# Alternative H2O2 Production Processes: An Outlook on Candidate Technologies Beyond the Anthraquinone Process

Stavros-Alexandros Theofanidis, Amvrosios Georgiadis, Christianus Hop, Xiaobin Yu, Vasileia-Loukia Yfanti, Guillaume Fayet, Claire Villemur, Hank Vleeming, Evangelos Delikonstantis, Richard Heyn

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## To cite this version:

Stavros-Alexandros Theofanidis, Amvrosios Georgiadis, Christianus Hop, Xiaobin Yu, Vasileia-Loukia Yfanti, et al.. Alternative H2O2 Production Processes: An Outlook on Candidate Technologies Beyond the Anthraquinone Process. ACS Omega, 2025, 10.1021/acsomega.5c07503. [hal-05425733](https://hal.science/hal-05425733v1)

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Submitted on 16 Feb 2026

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# Alternative H₂O₂ Production Processes: An Outlook on Candidate Technologies Beyond the Anthraquinone Process

Stavros-Alexandros Theofanidis, Amvrosios G. Georgiadis, Christianus J. W. Hop, Xiaobin Yu, Vasileia-Loukia Yfanti, Guillaume Fayet, Claire Villemur, Hank Vleeming,* Evangelos Delikonstantis,* and Richard H. Heyn*

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Cite This: ACS Omega 2025, 10, 61076–61095

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**ABSTRACT:** Hydrogen peroxide is currently made on a large scale via the auto-oxidation (AO) process, an indirect process via the hydrogenation of anthraquinone over a Pd catalyst, followed by reaction with O₂. The AO process has a significant climate footprint, since H₂ is usually sourced from methane steam reforming. A more sustainable H₂O₂ production process is, therefore, highly relevant. Potential alternative routes include the direct reaction of H₂ and O₂ and processes that use electricity, either directly via electrochemical or bioelectrochemical systems or indirectly via plasmas. Despite research on these routes, there are

currently no real challengers to the established AO process. An overview of these alternative processes is provided before considering the energetic, techno-economic, life cycle, and safety challenges facing them. By highlighting these challenges through comparison with the AO process, this review aims to inspire further work for scaling up these alternative processes for truly determining their industrial viability. As these processes may ultimately prove more advantageous than the AO process for on-demand production, their industrialization might not need to occur on the same production scale as that of the AO process.

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## INTRODUCTION

Hydrogen peroxide, H₂O₂, is a versatile chemical that is used in a variety of applications, primarily because of its strong oxidation potential and benign products, as it releases only H₂O and O₂ upon use. H₂O₂ is used as a chemical reagent (e.g., epoxidation, production of other oxidizers), for the bleaching of cellulosic materials (e.g., cotton and wood) and for disinfection and cleaning.¹ European industry uses 50% of the produced H₂O₂ in the chemical and pharmaceutical sectors, 41% in the pulp and paper sector, and 8% in the textiles sector.² The volume of the global H₂O₂ market in 2023 was estimated to be 4.3 million tons (Mt) with a value of USD 1.82 billion, with an anticipated compound annual growth rate of 3.8% from 2023 to 2032.³

The first industrial H₂O₂ process, the Weissenstein process, was electrochemical and was established in 1908. In this process, H₂SO₄ was oxidized to peroxodisulfuric acid, HO₃SOOSO₃H, and then hydrolyzed to H₂O₂.⁴ Improvements to this process evolved from the use of sulfate salts. For example, the Löwenstein and Laporte processes used (NH₄)₂SO₄ rather than H₂SO₄, while the Pietzsch–Adolph process precipitated the intermediate ammonium peroxydisulfate, (NH₄)₂(O₃SOOSO₃), as a potassium salt that was then isolated, acidified, and hydrolyzed.⁴ These electrochemical processes were gradually replaced by the Riedl–Pfleiderer, anthraquinone, or auto-oxidation (AO) process, the first plant

of which was commissioned in 1953. The AO process is now the global standard for industrial H₂O₂ production. While the change from an electrochemical to the thermochemical AO process was undoubtedly driven by the unfavorable economics of high capital expenditures, electricity consumption, and the required purity of chemicals and electrolytes, the original electrochemical processes would also no longer meet today's health and safety considerations. For example, it is reported that the cathode in the Pietzsch–Adolph process was wrapped with asbestos rope to eliminate the need for a catholyte and that the produced ammonia and hydrogen gases were vented directly to the atmosphere, requiring well-ventilated cell rooms.⁴

The AO process accounts for 95% of the worldwide H₂O₂ production and is an indirect synthesis starting with the hydrogenation of an alkylanthraquinone over a Pd catalyst, separation of the hydroquinone, and finally oxidation with O₂ (air) back to the anthraquinone, producing H₂O₂ quantitatively. A whole host of solvents, including alkylated benzenes,

Received: July 29, 2025

Revised: November 28, 2025

Accepted: December 2, 2025

Published: December 10, 2025

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<figure><img src="image_11.png" /><figcaption>**Figure 1.** Overview of alternative H₂O₂ production processes. Gray, thermal; blue, electrical; orange boxes, technologies; green circles, sustainable reactants; and black circles, auxiliary reactants.</figcaption></figure>

alkyl phosphates, tetraalkyl ureas, and other organics, are required to keep all of the components in solution and facilitate the reaction and separation steps in the process.⁵ Paradoxically, although H₂O₂ is considered one of the greenest oxidizing agents⁶ and is not a carbon-containing chemical, fossil carbon is one of the key reagents in the AO production process, since the required H₂ is generated from steam methane reforming (SMR). This, together with a total energy consumption of 2.9–5.7 GJ/1000 kgH₂O₂,⁷ is the main reason that the CO₂-equiv emissions of the AO process exceed 2.8 Mt annually (1.33 kgCO₂/kgH₂O₂).⁸,⁹

Electrification has been identified as one of the main routes to reducing the CO₂ footprint of the chemical industry,¹⁰ particularly when combined with efforts to enhance the green electricity supply,¹¹ i.e., renewable energy sources such as wind, solar, and hydropower.¹² Of the different electrification strategies, electrochemistry can beneficially lower reaction temperatures, reducing thermal losses, decreasing material stability issues, and overcoming equilibrium conversion limitations in exothermic reactions.¹³ However, a challenge associated with electrification is the increased electricity demand that will accompany the transition of the chemical industry from conventional fossil-fuel-based to electrified routes. One estimate suggests that renewable electricity demand will increase 10-fold by 2050.¹⁴ Fortunately, electrification also allows the operational flexibility necessary for smaller, modular facilities that can be located in regions where renewable electricity is abundant¹⁴ and that can be more easily integrated into distributed power grids. Electrochemical processes can thus provide grid-balancing services that can be economically advantageous.¹³ Indeed, because of the energy demands of the chemical industry, collaboration and integration between the chemical and energy sectors are necessary to achieve emissions reductions through the electrification of the chemical industry.¹² Furthermore, if an electrified chemical process could make easily stored products, then the process could be run when sustainable electricity is cheap and available, provided that the cost savings outbalance additional investments.

While the hydrogen atoms in H₂O₂ and a reduced CO₂ footprint could come via a modified AO process using green H₂ produced from H₂O electrolysis, the direct use of electricity has the potential to be more efficient. A number of alternative syntheses of H₂O₂ are therefore under investigation.⁹,¹⁵,¹⁶ For such syntheses, there is no need for anthraquinone, a costly component that adds approximately USD 2 billion annually to the expenses of H₂O₂ manufacturers.⁸ Despite the potential for improved economics, there are still no examples of an alternative industrial process for producing the high-concentration H₂O₂ solutions (>30%) required for most applications. A frequently cited example of an active alternative process is the Headwaters-Evonik joint venture in Ulsan, Korea, where H₂O₂ was proposed to be produced by the direct reaction of H₂ and O₂ and supplied directly to a neighboring propylene oxide facility (the Hydrogen Peroxide to Propylene Oxide process).¹⁷ However, the implementation of the technology was referred to as a “future target” by the joint venture, without any recent reference about the progress in meeting this target. Although the Ulsan facility has more than doubled its capacity to over 68,000 t/yr since its acquisition from Kemira Oyi in 2006, there is no definitive public evidence confirming the industrial implementation of an alternative H₂O₂ synthesis at this site.¹⁸⁻²⁰ An alternative H₂O₂ production method has been commercialized by the Danish company HPNow, providing modular, electrochemical H₂O₂ production units primarily for water treatment. While the inputs are only H₂O and air, the limitation is that the obtained H₂O₂ concentrations of 0.25–0.5% are not relevant for the textiles and pulp and paper industries. Cost is another challenge.²¹

Environmental pressures thus suggest the need for industrial H₂O₂ production to complete the circle, starting from the initial electrochemical routes in the early to mid-1900s to the currently dominant, fossil-fuel-driven AO production process, and now back to an electricity-driven process. The direct use of electricity from renewable energy sources could offer a sustainable pathway for the industrial production of H₂O₂. However, the alternative H₂O₂ production technologies still suffer from limitations that have prevented them from challenging the supremacy of the AO process. After a brief
<table><caption>Table 1. H₂O₂ Direct Synthesis with Noble Metal Catalysts</caption><thead><tr><th>catalyst</th><th>reactor type</th><th>selectivity (%)</th><th>formation rate<sup>a</sup></th><th>H<sub>2</sub>O<sub>2</sub> wt %</th><th>T/bar/t (°C/bar/h)<sup>b</sup></th></tr></thead><tbody><tr><td>5% Pd@TiO<sub>2</sub><sup>34</sup></td><td>batch</td><td>70</td><td>194</td><td></td><td>27/89/0.28</td></tr><tr><td>0.4Au1.1Pd@TiNT<sup>e35</sup></td><td>batch</td><td></td><td>174.9</td><td>0.14</td><td>5/20/0.67</td></tr><tr><td>Pd@XC72<sup>d36</sup></td><td>batch</td><td>74</td><td>129</td><td>1.7</td><td>10/1/4</td></tr><tr><td>Pd/NPCs-PSS<sup>g37</sup></td><td>batch</td><td>71.9</td><td>328.4</td><td>0.18</td><td>0/1/3</td></tr><tr><td>Pd<sub>0.3</sub>/NPCS<sup>g38</sup></td><td>batch</td><td>76.7</td><td>219</td><td></td><td>0/1/1</td></tr><tr><td>Pd(2%)/S-AC<sup>g39</sup></td><td>batch</td><td>87</td><td>42.1</td><td></td><td>2/30/1</td></tr><tr><td>Pd(0.6%)/HHDMA<sup>h</sup>/AC<sup>40</sup></td><td>batch</td><td>80</td><td>50.4</td><td></td><td>0/40/0.5</td></tr><tr><td>Pd<sub>1.8%</sub>873<sup>i41</sup></td><td>batch</td><td>93</td><td>248</td><td></td><td>r.t./30/0.33</td></tr><tr><td>0.2Pt0.2Au4.6Pd<sup>i33</sup></td><td>batch</td><td></td><td>184</td><td></td><td>2/14/0.5</td></tr><tr><td>2.5Au2.5Pd<sup>i42</sup></td><td>batch</td><td></td><td>252</td><td>0.5</td><td>2/29/0.5</td></tr><tr><td>AuPd@SZ<sup>i43</sup></td><td>batch</td><td>55</td><td>13.6</td><td></td><td>r.t./1/5</td></tr><tr><td>1Au3Pd<sup>i44</sup></td><td>batch</td><td>56.6</td><td>1141</td><td>0.12</td><td>0/38/3</td></tr><tr><td>AuPd<sup>i45</sup></td><td>batch</td><td>60.3</td><td>988.8</td><td>5.8</td><td>10/10/30</td></tr><tr><td>AuPd<sup>i46</sup></td><td>batch</td><td>99</td><td>426.7</td><td>0.12</td><td>5/20/1</td></tr><tr><td>Co<sub>x</sub> Pd/C<sup>i47</sup></td><td>batch</td><td>88.1</td><td>83</td><td></td><td>2/30/1</td></tr><tr><td>1.0Au2.0Pd<sup>i48</sup></td><td>semibatch</td><td>48</td><td>69.9</td><td></td><td>10/1/n.a.</td></tr><tr><td>Au(2.2%)/@hollowSiO<sub>2</sub><sup>49</sup></td><td>semibatch</td><td>15</td><td>24.8</td><td></td><td>10/1/n.a.</td></tr><tr><td>Pd(5.6%)/s-TiO<sub>2</sub><sup>50</sup></td><td>semibatch</td><td>21</td><td>85.7</td><td></td><td>30/n.a./n.a.</td></tr><tr><td>Pd(4.1%)/Sn/s-TiO<sub>2</sub>/H<sub>2</sub> SO<sub>4</sub><sup>50</sup></td><td>semibatch</td><td>77</td><td>378.9</td><td></td><td>30/n.a./n.a.</td></tr><tr><td>Pd<sub>50</sub> Te<sub>1</sub>/Al<sub>2</sub> O<sub>3</sub><sup>51</sup></td><td>semibatch</td><td>52.9</td><td>84.7</td><td></td><td>10/1/n.a.</td></tr><tr><td>Pd<sub>1</sub> Au<sub>0.7</sub>/TiO<sub>2</sub><sup>52</sup></td><td>continuous</td><td>86</td><td>36</td><td>2.9</td><td>23/10/n.a.</td></tr><tr><td>Pd/Al<sub>2</sub> O<sub>3</sub><sup>53</sup></td><td>continuous</td><td></td><td>2.8</td><td>0.388</td><td>25/1/n.a.</td></tr></tbody></table>

