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Contents Table of Figures ...... 2 Introduction ...... 3 Why hydrogen? ...... 3 Why bimetallic catalyst? ...... 3 Steam reforming ...... 4 Overview ...... 4 Steam reforming ...... 4 Steam Methane Reforming: ...... 5 Catalysts Used in the Methane Steam Reforming ...... 6 Noble catalysts ...... 6 Ni-based catalyst ...... 7 Strategy development on catalyst design ...... 10 Coke formation ...... 11 Preparation method ...... 16 Activity and Sintering ...... 17 Sulfur poisoning ...... 18 In summary ...... 19 APR ...... 20 Process overview ...... 20 Reactions ...... 20 Process parameters...... 20 Feeds ...... 21 Catalyst in APR ...... 21 Noble catalysts ...... 21 Non-noble metal catalysts ...... 22 Strategy development on catalyst design ...... 24 Water electrolysis ...... 28 Water electrolysis technologies ...... 28 Alkaline exchange membrane water electrolysis (AEMWEs) ...... 28 Metallic electrocatalyst for oxygen evolution reaction (OER) ...... 30 Contributor ...... 35

Table of Figures Figure 1: Simplified flow sheet of industrial-scale steam reforming of hydrocarbons [10] ...... 5 Figure 2: TPR patterns of the series 800 (xPt(NiAl)y)z catalysts. The catalysts contained 2 different amounts (%wt) of Pt (x=0.01 and 0.3), and were calcined before Pt impregnation at y = 800oC, and at different temperatures after Pt impregnation (z= 300, 450 and 600oC [23] ...... 9 Figure 3: Temperature-programmed reduction profiles of Ni and noble metal-modified Ni catalysts supported on a-Al2O3 [22]...... 10 Figure 4: Relationship between catalysts' challenges [24] ...... 10 Figure 5: The proposed mechanism of steam reforming [27] ...... 11 Figure 6: Carbon whisker. The nickel particles have a diameter of 40nm[26] ...... 12 Figure 7: Conversion of n-butane as a function of time during steam reforming in a 3% n-butane-7% hydrogen-3%water in helium mixture. The dashed curve shows he n-butane conversion for the Ni and the solid curve is for Au/Ni-supported catalyst [29]...... 13

Figure 8: Data on Ni/La2O3-SiO2 on conversion of CH4 [30]...... 14 Figure 9: Representative perovskite unit cell [35]...... 15 Figure 10: Representative spinel unit cell [40] ...... 15 Figure 11: Representative hexaaluminate unit cell [42]...... 16 Figure 12: Typical catalytic materials with confinement environment for metal nanoclusters[44]. ... 17 Figure 13: Physicochemical properties of Ni-PS catalysts. TEM images of (a) lamellar Ni-PS, (b) tubular Ni-PSn, (c) reduced Ni-PS (d) reduced Ni-PSn [45]...... 17 Figure 14: Possible reaction pathways in APR. Asterisk represents a surface metal site [57]...... 20 Figure 15: TOF of several Pd supported catalysts in ethylene glycol reforming [65] ...... 22 Figure 16: Comparison of H2 and alkanes selectivities of Raney-NiSn (squares 480K, circles 283K) and Pt/Al2O3 catalyst (dashed lines) for APR of 1%wt oxygenated hydrocarbons [70]...... 23 Figure 17: SEM images of fresh and spent catalyst[71]...... 23 Figure 18. Comparison of bimetallic and monometallic catalysts during APR of 10%wt sorbitol at 200C and 20 bars. Rate of H2 formation (black) and gas-phase H2 selectivity (grey) [73]...... 24

Figure 19 H2 and alkane production by APR of glycerol [68]...... 25 Figure 20: Effect of the support on the APR of EG at 45.5 bar, over Pt-Fe (1:3) catalyst.[81] ...... 26 Figure 21: APR of 10% wt EG at 483 K and 25.4 bar over Pt:Ni (A) and Pt:Co (B) bimetallic catalysts. TOF H2 (black) and alkane selectivity (grey). [65] ...... 26 Figure 22: Schematic representation of alkaline exchange membrane water electrolysis process..... 29 Figure 23: Alkaline OER mechanism. The green line indicates O2 formation possibility from M-O instead of M-OOH intermediate: M= Metal [98] ...... 30 Figure 24: NiFe alloy in OER [106]...... 32 Figure 25: Volcano plot of the intrinsic activities of (oxyhydro) versus M-OH bond strength.[97]...... 33 Figure 26: Synergetic effect and strategies for the development of the high performance of multi- metal catalyst for OER [114] ...... 35

Introduction The rapid population growth and the paradigm shift of standard living of human beings have led to consuming massive world's energy. This heavy usage of energy navigated to an unprecedented energy crisis and global warming, intimidating the life of mankind [1]. The current primary energy source is extracted mainly from fossil fuels such as coal, natural gas, and crude oil. However, those non- renewable energy sources are limited in reservoirs and are also environmentally detrimental, which causes greenhouse gas emission, air pollution and acid rain. Those drawbacks mentioned above of fossil fuel triggered researchers to pursuit an alternative energy source. It becomes imperative to hunt for a new energy source alternative that could meet both sustainability and eco-friendly. Numerous approaches are underway or being practiced combatting the energy crisis and global warming. One of the most appealing alternative routes to obtain sustainable energy is shifting the energy source from non-renewable energy (fossil fuels) to renewable energy sources (i.e., wind, solar, hydro, and biomass) [2]. Renewable energy can be explained as one kind of energy source that confers electricity and heat without deteriorating the environment. These clean energy sources are expected to be the most reliable, and there is enormous devotion toward this energy source and are expected to offer at least 50% of the total globe's primary energy after 20 years. The idea of deploying hydrogen as an energy carrier was widely acknowledged starting from 1974 when the global energy crisis was observed. Hydrogen can be produced using various methods, including steam reforming [3], aqueous phase [4] , and water electrolysis [5], [6]. Why hydrogen? Hydrogen has been recognized as an interesting alternative to non-renewable fossil fuel, due to its potential for the production of several chemicals such as ammonia and also for the generation of electricity via a . Moreover, the usage of hydrogen as energy carrier owns several merits: (i) hydrogen which is found in water is earth-abundant, (ii) possess high gravimetric energy density (roughly three-fold higher than the liquid hydrocarbon, (iii) it is eco-friendly since its by-product during combustion is a water molecule [2], [7].

Why bimetallic catalyst? This question has different answers depending on the hydrogen-generating method. The three processes studied in BIKE need very different operational conditions, from working temperatures and pressures to reactants. A common point for all these cases is the need of enhancing long term activity and lowering catalysts cost. For example, since steam reforming uses methane as a hydrogen source, Sulphur in the form of H2S and other Sulphur-containing compounds can be present at trace levels. This can lead to the poisoning of the catalyst when operating with certain catalysts such as Ni and Pt based catalysts. In the case of water electrolysis, the use of bimetallic catalyst has been proven to be a reliable way of getting higher catalytic activity due to the change in the electronic structure of the catalyst when two different metals interact. On the other side, bimetallic catalyst don’t show a significative activity increment coming from the synergy of two metals when applied to aqueous phase reforming. However, as for steam reforming, the addition of certain metals can prevent Sulphur poisoning when using low purity biofeeds. Besides, as traditionally noble metals have been mainly used for this application, adding active and cheaper metals such as Ni can certainly make this process more economically competitive when aiming to hydrogen generation.

As it was said, this question has different answers, and you could find them in a more detailed way in the next lines. Welcome to BIKE project wiki page.

Steam reforming Overview In principle, there are three types of reforming processes: • Steam Reforming • Partial Oxidation o (Non-Catalytic) Partial Oxidation (POX) o Catalytic Partial Oxidation (CPO) autothermal reforming is grouped under partial oxidation, because partial oxidation may be carried by a combination of non-catalytic oxidation and steam reforming. Sometimes autothermal reforming is classified in a separate Partial oxidation and autothermal reforming are self-sustaining and do not require external provision of heat, but they have less efficient in producing hydrogen[8]. Steam reforming Steam reforming process is a well-known and used since the 30s for hydrocarbon reforming. Steam reforming is the most important route for large scale manufacture of synthesis gas for ammonia, methanol, and other petrochemicals and for the manufacture of hydrogen for refineries. The product of steam reforming is syngas and it depends on the feed composition. Steam reforming is often conducted with natural gas, heavier hydrocarbons such as naphtha are also used, but higher boiling point hydrocarbons than naphtha have not reached the commercial stage yet. Generally, steam reforming (SR) is a process of producing hydrogen by combining steam and hydrocarbon and reacting in a reformer at temperatures above 500°C in the presence of a metal-based catalyst [9].

