Photoelectrochemical devices for solar water splitting - materials and challenges.

The increasing demand of transportation fuels and challenges associated with the combustion of fossil fuels (emission, pollution) has led to the search for alternative sustainable fuels and fuel processing routes. The solar-driven production of synthetic fuels like methanol or kerosene can provide a solution to these problems if a carbon-neutral approach is chosen, i.e. the carbon-based source is recycled in the production-consumption cycle by capturing the CO2 directly from the air or concentrated sources such as exhaust of large power plants. These fuel synthesis routes are based on photo-electrochemical (PEC) water and CO2 electrolysis and subsequent reaction of the synthesis gas (a mixture of H2 and CO) in a Fischer-Tropsch (FT) process for the production of liquid hydrocarbons. FT reactions can produce hydrocarbon fuels only when an appropriate mixture of H2 and CO is provided (H2/CO ratios of 1.7 and 2.15 when FT reactors with iron and cobalt-based catalyst are used, respectively). The production of this exact mixture requires a well-designed PEC device incorporating water and CO2 splitting catalysts, or a combination of individual water-splitting PEC devices and CO2-splitting PEC devices. We expect that the simultaneous conversion of CO2 and water in a single device –if precisely designed– will operate at a reduced cost and a higher efficiency. We aim to provide quantifiable device design guidelines. A numerical multi-physics model of a characteristic PEC device for simultaneous CO2 and water splitting was developed. The catalysts were modeled by the Butler-Volmer correlations, for which exchange current densities, charge transfer coefficients, and limiting current densities were extracted from published experiments. Since most experimental studies focus on only one half-reaction, challenges were posed by finding operational conditions (electrolyte, temperature) simultaneously sustained by all components necessary for a working device. Current-potential performance characteristics of realistic PV cells were obtained by experimental fitting, and of idealized PV cells were obtained by the Shockley-Queisser limit and incorporating series and shunt resistances to account for transport losses. Steady and transient solar irradiation measured in Barstow-California were used for the calculations. The electrolyte and product-separating ion-conducting membrane were modeled as ohmic resistances. Electrolytes considered were 1M NaOH and 1M KOH for alkaline solutions, and 0.1M KHCO3 for pH-neutral solutions. Existing photoabsorbers and electrocatalyst materials were used for the investigation in order to assess the possibility of producing syngas at high efficiency and in the desired H2/CO ratio using readily available components. PEC devices using silver-cobalt catalysts in alkaline solution showed low and almost constant H2/CO ratios of around 0.13, regardless of the photoabsorber combination used. Silver-platinum catalysts showed H2/CO ratios below 0.13. Copper-platinum catalyst in pH-neutral solution showed H2/CO ratios between 5 and 11 but a lower solar-to-product efficiencies compared to devices operating in alkaline solutions due to larger reaction overpotentials and ohmic losses in pH-neutral environments. The influence of the photoabsorber choice on the performance of the PEC device using silver-cobalt catalysts was analyzed in-depth by considering hypothetical components made of dual-junction solar cells composed of materials with different band gaps, and different series and shunt resistances. The performance was improved as the band gap of the bottom cell was smaller. This, performance increase was limited when further reducing the bottom cell band gap due to similar generation rates of H2 and CO at high operating currents, and due to mass transfer limitations in the catalyst. The desired catalytic characteristics (charge transfer coefficient and exchange current density) required to produce H2/CO ratios of about 2 were quantified in order to guide the research of suitable-catalysts. As an engineering solution to increase the H2/CO ratio, the addition of a hydrogen-selective catalyst (i.e. nickel) was investigated. The positioning and the amount of this second catalyst was analyzed, allowing to provide design guidelines for reaching H2/CO ratios of around 2 at minimized cost. Finally, a silver-nickel-cobalt PEC device was isothermally modeled under realistic, transient irradiation conditions. The results showed a large variation of the product ratio during the day and year, which provided evidence that adequate on-site H2 and CO storage options are required to smooth these variations. During cold seasons an average daily H2/CO ratio of 1.66 was achieved (see figure 1-left side), whilst in warm seasons an average daily H2/CO ratio of 2.24 was obtained (see figure 1-rigth side). The developed model proved to be useful for the analysis and the optimization of PEC devices for simultaneous CO2 and water splitting under different operating conditions and material choices, and allowed for the formulation of design guidelines for practical device development. Figure 1