<sup>a</sup>The H₂O₂ production rates have been converted to mmol/g<sub>cat</sub>-h from the original sources. <sup>b</sup>r.t.” = room temperature; “n.a.” = not applicable.

<sup>c</sup>Nanotubes. <sup>d</sup>A carbon black. <sup>e</sup>Nitrogen-doped porous carbons stabilized by poly(styrenesulfonate). <sup>f</sup>N-doped porous carbon sphere. <sup>g</sup>Sulfonate activated carbon. <sup>h</sup>Hexadecylhydroxydimethylammonium. <sup>i</sup>N-containing carbon nanotubes heated to 873 K. <sup>j</sup>Sulfonated zirconia.

overview of the current state-of-play of four alternative processes for H₂O₂ production, thermochemical, electrochemical, bioelectrochemical, and plasmatic, the main economic, environmental, and safety challenges of these alternative processes are critically reviewed and compared against the AO process. The aim is to highlight the deficiencies that currently hamper the market penetration of these alternative technologies and thus provide impetus to address them. An overview of the alternative processes is shown in Figure 1.

## ALTERNATIVE TECHNOLOGIES FOR H₂O₂ PRODUCTION

**Thermochemical.** Thermochemical processes, specifically the direct synthesis of H₂O₂ (DSHP) and the synthesis from CO/O₂/H₂O mixtures, offer promising alternatives for H₂O₂ production. DSHP involves the catalytic reaction of H₂ and O₂ to form H₂O₂, offering a simplified, one-step process. However, challenges such as the explosive nature of H₂/O₂ mixtures and the competitive formation of H₂O necessitate stringent safety measures and catalyst optimization. On the other hand, synthesis from CO/O₂/H₂O mixtures leverages the in situ formation of H₂O₂ via intermediate species, providing improved control over reaction conditions and potentially mitigating safety concerns. The primary distinction lies in the reactants and their implications for process safety and scalability, with the CO-based method offering enhanced versatility under milder reaction conditions.

While a one-step DSHP (eq 1) could simplify H₂O₂ production, this reaction is complex, with exothermic and thermodynamically favorable side reactions (eqs 2–4). Reaction pathways depend on the catalyst, reaction medium additives, and conditions.⁶,²² Despite initial findings in 1914²³ and multiple patents,⁶ industrial-scale implementation remains

elusive. The main challenges of DSHP include managing the explosive H₂/O₂ mixture (*vida infra*) and achieving higher selectivity toward H₂O₂ instead of H₂O, as common catalysts also promote H₂ combustion and H₂O₂ decomposition.²⁴ Due to H₂O₂ instability, DSHP needs a solid catalyst and a H₂O/MeOH solvent to dissolve H₂ and O₂. To address mass transfer barriers, microbubbles of H₂ and O₂ have been suggested.²⁵ Viable H₂O₂ concentrations have been achieved with a decane-1-ol/H₂O solvent,²⁶ and proton-electron transfer can enhance H₂O₂ production in MeOH/H₂O.²⁷ However, these solutions do not fully mitigate explosion risks associated with high catalyst activity.²⁸

$$ \begin{gathered} \text{H}_2\text{O}_2 \text{ formation: } H_2 + O_2 \rightarrow H_2O_2; \Delta H = -135.8 \\ \text{kJ/mol}; \Delta G^\circ = -120.4 \text{ kJ/mol} \end{gathered} \tag{1} $$

$$ \begin{gathered} \text{direct oxidation to H}_2\text{O: } H_2 + 0.5 O_2 \rightarrow H_2O; \\ \Delta H = -241.6 \text{ kJ/mol}; \\ \Delta G^\circ = -237.2 \text{ kJ/mol} \end{gathered} \tag{2} $$

$$ \begin{gathered} \text{H}_2\text{O}_2 \text{ decomposition: } H_2O_2 \rightarrow H_2O + 0.5O_2; \\ \Delta H = -105.8 \text{ kJ/mol}; \\ \Delta G^\circ = -116.8 \text{ kJ/mol} \end{gathered} \tag{3} $$

$$ \begin{gathered} \text{H}_2\text{O}_2 \text{ hydrogenation: } H_2O_2 + H_2 \rightarrow 2 H_2O; \\ \Delta H = -211.5 \text{ kJ/mol}; \\ \Delta G^\circ = -345.0 \text{ kJ/mol} \end{gathered} \tag{4} $$

In DSHP, the solvent plays a crucial role in facilitating the reaction. Acidic additives, such as phosphoric acid, stabilize
<figure><img src="image_12.png" /><figcaption>**Figure 2.** H₂O₂ synthesis using a Pd membrane reactor. H₂ is generated from water and electricity and is separated from O₂ for hydrogenation via a Pd membrane. Adapted and reprinted with permission.⁸ Copyright 2022 American Chemical Society.</figcaption></figure>

<table><caption>**Table 2.** Comparison of Thermochemical Synthesis Methods for H₂O₂ Production</caption><thead><tr><th>advantages</th><th>disadvantages</th><th>H₂O₂ productivity and concentration or selectivity</th></tr></thead><tbody><tr><td><strong>DSHP</strong><ul><li>high efficiency and selectivity with noble metal catalysts like Pd and Au</li><li>potential for high H₂O₂ production rates</li><li>versatility in catalyst and support modifications to enhance performance</li></ul><strong>catalytic membranes</strong><ul><li>high efficiency and selectivity with noble metal catalysts like Pd and Au</li><li>potential for high H₂O₂ selectivity</li><li>facilitates complete consumption of H₂, reducing explosive risks</li><li>innovative designs can potentially improve productivity and selectivity</li></ul><strong>synthesis from CO/O₂/H₂O</strong><ul><li>utilizes cost-effective and widely available CO as feedstock</li><li>potential for lower energy usage</li><li>homogeneous catalysts offer good selectivity with proper ligand choices</li><li>noble-metal-free heterogeneous catalysts also show promising activity</li></ul></td><td><ul><li>high cost of noble metal catalysts</li><li>catalyst deactivation due to dissolution in acidic environments</li><li>safety concerns with highly concentrated acidic solutions</li><li>potential for secondary reactions reducing H₂O₂ yields</li><li>the reaction rate is largely dictated by mass transport limitations</li><li>challenges in achieving high H₂O₂ concentrations in the liquid phase</li><li>need for further optimization for industrial scalability</li><li>homogeneous catalysts face rapid deactivation and limited turnover numbers, preventing industrial use</li><li>limited research on heterogeneous catalysts with issues in selectivity</li><li>deactivation of catalysts by water</li><li>mass transfer limitations affect overall efficiency</li></ul></td><td><ul><li>productivity: up to 426.7 mmol/g<sub>cat</sub>·h (e.g., Au-Pd catalysts)<sup>46</sup></li><li>concentration: up to 10 wt %<sup>17</sup></li><li>productivity: up to 16.8 mol·h·m<sup>2</sup> assuming 70 m<sup>2</sup>/g surface area<sup>54</sup></li><li>selectivity: 80–90% with Pd/Ag alloy membrane and hydrophobic polymer layer<sup>54</sup></li><li>productivity: up to 74.6 mmol/g<sub>cat</sub>·h with Au/TiO<sub>2</sub><sup>61</sup></li><li>concentration: achieved up to 32 wt % with optimized conditions<sup>61</sup></li></ul></td></tr></tbody></table>

H₂O₂ by lowering the pH and mitigating decomposition in the presence of basic impurities or reactive surfaces.²⁹ HCl and HBr further enhance H₂O₂ selectivity by providing both acidic and anionic effects, which suppress side reactions like decomposition and overhydrogenation. However, the highly acidic conditions, combined with halides, necessitate the use of corrosion-resistant materials.³⁰

Most catalysts for DSHP rely on noble metals supported on materials such as alumina, silica, and carbon (see Table 1).²⁴ Palladium is often used in combination with supports designed to reduce corrosion or enhance stability, such as tungsten oxide on zirconia or functionalized carbons.³¹,³² Hydrophobic supports, like fluorine-treated carbon, reduce H₂O₂ side reactions by improving mass transport and preventing degradation.³¹ Advanced designs include Pd catalysts anchored on sulfonic acid resins, which offer high yields at moderate temperatures (40 °C) in MeOH.²⁹ Alloying palladium with gold or platinum further enhances selectivity and stability, as evidenced by trimetallic Pt-Au-Pd catalysts that suppress H₂O₂ decomposition and increase overall yields.³³ However, challenges persist, particularly at high H₂O₂ concentrations, where side reactions and catalyst degradation are exacerbated.

Efforts to address these challenges focus on optimizing catalyst composition and structure, improving recovery and reuse of noble metals, and enhancing long-term stability under industrial conditions.²² H₂O₂ concentrations up to 10 wt % are achievable via DSHP but are limited by safety concerns and the high costs of noble metal catalysts, constraining broader industrial application.