Figure 1: Simplified flow sheet of industrial-scale steam reforming of hydrocarbons [10]

Part of the hydrocarbon feed is burnt with air to heat up the reformer, and the recovered heat at the reformer outlet is used for steam production and feed preheating. The hydrocarbon feed must first be desulfurized and preheated before entering the catalyst reformer tubes. The preheated feed passes through the catalytic bed in a multi-tubular reformer reactor. Usually, steam reforming is performed at around 773-1173K (500-900°C), above20bar, and with a steam-to-carbon ratio, of around 2.5-3 to ensure coke-free operations [10]. If maximum hydrogen production is desired, the reformer outlet undergoes first the WGS reaction to maximize the hydrogen production. The hydrogen is then purified to above 99.95% purity by getting rid of steam and CO2 through condensation and Pressure Swing Adsorption (PSA), respectively. Instead of PSA for CO2 recovery, a permeable reactor membrane can be used. The tail gas after PSA is used to enrich the fuel and provide further energy to heat up the reformer[11].

Steam Methane Reforming: Steam Methane Reforming (SMR) has two steps: 1. The reforming reaction (Eq.1), which is strongly endothermic (206 kJ.mol−1). 2. Water Gas Shift (Eq.2) that is somewhat exothermic (−41 kJ.mol−1). • Steam Methane Reforming (SMR - Eq. 1) 퐶퐻4 + 퐻2푂 퐶푂 + 3퐻2 ∆퐻 = +206푘퐽/푚표푙

• Water Gas Shift (WGS - Eq. 2)

퐶푂 + 퐻2푂 퐶푂2 + 퐻2 ∆퐻 = −41푘퐽/푚표푙 results from the combination of reaction 1 and 2 (Eq.3): 퐶퐻4 + 2퐻2푂 퐶푂2 + 4퐻2 ∆퐻 = +165푘퐽/푚표푙

CO2 is not only produced through the shift reaction (Eq.2), but also directly through the steam reforming reaction (Eq.3). Reactions 1 and 3 are reversible and normally reach equilibrium over an active catalyst, at high temperatures. The overall product gas is a mixture of , carbon dioxide, hydrogen, and unconverted methane and steam.

The basic reforming reaction for a generic hydrocarbon CnHm may be written as (Eq.4): 푚 퐶 퐻 + 푛퐻 푂 푛퐶푂 + ( + 푛)퐻 푛 푚 2 2 2

The product from the reformer depends on the temperature of the reactor, the operating pressure, the composition of the feed gas, and the proportion of steam fed to the reactor. The amount of carbon monoxide is quite high; because the water gas shift reaction is thermodynamically favorable at higher temperatures. The amount of carbon monoxide in the final product is determined by the thermodynamics and kinetics of the reaction within the reformer. Steam reforming process is divided into two steps, first is at high temperature, typically 800- 1000°C, and pressure, 30-40 bar, the reforming and shift reaction occurs in first step and followed by an additional two-step shift section at a lower temperature typically at 200-400°C in order to maximize the CO conversion. Typically, a 3:1 ratio of steam to carbon (S/C) is used to obtain high methane conversion and carbon monoxide and carbon dioxide selectivity. In order to maintain a high methane conversion, it is necessary to operate the system at high temperature, low pressure, and relatively high steam to carbon ratio[12]. Catalysts Used in the Methane Steam Reforming: The catalyst mainly used for steam reforming is a heterogeneous catalyst at which the reactants and the catalysts exist in different phases. The catalyst consists of either a single substance or more than one component distinguished as: active metal component, a support, and promoters. In general, reforming reactions are catalyzed by group 8-10 metals with nickel as the preferred metal for industrial application because of its activity ready availability and low cost. Methane is activated on the nickel surface. The resulting CHx species then reacts with OH species adsorbed on the nickel or on the support. In the steam reforming of methane, a catalyst can reduce the reaction temperature range and thus avoid the occurrence of side reactions. Moreover, the catalyst should be stable under the rather extreme conditions under which high CH4 conversions can be reached (i.e., high temperatures and high probabilities of unwanted side reactions involving carbon deposition). Nickel-based catalysts showed an important methane conversion and a high H2/CO ratio, and it has been widely employed as the conventional industrial steam reforming catalyst. However, because of side reactions that are Inevitable, it suffers from coke formation. Therefore, the development of active steam reforming catalysts with high durability against coking is thus desirable[13].

Noble metals catalysts Noble metals (Ru, Rh, Pd, Pt…) have high hydrogen production rates, important activity and stability, and strong resistance to coke formation compared to nickel catalysts6. One of the first studies of ranking within steam reforming activity was done by Kikuchi et al.[14], where they measured the relative order in activity at atmospheric conditions and 350-600°C and observed the following order: Rh ~ Ru > Ni > lr > Pd ~ Pt >> Co, Fe. Later studies were reported by Rostrup Nielsen and Hansen. They conducted a series of experiments with Ru, Rh, Pd, Pt and Ni on MgO support, measuring steam reforming activity at 550°C and under atmospheric pressure [6]. Relative activities were reported to be: Ru ~ Rh > Ir > Pt ~ Pd. Qin and Lapszewicz performed similar experiments to those of Rostrup Nielsen with noble metals on a MgO support in the temperature range of 600-800°C and under atmospheric pressure and found an almost identical activity relationship for the noble metal catalysts in methane steam reforming reaction as the two previous mentioned studies: Ru > Rh > Ir > Pt > Pd.

Jakobsen performed several studies on noble metals supported on ZrO2, Al2O3 and MgAl2O4. He also found the following ranking: Rh ~ Ru > Ni ~ Pt ~ Ir ~ Pd [15]. To conclude, the above mentioned studies seem to point to a general trend for the order of reactivity among the noble metals: Ru and Rh based catalysts are the most active, Ni and Ir have intermediate activities and Pd and Pt are less active. has been widely used in the methane steam reforming reaction [6]. Ru catalysts are also very active in steam reforming reactions under water deficient conditions and very low amount of carbon is formed on catalysts during the reaction without any change to the mechanical properties of the catalyst. They increased the conversion percentage and strongly improved the selectivity for syngas production. It was also reported that Ru-based catalysts, especially Ru supported on magnesia and alumina present high activity and high hydrogen selectivity as well as stable performance. The pronounced reactivity of ruthenium catalysts seems to result from the reducibility of the oxide itself. For example, Ru doped in Ni/Al2O3 and Ni/Mg(Al)O exhibited self-activation resulted from the hydrogen spillover via Ru metal and Ru Ni alloy. In addition, the incorporation of Ru in the lattice of the support favored their reduction behavior and increased their stability during the reaction leading to a negligible formation of carbonaceous deposits[16]. Ru-based catalysts have been also proved to be among the best catalytic systems for partial oxidation of methane reaction with excellent coke resistance at elevated temperature [9].

Several studies were done in the laboratory on the effect of the addition of ruthenium on CeO2-Al2O3,

CeO2-ZrO2 supports. They concluded that the presence of ruthenium enhanced the activity of the dry and steam reforming of methane [17]. However, even though noble metal catalysts present higher activity than other metal catalysts, their manufacturing cost remains relatively high. Therefore, efforts are done to develop transition metal oxides catalysts with high activity. is a transition metal that is extensively used as an active phase to catalyze different types of reactions; however, it is not well-known in the methane steam reforming reaction. It was shown that CuO exhibits a high activity in the catalytic reactions by the release of active oxygen species from Cu (II) species and the increase of the reducibility of the support [18]. In addition, it is reported that the presence of copper as an active metal can suppress carbon formation step and can improve the water gas shift reaction (WGS) in methane steam reforming. In fact, Cu played a role in promoting the gasification of deposited carbon, and its addition to Ni led to the enhancement of the WGS activity in the overall process and decreased the coke deposit. In addition, a system based on doping LaCoO3 with copper showed that the presence of this metal increased the reducibility of the support in the reaction of alcohols and hydrocarbons synthesis from syngas[19]. Copper is also used in the dry reforming and partial oxidation of methane leading to a high conversion of this latter. Copper is also used in the dry reforming and partial oxidation of methane leading to a high conversion of this latter [20]. Ni-based catalyst Nickel is the most commonly used catalyst in steam reforming thanks to its high activity, stability and low cost. Co and Fe are also active for steam reforming, however, being susceptible to oxidation in reaction condition [21]. Ni is supported on ceramic oxides such as α-alumnia, calcium aluminate, magnesia, or magnesium-aluminate. For low temperature reforming (< 500oC), γ-alumina or chromia can be utilized. A typical Ni catalyst contains 15-25 wt% Ni.

