The development of cost-effective, highly efficient, and durable electrocatalysts has been a paramount pursuit for advancing the hydrogen evolution reaction (HER). Herein, a simplified synthesis protocol was designed to achieve a self-standing electrode, composed of activated carbon paper embedded with Ru single-atom catalysts and Ru nanoclusters (ACP/RuSAC+C) via acid activation, immersion, and high-temperature pyrolysis. Ab initio molecular dynamics (AIMD) calculations are employed to gain a more profound understanding of the impact of acid activation on carbon paper. Furthermore, the coexistence states of the Ru atoms are confirmed via aberration-corrected scanning transmission electron microscopy (AC-STEM), X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS). Experimental measurements and theoretical calculations reveal that introducing a Ru single atom site adjacent to the Ru nanoclusters induces a synergistic effect, tuning the electronic structure and thereby significantly enhancing their catalytic performance. Notably, the ACP/RuSAC+C exhibits a remarkable turnover frequency (TOF) of 18 s-1 and an exceptional mass activity (MA) of 2.2 A mg-1, surpassing the performance of conventional Pt electrodes. The self-standing electrode, featuring harmoniously coexisting Ru states, stands out as a prospective choice for advancing HER catalysts, enhancing energy efficiency, productivity, and selectivity.

The realization of a complete techno-economy through a significant carbon dioxide (CO2) reduction in the atmosphere has been explored in various ways. CO2 reduction reactions (CO2RRs) can be induced using sustainable energy, including electric and solar energy, using systems such as electrochemical (EC) CO2RR and photoelectrochemical (PEC) systems. This study summarizes various fabrication strategies for non-noble metal, copper-based, and metal-organic framework-based catalysts with excellent FE for target carbon compounds, and for noble metals with low overvoltages. Even though EC and PEC systems exhibit high energy-conversion efficiency using excellent catalysts, they are not completely bias-free operations because they require external power. Therefore, photovoltaics, which can overcome the limitations of these systems, have been introduced. The utilization of silicon and perovskite solar cells for photovaltaics-assisted EC (PV-EC) and photovaltaics-assisted PEC (PV-PEC) CO2RR systems are cost efficient, and the III-V semiconductor photoabsorbers achieved high solar-to-carbon efficiency. This review focuses on all the members composed of PV-EC and PV-PEC CO2RR systems and then summarizes the special cell configurations, including the tandem and stacked structures. Moreover, current problems such as a low energy conversion rate, expensive PV, theoretical limitations, and scale-up to industrialization are discussed with the suggested direction.