Membrane catalysts offer a safer method for DSHP by keeping H₂ and O₂ separated, thus mitigating H₂/O₂ mixture explosion concerns.⁵⁴ Dense palladium membranes are particularly promising, enabling liquid-phase H₂O₂ synthesis by supplying atomic hydrogen to a catalyst layer on the membrane's surface. A Pd/Ag alloy membrane on α- or γ-Al₂O₃ substrates facilitates controlled oxidation, enhancing H₂O₂ selectivity to over 80%.³¹ Adding a hydrophobic polymer layer further improves selectivity by allowing H atoms to pass while blocking molecular H₂, thus favoring H₂O₂ formation on the catalyst surface.⁵⁵ The mechanism of H₂O₂ formation on Pd surfaces and clusters has been studied experimentally and theoretically.⁵⁶,⁵⁷

An advanced “catalytic diffuser” membrane can safely supply reactants and overcome the hydrogen mass transfer limitations
<table><caption>Table 3. Summary of Cathode and Anode Materials for H₂O₂ Production via 2e⁻ ORR</caption><thead><tr><th>materials</th><th>electrolyte</th><th>E versus RHE (V)</th><th>J (mA/cm²)</th><th>FE<sup>a</sup> (%)</th><th>H₂O₂ productivity (mg/L·h)</th><th>refs</th></tr></thead><tbody><tr><td colspan="7" style="text-align: center; font-weight: bold;">cathode</td></tr><tr><td rowspan="2">noble metals</td><td>Pt-Hg</td><td>0.1 M HClO₄</td><td>0.6</td><td>3.0</td><td>90</td><td>65</td></tr><tr><td>Pd/TiO₂</td><td>0.1 M KOH</td><td>0.88</td><td></td><td>86</td><td>63</td></tr><tr><td rowspan="5">carbon</td><td>MCHS<sup>b</sup></td><td>0.1 M PBS<sup>c</sup></td><td>0.62</td><td>1.5</td><td>99</td><td>594</td></tr><tr><td>N-FLG<sup>d</sup></td><td>0.1 M KOH</td><td>0.8</td><td>15.4</td><td>95</td><td>936</td></tr><tr><td>O-CNT<sup>e</sup></td><td>0.1 M KOH</td><td>0.72</td><td>20</td><td>90</td><td>3950</td></tr><tr><td>GF<sup>f</sup></td><td>0.05 M Na₂SO₄</td><td></td><td>12</td><td>63</td><td>160</td></tr><tr><td>SACs</td><td>Co-N-C</td><td>0.5 M H₂SO₄</td><td>0.1</td><td>2.9</td><td>80</td><td>80</td></tr><tr><td rowspan="3">metal oxide</td><td>Fe-C-O</td><td>0.1 M KOH</td><td>0.8</td><td>0.1</td><td>95</td><td>~500</td></tr><tr><td>Mo-OSG<sup>g</sup></td><td>0.1 M KOH</td><td>~0.7</td><td>0.1</td><td>95</td><td>81</td></tr><tr><td colspan="6" style="text-align: center; font-weight: bold;">anode</td></tr><tr><td rowspan="5">carbon</td><td>BiVO₄</td><td>1 M KHCO₃</td><td>3</td><td></td><td>35</td><td>83</td></tr><tr><td>Gd-BiVO₄</td><td>2 M KHCO₃</td><td>3.1</td><td>5.2</td><td>78</td><td>84</td></tr><tr><td>CaSnO₃-CF<sup>h</sup></td><td>2 M KHCO₃</td><td>2.9</td><td></td><td>90</td><td>73</td></tr><tr><td>CNFs<sup>i</sup>/NF<sup>j</sup></td><td>1 M Na₂CO₃</td><td>2.9</td><td>72.6</td><td>41</td><td>165.9</td></tr><tr><td>BDD<sup>k</sup>/Ti</td><td>1 M KHCO₃</td><td>3.5</td><td>88.2</td><td>28</td><td>11,895</td></tr><tr><td></td><td>BDD/Nb</td><td>1 M KHCO₃</td><td>3.0</td><td>300</td><td>87</td><td>1,55,856</td></tr></tbody></table>

<sup>a</sup>Faradaic efficiency. <sup>b</sup>Mesoporous carbon hollow sphere. <sup>c</sup>Phosphate-buffered saline. <sup>d</sup>Few-layered graphene. <sup>e</sup>Carbon nanotubes. <sup>f</sup>Graphite felt. <sup>g</sup>O/S dual coordinated graphene. <sup>h</sup>Carbon felt. <sup>i</sup>Carbon nanofiber. <sup>j</sup>Nanofiber. <sup>k</sup>Boron-doped diamond.

seen in dense Pd alloys.⁵⁴ This structure, with graded pore sizes and a fine-porous catalyst layer, boosts H₂O₂ productivity to 16.8 mol·h·m² in MeOH and selectivity to 80–90%. An electrically driven membrane-catalyst system sources H₂ directly from H₂O (Figure 2).⁸ A Pd-foil membrane separates the electrolysis and hydrogenation chambers, producing H atoms that react with O₂ to form H₂O₂. Optimizing the MeOH/H₂O ratio and catalyst design increased the H₂O₂ concentration 8-fold (from 56.5 to 443 mg/L), with Au-Pd catalysts reducing decomposition. Membrane-based synthesis allows for complete H₂ consumption, with H₂ transport through the membrane as the rate-limiting step. While this approach mitigates explosive mixtures, this technology remains far from industrialization.

H₂O₂ production can be also attained from CO, O₂, and H₂O mixtures according to eq 5.⁵⁸

$$CO + O_2 + H_2O \rightarrow CO_2 + H_2O_2 \quad (5)$$

This reaction is thermodynamically favorable ($\Delta G° = -134.2$ kJ/mol) and presents an option when a source of CO is readily available.⁶ The feasibility of this concept was first presented using homogeneous palladium triphenyl phosphine complexes as catalysts; however, these showed no potential for an upscaled process. Catalyst deactivation occurred rapidly due to the oxidation of phosphine ligands, resulting in the formation of colloidal palladium nanoparticles in the liquid phase.⁵⁹,⁶⁰ Au/TiO₂ can heterogeneously catalyze this reaction, with yields up to 9000 mmol·H₂O₂/g·Au·h. The mechanism and reaction kinetics of the reaction with this catalyst have been investigated.⁶¹

The advantages, disadvantages, and key product properties for the different thermochemical routes are summarized in Table 2.

**Electrochemical.** The electrochemical production of H₂O₂ proceeds primarily via the two-electron oxygen reduction pathway (2e⁻ ORR, eqs 6 and 7). However, this route is challenging because of the competitive reaction of O₂ to H₂O or OH⁻ through the four-electron pathway (4e⁻ ORR, eqs 8 and 9). For both pathways, the use of both acidic and alkaline media is well described in the literature.⁶²⁻⁶⁴ H₂O₂ can also be

produced via the two-electron water oxidation reaction (2e⁻ WOR, eqs 10 and 11) at the anode.⁶²,⁶⁵,⁶⁶ Due to the absence of gaseous reagents, the 2e⁻ WOR requires a simpler experimental setup than the 2e⁻ ORR. However, the 2e⁻ WOR requires a high overpotential (1.76 V vs RHE), and it also has a competitive four-electron oxidation (4e⁻ WOR) pathway (eq 12).

$$O_2 + 2H^+ + 2e^- \rightarrow H_2O_2 \quad E^0 = 0.68 \quad V_{\text{RHE}} \quad (6)$$

$$O_2 + 2H_2O + 2e^- \rightarrow HO_2^- + OH^- \quad E^0 = 0.06 \quad V_{\text{RHE}} \quad (7)$$

$$O_2 + 4H^+ + 4e^- \rightarrow H_2O \quad E^0 = 1.23 \quad V_{\text{RHE}} \quad (8)$$

$$O_2 + 2H_2O + 4e^- \rightarrow 4OH^- \quad E^0 = 0.4 \quad V_{\text{RHE}} \quad (9)$$

$$2H_2O \rightarrow H_2O_2 + 2H^+ + 2e^- \quad E^0 = 1.765 \quad V_{\text{RHE}} \quad (10)$$

$$H_2O \rightarrow OH^\bullet + H^+ + e^- \quad E^0 = 2.38 \quad V_{\text{RHE}} \quad (11)$$

$$2H_2O \rightarrow O_2 + 4H^+ + 4e^- \quad E^0 = 1.23 \quad V_{\text{RHE}} \quad (12)$$

The electrocatalyst is a vital component for the production of H₂O₂ via 2e⁻ ORR and 2e⁻ WOR. Electrocatalysts should have high activity, operating at low overpotential with high current density, and high selectivity, maximizing the 2e⁻ pathways while inhibiting the 4e⁻ pathways. Furthermore, they should display high stability, enabling long-lasting performance and long-term durability at low cost, for the feasibility of industrial-scale applications.⁶²,⁶⁷

**Cathode Materials.** To date, numerous catalysts consisting of noble metals, carbon, or their corresponding composites have been developed for the 2e⁻ ORR pathway. Single atom catalysts (SACs) have emerged as a next-generation option. Commonly used cathode materials for H₂O₂ production via the 2e⁻ ORR are summarized in Table 3. Generally, noble metal-based catalysts possess excellent activity and selectivity, but the applied metals are costly and scarce.⁶⁸,⁶⁹ Carbon-based catalysts provide numerous advantages because of their high activity and stability, but structure-performance relationships
<figure><img src="image_13.png" /><figcaption>**Figure 3.** Illustrations of the different configurations of electrochemical cells for H₂O₂ production. The blue color represents liquid-phase flows, while the colorless areas represent gas-phase flows. (a) H-cell, adapted with permission.⁷⁶ Copyright 2020 American Chemical Society; (b) gas-liquid separated flow cell, reproduced¹⁰⁰ with permission from the Royal Society of Chemistry; (c) dual-chamber reactor with membrane, reproduced⁸⁹ with permission from Springer Nature BV; and (d) dual-chamber reactor with venturi-based jet aerator, reproduced⁹⁴ with permission from Elsevier Science and Technology Journals.</figcaption></figure>

are challenging due to the complex carbon-based catalytic pathway. Efforts are needed to regulate the structure and functional groups to promote the 2e⁻ ORR.⁷⁰⁻⁷⁴ SACs maximize atom utilization and minimize size effects, which strongly enhance electrochemical H₂O₂ production, but fundamental knowledge on the reaction mechanism is still insufficient.⁷⁰,⁷⁵⁻⁷⁸

**Anode Materials.** Compared with the 2e⁻ ORR pathway, the 2e⁻ WOR requires no O₂ or aeration and is therefore simpler with greater prospects for practical applications. However, suitable anode catalysts that improve the selectivity of the 2e⁻ WOR and inhibit the competitive 4e⁻ WOR are required. Metal oxides and carbon-based materials have been used as catalysts (Table 3). Generally, stability is the crucial metric due to the high oxidizing potential required for the 2e⁻ WOR. Many metal oxides, like BiVO₄ and CaSnO₃, show good results regarding overpotential and H₂O₂ production rate. However, high efficiencies are usually based on low current densities.⁸⁰,⁸⁵⁻⁸⁹ Carbon materials demonstrate good H₂O₂ activity and selectivity.⁹⁰,⁹¹ The stability can be improved with composite carbon materials or diamond-like carbons.⁹²⁻⁹⁵ The effects of heteroatom dopants and the electrolytes on the catalytic mechanism have been studied.⁹⁶,⁹⁷

