Shengwei Shi

Dongge Ma

Investigation on internal electric field distribution of organic light-emitting diodes (OLEDs) with Eu 2 O 3 buffer layer

Investigation on internal electric field distribution of organic light-emitting diodes (OLEDs) with Eu2O3 buffer layer

Shengwei Shi, Dongge Ma

To cite this version:

Introduction Since the first demonstration of

OLEDs [1], a large research effort has focused on improving the device performance and understanding the device physics. Introduction of buffer layers at the interface between the electrode and organic active layer is indeed an efficient approach for OLEDs. On the one hand, a buffer layer inserted between the organic active layer and cathode can enhance electron injection and improve device performance [2][3][4][5]. On the other hand, many researches are focus on a buffer layer inserted between the anode and the organic active layer to control hole injection [6][7][8][9][10]. In general, the basic physical processes for OLEDs include injection, transport and recombination of charge carriers. To improve the device performance, a detailed understanding of the role that buffer layers in OLEDs on the charge carrier injection and transport processes is important.

Although LiF/Al is usually used as the cathode in OLEDs, and it is considered an effective way to improve the electroluminescence (EL) efficiency of devices [11], the injection of holes and electrons is still unbalanced [12]. In this letter, we investigate the effects of Eu 2 O 3 as an efficient buffer layer on device performance and the internal electric field distribution in the device, We have found that the introduction of the buffer layer balanced the internal electric field distributions in hole transport layer (HTL) and electron transport layer (ETL), leading to an improvement in the device performance. Our results show that the hole injection into NPB from anode is FN tunnelling and the electron injection into Alq 3 from cathode is RS thermionic emission.

Experiments

We have fabricated OLEDs with the following structure: ITO / Eu 2 O 3 (0, 0.5, 1.0, 1.5 nm) / N,N`-bis(1-naphthyl)-N,N`-diphenyl-1,1` biphenyl -4,4`dimaine (NPB) (50 nm) / tris-(8-hydroxyquinoline) aluminium (Alq 3 ) (60 nm) / LiF (1 nm) / Al(150 nm). To investigate the internal electric field distribution of the device, we also fabricated two types of devices with different thickness of NPB (20, 40, 60, 80 nm) for each one: A: ITO/NPB(x)/Alq 3 (60nm)/LiF(1nm)/Al(150nm); B: ITO/Eu 2 O 3 (1nm)/NPB(x)/Alq 3 (60nm)/LiF(1nm)/Al(150n m). The evaporation base pressure is at a vacuum of 5×10 -4 Pa. The buffer layer Eu 2 O 3 (deposition rate 0.04~0.06 nm/s) was deposited on the cleaned ITO glasses firstly, and the NPB was thermally evaporated (deposition rate 0.2~0.4 nm/s) as a hole transport layer. A 60-nm Alq 3 film was evaporated at a rate 0.2~0.4 nm/s as an electron transporting and light emitting layer. Then an electron injection layer (EIL) of thin LiF was deposited at a rate 0.04~0.06 nm/s without breaking the vacuum. Finally, 150 nm Al electrodes were deposited at a rate of 0.5~0.6 nm/s. The deposition rates were controlled by a quartz oscillating thickness monitor. The luminance-current-voltage characteristics were measured using a Keithley (2400 and 2000) source with a calibrated silicon photodiode. All measurements were performed at ambient atmosphere without encapsulation. The active area of the devices was 9 mm 2 .

Experimental results

The incorporation of a thin layer of Eu 2 O 3 between ITO and NPB layer, and a thin layer of LiF between Al cathode and Alq 3 layer, produced a significant improvement in the device performance. Fig. 1 shows the current density-voltage characteristic of devices with different thicknesses (0, 0.5, 1.0, 1.5 nm) of the Eu 2 O 3 buffer layer, including a reference device without Eu 2 O 3 . Compared with the reference device, the current at a certain voltage was enhanced upong introducing Eu 2 O 3 . For Eu 2 O 3 thicknesses greater than 1.0 nm, the current began to decrease. This means that the optimal Eu 2 O 3 thickness is 1nm.

We present in Fig. 2 the bias voltage dependence of luminance. Upon introducing Eu 2 O 3 , the luminance is also greatly enhanced. Indeed, the luminance for 1.0 nm Eu 2 O 3 is nearly two times that of the reference device, while the turn-on voltage decreased from 5.5 V to 3.5 V. The insert to Fig. 2 shows the relation between current efficiency, power efficiency with current density. Device efficiencies were also improved with the buffer layers. We present in Table 1 device performance as a function of Eu 2 O 3 thickness in the range 0-1.5nm that yields the best performance.

To elucidate the performance improvement of OLEDs with Eu 2 O 3 buffer layer, we investigate the effects of buffer layers on average internal electric field distributions within the ETL and HTL in OLEDs, by studying steadystate J-V characteristics [12,13].

We first processed two types of devices A and B that either contain or do not contain the Eu 2 O 3 buffer layer. For each type of device, we investigated four different thickness of NPB HTL, while keeping the thickness of Alq 3 ETL fixed at 60nm. We present the J-V characteristics of these devices in Fig. 3. For a given current density, the corresponding voltage values may thus be extracted from all J-V curves. The insert to Fig. 3 presents the total voltage drop on the diode plotted against the thickness of the NPB layer for a current density of 0.001 A/cm 2 . Obviously, the voltage drop exhibits a nice linear relation with the NPB layer thickness for every device, which indicates that the space charge effects in both HTL and ETL can be negligible at within certain current range.

Because of the large difference in the hole and electron mobilities in NPB and Alq3 [14,[START_REF] Tsutsui | Proc. SPIE[END_REF], respectively, one expects that the Alq3 layer limits the current, implying that this layer takes on most of the voltage drop [START_REF] Riess | [END_REF]. Indeed, the electroabsorption measurements by Rohlfing et al. have shown that the voltage drop at the Alq3 layer for positive bias is almost ten times larger than that at the NPB layer [17]. Thus, the voltage drop across the entire device occurs mostly across the Alq 3 layer. Although LiF [18] and Eu 2 O 3 [19] are wide band gap dielectrics, so that the voltage drops across them could be significant, this voltage drop is nevertheless negligible compared with the voltage drop across Alq 3 layer. We may therefore approximate that the voltage drop across the devices occurs only across the HTL and ETL layers. In this case, the voltage (V) across the devices can then be written as V = V HTL +V ETL = F HTL L HTL + F ETL L ETL , where F HTL and F ETL represens the average electric field within HTL and ETL layers, respectively. We can then extract the average internal electric field distributions within the HTL and ETL from the slope and intercept of the linear relation at different values of current density [12,13].

Although LiF/Al is usually used as the efficient cathode in OLEDs, hole and electron injection is still unbalanced for the device [12]. For device A, the introduction of the LiF EIL enhances electron injection [1], so the internal electric field of HTL will be raised, although the internal electric field of ETL is reduced, the average internal electric field distributions in HTL and ETL are pulled away [12] (see Fig. 4). Regarding device B containing the Eu 2 O 3 buffer, the internal electric field in the HTL is reduced greatly compared with device A, and the average internal electric field distributions in HTL and ETL become much more similar, which helps balance charge carrier injection and improve carrier recombination efficiency. The low resulting internal electric field in device B will also help to reduce the exciton quenching induced by the large internal electric field.

From Fig. 4, we can extract the dependencies of the total current density on the average electric field in HTL. For devices B, this dependence goes as logJ/F 2 -F -1 av, NPB plot, while the dependence of the total current density on the average internal electric field in ETL goes as log J- F 1/2 av, Alq3 plot (Fig. 5) [12,13]. These dependencies imply that the hole injection into HTL is controlled by Fowler-Nordheim (FN) tunnelling, whereas the electron injection into ETL is controlled by Richardson-Schottky (RS) thermionic emission.

Conclusions

In this work, we have introduced a thin film of Eu 2 O 3 as an efficient hole injection layer into OLED heterostructures, which can greatly improve device electroluminescent performance. The optimal thickness of Eu 2 O 3 was found to be 0.5~1.5 nm. The peak luminance, current efficiency and power efficiency can reach 27994 cd/m 2 , 6.4 cd/A and 3.1 lm/w, and the turn-on voltage can decrease from 5.5 V to 3.5 V compared with a reference device without the Eu 2 O 3 buffer. Our experimental results clearly demonstrate that the hole injection from anode and the electron injection from cathode are FN tunnelling and RS thermionic emission, respectively. The introduction of both interfacial layers Eu 2 O 3 and LiF not only lowers the average internal electric field inside the device, but also results in a balancing between the average internal electric field in the HTL and in ETL over the entire current range, which will help to balance charge carrier injection and improve device efficiency.

Figure 1

 1 Figure 1 Current density-voltage characteristics of ITO/ Eu 2 O 3 (x nm)/NPB(50 nm)/Alq 3 (60 nm)/LiF(1 nm)/Al(150 nm) devices with different Eu 2 O 3 thickness.

Figure 2

 2 Figure 2 Luminance-voltage characteristics of ITO/ Eu 2 O 3 (x nm)/NPB(50 nm)/Alq 3 (60 nm)/LiF(1 nm)/Al(150 nm) devices with different Eu 2 O 3 thickness. Inserts: relations between current density and the current and power efficiencies.

Figure 3

 3 Figure 3The total voltage drop on the diode is plotted against the thickness of NPB layer for the current density of 0.001 A/cm 2 . Their slopes and intercepts are related to the average electric field in NPB and Alq 3 layer, respectively. Insert: current density-voltage characteristics of device A and B with different thicknesses of NPB.