Attempts to improve Ni catalysts have been stated in publication. Addition of other metals to the catalysts can not only improve activity, stability, lifetime of the catalysts but also increase its versatility in different modes of reforming.

Addition of noble metals Despite high activity and stability, noble metal remains unattractive as its high cost. However, by doping with small amount of precious metals (Pt, Pd, Ru, Rh) the Ni-based catalysts can harness their characteristics cost-effectively.

Li et al. published a review on Ni catalysts modified with noble metals[22]. The list can be seen in Table 1

:

Table 1: Ni modified with noble metal catalysts [22]

In general, noble metal increase reducibility of Ni and prevent coke formation on Ni surface. It is well- known that Ni interacts strongly with Al2O3 forming NiAl2O4 spinel that is inactive towards MSR and it is very difficult to reduce leading to deactivation and loss of metallic Ni, which dampen activity of catalysts and increase energy demand in catalyst activation step. Doping noble metals can greatly reduce the reduction temperature of the catalysts thanks to “hydrogen-spillover effect”: noble metals can easily absorb H2 and dissociate to H, the reactive H atom can be transferred to nickel and reduce it to metal state. For instance, de Souza et al. prepared Pt-doped α-Al2O3 for steam reforming [23]. The TPR profile of different catalysts shown in Figure 2 :

Figure 2: TPR patterns of the series 800 (xPt(NiAl)y)z catalysts. The catalysts contained 2 different amounts (%wt) of Pt (x=0.01 and 0.3), and were calcined before Pt impregnation at y = 800oC, and at different temperatures after Pt impregnation (z= 300, 450 and 600oC [23]

It can be clearly seen that comparing to NiAl sample, Pt-containing samples have lower reduction peak temperature, even with small doping amount (0,01wt% Pt). It is noted that the author states that when spinel phase between Ni and Al is formed as NiAl2O4, the enhanced reducibility by Pt is negligible.

The increase in reducibility is also observed with other noble metals (Figure 3).

Figure 3: Temperature-programmed reduction profiles of Ni and noble metal-modified Ni catalysts supported on a-Al2O3 [22].

In term of activity, including noble metal during catalyst synthesis improve dispersion of the catalysts, as a result, improve activity during steam reforming. Luna et al. show addition of Rh to Ni/Al2O3 improves number of surface atoms, but the turnover frequency remains unchanged. Pt addition to

Ni/MgAl2O4 also improves methane conversion, as illustrated by Foletto et al. [22] Rh addition improves activity as well as coke resistance in dry reforming.

Beside doping with noble metals, other approaches have been applied to nickel catalyst increase its performance. Doping with rare earth element such as La or modified support with CeO2 have shown improvement in coke resistance of the catalyst and attract many research attentions.

Strategy development on catalyst design

Continuous development on catalysts have been observed for steam reforming for more efficient and economical H2 production in recent decades. Since the most widely used catalysts in steam reforming is Nickel and research development has been focused on around this element for improved reforming catalyst, this section is about strategy in catalyst design to overcome limitation of Ni-based catalysts. A few developments have been introduced in the previous section. In this section, a systematic strategy to overcome problems in steam reforming is discussed. There are four major challenges for steam reforming catalysts to be solved: Activity, Sintering, Coke formation and Sulfur poisoning [24].

Figure 4: Relationship between catalysts' challenges [24]

The following sections will discuss each types of challenges and its current overcoming strategy. However, it must be noted that all four factors are influencing each other. For example, catalyst poisoned by sulfur are more susceptible to sintering, coke formation and activity reduction while big Ni particles (due to sintering) increase coke formation, reduce activity and are more easily to be deactivated by sulfur [25].

Table 2: Routes to carbon formation [26]

Coke formation For Ni-based catalyst, coking is one of the most serious problem in reforming reaction. The carbon can be in different forms (whisker, gum, pyrolytic coke) and affect wide ranges of catalyst properties, summarized in Table 2. Coke formation mechanism Coke is formed via two main pathways: The methane dissociation: 퐶퐻4 ⇔ 퐶 + 2퐻2 ( ∆퐻 = +75푘퐽/푚표푙)

Figure 5: The proposed mechanism of steam reforming [27]

The CO disproportionation (Boudouard reaction): 2퐶푂 ⇔ 퐶푂2 + 퐶 ( ∆퐻 = −172푘퐽/푚표푙) Steam reforming is normally done in high temperature, therefore, Boudouard reaction is less likely to occur due to its exothermicity. In principle, methane dissociates into reactive 퐶훼 and hydrogen. The 퐶훼 can be either gasified into CO, CO2 or converted to less reactive 퐶훽. 퐶훽 can be further gasified or transform into coke. If coke formation rate is faster than gasification process, overall coke accumulation will occur [27]. DFT calculation and in-situ electron microscopy also revealed the mechanism of coke formation at atomic level. It has been shown that coke nucleation is formed preferentially in step site. However, step site Ni is also the most active sites for hydrogen generation. Therefore, decrease coke formation might be as well decrease activity of the catalyst. As the mechanism of coke formation is known, different strategies have been developed: reduce number of step sites Ni, enhance Ni particle dispersion and interaction with support or increase oxygen mobility to facilitate gasification. These strategies can be done via promoter addition, support change, preparation method or unique catalyst structure.

Figure 6: Carbon whisker. The nickel particles have a diameter of 40nm[26]

Promoter Alkaline metal It is well-known that basic catalyst promotes reaction between steam and carbon, therefore, doping Ni catalyst with alkaline (typically K) and alkaline earth elements have been widely used. In industrial scale, ICI caldaie SpA uses potassium hydroxide, Haldor Topsoe favors magnesia and British Gas uses Urania [28]. One main challenge to use alkaline dopant is its evaporation under reforming condition (e.g. creation and decomposition of potassium carbonate). In order to solve the problem, slowing down potassium hydrolysis by binding a reservoir alkali such KAlSiO4 or β-Al2O3 or bound to a calcium aluminate support [26]. Noble metal Noble metal is well-known for its coke-resistant ability. However, due to its high cost, it has not been used widely for hydrogen production [26]. By doping Ni-based catalyst with small amount of noble metal, it can enhance catalyst stability against coke formation while keep the cost low. Besenbacher et al. investigate addition of Au in Ni catalysts in butane steam reforming. They have found that addition of Au significantly enhances coke resistance of the catalyst with small expense of activity. The catalyst is stable for 80hrs of running time while pure Ni catalyst deactivates quickly [29].

Figure 7: Conversion of n-butane as a function of time during steam reforming in a 3% n-butane-7% hydrogen-3%water in helium mixture. The dashed curve shows he n-butane conversion for the Ni and the solid curve is for Au/Ni-supported catalyst [29].

Rare earth elements Addition of rare earth elements have shown a slight improved activity and good coke resistance. Ceria and Lanthanum are among the most investigated rare earth elements in coke-minimized catalysts. La can stabilize Ni particles while its basic properties promote CO2 or H2O adsorption, leading to improved carbon gasification. For example, Gao et al. synthesize Ni supported La2O3-modified SiO2 and test on dry reforming. The La-containing shows significant stability towards coke formations. While pure Ni sample rapid deactivates, La-containing samples remain its activity for 5 hours. The coke analysis show negligible about coke formed in used catalysts [30].

Figure 8: Data on Ni/La2O3-SiO2 on conversion of CH4 [30].

Ceria possess a unique oxygen storage capacity (OSC) that enables self-decoking properties when used in reforming reaction. The carbon species reacts with lattice oxygen of the catalyst. Moreover, ceria shows a strong interaction with Ni, assisting and stabilizing the active sites during the reaction. ZrO2 is usually used with CeO2 as its stability and ability to enhance oxygen storage capacity of CeO2. For instance, Purnomo et al. investigate the effect of Ceria loading on carbon formation during steam reform over Ni/CeO2-ZrO2 catalyst [31]. The group shows that addition of ceria increases the surface area and basicity of the catalyst and coke formation is reduced significantly.

Table 3: Properties of the series of Ce-Zr doped reduced catalyst [31]

Other elements Sulfur: Sulfur-passivated catalyst have been used practiced to minimized coke formation. Sulfur atoms are preferentially located at step sites, inhibiting active Ni step sites to form carbon. However, like other step-blocking site dopants, it decreases activity of the catalyst as well [24].

Gallium: Pan et al. doped Ga2O in Ni/SiO2 in dry reforming of methane [32]. The formation of carbonate and bicarbonate species enhances activation of CO2 and reverse Boudouard reaction

Indium: Károlyi et al investigated Ni-In/SiO2 in dry reforming of methane and found that with In inclusion, the coke formation decreases significantly [33]. This could be thanks to dilution effect of In to remove ensemble effect of Ni to create coke. Well-defined structure Strong interaction between active sites and support is one of the criteria to make a good anti-coking catalyst. Ni can be incorporated in well-fined structure such as perovskite, spinel, hexaaluminate or solid solutions. Incorporating Ni into such structure will make it harder to activate Ni (i.e. reduce Ni to its active metal form), but increase its dispersion and thermal stability [34]. Perovskite

Figure 9: Representative perovskite unit cell [35].

