From Waste to Hydrogen: Utilizing Waste as Feedstock or Catalysts for Hydrogen Generation

Abstract

With the world facing the twin pressures of a warming climate and an ever-increasing amount of waste, it is becoming increasingly clear that we need to rethink the way we generate energy and use materials. Despite growing awareness, our energy systems are still largely dependent on fossil fuels and characterized by a linear ‘take-make-dispose’ model. This leaves us vulnerable to supply disruptions, rising greenhouse gas emissions, and the depletion of critical raw materials. Hydrogen is emerging as a potential carbon-free energy vector that can overcome both challenges if it is produced sustainably from renewable sources. This study reviews hydrogen production from a circular economy perspective, considering industrial, agricultural, and municipal solid waste as a resource rather than a burden. The focus is on the reuse of waste as a catalyst or catalyst support for hydrogen production. Firstly, the role of hydrogen as a new energy carrier is explored along with possible routes of waste valorization in the process of hydrogen production. This is followed by an analysis of where and how catalysts from waste can be utilized within various hydrogen production processes, namely those based on using fossil fuels as a source, biomass as a source, and electrocatalytic applications.

1. Introduction

Currently, the world is increasingly concerned about matters such as greenhouse gas (GHG) emissions, changes in climate, energy security, and availability [1]. The continuous growth and rapid urbanization of the world’s population and economy have led to an ever-increasing demand for energy [2]. Fossil fuels are the most widely used energy source [3] and account for 80% of global energy demand in 2023 [4]. It is estimated that fossil fuels will remain the dominant source until 2050 [3]. The global energy share in 2023 is shown in Figure 1. As can be seen, the majority of energy is generated by fossil fuels such as oil (30%), coal (27%), and natural gas (23%). The share of renewable energies is 12% [4].

The reliance on fossil fuels in producing energy or chemicals results in the emission of GHGs like CO₂, nitrogen oxides, volatile organic compounds, and solid particles, which in turn drive global climate change [5]. One of the most controversial issues in the energy sector today is the amount of CO₂ emissions released into the atmosphere from the use of fossil fuels [6], which necessitates a transition to cleaner alternatives [2]. In 2023, CO₂ emissions reached a new record high of 37.4 Gt [7], with coal (45%), oil (33%), and natural gas (22%) being the main contributors [4]. Consequently, global decarbonization in the transport, industry, and power generation sectors is crucial to mitigate anthropogenic climate change [2]. The world is expected to gradually switch to renewable or alternative energies to reduce GHG emissions. The leading renewable energy sources to replace fossil fuels are solar, wind, hydro, geothermal, and biomass [8]. Power generation using wind and solar sources is marked by considerable intermittency [9]. This greatly affects the current power generation and transmission grid, which was not initially developed to manage variable energy inputs. To combat this problem, research is currently underway to find a suitable technology to store energy from intermittent sources [1]. One of the possible solutions is the storage of energy in the form of chemical carriers, of which hydrogen (H₂) is particularly attractive for large-scale grid storage [10]. In addition, H₂ can be used as an alternative fuel to fossil-based fuels because, unlike fossil-based fuels that release CO₂, it only releases water [11]. In this context, there has been a growing interest by scientists and industry in versatile production routes. Renewable energy sources for H₂ production are available in abundance [2]. According to the forecast of the International Renewable Energy Agency (IRENA), H₂ will be one of the few energy carriers that will play a decisive role in reducing CO₂ emissions [6].

Such a transition requires a large number of critical and precious metals that are essential for the development, deployment, and operation of modern energy technologies, but are at risk of supply disruption due to geological scarcity, geopolitical issues, or limited production capacity [12]. These elements are labeled as energy critical elements (ECE). ECEs consist of rare earth elements (REEs), platinum group metals (PGMs), elements specifically related to photovoltaic systems (Photovoltaic ECE), and other ECEs, as presented in Figure 2 [13]. As a result, global demand for ECEs is expected to rise sharply in the coming decades [14]. This raises the question of the availability of resources for the large-scale implementation of these technologies [12].

These materials will be essential in the coming decades for the transition from today’s carbon-based energy system to a zero-emission, decarbonized system that draws on many elements from the periodic table. Their indispensability arises from the unique properties of ECEs, which often make them irreplaceable in critical applications [15]. Each group of ECEs has its own importance in the transition from fossil fuels. PGMs are mostly used in electrocatalytic applications for fuel cells or electrodes in electrolysis, while Pt and Ru are also used as catalysts in multiple chemical reactions for cleaner fuels [13]. Rare earth elements (REEs) play a vital role in industry and are essential for the advancement of modern defense systems, green technologies, and electronic devices. Their importance is illustrated by REE-based alloys and permanent magnets, which are fundamental components in renewable energy technologies such as electric vehicles, energy-efficient lighting, and wind turbines. [16]. Photovoltaic ECEs are tied closely to semiconductor technology and are also indispensable in solar cell technology [17]. Other ECEs have a variety of uses. Li is essential in battery technology, which is necessary in the transition from fossil fuels [14]. Co is also important in battery technology, along with being utilized as a catalyst [17]. Ag is an important conductor, Re is used in superalloys for ultra-efficient gas-turbine blades, and, lastly, He is used for superconducting magnets. Out of the ECEs, Pt and Li are the most crucial elements for a transition into H₂ technology [13].

Due to their importance for the transition to a sustainable technology system, many of these elements are classified as critical raw materials (CRM) in the EU [17]. The ECEs based on the CRM list for 2020 are shown in Figure 3.

The current and traditional linear model of material and energy flow of the modern economic system—extract–produce–use–discard—is not sustainable [19]. Inefficient recycling leads to significant amounts of CRMs being discarded in landfills, demonstrating resource misuse. Globally, lithium consumption is anticipated to rise by 18 times and platinum by 30 times from 2020 to 2030 [15]. Initially, the use of these materials can be reduced by improving the design of the products that contain them, making them more robust and durable [20]. Another method is to design products in such a way that they have higher material efficiency, and CRMs are used more wisely to reduce overall demand. This can be achieved, for example, by reducing the precious metal load of the catalysts. An example of doing so is in electrolysis, where single-atom catalysts, nanoparticles, or extended surface structures could be used to reduce the amount of Ir and Pt required [15]. Finally, CRMs can be replaced by materials that are abundant on Earth [20]. For instance, MoS₂ is considered a possible and cost-effective replacement for Pt in the catalysis of H₂ production from water [21]. Finally, more effective collection and recycling strategies are needed [20].

With the quality and quantity of finite resources progressively falling [22], along with an ever-increasing production of municipal solid waste (MSW) [23], a circular economy (CE) is an inevitable path to address the mentioned trends. It provides the economic system with an alternative flow model, one that is cyclical [19]. This concept describes an economic framework built on business models that eliminate the idea of ‘end-of-life’ by emphasizing material reduction, reuse, recycling, and recovery within production, distribution, and consumption. It operates on three scales: micro (products, enterprises, consumers), meso (eco-industrial parks), and macro (urban, regional, national, and global), targeting sustainable development that ensures environmental integrity, economic well-being, and social justice for current and future generations [24]. The CE model seeks to redefine progress, focusing on constructive society-wide advantages. It entails gradually decoupling economic activity from consuming finite resources and designing waste out of the system. Underpinned by a transition to renewable energy sources, the circular model builds economic, natural, and social capital [25]. Its main objective is to restore and regenerate material cycles, i.e., to maintain the value of materials at each point in a product’s life [26].

At the current rate of waste generation, global MSW generation is expected to increase by 70% compared to 2016 to 2.59 Bt in 2030 and 3.4 Bt in 2050 [27]. This worrying trend calls for innovative solutions for waste management in the context of a CE [28]. In terms of its physical form, waste can be divided into solid, liquid, and gaseous waste. According to the United Nations Environment Program (UNEP), solid waste contributes to about 5% of greenhouse gas emissions, especially CO₂ and CH₄ [29]. Currently, the majority of MSW is generated in industrialized countries. The energy contained in the MSW can be recovered through so-called waste-to-energy (W-t-E) technologies, which can produce usable energy in the form of electricity, heat, and fuels [27]. The utilization of waste as an energy source or by-product could help to reduce waste disposal problems in the future [30]. There are two main recovery or conversion processes for W-t-E technologies (i.e., biochemical and thermochemical), which depend on the composition and moisture content of the waste [31]. Thermochemical methods for waste transformation usually include biomass gasification [32], pyrolysis [33], and hydrothermal liquefaction [34]. However, biochemical methods of waste transformation consist of dark fermentation [35], photo fermentation [36], and microbial electrolysis cell [37]. The type of waste used as feedstock and the specific W-t-E process applied determine the form of energy produced. Typically, waste with high moisture levels—such as industrial wastewater, animal manure, or sewage sludge—is treated through anaerobic digestion to generate biogas. In contrast, solid waste streams, including MSW and lignocellulosic residues, are mainly utilized to produce thermal energy or converted into intermediate products like syngas and H₂ [30]. With solid waste containing about 40–60% organic matter, it represents a valuable resource for energy recovery via appropriate W-t-E technologies. Despite the progress and maturity of these technologies, challenges persist due to waste heterogeneity, the complexity of treatment systems, and the issue of air pollutant emissions [38]. Another possibility of waste utilization is to convert waste into catalysts. A plethora of wastes have shown catalytic activity, from waste generated by large-scale industrial processes to wastes derived from biological sources (biomass) [39]. The waste-derived catalyst could be used in the production of biodiesel, bioethanol, syngas, bio-oil, H₂, and energy [40]. The possible routes of waste utilization towards H₂ are shown in Figure 4.

Extensive research has already explored H₂ production methods from diverse resources [1,2,3], including fossil fuels [6], biomass [23,27,28], and water electrolysis [41,42]. However, to date, there has been no consolidated review that specifically addresses the role of waste-derived catalysts in various H₂ production processes. This review aims to fill this research gap by systematically evaluating the application of waste catalysts across various H₂ production methods. The article structure is as follows: firstly, H₂ is outlined as an energy carrier and its importance in the industrial sector, along with the most common production methods. The review then continues onto waste valorization for H₂ production. Three possible routes are explored as depicted in Figure 4. The routes explored are thermochemical and biochemical conversion methods, followed by the utilization of waste as a catalyst for H₂ production, as the main topic of this review. Waste-derived catalysts are organized into chapters by their difference in feedstocks utilized: (i) fossil fuel-based processes; (ii) biomass-based processes; and (iii) electrochemical H₂ production. In each chapter, we first provide a brief introduction and overview of the relevant technologies. Then, the benchmark waste-derived catalysts against commercial or non-waste catalyst counterparts using process-appropriate metrics are discussed. The review concludes with future prospects and research directions for deploying waste-derived catalysts for H₂ production.

