Sustainable alkane production from waste fatty acids via electrochemically coupled decarboxylation and hydrogenation

Domain: physics
Published: 2025-12-05
Editorial outcome: major_revision

Abstract
Upgrading fatty acid derivatives is a promising route to sustainable diesel and jet fuels, but conventional processes require high temperature, pressure, and large H₂ input, while contaminants in waste oil deactivate catalysts, demanding costly purification. Here, we present an integrated electrochemical strategy combining anodic decarboxylation, cathodic proton reduction, and olefin hydrogenation in one reactor, upgrading fatty acid derivatives to long-chain alkanes under mild conditions (60°C, 1 atm) without external hydrogen. A high alkane yield of 88.9% is achieved and the reported performance is competitive. Experiments and calculations reveal that fatty acid chain length governs product yield and distribution by influencing –H departure and Cγ –Cβ –COOH bond cleavage barriers. This approach shows high activity and selectivity toward diverse feedstocks, including unsaturated fatty acids, esters, mixtures, crude acids, and waste oils. Powered by solar energy, approximately 40 g of long-chain alkanes are produced in a 1 L reactor, highlighting its scalability and potential for green fuel synthesis.

Introduction
Long-chain hydrocarbons, including long-chain alkanes used as diesel and jet fuels, as well as long-chain olefins employed as chemical intermediates, are extensively utilized. Currently, large-scale production of long-chain hydrocarbons primarily relies on petroleum refining, which consumes a significant amount of electricity and emits substantial amounts of toxic and greenhouse gases1–4. Additionally, the resulting products often contain high levels of sulfur and harmful polycyclic aromatic hydrocarbons, failing to meet with the stringent international environmental and safety standards5–7. Therefore, developing high-yield, mild, and environmentally friendly synthetic routes for long-chain alkanes has become a priority.

Fatty acids (CHCOOH), derived primarily from edible oils, represent valuable feedstock for synthesizing long-chain hydrocarbons due to their structural similarity with –CH –unit8–11. In contrast to edible oils, non-edible oils are more cost-effective as they do not encroach upon food resources, thus avoiding competition with the food industry for raw materials. Annually, factories and catering services generate thousands of tons of waste oils, rich in fatty acids and their derivatives, highlighting a significant source of potential raw materials for sustainable energy solutions12. Typically, waste oil undergoes some treatment processes such as anaerobic digestion, aerobic treatment, and co-incineration, which can nonetheless give rise to considerable environmental concerns. Besides, waste oil can be transformed into sustainable fuels through the HEFA (Hydroprocessed Esters and Fatty Acids) process, which includes the steps like pretreatment (Fig. S1), catalytic hydrogenation, cracking, isomerization, and fractionation (Fig. 1a)13–20. However, these methods require harsh conditions, such as temperatures exceeding 250°C and elevated H₂ pressures (≥2 MPa), to reduce the carboxyl group (–COOH) or promote the cleavage of carbon-carbon bonds. Such requirements make these methods unsuitable for achieving commercial viability while simultaneously reducing energy consumption and reliance on fossil fuel-derived hydrogen. In reality, integrating hydrogen species into the reaction system is crucial as it significantly reduces the need for extra H₂ input, addressing a common challenge in many catalytic systems.

For instance, the decarboxylation of fatty acids to long-chain alkanes can be facilitated by hydrogen species derived from the photocatalytic reduction of water on the Pt/TiO catalyst9. To fully eliminate the reliance on fossil fuel–derived H₂, an electrocatalytic system can be employed to achieve both higher efficiency and an adequate supply of hydrogen species21. In such a system, the hydrogen species generated at the cathode are expected to be directly utilized for olefin hydrogenation. However, there is no report on an electrochemical decarboxylative coupled with hydrogenation strategy for the transformation of waste oils into alkanes, which entirely eliminates fossil fuel-derived H₂ input, while delivering high product yields22,23. Therefore, developing methods for such high-profit alkane production under mild conditions and without relying on fossil fuel-derived H₂ is imperative for advancing sustainable synthesis.

Here, we demonstrate a highly efficient electrochemically coupled approach that can proceed smoothly even with elevated contaminant levels (like water, salt (mainly NaCl), lipid polymers, food particles/colloid) in the feedstock, obviating the need for costly purification (Fig. 1b). Moreover, the hydrogen species generated through cathodic reduction is directly supplied for the hydrogenation of long-chain olefins produced via anodic oxidation, ensuring efficient integration of these processes. The productivity can be readily optimized. Our method consistently achieves high yields for saturated and unsaturated fatty acids, as well as their esters and mixtures. Investigations reveal that the chain length determines the transformation from carbocations to alkenes accounting for the barriers of –H departure and Cγ –Cβ –COOH bond scission. Importantly, our approach facilitates the conversion of crude fatty acids and waste oils to valuable products. Besides, we have successfully scaled up long-chain alkane production by seamlessly integrating decarboxylation and hydrogenation within a single reactor. This study introduces a comprehensive strategy for the conversion of waste oils into long-chain hydrocarbons, showcasing its potential for industrial applications.

Results
The production of long-chain alkanes from fatty acids via electrochemical coupling. In the process of electrochemically coupled conversion from fatty acids to long-chain alkanes, three key reactions occur, including anodic decarboxylation to olefins, cathodic proton reduction, and olefin hydrogenation. During anodic decarboxylation, solvents and reactants are prone to competitive adsorption and oxidation, which would result in a markedly low conversion of fatty acids. In addition, it is critical to avoid hydrogen evolution reactions (HER) at the cathode, as this would limit the availability of H species to complete olefin hydrogenation. We began by exploring the electro-decarboxylation of fatty acids in the conditions, including stearic acid (C ), KHCO , n-Bu NI, and hexafluoroisopropanol (HFIP), but trace of product was observed. Extensive testing of various solvents and bases, detailed in Tables S1, S2, identifies EtOH and KOH as the most effective for the conversion process.

Notably, a reduced KOH concentration of 0.4 equivalent also maintains yields (Fig. S2), which is beneficial for scale-up production. In contrast, the higher KOH concentrations decay yields due to local fatty acid depletion caused by OH⁻ ion buildup at the anode. In the term of electrolyte stability and conductivity, the n-Bu NPF electrolyte with the concentration of 0.06 M appears optimal, as indicated in Table S3, Fig. S3. Under these refined conditions, the yield of long-chain olefins can reach 83%. Additionally, to minimize solvent usage in a large-scale process, the concentrations of fatty acid are increased to 0.1 M and 0.2 M, which both yield satisfactory results (Fig. S4). The post-electrolysis analysis identifies 1-n-heptadecene as the major product at 54%, alongside 2-n-heptadecene (23%), 3-n-heptadecene (10%), 4-n-heptadecene (4%), and branched olefins (9%). A small portion of the feedstock is converted into heptadecanol ether, constituting less than 10% of the total products.

To increase the decarboxylation efficiency, various electrode materials were evaluated (Fig. 2a). Notably, under identical conditions, the yield obtained with a platinum (Pt) electrode is disappointingly low at less than 10% within 10-hour period. This poor yield is attributed to the propensity to catalyze the oxidation of ethanol24–26, which diverts the reaction away from the desired decarboxylation pathway. In contrast to metal, low-cost carbon materials without heteroatom (N, P, B) doping exhibit weak adsorption and minimal alcohol oxidation27–29. However, these materials possess a favorable affinity for electrodecarboxylation of carboxylic acids30. The performances of carbon-based materials show significant improvement: Acetylene black (AB) and carbon black (CB) achieve yields of 20–30%, whereas graphite (G) delivers the higher yield of approximate 60%. Furthermore, reduced graphene oxide (rGO), with its large specific surface area attributed to layered structure, enhances electron transfer and mass transport at the solid-electrolyte interface, further boosting the yield to 78%. To further optimize performance, porosity was introduced into the GO structure by etching with KOH at 100°C, producing a material referred to as p-rGO31. The successfully synthesized p-rGO exhibits a well-defined porous structure, as demonstrated by transmission electron microscope (TEM) (Fig. S5), X-ray diffraction (XRD) (Fig. S6), and Brunauer-Emmett-Teller (BET) analyses (Fig. S7). One point to note is that BET shows the reduced surface area for p-rGO considering that general BET characterization has limitations in micropores. Because electrolyte ions can effectively access microporous structures, the electrochemically active surface area (ECSA) measurement can be a more representative evaluation of electrochemically active surface sites of porous structures. As shown in Fig. S8, the ECSA of p-rGO remains comparable to that of rGO, most likely indicating similar density of electrochemically active surface sites. As reported, the presence of carbon defects can significantly enhance substrate adsorption and accelerate charge-transfer rate32,33. As shown in Fig. S9, Raman spectra demonstrate the D band at 1350 cm−1 and G band at 1570 cm−1, which corresponds to the breathing mode of structural disorder and the in-plane stretching mode of sp2 hybridized carbon atoms, respectively. Compared with rGO, the higher peak intensity (I) ratio (I /I ) of p-rGO corresponds to the higher concentration of structural defects. Further characterization using Fourier-transform infrared (FT-IR) spectroscopy reveals that the rGO contains a mix of sp³C–O and sp²C–O bonds, while the p-rGO is primarily composed of sp²C–O bonds (Fig. S10). The absence of a distinct C–H bending vibration in p-rGO, observed in rGO, indicates the removal of sp³CHₓ units, which associates with the formation of the porous structure. High-resolution C1s X-ray photoelectron spectroscopy (Fig. S11) furthers this finding, with the p-rGO exhibiting a lower binding energy and higher sp²C/sp³C ratio, directly linking its improved performance to the porous architecture. These structural and chemical improvements underline the advantage of p-rGO over commercial carbon materials, as reflected in the enhanced electrochemical performance. However, due to its low adhesion to carbon paper, a composite material comprising rGO and p-rGO in a 5:1 mass ratio was developed. As expected, this composite material significantly outperforms its individual counterparts, achieving a yield of ~90%. In contrast, the bare carbon paper (CP), composed of carbon fibers, affords only ~10% product yield. Throughout the electrochemical process, the FEs remain consistently high (Fig. S12), demonstrating high stability of electricity consumption during the entire process.