Perovskite structure is a class of oxides with similar crystal structure to CaTiO3 (general formula ABO3). In this structure, an A-site ion, is usually an alkaline earth or rare-earth element while B-site could be transitional metal. The perovskite site can be partially substituted [36]. In case of Ni catalyst, rare-earth element (La, Ce) is at A-site and Ni is at B-site. In general, perovskite- based Ni catalysts perform comparably to commercial catalysts in steam reforming [34].

For example, Pereñíguez et al. prepared LaNiO3 perovskite and tested for dry reforming and steam reforming. The catalyst shows remarkable stability, especially in dry reforming [37]. One of the limitations of perovskite catalyst is its low specific area. Few research groups have tried overcoating or incorporating perovskite precursor in porous materials to overcome this challenge [38], [39]. Spinel

Figure 10: Representative spinel unit cell [40] Spinel is a class of magnesium/aluminum crystal with formula AB2O4 with A is a co-valent ion and B is a tri-valent ion (e.g. MgAl2O4). Spinel has high chemical inertness, good thermal and mechanical stability, and large surface area.

Ni can form spinel phase with Al as NiAl2O4. This spinel is easily formed but difficult to reduce. As a result, formation of NiAl2O4 is believed to reduce activity of the catalyst. However, research have shown that formation of NiAl2O4 reduces coke formed during the reaction [41]. Hexaaluminate

Figure 11: Representative hexaaluminate unit cell [42].

Hexaaluminate is a class of hexagonal aluminate compounds consisting of layered spinel blocks of close packed oxide ions and mirror planes. The general formulation of hexaaluminate is ABxAl12-xO19 where A can be mono-, di-, or trivalent large cation residing in the mirror plane. B represents transitional metal or noble metal, which substitute Al sites [42]. Solid solution

Ni can form solid solution with various oxides such as CeO2, ZrO2, and MgO. The resulting mixture show coke resistance, with the best performance is of NiO-MgO solution [34]. However, formation of solid solution also decreases reducibility of the catalysts. An optimal ratio between NiO and MgO is desirable to both maintain reducible Ni for activity (not too much MgO) and avoid “free” NiO (not incorporated into solid solution, which leads to deactivation). Preparation method The preparation method of catalyst can alter interaction between active site and support, therefore, greatly affects catalyst performance. While the most common methods for catalyst synthesis are impregnation and sol-gel, different approaches are being explored to have a high coke-resistant catalyst. These methods consist of supercritical water, plasma treatment, combustion, and microwave radiation [34]. Plasma treatment has shown promising results in coke-resistant Ni catalysts. There are different plasma methods, consisting of plasma jet, glow discharge, thermal plasma chemical vapor deposition, dielectric-barrier discharge. The resulting catalysts by plasma technique possesses small size, narrow size distribution and enhanced metal-support interaction, which attributes to improved coke-resistant characteristic. For example, Pan et al compared plasma-treated Ni/SiO2 to conventional prepared

Ni/SiO2 and found a significant decrease in coke formation in the former [43]. Activity and Sintering Sintering describes a process in heterogeneous catalysts when small particles grow in size. In general, this process is undesired since the active site number on particles surface reduces as the particle grows. Two processes are involved in sintering: Ostwald ripening (where metal atoms are emitted from particles and captured by another particles) and particle migration (where particles migrate, collide, and combine). In principle, in order to reduce sintering, one must prevent mobility of active site toward each other. There are two major strategies to achieve: strong metal-support interaction and geometric confinement. To achieve strong metal-support, as mentioned in coke formation section, several designs can be used such as well-defined structure, novel preparation method, etc. [44].

Figure 12: Typical catalytic materials with confinement environment for metal nanoclusters[44].

Figure 13: Physicochemical properties of Ni-PS catalysts. TEM images of (a) lamellar Ni-PS, (b) tubular Ni-PSn, (c) reduced Ni- PS (d) reduced Ni-PSn [45].

To achieve geometric confinement, nanoconfinement or engineering nanostructured supports are being investigated. For example, Ni supported on phyllosilicate SiO2, a layered mineral, was investigated in ethanol reforming by Zhang et al. [45]. It Is found that the catalyst shows high metal dispersion, narrow size distribution, enhanced strong metal-support interaction. The conversion from lamellar phyllosilicate into tubular phyllosilicate further help confining Ni-particle and reduce sintering of the catalyst.

Sulfur poisoning Group VIII metals, particularly Ni, are susceptible to sulfur poisoning [46]. The sulfur compound, under reforming condition will be converted to , is chemisorbed on metal surfaces: 퐻2푆 + 푀푠푢푟 푀푠푢푟 − 푆 + 퐻2 The sulfide compound blocks active site for reforming reaction and deactivates the catalysts. Consequently, sulfur must be removed in advance to catalytic reforming unit. Hydrocarbon fuel for steam reforming contains sulfur impurity. In conventional industrial steam reforming unit, the feed is treated in a sulfur-removal (desulfurization) unit before entering reforming unit [26]. However, recently, more focus is put on small-scale hydrogen regenerators such as fuel cell. For such devices, additional desulfurization unit is not practical. As a result, a development for sulfur-resistant catalysts is desirable [25]. Noble metals catalyst: Most of the catalysts developed for small-scale reforming are noble metal-based as they are less susceptible to sulfide formation than Ni [25]. Rh is the most common studied noble metal for sulfur- laden operation as its superior stability against sulfur compared to other noble metal [47]. Combining different noble metals for synergistic effect has also shown promising results. For example, Farrauto et al. found that Rh-Pt catalyst activity is stable under 5 ppm sulfur-containing reforming stream. Similarly, Cimino et al. doped Rh with Pt and found that adding Pt to the catalysts decrease deactivation by sulfur and the sulfur favored adsorbing on small-size Rh particles [48]. Regarding to the support, Ceria has been a focus on research. Thanks to its oxygen storage ability

(OSC), CeO2 enhances stability of catalysts. The supports are usually doped with Zr, Gd, Y, La, Sm, etc. to enhance its oxygen storage ability as well as its thermal stability in reforming condition [25]. Laosiripojana et al. investigated CeO2 doped with Gd, Y, Nb, Ga and Sm. It was found that the formation of Ceria sulfates is beneficial for oxygen storage capacity while Ceria sulfides negatively affect ceria OSC [49]. Ni-based catalyst One disadvantage of using noble metal is its high cost. Therefore, researchers have investigated on cost-effective Ni-based catalysts for sulfur-resistant characteristics. There are two main approaches: Defined-structure catalysts: It is believed that incorporating active site to proper crystal structure will increase dispersion (therefore, activity) and sulfur resistance [25]. Ni can be incorporated in different crystal structures such as perovskites, pyrochlore or hexaaluminates. For example, Ni in pyrochlore (Ni/La/Sr/Zr) was shown to lose its initial activity but remained stable under sulfur-containing feed [50]. On the other hand, thanks to its increase oxygen mobility, perovskite of Ni-La-Ce performed a good resistance to low sulfur concentration (5 ppmw). However, the catalyst lost its activity significantly with higher sulfur concentration [51]. Bimetallic catalysts: combining Ni with other metal to enhance sulfur withstand ability is also considered. Ni-Rh and Ni-Re have shown some resistance to sulfur-laden environment. For example, Wang et al. compare different bimetallic system Ni-Co, Ni-Mo, and Ni-Re in kerosene steam reforming under sulfur-laden condition. They found that Re is the most promising dopant in term of activity and sulfur resistance [52]. Recently, Theofanidis et al. showed the activity of Ni and Rh-Ni catalyst dropped when introducing H2S to the feed stream. However, Rh-Ni catalyst show less reduction in activity and is able to regenerate partly its activity (~35% for their best catalyst) when H2S feed is shut off [53]. Molybdenum catalyst:

The use of molybdenum catalysts is also investigated. Mo2C has been shown to have a similar catalytic properties to group VIII noble metals [54] and can be active in reforming reaction. Moreover, under sulfur-laden condition, Mo compounds can be converted to MoS2, which shows activity on reforming itself. Cheekatamarla et al. investigated activity and stability of Mo2C in diesel reforming with up to 500wppm benzo-thiophene. The deactivation of the catalyst is completely reversible with regeneration under inert or H2 gas [55]. Haynes et al. also performed a study on Co-Mo carbide catalyst and showed that the catalyst is stable under 50 wppm benzo-thiophene in partial oxidative reforming of n-tetradecane [20]. One of the challenges regarding to carbide catalyst is their stability under oxidizing environment as gasification, converting carbide to oxide and carbon dioxide, can occur.