Figure 4

 4 Figure 4 Average internal electric field distributions in NPB and Alq 3 layers for devices A and B.

1 Figure 5

 15 Figure 5 Dependence of the current density on the average internal electric field in NPB and Alq 3 layers for device B.

Table 1

 1 Device performance with varying thickness of Eu 2 O 3 .

	Thickness (nm)	0	0.5	1.0	1.5
	Luminance (cd/m 2 )	14260	21600 27994 22960
	Turn-on voltage (V)	5.5	4.7	3.5	4.9
	Current efficiency, CE, (cd/A) 4.7	5.6	6.4	6.0
	Power efficiency, PE, (lm/w) 1.9	2.7	3.1	2.4
	CE at 500 mA/cm 2	2.8	4.0	4.9	4.3
	PE at 500 mA/cm 2	0.4	0.7	1.0	0.8

Acknowledgements We thank M. Bowen for a careful reading of the manuscript. D. Ma thanks the support of Hundreds Talents Program, Chinese Academy of Sciences and the National Science Fund for Distinguished Young Scholars of China (50325312).


Greg De Temmerman

Thomas Boigontier

Julien Lamarque-Lacoste

Saint Manon De La Motte

Anouck Dubois

NATURAL HYDROGEN COMPARISON OF HYDROGEN PRODUCTION TECHNOLOGIES KEY TAKEAWAYS

Hydrogen (H 2 ) has long been considered a key for the future energy system -the concept of "H 2 key economy" was first coined at the beginning of the 1970s 1 . This is because H 2 can in principle be used for many applications such as fuel, steel production, energy storage etc 2 . This has led to a renewed interest in H 2 in the context of the energy transition 3 . This means that the demand for H 2 will increase in the future, although scenarios differ about the extent of this demand for 2050 4 . This poses a double challenge. First, to comply with climate pledges, the current production of H 2 needs to be decarbonised as quickly as possible. At the same time, the production will need to increase sharply to allow the development of new end-uses technologies, and this additional H 2 will also need to be low-carbon.

Current hydrogen production: a carbon-intensive industry

About 94 Mt of hydrogen (H 2 ) are produced every year in the world, currently used for different purposes dominated by chemicals (70% for methanol and 30% for ammonia production) and refineries. Today's production is dominated by processes based on fossil fuels: steam methane reforming (SMR, using natural gas), and coal gasification (CG). Those two processes are particularly carbonintensive: the production of 1 kg of H 2 emits on average 9 kg CO 2 e/kgH 2 for SMR, and 20 kg CO 2 e for CG [START_REF] Longden | Clean' hydrogen? -Comparing the emissions and costs of fossil fuel versus renewable electricity based hydrogen[END_REF] . Less than 1% of the total H 2 demand is currently met by low-carbon production technologies (mostly fossil fuels with carbon capture) [START_REF]Global Hydrogen Review 2022 -Analysis[END_REF] .

Navigating the variety of H 2 production methods

Hydrogen production methods are usually referred to by colours. 'Green H 2 ' from water electrolysis, one of the most mature processes, in which water is decomposed into H 2 and oxygen using renewable electricity, is considered in most net-zero scenarios. There is, however, a large variety of methods at various stages of development.

This report aims at exploring the different routes and individual technologies in terms of their working principles, CO 2 emissions, current maturity and evolutions perspectives.

THERMOCHEMICAL ROUTE

Thermochemical production of hydrogen (H 2 ) encompasses a very broad range of processes involving chemical reactions that occur when applying a certain amount of heat to an H-containing feedstock.

This route includes the two most common methods for producing H 2 today, namely Steam Methane Reforming (SMR) and Coal Gasification (CG), but also other techniques such as Autothermal Reforming (ATR), Partial Oxidation (PO), Biomass Gasification (BG) and finally Thermochemical Water Splitting (TCWS).

Apart from TCWS, all of these pathways generate CO 2 directly -coupling them with carbon capture utilisation and storage (CCUS) is therefore necessary for such processes to produce H 2 in a low-carbon way (or even carbon-negative, in the case of BG).

Below is a simplified flow diagram that applies for all thermochemical technologies except TCWS. These diverse processes share some fundamental steps. First, the fuel is pretreated (depending on its type: de-sulfuration, pre-heating…) and fed into the reformer or gasifier. The heat necessary for oxidation and gasification is provided either by the fuel itself or by an external heat source in the case of reforming. These reactions produce a syngas composed of CO, H 2 , CH 4 and CO 2 which then enters a water gas shift unit (WGS) where it reacts with water at lower temperatures to generate more H 2 and CO 2 . The mixture is then flowed into the pressure swing adsorption unit (PSA) where calcium oxide (CaO) is combined with CO 2 to form calcium carbonate (CaCO 3 ). A high purity, high-pressure flow of H 2 is obtained, along with another low-pressure stream after the desorption phase, called "tail gas", containing the impurities and some of the H 2 . In the case of SMR, the tail gas is then fed to the reformer and burnt with natural gas to provide heat for the reforming process. Eventually, burned gasses are vented into the atmosphere at the end of the reformer (flue gas) [START_REF]Hydrogen production with CO2 capture[END_REF] .

Steam Methane Reforming (SMR)

Steam methane reforming is a catalytic reaction where methane (CH 4 ) reacts with high temperature steam (700-900°C) at pressures of 3-35 bar to generate H 2 and syngas (CO). The reaction is endothermic and thus requires the input of external heat provided by steam and/or the burning of some of the natural gas along with the tail gas.

A. The war in Ukraine that has resulted in rapidly rising cost for natural gas in Europe (from <30€/MWh before May 2021 to as high as 343€/MWh in August 2022). But prices are gradually coming down (56€/MWh on January 25th 2023) and prices in the US and in Russia are still low, respectively USD$10/MWh on January 25th 2023 and USD$9/MWh for businesses in June 2022, so these two countries can still produce hydrogen at a low cost.

B. Global warming potential (GWP) is defined as the cumulative radiative forcing, both direct and indirect effects, over a specified time horizon resulting from the emission of a unit mass of gas related to some reference gas (CO 2 ).

Historically, SMR has imposed itself as the most attractive way to produce H 2 , due to a number of key factors:

• SMR has long benefited from the development of the natural gas industry, with cheap prices and widespread infrastructure, making it a very cost-competitive method -despite recent fluctuations in Europe due to the war in Ukraine a ;

• SMR is very efficient (57-75%) 10 , which contributed to a low Levelized Cost of Hydrogen (LCOH) and rapid market adoption;

• Hydrogen yield is important since the molar ratio of H 2 with SMR is 4:1, meaning that for every mole of methane, 4 moles of H 2 are produced 59 ;

• SMR can produce H 2 continuously, making it suitable for industrial applications such as ammonia production or fossil fuel refining.

However, SMR is very carbon intensive. The process itself emits ~9 kg CO 2 e/kg H 2 and ~10.5 kg CO 2 e/kg H 2 when taking supply chain emissions into account (100 years GWP b , standard IPCC natural gas leakage rate of 1.7%) [START_REF] Longden | Clean' hydrogen? -Comparing the emissions and costs of fossil fuel versus renewable electricity based hydrogen[END_REF] . Hence, carbon capture technologies are particularly adapted to these fossil fuels technologies.

Air Liquide's Cryocap CO2 -capture technology is in use at its hydrogen plant in France -Credit Air Liquide

Partial Oxidation (PO)

Partial oxidation (PO) is another commercial pathway to produce H 2 from natural gas. The difference with SMR is that the oxidation process is done using oxygen instead of steam. Oxygen is supplied in proportions lower than the stoichiometric ratio, inducing an incomplete oxidation of methane which as a result gives a CO and H 2 mixture [START_REF]Review and comparison of various hydrogen production methods based on costs and life cycle impact assessment indicators[END_REF] . Compared to SMR, PO does not require external heat supply since the reaction is exothermic, thus reducing energy consumption. PO reaction does not necessarily require catalysts contrary to SMR thus decreasing the cost of the process. It also has a better tolerance to impurities towards sulphur generally present in fossil fuels and the efficiency of the process generally reaches 55-75% [START_REF]Review and comparison of various hydrogen production methods based on costs and life cycle impact assessment indicators[END_REF] . However, the current H 2 yield of PO is not comparable to that of SMR making it difficult to compete with SMR (SMR produces 4 moles of H 2 for every mole of CH 4 while PO produces only 2 moles) [START_REF] Goodman | Low-Temperature Methane Partial Oxidation to Syngas with Modular Nanocrystal Catalysts[END_REF] .

Autothermal Reforming (ATR)

Autothermal reforming (ATR) combines the advantages of SMR and PO processes by connecting the devices of the two processes in series: the heat produced by the partial oxidation step is used to power the reforming process. Given the high thermal efficiency of this process, it requires less energy than SMR or PO individually [START_REF] Megía | Hydrogen Production Technologies: From Fossil Fuels toward Renewable Sources. A Mini Review[END_REF] . However, ATR is characterized by a higher H 2 yield than PO but lower than SMR. The efficiency of ATR is generally 60-75% [START_REF]Review and comparison of various hydrogen production methods based on costs and life cycle impact assessment indicators[END_REF] .

Biomass gasification is one of the most efficient processes of biomass conversion to H 2 (40-60%) 10 . It can convert a variety of biomass sources (sometimes with a high moisture content) into H 2 with a relatively high yield depending on the feedstock (0.068 to 0.080 kg H 2 /kg-feedstock) [START_REF] Lee | The Potential Role of Biohydrogen in Creating a Net-Zero World[END_REF] . However, it still needs to solve H 2 separation issues as well as catalyst corrosion and tar formation issues before becoming a competitive production route.