Further exploration into the voltage-dependent performance (Fig. S13a) reveal that, while the yields for the p-rGO and commercial G are nearly identical at 2 V, FE of p-rGO is 10% higher. As the applied voltage is increased, the difference in both yields and FEs becomes more pronounced, indicating that elevated voltages are more conducive to the decarboxylation process. This can be attributed to higher anode voltages enhancing interaction with negatively charged carboxylates, which in turn form carboxylate layers on the electrode surface that prevent other substances from accessing the electrode (Fig. S13b), consistent with the Frumkin effect. Additionally, the p-rGO exhibits high stability and durability, maintaining high performance over five consecutive cycles without any significant activity loss, as documented in Fig. S14. Moreover, the E–t curves over five consecutive cycles were shown in Fig. S15. The highly consistent profiles of these curves indicate minimal fluctuation, thereby confirming the high stability of the material during the electrocatalytic decarboxylation process. Furthermore, the Raman and FT-IR spectra demonstrate that the structure of the p-rGO can be well preserved (Figs. S9, S10). This stability underscores the potential of the optimized system for scalable and sustainable production processes.

To couple electrochemical decarboxylation and hydrogenation, we introduced a Pd/activated carbon (Pd/C, 20 wt.% Pd) catalyst into the solution, known for its hydrogenation performance on olefins34–36. Even when the Pd/C vs. olefin mass ratio is reduced from 0.01 to 0.001, there is no discernible impact on the completion time and yield (Fig. S16), underscoring the robust hydrogenation capacity of Pd/C.

To investigate the cathodic proton reduction process, we examined the electrochemically-assisted hydrogenation of 1-n-heptadecene via using ethanol oxidation as sacrificial anodic reaction. Both ethanol and water, as proton donors, can undergo cathodic electron transfer, converting protons into adsorbed hydrogen atoms that act as reducing agent in the electrolytic hydrogenation of organics37. Also, studies indicate that ethanol and water together enhance hydrogen species adsorption and that adding Pt/C or Pd/C catalysts to the solution promotes hydrogenation of alkynes to alkenes and ketones to alcohols38. Moreover, the absence of alkane formation in aprotic solvents like CH CN and dimethylformamide (DMF) (Table S4) suggests that the reductive hydrogen species originates from ethanol rather than hydroxide ion (OH⁻). As reported, on Pt/C cathode—the common catalyst for HER across nearly all pH conditions—hydrogen atoms readily combine to form H gas (with ΔG close to zero)39, yet the alkane yield is only 7.6% (Fig. 2b). Instead, graphite’s sp² hybridized carbon atoms, forming strong in-plane covalent bonds, exhibit the weaker interaction with hydrogen atoms, which aids in hydrogen atom diffusion and utilization40,41. The barrier for hydrogen gas formation on graphite is rather higher than on platinum, resulting in a higher concentration of weakly adsorbed hydrogen species on the graphite surface over the larger voltage range. Significantly, our experiments demonstrate that alkane yield increases to 82% when graphite rod is used as the cathode. Density functional theory (DFT) calculations were employed to suggest barrierless olefin hydrogenation for separated H atoms. It is evident that when H species transition from H molecule to separated H atoms, the barrier of olefin hydrogenation decreases significantly (Fig. 2c).

This brief overview presents a synopsis of the coupled synergy between electrochemical decarboxylation and hydrogenation. At the anode, the high potentials enhance the adsorption of carboxylates, forming a shielding layer that minimizes adsorption and oxidative competition with other substances. As shown in Fig. 2d, the productivity at the voltage of 5.5 V is 2.2 times higher than that at the voltage of 3.5 V, meaning that the productivity can be improved significantly through increasing the voltages. Another phenomenon reflecting the promoting effect of high-concentration substrate adsorption on the reaction is the concentration-dependent performance test. As shown in Fig. 2d, the initial reaction rate increases by 10 times and 5 times when the mole of fatty acids only increases from 0.1 to 0.3 mol and from 0.3 to 0.6 mol, respectively. These results suggest that increasing the voltage and concentration of fatty acid is very important to accelerate the electrochemical kinetics. On the other hand, for electrochemical cathodic reaction, EtOH can be reduced on the cathode through electrons supply and hydrogen ion acceptance, producing reductive hydrogen species. The high barrier required to release H₂ gas on the graphite means that these species are not primarily volatilized as H₂; instead, they are directed to Pd catalyst, which facilitates the hydrogenation of olefins to alkanes. As a consequence of coupling decarboxylation and hydrogenation, the selectivity for long-chain alkanes exceeds 95% at the voltages of 3.5, 4.5, and 5.5 V, with ~80% conversion, signifying a seamless conversion from fatty acids to long-chain alkanes without interference (Fig. 2e). Note that the higher voltage promotes more H species at the cathode to well synergize the hydrogenation and decarboxylation rate. Therefore, the reported productivity of long-chain alkanes is competitive, compared with relevant photocatalytic and thermocatalytic studies (Fig. S17, Table S5)18,42–52.

Conversion of fatty acids with different chain length and waste fatty acid derivatives. The composition of waste oil is complex, containing a variety of fatty acids and derivatives with different chain lengths. To better understand the mechanism of electrochemical coupling conversion of waste oil and further improve the conversion rate of waste oil, we probed the conversion behavior of fatty acids with different chain length. Fatty acids ranging from C to C were utilized in this study (Fig. 3a). Fatty acids with under 10 carbons primarily produced nucleophilic products, such as ethers and alcohols, rather than long-chain olefins. C and C fatty acids achieve moderate yields (30~40%) of C and C olefins, respectively. In contrast, for C –C fatty acids, the yields of C –C olefins are significantly higher, reaching 60–90% and peaking with C fatty acid. The GCMS spectra of products are shown in Figs. S18~S29.

The cyclic voltametric results also indicate non-trivial differences in product distribution based on chain length (Figs. 3b, S30). For C to C fatty acids, 1-n-olefin accounts for only 30~40% of the olefins, with the remainder being thermodynamically more stable olefins. Instead, for C to C fatty acids, the 1-n-olefin proportion rises to around 50%, indicating that long-chain fatty acids are more likely to retain the α-carbocations after decarboxylation. Intriguingly, with further chain elongation in C to C fatty acids, a two-carbon atom departure in the carbocations follows –COOH group removal. Figure 3c illustrates the reaction pathways of carbocations with different carbon length. Obviously, the kinetics of α-carbocation transformation partially determines the olefin production, thereby affecting the efficiency of final alkane.

The chain-length-dependent kinetics of α-carbocation (R–CH +) is crucial for improving the yield and selectivity of olefins. The initial intermediate species formed during fatty acid decarboxylation is the α-carbocation, and its transformation pathways may be: (1) Directly converting into 1-n-olefins; or (2) Undergoing rearrangement to form other, more stable carbocations, which then convert into other types of olefins. The higher proportion of 1-n-olefins is primarily attributed to α-carbocations, which are favored by reaction kinetics. In contrast, rearranged olefins originate from rearranged carbocations, such as β- and γ-carbocations, and their formation is driven by the thermodynamic stability of internal alkenes. Collectively, the longer the carbon chain, the more favorable the stabilization and dehydrogenation kinetics of the α-carbon carbocation, thereby improving the yield and selectivity.

DFT calculations were performed to examine the dehydrogenation behaviors of α- or β-carbocations derived from fatty acids of varying chain length (Fig. S31, the atomic coordinates of the optimized computational models refer to the provided supplementary data). (a) Short-chains (e.g., C ): Geometry optimization failed to stabilize the α-carbocation, indicating its tendency to rearrange into the more stable β-carbocation. The β-carbocation favors OH quenching over high-barrier dehydrogenation, aligning with the lower olefin yield and selectivity observed for C fatty acid. (b) Long-chains (e.g., C , C ): The α-carbocation is structurally stabilized, enabling low-barrier dehydrogenation to olefins without OH assistance, consistent with the higher olefin yield and selectivity from C and C fatty acids. (c) Longer-chains (e.g., C , C ): Besides olefin formation, the α-carbocation can undergo low-barrier Cβ –Cγ bond cleavage, generating a truncated species with two fewer carbons, explaining the chain-shortened products observed.

Calculations above have mentioned that fatty acids with different chain length follow different pathways, thus mixtures encompassing a spectrum of fatty acids were used to suggest that different fatty acids would not interfere with each other. Indeed, all mixtures yield olefins at yields of 70~95%, underscoring the industrial viability of our method in manufacturing long-chain hydrocarbons. Besides, for unsaturated fatty acids, it is worth noting that our decarboxylation approach can circumvent the formation of alkyl radicals through employing porous carbon material, which are known to non-Kolbe, heterolytic decarboxylation pathways and minimize surface radical accumulation, thus preventing unwanted radical self-coupling and coupling with unsaturated fatty acids53–55. As shown in Table S6 and Fig. S32, unsaturated fatty acids achieve high olefin yields of 70~80% with high selectivity. Also, fatty acid esters were leveraged in one pot, commonly sourced from animal fats and plant oils, in our decarboxylation process. By elevating the KOH to 1.4 equivalent and incorporating H O to facilitate preliminary ester hydrolysis, the yields of 70~80% can be successfully achieved (Table S6), highlighting the efficiency in converting esters to valuable olefins.