In summary the presence of sulfur in hydrogen production feedstock is detrimental to the catalysts’ activity. Some of the deactivation is reversible and the catalyst activity can be recovered (by either removing sulfur feed or redox circle) and some deactivation is irreversible. General strategy to design a highly sulfur-resistant catalyst can be summarized: - Doping with noble metals, rare earth element (Ce, La, Y, Gm, etc.) or Group VI (Mo, W) - Increasing active site-support interaction via crystal structures Future challenges: - Understand between sulfur-containing species and active metal - Computational approach to rationally discover new elements and synthesis to overcome sulfur poisoning

APR Process overview Aqueous Phase Reforming (APR) is a liquid phase process based on the conversion of organic compounds containing oxygen – usually obtained from biomass – to produce hydrogen gas (H2) and carbon dioxide (CO2) using a solid metallic catalyst. The heterogeneous nature of this catalytic process strongly enhances the chances of catalyst recovering when using batch reactors. The process generates H2 without volatilizing water, which represents a major energy savings. while low operating temperatures and moderate working pressures favour overall energy efficiency and lower operational costs [57]. Additionally, H2 is produced throughout two different reactions with, which constitutes an extra input when aiming to generate this gas from renewable sources [58].

Reactions

Figure 14: Possible reaction pathways in APR. Asterisk represents a surface metal site [57].

As seen from Figure 14, H2 is produced by two main reactions. In a first step, C-C bond cleavage of an organic molecule from the liquid phase takes place on the catalyst surface, producing carbon monoxide (CO) and hydrogen as gas products. The second reaction is the well-known water gas shift reaction and CO and H2O in gas state react, producing CO2 and an extra hydrogen input [58]. CO consumption in this reaction contributes to decrease overall CO levels of the final gas mixture and makes APR an ideal method to produce H2 for CO sensitive applications [59]. H2 can be also consumed through Fischer-Tropsch reaction and CO and CO2 methanation, producing alkanes and methane, respectively. To effectively avoid these side reactions, a suitable catalyst, feed and operating conditions are needed. Process parameters Moderate pressures and low temperatures ranging from 10 bar to 50 bar and 200 °C to 250 °C respectively are usually employed in this process to promote water gas shift reaction. These soft conditions are the main advantage when comparing other methods such as the widely exploited steam reforming of syn-gas, which usually operates at higher temperatures and similar working pressures [59].

When aiming to maximize H2 production, it must be noticed that higher temperatures promote hydrogen consumption via Fischer-Tropsch reaction and lower water gas shift yield, penalizing hydrogen generation. Additionally, lower temperatures imply lower conversion of organics and enhances methanation reaction [57]. Therefore, the key parameters are the choice of suitable catalysts and feeds. Feeds

There are a lot of parameters affecting H2 production when it comes to feeds. For example, the reforming of concentrated feeds needs harsher conditions that the described above [57]. Therefore, low concentration feeds – 10 wt% approximately – are preferred. With regards to molecule structure and carbon chain size, methanation decreases and more CO2 is produced as hydroxyl groups in the organics molecules increases [60] while it is easier to produce H2 when short oxygenates with few carbon atoms are used. This make hydroxylated compounds such as methanol and ethylene glycol better candidates to be used as feeds than sorbitol or glucose.

Catalyst in APR Noble metal catalysts Pt-based catalysts The most commonly used for APR is supported on g-alumina [61]. The catalyst has been selected thanks to high carbon conversion and selectivity towards hydrogen, and low production alkane. The catalysts show good activity and selectivity in APR. For example, Shabaker et al. showed 100% selectivity to hydrogen. However, the support is rapidly deactivated due to support transformation, metal sintering and carbon formation. Boehmite (AlO(OH)) is considered to be an alternative for alumina support.

Support change has been investigated to improve Platinum-based catalysts. CeO2 is well-known for its oxygen storage capacity and strong interaction with active site. CeO2-Al2O3 and CeO2-ZrO2-Al2O3 have been used in reforming of APR. The redox properties of the mixed oxides result in enhance water gas shift reaction in APR, significantly increase H2 production. Pt/C is also investigated for APR of cellulose, glucose, sorbitol, and lignocellulosic biomass. The hydrogen yield of Pt/C is lower than Pt/Al2O3. However, carbon-based materials are more stable in hydrothermal condition and therefore, attract development to improve its activity. Pt supported ordered mesoporous carbons (CMK-3 and CMK-5) exhibits good stability and high H2 production thanks to high dispersion of Pt in porous support. Bimetallic catalysts are investigated by adding another metal to developed Pt-based catalyst. Researchers have doped Pt catalysts with various metals such as Pd, Co, Ni, Fe, Mo, Mn, Re, Cs, Ba, Ga, Ag. In general, bimetallic catalysts show higher activity than monometallic counterparts. There are few combinations express significant enhancement. Pt-Ni alloy system is widely researched because of its synergistic behaviors and application. In APR, research shows that Ni can enhance Pt activity while Pt increase Ni stability. Addition of Ni while keeping high yield means reduction of Pt usage, resulting in economic advantage. For example,

Rahman et al. showed that Pt-Ni/CeO2-Al2O3 can reduce Pt content from 3 to 1wt% with increased stability in the long run [62]. Pt-Ni supported on MWCNTs are also tested and showed superior conversion compared to Pt-Cu, Pt-Co, Pt-Pd, Pt-Ru [63]. The effect of Pt on Ni will also be mentioned in non-noble metal for APR section. Pt-Re attracts a lot of attention recently. The addition of Re in Pt catalysts increase its conversion, which is particularly useful for highly selective catalysts such as Pt/C. Pt-Re also exhibits synergistic effect in water gas shift reaction [64]. Pd-based catalysts Pd is also active to APR. Even though its intrinsic activity is lower than Pt, Pd is more selective. By screening 130 Pt and Pd catalyst using high-throughput reactor for ethylene glycol reforming, Huber et al. found that beside Pt-based catalyst, Pd supported on Fe2O3 shows high turnover frequency for

H2. The enhanced activity is attributed to catalytic WGS by Fe2O3 [65]. Liu et al. found that Pd/Fe3O4 expresses better H2 generation compared to Pd supported on Fe2O3, NiO, Cr2O3, Al2O3 or ZrO2 [66].

Figure 15: TOF of several Pd supported catalysts in ethylene glycol reforming [65]

Pd-Zn is also tested for ethanol APR. The catalyst favored the production CO-free hydrogen [67]. Other noble metals are also investigated but less extensively. The reason could be relative reactivity of the noble metal in reforming reaction: Pt > Ru > Rh ~ Pd > Ir and Pt and Pd prefer H2 generation while Ru and Rh fav or alkane formation [61]. Non-noble metal catalysts

Even though the noble metal catalyst present high activity and selectivity to produce H2 from biomass derivatives, improvements are necessary to lower catalyst cost and achieve higher activities [60]. Other metals have been studied to develop less expensive catalysts. The replacement of these noble metals tends to produce catalysts with lower selectivity and reduced life cycles [68]. However, the addition of a second metal has shown an improvement in these deficiencies. Several studies have assessed the efficacy of Nickel as a catalyst for the APR process due to its ability to cleave C-C bonds and high activity for the WGS reaction, besides its relatively low price. However, Ni presents a high activity for C-O bonds, leading to the formation of alkanes and lower stability, related to metal leaching, sintering of the metal particles and nickel oxidation under APR process conditions [68]. To date, several studies have demonstrated that the addition of a second metal can intensify the competence of the catalyst [69]. Adding Sn as a promoter to a Ni catalyst reduces the selectivity to alkanes and methane formation. It enhances the rate of C-C bond cleavage, achieving almost 90% of

H2 selectivity with no production of alkanes using glycerol or ethylene glycol as a feedstock [58]. Nonetheless, this catalyst was not stable during the APR environment. Thus, Hubert et al. developed Raney-NiSn catalyst, which demonstrated high conversion and selectivity for over 340 h, suppressing sintering and dissolution of Ni by improving the oxidation of Ni surfaces [70]. This catalyst showed comparable achievements with the monometallic platinum-based catalyst, illustrated in Figure 16.

Figure 16: Comparison of H2 and alkanes selectivities of Raney-NiSn (squares 480K, circles 283K) and Pt/Al2O3 catalyst (dashed lines) for APR of 1%wt oxygenated hydrocarbons [70].

