/10.1016/S1464-2859(14)70204-4 ENGIE. (2016). The GRHYD demonstration project. ENGIE. Retrieved from https://www.engie.com/ Esposito, D. V. (2017). Membraneless Electrolyzers for Low-Cost Hydrogen Production in a Renewable Energy Future. Joule, 1(4), 651-658. https://doi.org/10.1016/j.joule.2017.07.003 FCW. (2020). News by Fuel Cells Works. Retrieved from https://fuelcellsworks.com/news/several-hundred-hydrogen-scooters-will-circulate-in-marrakech-i n-the-next-few-days/ Gannon, W. J. F., Warwick, M. E. A., & Dunnill, C. W. (2020). Woven Stainless-Steel Mesh as a Gas Separation Membrane for Alkaline Water-Splitting Electrolysis. Membranes, 10(5). https://doi.org/10.3390/membranes10050109 Ghosh, S., Nakada, A., Springer, M. A., Kawaguchi, T., Suzuki, K., Kaji, H., … Seki, S. (2020). Identification of Prime Factors to Maximize the Photocatalytic Hydrogen Evolution of Covalent Organic Frameworks. Journal of the American Chemical Society, 142(21), 9752-9762. https://doi.org/10.1021/jacs.0c02633 https://doi.org/10.1039/c2ee22248e https://doi.org/10.1016/j.energy.2019.116671 https://doi.org/10.1038/ncomms12555 https://doi.org/10.1016/j.ijhydene.2019.08.168 https://doi.org/10.1016/j.ijhydene.2020.04.112 https://doi.org/10.1016/S1464-2859(14)70204-4 https://doi.org/10.1016/j.joule.2017.07.003 https://doi.org/10.3390/membranes10050109 https://doi.org/10.1021/jacs.0c02633 www.scholink.org/ojs/index.php/ees Energy and Earth Science Vol. 3, No. 2, 2020 59 Published by SCHOLINK INC. Gondal, I. A. (2019). Offshore renewable energy resources and their potential in a green hydrogen supply chain through power-to-gas. Sustainable Energy & Fuels, 3(6), 1468-1489. https://doi.org/10.1039/C8SE00544C Jones, D. R., Al-Masry, W. A., & Dunnill, C. (2018). Hydrogen-enriched natural gas as a domestic fuel: An analysis based on flash-back and blow-off limits for domestic natural gas appliances within the UK. Sustainable Energy & Fuels, 2(4), 710-723. https://doi.org/10.1039/C7SE00598A Kuehnel, M. F., & Reisner, E. (2018). Solar Hydrogen Generation from Lignocellulose. Angewandte Chemie International Edition, 57(13), 3290-3296. https://doi.org/10.1002/anie.201710133 Landman, A., Halabi, R., Dias, P., Dotan, H., Mehlmann, A., Shter, G. E., …Rothschild, A. (2020). Decoupled Photoelectrochemical Water Splitting System for Centralized Hydrogen Production. Joule, 4(2), 448-471. https://doi.org/10.1016/j.joule.2019.12.006 Mehr, A. S., Moharramian, A., Hossainpour, S., & Pavlov, D. A. (2020). Effect of blending hydrogen to biogas fuel driven from anaerobic digestion of wastewater on the performance of a solid oxide fuel cell system. Energy, 202, 117668. https://doi.org/10.1016/j.energy.2020.117668 METD. (2017). Japan’s Energy White Paper, Ministry of Economy, Trade and Industry. Mulla, R., & Dunnill, C. W. (2019). Powering the Hydrogen Economy from Waste Heat: A Review of Heat-to-Hydrogen Concepts. ChemSusChem, 12(17), 3882-3895. https://doi.org/10.1002/cssc.201901426 Mulla, R., & Dunnill, C. W. (2020). Graphite-loaded cotton wool: A green route to highly-porous and solid graphite pellets for thermoelectric devices. Composites Communications, 20, 100345. https://doi.org/10.1016/j.coco.2020.04.011 Mulla, R., & Rabinal, M. H. K. (2019). Copper Sulfides: Earth-Abundant and Low-Cost Thermoelectric Materials. Energy Technology, 7(7), 1800850. https://doi.org/10.1002/ente.201800850 Mulla, R., & Rabinal, M. K. (2018). Defect-Controlled Copper Iodide: A Promising and Ecofriendly Thermoelectric Material. Energy