Coal gasification is generally less efficient (50-80%) and more expensive than SMR 10 . This is why this method is mostly used in countries with abundant coal reserves like China. This process is also the most carbon intensive one, producing as much as 20.4 kg CO 2 e/kg H 2 (brown coal), double the emissions of SMR [START_REF] Longden | Clean' hydrogen? -Comparing the emissions and costs of fossil fuel versus renewable electricity based hydrogen[END_REF] .

Coal Gasification (CG)

Coal and biomass gasification are very similar in the process and only differ on the pre-treatment step. Depending on the type of biomass, it is first washed, dried, sieved, leached or thermally pre-treated while coal is only dried and grounded before being fed into a gasifier, where they react with oxygen and steam under high temperature conditions (usually >900°C), producing a gas mixture containing H 2 , CO and CO 2 . This reaction is also endothermic-heat being often provided by the (partial) combustion of coal. The H 2 content of the gas mixture is then increased using a water gas shift process (WGS), converting the remaining CO into CO 2 as for SMR. Finally, H 2 is separated from the CO 2 via the pressure swing adsorption unit (PSA)

Carbon capture rates

Usually, carbon capture rates as high as 90% can be achieved for CG, PO and ATR with one unit positioned either after the WGS or after the PSA. Other studies even mention a maximum of 99.9% 10 and some ATR projects have reported 95% capture rates [START_REF] Institute | Carbon intensity of blue hydrogen production[END_REF] .

The current practice for SMR is also to install the unit after the WGS or the PSA as the stream has a high concentration of CO 2 , making it easier and cheaper to capture. By doing so, 56% of the total CO 2 of the whole process can be captured.

The option that has the biggest potential for carbon capture is after the reformer, where 90% capture rates can usually be achieved.

However, as the stream is less concentrated and contains more pollutants and impurities, this option is the most expensive, potentially doubling the cost of the other two options, which is significant because CCUS already represents a significant portion of the cost of low-carbon H 2 production 10 18 .

CARBON CAPTURE, UTILISATION AND STORAGE (CCUS) FOR THERMOCHEMICAL PROCESSES

Carbon capture, utilisation and storage (CCUS) can help mitigate the emissions of H2 production routes described above and lead to H 2 certified as low-carbon 16 .

Considering that most of the current production is from fossil fuel, implementing CCUS can make use of existing infrastructure of natural gas in Europe and "only" require installing CCUS units on existing facilities, with minimal changes to consumption patterns.

Production routes with carbon capture

In the case of CG, PO and ATR, the fuel is oxidized inside the reactor that provides the heat [START_REF]Hydrogen production with CO2 capture[END_REF] . As for SMR, the reactor is externally heated by a furnace burning fossil fuels (usually natural gas) and thus produces a flue gas stream containing CO 2 .

Hence, about 60% of the total CO 2 generated by SMR is contained in the syngas or tail gas and the other 40% is produced during the combustion process in the furnace. A carbon capture unit can then be positioned after the reformer to prevent the remaining 40% from entering the atmosphere.

A general diagram for fossil fuel H 2 production + CCUS is given below:

The emissions of the SMR process itself is 74 kg CO 2 e/GJ or ~9 kg CO 2 e/kg H 2 and range between 9.6 and 12.1 kg CO 2 e/kg H 2 for the whole value chain depending on the level of fugitive emissions [START_REF] Longden | Clean' hydrogen? -Comparing the emissions and costs of fossil fuel versus renewable electricity based hydrogen[END_REF] . The warming effect from these emissions is even higher when considering a 20 years period, as CH 4 is a potent greenhouse gas whose half-life in the atmosphere is about 9 years (while that of CO 2 is 100 years). Fugitive emissions are also higher, thus reducing the impact of CCUS on the process total emissions. [START_REF] Longden | Clean' hydrogen? -Comparing the emissions and costs of fossil fuel versus renewable electricity based hydrogen[END_REF] . The global warming effect of the emissions was studied on a 100 years and a 20 years period. The lower bound was calculated using the IPCC low value of 0.9%, the middle value used the IPCC default level of 1.70% and the upper bound used the 3.50% conservative value found in Howarth & Jacobson (2021) paper 20 21 .

CCUS is also effective for production based on coal gasification, but the emissions are still high compared to SMR with CCUS or other low-carbon production methods, respectively 7.8 and 5. .

The copper-chlorine cycle is also very promising, as it operates at lower temperatures than the sulphur-iodine cycle, allowing it to be integrated with a wider range of heat sources.

A simplified flow diagram of a sulphur-iodine thermochemical cycle is given below: High efficiencies of 20-45% for thermochemical water splitting have been reached under lab conditions and the advantage of this thermochemical technology is that it doesn't produce any carbon dioxide [START_REF]Comparative review of hydrogen production technologies for nuclear hybrid energy systems[END_REF] .

Thermochemical Water Splitting (TCWS)

Thermochemical water splitting is a process that relies on high temperature heat to drive chemical reaction cycles which as an output produce H 2 and oxygen. The minimum temperature to generate H 2 from water is 1700°C for partial dissociation and 4000°C for total dissociation, which is hard to reach for most heat sources, hence the use of cycles to reduce the temperature needed for the reaction [START_REF]Comparative review of hydrogen production technologies for nuclear hybrid energy systems[END_REF] .

Between 2000 and 3000 thermochemical cycles have been studied in the literature, of which only about 10% are available for large-scale H 2 production.

Thermochemical Routes: maturity and projects Mature, yet currently a very limited share of H 2 production.

PO & ATR + CCUS 5

Because of their design, only high capture rates are foreseen. Some projects with 95% capture rates have been announced in the EU.

TCWS 6

Large prototypes are being built (50 kW demonstrator for kerosene production using CO2 and water currently under construction) [START_REF] Zoller | A solar tower fuel plant for the thermochemical production of kerosene from H2O and CO2[END_REF] .

Announced projects

ELECTROLYTIC ROUTE

Water electrolysis for hydrogen (H 2 ) production was discovered as early as 1789 by van Troostwijk and Deiman and electrolysers began producing H 2 for the industry by the end of the 19th century 29 30 . The principle is very simple: a direct electric current decomposes water into H 2 (forming at the anode) and oxygen (at the cathode).

There are currently four main electrolyser technologies based on the type of electrolyte used (charge carriers): Alkaline (ALK), Proton Exchange Membrane (PEM), Solid Oxide (SOEC) and Anion Exchange Membrane (AEM).

Alkaline Electrolysis (ALK)

In ALK, two electrodes are immersed in a concentrated alkaline solution, typically KOH or NaOH. Negatively charged hydroxide ions (OH -) act as charge carriers and cross an anion-selective porous diaphragm, transfer from the cathode to the anode in solution, and then generate water with the H 2 ions produced at the anode.

Oxygen is thus produced at the cathode and H 2 at the anode, allowing for easy separation of the gasses. The porous diaphragm assumes a very important role, separating the H 2 and oxygen produced. It must be durable and highly wettable in alkaline electrolytes, otherwise gas bubbles would be produced and increase resistance, affecting the purity of gas products. ALK efficiency is usually 58% to 65% [START_REF]Cost-competitive green hydrogen: how to lower the cost of electrolysers? Oxford Institute for Energy Studies[END_REF] .

Figure 6 : Simplified representation of an ALK electrolyser

Thanks to continuous improvements, it is currently the cheapest option (CAPEX 800 -1500 USD/kW) and offers the longest lifetime (~100,000h) [START_REF]Cost-competitive green hydrogen: how to lower the cost of electrolysers? Oxford Institute for Energy Studies[END_REF] . However, it generally has a low operating pressure (<10 bars) [START_REF]Review and comparison of various hydrogen production methods based on costs and life cycle impact assessment indicators[END_REF] which necessitates an additional compression step for transport and/or storage. Compressing H 2 from 1 to 30 bar results in a 3.5%-4% increase of the energy consumption of H 2 production based on H 2 LHV C . Increasing the operating pressure to 30 bar is also technically possible and would raise its price by ~$USD 50/kW 31 , which is quite low compared to the total price of the electrolyser.

C Lower Heating Value (LHV): The LHV of a fuel is defined as the amount of heat released by combusting a specified quantity (initially at 25 °C or another reference state) and returning the temperature of the combustion products to 150 °C. The LHV assumes that the latent heat of vaporization of water in the fuel and the reaction products is not recovered. By contrast, the higher heating value (HHV) includes the heat of condensation of water in the combustion products 34

Proton Exchange Membrane Electrolysis (PEM)

The chemical reaction is the same as ALK but the technology differs in the electrolyte. PEM efficiency is usually 59 to 68% [START_REF]Cost-competitive green hydrogen: how to lower the cost of electrolysers? Oxford Institute for Energy Studies[END_REF] . It uses a solid acidic electrolyte membrane where the charge carriers are H 2 ions (protons) instead of hydroxide ions (anions). The advantages of PEM are its higher flexibility, higher operating pressure (lower need for compression), smaller footprint (relevant for coupling with offshore wind), higher current densities, faster response and lower degradation rate with load changes, making it particularly suitable for renewable energy integration (see Table 2). However, PEM uses noble metal catalysts like palladium and iridium (which increases the total cost of the system), has a shorter lifetime than ALK (50,000 -90,000 h) because of the harsh oxidative environment, high voltages, and oxygen evolution at the anode [START_REF] Shiva Kumar | Hydrogen production by PEM water electrolysis -A review[END_REF] . State-of-the-art PEM electrolyser efficiencies are currently lower than ALK electrolysers but there is great potential for improvement and cost reduction, as small improvements around the cell design or in reducing the membrane thickness would greatly increase the current density [START_REF]Green hydrogen cost reduction[END_REF] .