Previous studies have assessed the addition of Co to a Ni catalyst. Luo et al. reported an improved hydrogen selectivity in the APR of glycerin. The interaction between these two metals with the addition of Ce reduced the sintering of the particles, decreasing catalyst deactivation, and reducing methane formation. Figure 17 presents the SEM images of this catalyst before and after the aqueous phase reforming of 5%wt glycerin. The sintering of the particles is more visible for the catalyst without the presence of Ce, demonstrating a reduction in the sintering phenomenon [71]. However, the reaction rates are still low for developing an economically viable process.

Figure 17: SEM images of fresh and spent catalyst[71].

In the case of Cu, as a promoter in Ni catalysts, showed activity and stability during the APR of glycerol with a reduction in methane formation using hydrotalcite-like compounds as support. It was observed that Cu promoted the WGS reaction towards hydrogen and carbon dioxide production. However, low

H2 selectivity was obtained due to the acidity of the support that increased the formation of intermediate liquid by-products [72]. Another approach tested to improve the activity of a Ni-based catalyst is the addition of a small amount of noble metal, Pt or Pd, displaying an enhancement in selectivity and hydrogen production. The APR of sorbitol using the Ni-Pt catalyst was 3 to 5 times higher in comparison with the monometallic catalyst, as can be seen in Figure 18. The observed increase in the number of active sites is related to the assisted reducibility of the Ni catalyst by the presence of the noble metal. Besides, an observable decrease in the absorption of CO was a positive effect to avoid the poisoning of active sites [73].

Figure 18. Comparison of bimetallic and monometallic catalysts during APR of 10%wt sorbitol at 200C and 20 bars. Rate of H2 formation (black) and gas-phase H2 selectivity (grey) [73].

These studies provide an insight into the synergistic effects that are present in bimetallic catalysts, presenting Ni-modified catalyst as a promising and cheaper alternative to improve the selectivity and stability of the catalyst to produce H2 from the aqueous phase reforming. Nonetheless, more considerable efforts are needed to overcome the barriers of catalyst deactivation, mass transfer limitations, and increase the selectivity towards H2 to further develop the application of aqueous phase reforming of oxygenated hydrocarbons.

Strategy development on catalyst design Catalysts are used to facilitate the formation of the desired product by generating energetically favorable reaction routes in a chemical process. The critical aspects of catalyst design are based on the development of active, stable and selective materials that withstand operating conditions [57]. In aqueous phase reforming (APR), the presence of water and the diversity of the feed introduce significant challenges to designing a selective and stable catalyst [58]. Thus, diverse approaches have been studied to be able to overcome these barriers. Selectivity challenges The high functionality of the biomass used as a feedstock on the APR process leads to the formation of non-desired products that reduce the overall yield and can affect the catalyst stability [3]. A purpose in catalyst development is to facilitate these transformations by being able to selectively break specific chemical bonds in the feed [60]. In APR conditions, selectivity challenges on the catalyst surface decrease the hydrogen production: consecutive reactions take place, such as methanation and Fisher- Tropsch, that produce methane and alkanes [59]. Consequently, the selectivity and activity of the catalyst play a crucial role in the APR process. The selectivity can be tuned to produce H2 or alkanes as a function of the active metal, the catalyst support, the nature of the feed and process conditions [68]. As Figure 19 shows, when C-C bond cleavage and WGS reaction occur faster than the rate of C-O cleavage and methanation/fisher-Tropsch reactions, the production of H2 will predominate in the process. Thus, the selection of metals that favors C-C cleavage over C-O cleave is preferred for APR applications. In general, metals from Group VIII present high activity towards C-C bond cleavage and have been widely tested for its use in the APR process, highlighting platinum as the most promising metal [59], [74].

Figure 19 H2 and alkane production by APR of glycerol [68].

Another variable considered that influences the selectivity of the catalyst is the nature of the support used; supports with high stability and high surface area are preferred for the APR process [61]. Various studies have assessed the influence of the acidity of the catalytic system over the selectivity in APR [75], [76]. Previous research has established that high selectivity for alkanes and faster deactivation is obtained using acidic supports (silica-alumina, TiO2). It appears that acidic supports facilitate dehydration reactions, leading to the formation of alcohols by hydrogenation and, subsequently, the production of alkanes. Thus, higher alkanes selectivity is obtained with the consumption of hydrogen [77]. Whereas, more basic-neutral supports favor the WGS reaction by facilitating the dissociation of water to provide hydroxyl groups on the surface of the catalyst, promoting the production of hydrogen [75].

Alumina is the predominant support studied in APR; however, it has been demonstrated that it loses its activity by going through a phase transition to boehmite under APR reaction conditions [78], [79].

Hydrothermal deactivation was observed in supports like CeO2, ZnO and SiO2 [77]. Currently, carbon- based supports have gained attention due to its hydrothermal stability and tunable chemical and surface properties, making them suitable for the APR process [80]. An example of the effect of the support on the APR of ethylene glycol (EG) can be seen in Figure 20, comparing an ordered mesoporous carbon (CMK-9), activated carbon (AC) and alumina performance.

Figure 20: Effect of the support on the APR of EG at 45.5 bar, over Pt-Fe (1:3) catalyst.[81]

An interesting approach to improve the catalytic properties of a catalyst is the addition of a second metal to form bimetallic systems. In APR, the synergistic effects of bi-metallics catalyst can enhance one desired outcome, such as C-C bond cleavage, and suppress the rate of C-O cleavage. Furthermore, it could introduce or improve a catalytic pathway; for example, the addition of Ni, Fe or Co to the metal catalyst in APR are tested to improve the WGS reaction rate [81]. As can be seen in Figure 21, the addition of Ni or Co to a platinum-based catalyst significantly increases the turnover frequency of

H2 production, maintaining a high H2 selectivity compared to the platinum-based catalyst [65].

Figure 21: APR of 10% wt EG at 483 K and 25.4 bar over Pt:Ni (A) and Pt:Co (B) bimetallic catalysts. TOF H2 (black) and alkane selectivity (grey). [65]

Stability challenges Moreover, bimetallic catalysts can be designed to address typical problems causing the deactivation of the catalyst, preventing coke deposition, active metal phase oxidation or sintering of the metal particles, taking advantage of the positive effects caused from the metal interactions [82]. At APR conditions, the presence of acid products may lower the pH in the liquid phase, increasing the risk of catalyst leaching by the oxidation of the metal, as it is observed in Ni-based catalysts [78]. The introduction of a second metal can increase the dispersion and interaction of the metals with the support, improving the thermal stability of the catalyst. This has been seen in the case of Pt added to a Ni-based catalyst. A reduction in the number of strong CO adsorption sites is observed due to the formation of Ni-Pt alloys, improving the catalyst stability [73]. This section has mentioned aspects considered in the selection of a catalyst for APR of oxygenated compounds. Bimetallic catalysts seem a promising alternative to develop an economical and viable process to upgrade biomass, producing valuable products at high yield. Currently, research on the APR process is directing its attention to improve catalyst activity by the implementation of bimetallic catalysts and modified supports, taking advantage of the positive effect of the distinctive interactions [59].

Water electrolysis Water electrolysis technologies Hydrogen production using water electrolysis is widely documented as the most promising strategy to obtain green energy. Electrochemical water splitting is a process that splits water into hydrogen and oxygen by the aid of electrical/thermal energy: 2H2O ↔2H2+ O2. The produced hydrogen is considered as a renewable source of energy if the water-splitting process takes place using “non- fossil” energy source such as solar, geothermal or wind. Since 2019, it was claimed that approximately 500 billion cubic meter per year (bm3/year) of hydrogen is produced worldwide, and only 4 % of the total hydrogen produced is obtained by water electrolysis due to the challenge of economical cost [83]. Clean and eco-friendly hydrogen without sulfur contaminant or carbon can be produced through water electrolysis. However, this method has some disadvantages, such as high economic aspects and higher energy requirements than fossil fuel [84]. Despite this, due to their compactness and flexibility for a small- and large-scale application, electrolysis remained to be cost-effective for hydrogen production. In general, there are four approaches to water electrolysis (i) proton exchange membrane water electrolysis [83] (ii) alkaline water electrolysis [85], (iii) solid oxide electrolysis [86] and (iv) microbial electrolysis cell [87]. In this section, an introduction to anion exchange membrane water electrolysis (AEMWEs) with some underlying reaction mechanism, and the common electrocatalyst (both noble and non-noble metal-based) used to facilitate the reaction is presented. Detail information on the solid oxide electrolyte, proton exchange membrane and microbial fuel cell can be found in the reference mentioned above.