Further, the conversion of industrial crude fatty acids was operated to verify the feasibility of electrochemical approach. For the production of ‘green’ alkanes, it is advantageous to utilize non-food feedstock to prevent competition with the food sector56,57. Crude soybean and tall oil fatty acids were directly reacted, which are by-products from soybean oil refining and the pulp industry, respectively. Soybean oil is predominantly composed of C and C acids/esters, with 48.8% additives, whereas tall oil mainly contains C acids/esters and 10% additives (Figs. 3d, S33). Both oils demonstrate high yields of around 85%. The conversion of soybean oil yields a mix of C , C , and other olefins, whereas tall oil primarily produces C olefins (Fig. 3e). Typically, a variety of organic compounds in waste fatty acids impede the conversion of fatty acids in the way of thermocatalysis and photocatalysis for deactivating catalysts and outcompeting for transformation reactions. Therefore, in the study of contaminants, various additives were introduced into the system. Significantly, the contaminants do not adversely affect the electrolytic process (Fig. S34). This resistance to interference could be due to the isolating effect of carboxyl monolayers on the electrode58, and the high local concentration of fatty acids at the solid-electrolyte interface. As supported in Fig. S13, the higher potential significantly boost the FE.

Waste non-edible oils from catering, a mixture of triglycerides, phospholipid, C and C fatty acids/esters, along with food residues, salt, water, flavors, and other organic compounds (Fig. 3d), can be effectively processed using our method. As shown in Fig. S35, the waste oil does not include fatty acids with low carbon-chain length, whereas the main component of fatty acids is C and C acids which can be converted with high yields as displayed in Fig. 3a. Meanwhile, triglyceride and phospholipid readily undergo saponification in strong alkaline solutions, and then exist in two distinct forms in the system after hydrolysis: Glycerine/phosphate and C fatty acid that further proceeds decarboxylation. Unlike conventional technique, our electrolytic approach allows for direct use of the waste oil without any prior treatment. It achieves good conversion and high selectivity, even at the feedstock scale of up to 10 grams (Fig. 3e). In general, these clearly illustrate the substantial advantage of our method over traditional methods.

Production of long-chain alkanes from solar cells. Based on the previous confirmation of high activity and stability in the two-electrode system, we have scaled up and utilized solar cells to explore its potential for practical production. Here, utilizing a custom-built reactor (1L) with three pairs of electrodes (Fig. 4a), large-scale production was successfully demonstrated via this approach. The construction of the electrochemical setup is provided in Fig. S36. Upon the consecutive cycles, 40 grams of long-chain olefins are obtained with 67% yield, which are subsequently converted into approximate 40 grams of long-chain alkanes through hydrogenation. Moreover, the electrochemically coupled decarboxylation and hydrogenation in the reactor yields 12% long-chain olefins and 55% long-chain alkanes (Fig. 4b). It is worth noting that the resulting alkenes or alkanes from the reaction have very low polarity, with retention factors (R) values of almost 1 when using petroleum ether as the eluent, while other impurities have R values less than 0.1. The significant difference in R values makes it easy to separate them from other polar molecules using column chromatography. This greatly reduces the difficulty of subsequent purification, resulting in alkanes with the purity of over 95% after purification. Alternatively, product can be purified through slurry washing in petroleum ether combined with vacuum distillation. The product obtained from slurry washing has the purity of around 70~80%, and after distillation, the product with the purity as high as 99% can be achieved. These purification methods can be completed in just one hour, significantly shortening the production time from waste oil to long-chain alkanes, thereby reducing the cost of products. According to the comparison presented in Table S8, our product can serve as a viable substitute for diesel fuels. Furthermore, compared to commercial gasoline, petro-diesel, biodiesel, and ethanol fuels, it exhibits the higher calorific value, which translates to greater energy output and improved fuel economy at a lower cost. Notably, the reactor design is compatible with photovoltaic cells, enhancing the sustainability and economic viability of the process. This confirms that our method effectively scales up the production of long-chain alkanes.

A life-cycle assessment utilizing GREET software reveal that our process is more sustainable than conventional diesel (CD) production and photocatalytic methods, with significant savings in fossil energy depletion (FED) and reduced greenhouse gas emissions (GGE) (Figs. S37, 4c, d, more details in the supplementary information). The FED of our method is marginally lower than that of CD production and nearly five times lower than photocatalytic approach. Similarly, GGE is approximately two and ten times less than those of CD and photocatalytic methods, respectively9. In addition, when solar energy is utilized through solar cells in the system, the FED and GGE are decreased. Moreover, with the inventories from China, the USA, and the European Union (EU) provided for comparative analysis, the FED and GGE present similar results with China (Fig. S38), suggesting that such sustainability can also be realized in other countries when they employ our approach to produce fuels. Significantly, compared with the default production method in GREET, implementing this system on an industrial scale for 100 kt of long-chain alkanes could save around 2192 TJ of fossil energy and reduce GGEs by 112.2 kt, offering an overwhelming advantage over CD production.

In summary, a strategy of electrochemically coupled decarboxylation and hydrogenation was developed for converting fatty acids into long-chain alkanes under ambient conditions (open system, 60°C, atmospheric pressure and zero H input), achieving high productivity and selectivity across a range of carbon-chain lengths. Moreover, the electrochemical performance can be flexibly adjusted by voltage and concentration of fatty acid to avoid competitive adsorption and oxidation of other substances, to achieve high Faradaic efficiency. Additionally, the transformation mechanism from carbocations to alkenes evolved with chain length, progressing through OH–-assisted dehydrogenation, self-dehydrogenation, and β-carbon-carbon bond scission adjacent to the COO– group. Significantly, our system effectively handled various feedstocks, including unsaturated fatty acids, esters, mixtures, crude fatty acids, and waste oil, yielding high selectivity and productivity. Furthermore, the scalable production of long-chain olefins and alkanes was feasible in a straightforward reactor setup. Further improvement was proposed: the reaction setup could be improved by adopting a flow device to maintain a high concentration of fatty acid59; and a base that is soluble in ethanol at room temperature (25°C) could be used, such as N(n-Bu) OH. Our approach offers significant reduction in fossil fuel usage and greenhouse gas emissions, outperforming conventional diesel production and photocatalytic process.

Methods
Reagents and materials
Stearic acid was obtained from Meryer (lot #M20114). Potassium hydroxide (KOH) and ethanol were sourced from China National Pharmaceutical Group Corp (lots #10017018 and #10009218, respectively). n-Bu NPF was acquired from Alfa (lot #A17196). All reagents were purchased in their highest commercial purity and used without additional purification. Thin-layer chromatography (TLC) was performed on 0.25 mm E. Merck silica gel plates (60 F-254), with visualization through 254 nm UV light and the iodine fuming technique. Column chromatography utilized Aladdin silica gel (100–200 mesh, particle size 0.075~0.147 mm). All the electrolytes should be prepared freshly.

Preparation of p-rGO
Graphene oxide (GO, 100 mg) was dispersed in ethanol (EtOH, 10 mL), and sodium borohydride (NaBH , 100 mg) was introduced in a single batch while stirring. The mixture was continuously stirred at 25°C for 12 h. Post-centrifugation, the precipitate was washed with copious amounts of distilled deionized water and EtOH, then dried under vacuum at 60°C for 24 h to yield the reduced graphene oxide (rGO). GO (90 mg) was dispersed in a 1 wt.% sodium hydroxide (NaOH) solution (60 mL) and refluxed in a round-bottom flask under continuous stirring for 1 h. The resulting treated GO was collected by centrifugation at 10,000 rpm and re-dispersed in 60 mL of deionized water. Concentrated hydrochloric acid (36~38%, 1.5 mL) was then slowly added to the dispersion. The mixture was refluxed for an additional hour, followed by filtration and thorough washing with distilled water and ethanol. Finally, the residue was treated with sodium NaBH (100 mg) in ethanol (15 mL) and stirred for 10 h. Afterward, the mixture was centrifuged, and the precipitate was washed extensively with distilled water and EtOH. The product was dried under vacuum at 60°C for 24 h to obtain porous reduced graphene oxide (p-rGO).

A mixture of rGO and p-rGO in a 5:1 mass ratio was dispersed in EtOH and stirred for 12 h. The mixture was then centrifuged, and the resulting precipitate was washed twice with ethanol. Finally, the product was dried under vacuum at 60°C for 24 h to yield the composite material.

Preparation of the working electrodes
Prior to the fabrication of the working electrodes, carbon papers were cleaned with a water-EtOH mixture. 10 mg of the catalyst was then dispersed in 0.5 mL of EtOH using ultrasonication for 30 min. Subsequently, 5 μL of a polytetrafluoroethylene (PTFE) solution (60%) was added. The resulting ink was drop-casted onto the conductive side of a carbon paper substrate (2 cm by 2 cm).

Optimization of reaction parameters for electrochemical decarboxylation into olefin
Unless otherwise specified, all optimization reactions were performed on a 0.30 mmol scale. The initial screening of solvents, electrolytes, and bases was conducted using a standard protocol: Stearic acid (1.0 equivalent), base (1.2 equivalents), electrolyte (0.06 M), solvent (20 mL), at 60°C and 3.5 V. For each optimization step, one parameter was varied while the others were held constant. Specifically, the concentration of n-Bu NPF was optimized with the following conditions: Stearic acid (1.0 equivalent), KOH (1.2 equivalent), varying electrolyte concentrations (0.01, 0.06, 0.1, or 0.5 M), in ethanol (20 mL), at 60°C and 3.5 V. Similarly, the KOH concentration was optimized under the following conditions: Stearic acid (1.0 equivalent), varying KOH amounts (0.4, 1.2, 2.0, or 2.8 equivalents), a fixed electrolyte concentration (0.06 M), in ethanol (20 mL), at 60°C, and 3.5 V.

