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Abstract Renewable fuel generation is essential for a low carbon footprint economy. Thus, over the last five decades, a significant effort has been dedicated towards increasing the performance of solar fuels generating devices. Specifically, the solar to hydrogen efficiency of photoelectrochemical cells has progressed steadily towards its fundamental limit, and the faradaic efficiency towards valuable products in CO₂reduction systems has increased dramatically. However, there are still numerous scientific and engineering challenges that must be overcame in order to turn solar fuels into a viable technology. At the electrode and device level, the conversion efficiency, stability and products selectivity must be increased significantly. Meanwhile, these performance metrics must be maintained when scaling up devices and systems while maintaining an acceptable cost and carbon footprint. This roadmap surveys different aspects of this endeavor: system benchmarking, device scaling, various approaches for photoelectrodes design, materials discovery, and catalysis. Each of the sections in the roadmap focuses on a single topic, discussing the state of the art, the key challenges and advancements required to meet them. The roadmap can be used as a guide for researchers and funding agencies highlighting the most pressing needs of the field. 

Numerous efficient semiconductors suffer from instability in aqueous electrolytes. Strategies utilizing protective coatings have thus been developed to protect these photoabsorbers against corrosion while synergistically improving charge separation and reaction kinetics. Recently, various photoelectrochemical (PEC) protective coatings have been reported with suitable electronic properties to ensure low charge transport loss and reveal the fundamental photoabsorber efficiency. However, protocols for studying the critical figures of merit for protective coatings have yet to be established. For this reason, we propose four criteria for evaluating the performance of a protective coating for PEC water-splitting: stability, conductivity, optical transparency, and energetic matching. We then propose a flow chart that summarizes the recommended testing protocols for quantifying these four performance metrics. In particular, we lay out the stepwise testing protocols to evaluate the energetics matching at a semiconductor/coating/(catalyst)/liquid interface. Finally, we provide an outlook for the future benchmarking needs for coatings. 

One key objective in electrocatalysis is to design selective catalysts, particularly in cases where the desired products require thermodynamically unfavorable pathways. Electrochemical synthesis of hydrogen peroxide (H 2 O 2 ) via the two-electron water oxidation reaction (2e − WOR) requires a +0.54 V higher potential than four-electron O 2 evolution. So far, best-performing electrocatalysts require considerable overpotentials before reaching peak faradaic efficiency. We present Mn-alloyed TiO 2 coatings prepared by atomic layer deposition (ALD) and annealing as a stable and selective electrocatalyst for 2e − WOR. Faradaic efficiency of >90% at < 150 mV overpotentials was achieved for H 2 O 2 production, accumulating 2.97 mM H 2 O 2 after 8 hours. Nanoscale mixing of Mn 2 O 3 and TiO 2 resulted in a partially filled, highly conductive Mn 3+ intermediate band (IB) within the TiO 2 mid-gap to transport charge across the (Ti,Mn)O x coating. This IB energetically matched that of H 2 O 2 -producing surface intermediates, turning a wide bandgap oxide into a selective electrocatalyst capable of operating in the dark. However, the high selectivity is limited to the low overpotential regime, which limits the system to low current densities and requires further research into increasing turn-over frequency per active site.