Anion Exchange Membrane Electrolysis (AEM)

AEM uses a low concentration alkaline membrane, where OH -ions serve as charge carriers, which allows to avoid the high concentration corrosive electrolytes used in ALK (see Table 2). Also, contrary to PEM electrolysers, AEM does not use noble metal catalysts but rather cheap and abundant transition metals, reducing the cost. It shares the high flexibility, compact cell design, high operating pressure and a leakage-free design with PEM and the low cost and high durability with ALK. But it also suffers from membrane degradation, low current densities compared to PEM (200-1000 mA/cm -2 ) and excessive catalyst loading (amount of catalyst per unit amount of reactant).

The IRENA reported LHV efficiencies of 48%-58% for AEM in 2020 [START_REF]Green hydrogen cost reduction[END_REF] . Hence, some improvements around the efficiency, membrane design and catalyst stability, ease of handling as well as reduction of cell cost are still required for AEM to become a competitive technology in the future [START_REF]Low cost hydrogen production by anion exchange membrane electrolysis: A review[END_REF] .

MultiPHLY project in Rotterdam, Netherlands. Multi megawatt SOEC electrolyser for the production of green hydrogen -Courtesy of Sunfire.

Solid Oxide Electrolysis (SOEC)

Figure 8 : Simplified representation of a SOEC electrolyser SOEC is quite different in its design because it doesn't use a liquid electrolyte. Instead, it uses high temperature steam water (650-1000°C) as an electrolyte and a ceramic solid oxide membrane to make the bridge between the cathode and the anode.

O 2 -ions and H 2 are formed at the cathode and transported towards the anode to complete the electrolysis process. Using high temperature steam, generated by industrial waste heat for example, greatly decreases the power consumption needed for the reaction. SOEC is thus the most efficient electrolysis process, with an efficiency of 68% to 77% [START_REF]Cost-competitive green hydrogen: how to lower the cost of electrolysers? Oxford Institute for Energy Studies[END_REF] . Genvia plans to deliver its first commercial units in 2027. Elcogen, in Estonia, is a manufacturer of SOEC that has also developed a reversible cell that can be used as a fuel cell or an electrolyser depending on the needs. It received €24 Million of investment from HydrogenOne in mid-2022 to scale-up their production line and to continue the development of its technology [START_REF]Elcogen receives €24 million investment from HydrogenOne. Elcogen | Solid Oxide Cells and Stacks[END_REF] .

The AEM concept was only introduced at the beginning of the 21 st century [START_REF]Review and comparison of various hydrogen production methods based on costs and life cycle impact assessment indicators[END_REF] , and this technology is still at the lab stage (TRL 6). In Europe, the German company Enapter is developing solutions based on the AEM technology, betting on the modular aspect of electrolysers to progressively scale-up the size of the H 2 production plants by stacking modules 46 . In July 2022, Enapter raised $USD 53 million to support the construction of its electrolyser factory, with the goal to produce 10,000 AEM electrolysers per month [START_REF]Enapter nets EUR 53m to build electrolysers factory[END_REF] . In France, Gen-hy is working on a new high efficiency H 2 production system that also uses AEM technology, reaching record efficiencies comparable to those of SOEC electrolysers (72% LHV efficiency) [START_REF] Hydrogen | Gen-Hy • Hydrogen Solutions[END_REF] . It will also be the first company to fully control the value chain of their electrolysers [START_REF]Gen-Hy va industrialiser ses électrolyseurs à hydrogène dans le Doubs. Les Echos[END_REF] . Gen-hy announced in May 2022 it invested 15 M€ for a new factory with 3 production lines that will come in operation mid-2023.

ALK & PEM ELECTROLYTIC ROUTE: MATURITY AND PROJECTS

TRL 9 ALK electrolysis is the oldest and most mature production technology [START_REF]Cost-competitive green hydrogen: how to lower the cost of electrolysers? Oxford Institute for Energy Studies[END_REF] (TRL 9 [START_REF]ETP Clean Energy Technology Guide -Data Tools[END_REF] ) and also the most used today (59% of market shares in 2021 [START_REF]Electrolyzer Market Size, Trends, Growth[END_REF] ). PEM electrolysis is also a mature technology (TRL 9 [START_REF]ETP Clean Energy Technology Guide -Data Tools[END_REF] ) which was developed during the 60's 33 to replace ALK. It has for the moment a lower market share (27% of market shares in 2021 [START_REF]Electrolyzer Market Size, Trends, Growth[END_REF] ) although it is expected to grow very fast in the next decade. NEL (Norway) is the leading ALK and PEM electrolyser manufacturer, with more than 3,500 units installed around the globe (2023) [START_REF] Hydrogen | Unlocking the potential of renewables[END_REF] . Thyssenkrupp (Germany), is also one of the biggest suppliers of electrolysers, with over 10 GW of installed power in 2019 [START_REF]Home -thyssenkruppp Uhde[END_REF] .

Concerning the biggest operating electrolysers in Europe, the 20 MW alkaline electrolyser of the HySynergy project in Denmark began producing H 2 in December 2022 and Iberdrola also started up its 20 MW PEM electrolyser in Spain in May 2022 [START_REF]ETP Clean Energy Technology Guide -Data Tools[END_REF] . Gigawatt scale projects have also been announced for the mid-2020s like the Green Marlin project in Bantry Bay, southwest Ireland, planned for 2026 with its 3.2GW electrolyser powered by a 4GW offshore wind farm, or the 1GW electrolyser project planned for 2024 in Esbjerg, Denmark 40 41 . From a pure stoichiometric perspective, producing 1 kg of H 2 requires 9 kg of water. Some electrolyser technologies like PEM require high purity demineralised water and along with the system inefficiencies, the water consumption can reach between 18 kg and 24 kg of water per kg of H 2

ELECTROLYTIC ROUTE: GENERAL CONSIDERATIONS

Shares of H 2 production

50

. Using electrolysis to produce the 94 Mt of H 2 produced in 2021 would consume between 1.7 and 2.26 billion cubic meters of water. For comparison, this represents less than 0.06% of the global freshwater use (2014 figures) [START_REF] Ritchie | Water Use and Stress[END_REF] . Hence the water consumption may be a local issue, depending on where electrolysers are installed, but is not a resource problem. In regions where freshwater is scarce, using seawater could provide an interesting alternative, as it represents 96.5% of the Earth water resources 52 .

9-24 kgH 2 O/kgH 2

Water requirements

This normally require desalinating it first but some new technologies allow for directly feeding it to the electrolyser. The former is not a technical challenge as many desalination plants are already operating around the world and would only induce a marginal increase of the overall energy consumption (+0.06-0.13%) and of the cost of H 2 produced (+$0.01/kg H 2 ) [START_REF] Beswick | Does the Green Hydrogen Economy Have a Water Problem?[END_REF] . The latter is much less attractive as it less developed (TRL 3) and show multiple issues such as catalyst degradation, low current densities, lower efficiency [START_REF] Khan | Seawater electrolysis for hydrogen production: a solution looking for a problem?[END_REF]

Key characteristics

As for SMR, methane is supplied in the form of natural gas or biogas but pyrolysis requires 2 times more natural gas than SMR to produce 1 kg of H 2 , due to the molar ratio of H 2 to CH4 being 2:1 instead of 4:1 for SMR. Indeed, in SMR, the steam provides another source of H 2 atoms contrary to methane pyrolysis (MP). The yield is even lower with biomass due to its low H 2 to carbon ratio compared to methane (0.04 kg H 2 /kg-biomass as opposed to 0.3 kg H 2 /kg CH 4 ) [START_REF]ETP Clean Energy Technology Guide -Data Tools[END_REF] . However, the upside is that pyrolysis does not produce CO 2 but only solid carbon that can be used for diverse applications depending on its structure. Consequently, utilizing the carbon by-product will be crucial for economic feasibility as well as environmental performance of the process.

PYROLYTIC ROUTE

Pyrolysis is the process of chemical decomposition of an organic compound that happens when elevating its temperature higher than 200°C in the absence of oxygen, which as a result produces other gasses and compounds depending on the reaction conditions.

Producing hydrogen (H 2 ) through the pyrolytic route relies on the pyrolysis of methane or biomass, also referred to as methane/biomass decomposition, producing gaseous H 2 and solid elemental carbon. This reaction is highly endothermic, thus requiring external heat supply.

Monolith's Olive Creek 1 (OC1) commercial-scale facility -Courstesy of Monolith Corp The other upside of methane pyrolysis compared to SMR is that it is thermodynamically less energy intensive: 37.8 kJ/mol H 2 compared to 63 kJ/ mol H 2 respectively. Compared to water splitting, it is even 8 times less energy intensive, although the feedstock is different.

There are 3 main methane pyrolysis technologies at different development stages. They differ from each other by the way methane is decomposed in the reactor and how the energy needed for the decomposition is provided [START_REF]ETP Clean Energy Technology Guide -Data Tools[END_REF] .

Some companies like Monolith even built their business model on the production of carbon black, a very pure and valuable form of carbon used for diverse applications such as rubber reinforcement, paints & coatings, plastics, battery electrodes, inks & toners and soil enhancers [START_REF] Gautier | Direct decarbonization of methane by thermal plasma for the production of hydrogen and high value-added carbon black[END_REF] . The carbon black market was estimated at $18.6Bn in 2020 and is expected to continue to grow because of its increasing use in tires [START_REF]Carbon Black Market[END_REF] . But considering the expected consumption of H 2 in the future, one should also be careful not to saturate the carbon black market. The worldwide annual carbon black consumption in 2025 is forecasted to grow beyond 15 Mt/a. In comparison, meeting the current annual demand of pure H 2 of around 70 Mt 56 by methane pyrolysis would lead to approximately 210 Mt/a of carbon by-product and, thus, exceed the estimated market demand for carbon black by a factor of 14 [START_REF] Hermesmann | Turquoise, Blue, or Grey? Environmentally friendly Hydrogen Production in Transforming Energy Systems[END_REF] . This should not happen as some technologies are currently being developed with the goal of producing other carbon compounds like graphene, which have many other applications and whose demand is expected to grow particularly because of its increasing use in batteries [START_REF] Levidian | Powered by hydrogen, built on graphene[END_REF] .