General procedure for electrochemical decarboxylative olefin with fatty acid as substrate
The reactor was loaded with carboxylic acid (0.3 mmol), KOH (1.2 equivalents), n-Bu NPF (0.06 M), and EtOH (20 mL). A reactor cap fitted with a working electrode (anode) and a counter electrode (graphite rod) was then inserted into the reaction mixture. The mixture was stirred at 60°C for 30 min without an electric current to ensure complete dissolution of the reagents. Following this, the reaction system was subjected to electrolysis at a constant voltage of 3.5 V or a constant current of 10 mA at 60°C for 10 h. Subsequently, the solvent was removed under vacuum, and the residue was purified via column chromatography using petroleum ether as the eluent to yield long-chain olefins. Time-dependent and voltage-dependent reactions were also conducted under identical conditions, followed by the same post-treatment process. The resulting products were characterized using GCMS to determine the olefin types.

General procedure for electrochemical decarboxylative olefin with fatty acid ester as substrate
The reactor was loaded with ester (0.3 mmol), KOH (1.4 equivalents), n-Bu NPF (0.06 M), and a solvent mixture of EtOH (18 mL) and distilled water (2 mL). A reactor cap, fitted with a working electrode (anode) and a counter electrode (graphite rod), was then installed into the reaction mixture. The mixture was stirred at 60°C for 30 min without an electric current to ensure complete dissolution of the reagents. Subsequently, the reaction system was subjected to electrolysis at a constant voltage of 3.5 V at 60°C for 20 h. After the reaction, the solvent was removed under vacuum, and the residue was purified by column chromatography using petroleum ether as the eluent to yield long-chain olefins.

General procedure for electrochemical decarboxylative olefin with crude soybean oil and tall oil as substrates
The reactor was loaded with oil (100 mg), KOH (20 mg), n-Bu NPF (0.06 M) and EtOH (20 mL). A reactor cap, complete with an anode (working electrode) and a cathode (graphite rod), was then placed into the reaction mixture. The mixture was stirred at 60°C for 30 min without an electric current to dissolve the reagents. Following this, electrolysis was conducted at a constant voltage of 3.5 V at 60°C for 10 h. Post-reaction, the solvent was removed under vacuum, and the residue was subjected to purification via column chromatography using petroleum ether as the eluent. In crude soybean oil, the mass fraction of fatty acids is 51.2%, which includes 2.3% of C fatty acids/esters, 30% of C fatty acids/esters, 11.4% of C fatty acids/esters, and 7.5% of other fatty acids/esters. In addition to these, there are 48.8% unidentified additives. In crude tall oil, the mass fraction of fatty acids is 90%, which includes 5.5% of C fatty acids/esters, 79.8% of C fatty acids/esters, 3.2% of C fatty acids/esters, and 1.5% of other fatty acids/esters. In addition to these, there are 10% unidentified additives.

The description of specific parameters for GCMS
Column oven temperature: 25°C; injection port temperature: 25°C; carrier gas: He; inlet pressure: 100 kPa; total flow rate: 50 mL/min; column flow rate: 1.93 mL min−1; linear velocity: 49.7 cm s−1; purge flow rate: 3 mL/min; split ratio: −1. GC column: Rtx-5MS capillary column (30 m × 0.25 mm i.d., film thickness: 0.25 μm). Ion source temperature: 230°C; interface temperature: 280°C; mass scan range: m/z 50–550.
Some specific formulas were described as following:
Yield= n p / n0 ×100% (1)
Selectivity= n i / n ×100% (2)
FE= n p ×2× F / (I×t) ×100% (3)
μ= n p / (m cat ×t) (4)
where n is the mole of the product, n’ is the theoretical moles of the product, n is the mole of the specific product, F is the Faraday constant (96,485 C mol−1), I is the current (A), t is the reaction time (s or h), m is the catalyst mass (g).
Note that the moles of the product were determined by weighing actual mass of the purified product after purifying the reaction mixture through flash column chromatography. The measurements of Faraday efficiency were only performed once.

Coupling reaction of decarboxylation and hydrogenation
The electrochemical reaction conditions were as following: Stearic acid (1.0 equivalent), KOH (1.2 equivalents), n-Bu NPF (0.06 M), and EtOH (20 mL) at a temperature of 60°C and voltages of 3.5, 4.5, or 5.5 V. The mixture was stirred for 30 min at 60°C without an electric current to ensure the reagents were fully dissolved. Following this, a palladium on carbon (Pd/C, 20 wt.% Pd) catalyst was introduced with a substrate-to-catalyst mass ratio of 1:0.001. The reaction was then stirred for 10 to 15 h at the specified voltages. Post-reaction, the solvent was removed under vacuum, and the residue was purified by column chromatography using petroleum ether as the eluent. This process yielded long-chain alkanes along with a small quantity of long-chain olefins.

General procedure for electrochemical transformation from waste catering oil to alkanes
The reactor (1 L) was loaded with waste catering oil (10 g), KOH (2 g), n-Bu NPF (0.06 M), and EtOH (450 mL) + H O (50 mL). A reactor cap, fitted with three pairs of anodes (working electrodes) and cathodes (graphite rods), was then installed into the reaction mixture. The mixture was stirred at 60°C for 30 min without an electric current. Then a palladium on carbon (Pd/C, 20 wt.% Pd, 20 mg) catalyst was added. The reaction system was then subjected to electrolysis at a constant current of 40 mA at 60°C. After the reaction, the solvent was removed under vacuum, and the residue was purified by column chromatography using petroleum ether as the eluent.
We sourced the waste catering oil from the slop bucket in the canteen of Central China Normal University. The waste oil could contain H O, salt (mainly NaCl), food residue, fatty acids/esters (including C , C , C ), triglyceride, and amide. The mass fraction of fatty acids is 25.7%, which includes 4.5% of C fatty acids/esters, 1% of C fatty acids/esters, and 21.2% of C fatty acids/esters.

Contaminant study
In the contaminant study, using stearic acid as the feedstock and maintaining the previously mentioned conditions, various additives (each at 0.3 mmol) were introduced into the reaction system. The additives tested included isopropylbenzene, methylstyrene, phenylacetylene, benzaldehyde, acetophenone, acetonitrile, epoxy phenylethane, and bromobenzene.

Electrochemical measurements
Electrochemical experiments were performed using a CHI760E workstation with a standard three-electrode configuration. The working electrode consisted of carbon paper loaded with the catalyst material, while a graphite rod and an Ag/AgCl electrode served as the counter and reference electrodes, respectively. Prior to testing, the solution was purged with pure nitrogen for 5 min to remove dissolved oxygen.
Cyclic voltammetry (CV) tests were carried out at a scan rate of 5 mV s−1.

Catalyst characterization
Morphological analysis was performed using a transmission electron microscope (TEM; H-7000FA, 100 kV). X-ray diffraction (XRD) patterns were obtained using an X-Pert diffractometer (D8 ADVANCE) with graphite monochromatized Cu-Kα radiation. Proton nuclear magnetic resonance (1H NMR) measurements were conducted on a VARIAN 600M spectrometer. Gas chromatography-mass spectrometry (GC-MS, EI) analyses were carried out using a SHIMADZU QP2020 smart chromatograph. Fourier-transform infrared (FT-IR) spectra were recorded on a Nicolet iS50 FTIR spectrometer (Thermo, U.S.A.). Raman spectra were examined with a Thermo Scientific DXR Raman Microscope utilizing a 534 nm laser wavelength. X-ray photoelectron spectra (XPS) were collected on a Thermo Fisher EscaLab 250Xi X-ray photoelectron spectrometer equipped with a monochromatic Al Kα source (E = 1486.6 eV), operated at a 10 mA filament current and 14.7 keV photon filament voltage. The Si peak (103.7 eV) of SiO additive was employed as a reference for calibration. Specific surface area (SSA) and pore size distribution were determined by Brunauer-Emmett-Teller (BET) analysis using a high-speed automated surface area and pore size analyzer (TriStar II 3020 V1.03.01).

DFT calculations
Computational simulations were performed using Gaussian 09 and Gauss View 5.08, employing the B3LYP hybrid functional. Initial model estimations were made based on the reaction characteristics. Geometry optimization and frequency calculations for transition states were initially conducted at the B3LYP/6-31G(+)** level with optimization settings specified for a transition state (opt=(calcfc, ts, noeigen)). The calculations targeted a Berny-type transition state. Force constant calculations were performed once. For models containing OH, the charge and multiplicity were set to 0 and singlet, respectively. Conversely, for models without OH, the charge and multiplicity were set to 1 and doublet. Once the transition state was identified, intrinsic reaction coordinates were calculated using IRC at the same computational level, exploring both forward and reverse directions. Force constant calculations were also performed once for these steps.