Utilization of solar light energy for water splitting and production of hydrogen, on the future industrial basis, will be performed in a mass-scale chemical plant involving lots of photocatalytic light-harvesting panels. Within those panels, not only the photocatalysts but also many other functional mechanisms will be included in order to feed and circulate the reactant water, to eject and lead the product gases safely, and to maintain the conditions of the catalysts and water for stable operation. All those functionalities consume energy, whereas the energy production is made only by the photocatalysts. As an energy-producing plant, the production and self-consumption of energy should be balanced so as to gain a profitable amount of energy. We are now focusing on the operational technologies for our laboratory-class solar light-driven water splitting devices towards higher energy conversion efficiency and durability, taking this balance into our consideration. Choice of the light-absorbing materials as well as co-catalysts for H2/O2evolution is the most important issue to realize satisfactory efficiency. Indeed most of our efforts are dedicated for research of materials suitable for light harvesting. To foster material research in this sense, we adhere to photocatalytic or photoelectrochemical devices without photovoltaics or external power supply as the main power source. We are intensively investigating several semiconductive candidate materials, such as oxides [1,2], nitrides, oxynitrides [3-5] and other chalcogenides [6-9] of transition metals for both photocathodes and photoanodes. The key issues in selecting the target materials are the light absorption spectrum in the solar region, the optimized photoelectrochemical current density, the onset potential of photoelectrochemical curves, and the durability after appropriate co-catalyst and protective coating. We chose pairs of photocathodes and photoanodes out of these materials of our own development, and assemble prototype water-splitting photo-cells. A finite non-zero photo-electrolysis current is obtained when the cyclic photo-voltammetric curves of the cathode and the anode cross over each other under light irradiation in the same electrolytic solution. This is a prerequisite of our water splitting device and is still a limiting issue for the choice of electrode materials for a highly efficient water-splitting device. The content of operational solution and, in particular, the pH of the solution are also important in the dual electrode operation. Naturally the optimal pH for the photocathode alone is different from that for photoanode alone, and we must adjust the solution pH at the median of those two, as long as the electrodes are not deactivated. The reactivity of co-catalysts are often dependent on the solutes. Phosphate, sulfate and borate buffers are the typical choices. The durability of water-splitting devices is related to the maintenance of catalyst surface activity and stabilization of chemical and physical status of the aqueous solution near the catalyst surface. To prevent or delay the chemical degradations of photocatalyst surfaces, we are developing protective layers that are electron/hole conductive and chemically inert. Physically, the sizes and arrangement of photocatalytic plates can facilitate the microscopic circulation of aqueous species near them. The rapidest decay of reactivity is usually due to building-up inhomogeneity of solute distribution between the two photoelectrodes during operation, often called “pH gradient”. This inhomogeneity will be cancelled by stirring, pumping or circulating the solution, or by applying the inverse voltage between the electrodes. We are trying to work out how efficiently these mechanical/fluidic and electrochemical issues are solved in order to conserve the extra energy for those costs. In this talk some specific examples of materials and technologies from our group will be presented, and some benchmark devices with notable efficiency and durability for solar powered water splitting will be described.

