Realizing solar‐to‐hydrogen (STH) efficiencies close to 20% using low‐cost semiconductors remains a major step toward accomplishing the practical viability of photoelectrochemical (PEC) hydrogen generation technologies. Dual‐absorber tandem cells combining inexpensive semiconductors are a promising strategy to achieve high STH efficiencies at a reasonable cost. Here, a perovskite photovoltaic biased silicon (Si) photoelectrode is demonstrated for highly efficient stand‐alone solar water splitting. A p+nn+ ‐Si/Ti/Pt photocathode is shown to present a remarkable photon‐to‐current efficiency of 14.1% under biased condition and stability over three days under continuous illumination. Upon pairing with a semitransparent mixed perovskite solar cell of an appropriate bandgap with state‐of‐the‐art performance, an unprecedented 17.6% STH efficiency is achieved for self‐driven solar water splitting. Modeling and analysis of the dual‐absorber PEC system reveal that further work into replacing the noble‐metal catalyst materials with earth‐abundant elements and improvement of perovskite fill factor will pave the way for the realization of a low‐cost high‐efficiency PEC system.

While direct solar‐driven water splitting has been investigated as an important technology for low‐cost hydrogen production, the systems demonstrated so far either required expensive materials or presented low solar‐to‐hydrogen (STH) conversion efficiencies, both of which increase the levelized cost of hydrogen (LCOH). Here, a low‐cost material system is demonstrated, consisting of perovskite/Si tandem semiconductors and Ni‐based earth‐abundant catalysts for direct solar hydrogen generation. NiMo‐based hydrogen evolution reaction catalyst is reported, which has innovative “flower‐stem” morphology with enhanced reaction sites and presents very low reaction overpotential of 6 mV at 10 mA cm−2. A perovskite solar cell with an unprecedented high open circuit voltage (Voc) of 1.271 V is developed, which is enabled by an optimized perovskite composition and an improved surface passivation. When the NiMo hydrogen evolution catalyst is wire‐connected with an optimally designed NiFe‐based oxygen evolution catalyst and a high‐performance perovskite‐Si tandem cell, the resulting integrated water splitting cell achieves a record 20% STH efficiency. Detailed analysis of the integrated system reveals that STH efficiencies of 25% can be achieved with realistic improvements in the perovskite cell and an LCOH below ≈$3 kg−1 is feasible.

Transition metal nitrides are a fascinating class of catalyst materials due to their superior catalytic activity, low electrical resistance, good corrosion resistance and earth-abundance; however, their conventional synthesis relies on...

and photovoltaic-electrochemical (PV-EC) [11][12][13] water splitting have reignited interest in the prospect of a sustainable hydrogen economy, with several efficiency records being broken in quick succession. Immersed photoelectrodes and PEC systems have long been a tantalizing goal, simultaneously offering the advantages of both PC and PV-EC systems while avoiding many of their respective drawbacks. First, photoelectrodes are compact, lowering material usage and electrical losses. This also minimizes the distance that photogenerated charge carriers must travel to reach the electrolyte, reducing the ohmic losses associated with carrier collection and subsequent redistribution across the catalytic surface. Second, H 2 and O 2 are evolved at opposite electrodes and can therefore be collected separately. Third, photoelectrodes do not require additional thermal management, as the electrolyte itself can act as a coolant. [14] This becomes particularly useful when operating under concentrated light. Despite recent breakthroughs, the stabilization of highefficiency photoelectrodes remains a key issue; no previously reported immersed system has maintained a solar-to-hydrogen (STH) efficiency of over 10% for longer than 5 days. [9] Instability is often encountered due to the fact that many proven An ideal catalytic interface for photoelectrodes that enables high efficiency and long-term stability remains one of the keys to unlocking high-performance solar water splitting. Here, fully decoupled catalytic interfaces realized using surfacestructured cocatalyst foils are demonstrated, allowing optimized photoabsorbers to be combined with high-performance earth-abundant cocatalysts. Since many earth-abundant cocatalysts are deposited via solution-based methods, deposition on chemical-sensitive photoabsorbers is a significant challenge. By synthesizing cocatalyst foils prior to device fabrication, photoabsorbers are completely isolated from corrosive chemical environments and are provided with outstanding protection during operation. Si and GaAs photoelectrodes prepared using Ni-based cocatalyst foils achieve excellent half-cell efficiencies and generate stable photocurrents for over 5 days. Furthermore, a GaAs artificial leaf achieves a solar-to-hydrogen efficiency of 13.6% and maintains an efficiency of over 10% for longer than nine days, an accomplishment that has not been previously reported for an immersed solar water splitting system. These results, together with theoretical calculations of other photoelectrode systems, demonstrate that cocatalyst foils offer a very attractive method for fabricating high-performance solar water splitting systems.

The design and development of materials at the nanoscale has enabled efficient, cutting‐edge renewable energy storage, and conversion devices such as solar cells, water splitting, fuel cells, batteries, and supercapacitors. In addition to creating new materials, the ability to refine the structure and interface properties holds the key to achieving superior performance and durability of these devices. Atomic layer deposition (ALD) has become an important tool for nanofabrication as it allows the deposition of pin‐hole‐free films with atomic‐level thickness and composition control over high aspect ratio surfaces. ALD is successfully used to fabricate devices for renewable energy storage and conversion, for example, to deposit absorber materials, passivation layers, selective contacts, catalyst films, protection barriers, etc. In this review article, recent advances enabled by ALD in designing materials for high‐performance solar cells, catalytic energy conversion systems, batteries, and fuel cells, are summarized. The critical issues impeding the performance and durability of these devices are introduced and then the role of ALD in addressing them is discussed. Finally, the challenges in the implementation of ALD technique for nanofabrication on industrial scale are highlighted and a perspective on potential solutions is provided.