Alkaline exchange membrane water electrolysis (AEMWEs) The concept of water electrolysis was introduced in 1789 by Troostwijk and Diemann, since then, alkaline water electrolysis (AWE) was evolved and was eventually commercialized worldwide for mass hydrogen production up to megawatt scale. AWE consists of two electrodes (anode and cathode) immersed in alkaline solution at a concentration of 20-30% KOH/NaOH. A diaphragm is employed to separate the two electrodes and prevent the produced gases (H2 and O2) from mixing and allows OH ions and water ion to pass through [88]. This technology faces some challenges such as low current density, limited partial load range, and low operating pressure. To elaborate these terms, firstly, the low partial load range is caused by the imperfection of the diaphragm that does not hinder the produced gases (hydrogen and oxygen) completely from crossing through it. The crossing of the gases through the diaphragm deteriorates the efficiency of the electrolysis. Since the diffusion of O2 to the cathode compartment will be mixed with H2, returning to the water. Also, hydrogen diffusion from cathode to the anode can happen, which causes a problem of safety and efficiency. These phenomena become so worse at low load (less than 40%) when the O2 production rate is declined, then the hydrogen content increases to an undesired and dangerous level. Secondly, due to ohmic loss resulted from the solution resistance, the usual current density achieved is low. Thirdly, due to the liquid electrolyte, the system cannot be operated at high pressure. Currently, there is a novel approach in the alkaline water electrolysis with the anion exchange membrane (AEM) instead of the diaphragm. The system is knowns as Anion exchange membrane water electrolysis (AEMWEs). The membrane act as a separator between the anode and cathode chambers and allows for the passage of hydroxide ion through it. This technology is interesting in the field of AWE that alleviates the problem associated with the diaphragm [83], [85]. A schematic flow diagram of AEMWE is shown in Figure 22, revealing all the components required to develop the system: Two electrodes (anode and cathode), AEM, power supply, and an alkaline electrolyte. Most of the time, KOH is commonly used in alkaline water electrolysis due to its’ low compared with acid electrolyte. Nickel is widely employed as electrode materials because of its good activity and abundance, and affordable cost. In the following section, the commercialized and ongoing electrocatalyst used as an electrode for water electrolysis is elaborated.

Figure 22: Schematic representation of alkaline exchange membrane water electrolysis process

Anion Exchange Membrane Water Electrolysis (AEMWE), employing a hydroxide conducting polymer membrane electrolyte, shows several advantages such as the use of non-noble metals as catalysts, low ohmic resistance, and good gas separation characteristics of membrane electrolyte. Two main reactions take place simultaneously in this system: Hydrogen Evolution Reaction (HER) and Oxygen Evolution Reaction (OER), as given by the following equations [89]–[91]. − − HER: 2퐻2푂 + 2푒 → 2푂퐻 + 퐻2

− − OER: 4푂퐻 → 2퐻2푂 + 4푒 + 푂2

Thermodynamically, 1.23 V of potential is required to split water to produce O2. However, in practice, a high potential is required to complete the reaction process, mainly caused by the sluggish kinetics of the OER found in the anodic compartment. Many researchers have devoted to enhancing the efficiency of water splitting by using electrocatalyst in the electrode. Noble metal oxide primary

RuO2 and IrO2 have shown promising results for OER. They are documented as a benchmark of electrocatalyst for OER due to their high performance and stability, but the high price and low availability hinder them for intensive scale application [92]–[94] . A substantial effort has been made in searching for electrocatalyst with low cost, high abundance, high catalytic activity, and high performance, and it was revealed that 3d transition metals are good candidates for OER electrocatalyst [95]–[97]. In general, an ideal electrocatalyst holds of the following fundamental characteristics (i) offers low overpotential for the desired reaction: HER and OER, (ii) A highly active surface which is essential to access reactants and to remove products, (iii)High electrical conductivity intended for fast electron transfer, (iv) Good chemical, electrochemical and mechanical stability for prolonged operation [2]. In the following section, the fundamental chemistry of oxygen evolution reaction in alkaline water electrolysis, and the noble and non-noble electrocatalyst used for OER are discussed.

Metallic electrocatalyst for oxygen evolution reaction (OER) Chemistry of OER The oxygen evolution reaction is a half-reaction of water electrolysis that occurs in the anode compartment. It is the slowest kinetic reaction in water electrolysis that limits the efficiency of the overall water splitting. This is due to the reaction mechanism of the anode part, which possesses a four-electron/proton-coupled reaction. Simply, the reaction can be expressed as follows: 4OH- ↔ - 2H2O + O2 + 4e . This reaction can be further elaborated into four consecutive steps as follows [7]:

4푂퐻− → 푂퐻∗ + 3푂퐻− + 푒− (1) ∗ − ∗ − − 푂퐻 + 3푂퐻 → 푂 + 2푂퐻 + 퐻2푂 + 푒 (2) ∗ − ∗ − − 푂 + 2푂퐻 + 퐻2푂 → 푂푂퐻 + 푂퐻 + 퐻2푂 + 푒 (3) ∗ − − 푂푂퐻 + 푂퐻 + 퐻2푂 → 푂2 + 2퐻2푂 + 푒 (4)

As it clear from the above mentioned four consecutive reaction steps, at least four-electron is needed to produce one mole of oxygen and, the numerous electron transfer at a time is kinetically sluggish, which made OER process slow. This is due to the energy barrier emanated from each step, which severs the reaction kinetics of OER, requiring massive overpotential to overcome the energy barriers [98]. Three intermediates (OH*, O*, and OOH*) of entities are evolved and the four steps are thermodynamically non-spontaneous, necessitating considerable energy to headway to the subsequent step. The step with a high greater energy barrier becomes the rate-limiting steps that ultimately control the efficiency of the process. Figure 23 depicts the schematic representation of the electron transfer process in OER and includes an additional O2 evolution pathway that can be evolved from the M-O, where M is the catalyst.

Figure 23: Alkaline OER mechanism. The green line indicates O2 formation possibility from M-O instead of M-OOH intermediate: M= Metal [98]

Both OER and HER are a vital reaction in water electrolysis, yet the slow reaction of OER due to the multi electron transfer precludes water electrolysis from the widespread application. Enormous electrocatalysts have been innovated to improve the performance for these two reactions, mainly Pt is used for HER [99], and precious metals of Ru and Ir [100], [101] their corresponding oxides have been utilized for OER. Though those mentioned metallic based electrocatalyst exhibited superb performance for water electrolysis, their high cost, limited natural abundance obstruct them for broad-scale usages. The Pt-based electrocatalyst is the well-known material for hydrogen evolution reaction due to its high catalytic activity. However, the low abundance and high cost of Pt hamper it for extensive large-scale applications. Nowadays, transition metal-based electrocatalysts such as Ni, Mo, Cu, Fe, Co, and their combinations have shown a profound catalytic activity for HER. In this recap, a brief introduction and elaboration about the noble metals used for OER and the recent advance research in non-noble bimetallic based electrocatalyst for OER are presented.

Noble metal for OER

Iridium oxide (IrO2) and ruthenium oxide (RuO2) electrocatalyst are considered as a benchmark for the OER process exhibiting high catalytic activity in both acidic and alkali media. These two metal oxides have a rutile structure in which the Ir and Ru metals are located in the center of the octahedral site with oxygen moieties in each corner, and each octahedron joins to each other by sharing the corners[100], [101]. It was reported that Ru based electrocatalyst is more stable than Ir based electrocatalyst during OER. For example, Serhiy and coworkers [102] demonstrated that Ru displayed high OER catalytic activity compared to Ir, while Ir based electrocatalyst exhibited better stability in both acid and alkaline media. The order of the catalytic activities of the was claimed to be as follows: Ru > Ir ≈ RuO2 > IrO2, whereas the dissolution of the noble metal in both acidic and alkaline media was described as IrO2 ≪ RuO2 < Ir ≪ Ru, indicating that IrO2 is more stable than

RuO2. The exact degradation mechanism of those precious metals is not yet disclosed. However, some 4+ researchers advocated that under high anodic potential (Ru ) O2 will change into the hydrous 8+ compound RuO2(OH)2 and deprotonate into a high (Ru ) O4, deteriorating its stability 6+ in the electrolyte, and also (Ir ) O3 can be formed from IrO2 under high anodic potential leading to dissolution in the electrolyte [101]. Lee and coworkers [90] demonstrated that both IrO2 and

RuO2 nanoparticles have high OER catalytic activities in acidic media than alkaline media. Moreover, at low overpotential, the OER activities of IrO2 are slightly lower than RuO2 in both electrolytes. –2 –2 Particularly, the intrinsic OER activities of RuO2 are 10 μAcm ox in acid electrolyte and ∼3 μA cm ox in –2 –2 the alkaline electrolyte and IrO2 nanoparticle displayed 4 μA cm ox in acid and ∼2 μA cm ox in alkaline media at an overpotential of 250 mV. The superior electrocatalyst activities of RuO2 for OER is attributed to the optimum bonding strength of the substrate (intermediate species) and catalyst during the electrochemical process from the perspective of thermodynamics over IrO2, which is consistent with the density functional theory (DFT) computational results reported by Rossmeisl and coworkers [103]. In general, researchers’ finding indicates that the overall OER performance of RuO2 is higher than IrO2 catalyst but less stable than that of IrO2. Though those precious metals are accredited as the benchmark electrocatalyst for water oxidation, their limited abundance precludes them for large scale production.