**Cell Configurations.** Apart from efficient anode and cathode catalysts, the cell configuration is critical for effective electrosynthesis of H₂O₂, particularly in industrial-scale applications.⁹⁸ Extensive efforts have been made to design effective cell devices. A continuous flow of electrolyte to prevent accumulation and decomposition of in situ-generated H₂O₂ is essential. Additionally, the cell setup should enhance mass transfer rates for an efficient 2e⁻ ORR. Different cell configurations for H₂O₂ production have been investigated (Figure 3). A review of the performance of the different cell configurations is available.⁹⁹

The H-type cell (Figure 3a) is commonly used in the laboratory. A proton-exchange membrane (PEM) is incorporated to separate the anode and cathode, which makes the synthesis of pure H₂O₂ solution feasible. However, the small, fixed volume and insufficient oxygen mass transfer limit the production of H₂O₂ at practical rates. Additionally, the low mass transfer and accumulation of H₂O₂ near the electrode surface lead to H₂O₂ decomposition.⁸⁰

Alternative flow cell configurations have been developed,⁶⁶,⁸³ e.g., a gas-liquid separated flow cell (Figure 3b). Due to the absence of gas transfer in the liquid, O₂ can pass through the electrode rapidly and immediately be reduced at the active sites. Additionally, the circulating liquid favors the transfer and collection of the produced H₂O₂, thus avoiding its decomposition at the electrode while achieving a high concentration.¹⁰¹ Similarly, a gas diffusion electrode (GDE) or natural air diffusion electrode (NADE), which contains a hydrophobic gas diffusion layer and acts as a membrane between O₂ and liquid electrolyte, achieves rapid O₂ transport to the active sites and enhances the stability of the electrodes by mitigating H₂O₂ corrosion of the catalyst.¹⁰²,¹⁰³ Compared to the normal GDE, a superhydrophobic NADE enables O₂/air to naturally diffuse to the electrode interface and shows longer operation stability.¹⁰⁴,¹⁰⁵

In a dual-chamber reactor with the cathode and anode chambers separated by a PEM (Figure 3c), the ion diffusion path and solution resistance are reduced by a narrow-gap cell based on a membrane electrode assembly (MEA). The liquid electrolyte is introduced into a narrow channel between the electrode and the membrane.⁹⁹ An alternative solution is a flow-through device that enhances the H₂O₂ desorption from the active sites, reducing the possibility of H₂O₂ degradation.⁹⁹ The addition of a venturi-based jet aerator (Figure 3d) can supply extra O₂ without additional energy consumption.⁹⁴

A solid-state electrolyte (SSE) device is much like a dual-chamber reactor (Figure 3c) and contains a functionalized proton-conducting copolymer microsphere in the middle chamber, separated from the electrodes by an anion and a cation exchange membrane (AEM and CEM, respectively). Produced ions are driven toward the SSE layer by Coulombic attraction and the internal electric field and recombine rapidly to form H₂O₂. Flushing the SSE with pure H₂O can provide H₂O₂ solutions without ionic impurities.¹⁰⁶ Based on the above cell structure, a wide range of H₂O₂ concentrations from 0.3 to 20 wt % were obtained while maintaining activity and selectivity in the SSE for 100 h.⁹⁵ By adding a boron-doped carbon cathode catalyst, the more economical 2e⁻ ORR was favored over the 4e⁻ ORR.¹⁰⁶ However, large-scale implementation remains difficult due to the internal resistance and permeability challenges.⁷²
<figure><img src="image_14.png" /><figcaption>**Figure 4.** Mechanistic representation of H₂O₂ production via BESs. Reprinted¹¹¹ with permission from Elsevier.</figcaption></figure>

<table><caption>**Table 4.** Summary of Different Types of BESs for H₂O₂ Synthesis</caption><thead><tr><th rowspan="2">system</th><th rowspan="2">anode/cathode</th><th rowspan="2">substrate/catholyte salt</th><th rowspan="2">reactor volume (anodic/cathodic)</th><th rowspan="2">membrane</th><th colspan="2">H₂O₂ production</th><th rowspan="2">refs</th></tr><tr><th>rate (mg/L·h)</th><th>conc.<sup>a</sup> (mg/L)</th></tr></thead><tbody><tr><td>MFC</td><td>carbon felt/graphite</td><td>acetate/Na₂SO₄</td><td>32 mL/50 mL</td><td>CEM</td><td>1.5</td><td>37.3</td><td>123</td></tr><tr><td>MFC</td><td>graphite/graphite</td><td>acetate Na₂SO₄</td><td>125 mL/125 mL</td><td>CEM</td><td>0.25</td><td>2.1</td><td>124</td></tr><tr><td>MFC</td><td>graphite/graphite</td><td>glucose/Na₂SO₄</td><td>80 mL/80 mL</td><td>CEM</td><td>6.6</td><td>80</td><td>125</td></tr><tr><td>MEC</td><td>carbon cloth/GDE</td><td>acetate/NaCl</td><td>200 mL/18 mL</td><td>AEM</td><td>4.2</td><td>3800</td><td>118</td></tr><tr><td>MEC</td><td>graphite/GDE</td><td>domestic ww<sup>b</sup>/NaCl</td><td>9.4 mL/9.4 mL</td><td>CEM</td><td>3.2</td><td></td><td>126</td></tr><tr><td>MEC</td><td>carbon felt/graphite</td><td>acetate/Na₂SO₄</td><td>32 mL/50 mL</td><td>CEM</td><td>88.2</td><td>705.6</td><td>127</td></tr><tr><td>MEC</td><td>carbon fiber/GDE</td><td>acetate/tap water</td><td>100 L/10 L</td><td>AEM</td><td>0.02</td><td>9</td><td>113</td></tr><tr><td>MEC</td><td>graphite/graphite</td><td>acetate/Na₂SO₄</td><td>10 L/10 L</td><td>CEM</td><td>10.8</td><td>420</td><td>128</td></tr></tbody></table>

<sup>a</sup>Concentration. <sup>b</sup>Wastewater.

To date, the development of electrochemical reactor system designs is less advanced than the catalyst design. H-cells are useful for catalyst screening, while flow cells are more promising for high-throughput H₂O₂ production and improve selectivity by carrying H₂O₂ away from the cathode surface before further reduction occurs. The reactor design should also consider the energy cost for the circulation of O₂, which can be assisted with the use of GDEs. Additionally, the MEA design tends to have a high single-pass efficiency due to a decreased diffusion path. While SSE designs enable highly pure H₂O₂ products with low concentrations, there are still permeability issues to be solved.

**Bioelectrochemical.** Bioelectrochemical systems (BESs), which integrate microbial metabolism with electrochemistry, are considered a promising approach for obtaining energy from biomass. Two main types of BESs are microbial fuel cells (MFCs) and microbial electrolysis cells (MECs). The common principle is that electroactive bacteria initiate electrochemical reactions from the organic substrate in the anodic chamber. The electrons are transferred to the cathodic chamber, which is used to produce bioelectricity in MFCs or H₂ in MECs, see Figure 4. MECs require exogenous energy to overcome thermodynamic barriers.¹⁰⁷,¹⁰⁸ When O₂ is utilized as the electron acceptor at the cathode, H₂O₂ is produced via the 2e⁻ ORR.¹⁰⁹ By adjusting working modes, both MFCs and MECs have been applied for H₂O₂ synthesis via the 2e⁻ ORR.¹⁰⁸,¹¹⁰

The anodic electrode surface should enable the growth of electroactive bacteria and allow a high electron transfer rate. Thus, certain characteristics, such as biocompatibility, appropriate mechanical strength, good conductivity, chemical stability, and large surface area, are important for the selection of anode materials.¹¹² The most commonly used material that

satisfies the above requirements is carbon, including carbon cloth, carbon felt, carbon fiber, graphite felt, and graphite fiber.¹¹²,¹¹³ To strengthen bacteria attachment on the anode, modifications of the electrode surface by NH₃ pretreatment and nitrogen doping have been investigated, which enhance the microbial interaction through hydrogen bonds or van der Waals forces.¹¹⁴⁻¹¹⁶ In addition, nanomaterials such as carbon nanotubes (CNTs) have proven to enhance the bacterial attachment to the electrodes.¹¹⁷ In general, the electrode materials used in MFCs and MECs possess similar properties. CNT and graphene are commonly used in the anodic materials of MFCs, while graphene-based derivatives and composites are mostly reported in MECs.¹⁰⁷

Both pure organic and waste organic substrates can be used as anolytes in H₂O₂-producing BESs. Substrates such as acetate, glucose, and cellulose are commonly used as a carbon source for the microorganisms. An H₂O₂ production rate of 3.3 mg/L·h was achieved in an MEC with the use of municipal wastewater.¹¹⁸ However, primary sludge produces more H₂O₂ due to its higher chemical oxygen demand content and an increase in electrons transferred for H₂O₂ generation.¹¹⁸ Nonetheless, the highest production rate was achieved with a pure substrate. The addition of acetate resulted in a H₂O₂ production rate 40 times higher than that for real wastewater,¹¹⁹ which could be attributed to the high biodegradability of acetate or to the low conductivity and high internal resistance of wastewater.¹¹²

Like anode materials, carbon-based materials are also preferred for cathodes.¹¹³ It was found that graphite particle electrodes exhibited the optimum performance for H₂O₂ synthesis,¹²⁰ since graphite electrodes inhibit the 4e⁻ ORR and prevent further H₂O₂ decomposition to H₂O.¹⁰⁸ An extra coating layer, such as a nonprecious metal, often enhances
<figure><img src="image_15.png" /><figcaption>Figure 5. Schematic of different discharges used in plasma-liquid interactions, redrawn.¹³⁵ ©IOP Publishing. Reproduced with permission. All rights reserved.</figcaption></figure>

H₂O₂ production.¹¹³ Studies indicated a 10-fold higher H₂O₂ production rate using a GDE over a graphite cathode electrode.¹¹⁹ Additionally, several air-cathodes, such as a graphene-based air-cathode¹²¹ and a pure carbon black air-cathode,¹²² showed promising results in H₂O₂ production.

