Over the last decades, hydrogen has been produced mainly for industrial applications through conventional production processes based on fossil fuels. Global warming, increasing energy demand, and energy storage challenge made hydrogen a potential solution for future energy transition scenarios. Therefore, on the one hand, a significant amount of hydrogen should be produced to satisfy the growing market demand. On the other hand, these production methods should meet economic and environmental needs. According to hydrogen production data available from International Energy Agency (IEA) [1], in 2021, less than 1% of the produced hydrogen can be considered low-carbon. Low-carbon hydrogen refers to the hydrogen obtained through technologies with a low climate profile, specifically, low CO₂ equivalent emissions. Figure 1, based on the IEA predictions, shows the current and future status of hydrogen production from different sources. Around 94 Mt of H₂ was produced in 2021, of which more than 80% has been obtained directly from fossil fuels without the application of carbon capture, utilization, and storage (CCUS). Currently, the chemical sector demands the largest share of hydrogen in the market to produce ammonia (36%) and for hydrocracking and desulfurization of fuels in refineries (42%) [2]. Based on a net-zero emissions (NZE) scenario, it is forecasted that in 2030, hydrogen production will reach up to 180 Mt. Production via the application of electricity (34%), and fossil fuels with CCUS (18%) will be responsible for this increase in supply. The corresponding demand for this increase in the production volume comes from new applications (heavy industry, power generation, and synthesis of H₂-based fuels) [1].

Hydrogen can be produced using a wide range of processes, including thermochemical, electrochemical, and biological methods. Regarding the diffusion of these processes in new installations, based on the statistical analysis of the available raw data for hydrogen production projects in different stages (operational, under construction, feasibility study, and concept) from IEA [3], more than 80% of new projects are based on electrolysis and less than 10% are powered with natural gas with CCUS. Achieving a low impact from hydrogen production is particularly important in view of its increasing role in the future energy system. For example, the current EU classification for sustainable activities (EU Taxonomy Regulation 2020/852) requires hydrogen production to remain below 3 kgCO₂ eq./kg H₂ [4] to comply with a criterion of “substantial contribution to climate change mitigation”. Figure 2 shows the number of new hydrogen production projects in Italy and Europe (without Italy). Among the 32 H₂ projects in Italy, 28 of them use electrolyzers. The type of electrolysis system is not disclosed for a considerable number of projects. However, the alkaline electrolyzer has a larger share in Italy for those plants with the known electrolyzer type, followed by proton-exchange membrane (PEM) electrolysis systems. In contrast, in Europe, PEM electrolysis systems are used more frequently.

The electricity powering the electrolyzers to produce H₂ can be supplied from the grid, nuclear, or renewable sources (e.g., wind, photovoltaic, hydro, biomass), usually labeled in the literature as yellow, pink, and green hydrogen. Table 1 introduces the colorful classification of hydrogen production pathways and briefly describes the related process and energy source for each color. The first two lines represent the production processes with high environmental impacts. In contrast, the second group shows the potential of low-impactful production approaches, including the green pathway considered herein.

Photovoltaic (PV) and wind-based power plants are today the main sources of renewable electricity for new installations. This text focuses on the case of PV electricity and its utilization for green hydrogen production. The environmental impacts of producing H₂ via electrolysis systems powered by renewable electricity have been studied using the process-based life-cycle assessment (LCA) method in several publications (attributional modeling). In 2014, Bhandari et al. [5] reviewed some LCA studies on H₂ production via electrolysis and reported that the global warming potential (GWP) for PV-based hydrogen could vary from 2 to 7 kg CO₂ eq./kg H₂. They mention that higher GWP values for solar PV than hydro or wind are caused by the emissions related to PV module manufacturing processes. In 2018, Parkinson et al. [6] re-estimated the life cycle GHG emissions of a solar PV-based electrolysis system around 2.21 kg CO₂ eq. per each kg of hydrogen. Their calculations assumed a generalized overall energy requirement of 51.2 kWh_el/kg H₂ and a 40 g CO₂ eq./kg H₂ for electrolyzer contribution. The related emission of PV systems was estimated based on the review work of Nugent and Sovacool [7].

2. Hydrogen Production

The water electrolysis process to produce hydrogen and oxygen has been known for two centuries, with the advantage of producing extremely pure hydrogen and a relatively straightforward process based on electricity consumption as energy input. Most applications have been limited to small-scale and unique situations in which access to large-scale fossil fuel-based hydrogen production plants—which dominate today’s production, as reported in Figure 1—was not possible or not justified (e.g., electronic industry, food industries as well as medical applications) [11]. Recent developments towards a lower cost of electrolyzers and the perspective availability of low-cost renewable electricity are making their application more attractive. The electrolysis processes can be grouped based on the electrolyte, which may feature different pH (alkaline and acid) or physical state (liquid or solid) [12]. In the following, the 4 most common types of electrolyzers proposed for hydrogen production are shortly explained.

• Alkaline

Among hydrogen production technologies, alkaline water electrolysis is a mature process [13]. Troostwijk developed the first design of an alkaline electrolyzer in 1789 [14], and the technology has followed a long trajectory of research and development, today showing the lowest plant-specific cost (€/kW) with respect to the other types [15]. An aqueous potassium hydroxide (KOH) solution with typical concentrations of 20–40% KOH is used as an electrolyte in alkaline water electrolysis [11]. The cathode and anode electrodes usually are based on low-cost materials (iron or nickel). The mature and low-cost alkaline electrolyzers show higher lifetime and production capacity. However, this electrolysis system is mostly designed for a continuous power supply and limited turndown capacity to avoid damage and safe operation [20]. Corrosive environment conditions, risks of leakage of liquid electrolytes, gas permeability, and low current density, bringing to relatively heavy and high-footprint installations, are other disadvantages of this system.

