Technologies for harvesting and storage energy from renewable sources need to be scaled up at least six times faster for the world to meet the decarbonization and climate mitigation goals set out in the Paris Agreement, states IRENA in Global Energy Transformation, a Roadmap to 2050. Sunlight is the most abundant and probably as well the most convenient local renewable energy source for producing thermal and electrical energy [1]. Although photovoltaic (PV) electricity is already the cheapest if produced in countries with high solar irradiance, technology gaps still exist for achieving cost-effective scalable deployment combined with storage technologies to provide reliable and dispatchable energy. More recently, the direct conversion of sunlight into storable fuels and feedstock chemicals has been attracting the attention of scientists and entrepreneurs; the name solarchemistry has been coined in this context.

One of the most important technologies which has emerged for converting sunlight into fuels is the photoelectrochemical (PEC) cells which can serve for: a) water splitting; b) charging redox flow cells and; c) CO₂ photoelectroreduction. This talk addresses the latest developments in PEC cells for solar water splitting and direct charging redox flow cells. Critical pathways towards economically viability of both technologies in energy markets require: i) the development of stable and efficient earth-abundant photo-absorber materials and electrocatalysts; and ii) the successful optimization of PEC cell architectures suitable for large-scale solar fuels production. These challenges have yet to be overcome and both are still in the research and development stage. Latest developments on hematite photoelectrodes concerning long-term stability [2] and high photovoltage [3] will be discussed along with their larger-scale implementation of these sunlight energy harvesting and storage processes. Photoelectrochemical “CoolPEC” cell, optimized for continuous operation and including improved key features such as: i) simultaneous photoelectrode and cell window; ii) electrolyte feeding manifold, assuring efficient gas bubbles detachment from windows and efficient heat dissipation if concentrated sunlight is used; and iii) narrow, easy-assemble and cost‑efficient embodiment [4], was reported. “CoolPEC” cell was operated over 42 days (1008 h) using a 50 cm² hematite photoelectrode in a tandem arrangement with silicon heterojunction solar cells, under 1000 W∙m⁻² and with constant electrolyte feeding at 45 °C [4]. More recently, a 200 cm² PEC module (Figure 1), comprising four 50 cm² PEC cells based on the CoolPEC cell design, was constructed and operated continuously outdoor under concentrated sunlight (up to 14 kW∙m^-2), provided by the SoCRatus (DLR. Cologne, Germany), with electrolyte recirculation over four days. When assembled with four multi-photoelectrode windows, the module generated a stable current density of ca. 2.0 mA∙cm^-2 at 1.45 V and reached a plateau current of ca. 4.0 mA∙cm^‑2 before dark current onset, resulting in a hydrogen production rate of 5.5 × 10^‑5 g_H2∙h^-1·cm^-2 (based on the active area). The technology of solar charging redox flow batteries, named solar redox flow cells (SRFC), received recently a boost with the publication of an article reporting the first truly stable aqueous alkaline redox flow battery with a hematite photoanode [5]. SRFC is now a hot research topic with great potential for building integrated since this technology can simultaneously harvest efficiently sunlight to two forms of energy, an electrochemical fuel easily converted in electricity and heat. This technology is expecting to display soon over 10 % of solar to electrochemical energy conversion efficiency and ca. 60 % of heat harvesting for sanitary and thermal comfort uses.