Photoelectrochemical (PEC) water splitting is a promising renewable energy technology to produce green hydrogen for the future fossil-fuel-free society. Over the past decade, research on PEC water splitting devices has achieved significant improvements in the demonstrated solar-to-hydrogen (STH) efficiencies. The improved efficiencies have led to the development of large-scale devices [1,2] and the coupling of hydrogen production with the synthesis of valuable chemicals [3,4]. The co-generation approach offers a potential route towards achieving a levelized cost of hydrogen (LCOH) that is competitive with the current market price of hydrogen and increases the overall economic feasibility of the PEC technology.This study evaluates the potential of co-producing hydrogen and methyl succinic acid (MSA) by coupling the hydrogenation of itaconic acid (IA) into MSA inside a PEC water splitting reactor. We used a PEC device that uses BiVO4 as the top absorber and a silicon solar cell as the bottom absorber, as reported previously [1,5]. To address the feasibility of this approach, a net energy balance assessment is conducted, and the results are compared with the benchmark values for conventional MSA production. We follow the Techno-Economic Assessment & Life Cycle Assessment Guidelines for CO2 Utilization (Version 1.1) which provides a specific protocol for multi-functional PEC devices [6]. Life cycle inventory (LCI) values from the literature and Ecoinvent database [7] are used to construct the target scenarios in Simapro v9.2.0.Our results show that the energy demand of our PEC device is ca. 3800 MJ/m2, and the most energy intensive components are the photoelectrode (~70%) and the Nafion membrane (8%). Under the base case condition (i.e., STH = 5%, device longevity = 10 years) and when H2 is the only product, a negative net energy balance of ca. -160 MJ/m2/year is obtained. However, with a coupled hydrogenation reaction, a zero net energy balance (i.e., energy breakeven) can already be achieved when only 2% of the produced H2 molecules are converted into MSA (see red circle in Fig. 1a). Figure 1b shows the cumulative energy demand to produce one kg of MSA under a more optimistic scenario, in which the H2-to-MSA conversion efficiency is 0.4. Under this condition, the net energy production is ca. 3500 MJ/m2/year, which translates to a cumulative energy demand of ca. 13 MJ/kg of MSA (see red circle in Fig. 1b). This is much lower compared to MSA produced using conventional hydrogenation methods (i.e., ~90 MJ/kg MSA), which underlines the attractiveness of the coupled PEC approach.Finally, we analyze the potential for further improvement of the net energy balance. We explore possibilities of replacing device components (e.g., photoelectrode, membrane) and assess the impact to the net energy balance of the device. The result of this optimization study will be presented, and the most effective strategy will be outlined. Keywords : water splitting, (photo)electrochemistry, net energy assessment, coupled catalysis, hydrogenation References [1] Ahmet IY, Ma Y, Jang JW, Henschel T, Stannowski B, Lopes T, Vilanova A, Mendes A, Abdi FF, van De Krol R. Demonstration of a 50 cm2 BiVO4 tandem photoelectrochemical-photovoltaic water splitting device. Sustain Energy Fuels. 2019;3(9):2366–79.[2] Tolod KR, Hernández S, Russo N. Recent advances in the BiVO4 photocatalyst for sun-driven water oxidation: Top-performing photoanodes and scale-up challenges. Catalysts. 2017;7(1).[3] Mei B, Mul G, Seger B. Beyond Water Splitting: Efficiencies of Photo-Electrochemical Devices Producing Hydrogen and Valuable Oxidation Products. Adv Sustain Syst. 2017;1(1–2).[4] Luo H, Barrio J, Sunny N, Li A, Steier L, Shah N, Stephens IEL, Titirici MM. Progress and Perspectives in Photo- and Electrochemical-Oxidation of Biomass for Sustainable Chemicals and Hydrogen Production. Adv Energy Mater. 2021;11(43).[5] Abdi FF, Han L, Smets AHM, Zeman M, Dam B, van De Krol R. Efficient solar water splitting by enhanced charge separation in a bismuth vanadate-silicon tandem photoelectrode. Nat Commun. 2013;4:1–7. Available from: http://dx.doi.org/10.1038/ncomms3195[6] Zimmermann AW, Wang Y, Wunderlich J, Buchner GA, Schomäcker R, Müller LJ, Langhorst T, Kätelhön A, Bachmann M, Sternberg A, Bardow A, Armstrong K, Michailos S, McCord S, Zaragoza AV, Styring P, Marxen A, Naims H, Cremonese L, Strunge T, Olfe-Kräutlein B, Faber G, Mangin C, Mason F, Stokes G, Williams E, Sick V. Techno-Economic Assessment & Life Cycle Assessment Guidelines for CO2 Utilization (Version 1.1). 2020;(September).[7] Jungbluth N, Stucki M FR. Photovoltaics. In Sachbilanzen von Energiesystemen: Grundlagen für den ökologischen Vergleich von Energiesystemen und den Einbezug von Energiesystemen in Ökobilanzen für die Schweiz. ecoinvent report No. 6-XII. Swiss Cent Life Cycle Invent Dübendorf, CH. 2009;16–69(6–XII). Figure 1