2. Hydrogen as an Energy Carrier

H₂ is the most abundant element in the universe. Due to its reactivity, it only occurs on Earth in compounds such as water and organic materials [2]. The physical and molecular properties of H₂ differ considerably from those of conventional fossil fuels [43]. When used in a fuel cell, H₂ can convert chemical energy directly into electricity, bypassing the inefficiencies of conventional combustion processes [44]. The heating value of H₂ is higher than that of traditional hydrocarbon-based fuels. However, due to its lower density, it requires a larger storage capacity or high compression for equivalent energy storage [2]. A comparison of important parameters for fuels between H₂, hydrocarbon-based fuels, and alternative fuels such as ammonia and methanol is shown in

Table 1. Hydrogen properties compared to other fuels [8,9,43,45,46,47,48].

Parameter	Hydrogen	Methanol	Ammonia	Gasoline	Methane	Diesel
Higher heating value (MJ/kg)	142	22.7	22.5	48.29	55	44.8
Lower heating value (MJ/kg)	119.9	18.0	18.8	43.9	50	42.5
Density at STP (kg/m3)	0.089	790	0.730	730–780	0.720	830.0
Auto-ignition temperature (K)	853	733	931	623	813	523
Flammability limits in air (%)	4–76	6.7–36	15–28	1–7.6	5.3–15	0.6–5.5
Flame temperature (K)	2480	2143	1850	2580	2187	2600

.

Everything indicates that the H₂ industry is set for significant development in the coming decades. However, there are many uncertainties and risks associated with the use of H₂ as part of future decarbonization strategies [6]. One such risk is the explosive nature of H₂. Since H₂ is an odorless, colorless, and flammable gas, there is a risk of an undetected leak that could lead to an explosion in an enclosed space [2]. The transportation and storage of H₂ could also pose a problem [6]. The biggest challenge for the transportation and storage of H₂ is its low volume density and the possibility of H₂-induced rupture in pipelines and storage facilities [49]. H₂ can be stored as a gas under high pressure [50], as a cryogenic liquid [51], as metal hydrides [52], and in the form of other chemicals such as ammonia [53] and carbon-based fuels [54]. A promising option for H₂ transportation is via the existing natural gas pipeline infrastructure, which also has the added benefit of reducing CO₂ emissions [8].

H₂ is also an important raw material for various industries [29]. It is used for hydrotreating, hydrocracking, and hydro conversion of crude oil and as a direct participant in the production cycles of synthetic and rocket fuel [6]. The chemical industry is highly dependent on H₂ [10]. It is used in the metallurgical sector [55] and in the production of methanol [56] and plays a crucial role in the fertilizer industry in the production of ammonia using the Haber–Bosch process [57]. Global H₂ production for 2023 reached 97 Mt, with use mainly concentrated in industry and the refinery sector. The industrial sector accounted for 54 Mt (56%) of total demand. Within this sector, ammonia production consumed around 60%, while methanol production and the production of directly reduced iron steelmaking consumed 30 and 10% respectively. The refinery sector consumed 43 Mt (44%) of H₂, mainly for desulphurization and oil refining. A small amount of H₂ was also used in the transport sector (0.06 Mt) and in electricity generation [58]. In the energy sector, H₂ is suitable for both stationary and mobile applications. H₂ can also be used to generate electricity by burning individual fuels and co-combustion (mixing) with other fuels and fuel cells [29].

H₂ can be produced from various resources, including both renewable and non-renewable ones [6]. The most important resources for H₂ production are currently fossil fuels [59], which account for 96% of all H₂ produced [60]. Natural gas is the most commonly used source of H₂, accounting for 47% of global production. The other two most commonly used fossil fuels are coal (27%) and oil (22%). The remaining 4% of H₂ production is provided by water electrolysis [58]. The most common and widely used methods for H₂ production are steam methane reforming (SMR) and autothermal reforming (ATR) of methane. Other production methods from fossil fuels are partial oxidation of methane (POM), dry methane reforming (DMR), coal gasification (CG), and pyrolysis (cracking) of methane and oil [8]. Consequently, the co-production of CO₂ is unavoidable, and about 830 Mt of CO₂ per year is produced by these methods [59]. Alternatively, H₂ can be produced by water electrolysis, thermal, and biochemical conversion of biomass [60].

3. Waste Valorization in the Process of Hydrogen Production

There is a wide range of waste that is suitable for H₂ production [5]. Pre-treatment steps like drying and impurity removal are crucial for enhancing energy density and regulating the physical and chemical characteristics of waste prior to further processing [60]. First, the biomass is converted into an intermediate H₂ substrate, and then H₂ production is achieved by catalytic conversion of the substrates [59]. The methods for converting waste into H₂ are usually divided into thermochemical and biochemical methods [29]. These methods differ in terms of energy requirements, operating conditions, use of raw materials, efficiency, reaction time, and final yield [60]. H₂ production from biomass and residual waste is technically and economically feasible, given the existing technical and economic conditions in many industrialized countries [5]. The utilization of biomass encounters problems such as seasonal availability and varying composition from one source to another. Although biomass composition may vary, it is composed of three components: hemicellulose, cellulose, and lignin. The proportions of these components set conversion behavior and product yields during processing [61]. The biggest contributor of H₂ among the components is lignin, while from cellulose and hemicellulose, more CH₄ and CO are derived [62]. As mentioned in the introduction, waste can also be utilized in the form of catalysts for the production of H₂. This chapter provides a short overview of said pathways of waste utilization in said concept of H₂ production.

3.1. Thermochemical Methods

Thermochemical processes are among the more effective methods for producing H₂-rich gases from biomass [5]. They have the potential to decompose industrial and agricultural waste [63]. They are essential for a sustainable, integrated MSW management system [60] and are considered low-carbon methods of H₂ production [6]. A distinct advantage of thermochemical processes is their ability to process a wider range of feedstocks [29]. Thermochemical processes are faster than biochemical processes, have higher stoichiometric H₂ yields, higher conversion efficiencies, and shorter reaction times [60]. When subjected to gasification or pyrolysis, the thermochemical conversion of dried biomass resembles that of fossil fuels. These processes yield CO and CH₄, which can be further transformed to enhance H₂ production through steam reforming and water–gas shift reactions [5]. The quality of the H₂ depends on the composition of the feedstock, the water content, the temperature, the heating rate, the type of gasifier, and the oxidation of the pyrolysis products [63]. Thermochemical processes offer clear benefits for waste management, achieving a 70–80% reduction in mass and an 80–90% decrease in volume, thereby conserving landfill capacity [60].

3.2. Biochemical Methods

Biochemical processes use microorganisms to decompose waste products containing organic material, biomass, or wastewater as substrates to produce H₂ [60]. Due to the simple operating conditions, uniform H₂ yield, and low energy consumption, this is an environmentally friendly method for producing H₂ from various organic wastes [64]. The main mechanism in the biochemical production of H₂ is the microbiological conversion of organic substrates and water into H₂-containing gas by the action of the enzyme hydrogenase or nitrogenase [6]. Numerous bacteria colonies or consortia are capable of degrading various organic substances to produce H₂ as their metabolic product [31]. Most biochemical processes are less energy-intensive than thermochemical processes, as they take place at ambient temperatures and pressures, but have a low H₂ yield and low reaction rates [60]. Biological methods offer a way to produce H₂ through fermentation, microbial electrolysis [29], and hybrid systems [60]. Based on the light requirement during the fermentation process, the process can be divided into dark fermentation (light-independent) and photo fermentation (light-dependent) [29]. Fermentative methods of H₂ production are considered particularly advantageous as they can acclimatize to various organic substrates [31].

3.3. Waste as a Catalyst

The field of catalysis has recently experienced renewed interest in the development of catalytic materials derived from abundant and low-cost elements. Although catalysts are required in small quantities, they can still pose a significant expense. By generating catalysts from waste materials alongside the target product, the overall system becomes more cost-effective and environmentally sustainable [65], particularly in those instances where the waste finds no additional use. Recently, an increased interest has been in the utilization of waste material as a resource for the application and preparation of heterogeneous catalysts. Waste-derived catalysts fall into three main categories. The first involves the direct use of waste as a catalyst or support without any pre-treatment. The second category includes waste that undergoes pre-treatment, such as calcination or chemical activation, before being used. The third approach utilizes waste as a chemical source, extracting specific components for the synthesis of catalysts [66].

Industrial and agricultural residues are increasingly being re-imagined as low-cost catalysts rather than landfill liabilities. A prime example is bauxite residue, or red mud (RM), which is generated during the Bayer process [67], at an estimated 150 Mt per year and typically stored in lagoons [66]. It is an alkaline waste material rich in ferric oxide (Fe₂O₃), aluminum oxide (Al₂O₃), titanium oxide (TiO₂), silica (SiO₂), calcium oxide (CaO), and sodium oxide (Na₂O). Among these, the high Fe₂O₃ content makes RM particularly suitable for catalytic applications, as iron-based catalysts are commonly used in industrial chemical reactions. Additionally, it possesses a relatively large surface area, thermal stability, and resistance to sintering, which prevents degradation at high temperatures [67]. Another potential waste that could be used for catalysts is steel slag (SS), a by-product of the steel industry, which has shown significant potential as a catalytic material due to its rich composition of metal oxides such as CaO, magnesium oxide (MgO), SiO₂, iron oxides (Fe₂O₃, FeO), and Al₂O₃. Through various modification processes, it can be transformed into an efficient catalyst for applications such as catalytic pyrolysis and electrocatalysis [68]. Another industrial waste fit to be used as a catalyst is fly ash (FA) as well as bottom ash [66]. FA, a residue of combustion, is composed of fine particles whose makeup reflects both the inorganic constituents of the burned fuel and the technology used in the process [39]. It is predominantly a SiO₂-Al₂O₃ material that also contains other components such as Fe₂O₃, CaO, and MgO [66]. Due to its alumina and silica content, numerous studies have examined the use of coal FA as a support [39].