The properties of the catholyte also substantially influence the production and stability of H₂O₂.¹⁰⁸ The stability of H₂O₂ is quite sensitive to the catholyte pH, where a neutral pH is favorable for the microorganism metabolism and lowers the overpotential (pH gradient) between the two chambers, even though most studies report the highest H₂O₂ concentration in an acidic environment.⁶⁸ As an alternative, tap water was utilized as a catholyte for H₂O₂ production to avoid secondary pollution and the high cost of electrolytes.¹⁰⁸

BESs generally use the dual-chamber system separated with an ion exchange membrane, preventing the exposure of microorganisms to H₂O₂.¹¹¹ Table 4 lists examples of BESs for H₂O₂ production. The stability of H₂O₂ could be maintained when CEMs were used instead of AEMs,¹²³ due to the transport of protons from the anode to the cathode chamber to lower the cathodic pH. However, AEMs show unique advantages with real wastewater,¹¹⁸,¹¹⁹ as it avoids the diffusion of unwanted cations (e.g., Ca²⁺, Mg²⁺) and the acceleration of H₂O₂ decay by metal catalysis.¹¹³,¹¹⁴

**Plasmatic.** Unlike most other processes that employ catalysts, plasma-assisted processes generate H₂O₂ by igniting plasma with electrical energy. This partially ionized gas forms H₂O₂ under ambient conditions without complex catalysts or solvents, providing a cleaner production method. H₂O₂ can be formed either through the direct plasma activation of H₂ and O₂ in the gas phase (eq 13) or by a plasma/H₂O interaction (eq 14).¹²⁹

$$H_2 + O_2 \rightarrow H_2O_2 \quad \Delta H = 1.95 \text{ eV/molecule or } 1.6 \quad \text{kWh/kg} \quad (13)$$

$$2H_2O \rightarrow H_2 + H_2O_2 \quad \Delta H = 3.2 \text{ eV/molecule or } 2.5 \quad \text{kWh/kg} \quad (14)$$

Activation of H₂/O₂ in a dielectric barrier discharge (DBD) is generating interest since it promotes H₂O₂ formation at atmospheric pressure.¹³⁰ A proposed reaction mechanism for H₂O₂ formation in a double DBD reactor¹³¹ is shown in eqs 15–19.

$$H + O_2 \rightarrow HO_2 \quad (15)$$

$$2HO_2 \rightarrow H_2O_2 + O_2 \quad (16)$$

$$O + H_2^* \rightarrow OH + H \quad (17)$$

$$H + O_2^* \rightarrow OH + O \quad (18)$$

$$OH + H_2 \rightarrow H + H_2O \quad (19)$$

HO₂ is the main intermediate species in H₂O₂ formation (eqs 15 and 16). Concurrently, H₂O is also formed via a second pathway (eqs 17–19). The synthesis of H₂O₂ is mainly driven by the composition of the gas mixture, the reactor geometry, and the discharge characteristics. Low H₂O₂ selectivity, relatively high energy demand, and safety issues are the main bottlenecks of H₂/O₂ gas-phase plasma activation.

Direct H₂O₂ synthesis in a nonequilibrium plasma environment has been studied using an atmospheric pressure DBD reactor.¹³² This system features a high-voltage (HV) copper electrode covered with a Pyrex tube and a 1.4 wt % NaCl aqueous solution as a liquid grounding electrode, which also serves as a coolant. The grounding electrode prevents thermal H₂O₂ decomposition, while the Pyrex tube avoids reactions between O₂, H₂O₂, and the metal electrode, improving yield and selectivity.¹³² Using a H₂/O₂ mixture with 3 mol % O₂ (below the 6% explosive limit of H₂/O₂ mixtures, vida infra), H₂O₂ selectivity increases from 3.5 to 56.3%. However, despite the selectivity enhancement, the energy efficiency is poor due to significant heat loss (80 kWh/kgH₂O₂ versus the theoretical energy consumption of 1.6 kWh/kgH₂O₂), and the H₂O₂ productivity is restricted by the low O₂ content. By replacing the metal HV electrode in the DBD reactor with a saturated NaCl solution covered by a thin Pyrex tube (Figure S1), high-purity H₂O₂ can be safely produced with 64% selectivity and 62 wt % concentration (37 gH₂O₂/L·h) using 14.3 mol % O₂ content and 19 kWh/kgH₂O₂ energy demand.⁷¹,¹³¹

H₂O₂ selectivity can also be controlled by adjusting the specific energy input (SEI), with a lower SEI favoring H₂O₂ selectivity. However, low SEI leads to low O₂ conversions and H₂O₂ yields featuring high energy demand.¹³³ A potential solution may be the injection of Ar and H₂O into the plasma, serving as an effective energy carrier and enhancing the plasma chemistry, respectively. Up to 3.5 times higher O₂ conversion (60%) and H₂O₂ yield (53.4%) can be attained by applying 12.8 kWh/kgH₂O₂.¹³⁴

Discharge inside or in contact with the liquid phase triggers reactions at the plasma-liquid interface, generating reactive species in both the gas and bulk liquid phases and enabling electrochemical phenomena at the interface. Plasma-liquid interactions influenced by the discharge type are shown in Figure 5.¹³⁵

In plasma-liquid (aqueous solution) systems, H atoms and hydroxyl radicals are generated via H₂O dissociation. The H₂O₂ formation rate is driven by the H₂O concentration in the plasma phase and increases almost linearly with the amount of applied power,¹²⁹ as evaporation, sputtering, and the electric
field are enhanced (Figure S2). It is not clear, however, which factor is dominant. Details on H₂O₂ formation pathways in plasma-liquid/aqueous solutions can be found elsewhere.¹³⁵⁻¹³⁸

The estimated energy demand for H₂O₂ formation can range between 125 and 5000 kWh/kgH₂O₂ depending on the discharge type,¹²⁹ but recent results have improved that, e.g., 116.3–212.8 kWh/kgH₂O₂.¹³⁹,¹⁴⁰ The H₂O₂ concentration is affected by the pH of the liquid¹³⁹,¹⁴¹ and increases linearly with the total energy input.¹⁴⁰ By varying those parameters, the energy yield can be maximized, but erosion phenomena on the metal electrodes may occur via electrochemical reactions (anodic oxidation followed by electrolyte dissolution) or thermal effects.¹⁴⁰,¹⁴²⁻¹⁴⁴

In gas-phase discharges above aqueous solutions, both the H₂O₂ production rate and energy yield increase with higher discharge current and liquid conductivity, which can be adjusted using chemicals like NaCl or NaOH.¹⁴⁵ The H₂O₂ concentration, production rate, and energy yield (energy efficiency) are enhanced by longer residence times, higher power, larger gas-liquid interface areas, and higher reactor temperatures, which promote H₂O evaporation (up to the decomposition point of H₂O₂).⁹ Inert gases also influence H₂O₂ production by expanding the plasma interface film region at the plasma-liquid boundary and reducing H₂O₂ degradation due to a lower specific energy density.¹⁴⁸ DBDs are the common plasma reactors used, while recently a modular, adaptable and continuous DBD plasma tubular microreactor was developed providing advantages like increased heat and mass transfer rates, improved reaction control, and limited waste generation.⁹ Indicatively, H₂O₂ energy demand and concentrations recently achieved ranges between 143 and 588 kWh/kgH₂O₂ and 0.4 and 27 mM, respectively, by applying 0.9–5 W.⁹,¹⁴⁵⁻¹⁴⁷ The main disadvantages of gas-phase discharges over a liquid are the erosion of the electrodes associated with direct liquid-phase electric discharge¹⁴⁰ and the low mass transfer and energy yield, making the upscaling for industrial application difficult.¹²⁹

Plasma discharge in bubbles enhances yields and reduces electrode erosion¹²⁹ by improving gas-to-liquid mass transfer through larger interfacial areas, longer residence times, and higher internal pressures.¹²⁹,¹³⁷ Microsized O₂ or Ar bubbles are preferred for better mass transfer.¹³⁷,¹⁴⁸ This method generates large plasma volumes in water with lower voltage and energy use.¹⁴⁸,¹⁴⁹ The H₂O₂ formation rate is proportional to the input power.¹⁴⁹ Recently, a 164.6 mgH₂O₂/h production rate was achieved, with 111 kWh/kgH₂O₂ energy demand, using an eight-microhole reactor powered by solar panels, connected with a secondary reactor equipped with a 2D-TiO₂/g-C₃N₄ photocatalyst.¹⁵⁰ Table 5 summarizes the different plasma-assisted synthesis reactors and their SEI.

## ■ HOLISTIC PROCESS BENCHMARKING

**Energy Footprint.** Energy consumption is one of the indicators to comprehensively understand the feasibility of different processes, and the corresponding required energy inputs for both the AO process and emerging technologies are presented in Table 6. It has been reported that 0.8–1.6 kWh/kgH₂O₂ (2.9–5.7 GJ/1000 kgH₂O₂) at 50% product concentration is required for the AO process¹⁵¹ after energy optimizations, which saved up to 1.1 kWh/kgH₂O₂. The biggest energy contributors are purification (~0.4 kWh/kgH₂O₂) and

<table><caption>**Table 5.** Comparison of Specific Energy Input (SEI) for H₂O₂ Production Using Different Plasma Reactors</caption><thead><tr><th>reactor type</th><th>feed</th><th>SEI (kWh/kgH<sub>2</sub>O₂)</th><th>refs</th></tr></thead><tbody><tr><td>DBD with metal HV electrodes</td><td>gas-phase H<sub>2</sub>/O₂</td><td>80</td><td>132</td></tr><tr><td>DBD with liquid (aqueous solution) electrodes</td><td>gas-phase H<sub>2</sub>/O₂</td><td>19</td><td>131</td></tr><tr><td>DBD with metal HV electrodes</td><td>gas-phase H<sub>2</sub>/O₂/Ar/H<sub>2</sub>O</td><td>12.8</td><td>134</td></tr><tr><td>pin to pin (underwater plasma discharge); pin hole</td><td>liquid H<sub>2</sub>O; phosphate-based liquid electrolyte</td><td>116.3–212.8</td><td>139,140</td></tr><tr><td>Eight-microhole reactor (underwater plasma bubbles)</td><td>liquid H<sub>2</sub>O with Ar bubbles</td><td>111</td><td>150</td></tr></tbody></table>

gas compression (~0.4 kWh/kgH₂O₂). However, these numbers exclude the production of H₂. If H₂ is produced by SMR, the total energy consumption becomes 1.3–2.1 kWh/kgH₂O₂ and, for electrochemically produced H₂, 3.8–5.5 kWh/kgH₂O₂.¹⁵²

The reported energy consumption data for electrochemically produced H₂O₂ varies significantly, e.g., from 3.1–60 kWh/kgH₂O₂. This depends on many aspects, such as the applied electrode materials, cell configurations, and operating conditions. For instance, an energy input of 4.6 kWh/kgH₂O₂ at 20 mA/cm² was reported, compared with 19.4 kWh/kgH₂O₂ at 240 mA/cm², which is a direct influence of the decrease in the current efficiency.¹⁵³ An average energy consumption of 9.3 kWh/kgH₂O₂ was reported for a 46-day test.¹⁵⁴ Therefore, when operating more efficiently, the energy consumption is more likely to range from 3.1–10 kWh/kgH₂O₂. For BESs, an external potential is applied to enhance H₂O₂ production via an MEC.¹⁰⁷ Most studies on MECs are still conducted at lab scale, and maximum energy inputs up to 56 kWh/kgH₂O₂ were found, although for a small-scale 26 cm² prototype and 100 mL solution, an energy input of 0.3 kWh/kgH₂O₂ was reported.¹²⁸

Energy consumption for plasma-assisted H₂O₂ formation ranges between 12.8 and 1429 kWh/kgH₂O₂, making it the most energy-intensive emerging process. The energy demand for H₂O₂ synthesis through H₂ and O₂ plasma activation is lower (12.8–80 kWh/kgH₂O₂) than that for H₂O activation (111–1429 kWh/kgH₂O₂), as more energy is required to facilitate phenomena such as H₂O evaporation. Enhancing gas-liquid mass transfer and extending the residence time can significantly improve the energy efficiency of plasma-based processes.

However, for various emerging technologies, the energy input values shown in Table 6 were only calculated from the voltage input of the electrochemical cell and did not account for the energy requirements of other equipment such as pumps, heaters, cooling equipment, and concentration and purification units. Additionally, for the AO process, the best state-of-the-art techniques are used and emerging technologies still need to be further optimized, which could lead to a reduced energy input.