Immobilized photocatalyst devices for overall water splitting (OWS) have emerged as a promising strategy for practical hydrogen production. Compared with traditional powder suspension systems, they offer great advantages, including scalability, facile recovery/replacement of photocatalysts, and the elimination of extra dispersion operations. Over the past decade, significant progress has been achieved in this field, especially in material screening, device construction, and system integration for scalable applications. However, up until now, there have been no related reviews focusing on this topic. This review aims to fill this gap by providing a comprehensive and structured overview of the immobilized photocatalyst devices for efficient OWS. Firstly, the basics of OWS process, including one-step and two-step photoexcitation mechanisms, are elaborated. Subsequently, recent advances in immobilized photocatalyst devices for scalable OWS via these two different approaches are summarized, based on which various solid-state electron mediators are classified and exemplified. The essential structure-performance relationship is also analyzed and revealed. Finally, the future prospects and challenges in this field are proposed and discussed.

A novel “photovoltaics (PV) + electrolyzer” concept is presented using a simple, small, and completely stand-alone non-biased device for solar-driven overall water splitting. Three or four spherical-shaped p-n junction silicon balls were successfully connected in series, named “SPHELAR.” SPHELAR possessed small projected areas of 0.20 (3PVs) and 0.26 cm2 (4PVs) and exhibited working voltages sufficient for water electrolysis. Impacts of the configuration on the PV module performance were carefully analyzed, revealing that a drastic increase in the photocurrent (≈20%) was attained by the effective utilization of a reflective sheet. Separate investigations on the electrocatalyst performance showed that non-noble metal based materials with reasonably small sizes (<0.80 cm2) exhibited substantial currents at the PV working voltage. By combining the observations of the PV characteristics, light management and electrocatalyst performance, solar-driven overall water splitting was readily achieved, reaching solar-to-hydrogen efficiencies of 7.4% (3PVs) and 6.4% (4PVs).

Empowering metal oxide photoanodes via zeolitic imidazolate frameworks for efficient photoelectrochemical water splitting: Current advances and future perspectives

Photoelectrochemical water splitting is a promising pathway for solar-driven hydrogen production with a low environmental footprint. The utilization of solar concentrators to supply such water splitting devices with concentrated solar irradiation offers great potential to enhance the economic viability of water splitting at “sunny” site locations. In this work, we defined a set of functional requirements for solar concentrators to assess their suitability to power such water splitting devices, taking into account concentrator optical performance, device coupling efficiency, perceived system complexity, as well as technological costs and risks. We identified, classified and compared a broad range of existing solar concentrator design concepts. Our geometrical analysis, performed on a yearly basis with a one-minute time step, shows that two-axis tracking concentrators with water splitting devices positioned parallel to the optical aperture plane exhibit the highest potential, given the initial conditions applied for the device tilt constraints. Demanding an angle of at least 20° between horizontal and the front side of the water splitting device, allows the device to be operational for 97% of the daylight time in Seville, Spain. The relative loss with respect to the available direct normal irradiance is estimated to 6%. Results moderately depend on the location of application, but generally confirm that the consideration of tilt angle constraints is essential for a comprehensive performance assessment of photoelectrochemical water splitting driven by concentrated sunlight.