• Proton-exchange membrane

A newer generation of electrolyzers with respect to alkaline electrolysis systems is known as proton-exchange membrane (PEM) electrolyzers. PEM electrolyzers use a thin solid polymer electrolyte (membrane) instead of a liquid electrolyte. Commercial systems use Nafion^® as the proton-conducting membrane with a typical thickness of 60–200 µm [21]. Nafion is prepared as a copolymer from tetrafluoroethylene (TFE) and fluorinated vinyl ether, e.g., perfluoro (4-methyl-3,6-dioxane-7-octene-1-sulfonyl fluoride) [19]. Compact design, high current density, faster response to load variation, wide turndown capacity, and dynamic operation are obtained by the use of a solid membrane [25]. PEM electrolyzers’ other advantages are the possibility of producing highly compressed and pure hydrogen and high efficiency [26]. On the downside, the technology relies on expensive noble metals, such as platinum and iridium (PGM) [16]. The application of such noble metals in addition to titanium and the proton-exchange membranes makes the PEM electrolyzers a tendentially more expensive option than conventional alkaline electrolysis systems.

• Solid oxide electrolyzer cells

Solid oxide electrolyzer cells (SOEC) operate in temperatures higher than 500 °C, typically in the range of 600–850 °C [22]. They can produce hydrogen with a lower electricity input than alkaline and PEM electrolyzers. At the same time, industrial waste heat can be utilized to provide the thermal energy demand of the high-temperature SOEC electrolysis. A SOEC consists of a dense ionic conducting electrolyte and two porous electrodes. Zirconia with different dopants (typically yttria) is the most used material for electrolytes, which allows the conduction of oxygen ions [19]. SOEC electrolysis systems offer significantly higher efficiency and may achieve a lower total cost of hydrogen production compared to conventional low-temperature electrolysis, thanks to favorable thermodynamics [29]. Additional opportunities are given by the possible simultaneous electrolysis of CO₂ and H₂O for the production of synthesis gas and the possibility of reversible operation as a fuel cell (particularly interesting for energy storage applications) [28].

• Anion-exchange membrane

An anion-exchange membrane (AEM) is an alkaline solid polymeric membrane, which is the core component of an AEM electrolyzer [16]. Applying AEM instead of the conventional diaphragms used in alkaline electrolyzers is the main difference between these technologies [30]. A dilute alkaline electrolyte (KOH) is used on the anode side, while no solution is supplied for the cathode side [16]. One of the advantages of AEM electrolysis is its overall lower cost due to the application of transition metal catalyst and the quaternary ammonium ion-exchange-group-containing membrane instead of more expensive noble metal (PGM) catalysts and the Nafion-based membranes [30]. Also, applying a low-concentration alkaline solution as an electrolyte instead of concentrated KOH (without a corrosive liquid) results in the absence of leaking, volumetric stability, ease of handling, and a reduction in the size and weight of the electrolyzer [30]. Catalyst performance deterioration may occur due to the intermittent nature of solar and wind power supplies in the AEM electrolysis. AEMs are susceptible to damage if frequent shutdowns occur on the electrolyzer. Therefore, the durability and conductivity stability of the AEMs should be reinforced considering the large-scale applications and real-world size and conditions within the desired life duration [32].

3. Electricity Source—Photovoltaic

Utility-scale photovoltaic (PV) plants include modules, mounting systems, inverters, transformers, cables, electrical protection systems, measurement equipment, and system monitoring [33]. The PV modules produce direct current electricity using solar irradiance and then convert it into alternating current in an inverter for further applications. Crystalline silicon PV panels production starts with silica mining, carbothermic/quartz reduction (removing oxygen from silica), metallurgical-grade silicon (MG-Si) purification, solar-grade (SOG) silicon construction, silicon ingot crystallization, wafer slicing, PV module assembly and finishes with module and laminate construction [34]. The silicon ore is reduced to metallurgical-grade silicon with silicon purity of around 99% via the use of carbon in a large arc furnace [35]. Metallurgical-grade silicon is further refined to 99.999% purity using the modified Siemens process, which is more advanced and less energy-consuming than the original Siemens process [36]. To produce high-purity silicon, the modified Siemens process consumes a considerable amount of energy due to the operation of the reaction chamber at a high working temperature (typically between 1100 °C and 1200 °C) [37], while over the last decade, the energy and material efficiency of the Siemens process has improved remarkably [38]. Single-crystalline and multi-crystalline silicon ingots are obtained by crystallization through the Czochralski process and casting of solar-grade silicon, respectively. These ingots are sliced using a multi-wire saw combined with a slurry of cooling liquid and abrasive particles and then treated by subsequent etching with sodium hydroxide (NaOH) and washing with water and sulfuric acid (H₂SO₄) [36]. PV cells are produced via different steps, surface preparation, dopant diffusion, junction formation, and coating, in which some materials like electric pole printing ribbons, nitrogen, oxygen, and argon are used [35]. Finally, solar cells are connected into a string and then encapsulated by two layers of glass and plastics (ethylene-vinyl acetate) to form the PV module [37].

4. Life-Cycle Assessment

The process-based life-cycle assessment (LCA) method is considered one of the most analytical methods to evaluate the environmental profile of products (goods and services), such as hydrogen. Since then, numerous directives, communications, and recommendations of the European Commission have been referred to LCA. The LCA is an internationally recognized method according to the principles defined in ISO 14040 and 14044 [39,40]. Italy is one of the 6 European countries with an annual renewable electricity potential capacity higher than 600 TWh [42]. The outcomes were calculated using SimaPro 9.4 software [43], the ecoinvent 3.5 library cut-off method as the Life Cycle Inventory database [