Lifecycle assessment
The GREET 2021 software was utilized to construct calculation models. The energy contribution of each input resource was calculated as a proportion of the total energy input. Data regarding soy oil production, soybean farming, transportation, H , and solvent alcohol were sourced from the GREET software database. The electricity considered was generated from a mix of sources, with inventories from China, the USA, and the European Union provided for comparative analysis. Production data for the Pd/C catalyst was derived from literature47. The raw materials for electrolyte production included ethylene, methane, ammonia, and electricity, enabling us to construct an LCA for the life cycle of electrolyte. The life cycle FED and GHG emissions were calculated by integrating the estimated materials and energy inputs into the GREET 2021 software.
GHG and FED were calculated based on the equations as following:
GHG=m +25m +296m (5)
FED= Σ (LHV i ×C i ) (6)
where m , m and m are emissions of CO , CH and N O, respectively, kg; LHV is the lower heating value of primary fossil energy i, MJ kg−1; C is consumption amount of primary fossil energy i, kg.

Data availability
Source data are provided with this paper.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The article developed a new efficient strategy to upgrade fatty acids in an electrochemical setup coupled with cathodic proton reduction and olefin hydrogenation, obtaining long-chain alkanes under mild conditions. The authors first explored the optimized condition and electrocatalyst. Furthermore, the transformation mechanism from carbocations to alkenes was investigated as progressing through OH-assisted dehydrogenation. The authors achieved a long-chain alkane yield of 88.9% and highest-record productivity compared with the literature. Finally, the article combined the process with the solar cells, demonstrating its availability and compatibility. The article is suitable for the journal after solving the following concerns.\n1. The full name of n.d. in Table S2 should be given.\n2. In Fig. S6, the definition of ID/IG should be given, explaining the ID/IG standing for peak intensity or peak area. In addition, the signal/noise ratio of p-rGO and p-rGO spent are inconsistent. More explanation should be given.\n3. In Fig. S8, the C 1s XPS spectra showed the slight peak shift. However, the methods of XPS demonstrated the calibration by C 1s peak to 284.8 eV, which cannot be used for C 1s XPS analysis especially in analyzing peak position in carbon materials. The peak position needs to be calibrated by other elements like Si or SiO2 additives.\n4. The stability test in Fig. S9 should provide E-t or i-t curve without fluctuation.", "reviewer_2": "Sustainable Alkane Production from Waste Fatty Acids via Electrochemically Coupled Decarboxylation and Hydrogenation\", the authors have developed an electrochemically coupling approach involving the anodic decarboxylation and hydrogenation of fatty acids to produce value-added long-chain alkanes. Compared to conventional method, this strategy exhibits more significant advantages including high yield, high selectivity, no external H₂ input, easy scale-up, and low carbon consumption and emission. The manuscript has discussed the subject from exploring the reaction condition and catalyst optimization for fatty acid transformation, the transformation of crude fatty acids and waste oil to potential for practical application. Overall, the work is interesting and the manuscript has a clear logical structure. This study has a large potential to address the key issue in conventional fatty acid conversion and makes a significant contribution to the production of long-chain alkanes from waste oils.Therefore, it is recommended to be published in Nature Communications. Despite the electrochemical strategy is well verified and systematical, some suggestions and comments are provided as following for the further improvement of the manuscript.\n1. It is meaningful to directly convert waste oils with many impurities into diesel or aviation fuels. Can the authors summarize relevant work in the literature and compare them with this work to better demonstrate its features, possibly like purification step? According to the data, it can be deduced that the purification time can be largely reduced through using the electrochemical strategy.\n2. The electrochemical strategy can produce long-chain alkanes without H₂ input, marking significant progress in catalytic waste-to-fuel reaction. The authors are suggested to analyze the difference between olefin hydrogenation of this strategy and direct cathodic hydrogenation on the cathodic electrode.\n3. In Experimental section, the authors should describe specific parameters for the product detection, such as the specific mobile phase ratio, flow rate, and detection wavelength.\n4. For several mentioned performance evaluation indicators (such as yield, selectivity, Faraday efficiency, and reaction rate), the authors need to specify the specific formula. Meanwhile, they need to clarify the quantitative methods for crude products and waste oil components.", "reviewer_3": "In this submitted manuscript, Wei et al. report a strategy to integrate anodic electrochemical decarboxylation, cathodic proton reduction, and olefin hydrogenation, which converts fatty acid derivatives into long-chain alkanes. This approach can contribute to a good alkane yield of 88.9%, and is specifically interesting to upgrade the waste carbon source. However, from the viewpoint of scientific demonstration, the manuscript is required to be substantially improved through a major revision, before it becomes acceptable by Nature Communications.\n1. The referee strongly suggests the author to improve the quality of figure images carefully.\n2. I recognize carbon paper is used as the anodic electrode. As it is a carbon material with graphite nature, does this bare electrode substrate contribute to the electrochemical activity?\n3. The authors calculated FE. So how the consumed charge is recorded?\n4. (Electrochemical) surface area matter. The author synthesized p-rGO to exhibit a well-defined porous structure. It is commonly regarded that the high porosity can contribute to the high surface area. However, the BET characterizations give the contrary results. In addition, electrochemical active surface areas of all GO-based samples are determined to be similar. The authors should give more interpretation.\n5. I am confused by the product distribution. When the authors conducted the anodic decarboxylation, olefin products were generated. In these electrochemical experiments, I notice that graphite rod was used as cathode. However, the authors further declared that graphite rod can catalyze the olefin hydrogenation with considerable activity (Figure 2c). So, how the author obtained the olefin products without hydrogenation by cathodic graphite rod?\n6. “The O–H bond of ethanol is cleaved on the cathode”. This statement is controversial. The dehydrogenation of alcohol though O–H bond cleavage should be an oxidation reaction.\n7. As H is supplied by ethanol, what does an it become after providing H? The authors should give a clear picture about the charge and mass balance in the whole reactor.", "reviewer_4": "Wei, Ma, Ouyang, Ren, and coworkers present the development of an electrochemical decarboxylation methodology for the generation of long chain alkanes from waste fatty acids and fatty acid esters. Their method utilizes a porous reduced graphene oxide (p-rGO) anode and graphite cathode to promote decarboxylative carbocation formation followed but either nucleophilic attack, beta-hydride elimination, or beta-scission to generate alkenes. These alkenes are then reduced to alkanes with the hydrogen atoms generated at the cathode via reduction of the ethanol solvent with the help of a Pd/C catalyst. The authors were able to demonstrate broad applicability not only by successfully generating alkanes from a large variety of fatty acid chain lengths, but also by performing this transformation with crude mixtures that include other additives such as salts, amides, and phospholipids. Finally, the authors were able to demonstrate the scalability of this method by processing 40 g of fatty acids providing a 67% yield of long-chain olefins. Some notes are provided below to aid in clarification of points made by the authors.\n1. Optimization tables in the supplementary information would benefit from clear reaction schemes, reminding the reader of the variables that are not being changed as mentioned in lines 121-129 of the SI. Some of the figures also contain too many subheadings and inserts. This should be broken up into figures specific to the discussion.\n2. The reaction is fairly dilute with regards to the long chain fatty acid (~15 mM). In order to minimize solvent usage in a large scale process, what is the highest concentration this methodology can tolerate with regards to the fatty acid? Are there any hypotheses as to why the organic amine bases cannot be used? This seems highly impractical for scale.\n3. Figure S13: The onset of oxidation is identical between the two fatty acids. The anodic peak is different, but that difference is an effect of the mass transport kinetics rather than the thermodynamics of C11 vs C18 redox. This should be corrected in the discussion.\n4. There is no explanation as to how this process circumvents the formation of alkyl radicals, which are frequently invoked in such electroxidative decarboxylation reactions.\n5. The DFT calculations require a proper discussion and summary in an energy landscape, rather than a series of simple mol2 structures in the SI.\nOverall, this is a nice report but has a number of shortcomings that ignored. A number of poorly justified arguments are made regarding the selectivity of the reaction. The DFT is unclear, and the experimental mechanistic study is buried in the SI. Finally, the system is completely impractical as the reaction is performed at 15 mM concentrations. This is a huge waste of EtOH solvent, which his likely the most expensive and valuable component of the reaction. Given the precedent for such reactions/systems, this work is more suitable for a more specialized journal.", "reviewer_5": "In this paper, Ma and co-workers describe an electrochemical method for upgrading fatty acids from waste and crude oil, into sustainable fuels including long-chain olefins and alkanes. This method is advantageous relative to previous methods in that it does not require exogenous H2, high temperature, or pressure, and is effective against crude mixtures. This work shows high potential for the green upcycling of waste feedstocks into fuels with high efficiency and scalability. The authors also include DFT analysis of alkene isomerization after the decarboxylation process and begin to explain the selectivity of carbon chains of different lengths. I found the method to be useful and interesting, with high potential for impact, but found the manuscript somewhat difficult to follow. Many experiments were shown, but their (i.e. DFT studies) importance was not well justified. Figures were dense, and included many bar/pie charts that would be better suited as tables in the SI. The authors report the use of a new electrode material (p-rGO) and do a good job characterizing and explaining its use. However, they were less successful in demonstrating clearly their exact electrochemical set-up, including the composition of their custom reactor. These details are critical for replicability. There are also several examples through the text where the authors make claims about their method relative to others, but do not reference any citations, leading to skepticism in some places. Finally, the authors only show the scaled-up conversion of stearic acid using their method. I recommend also demonstrating application of the crude inputs to the full method, going from input to alkane mixture.