Biomass sources can also be utilized for catalyst synthesis. Eggshell waste is being widely recognized as a valuable resource for catalytic applications, particularly in energy-related processes such as H₂ production, syngas generation, and biodiesel synthesis. Eggshells are composed of approximately 96% calcium carbonate (CaCO₃), 1% magnesium carbonate (MgCO₃), 1% calcium phosphate (Ca₃(PO₄)₂), and a small fraction of organic material and water. When subjected to calcination at temperatures between 800 °C and 1000 °C, CaCO₃ decomposes into CaO, which exhibits strong basicity, high surface area, and excellent reactivity [69]. Various types of biomass can also be transformed into nanocomposites that can be used for H₂ production due to their unique physicochemical properties, sustainability, and cost-effectiveness [70]. These catalysts are produced through processes such as pyrolysis, electrochemical treatments, and hydrothermal carbonization. Their ability to be modified with metals or heteroatoms further enhances their performance [71], enabling their use in energy conversion and storage technologies such as water electrolyzers and fuel cells [72]. They can be utilized for H₂ production via electrochemical water splitting and thermochemical processes [73]. These materials make excellent candidates for electrocatalytic applications, particularly in hydrogen evolution reactions (HER) [74]. A visual representation of which waste can be utilized for which process for H₂ production is shown in Figure 5.

4. Waste-Derived Catalysts for Fossil Fuel Processes

Regarding waste-derived catalysts for H₂ production via fossil-based fuels, two distinct feedstocks were utilized: methane and coal. With methane as the feedstock, several different methods were employed: dry reforming, steam reforming, pyrolysis, and partial oxidation. When using coal, steam gasification was the preferred method.

4.1. Waste-Derived Catalysts in Dry Methane Reforming

DMR is a hot research topic as it is a possible pathway for producing clean H₂ via the conversion of GHG [75]. DMR is a chemical conversion process via which CO₂ and CH₄ are simultaneously converted, using Ni-doped active materials to generate syngas [76], usually at temperatures above 600 °C [77]. The DMR reaction is shown in Equation (1) [78]:

$${\mathrm{C}\mathrm{H}}_4+{\mathrm{C}\mathrm{O}}_2\leftrightarrow 2{\mathrm{H}}_2+2\mathrm{C}\mathrm{O};∆{\mathrm{H}°}_{\mathrm{r}}=247\mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}$$

(1)

As a slow process, the DMR reaction requires significant energy input to dissociate the stable CO₂ and CH₄ molecules, thereby achieving equilibrium and producing syngas [78]. Although noble metals achieve high catalytic activity, their use is limited due to high cost. Non-noble metals like Co and Ni offer a cheaper alternative while also achieving good catalytic performance [77].

DMR has been extensively studied using waste-derived catalysts. For example, Kanamori et al. [79] used Ni-containing battery waste to produce NiO, CeO₂, and LaCoO₃ catalysts. Out of the catalysts, the NiO catalyst achieved the highest CH₄ conversion of 100% and maintained stable performance for 9 days, achieving a H₂/CO ratio of slightly above 1, outperforming the CeO₂ and LaCoO₃ catalysts, which achieved a CH₄ conversion of 45–80% and 28–62%, respectively. Huang et al. [80] also reported that adding basic oxides to waste catalysts improves performance: they modified FA with MgO and Ni. The catalyst with 20% loading achieved the highest methane conversion of 75–80%, reaching a H₂:CO ratio slightly below 1. Likewise, Gao et al. [81] used coal fly ash (CFA) as Ni support. The Ni-CFA catalyst achieved a 96% CH₄ and 97.2% CO₂ conversion at 850 °C. The catalyst also maintained stability with minimal carbon formation for 12 h, while achieving a H₂:CO ratio of slightly below 1. Chen et al. [82] demonstrated that photovoltaic waste (SiCl₄) could be converted to SiO₂ as a support for Ni. The Ni/SiO₂ waste-derived catalyst achieved a CH₄ conversion of 92.3% at 850 °C; in addition, the high conversion also maintained stability for 40 h on stream and a H₂:CO ratio of 0.95. It also outperformed the Ni catalyst on commercial SiO₂. In another study, Dega et al. [83] investigated autothermal dry reforming using a Ni-based spinel catalyst derived from SS. The slag was transformed into a multi-oxide Ni-Mg-Fe-Al spinel. The catalyst achieved a 95% CH₄ conversion while also achieving a H₂:CO ratio of 1.62. The catalyst also showed no coke formation over 48 h on stream. The process parameters and performance metrics of waste-derived catalysts for the DMR reaction are shown in

Table 2. Comparisons of parameters of waste-derived catalysts in the process of DMR.

Catalyst	Preparation Method	Process Parameters	Substrate Conversion	H2:CO Ratio	Reference
Non-waste 12% Ni-Al2O3	Wet impregnation Calcination	850 °C CH4:CO2 = 1 10 h	94%	1.2	[84]
NiO	Acid/Base treatment Precipitation Calcination	780 °C CH4:CO2 = 1 24 h	100%	Slightly over 1	[79]
20% Ni/MgO-FA	Alkali treatment Sol-gel synthesis	750 °C CH4:CO2 = 1 9 h	75–80%	Slightly below 1	[80]
10% Ni-CFA	Alkali/acid treatment Wet impregnation Calcination	850 °C CH4:CO2 = 1 1 h	96%	Slightly below 1	[81]
10% Ni-SiO2	Wet impregnation Calcination	800 °C CH4:CO2 = 1 1 h	92.3%	0.95	[82]
13% Ni-SS oxide	Ni doping Calcination	850 °C CH4:CO2 = 3 24 h	95%	1.62	[83]

, along with a non-waste Ni-based catalyst developed by Manabayeva et al. [84] as a benchmark.

As a benchmark, the non-waste catalyst reported by Manabayeva et al. [84] achieved a 94% CH₄ conversion at 850 °C with a H₂:CO ratio of 1.2. Compared to the benchmark, several waste-derived catalysts achieved similar, if not better, performance. The NiO catalyst developed by Kanamori et al. [79] achieved a 100% conversion at a lower temperature of 780 °C. The 10% Ni-CFA catalyst developed by Gao et al. [81] and the 13%Ni-SS catalyst developed by Chen et al. [82] also outperformed the non-waste catalysts in terms of CH₄ conversion at the same temperature, achieving 96% and 96% CH₄ conversion, respectively. The waste-derived catalysts mostly achieved a H₂:CO ratio of around 1, which is common for the process of DMR. The exception is the 13%Ni-SS catalyst developed by Chen et al. [82], which achieved a H₂:CO ratio of 1.62, which can be explained by the increase in the feed ratio of CH₄:CO.

4.2. Waste-Derived Catalysts in Other Processes Utilizing Methane

SMR is the most widely used hydrocarbon-based reforming process [5]. It is the most common method used for H₂ production [3], accounting for more than half of the world’s production [85]. Its attractiveness is attributed to its large-scale capabilities and relatively inexpensive production costs [86]. The reaction is highly endothermic, and a large amount of heat is required [85]; hence, the reactor operates at a temperature of 700–1000 °C [87]. The SMR reaction is shown in Equation (2) [88]:

$${\mathrm{C}\mathrm{H}}_4+{\mathrm{H}}_2\mathrm{O}\leftrightarrow \mathrm{C}\mathrm{O}+3{\mathrm{H}}_2;∆{\mathrm{H}°}_{\mathrm{r}}=206\mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}$$

(2)

Group 8 metals, typically Ni, serve as the active catalysts for the SMR owing to their high availability and low cost [85]. Among them, Ni/Al₂O₃ is the most widely employed [89]. An important parameter of the SMR reaction is the S/M (steam to methane) ratio, which positively influences the activity and selectivity of the process [85]. Usually, a ratio of 2.5–3 is employed [88]. In the process of SMR, multiple waste-derived catalysts have been explored. For instance, Ayesha et al. [90] synthesized a hydrotalcite-like Mg-Ni-Al catalyst promoted with CaO from waste eggshells (WE). The optimized 10% CaO catalyst exhibited a 77% CH₄ conversion at 650 °C. The catalyst also had the added benefit of reducing carbon emissions, while stability tests over three cycles confirmed negligible carbon deposition and strong regenerability. In another study, Minhas et al. [91] investigated hemp-derived activated carbon (AC) as a catalyst support for Ni- and Co-based catalysts. The catalysts were loaded with 5% Ni, Co, and NiCo. At 750 °C the Co-based catalyst achieved the highest CH₄ conversion (97.2%), outperforming the Ni-based (90.3%) and NiCo-based (92.5%) catalysts. The Co-based catalyst also showed superior stability over 44 h, maintaining an average of 93.2% CH₄ conversion. As the benchmark, the non-waste-derived NiO/γ-Al₂O₃ catalysts developed by De Freitas Silva et al. [92], which achieved a 78% CH₄ conversion at 700 °C, along with a H₂:CO ratio of 6.8, was chosen. The catalyst by Ayesha et al. [90] achieved a similar CH₄ conversion at lower temperatures, while the Co-AC catalysts developed by Minhas et al. [91] achieved a much higher CH₄ conversion of 97.7%, albeit at a higher temperature. When comparing the H₂:CO ratio achieved the non-waste catalyst achieved the highest ratio of 6.8, which is most likely due to the higher S/M ratio of the feed.

Partial oxidation serves as an alternative to steam reforming [93]. This process can operate with various feedstocks ranging from CH₄ to heavy fuel oil and coal [5]. Due to the mild exothermicity of the partial oxidation, this reaction can be conducted autothermally [93], and usually in the temperature range of 800–900 °C [94]. POM can be described by Equation (3) [95]:

$${\mathrm{C}\mathrm{H}}_4+0.5{\mathrm{O}}_2\leftrightarrow \mathrm{C}\mathrm{O}+2{\mathrm{H}}_2;∆{\mathrm{H}°}_{\mathrm{r}}=-36\mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}$$

(3)

To achieve a significant degree of methane conversion (more than 90%), a mixture of methane and air undergoes a heat treatment in the presence of supported metal catalysts made of transition metals (Co, Ni) [6], or noble metals (Pt, Rh, Ir, Pd) [93]. Hasanat et al. [96] studied POM using a biomass FA-supported catalyst. The study conducted research on the catalytic ability of biomass FA, as a stand-alone catalyst, as well as different dopings of Ni/La₂O₃. The best performing catalyst (5% Ni/La₂O₃) achieved a CH₄ conversion of 87%, while maintaining stability for over 30 h. On the other hand, the stand-alone biomass FA achieved a methane conversion of 55%. For the benchmark catalyst, the non-waste-derived Ni/Al₂O₃ developed by Vermeiren et al. [97] was used. The benchmark catalysts achieved a CH₄ conversion of 95.7% and a H₂:CO ratio of 2 at 780 °C. When comparing the waste-derived catalyst to the benchmark one, it can be seen that the waste-derived catalyst developed by Hasanat et al. [96] fell short in terms of CH₄ conversion but achieved the same H₂:CO ratio at a higher temperature.