Plasma Decomposition

In thermal-plasma pyrolysis processes, the energy demand of the pyrolysis is supplied by electricity. The electric energy ignites the plasma (an ionised gas), which reaches temperatures in the range of 1,000-2,000°C and splits CH 4 into its elements. Two types of plasma are applied for methane decomposition 63 :

1) In a thermal plasma or a "hot" plasma the temperature is homogenously distributed. The chemical decomposition process of methane is observed within a high-temperature environment allowing it to reach chemical equilibrium.

2) In a non-thermal or "cold" plasma, the electrons show a much higher temperature than the heavier species such as neutrals and electrons. The electrons can have temperatures of several 10,000°K whereas the gas is much colder (e.g. room temperature).

Based on these two plasma types, plasma applications for methane decomposition are classified into cold (less than 1,000 K) and hot (>1,000 K) processes. Cold plasma processes typically show lower conversion efficiencies compared to hot plasma processes. However, cold plasma processes show a higher selectivity.

Possible by-products of the methane decomposition reaction include ethyne, benzene, and ethane. Most plasma systems operate without a catalyst.

Thermal & Catalytic Decomposition

In thermal pyrolysis, methane breaks down into H 2 and carbon in reactors that reach temperatures of 1,000-1,500°C. Catalytic decomposition is often preferred because it reduces the operating temperature and thus the energy required for the process.

In catalytic pyrolysis, methane breaks down into H 2 and carbon over a metal catalyst, which is typically nickel-or iron-based, at temperatures typically under 1,000°C [START_REF]Methane Pyrolysis for Zero-Emission Hydrogen Production: A Potential Bridge Technology from Fossil Fuels to a Renewable and Sustainable Hydrogen Economy[END_REF] , making it more suitable for coupling with renewable energy systems like concentrated solar.

Biomass pyrolysis is usually performed at temperatures between 200-600°C in presence of a catalyst. The energy efficiency of this process is usually 56%-64% [START_REF] Lee | The Potential Role of Biohydrogen in Creating a Net-Zero World[END_REF] . Contrary to methane pyrolysis, biomass pyrolysis does not produce pure solid carbon but rather solid biochar, liquid bio-oil and other non-condensable gas products along H 2 . The type of biomass as well as operational conditions (heating rate, solid residence time) have a great influence on the quantity and the type of products. A fast pyrolysis (high temperature, short residence time) targets liquid fuel as the output, whereas slow pyrolysis (low temperature, long residence time) targets solid biochar. Biomass pyrolysis, however, faces several issues such as tar formation (10-35%), high corrosiveness, low heat stability and low H 2 concentration in the gaseous products [START_REF] Lee | The Potential Role of Biohydrogen in Creating a Net-Zero World[END_REF] .

PYROLYTIC ROUTE: MATURITY AND PROJECTS Thermal decomposition

Molten Industries, a startup based in Oakland CA, plans to produce H 2 by heating up methane using renewable electricity and resistive heating [START_REF] Timmerberg | Hydrogen and hydrogen-derived fuels through methane decomposition of natural gas -GHG emissions and costs[END_REF] . It is currently developing a new type of reactor that fluidizes the carbon produced so that it does not clog during operation one of the major issues with methane decomposition. TRL

3-4

Catalytic decomposition

Startups like C-Zero in Santa Barbara (CA) and Hycamite in Finland are developing H 2 production processes based on methane catalytic decomposition 65 66 . C-Zero recently raised $USD 34 million to build a first pilot plant capable of producing 400 kg of H 2 per day, expecting to come online as soon as Q1 2023 [START_REF]C-Zero Closes $34 Million Financing Round Led by SK Gas to Build Natural Gas Decarbonization Pilot[END_REF] . Hycamite also secured a $USD 3 million investment round to build an industrial pilot plant next to their small test facility in Kokkola Industrial Park (KIP), although the operation date has not been disclosed [START_REF]More investors for hydrogen producer Hycamite[END_REF] . TRL 6

Biomass pyrolysis

SoCalGas and Kore Infrastructure have been testing and demonstrating their carbon-negative, waste-to-energy modular system based on biomass waste pyrolysis technology in Los Angeles (CA) since July 2022 [START_REF]SoCalGas Announces the Commissioning of Carbon-Negative Waste-to-Energy Technology at Los Angeles Facility | SoCalGas Newsroom[END_REF] . The facility can process 24 t/day of organic feedstock and produce up to 1 t H 2 /day dedicated to fuel cell electric vehicles.

TRL 6

Plasma decomposition

Plasma decomposition is the only methane pyrolysis technology that has reached commercial scale (TRL 8), others still being at the stage of development with some demonstration plants on the way.

Monolith produces 600 kg/hr of H 2 using its plasma decomposition technology at their Olive Creek 1 facility since 2020 and has recently received a conditional approval from the US Department of Energy for a $USD 1 billion guaranteed loan to expand the plant which will make Olive Creek the US's largest carbon black production facility with a production capacity of around 194,000 tonnes/year 70 . Although Monolith's economic model is for the moment only based on carbon black, it shows that producing H 2 at a commercial scale is already possible.

Other plasma decomposition technologies are less mature. In Europe, many startups are working on this technology, such as Sakowin (microwave plasma technology, TRL 3), Spark Cleantech (cold nanopulsed plasma, TRL 4), Hiiroc (hot plasma, TRL 7) or Levidian (microwave plasma, TRL 8), although they have not reached commercial scale production yet 71 72 60 . They also focus on the technology as a product rather than H 2 or carbon as a product like Monolith does.

Photo-Electrochemical Catalysis (PEC)

Photo-electrochemical cells could be considered as the combination of an electrolyser and a PV module in a single device, thus having the potential to reduce capital costs and energy losses (direct photo-chemical conversion). In PEC systems, semiconductors are immersed in a water-based electrolyte, where sunlight drives the process of splitting H 2 O into H 2 and O 2 -oxidation occurs at the photoanode and reduction at the photocathode, similar to alkaline electrolysis.

Figure 12 : Simplified representation of a PEC unit 76

There are two main photolytic processes: particulate photocatalysis (PC) and photo-electrochemical catalysis (PEC) [START_REF] Frowijn | Analysis of photon-driven solar-to-hydrogen production methods in the Netherlands[END_REF] . Both use photocatalysts, a substance that absorbs photons and generates excited states, which then cause photophysical and photochemical processes as they return to their original ground states [START_REF]Photocatalytic Water Splitting: Quantitative Approaches toward Photocatalyst by Design[END_REF] . They are most often coupled with electrocatalysts that induce the redox reactions. In essence, the same steps occur for both methods, but for PC all processes take place in single particles in an electrolytic solution, while the different components a separated for PEC. Photolytic methods generate no CO 2 and could hence play a role in the long term as a low-carbon H 2 source, but only if their efficiency and the durability of the catalytic materials improves rapidly.

Particulate Photocatalysis (PC)

In this process, the photocatalyst is in the form of a powder dispersed in an aqueous solution. These particles are composed of a photocatalyst, an oxidation electrocatalyst, and a reduction electrocatalyst. The charge carriers thus have a shorter pathway compared to PEC or electrolysis, reducing the power losses of the process. As photocatalysts produce H 2 and O 2 at the same place, a gas separator is needed.

Light radiation

Although PC is the simplest solar to H 2 process, main challenges need to be overcome. It proves very difficult to find light absorbers with a proper band gap, band edge potentials, and to maintain their stability in the aqueous medium. In addition, it is difficult to control the charge carriers since there is no applied bias. Hence, the solar to H 2 efficiency of these kind of systems has only reached maximum values of around ~5% under lab conditions [START_REF] Lakhera | A review on particulate photocatalytic hydrogen production system: Progress made in achieving high energy conversion efficiency and key challenges ahead[END_REF] .

Current PEC efficiencies range between 14 and 40% but systems quickly increase in complexity along with increased efficiency, where current electrolysers are already twice as efficient from 58% to 77% (if the efficiency of electricity production is not taken into account) 75 29 . However, it has the advantage of operating at near ambient temperatures, between 25 and 65°C, and does not need any noble metal catalysts. The real challenge with these systems lies in the scale up part, as bigger modules can have more spatial defects, require different fabrication methods and also increase total ohmic resistance thus greatly reducing the overall efficiency.

Photolytic routes: maturity and projects

Solar fuel production was rated at a low TRL of 1-3 in 2018 [START_REF] Cheng | How Green Are the National Hydrogen Strategies?[END_REF] , but this field is evolving very rapidly as the technology was reported to be at TRL 5 in 2021 as multiple prototypes have been built since then [START_REF] Frowijn | Analysis of photon-driven solar-to-hydrogen production methods in the Netherlands[END_REF] . Recently, a PEC pilot plant was built by Sunrgyze in 2021 [START_REF]Sunrgyze -Fotoelectrocatálisis[END_REF] , a company owned by Repsol and Enagas, and has been operating for more than 4,000 hours in real conditions reaching >20% solar-to-H 2 (STH) efficiency [START_REF] Delgado | A. Waste to Sustainable Biohydrogen Production Via Photo-Fermentation and Biophotolysis -A Systematic Review[END_REF] . This technology can hence now be considered at TRL 6. Sunrgyze is also planning to build a first pre-demonstration plant funded by the European Innovation fund which will be operational in 2025, with the goal to produce 100 kg of H 2 per day (efficiency >13%).