**Techno-Economic Feasibility.** Although various emerging technologies such as electrochemical, BES and plasma processes are potentially interesting alternatives for industrial H₂O₂ production, most reports mainly cover small-scale studies with very low H₂O₂ concentrations. Only limited information was found on economic assessments of capital and
<table><caption>Table 6. Comparison of Energy Consumption between Different H₂O₂ Production Processes</caption><thead><tr><th>process</th><th></th><th>energy input (kWh/kgH₂O₂)</th><th>scale</th><th>refs</th></tr></thead><tbody><tr><td>AO process (SMR-H₂)</td><td></td><td>1.3–2.1</td><td>state-of-the-art, industrial scale</td><td>155,156</td></tr><tr><td>AO Process (e-H₂)<sup>a</sup></td><td></td><td>3.8–5.5</td><td></td><td>152</td></tr><tr><td rowspan="3">emerging technologies</td><td>plasma</td><td>12.8–1429</td><td>lab scale</td><td>134,157</td></tr><tr><td>electrochemical</td><td>3.1–10<sup>b</sup></td><td>lab scale</td><td>99,154</td></tr><tr><td>MECs</td><td>0.3–56<sup>b</sup></td><td>lab scale, pilot scale</td><td>111,128</td></tr></tbody></table>

<table><caption>Table 7. H₂O₂ Production Cost Estimates for Emerging Technologies versus State-of-the-Art</caption><thead><tr><th>type of process</th><th></th><th>GDE<sup>154</sup></th><th>GDE<sup>150</sup></th><th>2e- ORR<sup>152</sup></th><th>2e- ORR (reworked)<sup>152</sup></th><th>AO process<sup>158</sup></th><th>AO process<sup>156</sup></th></tr></thead><tbody><tr><td rowspan="2">capital cost (EUR/ton)</td><td>electrolyzer</td><td>276</td><td>n.a.</td><td>121</td><td>1103</td><td>276</td><td>n.a.</td></tr><tr><td>other units<sup>a</sup></td><td>OSBL</td><td>OSBL</td><td>OSBL</td><td></td><td></td><td></td></tr><tr><td rowspan="3">operational cost (EUR/ton)</td><td>raw materials</td><td>1214</td><td>n.a.</td><td>14</td><td>811</td><td>368</td><td>n.a.</td></tr><tr><td>utilities</td><td></td><td></td><td>305</td><td>375</td><td></td><td></td></tr><tr><td>others</td><td></td><td></td><td>103</td><td>2247</td><td></td><td></td></tr><tr><td>manufacturing cost (EUR/ton)</td><td></td><td>1490</td><td>810</td><td>543</td><td>4536</td><td>644</td><td>1104</td></tr></tbody></table>

operating costs of emerging technology at an industrial scale. For some technologies (BESs, plasma), the technology readiness level is still too low and energy consumption too high for a meaningful or positive economic assessment. As one of the most representative emerging technologies, electrochemical processes exhibit excellent performance under certain conditions. Based on the results of H₂O₂ electrosynthesis obtained at lab scale, the techno-economic feasibility of industrial implementation has been evaluated by some researchers for different scenarios (Table 7).

For instance, a total cost of 1490 EUR/ton (~1620 USD/ton)<sup>a</sup> for H₂O₂ production with a GDE at 150 mA/cm² was estimated, which comprises a capital cost of 276 EUR/ton (~300 USD/ton) and an operating cost of 1214 EUR/ton (~1320 USD/ton).<sup>159</sup> In contrast, lower estimated H₂O₂ production costs of 810 EUR/ton (~880 USD/ton) were based on the required replacement of the GDE electrode every 46 d.<sup>154</sup> However, the production capacity was not mentioned in either scenario. These manufacturing costs were most likely underestimated by neglecting additional costs for heat exchangers, pumps, separation units, etc. A techno-economic analysis of a H₂O₂ electrosynthesis in combination with a purification process to produce 70 wt % H₂O₂ was performed.<sup>156</sup> The estimated production cost was 543 EUR/ton (~590 USD/ton), which is much lower than that of the AO process (1104 EUR/ton, ~1200 USD/ton). Although the target system boundary comprised separation and concentration units, the outcome is highly dependent on the economic input parameters that were used, such as an assumed electrolyzer life span of 30 years.

For an economic comparison of the reported H₂O₂ electrosynthesis process<sup>156</sup> with the AO process, the available experimental data from the literature and chosen industrial reference input values were recalculated (Table S1). The designed process, which is composed of electrochemical, separation, and concentration units, corresponds to a typical minimum industrial target to produce 35 wt % H₂O₂ (25 ton/ d on a 100% H₂O₂ basis). In the electrochemical cell, a Na₂SO₄ solution is used as the circulating electrolyte for both chambers. In the anode chamber, O₂ is generated via the 4e⁻ WOR, and 75% of the gas is assumed to be recovered in the cathodic reaction. Meanwhile, H₂O₂ is produced via the 2e⁻

ORR in the cathode chamber, and the concentration reaches 4.6 wt % under certain conditions. The H₂O₂ mixture is then fed into a stripping unit where H₂O₂ is separated from Na₂SO₄. The remaining Na₂SO₄ solution is sent to the evaporator to remove the excess H₂O before returning to the cathode chamber as the recycled electrolyte. The H₂O₂ is purified and concentrated to 35 wt % in a rectification column. All related process parameters are listed in the Supporting Information (Table S2). The recalculation of capital and operational costs was completed based on an industrial standard.<sup>160</sup>

The recalculated manufacturing cost for industrial H₂O₂ production via an electrochemical process is greater than eight times that of the reported value. This indicates that both the reported capital and operational costs are significantly underestimated. According to the study,<sup>156</sup> only the electrolyzer and separation units are considered for the calculation of the capital cost. In contrast, heat exchangers, various buffer and storage vessels, pumps, and other auxiliaries are also included in the recalculations. The recalculated value of the electricity cost is slightly higher than that reported in the literature, even though a lower electricity price was adopted during the recalculation, since only electricity from the electrolyzer is counted in the study, excluding the superheated steam and chillers needed for H₂O₂ separation and purification. Additionally, increases in the relatively low benchmark prices for Na₂SO₄ (1.8 EUR/ton) and O₂ (40 EUR/ton) adopted in the reference result in higher recalculated values. Finally, the costs for utilities, labor, and maintenance are considered to be underestimated, which again increases the recalculated values.

Others<sup>161</sup> confirm the challenge to make emerging technologies for the electrochemical production of H₂O₂ economically viable. Here, it was anticipated that the cost of electrochemical H₂ production could be lowered through a combination with selective H₂O oxidation to H₂O₂ over boron-doped diamond (BDD) electrodes. It was found that despite H₂O₂ being a more valuable coproduct than O₂, the levelized cost of H₂ was significant (62 USD/kg), mainly because of the high market price of BDD electrodes.

**Environmental Footprint.** While H₂O₂ is vital for many applications due to its strong oxidation potential and benign degradation products, its production through the AO process poses significant environmental challenges, including high

<footnote>ᵃElectrochemically produced. ᵇElectricity consumption from an electrochemical cell.</footnote>

<footnote>ᵃOSBL = outside battery limits (not considered).</footnote>
<table><caption>Table 8. Life Cycle Environmental Impact for H₂O₂ Production Process (AO) Utilizing H₂ Formed via SMR or CAE (Differentiated by the Energy Source and Geographical Area)<sup>a,b165,166</sup></caption><thead><tr><th rowspan="2">H₂ source</th><th rowspan="2">electricity and steam source</th><th rowspan="2">geographical area</th><th colspan="3">impact categories</th></tr><tr><th>global warming, kgCO₂-equiv/kgH₂O₂</th><th>Fossil resource scarcity, kgoil-equiv/kgH₂O₂</th><th>water consumption, m³/kgH₂O₂</th></tr></thead><tbody><tr><td>SMR</td><td>coal coking and gasification</td><td>China</td><td>2.5</td><td>0.455</td><td>0.035</td></tr><tr><td>SMR</td><td>grid electricity and fossil-based steam</td><td>Europe</td><td>1.79</td><td>0.60</td><td>0.076</td></tr><tr><td>CAE</td><td>coal coking and gasification</td><td>China</td><td>1.61</td><td>0.334</td><td>0.036</td></tr><tr><td>CAE</td><td>grid electricity and fossil-based steam</td><td>Europe</td><td>1.56</td><td>0.458</td><td>0.081</td></tr></tbody></table>

<sup>a</sup>Boundary limits are cradle-to-gate (from extraction of resources to production of H₂O₂), no mass or economic allocation applied. <sup>b</sup>Notes: Functional unit: 1 kg pure (100%) H₂O₂. LCIA Method: Recipe 2016 v1.09, midpoint (H). Data referring to the geographical area of Europe are retrieved from Ecoinvent version 3.10.1. system model cutoff (i.e., H₂O₂ production, product in 50% solution, chlor-alkali electrolysis, membrane cell).

energy consumption, hazardous waste generation, and fossil fuel reliance. While promising alternatives are the emerging electrochemical processes that are ideally fueled by renewable electricity, they are not yet industrialized. Currently, industry focuses on more energy-efficient and cost-effective applicable solutions, which involve transitioning to the consumption of lower carbon footprint H₂ and to a renewable electricity supply for the main utilities required to produce H₂O₂ via the AO process. Examples include green H₂ production via electrolysis powered by renewable electricity (e.g., photovoltaics) and the production of steam using heat pumps or electrified boiler technologies. Key H₂O₂ players, such as Evonik,<sup>162</sup> Solvay,<sup>163</sup> and Nouryon<sup>164</sup> have already invested in such technologies.

Studies focusing on the environmental performance of H₂O₂ production are limited; only one life cycle assessment (LCA) study<sup>165</sup> could be found in an international peer-reviewed journal, highlighting the research gap in the field. Alternatively, the environmental impact of the AO process can be retrieved from LCA databases, like Ecoinvent.<sup>166</sup> In addition, one can find publicly available environmental product declaration data<sup>167</sup> carried out for the benefit of H₂O₂ production companies. However, in that case, it is difficult to rationalize the data due to the limited detailed information provided.

According to the Ecoinvent database, the calculated global warming potential midpoint (GWP (100)) impact of the AO process is 1.79 kg of CO₂-equiv/kgH₂O₂. The data are based on the European market (average data from eight European producers), and the boundary limits extend from the extraction of natural resources to H₂O₂ ready to leave the gate of the production site ("cradle-to-gate" approach). The different process contributions are shown in Figure S3. The production process for the required H₂ (i.e., via SMR) is the main polluting driver, contributing 44% of the climate change impact of the AO process. H₂ impact is mainly attributed to the direct CO₂ emissions primarily generated from the combustion of natural gas to meet the energy requirements of the endothermic SMR process and from the CO₂ formed as a byproduct during the reforming stage.<sup>166</sup> Grid electricity, mixed-fossil-based steam, and natural gas-based heat are the main utilities used by the AO process, contributing 10.3, 27.8, and 7.8%, respectively, to the overall climate change impact.