An additional route is pyrolysis, a process of thermal breakdown without O₂ that converts various light hydrocarbons into H₂ and solid carbon [98]. The main advantage of this process is the generation of H₂ without a CO₂ by-product [99]. The reaction of methane pyrolysis (MP) is shown in Equation (4) [100]:

$${\mathrm{C}\mathrm{H}}_4\leftrightarrow \mathrm{C}+{\mathrm{H}}_2;∆{\mathrm{H}°}_{\mathrm{r}}=74.9\mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}$$

(4)

Solid carbon is the by-product of the methane pyrolysis (MP) reaction and is easy to store [101], and can be used as a raw material in various valuable materials (rubbers, tires, pigments, etc.) [102]. To achieve a high H₂ yield, pyrolysis requires a temperature above 1000 °C. This temperature can be lowered with the addition of a catalyst [101]. For MP, catalysts are generally classified into two groups: metal-based and carbon-based [103]. While the former enables lower-temperature operation but deactivates rapidly, the latter maintains activity for extended periods, albeit at elevated temperatures [98]. Transition metals like Ni, Fe, and Co have been widely studied as active species for MP due to their high activity and moderate operating temperature [104]. Raza et al. [105] investigated the catalytic pyrolysis of methane using biomass FA as a catalyst support. In the study, the performance of only biomass FA, as well as different Co and Co/CeO₂ dopings, was studied. The best performing catalyst was the 10% Co/CeO₂-doped biomass FA, achieving a conversion of 76%. While biomass FA as a stand-alone catalyst achieved only 36% CH₄ conversion. The benchmark catalyst for the MP process used was a FeCo/CeZrO₂ non-waste catalyst developed by Ramasubramanian et al. [106]; the catalyst achieved a CH₄ conversion of 90%. When compared to the benchmark, the waste-derived catalysts by Raza et al. [105] achieved a lower CH₄ conversion at a higher temperature.

The parameters for SMR, MP, and POM technologies via waste-derived catalysts, along with the parameters for the benchmark catalysts, are presented in

Table 3. Comparisons of parameters of waste-derived catalysts for processes of methane conversion.

Process	Catalyst	Preparation Method	Process Parameters	Substrate Conversion	H2:CO Ratio	Reference
SMR	Non-waste NiO/γ-Al2O3	Wet impregnation Calcination	700 °C S/C = 4	78%	6.8	[92]
Sorption-enhanced SMR	10% CaO-Mg/Ni/Al	Co-precipitation Wet impregnation Calcination	650 °C S/C = 2	77%	/	[90]
SMR	5% Co-AC	Wet impregnation Calcination	750 °C S/C = 2	97.7%	2.7	[91]
POM	Non-waste 5% Ni/Al2O3	Wet impregnation Calcination	780 °C CH4:O2 = 2	95.7%	2	[97]
POM	5% Ni/La2O3-FA	Wet impregnation Calcination	850 °C CH4:O2 = 2	85%	2	[96]
MP	Non-waste FeCo/CeZrO2	Wet impregnation Calcination	700 °C	90%	/	[106]
MP	10% Co/CeO2-FA	Hydrothermal processes Wet impregnation Calcination	850 °C	76%	/	[105]

.

4.3. Waste-Derived Catalysts in Coal Gasification

The composition of coal includes various phases such as volatile matter, char, ash, and moisture [107]. In CG, coal undergoes thermochemical transformation to yield gaseous products, including CO and H₂ [5], also defined as an incomplete combustion of coal [108]. It includes the processing of raw materials with air and steam in several stages at high temperatures [109]. Fan et al. conducted two complementary studies on using WE-derived CaO as a catalyst or catalyst support for H₂ production via CG using a fixed-bed reactor setting in both. In the first study [110], the CaO from WE was used as a stand-alone catalyst, also acting as a CO₂ sorbent. The addition of CaO (20%) at 900 °C almost doubled the H₂ yield to 1.24 mol H₂/mol C from 0.69 mol H₂/mol C from gasification without a catalyst. The process achieved a H₂:CO ratio of 3.65. In the second study [111], Fan et al. prepared a composite catalyst, combining CaO with K₂CO₃. The optimal mixture of 15% K₂CO₃ + 5% CaO gave the highest H₂ yield of 1.34 mol H₂/mol C at 800 °C, improving the yield given by only CaO as a catalyst, while also achieving a higher H₂:CO ratio of 4.19 at lower temperatures. Zhao et al. [112] also explored the usage of a CaO-rich catalyst for coal gasification. They used construction demolition waste as a source of Ca. The demolition waste-based catalyst achieved a H₂ yield of 1.64 mol H₂/mol C at 9% loading and a reaction temperature of 900 °C, while also achieving a H₂:CO ratio of 4.25. As the benchmark, the non-waste-derived K₂CO₃ + CaO catalysts developed by Zhou et al. [113] were chosen. The parameters and performance indicators of the benchmark and waste-derived catalysts are shown in

Table 4. Comparisons of parameters of waste-derived catalysts for processes of coal gasification.

Catalyst	Preparation Method	Process Parameters	Hydrogen Yield	H2:CO Ratio	Reference
Non-waste K2CO3 + CaO	Mechanical mixing	700 °C	≈1.53–1.58 mol H2/mol C	Not reported	[113]
20% WE CaO	Calcination	900 °C	1.24 mol H2/mol C	3.65	[110]
5% WE CaO + 15% K2CO3	Calcination	800 °C	1.34 mol H2/mol C	4.19	[111]
CaO-rich catalyst from demolition waste	Calcination	900 °C	1.64 mol H2/mol C	4.25	[112]

.

The benchmark catalysts developed by Zhou et al. [113] achieved a H₂ yield of 1.53-1.58 mol H₂/mol C at 700 °C. The WE-based catalysts developed by Fan et al. [111,112] achieved a lower hydrogen yield than the benchmark catalyst while also requiring higher temperatures. The CaO catalyst from demolition waste-developed by Zhao et al. [112] achieved a higher H₂ yield compared to the benchmark catalyst, but required a higher temperature of 900 °C to achieve that. In terms of H₂:CO ratios, the catalyst by Zhao et al. [112] achieved the highest ratio of 4.25, followed closely by the CaO + K₂CO₃ catalyst developed by Fan et al. [111], which achieved a ratio of 4.19.

5. Waste-Derived Catalysts for Biomass-Based Processes

For H₂ production from biomass-based sources using waste-derived catalysts, several different methods were employed: pyrolysis, a combination of pyrolysis and reforming, reforming of pyrolysis volatiles, and gasification. Across these biomass-based processes, the literature reports H₂ yield in a variety of different units. To allow for a direct comparison between studies, all reported yields have been converted to a common basis of mL/g. The converted values are presented in the tables in parentheses alongside the originally reported units. These conversions assume standard temperature and pressure.

5.1. Waste-Derived Catalysts in Biomass Pyrolysis

Pyrolysis is a thermochemical technique that converts carbon-rich materials into value-added products such as bio-oil, biochar, and syngas under oxygen-free conditions. The absence of oxygen reduces the formation of dioxides, while catalysts derived from inorganic salts—such as chlorides, carbonates, and chromates—can accelerate the process [60]. To increase H₂ yield, catalytic pyrolysis is proposed using metal (e.g., Ni or alkali metals) or non-metal-based catalysts (e.g., AC) [5].

In the process of biomass pyrolysis, several waste-derived catalysts have been tested. Wang et al. [114] used a dental ceramic wasted to synthesize Na₂ZrO₃ as a combined catalyst and CO₂ sorbent. The catalyst was tested on the pyrolysis of different biomasses (municipal sludge, spirulina algae, and methylcellulose). The highest H₂ yield of 205 mL H₂/g_biomass was achieved with the pyrolysis of spirulina algae at 900 °C. The pyrolysis of methylcellulose achieved a yield of 197 mL/g_biomass and waste sludge achieved a yield of 142 mL/g_biomass. In terms of H₂:CO ratios, spirulina algae pyrolysis achieved the highest one of 1.6, followed by municipal sludge (1.4) and methylcellulose (1). The catalyst also achieved excellent cyclic stability for over 30 cycles. In another study, Lan et al. [115] employed FA as a support for NiO in catalytic pyrolysis of rice straw. At 600 °C and an optimum loading of 20% NiO-FA, the H₂ concentration in the synthesis gas increased from 7 vol% (without catalyst) to 41 vol% with the catalyst. The addition of the catalyst also raised the H₂:CO ratio from 0.8 to 1.2. Zhang et al. [116] investigated two industrial wastes—spent fluid catalytic cracking catalyst (sFCC) and blast furnace ash (BFA)—as additives for the pyrolysis of sawdust. Adding 10 wt% of either waste shifted the product slate towards gases. At 700 °C, total gas yield reached 52.6% with BFA and 48.8 with sFCC, as compared to 45.2% without a catalyst. The H₂ also increased to 43 mL H₂/g_biomass with BFA and 40 mL H₂/g_biomass with sFCC, showing a big increase in comparison to the non-catalytic process, which produced a H₂ yield of 25 mL/g_biomass. The non-catalytic process yielded an extremely low H₂:CO ratio of 0.05, which later increased to 0.2 with the addition of either of the catalysts. Lastly, Zhou et al. [117] used RM as a catalyst for the pyrolysis of corn stover. Using 10–40 wt% RM in the feed, the H₂ yield steadily rose to 108 mL/g_biomass with 40 wt% of RM, as compared to 60 mL/g_biomass without RM present. The gas yield also increased to 643 mL/g_biomass from 412 mL/g_biomass. For the benchmark catalyst, the NiMo/Al₂O₃ catalyst developed by Qinglan et al. [118] was used. The benchmark catalyst achieved a 33.6 g H₂/kg_biomass yield at 450 °C while also achieving a H₂:CO ratio of 1.44. The key operating conditions and performance metrics of the studies on biomass pyrolysis using waste-derived catalysts and the benchmark catalyst are shown in

Table 5. Comparisons of parameters of waste-derived catalysts for processes of biomass pyrolysis.