\nDue to the reasons listed above, while I find this paper to be of potentially high importance and impact, I do not believe it rises to the level of rigor necessary for publication in Nature Communications, and recommend it be published elsewhere. I recommend authors perform significant revision of their writing/organization, and demonstrate a broader scope in their ultimate transformation.\n78-81: The discussion of these alternative electrocatalytic systems read as non-sequiturs, and details are given here that have no point of comparison – why is this important?\n81-83: this statement needs a reference\n81-85: The statement is given that alkane production from waste oils is “inefficient” and requires H2. However, no references are given. This comes across as obfuscating the relevant background to what this paper purports to show. If production of alkanes from waste oils truly has never been done and this work is fully novel, the authors should say that explicitly.\n87: Be more specific as to which contaminants are referred to here and are common in these feedstocks\n138-140: How are these yields determined? Constitutional isomers are notoriously difficult to differentiate, and no information is provided to prove this claim. The SI implicates GCMS, but shows few traces, and certainly not for most of the reactions described.\n203: should be “sacrificial”\n221-222: This discussion might be improved by being more explicit about what Fig 2d is showing. I did not follow this part, and do not understand Fig 2d.\n244: Need references here.\n266-274: I do not follow the rationale for the difference in CVs. I would not expect that significant a difference between fatty acids of these chain lengths – can this be explained another way?\n275-300: The authors discuss carbocation rearrangements of fatty acids of different chain lengths. However, the text is dense and difficult to follow. Fig 3b similarly attempts to draw conclusions about alkene selectivity, and subsequent discussion focuses on DFT calculations. I would like to understand why a detailed analysis of carbocation/alkene rearrangements is necessary especially given that the authors emphasize the practicality of the method (the alkenes should be reduced to the alkane anyway).\n347-371: This is supposed to be the heart of the method – direct, coupled net conversion of fatty acids to long chain alkanes. However, few details are given as to what the input is in the bulk reactor! Is this study only performed from stearic acid? What happens to waste oils on bulk scale? Ultimately, it would be very important to show that the method is applicable to crude inputs, or else explain why it is not.\n350: “custom built reactor” – details must be provided for replicability.\n367-369: “surpass” should be elaborated on. The physical properties differ, but authors need to justify why these are better.\n383-385: where are these numbers from?\nFig 2h: This appears unnecessary to me – it is simply a comparison of the productivity of this method compared to other methods. Should be kept in the SI and summarized in a sentence or two in the text.\nFig 3e: this figure shows pie charts that give the various components of different oils. This can be summarized in text or in the SI, but does not significantly enhance the story as a figure.\nFig 3f: this figure seems to show conversion of waste oils to different alkene components. Are they not then reduced further to the alkanes? There is ambiguity as to what the figure is showing.\nGeneral:\n-This manuscript never shows a general reaction scheme that describes the main components of the reaction. This would be beneficial in the main text.\n-The manuscript further gives very few details into the construction of the electrochemical reactor that purports to convert stearic acid on the 40g scale. Details must be provided to enable replicability.\n-Many schemes in this manuscript rely on bar graphs, when a simple table would suffice. Greater care should be taken in effective visual communication. (This applies to many parts of Fig. 2, for example).\n-What is “waste catering oil”? Is this the same as “cooking oil”? Need details as to where they source this material and its composition.\n-In general, the manuscript relies on claims about the components of complex crude mixtures, but is not explicit about how they are analyzing these mixtures. It appears, upon careful digging through the SI, that GCMS may be indicated, but is it truly quantitative? Details are needed throughout."}, "author_response": "Response to the reviewers’ comments\nGeneral remark: We sincerely thank all referees for taking the time to carefully review our manuscript and\nprovide constructive feedback, which has greatly contributed to improving its clarity, completeness, accuracy, and\noverall quality. We have thoroughly addressed all the comments and revised the manuscript accordingly. The\nrevisions are highlighted in yellow in the manuscript. Point-by-point responses are provided below under “Reply”,\nwith the referees’ original comments presented in italics.\nResponse to comments from Reviewer #1\nComments: The article developed a new efficient strategy to upgrade fatty acids in an electrochemical setup\ncoupled with cathodic proton reduction and olefin hydrogenation, obtaining long-chain alkanes under mild\nconditions. The authors first explored the optimized condition and electrocatalyst. Furthermore, the\ntransformation mechanism from carbocations to alkenes was investigated as progressing through OH-assisted\ndehydrogenation. The authors achieved a long-chain alkane yield of 88.9% and highest-record productivity\ncompared with the literature. Finally, the article combined the process with the solar cells, demonstrating its\navailability and compatibility. The article is suitable for the journal after solving the following concerns.\nReply: We sincerely thank the reviewer for the positive and encouraging comments on our manuscript. We have\ncarefully addressed all the concerns raised by the reviewer. Please see our point-by-point response below.\n1. The full name of n.d. in Table S2 should be given.\nReply: We thank the reviewer for pointing this out. We have now added the full name of \"n.d.\" as \"not detected\"\nin the captions of Tables S2, S3, and S4 in the revised Supplementary Information as following: “Note: “n.d.”\nrefers to not detected.”\n2. In Fig. S6, the definition of I /I should be given, explaining the I /I standing for peak intensity or peak area.\nD G D G\nIn addition, the signal/noise ratio of p-rGO and p-rGO spent are inconsistent. More explanation should be given.\nReply: We thank the reviewer for the constructive comment. In response, we have clarified the definition of I /I\nD G\nin the caption of Figure S6, specifying that the ratio is calculated based on the peak intensity of the D and G bands.\nGenerally, a higher I /I ratio indicates a greater concentration of carbon defects.\nD G\nIn the original measurement, the minor variation in signal-to-noise ratio (SNR) between the p-rGO and p-rGO\nspent samples may be attributed to minor inconsistencies in instrument focusing and laser alignment during the\ninitial measurements, which does not affect the I /I evaluation. In response to the reviewer’s suggestion, we\nD G\ncarefully re-optimized the Raman instrument settings by increasing the scan counts, adjusting the laser power to\n100%, and ensuring the laser is focused on a smooth and flat area of the sample surface, and remeasured both\nsamples under identical conditions. The updated spectra (Figure R1) now show consistent SNR, as presented in\nthe revised Figure S9. The I /I change trend remains unchanged, and the structural interpretation is unaffected.\nD G\nWe have also clarified in the figure caption that I /I is based on peak intensity.\nD G\nFigure R1 Raman spectra of rGO, p-rGO and p-rGO-spent. Note that I /I is based on peak intensity.\nD G\n3. In Fig. S8, the C 1s XPS spectra showed the slight peak shift. However, the methods of XPS demonstrated the\ncalibration by C 1s peak to 284.8 eV, which cannot be used for C 1s XPS analysis especially in analyzing peak\nposition in carbon materials. The peak position needs to be calibrated by other elements like Si or SiO2 additives.\nReply: We thank the reviewer for this important suggestion. As advised, we re-evaluated the C 1s XPS spectra by\nintroducing an internal reference (SiO ). Specifically, 10 mg of rGO or p-rGO was mixed with 2 mg of SiO₂, and\n2\nhigh-resolution C 1s spectra were re-collected. As shown in Figure R2 (new Figure S11 in the revised\nSupplementary Information), p-rGO exhibits a lower binding energy and a higher sp²C/sp³C ratio, which is similar\nwith the original results. We have updated the XPS measurement method in the revised Manuscript\nLine 464 Page 19:\n“[...] and 14.7 keV filament voltage. The Si peak (103.7 eV) of SiO additive was employed as a reference for\n2p 2\ncalibration. Specific surface area (SSA) [...]”\nFigure R2 High-resolution (a) Si 2p XPS spectra and (b) C 1s XPS spectra. Note that SiO additive was mixed\n2\nwith the catalysts for the calibration of C peak position.\n1s\n4. The stability test in Fig. S9 should provide E-t or i-t curve without fluctuation.\nReply: We thank the reviewer for this valuable comment. In response, we have now provided the E–t curves over\nfive consecutive cycles, as shown in Figure R3 (new Figure S11 in the revised Supplementary Information). The\nhighly consistent profiles of these curves indicate minimal fluctuation, thereby confirming the excellent stability\nof the material during the electrocatalytic decarboxylation process. To supplement this test, we have now added\nthe following discussion in the revised Manuscript\nLine 176 Page 8:\n“[...] as documented in Figure S14. Moreover, the E–t curves over five consecutive cycles were shown in Figure\nS15. The highly consistent profiles of these curves indicate minimal fluctuation, thereby confirming the excellent\nstability of the material during the electrocatalytic decarboxylation process. Furthermore, the Raman and FT-IR\nspectra [...]”\nFigure R3 E–t curves of the cycle experiments over p-rGO.