Non-noble metals for OER NiFe and CoFe electrocatalyst for OER NiFe-based materials have low cost and high activity in alkaline conditions. Even at an ultra-low Fe concentration (0.01%) decreases the OER overpotential and discharge capacity, this shows the high sensitivity nature of OER on Ni-based electrodes to Fe impurities. Also among non-noble metal electrocatalysts CoFe-based compounds, especially (oxy)hydroxides, have the most active OER electrocatalysts in alkaline electrolyte solutions, in competition with NiFe-based catalyst. conductive supports, suitable Co/Fe ratio, micro-/nanostructures, size, and crystal phase can influence the catalytic performance of CoFe-based compounds [104]. NiFe alloy One of the easiest methods of synthesizing NiFe-based electrocatalysts is to mix metallic Ni and Fe into NiFe alloy. By adjusting the electrolyte composition and electro-deposition parameter-such as current and time- we could be synthesized NiFe alloys with a variety of morphologies and catalytic properties. It is a good approach of mixing elements to study the synergistic effects but the physical mixture nature leads to large crystal size with low electrochemically accessible surface area and lack of chemical contact between them [105].

Figure 24: NiFe alloy in OER [106].

Cathodic electro-reduction of mixed metal salt solutions is an alternative way of synthesizing NiFe alloy. Suitable electrolytes for cathodic electro deposition of NiFe films are Ascorbic acid, boric acid and ammonium sulphate solutions containing iron and nickel sulphate. increasing Fe content by the boric acid approach could potentially improve the activity.

Also, electrodeposition synthesis of NiFe alloy is a popular approach for studying NiFe-based compound. However, the limitation of NiFe alloy lies in that structural transformation is needed before entering an OER active phase. This needs to NiFe deposits with the high surface area. The other problem is that it still remained unclear whether the oxidation process of NiFe film fully penetrated through the entire film or only on the surface. Therefore, NiFe alloy film could have optimal electrical contact with the underlying substrate but show moderate activity and low stability [107].

CoFe alloy Metallic CoFe alloy materials have high conductivity, but they often show low stability in acidic and alkaline media. Metallic CoFe nanoparticles are also not stable in the presence of oxygen. Their surface is often covered with a layer of oxides, especially when the size is small and/or iron content is high in the alloys. Thus it can be result that it is the oxidized surface that provides the active sites for OER and electron transfer between the substrate and surface OER active site via internal metal which provides high electronic conductivity[108].

NiFe oxide Transition metal oxides have shown high activity for OER in alkaline solutions. NiFe oxide often has oxidation states of +2 and +3 for Ni and Fe respectively, which are much closer to the oxidation states in OER overpotential region. NiFe oxide phase usually obtains from annealing at high temperature (except of amorphous NiFe oxide that preparing by thermal decomposition of the metal-organic compound at relatively lower temperatures) and therefore most NiFe oxides are crystalline with well- defined structure and they analyze with X-ray diffraction analysis. Owing to the highly corrosion- resistant nature (insoluble in most acids), nickel ferrite has attracted much attention as an OER electrocatalyst. NiFe mixed compound investigated in some studies, due to the heterogeneity of spinel structures, researchers have recently focused on substituting the NiFe2O4 by other cations (e.g. Cr, V, Mo and Co) for enhancing the electrocatalytic activity.

The important thing in NiFe-based OER electrocatalyst is the balance between crystallinity to obtain high durability with rigid structures under OER operation and amorphousness to obtain high activity with a large electrochemically active surface area [109].

CoFe oxide

CoFe-based oxides is one of the most active catalyst. In 1981 it was found that doping of Fe in Co3O4 improved the OER activity. There are various nanostructure to prepare CoFe-based oxides such as ultrathin nanosheets, nanoplates, porous structures, nanoarrays and nanofibers. Research shows that CoFe-based compounds likely indicate high OER activity, similar to NiFe-based materials, as you see in the plot (fig 2.) Co0.6Fe0.4Ox, Co0.8Fe0.2Ox, and Co0.5Fe0.4Ni0.1Ox are close to the top of the volcano and are the best catalysts[97].

Figure 25: Volcano plot of the intrinsic activities of transition metal (oxyhydro)oxides versus M-OH bond strength.[97].

The electrical conductivity of the CoFe-based oxide catalysts is another important factor that impacts the OER activity.

Another important factors that influence the catalysts is the Co/Fe atomic ratio. The CoxFe3-xO4 film at the Co/Fe ratio of ~1:4 indicated the highest catalytic activity. for ultrathin iron-cobalt oxide nanosheets that they prepared by a solution reduction method at the Co/Fe ratio of ~1:4 indicated the highest catalytic activity [110].

NiFe layered double hydroxide Layered double hydroxide (LDH) is a class of layered materials consisting of positively charged layers and charge-balancing anions in the interlayer region. It is highly accessible to the electrolyte by anion exchange. The positively charged layers are constructed by substituting divalent cations such as Ni2+, Mg2+, Ca2+, Mn2+, Co2+, Cu2+, and Zn2+ or monovalent cations by trivalent cations such as Al3+, Co3+, Fe3+, 3+ 2- and Cr . The intercalated anions are usually carbonate (CO3 ), but it could be easily replaced by other 3- 2- - - anions such as NO , SO4 , Cl , and Br . A common problem with hydroxides is its low conductivity[111]. Hybridizing with conductive material or directly growing LDH on conductive substrate can almost solve the problem. CoFe-based (oxy-)hydroxides During OER, the catalyst surface is covered with one or a few layers of OH groups in alkaline electrolyte, and the actual catalytic active species are the in situ formed (oxy-)hydroxides.

If CoFe(OH)x changes to ultrathin nanosheets by nitrogen plasma, shows higher activity. Also loading

CoFe(OH)x product on reduced graphene oxide will improve the catalytic performance. In addition, the introduction of intercalation ions in the LDH layer can enlarge the layer spacing, create more coordinatively unsaturated metal sites, and improve electronic conductivity. In addition, the introduction of intercalation ions in the LDH layer can enlarge the layer spacing, create more coordinatively unsaturated metal sites, and improve electronic conductivity. Also hybrid materials composed of Co and Fe-based hydroxides also show improved catalytic performance due to the synergistic effects between Co and Fe species [112].

CoFe-based phosphides, sulfides, selenides, and nitride The anions in the CoFe-based OER catalysts influence the electrocatalytic performance. Co-Fe sulfides, phosphides, selenides, and nitrides have better OER catalytic activity than the oxide and hydroxide counterparts because of their better conductivity. In these OER catalysts, the Co or Fe site acts as the catalytic active centers. The counter anions act as electron donors and manipulate the electron interactions with the cations. Thus the nature of the anions may influence the interaction and the reactivity of the active centers [113].

Advantages of bimetallic system in OER Bimetallic electrocatalyst exhibits superior electrochemical performance in OER compared with the unary metal oxide counterparts since the coupling of two metal revolutionizes the electronic structure, favoring for the adsorption of the intermediates on the surface of the catalyst hence improving the overall of catalytic performance [114]. This can be explained in simple terms; Figure 26 shows a scheme of multi-metallic catalyst in OER. As it is apparent from the figure, single metal requires a lot of potential/energy to obtain the desired current, but by coupling of two or more metals, the amount of potential applied to the system can be substantially reduced to reach the same current density. This is due to the synergetic effect of the two metals that made the reaction faster. Although prominent effort has been made to innovate and develop an efficient catalyst for OER, it is still unambiguously desired to develop new catalysts with low cost, superior performance, and stability for sustainable and widespread penetration of water electrolyser in the market.

Figure 26: Synergetic effect and strategies for the development of the high performance of multi-metal catalyst for OER [114]

Contributor Introduction Monica - Jonathan

Steam reforming Soroosh (SRM, Noble metals)

Trung (Ni-based catalysts, development strategy)

APR Jonathan (Process overview)

Trung (Noble metal catalysts)

Monica (Non-metal, development strategy)

Water electrolysis Abram (Water electrolysis technology)

Soroosh (Non-noble metal for OER)

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