Catalyst	Preparation Method	Substrate	Process Parameters	Hydrogen Yield	H2:CO Ratio	Reference
Non-waste NiMo/Al2O3	/	Pine chips	450 °C 40 min	33.6 g/kgbiomass (374 mL/gbiomss)	1.44	[118]
Na2ZrO3	Calcination	Spirulina algae	900 °C 40 °C/min	205 mL/gbiomass	1.6	[114]
20% NiO-FA	Homogeneous precipitation Calcination	Rice straw	600 °C 20 min	41 vol%	1.2	[115]
10% BFA	Calcination	Sawdust	700 °C 20 min	43 mL/gbiomass	0.2	[116]
10% sFCC	Calcination	Sawdust	700 °C 20 min	40 mL/gbiomass	0.2	[116]
40% RM	Direct use	Corn stover	800 °C 1 h	107.7 mL/gbiomass	0.63	[117]

.

Relative to the non-waste benchmark catalyst developed by Qinglan et al. [118], which produced 374 mL H₂/g_biomass, the waste-derived catalysts fall short, even though most of them were tested at a higher temperature. Out of the waste-derived catalysts, the best performing one according to H₂ yield was the Na₂ZrO₃ catalyst from dental waste-developed by Wang et al. [114], which achieved a yield of 205 mL H₂/g_biomass. The other waste-derived catalysts achieved a much lower H₂ yield, below 108 mL H₂/g_biomass. When comparing the H₂:CO ratios, the dental waste catalyst [114] achieved the highest ratio of 1.6, followed by the non-waste NiMo-based catalyst [118], which achieved a ratio of 1.44.

5.2. Waste-Derived Catalysts in Reforming of Pyrolysis Volatiles

The process of reforming pyrolysis volatiles for H₂ production involves two main steps: biomass pyrolysis followed by catalytic steam reforming of the resulting vapors [119]. Reforming typically takes place in a fluidized bed reactor at temperatures between 600–800 °C. During the steam reforming stage, pyrolysis volatiles pass through the catalytic bed, where they react with steam at the catalyst’s active sites to produce a H₂-rich gas [120]. Catalysts play a crucial role in this process, with metal-based catalysts, particularly Ni-based commercial catalysts, being most commonly used [121].

Al-Rahbi and Williams [122] examined Ni-doped waste ashes as reforming catalysts of pyrolysis vapors from the pyrolysis of wood biomass. They prepared three catalysts by doping Ni (10%) onto CFA, refuse-derived fuel (RDF), and waste tire ash (WTA). All the catalysts improved gas and H₂ yield compared to non-catalytic operation, which produced 3.39 mmol H₂/g_biomass along with a gas yield of 39.9%. Out of the three catalysts tested, the WTA catalyst was the most effective, increasing H₂ yield to 10.55 mmol/g_biomass along with gas yield to 59.3%, outperforming both CFA (7.3 mmol H₂/g_biomass and 54.9% gas yield) and RDF (9.66 mmol H₂/g_biomass and 58% gas yield) catalysts. All three catalysts improved the base H₂:CO ratio of 0.46 to 0.7 in the case of the Ni-CFA catalyst, and a ratio of 1 in the case of the WTA and RDF catalysts. Li and Williams [123] also used WTA and RDF for the reforming of pyrolysis volatiles, but in the case of volatiles from the pyrolysis of waste plastic. They used only WTA and RDF without Ni-doping. Both wastes increased the H₂ yield from the non-catalytic process, which produced 72 mmol H₂/g_plastic. The WTA catalyst increased the H₂ yield to 83.2 mmol/g_plastic, while achieving a H₂:CO ratio of 1.4, and RDF increased the yield to 81 mmol/g_plastic, while achieving a ratio of 1.5. Furthermore, Ryczkowski et al. [124] employed a two-stage fixed-bed setup. The first step involved the pyrolysis of either pine pulp or cellulose. After that, the vapors passed through a Ni/La-derived catalyst on an FA-derived zeolite. The Ni/La catalyst achieved a H₂ yield of 15.0 mmol H₂/g_cellulose and 10.8 mmol H₂/g_{pine pulp}, significantly outperforming the non-catalytic process, which produced only 1.3 mmol H₂/g_cellulose and 0.8 mmol H₂/g_{pine pulp}. Total gas production was also enhanced, increasing from 220 cm³ without a catalyst to 398 cm³ with Ni/La-A for cellulose conversion, and from 194 cm³ to 356 cm³ for pine pulp. The addition of the catalyst also increased the H₂:CO ratios in the process for both biomasses, achieving a ratio of 1.24 for pine pulp (0.10 in the non-catalytic process) and 1.43 for cellulose (0.21 in the non-catalytic process). Wu et al. [125] also explored the reforming of pyrolysis volatiles from pine sawdust. They found that Ni-Fe alloy on SS can reform pine sawdust volatiles, increasing total gas output to 1.138 Nm³/kg from 0.987 Nm³/kg. The H₂ yield of the process also increased to 386.52 mL/g_biomass from 321 mL/g_biomass. Guo et al. [126,127] also conducted research on the usage of steel industry wastes for reforming biomass tar. In one article [126], they explored the effect of using SS to enhance syngas and H₂ production via catalytic reforming of pine sawdust pyrolysis tar. The experiments were conducted with and without the presence of steam. In the process of dry reforming, the syngas yield was 350 mL/g_biomass, with H₂ production reaching 48.6 mL/g_biomass, achieving a H₂:CO ratio of 0.24. With the addition of steam, the syngas yield increased to 493.5 mL/g_biomass, and H₂ production to 91.3 mL/g_biomass, the H₂:CO ratio also increased to 0.34. In a subsequent study, Guo et al. [127] investigated the effect of Ni-loading (5–10 wt%) onto the SS. The best results were obtained with a 10% Ni loading, the syngas yield achieved was 463 mL/g_biomass compared to 320 mL/g_biomass in the non-catalytic process. H₂ production was also significantly increasing from 55 mL/g_biomass in the non-catalytic process to 86 mL/g_biomass with the 10 wt% Ni-SS catalyst. The addition of Ni onto the SS also improved the H₂:CO ratio to 0.39. For the benchmark catalyst, the commercial Ni/Al₂O₃ catalyst used by Arregi et al. [121] was used. The catalyst achieved a high H₂ yield of 117 g/kg_biomass, at a pyrolysis temperature of 500 °C, followed by a reforming temperature of 600 °C. The key operating conditions and performance metrics of the studies on reforming of pyrolysis volatiles using waste-derived catalysts and the benchmark catalyst are shown in

Table 6. Comparisons of parameters of waste-derived catalysts for processes of reforming of pyrolysis volatiles.

Catalyst	Preparation Method	Substrate	Process Parameters	Hydrogen Yield	H2:CO Ratio	Reference
Non-waste Ni/Al2O3	/	Pine wood sawdust	Pyrolysis: 500 °C Reforming: 600 °C	117 g/kgbiomass (1300 mL/gbiomass)	/	[121]
10% Ni-WTA	Ashing Ni impregnation	Waste wood pellets	Pyrolysis: 600 °C Reforming: 800 °C	10.5 mmol/gbiomass (235.35 mL/gbiomass)	1	[122]
WTA	Oxidation	High Density Polyethylene	Pyrolysis: 600 °C Reforming:1000 °C	83.2 mmol/gplastic (1865.89 mL/gbiomass)	1.4	[123]
20% Ni/La-FA	Impregnation Calcination	Cellulose	Pyrolysis: 500 °C Reforming: 700 °C	15 mmol/gbiomass (336.21 mL/gbiomass)	1.43	[124]
20% Ni/La-FA	Impregnation Calcination	Pine Pulp	Pyrolysis: 500 °C Reforming: 700 °C	10.8 mmol/gbiomass (242.07 mL/gbiomass)	1.24	[124]
Ni-SS	Impregnation Calcination	Pine sawdust volatiles	Reforming: 800 °C	386.5 mL/gbiomass	/	[125]
SS	Calcination	Pine sawdust tar	Reforming: 800 °C	91.3 mL/gbiomass	0.34	[126]
10% Ni-SS	Impregnation Calcination	Pine sawdust tar	Reforming: 800 °C	86 mL/gbiomass	0.39	[127]

.

In terms of the process of combining pyrolysis and further reforming of pyrolysis volatiles with biomass as its feedstock, the benchmark catalyst used by Arregi et al. [121] achieved the highest H₂ yield of 1300 mL/g_biomass. Out of the waste-derived catalysts, the Ni-SS catalyst developed by Wu et al. [125] achieved the highest H₂ yield of 386.5 mL/g_biomass, followed by the 20%Ni/La-FA developed by Ryczkowski et al. [124] catalysts, which achieved a H₂ yield of 336.21 mL/g_biomass. This same catalyst also achieved the highest H₂:CO ratio of 1.43. The WTA-based catalyst developed by Li and Williams [123], which used HDPE instead of biomass as a feedstock, achieved a very high H₂ yield of 1865.89 mL/g_plastic. This H₂ yield seems unusually high when compared to other studies described. However, Li and Williams attribute their high H₂ is due to the high reforming temperature, which improved the conversion of hydrocarbons and steam into H₂ via steam reforming and water–gas shift reactions.

5.3. Waste-Derived Catalysts in Biomass Gasification

Biomass gasification is regarded as one of the most practical, sustainable, and potentially carbon-neutral alternatives to generate H₂ [2]. Lignocellulosic biomass such as oil palm, bamboo, sugar palm, and wood are common feedstocks for gasification. Additionally, food scraps and MSW can serve as alternative feedstocks, while plastic waste, in particular, has recently gained significant attention. Combining plastics with biomass can further enhance gasification efficiency [128]. In biomass gasification, biomass undergoes partial oxidation to yield a gas mixture containing H₂, CO, CH₄, and CO₂ [129]. Oxidizing agents such as air, oxygen, or steam are necessary for gasification [130]. Depending on the gasification agent used, there exists a difference in cost, gasification temperature, and product gas composition [131]. Catalysts commonly used in biomass gasification include alkaline earth metals (mineral oxides or carbonates made from calcium or magnesium), metal oxide-based catalysts (zeolites, silicates), and natural minerals (dolomite, olivine, limestone) [128].