BIOLOGICAL ROUTE

Biological hydrogen (H 2 ) production uses microorganisms that contain enzymes able to synthesize H 2 through biochemical reactions.

Biophotolytic Processes

In biophotolysis, microorganisms use light radiation to decompose water and produce H 2 and oxygen. It generates no CO 2 and consumes only water to produce H 2.

Direct biophotolysis relies on photosynthesis: microorganisms containing chlorophyll (green algae, cyanobacteria) are placed under sufficient light intensity, producing oxygen with a tenfold solar conversion efficiency compared to plants. Under sunlight energy, enzymes (hydrogenases) convert surplus electrons and protons (H 2 ions) from water into gaseous H 2 . Although this enzyme can theoretically convert sunlight to H 2 with 12-14% efficiency 80 , laboratory experiences based on green microalgae have not exceeded values of 3-4% [START_REF] Osman | Critical challenges in biohydrogen production processes from the organic feedstocks[END_REF] . Direct biophotolysis could become cost-effective once it reaches 5% conversion efficiency in microalgae.

Figure 14 : Simplified direct biophotolysis process [START_REF] Hitam | A review on biohydrogen production through photo-fermentation of lignocellulosic biomass[END_REF] Such technologies can perform at or close to room temperature and atmospheric pressure. There are two types of biohydrogen production processes: sunlightdriven ones relying on photosynthesis, and pathways using fermentation of organic substrates. Hybrid processes also exist, combining the advantages of each category [START_REF]Review and comparison of various hydrogen production methods based on costs and life cycle impact assessment indicators[END_REF] . Involved microorganisms differ depending on the type of process: sunlight-driven methods use microalgae, cyanobacteria or photosynthetic bacteria, whereas fermentation-based ones use fermentative bacteria.

Indirect biophotolysis is a two-step process where CO 2 is first fixed as carbohydrates by cyanobacteria using sunlight, which are then broken down by the hydrogenase enzyme to produce H 2 . The advantage of indirect biophotolysis is that oxygen production is separated from H 2 production. Indeed, oxygen has an inhibitory effect on the hydrogenase enzyme which is a major issue for direct biophotolysis systems. Cyanobacteria could in principle achieve a maximum 16.3% light conversion efficiency but the actual process efficiency is much lower.

Fermentation-based Processes

Fermentation is a process where microorganisms use sugar-containing organic matter as carbon source and break it down into simpler compounds, producing chemical energy along with H 2 and carbon dioxide. These processes can be classified into two types depending on whether the microorganisms need light to process the organic matter or not.

Fermentation processes that need light to occur are called photo-fermentative. Contrary to bio-photolysis, photo-fermentation takes place in anaerobic conditions. This process can consume carbohydrate-rich waste (effluents from the food and beverage industry, household and municipal organic waste, waste water) to produce a valuable energy vector. The downside is that the reaction requires strictly-controlled conditions, which makes bioreactors very expensive. Light conversion efficiency is also quite low compared to other H 2 production methods (1-5% 83 ). In dark fermentation, bacteria and microalgae degrade complex organic compounds under anaerobic conditions with no light radiation and produce H 2 among other molecules. This process can also use waste as an input. The overall efficiency of dark fermentation ranges between 19% and 28%, which is quite low compared to biomass-waste gasification or pyrolysis [START_REF] Islam | Bio-Hydrogen Production from Wastewater: A Comparative Study of Low Energy Intensive Production Processes[END_REF] . Low yields are also observed, with a theoretical maximum of 12 mol H 2 /mol glucose reduced to 4 mol H 2 /mol glucose in practice due to the co-production of organic acids. Dark fermentation also produces CO 2 , between 0.96 and 8.6 kg CO 2 /kg H 2 depending on the feedstock which reduces the environmental benefits of the process 12 . However, an advantage of such reactors is their overall reduced complexity, which makes them cheaper to build and operate.

Hybrid Processes

Dark-photo co-fermentation makes use of photo fermentation to further decompose the carbohydrates produced by dark fermentation, thus improving H 2 yields and minimizing waste production [START_REF] Mishra | Outlook of fermentative hydrogen production techniques: An overview of dark, photo and integrated dark-photo fermentative approach to biomass[END_REF] . Another process that has gained popularity in recent years is Microbial Electrolysis Cells (MEC), in which electrons are generated by the oxidation of organic matter in the anode and transported to the cathode where they combine with protons, generating H 2 . This method can reach very high efficiencies but suffers from the same issues as biophotolysis.

Biological route: maturity and projects

Despite the environmental advantages of biological methods, their low H 2 yield -about 0.01 kg H 2 /kg-feedstock for biophotolysis and between 0.01 and 0.07 kg H 2 /kg-feedstock for fermentation processes -makes scale-up and industrial applications difficult [START_REF] Lee | The Potential Role of Biohydrogen in Creating a Net-Zero World[END_REF] . Comparatively, steam methane reforming has a H 2 yield of about 0.25 kg H 2 /kg-feedstock. For the moment, most of these technologies are still at the lab level (TRL 1-3) and will only be able to get to demonstration if their efficiency and H 2 production yield are improved. TRL 1-7

A key issue common to all biological routes is that microorganisms are highly sensitive to light intensity and wavelength, which can lead to substantial disparities between experiments under optimal conditions in laboratories and real-life situations. Dark fermentation is more mature and was rated at TRL 7 in 2021 as multiple pilot projects have demonstrated good yields and rates of H 2 production, with some problems still to be addressed, such as process stability. H 2 properties have made measurements very difficult. H 2 is very volatile and diffusive so it cannot be retained for long periods of time in geological formations. It is also rapidly consumed by microorganisms and reacts with oxygen to form water when arriving in the atmosphere, which makes it even more difficult to detect [START_REF]The occurrence and geoscience of natural hydrogen: A comprehensive review -ScienceDirect[END_REF] .

For example, continuously burning gas seep near Antalya in Turkey with concentrations 7.5-11.3% of H 2 have been known for 2,500 years and "Los Fuegos Eternos" (the eternal flames) in the Philippines were discovered more than two centuries ago with concentrations ranging from 41.4 to 44.5% [START_REF]The occurrence and geoscience of natural hydrogen: A comprehensive review -ScienceDirect[END_REF] .

While today's estimations on resources are not very precise due to insufficient exploration and data, the latest estimates from 2019 give an annual production of 23 Mt 88 , much more than the previous ones of 6 Mt/y in 2005 [START_REF] Gilat | Primordial hydrogen-helium degassing, an overlooked major energy source for internal terrestrial processes[END_REF] . Historical records show that estimates have been increasing by an order of magnitude every one or two decades, implying that even these are likely to underestimate the resource [START_REF]The occurrence and geoscience of natural hydrogen: A comprehensive review -ScienceDirect[END_REF] . Some argue that the situation is similar to the beginnings of oil exploration at the end of the 19th century [START_REF] Smith | Hydrogen exploration: a review of global hydrogen accumulations and implications for prospective areas in NW Europe[END_REF] . By improving exploration techniques, the potential reserves might increase and their exploitation will allow us to better understand the underlying formation mechanisms.

Natural hydrogen: maturity and projects

For the moment, natural H 2 extraction is considered by the IEA to be at TRL 3 [START_REF]ETP Clean Energy Technology Guide -Data Tools[END_REF] , meaning that the concept has been formulated but the solution still needs to be prototyped and applied.

However, there is already an example of a commercial exploitation of natural H 2 in Mali since 2016 from a well that has produced continuously without a drop in pressure, implying a continuous recharging of the reservoir. In 2011, a company named Petroma (now Hydroma) acquired the rights to exploit the initial well and eighteen other small ones. The gases from these wells (98% H 2 purity, 1400 m3/ day) have been fuelling an internal combustion engine supplying the nearby village with clean electricity since 2018 91 92 .

A startup named 45-8 Energy in France is also working on natural H 2 and is currently estimating the potential for natural H 2 in the Pyrenees, along with other energy companies like Engie or Total Energies. Some permits have also been granted in Australia where the resource seems to be more available than elsewhere [START_REF]Début de l'exploration-production d'hydrogène naturel : une nouvelle ère pour l'hydrogène ? | Connaissances des énergies[END_REF] .

COMPARISON OF HYDROGEN PRODUCTION TECHNOLOGIES

As shown previously, there exists a lot of hydrogen (H 2 ) production technologies, using different feedstocks, with different maturity levels… What is important for the energy transition is the potential to decarbonize the current production of H 2 and to enable new end-use technologies based on low-carbon H 2 .