\nResponse to comments from Reviewer #2\nComments: Sustainable Alkane Production from Waste Fatty Acids via Electrochemically Coupled\nDecarboxylation and Hydrogenation\", the authors have developed an electrochemically coupling approach\ninvolving the anodic decarboxylation and hydrogenation of fatty acids to produce value-added long-chain alkanes.\nCompared to conventional method, this strategy exhibits more significant advantages including high yield,\nhigh selectivity, no external H₂ input, easy scale-up, and low carbon consumption and emission. The manuscript has\ndiscussed the subject from exploring the reaction condition and catalyst optimization for fatty acid transformation,\nthe transformation of crude fatty acids and waste oil to potential for practical application. Overall, the work is\ninteresting and the manuscript has a clear logical structure. This study has a large potential to address the key\nissue in conventional fatty acid conversion and makes a significant contribution to the production of long-chain\nalkanes from waste oils. Therefore, it is recommended to be published in Nature Communications. Despite the\nelectrochemical strategy is well verified and systematical, some suggestions and comments are provided as\nfollowing for the further improvement of the manuscript.\nReply: Thank you for your constructive feedback and valuable comments. We have carefully considered your\nsuggestions and address them as follows.\n1. It is meaningful to directly convert waste oils with many impurities into diesel or aviation fuels. Can the\nauthors summarize relevant work in the literature and compare them with this work to better demonstrate its\nfeatures, possibly like purification step? According to the data, it can be deduced that the purification time can be\nlargely reduced through using the electrochemical strategy.\nReply: We thank the reviewer for raising the importance of direct upgrading of highly contaminated waste oils.\nTo demonstrate our strategy’s advantages, we have surveyed key literature on waste‑oil‐to‐diesel/jet processes and\norganized a direct comparison in four aspects:\n(a) Pretreatment Requirements\nPrevious study (Environ. Chem. Lett., 2024, 22, 1005-1071), before converting waste cooking oils into biodiesel\nfuels, it is necessary to remove impurities and contaminants (such as water, salts (mainly NaCl), lipid polymers,\nfood particles/colloid), which would damage the production equipment and deactivate the catalysts at high\ntemperatures, through processes such as filtration and purification. Moreover, a portion of triglyceride in waste\noils is required to be transformed to ester or acid through transesterification or hydrolysis, ensuring better reaction\nefficiency. These additional steps increase the complexity and cost of biodiesel production.\nOur electrochemical route processes raw waste oil (≈50 % impurities) directly (10 g feed → 3 g diesel) with no\nfiltration, desalting or external pretreatment, since in situ alkaline hydrolysis and cathodic hydrogenation handle\nboth conversion and impurity tolerance simultaneously.\n(b) Product Quality\nLiterature benchmarks (Ind. Eng. Chem. Res. 2020, 59, 14935-14946; Sustainability 2021, 13, 12089; Clean\nTechn. Environ. Policy 2016, 18, 473-482; Energy Fuels 2011, 25, 4638-4642) report oxygenated esters or mixed\nhydrocarbon streams requiring further deoxygenation, often suffering from sulfur contaminants, cracking or\ncoking side‑reactions.\nOur products are long-chain alkanes(C -C ) with 80~90% selectivity. After flash column chromatography, the\n17 20\npurity of the products can reach up to 98%. They exhibit moderate density, boiling range and viscosity, higher\nvolumetric and gravimetric heating values, plus minimal oxygen/sulfur content—fully compatible with existing\nengines and blending protocols (Table R1):\nTable R1 The comparison of physical properties of the products, commercial gasoline, petro-diesel, biodiesel and\nethanol fuels.\nOur method\nParameter Gasoline Petro-diesel Biodiesel EtOH\n(C alkane)\n0.72~0.78 0.85~0.88 0.89 0.79 0.78\n(g cm-2)\nFreezing point\n-40 -40~-1 -3~18 -114 20\n(at 1 bar) (℃)\nBoiling range 299~346\n27~225 180~340 78 292\n(at 1 bar) (℃) (326~366)\nViscosity/rheological\nproperties (at 40 ℃) 0.5~0.6 1.3~4.1 4.1 (4.8) 1.5 3.0\ncSt (1 mm2 s-1)\nCalorific value\n4.4 8.3 10.8 1.2 16.6\n(per mol) (MJ mol-1)\nCalorific value\n43.4 42.8 37.0 27.0 69.2\n(per mass) (MJ kg-1)\nAutoignition\n257 315 177 423 300~350\ntemperature (℃)\n(c) Mild reaction conditions, and no additional H input.\ncost, and safety concerns (ACS Catal. 2017, 7, 1158-1169; CN105218289A, 2016).\nOur electrochemical system runs at 60 ℃, 1 atm, generating active H* at the cathode so no external H₂ is\nrequired—dramatically lowering capital/operational cost and safety risk.\n(d) Purification time.\nTraditional distillation of diesel‐range fractions from complex mixtures often takes many hours (or days) due to\noverlapping boiling points of impurities (Environ. Chem. Lett., 2024, 22, 1005-1071).\nIn our study, following completion of the reaction, the product can be rapidly purified via flash column\nchromatography using common petroleum ether as the eluent. This is highly viable because the long-chain alkane\nproducts exhibit extremely low polarity, with R values close to 1 on silica gel plates, whereas most impurities\nf\nshow R values below 0.5. For example, from 10 g of waste oils, high-purity diesel-range components can be\nf\nrapidly purified within one hour.\nTaken together, our work not only sidesteps costly pretreatment and high‐pressure hydrogenation, but also slashes\nproduct‐purification time—highlighting the practical potential of electrochemical upgrading for industrial\nwaste‑oil valorization.\n2. The electrochemical strategy can produce long-chain alkanes without H₂ input, marking significant progress in\ncatalytic waste-to-fuel reaction. The authors are suggested to analyze the difference between olefin hydrogenation\nof this strategy and direct cathodic hydrogenation on the cathodic electrode.\nReply: We thank the reviewer for the valuable question regarding the difference between our olefin\nhydrogenation strategy and the direct cathodic hydrogenation on the electrode. The differences are as follows:\n(a) Catalyst accessibility and dispersion: In our approach, Pd/C catalysts are uniformly dispersed in the reaction\nsolution, allowing for sufficiently contact with the olefin substrates. In contrast, in the direct hydrogenation\nstrategy, the contact between metal catalysts and olefins is limited to the cathode surface, restricting the accessible\nreaction area.\n(b) H₂ evolution vs. hydrogenation efficiency: At higher applied potentials, the graphite cathode in our system\nproduces minimal H₂ gas and mainly generates active hydrogen species, which are favorable for hydrogenation.\nIn contrast, metal cathodes in the direct strategy tend to release large amounts of H₂ gas at higher applied potentials,\nwhich diminishes the efficiency of surface hydrogenation due to rapid hydrogen desorption.\nThese features enable our approach to achieve more efficient and scalable olefin hydrogenation under mild, H₂-\nfree conditions, offering distinct advantages for waste-to-fuel applications.\n3. In Experimental section, the authors should describe specific parameters for the product detection, such as the\nspecific mobile phase ratio, flow rate, and detection wavelength.\nReply: Thank the reviewer for the suggestion. We have included the detailed GCMS parameters for product\ndetection in the revised Manuscript (Line 418 in Page 17): “Column oven temperature: 25 ℃; Injection port\ntemperature: 25 ℃; Carrier gas: He; Inlet pressure: 100 kPa; Total flow rate: 50 mL/min; Column flow rate: 1.93\nmL/min; Linear velocity: 49.7 cm/s; Purge flow rate: 3 mL/min; Split ratio: -1. GC column: Rtx-5MS capillary\ncolumn (30 m × 0.25 mm i.d., film thickness: 0.25 μm). Ion source temperature: 230 ℃; Interface temperature:\n280 ℃; Mass scan range: m/z 50-550.”\n4. For several mentioned performance evaluation indicators (such as yield, selectivity, Faraday efficiency, and\nreaction rate), the authors need to specify the specific formula. Meanwhile, they need to clarify the quantitative\nmethods for crude products and waste oil components.\nReply: We appreciate the reviewer’s precious suggestion.\n(a) We have specified the formula as following:\nYield = Actual moles of the product / Theoretical moles of the product × 100%;\nSelectivity = Moles of the specific product / Moles of all the products × 100%;\nFaraday efficiency = Moles of the product × n × F / (Current (A) × Reaction time (s)) × 100%, where n is the\nnumber of electrons transferred per molecule (e.g., n = 2 for fatty acid decarboxylation), and F is the Faraday\nconstant (96485 C mol⁻¹);\nInitial reaction rate = Moles of the product / (Catalyst mass (g) × Reaction time (h)).\n(b) The quantitative method for crude oils is described as following:\nThe components of crude soybean oil and crude tall oil were analyzed by GC-MS to identify their fatty acid\ncontents. These industrial crude oils contain not only a variety of fatty acids, but also unidentified compounds and\ninsoluble substances. Due to the complexity of the mixtures—along with peak overlap and limited\nchromatographic resolution—it is only possible to obtain the approximate composition. This represents an\ninherent limitation in the quantitative analysis of crude oils and waste oils.\nBelow, we provide further details on the quantitative analysis conducted using GC-MS:\n(1) For major fatty acids with well-resolved and distinct peaks—such as peaks d, i, and j in Figure R4a, and\npeaks a and f in Figure R4b—the peak areas were directly integrated and concentrations calculated based on the\ncalibration curve.\n(2) For major fatty acids overlapping with other peaks—such as peak d* in Figure R4a—the peak areas were\nobtained through peak deconvolution by Origin software.\n(3) For minor peaks—such as peaks b, c, f, g and h in Figure R4a, and peaks b, c, d, e, g, h, i, j, k, and l in Figure\nR4b—the peak areas were integrated within defined boundaries and then quantified using the calibration curve.\n(c) The quantitative method for waste oils is described as following:\nThe waste catering oil was sourced from the slop bucket in the canteen of Central China Normal University. It\nmay contain water, salts (primarily NaCl), food residues, fatty acids/esters (including C , C , and C ),\n16 17 18\ntriglycerides, and amides. The quantitative analysis of the components was conducted as follows:\n(1) Quantification of water, salts, and food residues: The solid residues were first filtered and washed with\ndichloromethane. A known amount of water was then added to the oil phase, followed by vigorous stirring and\nphase separation. The aqueous phase was removed, and this extraction was repeated three times. The resulting\norganic phase was dried under vacuum at 60 ℃. The mass difference before and after this treatment was used to\nestimate the total mass of water, salts, and food residues present in the waste oils.