Raheem et al. [132] used eggshell-derived CaO for the gasification of microalgae biomass. Two microalgae species, Spirulina platensis and Chlorella vulgaris, were used as feedstock, with CaO loadings in the range of 0–50 wt%. The base H₂ yield (0% CaO) was 120 mL/g_biomass and 161 mL/g_biomass for Spirulina platensis and Chlorella vulgaris, respectively. With a 50 wt% CaO addition, the H₂ yield increased to 252 mL/g_biomass with Spirulina platensis, and to 344 mL/g_biomass with Chlorella vulgaris. Along with the increase in H₂ yield, an increase in the H₂:CO ratio also followed. The gasification of Spirulina platensis achieved a ratio of 1.21, while the gasification of Chlorella vulgaris achieved a ratio of 2.28. Another study by Chen et al. [133] explored SS as a support for Ni as a catalyst in sewage sludge steam gasification. The addition of 20% Ni-doped SS increased gas and H₂ yield, from the base values of 0.56 Nm³ syngas/kg_biomass and 9.8 mol H₂/kg_biomass to 0.78 Nm³ syngas/kg_biomass and 15.72 mol H₂/kg_biomass, respectively. The increase in H₂ yield also resulted in the increase in the H₂:CO ratio from 1.75 to 2.05 with the addition of the Ni-SS catalyst. Dong et al. [134] investigated the use of RM as a catalyst support for Ni in the process of bamboo sawdust steam gasification. The addition of RM and 10% Ni-RM increased the syngas yield to 170.9 and 210.3 mmol/g_biomass, respectively, from the non-catalytic yield of 165.1 mmol/g_biomass of syngas. The RM catalyst achieved a H₂ yield of 108 mmol/g_biomass and a H₂:CO ratio of 6.3l; the addition of 20% Ni increased the H₂ yield to 135 mmol/g_biomass and the ratio to 7.82. Vamvuka et al. [135] also explored the use of RM as a catalyst in the process of steam gasification. They combined RM with building demolition concrete for a dual-function system in the form of a catalyst and CO₂ sorbent. They tested the catalytic activity of said catalyst with two different biomass feedstocks (acacia pruning and helianthus residues). In the case of acacia pruning gasification, they achieved a H₂ yield of 1.5 m³/kg_biomass alongside a H₂:CO ratio of 13.9 with a catalyst of 30% RM loading. In the case of helianthus residues, they achieved a higher H₂ yield of 2.35 m³/kg_biomass alongside a H₂:CO ratio of 9.5 with a catalyst of 20% RM loading. Irfan et al. [136] investigated the effect of waste marble powder (WMP) in the steam gasification of MSW. The highest syngas yield of 1.23 Nm³/kg_biomass (0.55 Nm³ H₂/kg_biomass) was achieved with 50 wt% of WMP, where the amount of H₂ in the gas rose to 44.6% as compared to 32.4% in the process with zero WMP. Although the amount of H₂ rose, the H₂:CO ratio change was minimal from the non-catalytic process of 2.19 to 2.3 with the addition of WMP. Meanwhile, Amin et al. [137] investigated air gasification of MSW using WMP as a calcium-based catalyst. Various ratios of WMP/MSW (0–0.20) were tested. Optimal H₂ yield and gas quality were achieved at a ratio of 0.15, resulting in a gas yield of 1.46 Nm³/kg_biomass and a H₂ yield of 0.12 Nm³/kg_biomass. The H₂:CO ratio remained comparable at all the WMP/MSW ratios (0.63–0.67). Laghari et al. [138] researched the use of a CaO-based catalyst sourced from WE in the process of air gasification of MSW. Different catalyst loadings were tested (10–40 wt%) at a temperature of 950 °C, with the best catalytic performance achieved at 40 wt% of CaO, reaching a H₂ yield of 19.37 mol/kg_biomass and a H₂:CO ratio of 2.5. The optimal process parameters were later determined to be 34% CaO loading at 795 °C to achieve a H₂ yield of 22.74 mol/kg_biomass and a H₂:CO ratio of 2.83. As the benchmark catalyst, the Ni/CeO₂/Al₂O₃ catalyst developed by Peng et al. [139] was used. This catalyst achieved a H₂ yield of 0.706 Nm³/kg_biomass at 823 °C, whilst achieving a H₂:CO ratio of 1.84 in the process of steam gasification. The key operating conditions and performance metrics of the studies on biomass gasification using waste-derived and benchmark catalysts are shown in

Table 7. Comparisons of parameters of waste-derived catalysts for processes of biomass gasification.

Catalyst	Preparation Method	Substrate	Temperature	Hydrogen Yield	H2:CO Ratio	Reference
Non-waste Ni/CeO2/Al2O3	Impregnation Calcination	Wood residue	823 °C	0.706 Nm3/kgbiomass (706 mL/gbiomass)	1.84	[139]
WE	Calcination	Spirulina platensis	800 °C	252 mL/gbiomass	1.21	[132]
WE	Calcination	Chlorella vulgaris	800 °C	344 mL/gbiomass	2.28	[132]
20% Ni-SS	Impregnation Calcination	Sewage sludge	900 °C	15.7 mmol/gbiomass (351.1 mL/gbiomass)	2.05	[133]
10% Ni-RM	Impregnation Calcination	Bamboo sawdust	800 °C	135 mmol/gbiomass (3025.9 mL/gbiomass)	7.82	[134]
30% RM	Calcination	Acacia pruning	850 °C	1.5 m3/kgbiomass (1500 mL/gbiomass)	13.9	[135]
20% RM	Calcination	Helianthus residues	850 °C	2.35 m3/kgbiomass (2350 mL/gbiomass)	9.5	[135]
50% WMP	Calcination	MSW	900 °C	0.55 Nm3/kgbiomass (550 mL/gbiomass)	2.3	[136]
10% WMP	Calcination	MSW	700 °C	0.12 Nm3/kgbiomass (130 mL/gbiomass)	0.66	[137]
40% CaO (WE)	Calcination	MSW	950 °C	19.4 mmol/gbiomass (435.83 mL/gbiomass)	2.5	[138]

.

Compared to the benchmark catalyst studied by Peng et al. [139], which achieved a H₂ yield of 706 mL/g_biomass, most of the other waste-derived catalysts achieved a lower yield. The exceptions are the 10% Ni-RM catalyst developed by Dong et al. [134], which achieved a H₂ yield of 3025.7 mL/g_biomass and the RM catalyst developed by Vamvuka et al. [135], which achieved a H₂ yield of 1500–2350 mL/g_biomass, depending on the feedstock. Both these waste-derived catalysts vastly outperformed the benchmark catalyst, which could be explained by the presence of a higher amount of steam in the process, which in turn increased the production of H₂, which can also be seen when comparing the H₂:CO ratios, as they are much higher in the processes of these waste-derived catalysts. Besides these two catalysts, the next best performing waste-derived catalyst in terms of H₂ yield was the WMP-based catalyst developed by Irfan et al. [136], which achieved a H₂ yield of 550 mL/g_biomass.

6. Waste-Derived Catalysts for Electrochemical-Based Processes

In electrocatalytic water splitting, catalysts are essential for reducing the kinetic barrier. Electrocatalyst performance is typically evaluated by parameters such as Tafel slope, overpotential, and exchange current density [140]. The hydrogen evolution reaction (HER) is the initial step of water electrolysis for producing H₂. Efficient H₂ generation requires catalysts that minimize the overpotential. The HER pathway depends on the solution’s acidity or basicity because the underlying mechanism changes with pH. In acidic media, protons (H⁺) from the electrolyte adsorb on the catalyst surface and are then reduced to form H₂. In alkaline media, hydroxide ions (OH^-) from the electrolyte discharge at the surface and are subsequently reduced, yielding water and H₂ [141]. To surpass kinetic barriers, water electrolysis must operate above its thermodynamic potential of 1.23 V, where the additional voltage applied is known as the overpotential (η) [140]. Catalyst activities are typically compared using three specific η values (η₁, η₁₀, η₁₀₀), which correspond to current densities of 1, 10, and 100 mA/cm² each [142]. The current density of 10 mA/cm² is usually employed when comparing HER electrocatalysts in mediums of different pH [142]. A catalyst’s inherent feature that is closely linked to the rate of HER is the Tafel slope [141]. Tafel slopes are widely applied to evaluate both the kinetics and mechanisms of electrocatalytic reactions. By definition, the slope represents the millivolts needed to increase current by one order of magnitude and is expressed in mV/dec. A lower Tafel slope signifies a more active catalyst, since less overpotential is required to achieve higher current densities [143]. Electrocatalyst stability is a key factor in determining its potential for commercialization, typically evaluated by maintaining activity over long-term operation [142]. Stability is an important factor for the practical use of HER catalysts. It is usually evaluated in two ways: (1) by many cycles of cyclic voltammetry (CV) or linear sweep voltammetry (LSV) and (2) by galvanostatic (or potentiostatic) testing during prolonged electrolysis. In the voltammetric approach, the CV or LSV is performed around the onset potential, and the overvoltage is compared before and after a certain number of cycles (e.g., 10,000); small shifts in overvoltage indicate good stability. In galvanostatic (or potentiostatic) measurements, the potential is monitored over time while the current (or potential) is held constant to track any drift. Other important performance metrics are the turnover rate and Faradaic efficiency [141].

This chapter will be divided into subchapters based on the type of electrocatalyst: (i) pure carbon electrocatalysts; (ii) self-doped carbon electrocatalysts; (iii) metal-doped carbon electrocatalysts; and (iv) multiple-doped carbon electrocatalysts. Lastly, a subchapter comparing the best-performing electrocatalysts from each subcategory with commercial Pt/C and Ir/C electrocatalysts will be included.