COMPARISON OF LHV EFFICIENCIES

Efficiency, which is the ratio between the energy input and the energy output of a process, is also an important parameter to consider. The Although TRLs give information about the stage of development of a technology, a high TRL (7-8-9) doesn't necessarily mean that the technology will be a commercial success. For example, ATR and PO are currently at TRL 9 but don't gain much market shares compared to SMR or CG because the market is already saturated and the benefits they bring are not enough to justify a switch. Moreover, there can be big differences of readiness level for the same technology as the TRL is determined by the latest and most advanced projects. For example, plasma pyrolysis is at TRL 8 because Monolith has been operating its first-of-a-kind commercial plant Olive Creek 1 since 2020, but other companies are still at the development stage (see plasma pyrolysis section). Sources: SMR, CG, biomass gasification and pyrolysis efficiencies 10 . Direct and indirect biophotolysis and MEC efficiencies [START_REF] Osman | Critical challenges in biohydrogen production processes from the organic feedstocks[END_REF] . Photofermentation efficiency [START_REF] Suresh | An overview on light assisted techniques for waste-derived hydrogen fuel towards aviation industry[END_REF] . Dark fermentation efficiency [START_REF] Islam | Bio-Hydrogen Production from Wastewater: A Comparative Study of Low Energy Intensive Production Processes[END_REF] . PC efficiency [START_REF] Lakhera | A review on particulate photocatalytic hydrogen production system: Progress made in achieving high energy conversion efficiency and key challenges ahead[END_REF] . ALK, PEM, SOEC and PEC efficiencies [START_REF]Cost-competitive green hydrogen: how to lower the cost of electrolysers? Oxford Institute for Energy Studies[END_REF] . AEM efficiency [START_REF]Green hydrogen cost reduction[END_REF] . ATR and PO efficiencies [START_REF]Review and comparison of various hydrogen production methods based on costs and life cycle impact assessment indicators[END_REF] . TCWS efficiency [START_REF]Comparative review of hydrogen production technologies for nuclear hybrid energy systems[END_REF] . Thermal and catalytic decomposition efficiency [START_REF]Methane Pyrolysis for Zero-Emission Hydrogen Production: A Potential Bridge Technology from Fossil Fuels to a Renewable and Sustainable Hydrogen Economy[END_REF] . Plasma decomposition efficiency calculated based on plant energy balance [START_REF] Diab | Why turquoise hydrogen will Be a game changer for the energy transition[END_REF] .

(*) refers to Solar-to-Hydrogen (STH) efficiencies

COMPARISON OF GHG EMISSIONS

It is thus interesting to look at the emission reduction potential of each technology presented here and see if the H 2 produced can be qualified as low-carbon according to the current and the new regulation. Only the most mature technologies that are likely to play a role in the EU H 2 production mix this decade are considered. Figure 20 shows GHG emission associated with the different technologies described in this report, taking into account fugitive emissions of the fossil fuels supply chains, when applicable:

In terms of climate impact, the level of process emissions is a critical parameter.

In its REDII directive [START_REF] Hermesmann | Turquoise, Blue, or Grey? Environmentally friendly Hydrogen Production in Transforming Energy Systems[END_REF] . (The one showed here is the best-case scenario where the heat for the process is provided by German offshore wind electricity). For electrolysis embedded emissions, a 64% average LHV efficiency was chosen for the ALK electrolyser. Life-cycle emissions of the EU and France electricity generation were taken from the European Environment Agency 97 , from ADEME INCER-ACV [START_REF]INCER ACV -Impacts environnementaux de la filière photovoltaïque & évaluation des incertitudes[END_REF] for EU solar PV and from Carbon Neutrality in the UNECE Region 2022 report [START_REF]Carbon Neutrality in the UNECE Region: Integrated Life-cycle Assessment of Electricity Sources[END_REF] for EU offshore wind.Values showed here don't take into account the emissions associated with manufacturing, transport, construction and end-of-life of hydrogen production technologies.

With What is also interesting is that SMR with 90% capture rates is better on a climate standpoint than electrolysis powered by grid electricity, even in 2030, when considering the average carbon intensity of the EU power sector (275 g CO 2 e/ kWh) [START_REF]Greenhouse gas emission intensity of electricity generation -European Environment Agency[END_REF] . However, countries whose power sector has a low-carbon intensity (<85 g CO 2 e/kWh) could produce low-carbon H 2 using grid electricity directly. Only countries like Sweden, France, Luxembourg and Finland had an electricity mix with a carbon intensity lower than 85 g CO 2 e/kWh in 2021. However, emissions decrease a lot when using renewable electricity from solar PV or offshore wind, coming down as low as 0.62 kg CO 2 e/kg H 2 and every country could install dedicated renewable power or buy renewable electricity through PPA.

Depending on the type of energy used for the provision of heat, methane pyrolysis embedded emissions can vary but are still lower than 1 kg CO 2 e/kg H 2 even with natural gas, because the carbon produced is stored or used under solid form. However, the carbon intensity of the electricity used in important because emissions can for example raise to 4.4 kg CO 2 e/kg H 2 when using German grid electricity. Hence, to be qualified as low-carbon, this technology should be powered by renewable electricity. Using biogas instead of natural gas will also be crucial later as this technology has the advantage of being able to store carbon under a solid form and thus balance the emissions and even make the process carbon negative.

Methane pyrolysis produces no carbon emissions directly but has a high level of fugitive emissions, as it needs more natural gas than SMR for the same quantity of H 2 produced. According to the IEA, a significant share of these emissions could be abated at no net cost, because the value of the captured methane is sufficient to cover the cost of the abatement measure 100 . The precise share of emissions that can be avoided at no net cost is highly dependent on natural gas prices and on geographical location.

First, it is clear that H 2 produced from either black or brown coal is far too carbon emissive to be considered low-carbon even with a 90% capture rate and considering that there are no associated fugitive emissions. In the end, among the fossil fuel-based methods, only natural gas-based ones with high capture rates above 90% and standard fugitive emissions could be considered as lowcarbon by the EU. This is also true when considering that carbon dioxide is stored permanently after being captured without generating too much fugitive emissions during transport. In comparison, biomass gasification is already very low-carbon and could become carbon negative if CCS is added to the process.

KEY TAKEAWAYS

There is a wide range of methods to produce low-carbon H 2 • Thermochemical route technologies with partial CCUS are already available but do not sufficiently reduce the emissions of processes based on fossil-fuels to make them qualify as low-carbon by the EU. Several projects announced that they planned to install CCUS with high capture rates, which could turn some fossil fuel-based production routes into low-carbon ones, but the technology has not yet been proven at scale.

• Other H 2 production technologies have some potential as low-carbon alternatives but won't be available at scale until at least the 2030s.

• Producing H 2 via electrolysis is only low-carbon when it is powered by renewables. The average carbon intensity of EU electricity is too high and will stay so in 2030 for low-carbon H 2 to be produced using electricity from the grid, except for some countries which had an average electricity carbon intensity lower than 65 gCO 2 e/kWh (18 gCO 2 e/ MJ) during the past calendar year, like France or Norway.

Figure 2 :

 2 Figure 2 : Simplified flow diagram of thermochemical production processes (except TCWS) 9

Figure 4 :

 4 Figure 4 : Well-to-gate emissions of SMR using different fugitive emissions levels (kg CO 2 e/kg H 2 )

Figure 5 :

 5 Figure 5 : Simplified flow diagram of a Sulphur-Iodine thermochemical cycle for H 2 production[START_REF]Review and comparison of various hydrogen production methods based on costs and life cycle impact assessment indicators[END_REF]

Figure 7 :

 7 Figure 7 : Simplified representation of a PEM electrolyser

Figure 9 :

 9 Figure 9 : Simplified representation of an AEM electrolyser

Figure 10 :Further

 10 Figure 10 : Electrolyser technologies market shares in 2021 37 .

Figure 11 :

 11 Figure 11 : Simplified methane pyrolysis with liquid metal reactor diagram

  PHOTOLYTIC ROUTE

Figure 13 :

 13 Figure13: Simplified representation of a PC unit[START_REF] Lakhera | A review on particulate photocatalytic hydrogen production system: Progress made in achieving high energy conversion efficiency and key challenges ahead[END_REF]

TRL 5 - 6

 56 Solar water splitting device from HydroGEN Advanced Water Splitting Materials consortium -Courtesy of NREL

Figure 15 :

 15 Figure 15 : Simplified indirect biophotolysis process 82

Figure 16 :

 16 Figure16: Simplified photo-fermentation process[START_REF] Howarth | How green is blue hydrogen?[END_REF]

Figure 17 :

 17 Figure17: Simplified dark fermentation process21 23

TRL 3 First

 3 U.S hydrogen well drilled in 2019 by Natural Hydrogen Energy -Credit: Viacheslav Zgonnik COMPARISON OF TECHNOLOGY MATURITIES Technology innovation takes time. One tool to track the development of a technology is the technology readiness level (TRL). At TRL 9, technologies are commercially ready. Hence, technologies that currently have a TRL lower than 7 (pre-commercial demonstration) are unlikely to play a major role during the coming decade but could in the long term contribute to achieving climate targets by 2050.

Figure 18 :

 18 Figure 18 : Classification of H 2 production technologies based on their maturity level (TRL)

  [START_REF]Review and comparison of various hydrogen production methods based on costs and life cycle impact assessment indicators[END_REF] .

		CO 2
	Fossil fuel Biogas Biomass	CO 2 capture unit	Purified flue gas
			Syngas production		Gas separation
	Pre-treatment	Reforming / Gasification	Water gas shift (WGS)	CO 2 capture unit	Pressure swing adsorber (PSA)	H 2
	Air / pure oxygen
	Biogas Natural gas			Purified tail gas	unit CO 2 capture	Purified tail gas
		gives the path for SMR only
					CO

2 Figure 3 : Simplified flow diagram of fossil fuel with CCS H 2 production process 9

and carbon footprint Another

  2 kg CO 2 e/kg H 2 for black and brown coal gasification with 90% capture rates and IPCC default fugitive emissions levels[START_REF] Longden | Clean' hydrogen? -Comparing the emissions and costs of fossil fuel versus renewable electricity based hydrogen[END_REF] . important aspect of implementing CCUS is its energy consumption. From a whole plant perspective, the energy efficiency of a SMR plant with CO 2 capture drops by 14% compared with a reference SMR plant without CO 2 capture[START_REF] Spallina | Techno-economic assessment of membrane assisted fluidized bed reactors for pure H2 production with CO2 capture[END_REF] . Depending on the carbon intensity of the electricity used, the emission reduction offered by CCUS facilities could be lower than expected.