Technology, 6(6), 1178-1185. https://doi.org/10.1002/ente.201700708 Mulla, R., Jones, D. R., & Dunnill, C. W. (2020). Thermoelectric Paper: Graphite Pencil Traces on Paper to Fabricate a Thermoelectric Generator. Advanced Materials Technologies, 5(7), 2000227. https://doi.org/10.1002/admt.202000227 Passas, G., & Dunnill, C. W. (2015). Water Splitting Test Cell for Renewable Energy Storage as Hydrogen Gas. Journal of Fundamentals of Renewable Energy and Applications, 5, 188. Perni, S., Liu, D. W., Shama, G., & Kong, M. G. (2008). Cold Atmospheric Plasma Decontamination of the Pericarps of Fruit. Journal of Food Protection, 71, 302-308. https://doi.org/10.4315/0362-028X-71.2.302 Philibert, C. (2017). Producing ammonia and fertilizers: New opportunities from renewables. International Energy Agency. https://doi.org/10.1039/C8SE00544C https://doi.org/10.1039/C7SE00598A https://doi.org/10.1002/anie.201710133 https://doi.org/10.1016/j.joule.2019.12.006 https://doi.org/10.1016/j.energy.2020.117668 https://doi.org/10.1002/cssc.201901426 https://doi.org/10.1016/j.coco.2020.04.011 https://doi.org/10.1002/ente.201800850 https://doi.org/10.1002/ente.201700708 https://doi.org/10.1002/admt.202000227 https://doi.org/10.4315/0362-028X-71.2.302 www.scholink.org/ojs/index.php/ees Energy and Earth Science Vol. 3, No. 2, 2020 60 Published by SCHOLINK INC. Phillips, R., & Dunnill, C. W. (2016). Zero gap alkaline electrolysis cell design for renewable energy storage as hydrogen gas. RSC Advances, 6(102), 100643-100651. https://doi.org/10.1039/C6RA22242K Phillips, R., & Dunnill, C. W. (2016). Zero Gap Alkaline Electrolysis Cell Designs for Renewable Energy Storage as Hydrogen Gas. RSC Advances, 6, 100643. https://doi.org/10.1039/C6RA22242K Phillips, R., Edwards, A., Rome, B., Jones, D. R., & Dunnill, C. W. (2017). Minimising the ohmic resistance of an alkaline electrolysis cell through effective cell design. International Journal of Hydrogen Energy, 42(38), 23986-23994. https://doi.org/10.1016/j.ijhydene.2017.07.184 Quarton, C. J., Tlili, O., Welder, L., Mansilla, C., Blanco, H., Heinrichs, H., … Samsatli, S. (2020). The curious case of the conflicting roles of hydrogen in global energy scenarios. Sustainable Energy & Fuels, 4(1), 80-95. https://doi.org/10.1039/C9SE00833K Sampson, J. (2020). Green light for hydrogen-powered homes in Scotland. Retrieved from https://www.h2-view.com/ Staffell, I., Scamman, D., Velazquez Abad, A., Balcombe, P., Dodds, P. E., Ekins, P., Shah, N., & Ward, K. R. (2019). The role of hydrogen and fuel cells in the global energy system. Energy & Environmental Science, 12(2), 463-491. https://doi.org/10.1039/C8EE01157E Wang, B., Klemeš, J. J., Liang, Y., Yuan, M., Zhang, H., & Liu, J. (2020). Implementing hydrogen injection in coal-dominated regions: Supply chain optimisation and reliability analysis. Energy, 201, 117565. https://doi.org/10.1016/j.energy.2020.117565 Warwick, M. E. A., Barreca, D., Bontempi, E., Carraro, G., Gasparotto, A., Maccato, C., … Mathur, S. (2015). Pt-functionalized Fe2O3 photoanodes for solar water splitting: The role of hematite nano-organization and the platinum redox state. Physical Chemistry Chemical Physics, 17(19), 12899-12907. https://doi.org/10.1039/C5CP01636C https://doi.org/10.1039/C6RA22242K https://doi.org/10.1039/C6RA22242K https://doi.org/10.1016/j.ijhydene.2017.07.184 https://doi.org/10.1039/C9SE00833K https://doi.org/10.1039/C8EE01157E https://doi.org/10.1016/j.energy.2020.117565 https://doi.org/10.1039/C5CP01636C