6.1. Pure Carbon Electrocatalysts

With the case of pure carbon materials derived from biomass, Sekar et al. [144] obtained corrugated graphene nanosheets from rice husks that exhibited a low overpotential of 9 mV at 10 mA/cm² and a Tafel slope of 31 mV/dec, all while maintaining stable performance for 10 h. Prabu et al. [145] prepared porous carbon nanosheets from palm spathe and pollen waste. The carbon achieved an overpotential of 330 mV at 10 mA/cm² with a Tafel slope of 63 mV/dec, whilst showing negligible degradation over 10 h of operation. Similarly, Pandey and Jeong [146] produced activated porous carbon from coffee waste, which had an overpotential of 210 mV at 10 mA/cm² and 120 mV/dec Tafel slope. It sustained HER continuously for 24 h. Thirumal et al. [147] reported an AC from tamarind shells with an overpotential of 221 mV at 10 mA/cm² and a Tafel slope of 204 mV/dec, with moderate stability, with 51.7% of current retained after 2 h of operation. In another example, Fu et al. [148] obtained porous carbon nanosheets from walnut shells, reaching an overpotential of 170 mV at 10 mA/cm² and a 69.8 mV/dec Tafel slope, while maintaining stable H₂ generation over 15 h. The comparison of parameters of pure carbon-based electrocatalysts is shown in

Table 8. Pure carbon electrocatalyst performance indicators.

Waste Material	Catalyst	Overpotential (mV at 10 mA/cm2)	Tafel Slope (mV/dec)	Stability	Reference
Rice husk	Graphene nanosheets	9	31	10 h	[144]
Palm spathe Pollen waste	Porous carbon nanosheets	330	63	10 h	[145]
Coffe waste	Porous carbon	210	120	24 h	[146]
Tamarind shells	AC	221	204	51.7% after 2 h	[147]
Walnut shells	Porous carbon nanosheets	170	69.8	15 h	[148]

.

Table 8 shows that without any doping or added catalytic sites, carbon materials can drive HER but typically at the cost of higher overpotentials (often in the 170–330 mV range for biomass carbons [128,129,130,131]). The electrocatalyst designed by Sekar et al. [144] achieves a very low overpotential of 9 mV and the lowest Tafel slope of 31 mV. The pure carbon catalysts showed decent stability under operating conditions, maintaining stable performance for over 10 h. The only outlier was the tamarind shell carbon, developed by Thirumal et al. [147], which maintained only 52% of the current after 2 h of operation.

6.2. Self-Doped Carbon Electrocatalysts

Biomass often contains heteroatoms (like N, S, P) that can be incorporated into the carbon matrix during pyrolysis to create catalytically active sites. One such example was by Saravanan et al. [149], who developed N-doped carbon nanosheets from peanut shells, which showed an onset overpotential of 80 mV and a Tafel slope of 75.7 mV/dec, remaining active over 10 h of HER testing. Zhu et al. [150] derived microporous N-doped carbon frameworks from pine needles, achieving an overpotential of 62 mV at 10 mA/cm² with a Tafel slope of 45.9 mV/dec. The carbon frameworks showed excellent durability with a slight activity drop after 1000 cycles and 100 h. Meanwhile, Sekar et al. [151] used human hair to obtain graphitized carbon nanobundles self-doped with N (and minor S). These nanobundles required a low 16 mV of overpotential at 10 mA/cm² with a Tafel slope of 51 mV/dec. Lastly, Zhou et al. [152] converted peanut root nodules into S, N-co-doped porous carbon, reaching 116 mV at 10 mA/cm² and a Tafel slope of 67.8 mV/dec, with stable production over 12 h and over 1000 cycles. The comparison of parameters of self-doped carbon-based electrocatalysts is shown in

Table 9. Self-doped carbon electrocatalyst performance indicators.

Waste Material	Catalyst	Overpotential (mV at 10 mA/cm2)	Tafel Slope (mV/dec)	Stability	Reference
Peanut shells	N-doped carbon	80 (onset)	75.7	10 h	[149]
Pine needles	N-doped carbon	62	45.9	100 h 1000 cycles	[150]
Human hair	N-doped nanobundles	16	51	/	[151]
Peanut root nodules	S, N-doped carbon	116	67.8	12 h 1000 cycles	[152]

.

Nitrogen doping is particularly effective, as N atoms (with their lone pair electrons) can tune the electron density of adjacent carbon, creating active sites that facilitate proton adsorption and conversion to H₂. Sekar et al. [151] achieved the best results, delivering an extremely low overpotential of 16 mV at 10 mA/cm² with a 51 mV/dec Tafel slope. In general, the self-doped carbons outperform pure carbons: doping typically lowers the overpotential into the double-digit millivolt range and yields smaller Tafel slopes (often 40–80 mV/dec) due to faster kinetics.

6.3. Metal-Doped Carbon Electrocatalysts

Incorporating metal species onto carbon can improve HER performance by providing additional active sites akin to noble metal catalysts. Nikitin et al. [153] synthesized a SiC/C–carbon composite with 5 wt% Pt that achieved overpotentials as low as 22–24 mV at 10 mA/cm² and a Tafel slope in the range of 34.60 mV/dec. The Pt-SiC catalyst maintained stability for over 1500 cycles. Yang et al. [154] loaded CoO nanoparticles onto porous carbon from watermelon peels. The CoO-doped porous carbon achieved an overpotential of 111 mV at 10 mA/cm² with a Tafel slope of 93.9 mV/dec and a stable performance for over 20 h. Min et al. [155] also embedded Co nanoparticles into a pomelo-peel-derived carbon membrane, achieving an overpotential of 154 mV at 10 mA/cm² and a Tafel slope of 106.4 mV/dec. The material endured 12 h and over 2000 CV cycles with minimal degradation. Lu et al. [156] developed a NiO/C nanocomposite from eggshell membranes. The nanocomposite had a relatively high overpotential of 565 mV at 10 mA/cm² despite a moderate Tafel slope of 77.8 mV/dec. The material had minor changes in HER activity after 500 CV cycles. Chai et al. [157] deposited Ni nanoparticles on carbon derived from leaves, achieving a low overpotential of 32 mV at 10 mA/cm², albeit with a large Tafel slope of 125.6 mV/dec. The Ni-doped carbon achieved robust stability, maintaining HER activity over 48 h and 2000 cycles. Zhou et al. [158] also explored doping Ni onto carbon. In his case, he synthesized Ni-doped graphitic carbon from rose petals that achieved an overpotential of 220 mV at 10 mA/cm² and a 64 mV/dec Tafel slope. The catalysts remained stable for over 24 h of operation. Meanwhile, Fakayode et al. [159] added Mo₂C onto porous carbon from watermelon rind, resulting in an electrocatalyst that maintained durability for over 300 h. The catalyst achieved an overpotential of 133 mV at 10 mA/cm² with a Tafel slope of 25 mV/dec. Humagain et al. [160] also used the approach of adding Mo₂C onto carbon but instead used birch-derived biochar to create porous-Mo₂C. The porous-Mo₂C achieved a remarkable overpotential of 35 mV at 10 mA/cm² and a low Tafel slope of 25 mV/dec, whilst maintaining almost 100% activity for over 100 h. Mir et al. [161] also explored the use of Mo₂C, making a carbon-supported Mo₂C from waste plastic, which achieved an overpotential of 179 at 10 mA/cm² and a Tafel slope of 80 mV/dec. The molybdenum carbide catalyst remained stable for 2000 cycles and 10 h. On the other hand, Sekar et al. [162] introduced WO₃ nanoflakes into neem-leaf carbon, achieving an overpotential of 360 mV at 10 mA/cm² and a low Tafel slope of 14 mV/dec. The nanoflakes-doped carbon maintained stable operation for 12 h. The comparison of parameters of metal-doped carbon-based electrocatalysts is shown in

Table 10. Metal-doped carbon electrocatalyst performance indicators.

Waste Material	Catalyst	Overpotential (mV at 10 mA/cm2)	Tafel Slope (mV/dec)	Stability	Reference
Rice and oat husks	5% Pt-SiC	22–24	34–60	1500 cycles	[153]
Watermelon peels	CoO-porous carbon	111	93.9	20 h	[154]
Pomelo peel	Co-carbon	154	106.4	12 h 2000 cycles	[155]
Eggshell membranes	NiO-carbon	565	77.8	500 cycles	[156]
leaves	Ni-carbon	32	125.6	48 h 2000 cycles	[157]
Rose petals	Ni-carbon	220	64	24 h	[158]
Watermelon rind	Mo2C-porous carbon	133	25	300 h	[159]
Birch wood	Mo2C-porous carbon	35	25	100 h	[160]
Waste plastic	Mo2C-carbon	179	80	10 h 2000 cycles	[161]
Neem leaves	WO3-carbon	360	14	12 h	[162]

.

Multiple metal dopants were tested on a carbon platform in terms of electrocatalytic activity, showing varied results. The best performing catalysts were the 5% Pt-doped catalysts, developed by Nikitin et al. [153], and the Mo₂C electrocatalyst by Humagain et al. [160]. They achieved an overpotential and Tafel slope below 35 mV. A similarly low overpotential was also achieved by a Ni-doped carbon electrocatalyst developed by Chai et al. [157], but the catalyst had a large Tafel slope of 125.6 mV/dec. On the other hand, the electrocatalysts, developed by Fakayode et al. [159] and Sekar et al. [162], achieved low Tafel slopes of 25 mV/dec and 14 mV/dec, respectively, but suffered from high overpotentials. When trying to compare similar metal dopants on carbon, it is hard to find a common demotion in regard to the activity of the electrocatalyst, which could be a consequence of different preparation methods.