	It is crucial to evaluate the carbon footprint of the whole value chain and not
	only process emissions. For example, fugitive emissions associated with natural
	gas extraction, transport, storage and distribution cannot be mitigated by CCUS
	technology.
	The most promising thermochemical cycle is the sulphur-iodine cycle because
	of its prospects for high efficiency. It consists of three chemical reactions that
	results in the dissociation of water. First, sulphur oxide and iodine combine with
	water at temperatures of 20-100°C during an exothermic reaction called Bunsen
	reaction, forming two separated acids: sulfuric acid and hydriodic acid. Sulfuric
	acid is then fed into a Gibbs reactor where it is decomposed under temperatures
	of 600-900°C with a catalyst forming oxygen and sulphur oxide. This reaction is
	endothermic, so heat must be provided by another source (e.g. concentrated solar
	or nuclear heat). At the same time, the hydriodic acid goes into the equilibrium
	reactor where it is decomposed under temperatures of 300-450°C (endothermic)
	forming H 2 and iodine. The sulphur oxide from the Gibbs reactor and the iodine
	from the equilibrium reactor are then fed into the Bunsen reactor to close the loop.
	The net reactant is water and the net products of this cycle are oxygen and H 2 13
	kg C0 2 -e / kg H

2 Energy

Technology Description

  MtCO 2 /year before 2030 using residual gases from the industry and a small amount of natural gas 28 .

	FerroSilva (Sweden)	BG + CCUS	Aims at showing the feasibility of producing fossil-free sponge iron for green steelmaking out of H 2 produced from biomass 25 .
	H 2 morrow (Germany)	SMR + CCUS
	H 2 BE (Belgium)	ATR + CCUS	Announced by ENGIE and Equinor in February 2021. Aims to produce low-carbon H 2 at gigawatt scale before 2030. Capture rates: 95% 27 .
	H-Vision (Rotterdam)	ATR + CCUS	Plans to produce 300 ktH 2 /year and capture 2.7

Launched by Equinor and OGE. Aims at decarbonizing current SMR H 2 production for the industry and other end-users in North Rine-Westphalia

[START_REF]H2morrow[END_REF]

. Plans to produce 8.6 TWh/year by 2030, capturing about 1.9 MtCO 2 /year (capture rates 95%), and storing it in saline aquifers in the North Sea.

AEM

  and may produce the cheapest H 2 in the future. However, SOEC is less flexible and stable than PEM and has a shorter lifetime because of the high working temperatures.Contrary to the first two electrolyser technologies, SOEC (solid oxide electrolyser cell) has not yet reached commercial scale (TRL 7[START_REF]ETP Clean Energy Technology Guide -Data Tools[END_REF] ). A company in France, Genvia, is developing high-performance SOEC systems to decarbonize the industrial processes in the cement, steel and chemical sectors 42 . The company is part of the European Hy2Tech IPCEI supported by France 2030 that grants public funding for innovative projects around low-carbon H 2

	SOEC	TRL 7
		43 44
		TRL
		6

  … Hence desalination is by far the preferred option where freshwater is scarce.

		ALK	PEM	AEM	SOEC
	TRL 32	9	9	6	7
	Temperature 13 55	60-90°C	50-90°C	50-70°C	500-1000°C
	Pressure 13 55	2-10 bar	15-30 bar	30 bar	< 30 bar
	Charge carriers 13 55	OH-	H+	OH-	O2-
	Most common electrolyte/ membrane 29	20-40% aqueous KOH or Na-	Proton conductive polymer membrane	Anion exchange ionomer (e.g. AS-4) + optional dilute caustic solution	Ceramic: solid, nonpourous metal oxide (Y2O3-stabilized ZrO2)
	Most common electrode materials 13	Ni and Ni alloys	Ni and Ni alloys	Transition metal-based materials	Cermet and doped metal composites
	Most common catalyst 29	Pt and Ru	Pt black, Ir, Ru, Rh	Pt and Ru	ZrO2
	Current density 13 55	0.2-0.5 A/cm²	0.8-2.5 A/cm²	0.2-0.5 A/cm²	0.3-1 A/cm²
	Load range 13 56 50	15-100% nominal load	0-160% nominal load	5-100% nominal load	20-100% nominal load
	Start-up (cold) 13 50	10 minutes	5 minutes	< 20 minutes	> 600 minutes
	Energy consumption 29	50-78 kWh/kg H 2	50-83 kWh/kg H 2	57-69 kWh/kg H 2	45-55 kWh/kg H 2
	Stack lifetime 29	60,000-100,000 h	50,000-90,000 h	~30,000 h	20,000-90,000 h
	CAPEX 29	$800-1,500/kW	$1,400-2,100/kW	Unknown	> $2,000/kW
	Project scale 32	Hundreds of MW	Tens of MW	kW	Hundreds of kW to MW
					• Low power consumption
		• Cheapest option	• High flexibility	• No rare materials	needs (fatal heat)
		today	(adapted to renewable	• Non-corrosive	• High energy efficiency
		• No rare materials	energy)	electrolyte	• Feasible reversible
	Advantages	• Long lifetime	• High current density	• Compact cell design	operation
		• High maturity	• Quick response time	• No leaking	• Low expected cost
		• Tolerance to	• Compact cell design	• High operating	• Combination with other
		impurities	• Low minimal load	pressure	technologies
					• Low minimal load
				• Low technological
	Disadvantages	• Low current density • Corrosive electrolyte • Mixing of gases • High minimal load • Electrolyte leakage	• Rare materials (more expensive) • High membrane cost • Acidic corrosive components • CO poisoning	maturity • Low current densities • Medium durability • Membrane degradation • Excessive catalyst loading	• Short lifetime (ceramic membrane) • Severe environment (high temperatures) • Unstable electrodes • Sealing issues
				• High minimal load

Table 2 :

 2 Summary of electrolyser technologies characteristics

energynews NATURAL HYDROGEN Natural hydrogen

  (H 2 ) was discovered a long time ago although the topic has clearly been overlooked in recent years.Unlike oil and gas, natural H 2 is not a fossil resource: it is continuously produced inside Earth's crust. Two main processes are currently favoured by scientists. The first one, a water-rock interaction called serpentinization, produces H 2 through the oxidation of Fe(II) or magnesium minerals by seawater. Serpentinization is most effective in the temperature range 200 to 310°C. The reaction rate becomes limited at lower temperatures. The second one is the radiolysis of water, where the radioactivity of Earth's crust dissociates water molecules due to ionizing radiation[START_REF]The occurrence and geoscience of natural hydrogen: A comprehensive review -ScienceDirect[END_REF] .

EQTEC Plc 1.1 MWe Market Development Center (MDC) turning biomass waste into hydrogen energy, biofuel and electricity (Italy) -Credit:

  table below compares the energy efficiency of different production methods. The most efficient technologies are electrolysers and current fossil fuel-based methods. Methane pyrolysis, photo-electrochemical catalysis and thermochemical water splitting are the second most efficient methods and could show great potential for H 2 production. In general the most mature technologies appear to have the highest efficiencies among the other technologies. Below are given the efficiency range for each of the technologies presented before. It is important to mention that if electrolysis and direct biotphotolysis were to be compared, the efficiency of the electricity generation would have to be considered too (hence taking into account the conversion efficiency of the solar panel if electrolysers are powered by PV).

  [START_REF]2001 of the European Parliament and of the Council of 11 December 2018 on the promotion of the use of energy from renewable sources (recast)[END_REF] , the European Union considers a fossil fuel comparator for low-carbon fuels like H 2 . It corresponds to a 60% reduction of emissions compared to the "well-to-gate" emissions of H 2 produced by natural gas without CCUS.Moreover, it also launched in 2021 an organism name CertifHy16 whose aim is to certify that the H 2 produced is indeed low-carbon as defined by the REDII directive. The benchmark value given for well-to-gate emissions of SMR by CertifHy is 10.9 kg CO 2 e/kg H 2 (91 kg CO 2 e/MJ). But even more recently, the European Commission set out new rules through two delegated acts on February 10th of 2023 to define what constitutes renewable H 2 in the EU 96 .The first Act clarifies the principle of "additionality" for H 2 set out in the EU's Renewable Energy Directive, by imposing stricter temporal and geographic correlation between renewable electricity supply and H 2 production. It also allows the use of grid electricity for renewable H 2 production if the installation is located in a bidding zone where the emission intensity of electricity is lower than 65 g CO 2 e/kWh (with certain conditions). The second delegated Act defines the methodology by which to assess the greenhouse gas emissions savings of renewable fuels of non-biological origin (RFNBO), changing the threshold from 60% to 70% of reduction in emissions compared to the fossil fuel comparator of 11.3 kg CO 2 e/kg H 2 (94 g CO 2 e/MJ). The European Parliament and the Council have 2 months to scrutinise them and to either accept or reject the proposals. This scrutiny period can be extended by 2 months at their request.Black coal, brown coal and SMR process (with CCS) and fugitive emissions were extracted from calculations made by Longen et al. (2021)[START_REF] Longden | Clean' hydrogen? -Comparing the emissions and costs of fossil fuel versus renewable electricity based hydrogen[END_REF] based on data provided by the IPCC. Biomass gasification emissions were obtained from the ICCT white paper on biomethane and H 2 emissions in Europe of October 202110 . Methane pyrolysis fugitive and embedded emissions as well as the amount of carbon stored under solid form come from calculations made byHermesmann and Müller (2022)

24

Figure 20 : GHGs emissions comparison of the most mature H 2 production technologies

MtH 2 /year