The impact of the H₂ source on the environmental performance of the AO process was studied.<sup>165</sup> Focusing on H₂O₂ production in China, a cradle-to-gate LCA study investigated four different scenarios for H₂ production: SMR, coal coking and gasification, cracking of fossil-based hydrocarbons, and as a byproduct of the chlor-alkali electrolysis

(CAE). To develop a Chinese localized LCA, real life cycle inventory (LCI) data from seven Chinese companies were retrieved, except for information on organic chemical production that was obtained from Ecoinvent. Given that China's energy sector is mainly based on coal-fired power plants, electricity and steam were considered as sourced from coal. Climate change, water use, human toxicity-non cancer, and fossil resources were defined as the main impact categories, and the normalized results per 1 ton H₂O₂ (27.5 wt %) were presented using midpoint and end point indicators.

Using the same four H₂ sources listed above and adjusting the results<sup>165</sup> to 1 kg H₂O₂ (100%), the GWP (100) impact of H₂O₂ varies between 1.61 and 2.50 kgCO₂-equiv/kgH₂O₂, depending on the H₂ source. Figure S4 shows the breakdown of the carbon footprint (expressed as kgCO₂-equiv/kgH₂O₂) of these different H₂ sources on the AO process. The key pollutants were the coal-based production of electricity and steam, contributing 56–86% of the GWP (100) impact, depending on the H₂ source. The carbon footprint associated with H₂ production decreases in the order of 14.5 kgCO₂-equiv/kgH₂ (SMR) > 4.3 kgCO₂-equiv/kgH₂ (coal coking and gasification) > 1.9 kgCO₂-equiv/kgH₂ (cracking) > 0.74 kgCO₂-equiv/kgH₂ (CAE). The H₂ contribution is limited (3%) when green H₂ (originating from CAE) is used, leading to the lowest overall carbon footprint for the AO process (1.61 kgCO₂-equiv/kgH₂O₂).<sup>168–170</sup> On the other hand, while the highest footprints belong to SMR and coal coking and gasification, application of carbon capture and storage methods reduces the environmental impact of the H₂ production and consequently improves the environmental performance of H₂O₂ production, for example, by 69% for SMR and by 79% for coal gasification.<sup>169</sup>

An alternative, effective strategy to enhance the sustainability of the AO process involves minimizing its dependence on conventional electricity, a key pollution source. This can be achieved by increasing the penetration of renewable energy sources in the production process while still producing H₂ via SMR (see Figure S5). By switching from lignite-based electricity to 100% renewable electricity (wind-harvested), the overall AO GWP (100) decreases by 30%.

Table 8 summarizes the LCA of the AO process utilizing H₂ formed via SMR or CAE (differentiated by the energy source and geographical area) for the impact categories of global warming, fossil resource scarcity, and water consumption. The results are based on the above study<sup>165</sup> and on the Ecoinvent database.<sup>166</sup> When H₂ is formed via SMR, the use of coal-based electricity increases the GWP (100) impact of the AO process.
However, the effect on fossil resource depletion and water
consumption categories is lower than that for grid electricity.
The same applies when H₂ is produced through CAE. Using
green H₂, the AO process has the lowest environmental impact
among the different H₂ production routes discussed (e.g.,
SMR, coal coking, and gasification), which could be even lower
if grid electricity is replaced by 100% renewables (approx-
imately 11% lower in case of wind-harvested electricity).¹⁶⁶

**Safety.** The AO process has historically been favored for its safety, as it enables the production of H₂O₂ via an indirect reaction of H₂ with O₂, making it suitable for large-scale production.⁶,²² However, the process presents inherent risks related to the substances used and the operating conditions. One major concern is the exothermic decomposition of the produced H₂O₂, which can be initiated by elevated temperatures or incompatible substances such as transition metals or reducing agents, and lead under certain conditions to thermal runaway.¹⁷¹ The decomposition rate is influenced by the H₂O₂ concentration, making precise control of solution composition and temperature critical, particularly during extraction, purification, or distillation steps.¹⁷² The highly exothermic hydrogenation and oxygenation reactions also pose thermal runaway risks. Explosive atmospheres are another safety challenge. H₂ explosions may result from air/H₂ mixtures or leaks near the hydrogenator and auxiliary equipment. During the oxygenation, extraction, and regeneration phases, flammable organic solvents (C9–C10 aromatics) present a risk of air-organic vapor explosions. Special attention to H₂O₂/H₂O/ organic solvent mixtures during H₂O₂ extraction is required, as explosion limits depend on the chemical composition and operating conditions.¹⁷³ Similar risks have been identified in alternative auto-oxidation processes using other alcohols, such as isopropyl alcohol.¹⁷⁴ Finally, large-scale production using the AO process implies storage and transportation of H₂O₂, stages that present additional risks of thermal runaway.

Due to its extensive and long-standing application, the risks associated with the AO process are well-documented and effectively managed. In contrast, the more recently developed alternative processes have less comprehensive hazard documentation. Nonetheless, a first identification of the potential risks associated with these emerging processes is proposed based on the currently available information.

DSHP is a promising alternative to the AO process but poses substantial safety challenges linked to explosive gas mixtures, secondary reactions, and flammable solvents. First, the triphasic reaction involves a mixture of gaseous H₂ and O₂ that reacts on a solid catalyst in a liquid solvent. Consequently, DSHP presents a risk of explosion when H₂ and O₂ are mixed at concentrations within the explosive range. Ensuring safety requires meticulous control of the H₂:O₂ ratio, typically maintaining H₂ concentrations below 4 vol % or O₂ concentrations below 6% to remain outside the explosive range (Figure S6). However, such conditions can limit the H₂O₂ productivity. Higher H₂ and O₂ concentrations within the flammability range enhance both productivity and selectivity but require special and complex reactors for safe operation.¹⁷⁵

Second, side reactions, including H₂ combustion, H₂O₂
decomposition, and H₂O₂ hydrogenation to H₂O, further
complicate the process.⁶,²²,¹⁷⁶ These exothermic secondary
reactions reduce the overall efficiency and introduce additional
safety hazards, particularly in systems where heat removal is
challenging.²² The choice of catalyst is critical to inhibiting

undesired side reactions. Finally, solvents, typically MeOH or
other alcohols,¹⁷⁶ facilitate gas dissolution, improving reaction
efficiency.²² However, the high MeOH vapor pressure and
flammability pose explosion risks, with incidents reported
during experimental synthesis.¹⁷⁶ Alternatives such as H₂O-
based systems have been proposed but may sacrifice
efficiency.⁶

Innovative reactor designs aim to address these safety challenges. Membrane reactors physically separate H₂ and O₂, significantly reducing the risk of explosive mixtures. In these systems, H₂ is supplied as atomic H via a dense palladium membrane, which also acts as a catalyst. Similarly, designs like microreactors allow safe operation within the flammability domain of the H₂–O₂ mixture.¹⁷⁷,¹⁷⁸ The small dimensions of such processes both facilitate contact between the gas mixture and the reactor wall and inhibit the spread of flame and explosion inside the equipment.¹⁷⁷ Moreover, in the case of an explosion, less severe consequences are expected due to the small volumes and quantities operated. However, such systems remain complex and expensive for large-scale industrial deployment.¹⁷⁵ The alternative synthesis route using CO, O₂, and H₂O has shown potential, with heterogeneous catalysts like Au/TiO₂ achieving high H₂O₂ concentrations under mild conditions. While safer and suitable for integrated processes, this method is still in early development.⁶

The electrochemical synthesis of H₂O₂ has been extensively studied, yet safety considerations remain poorly addressed in the literature. The risks associated with this type of process vary widely, depending on the reactants, electrodes, electrolytes, and operating conditions involved. Fuel-cell electrochemical processes use H₂ and O₂ as reactants. The separation of gases by an electrolytic membrane minimizes the risk of explosive mixtures.¹⁷⁶,¹⁷⁹ However, the inherent hazards of handling H₂, such as leakage and potential ignition, persist and require safety measures. Other electrochemical processes use H₂O and O₂ as reactants, providing a safer alternative.¹⁷⁹ Commonly used electrolytes containing Na₂SO₄⁸⁰ and Na₂CO₃⁷² entail minimal risk. On the contrary, strong acids such as HClO₄⁶⁵ and H₂SO₄¹⁷⁷ present significant risks due to their corrosivity and potential for fast and exothermic reactions.

Finally, the electrochemical cell configurations can mitigate risks. For instance, flow cells designed to enhance mass transport reduce risks associated with H₂O₂ accumulation. Overaccumulation of H₂O₂ can lead to its decomposition, which generates heat and O₂,¹⁶ increasing explosion and fire hazards. Another example is the development of SSE systems that replace liquid electrolytes and offer a promising route for safer and more efficient H₂O₂ production. SSEs eliminate the need for postsynthesis separation steps, reducing process complexity and associated risks.

Available information is even more limited for other emerging processes that are at the lab scale, such as BESs. The unequal level of development and the diversity of the new technologies explored herein do not allow a direct comparison of their safety performance, particularly on an industrial scale. Advances in catalysts and reactor designs are notably essential to mitigate the safety issues.

Plasma-assisted processes present the advantage of operating directly with an H₂/O₂ feed without the need for any catalyst or solvent. For safety reasons, early developments on such a process were conducted strictly below 6 mol % O₂¹³¹ to stay out of the flammability domain. Moreover, the electric field in
the plasma reactor can be an ignition source for an explosion. Therefore, the control of both the O₂ content and the electric power represents important safety challenges¹³¹ that greatly limited the practicality of H₂/O₂ plasma methods in the past. To overcome these constraints, new optimized reactors were proposed for safer plasma production, allowing similar operations at higher concentrations, and a self-cooling-double discharge (SC-DDBD) reactor demonstrated the possibility to extend the concentration domain for safe use up to 30 mol % of O₂ in the feed gas mixture.¹³² As an explanation, the authors proposed that H₂/O₂ explosions in a plasma process would be mainly due to the formation of H₂O from OH and H₂, and the generation of OH from reactive species (mainly radicals) is fast and highly exothermic. On the contrary, in the SC-DDBD reactor, the concentration of reactive species could be reduced thanks to more uniform and diffuse discharges. This demonstrates well the critical importance of safety for the development of a plasma process suitable for industrial applications.

**Challenges and Future Perspectives.** While at present H₂O₂ production is dominated by the large-scale AO process, the drive toward more sustainable chemical processes with smaller CO₂ footprints continues. Although the AO process is economic, it can clearly be improved from energy and climate perspectives. Utilities (electricity) and H₂ production are the key sources of pollutants in the AO process. Increasing the share of renewable electricity into the AO process, either as utility (replacing fossil-fuel-based electricity and heat) or as the main energy source for raw material production (e.g., H₂), is necessary for reducing the environmental burden of the AO process. Sourcing enough H₂ via electrolysis to meet the production requirements of world-class AO plants has its own challenges. There are, in addition, efficiency and global sustainability questions related to the production of green H₂.