6.4. Multiple-Doped Carbon Electrocatalysts

Another approach used by researchers was to combine multiple dopants or phases to boost HER activity. For instance, Mulyadi et al. [163] engineered N, S, and P-doped carbon nanofibers from softwood pulp. Achieving an overpotential of 331 mV at 10 mA/cm². The Tafel slope was 99 mV/dec, and the catalyst showed slight degradation over time. Deng et al. [164] utilized animal bone waste to create N, P, and Ca-doped biochar, achieving an overpotential of 162 mV at 10 mA/cm² with a Tafel slope of 80 mV/dec. The animal bone-derived carbon showed slight deviation after 2000 cycles. To further better HER activity, besides adding multiple heteroatoms, metal species can be added along with them. Yang et al. [165] synthesized an electrocatalyst using edible amaranth-derived carbon, doped in situ with Fe and N. The catalyst demonstrated a HER overpotential of 92 mV at 10 mA/cm² and a Tafel slope of 95.8 mV/dec. The catalyst also maintained a stable current output over 10 h. On the other hand, Wang et al. [166] explored a bimetallic electrocatalyst. They introduced Ni-Fe co-doping along with N, P, and S heteroatoms on nanocarbon from alfalfa biomass. The resulting electrocatalyst achieved an overpotential of 250 mV at 10 mA/cm² and a Tafel slope of 84 mV/dec. While also showing stable performance after 1000 cycles and 50 h of operation. Abdolahi et al. [167] loaded Ni-Co oxide onto S, N-doped carbon from chicken feathers, achieving an overpotential of 87 mV at 10 mA/cm² and a Tafel slope of 50 mV/dec. The catalyst maintained stable operation after 20 h. Jiang et al. [168] transformed waste tires into a Zn, S, N-doped graphitic carbon, which exhibited an overpotential of 50 mV at 10 mA/cm² and a Tafel slope of 78 mV/dec, while maintaining stability for over 110 h. Sun et al. [169] reported a Co, N-doped carbon from office paper, which showed excellent stability over 3000 cycles and 14 h, while achieving an overpotential of 226 mV/dec at 10 mA/cm² and a Tafel slope of 91 mV/dec. Lastly, Zhao et al. [170] demonstrated a hybrid catalyst from cotton textiles, synthesizing a CoNiO₂/N, P-doped carbon substrate. The catalyst showed an overpotential of 247.6 mV at 10 mA/cm² and a Tafel slope of 120.8 mV/dec, while maintaining stability for over 50 h. The comparison of parameters of hybrid-doped carbon-based electrocatalysts is shown in

Table 11. Multiple-doped carbon electrocatalyst performance indicators.

Waste Material	Catalyst	Overpotential (mV at 10 mA/cm2)	Tafel Slope (mV/dec)	Stability	Reference
Softwood pulp	N, S, P-carbon nanofibers	331	99	/	[163]
Animal bones	N, P, Ca-biochar	162	80	2000 cycles	[164]
Amaranth	Fe3O4, N-carbon	92	95.8	10 h	[165]
Alfalfa	NiFe-N, P, S-nanocarbon	250	84	50 h 1000 cycles	[166]
Chicken feathers	NiCoO-S, N-porous carbon	87	50	20 h	[167]
Waste tires	Zn, S, N-carbon	50	78	110 h	[168]
Office paper	Co, N-carbon	226	91	14 h 3000 cycles	[169]
Cotton textiles	P, CoNiO2, N-carbon	247.6	120.8	50 h	[170]

.

The best performers in this group (e.g., Fe, N–C [165], Ni–Co–oxide/N, S–C [167], Zn, S, N–C [168]) achieved overpotentials well below 100 mV, some even near 50 mV, with Tafel slopes in the range of ~50–80 mV/dec. These metrics are comparable to, and in some cases better than, the simpler metal-doped carbons, indicating a successful synergy. Although the above-mentioned electrocatalysts achieved good performance, some hybrid doped electrocatalysts achieved a performance worse than that of pure carbon electrocatalysts when considering overpotential. In terms of Tafel slope, the performance of hybrid doped electrocatalysts was better than pure carbon-based ones.

6.5. Comparison of Commercial and the Best Performing Carbon Electrocatalysts

In this subchapter, the performance parameters of commercial and waste-derived carbon-based electrocatalysts will be compared. As the benchmark, the commercial Pt/C and Ir/C catalysts were used. The performance parameters for the commercial Pt/C catalyst were taken from the study of Zheng et al. [171], in which the overpotential of the catalysts was defined as 29 mV at 10 mA/cm² and the Tafel slope was 46 mV/dec. The catalyst maintained stability for 10 h. For the Ir/C commercial catalyst, the data were taken from the study by Peng et al. [172], where it was reported that the commercial Ir/C catalyst achieved an overpotential of 28 mV at 10 mA/cm² and a Tafel slope of 55 mV/dec, while maintaining stability for 8 h. The parameters of the commercial and the best-performing waste-derived electrocatalyst are shown in

Table 12. Parameters of commercial and waste-derived electrocatalysts.

Catalyst Type	Catalyst	Overpotential (mV at 10 mA/cm2)	Tafel Slope (mV/dec)	Stability	Reference
Commercial	Pt/C	29	46	10 h	[171]
/	Ir/C	28	55	8 h	[172]
Pure carbon	Rice husk-based Graphene nanosheets	9	31	10 h	[144]
Self-doped	Pine needle-based N-doped carbon	62	45.9	100 h 1000 cycles	[150]
/	Human hair-based N-doped nanobundles	16	51	/	[151]
Metal-doped	Rice and oat husk-based 5% Pt-SiC	22–24	34–60	1500 cycles	[153]
/	Birch wood-based Mo2C-porous carbon	35	25	100 h	[160]
Multi-doped	Chicken feather-based NiCoO-S, N-porous carbon	87	50	20 h	[167]
/	Waste tire-based Zn, S, N-carbon	50	78	110 h	[168]

.

From the data presented in Table 12, it can be seen that waste-derived electrocatalysts have a comparable performance to the commercial Pt/C and Ir/C electrocatalysts used. In terms of overpotential, several catalysts have outperformed the benchmark ones, with the pure-carbon rice husk-based electrocatalyst developed by Sekar et al. [144] achieving the lowest overpotential of 6 mV at 10 mA/cm², closely followed by the self-doped human hair-based electrocatalyst designed by Sekar et al. [151], which achieved 16 mV at 10 mA/cm². In terms of Tafel slope, the best performing electrocatalyst was the metal-doped birch wood-based one designed by Humagain et al. [160], achieving a Tafel slope of 25 mV/dec.

7. Future Prospects

Although this review shows significant progress, further efforts are needed to fully explore the potential of catalysts from waste materials for H₂ production. Key directions for future research and development include the following:

In-depth benchmarking: although the efficiency in catalytic parameters was compared between the commercial (non-waste) and waste-derived catalyst, further studies and research are needed for a more detailed comparison of catalyst performance. Besides the parameters for catalytic conversion, such as yield and conversion, studies on long-term performance metrics, such as catalyst stability and deactivation rates over time. Determining whether waste-derived catalysts can maintain their performance over time (comparable to existing industrial catalysts) is critical to confirming their commercial viability. Stability is a critical factor for use in practice. Besides the performance metrics, the effect of impurities in waste-derived catalysts should be extensively screened to provide a more in-depth understanding of their effects on performance variability, environmental, and safety risks.

Techno-economic and environmental assessment: Secondly, a rigorous techno-economic analysis and life cycle assessment (LCA) of waste-to-hydrogen technologies is needed. Initial results suggest that the use of waste materials can reduce the cost of catalysts and the overall cost of H₂ production, but a detailed financial assessment is needed to quantify the capital and operating costs on a large scale. This should take into account the costs of waste collection, processing, and treatment, as well as the savings from reduced feedstock consumption. In parallel, an LCA should be carried out to assess the net environmental impact, taking into account factors such as the energy required for waste processing, GHG throughout the supply chain, and the handling of spent catalysts at the end of their useful life.

Resource availability and location analysis: Thirdly, a spatial analysis of the availability of waste and biomass resources should be conducted to determine the strategic location of H₂ production plants. The distribution of suitable waste feedstock is uneven. By mapping the sources of waste materials using geographic information systems, optimal locations for processing facilities can be identified, ideally near abundant waste streams to minimize transportation costs. By planning facilities close to feedstock sources, supply chain logistics will be improved and the overall efficiency and carbon footprint of H₂ production can be reduced.

By pursuing these future directions—from performance validation and economic analysis to strategic resource planning—the remaining challenges can be addressed, and the viability of waste-to-hydrogen technologies can be improved upon. Such efforts will be instrumental in bridging the gap between laboratory innovations and real-world applications, ultimately advancing the twin goals of clean energy production and sustainable waste management.

8. Conclusions

H₂ can be used in a variety of ways, from alternative energy storage to fuel, and is an important feedstock in the chemical industry for the production of ammonia and methanol, as well as in reforming processes. Its use as an energy storage vector and fuel makes it a potential solution for decarbonizing the energy sector, along with other renewable energy alternatives. Currently, however, H₂ is mainly produced from fossil fuels, so a transition to renewable feedstocks such as biomass or water is a perspective alternative. Another aspect of the transition is the need for specific CRMs, some of which are geologically scarce, unevenly distributed, and prone to geopolitical instability, leading to significant supply issues. The current linear model exacerbates this problem through the inefficient use of these elements, resulting in significant quantities of these elements ending up in landfills.

In this context, waste recycling has been presented as a promising solution to reduce the dependence on virgin CRMs. The review focuses on the investigation of waste utilization for catalyst synthesis. Different catalytic routes have been explored, showing how industrial waste (e.g., red mud, steel slag, fly ash) and agricultural waste (e.g., eggshells, biomass waste) can serve as effective catalysts or catalyst support for H₂ production. The catalytic properties of these catalysts have been observed in various H₂ production processes, including fossil fuel-based processes, biomass conversion, and water electrolysis. For fossil fuel-based processes, waste-derived catalysts were effectively used in steam methane reforming, dry methane reforming, methane pyrolysis, partial oxidation of methane, and coal gasification. In the case of coal gasification, waste eggshells were researched. Many different wastes have been used in methane conversion processes, but they have the common characteristic of being doped with metals such as nickel and cobalt. Waste has also been successfully used as a catalyst in biomass conversion processes such as gasification and pyrolysis of biomass and the reforming of pyrolysis volatiles. In these thermochemical methods, industrial waste was mostly used as a catalyst or catalyst support, with the most common wastes being red mud, fly ash, and steel slag. In the context of electrochemical water splitting, a clear trend emerged where mainly carbon-rich biomass waste was used to synthesize electrocatalysts. These carbon-based materials have been used in various forms, including undoped carbons, self-doped carbons (e.g., N-, S-, P-doped), metal-doped carbons (e.g., Co, Ni, Mo), and hybrid-doped carbons, which contain both metallic and non-metallic dopants. By converting waste into valuable catalytic resources, this approach not only alleviates the problems of waste management but also has the potential to reduce production costs and improve sustainability.

Future research directions should focus on overcoming the remaining technical and economic challenges associated with waste-derived catalysts, such as long-term performance and stability of the catalyst, techno-economic analyses, and life cycle assessments of waste-derived catalysts. Additionally, regional mapping and analysis of waste availability should be conducted to identify strategically optimal locations for future H₂ production centers.