In search for new technology to preserve global warming caused by fossil fuel, solar energy driven devices have been high in demand. For instance, photovoltaics (PV) are a promising technology that can produce high power conversion efficiency (PCE). In addition, photoelectrochemical (PEC) water splitting, a promising artificial photosynthesis method, is a potential alternative technique to generate renewable fuel. PEC devices can generate renewable energy sources such as hydrogen, which can alleviate CO2 emission from fossil fuels. However, most readily employed PEC devices require external bias to function as a hydrogen producer. Thus, utilizing PV active layer as a photo absorber, PEC system is a suitable approach to the breakthrough of efficient hydrogen production.In the past decade of PV technology advances, organic-inorganic lead halide perovskite (LHP) can be arguably the most progressive research because of its tunable bandgap, fast charge separation, broad light absorption spectra, low cost, facile fabrication, and high efficiency. These advantageous properties led LHPs to be used as an active layer of water splitting devices. However, LHP-based PECs have flaws to be addressed to be viable material for PEC. LHP are known for their fragile nature from exposure to water. Hence, LHP-based PECs are commonly protected with metal foil encapsulation while covering the edges with epoxy resins. However, the metal foil provides poor contact with the LHP leading to poor charge transfer for water splitting. To solve this problem, we applied carbon conductive powder (CCP) as the interlayer to successfully facilitate charge transfer.With the proposed device modeling, monolithic PEC device that is capable of generating hydrogen in presence of low external applied bias is fabricated. Nonetheless, single monolithic PEC devices still need external applied bias to function as water splitting device which seek for further research. Therefore, adding another photoelectrode will eliminate the necessity of external bias resulting in an only solar powered water splitting device. Herein, we report unassisted solar to hydrogen conversion by using inverted and planar type LHP solar cells as photocathode and photoanode, respectively. With the integration of catalyst, 3D Ni.NiFe and NiPCoP, LHP-based PEC with exceptional performance was attained. We demonstrated unbiased PEC water splitting using a coupled LHP-based photoelectrode under alkaline conditions. Each LHP-based photoelectrode exhibited a outstanding performance of 22.42 mA cm-2 at 1.23 VRHE and 21.96 mA cm-2 at 0 VRHE for the photoanode and photocathode, respectively. Thus, we achieved solar to hydrogen (STH) of 10.64% and unassisted photo-current density of 8.65 mA/cm2. In addition, unbiased catalytic reactivity over 20 h retaining about 60% of photo-current density was attained.

Earth-abundant photoelectrodes for water splitting and alternate oxidation reactions: Recent advances and future perspectives

Recent progress in efficiency of hydrogen evolution process based photoelectrochemical cell

A cost-effective and high-efficiency photoelectrochemical (PEC) water splitting system based on colloidal quantum dots (QDs) represents a potential solar-to-hydrogen (STH) conversion technology to achieve future carbon neutrality. Herein, a self-biased PEC cell consisting of BiVO4 photoanode and Cu2 O photocathode both decorated with Zn-doped CuInS2 (ZCIS) QDs is successfully fabricated. The intrinsic charge dynamics of the photoelectrodes are efficiently optimized via rational engineering of the surface ligands capped on QDs with controllable chain lengths and binding affinities to the metal oxide electrodes. It is demonstrated that the short-chain monodentate 1-dodecanethiol ligands are beneficial to ZCIS QDs for suppressing charge recombination, which enables the construction of tight heterojunction with coupled metal oxide electrodes, leading to effective photo-induced charge transfer/injection for enhanced PEC performance. The QD decorated BiVO4 and Cu2 O photoelectrodes in pairs demonstrate a self-biased PEC water splitting process, delivering an STH efficiency of 0.65% with excellent stability under AM 1.5 G one-sun illumination. The results highlight the significance of synergistic ligand and heterojunction engineering to build highly efficient and robust QDs-based PEC devices for self-biased solar water splitting.