The transition to electricity-driven alternative technologies for H₂O₂ production could attain lower CO₂ footprints, considering that electrification will be coupled with the decarbonization of the grid electricity. However, the lack of demonstration at a larger scale is the current challenge to be overcome. Indeed, as fossil-fuel-based processes have an incumbency advantage from years of optimization, technological newcomers must offer other advantages that might offset higher costs, at least until these newcomers have had sufficient opportunity to optimize their processes near the extent fossil-fuel-based ones have. For example, while the DSHP reaction is aesthetically appealing, safety remains an inherent challenge since processes developed for the DSHP reaction must be keenly aware of the explosion potential of H₂/O₂ mixtures. The low acceptable concentrations of either one of the reactants limit H₂O₂ yields. While this can be avoided through the use of palladium-based membranes that provide atomic H rather than molecular H₂, these approaches suffer from low reaction rates and are costly to scale up due to the expense of palladium.¹⁷⁶

Despite the continuing optimization of catalysts, electrodes, and reactor configurations to achieve the electrosynthesis of H₂O₂ reported in the literature, relatively few reports have tested the electrochemical system on a practical industrial scale. Standard industrial operation requires high current density (>300 mA/cm²), low cell potential (<2 V), high selectivity toward H₂O₂ (>90%), long-term electrode stability (>1000 h), and relatively low material costs, all of which should be simultaneously achieved.¹⁰¹ Indeed, various

modifications, including heteroatom doping and the addition of supports, have been studied to improve the stability and durability of the electrodes. However, electrode stabilities are generally still too low (up to 100 h) for industrial applications. Hydrophobic carbon GDEs exhibit excellent H₂O₂ activity and selectivity, but flooding of the GDE might occur.⁹⁹ The electrode stability also faces a challenge under high current density for long-term operations.⁸⁹

For the deployment of BESs for H₂O₂ production at larger production scales, the complexity of the process is still a major challenge for upscaling. Stacking multiple BES modules in series is an appropriate option,¹¹¹,¹⁸⁰,¹⁸¹ but expensive. However, due to the system overpotentials, 2–3 orders of magnitude lower H₂O₂ production rate at a pilot scale was reported primarily due to its decomposition, as compared to that obtained for lab-scale studies. Additionally, the low efficiency and reduced rate of H₂O₂ production further impede the upscaling of BESs.¹⁸² Therefore, it has been considered that BES production of H₂O₂ for in situ oxidation for environmental remediation would be more practical than direct H₂O₂ synthesis.¹⁸²

Plasma-assisted H₂O₂ production may hold promise since, unlike other electrified technologies, plasma reactors and specifically DBD reactors are available for large-scale production. However, the poor energy efficiency, low yield, and high energy demand hinder wide industrial deployment. Minimization of the energy dissipation into the hardware (dielectric barrier) and selective boosting of the electron-impact reactions that effectively result in H₂O₂ formation may facilitate the commercialization of the technology. The use of an aqueous solution as a grounding electrode and cooling agent, removing resistive and reaction heat, has been suggested as one approach to improve the energy efficiency.¹³³ While plasma-based production of H₂O₂ presents a cleaner, safer, and more flexible alternative to traditional methods, the low yield and high energy demand currently limit the application of plasma-assisted H₂O₂ at an industrial scale.

Although the upscaling of emerging H₂O₂ technologies is a necessary step to determine their long-term commercial prospects, large-scale production along the lines of the AO process may not be needed. Issues related to transportation (logistics), safety (intermediate storage), and opportunities to benefit from fluctuations in renewable electricity supply may change the production landscape. Since H₂O₂ is produced centrally and in large quantities, transport of the product to customers, large and small, is required. This is perhaps an under-appreciated climate change contribution from H₂O₂ production. Since H₂O₂ can be unstable, particularly for concentrated solutions, decomposition hazards must be addressed during transport and storage. Hence, H₂O₂ is generally diluted to 50% (or lower), ensuring that at least half of the transported weight is H₂O.¹⁸³ While the climate change aspect of the transport of diluted H₂O₂ can be mitigated by using trucks running on electricity or renewable fuels, there are other environmental and social aspects that must still be considered, such as wear and tear on the transportation infrastructure and risks associated with the transport of a very oxidizing chemical through cities and towns. Additionally, stabilizers (like sodium phosphate, phosphoric acid, or acetanilide) to reduce further decomposition and contamination hazards are added to marketed aqueous solutions of H₂O₂. These stabilizers must then be removed or otherwise accounted for by H₂O₂ end-users, which places extra demands
on their processes. This extra handling decreases the overall energy efficiency and increases the climate footprint and cost of downstream applications.

Technologies with flexible capacities may serve the market better. Since electrified chemical production is less influenced by the economies of scale that have led to centralized chemical production process, a direct electrochemical production of H₂O₂ clearly has its advantages. Small and flexible H₂O₂ production facilities offer the potential of lower production cost by benefiting from periods when renewable electricity is abundantly available and cheap. This is of course under the precondition that operational savings compensate for the required investments in additional production and storage capacity. Such small, flexible facilities might additionally provide H₂O₂ locally to end-users, eliminating the need for long-term stabilization and transport and simplifying downstream production processes, as long as the desired H₂O₂ concentrations for the intended applications can be achieved. An advantage of modular, on-site, and on-demand H₂O₂ production might be the reduction of concerns about H₂O₂ decomposition, shipping, and the use of stabilizers and the ability to provide exact H₂O₂ concentrations as needed. All of these qualities will simplify downstream applications and improve their environmental and economic profiles.

Regardless of whether renewable electricity is integrated into the AO process or used to establish new H₂O₂ production technologies, the intermittency and geographic availability of renewable energy supplies that can be used to power electrified H₂O₂ production technologies will also affect production trends and subsequently dictate the features of the new technologies. For the use of renewable energy in industrial electrochemical plants, the process must be able to operate dynamically and in discontinuous mode. This dynamic operation is very challenging, as excessive dynamic operation (variable power) and frequent starts and stops may affect the efficiency and stability of the process and may decrease the material lifetimes. To improve this situation, the degree of robustness and the selectivity of any electrochemical reaction need to be addressed. Especially, the anode reaction has to tolerate a wide range of current densities. The cathode has to be tuned to optimize energy efficiency and downstream integration. At low current densities, the radical density might be low, and the formation of peroxo-compounds may not be efficient. If the current density is too high, a high gas fraction in the electrolyte will introduce mass transport limitations and, therefore, lead to the oxidation of H₂O and O₂ formation. Special attention has to be paid to the cell design to handle the variety of gas fraction ratios.

Future research and development efforts for electricity-
driven and on-demand H₂O₂ production processes should
focus on addressing these critical remaining issues.

* Design and synthesis of high-performance and stable electrocatalysts for both anode and cathode: a comprehensive understanding of the involved reaction mechanisms and catalyst structure is required to increase the selectivity toward H₂O₂ production and ensure stable performance.

* Superior electrochemical cell/reactor design: improved cell/reactor design can enable scalable, robust, and renewable energy integrated H₂O₂ production.

* Demonstration at a higher scale: all of the emerging H₂O₂ production processes are currently at lab scale.

Demonstration at a larger scale (i.e., pilot scale) is needed to gain hands-on experience on technical and integration challenges that might arise. Bridging the gap between lab- and large-scale applications will also be able to showcase the feasibility of the emerging processes.

* Product purification: current research efforts overlook H₂O₂ purification from byproducts or from liquid electrolytes. However, as the downstream product purification process steps depend on the targeted application of the produced H₂O₂ and can be the cost driver for large-scale production systems, they should be carefully considered.

* Minimization of the environmental impact across the entire electricity-driven H₂O₂ production process: key efforts should target reducing carbon emissions through increasing the penetration of renewable electricity and improving overall energy efficiency. Additionally, optimization of the system design to eliminate hazardous byproducts and to minimize resource consumption will be essential to achieve a truly sustainable, low-carbon H₂O₂ production pathway. Comprehensive LCA studies focused on fully electrified emerging technologies for H₂O₂ production are needed.

* Improved investigations of safety issues for alternative processes: analyses of these processes and their modular integration for on-demand production are needed to enable comprehensive risk management strategies, which are crucial for successful implementation at user sites. These must account for full integration at end-user sites, including, notably, substance storage and operator training, especially in industries unfamiliar with chemical synthesis.

In conclusion, it is hoped that this review of alternative H₂O₂ production processes has at least raised awareness of some of the technical, techno-economic, life cycle, and safety challenges that these technologies have to solve in order to compete with the entrenched AO process. As many of these challenges cannot be properly addressed based solely on lab-scale technology developments, efforts to scale up some of the more promising technologies and better evaluate their industrial fitness are required. For electrified solutions, the overall challenge above and beyond the pure technical challenges is one of infrastructure. Technologies reliant on renewable electricity generation and storage, necessary for any modular, "on-demand" H₂O₂ production process, must not be a slave to the inherent fickleness of renewable energy production. Grid stabilities that can tolerate the electricity supply fluctuations as increased amounts of renewable electricity enter the grid are also critical to the eventual success of electrified technologies. Furthermore, as more and more sectors of our society become electrified, the overall availability of renewable electrical energy and its best deployment between sectors for achieving the greatest reduction of society's CO₂ footprint must be addressed.

<figure><img src="image_16.png" /></figure>

ASSOCIATED CONTENT

<figure><img src="image_17.png" /></figure>

**Supporting Information**

The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acsomega.5c07503.

Figures of plasma reactors and processes, tables of recalculated values and various operational parameters for the techno-economic evaluations, figures of
GWP(100) breakdowns, the variation of electricity source on the H₂O₂ production footprint, and the explosion limits of H₂/O₂ mixtures (PDF)

<figure><img src="image_18.png" /></figure>

## AUTHOR INFORMATION

### Corresponding Authors

Hank Vleeming - Process Design Center BV, Breda NL-4824EH, The Netherlands; orcid.org/0000-0001-9526-9165; Phone: +31 76 5301 906; Email: vleeming@process-design-center.com

Evangelos Delikonstantis - AristEng S.à r.l., Luxembourg City L-2310, Luxembourg; orcid.org/0000-0002-4727-3327; Phone: +352 661 213 742; Email: evangelos.delikonstantis@aristeng.lu

Richard H. Heyn - SINTEF Industry, Oslo 0314, Norway; orcid.org/0000-0001-5670-4320; Phone: +47 982 43 927; Email: rhh@sintef.no

### Authors

Stavros-Alexandros Theofanidis - AristEng S.à r.l., Luxembourg City L-2310, Luxembourg; orcid.org/0000-0002-3859-7135

Amvrosios G. Georgiadis - AristEng S.à r.l., Luxembourg City L-2310, Luxembourg

Christianus J. W. Hop - Process Design Center BV, Breda NL-4824EH, The Netherlands

Xiaobin Yu - Process Design Center BV, Breda NL-4824EH, The Netherlands

Vasileia-Loukia Yfanti - AristEng S.à r.l., Luxembourg City L-2310, Luxembourg

Guillaume Fayet - Ineris, Parc Technologique ALATA, BP 2, Verneuil-en-Halatte 60550, France

Claire Villemur - Ineris, Parc Technologique ALATA, BP 2, Verneuil-en-Halatte 60550, France

Complete contact information is available at: https://pubs.acs.org/10.1021/acsomega.5c07503

### Notes

The authors declare no competing financial interest.

<figure><img src="image_19.png" /></figure>

## ACKNOWLEDGMENTS

This work was funded by the European Union (HYPER project—Grant Agreement No. 101091554). The views and opinions expressed are those of the authors only and do not necessarily reflect those of the European Union or the European Health and Digital Executive Agency (HaDEA). Neither the European Union nor the granting authority can be held responsible for them. The authors thank the HYPER consortium for helpful discussions and insights.

<figure><img src="image_20.png" /></figure>

## ADDITIONAL NOTE

<figure><img src="image_21.png" /></figure>

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