The development of a bifunctional electrocatalyst with high efficiency, high stability, and low cost is of great significance in practical applications of electrocatalytic water splitting. Herein, a self-supporting bifunctional electrocatalyst with a NiFe layered double hydroxide/Fe2O3/Ni3S2 heterostructure (NiFe LDH/Fe2O3/Ni3S2/IF) for hydrogen evolution and oxygen evolution reactions (HER/OER) is synthesized by the self-corrosion of iron foam (IF) and hydrothermal strategies. The constructed NiFe LDH/Fe2O3/Ni3S2/IF hierarchical heterostructure was not only beneficial to expose active sites and promote charge/mass transfer but also generate a superhydrophilic/superaerophobic surface, thereby accelerating the reaction kinetics to improve the HER/OER activity. Therefore, NiFe LDH/Fe2O3/Ni3S2/IF exhibited superior overpotentials of 226.2 and 162.8 mV for the OER and HER at 100 mA cm-2, respectively. NiFe LDH/Fe2O3/Ni3S2/IF was employed as both the cathode and the anode to assemble a device for overall water splitting and displayed a voltage of 1.55 V at 10 mA cm-2. The overall water splitting device was coupled with a solar cell to simulate a solar-powered water splitting system, resulting in a superior solar-to-hydrogen conversion efficiency of 15.16%. This work can promote the development of clean energy sources such as solar hydrogen production.

A two-step water-splitting thermochemical cycle using redox working material of iron-based oxide (ferrite) particles has been developed for converting solar energy into hydrogen. The two-step thermochemical cycle for producing a solar hydrogen from water requires a development of a high temperature solar-specific receiver-reactor operating at 1000–1500°C. In the present work, ferrite-loaded ceramic foams with a high porosity (7 cells per linear inch) were prepared as a water splitting device by applying ferrite/zirconia particles on a MgO-partially stabilized Zirconia (MPSZ) ceramic foam. The water splitting foam device was prepared using a new method of spin coating. A spin coating method we newly employed that has advantages of shortening preparation period and reducing of the coating process in comparison to previous preparation method reported. The water-splitting foam devices, thus prepared, were examined on hydrogen productivity and reactivity through a two-step thermochemical cycle. NiFe2O4/m-ZrO2/MPSZ and Fe3O4/c-YSZ/MPSZ foam devices were firstly tested for thermal reduction of ferrite using the laboratory scale receiver-reactor by a sun-simulator to simulate concentrated solar radiation. Subsequently, with another quartz reactor the light-irradiated device was reacted with steam by infrared furnace. As a result, it was possible to perform cyclic reactions over several times and to produce hydrogen through thermal-reduction at 1500°C and water-decomposition at 1100–1200°C. In further experiments, the NiFe2O4/m-ZrO2/MPSZ foam device was successfully demonstrated in a windowed single reactor for cyclic hydrogen production by solar-simulated Xebeam irradiation with input power of 1 kW. The NiFe2O4/m-ZrO2/MPSZ foam device produced hydrogen of 70–190μmol per gram of device through 6 cycles and reached ferrite conversion of 60% at a maximum.

Dual-bioinspired Janus mesh membrane with controllable bubbles manipulation property for efficient water splitting and pure gas collection

Photoelectrochemical water splitting (PEC) offers a promising path for sustainable generation of hydrogen fuel. However, improving solar fuel water splitting efficiency facing tremendous challenges, due to the energy loss related to fast recombination of the photogenerated charge carriers, electrode degradation, as well as limited light harvesting. This review focuses on the brief introduction of basic fundamental of PEC water splitting and the concept of various types of water splitting approaches. Numerous engineering strategies for the investgating of the higher efficiency of the PEC, including charge separation, light harvesting, and co-catalysts doping, have been discussed. Moreover, recent remarkable progress and developments for PEC water splitting with some promising materials are discussed. Recent advanced applications of PEC are also reviewed. Finally, the review concludes with a summary and future outlook of this hot field.