\n(2) Quantification of fatty acids/esters: The dried oil phase was analyzed by GC-MS to quantify the fatty\nacid/ester contents. The total mass fraction of fatty acids/esters was found to be 25.7%, including 4.5% C\n16\nspecies, 1.0% C species, and 21.2% C species.\n17 18\n(3) Quantification of triglycerides: The oil was treated with 0.5 M KOH at 100 ℃ to fully hydrolyze the\ntriglycerides, thereby releasing C fatty acid. The total amount of C fatty acid was determined by GC-MS. The\n18 18\ncontent of C fatty acid initially present in the oil was subtracted from this total, and the difference was assigned\n18\nto the C fatty acids derived from triglyceride hydrolysis, allowing the triglyceride content to be calculated.\nThe purified products were subsequently identified and quantified using GC-MS.\nWe have supplemented the relevant discussion:\nIn the revised Manuscript, Line 423 Page 17:\n“[...] Some specific formulas were described as following:\nYield = Actual moles of the product / Theoretical moles of the product × 100%;\nSelectivity = Moles of the specific product / Moles of all the products × 100%;\nFaraday efficiency = Moles of the product × n × F / (Current (A) × Reaction time (s)) × 100%, where n is the\nnumber of electrons transferred per molecule (e.g., n = 2 for fatty acid decarboxylation), and F is the Faraday\nconstant (96485 C mol⁻¹);\nInitial reaction rate = Moles of the product / (Catalyst mass (g) × Reaction time (h)). [...]”\nIn the revised Supplementary Information, Page 6:\n“[...] The quantitative methods for crude oils were described as following:\nThe components of crude soybean oil and crude tall oil were analyzed by GC-MS to identify their fatty acid\ncontents. These industrial crude oils contain not only a variety of fatty acids, but also unidentified compounds and\ninsoluble substances. Due to the complexity of the mixtures—along with peak overlap and limited\nchromatographic resolution—it is only possible to obtain the approximate composition. This represents an\ninherent limitation in the quantitative analysis of crude feedstocks and waste oils.\nBelow, we provide further details on the quantitative analysis conducted using GC-MS:\n(a) For major fatty acids with well-resolved and distinct peaks—such as peaks d, i, and j in Figure S34a, and\npeaks a and f in Figure S34b—the peak areas were directly integrated and concentrations calculated based on the\ncalibration curve.\n(b) For major fatty acids overlapping with other peaks—such as peak d* in Figure S34a—the peak areas were\nobtained through peak deconvolution by Origin software.\n(c) For minor peaks—such as peaks b, c, f, g and h in Figure S34a, and peaks b, c, d, e, g, h, i, j, k, and l in Figure\nS34b—the peak areas were integrated within defined boundaries and then quantified using the calibration curve.”\n“The quantitative methods for waste oil were described as following:\nThe waste catering oil was sourced from the slop bucket in the canteen of Central China Normal University. It\nmay contain water, salts (primarily NaCl), food residues, fatty acids/esters (including C , C , and C ),\n16 17 18\ntriglycerides, and amides. The quantitative analysis of the components was conducted as follows:\n(a) Quantification of water, salts, and food residues: The solid residues were first filtered and washed with\ndichloromethane. A known amount of water was then added to the oil phase, followed by vigorous stirring and\nphase separation. The aqueous phase was removed, and this extraction was repeated three times. The resulting\norganic phase was dried under vacuum at 60 ℃. The mass difference before and after this treatment was used to\nestimate the total mass of water, salts, and food residues present in the waste oils.\n(b) Quantification of fatty acids/esters: The dried oil phase was analyzed by GC-MS to quantify the fatty\nacid/ester contents. The total mass fraction of fatty acids/esters was found to be 25.7%, including 4.5% C\n16\nspecies, 1.0% C species, and 21.2% C species.\n17 18\n(c) Quantification of triglycerides: The oil was treated with 0.5 M KOH at 100 ℃ to fully hydrolyze the\ntriglycerides, thereby releasing C fatty acid. The total amount of C fatty acid was determined by GC-MS. The\n18 18\ncontent of C fatty acid initially present in the oil was subtracted from this total, and the difference was assigned\n18\nto the C fatty acids derived from triglyceride hydrolysis, allowing the triglyceride content to be calculated.\nThe purified products were subsequently identified and quantified using GC-MS. [...]”\nFigure R4 GC pattern of (a) crude soybean oil and (b) crude tall oil.\nResponse to comments from Reviewer #3\nComments: In this submitted manuscript, Wei et al. report a strategy to integrate anodic electrochemical\ndecarboxylation, cathodic proton reduction, and olefin hydrogenation, which converts fatty acid derivatives into\nlong-chain alkanes. This approach can contribute to a good alkane yield of 88.9%, and is specifically interesting\nto upgrade the waste carbon source. However, from the viewpoint of scientific demonstration, the manuscript is\nrequired to be substantially improved through a major revision, before it becomes acceptable by Nature\nCommunications.\nReply: We sincerely thank the reviewer for the valuable comments and for recognizing the significance of our\nstrategy in upgrading waste carbon sources.\n1. The referee strongly suggests the author to improve the quality of figure images carefully.\nReply: We thank the reviewer for this valuable suggestion. In response, we have carefully examined all the\nfigures in the revised manuscript and supplementary information and have made every effort to improve their\nquality, including resolution, clarity, labeling, and consistency. We hope that the updated figures now meet the\npublication standards and enhance the readability of the manuscript.\n2. I recognize carbon paper is used as the anodic electrode. As it is a carbon material with graphite nature, does\nthis bare electrode substrate contribute to the electrochemical activity?\nReply: We thank the reviewer for raising this important point. Indeed, as a carbon-based material with a graphitic\nstructure, bare carbon paper (CP) can exhibit some level of electrochemical activity. However, its contribution to\nthe overall catalytic performance is minimal under our reaction conditions. When bare CP is used as the anodic\nelectrode, the yield of the target product is only ~10%, as shown in our control experiments.\nTo clarify this, we have now included the discussion in the revised Manuscript\nLine 164 Page 8:\n“[...] impressive yield of ~90%. In contrast, the bare carbon paper (CP), composed of carbon fibers, affords only\n~10% product yield. Throughout the electrochemical process [...]”\n3. The authors calculated FE. So how the consumed charge is recorded?\nReply: We thank the reviewer for raising this important question. The Faraday efficiency (FE) is calculated using\nthe following equation:\nFE (%) = (Moles of electrons consumed by the product / Total moles of electrons passed) × 100%.\nThe method for determining the total charge depends on the electrolysis conditions:\n(a) Under constant current, the total charge passed is calculated using the equation: Q = Current (A) × Reaction\ntime (s), and the total moles of electrons is then obtained as: Total electrons = Q / 96485, where 96485 C/mol is\nthe Faraday constant.\n(b) Under constant potential, the Current–Time (i–t) curve is recorded by the electrochemical workstation. The\ntotal charge is obtained by integrating the Current over Time, i.e.: Q = ∫i(t) dt, and then converted to moles of\nelectrons by dividing by the Faraday constant.\n4. (Electrochemical) surface area matter. The author synthesized p-rGO to exhibit a well-defined porous structure.\nIt is commonly regarded that the high porosity can contribute to the high surface area. However, the BET\ncharacterizations give the contrary results. In addition, electrochemical active surface areas of all GO-based\nsamples are determined to be similar. The authors should give more interpretation.\nReply: We appreciate the reviewer’s insightful comment. Regarding the observation that the BET surface area of\np-rGO is relatively low while its electrochemical surface area (ECSA) remains comparable to that of rGO, this can\nbe attributed to the following factors:\n(a) The surface porosity of p-rGO is predominantly microporous. General BET measurements have limitations in\naccurately evaluating micropores: BET requires an “infinite plane + multilayer adsorption + reversible path”,\nwhereas a microporous system presents a “confined space + pore filling + irreversible path”.\n(b) Electrolyte ions in ECSA measurements can effectively access these confined regions, leading to a more\nrepresentative evaluation of real electrochemically active surface sites. (J. Electroanal. Chem. 1992, 327, 353-376;\nJ. Am. Chem. Soc. 2018, 140, 2397-2400; ACS Appl. Energy Mater. 2024, 7, 2906-2917) ECSA is largely\ndetermined by the number of electrochemically active surface sites, and both materials may exhibit comparable\ndensities of such sites.\n(c) The high electrical conductivity of p-rGO facilitates more efficient utilization of active sites during\nelectrochemical processes. (Sensors 2017, 17, 1539; Small 2019, 15, 1903780) Furthermore, the interconnected\nporous network and enhanced charge-transfer properties of p-rGO are the key contributors to its superior\nelectrochemical performance, rather than its physical surface area alone. (Nano Lett. 2012, 12, 1806-1812;\nChemosensors 2023, 11, 50; Small 2019, 15, 1903780)\nWe have now included a relevance discussion in the revised Manuscript\nLine 142 Page 7:\n“[...] and Brunauer-Emmett-Teller (BET) analyses. One point to note is that BET shows the reduced surface area\nfor p-rGO considering that general BET characterization has limitations in micropores. Because electrolyte ions\ncan [...]”\nLine 144 Page 7:\n“[...] has limitations in micropores. Because electrolyte ions can effectively access microporous structures, the\nelelectrochemically active surface area (ECSA) measurement can be a more representative evaluation of\nelectrochemically active surface sites of porous structures. As shown in Figure S5, the ECSA of p-rGO remains\ncomparable to that of rGO, most likely indicating similar density of electrochemically active surface sites. As\nreported, the presence of [...]”\nIn the revised Supplementary Information, Page 19:\n“[...] Regarding the observation that the BET surface area of p-rGO is relatively low while its ECSA remains\ncomparable to that of rGO, this can be attributed to the following factors:\n(a) The surface porosity of p-rGO is predominantly microporous. General BET measurements have limitations in\naccurately evaluating micropores: BET requires an “infinite plane + multilayer adsorption + reversible path”,\nwhereas a microporous system presents a “confined space + pore filling + irreversible path”.\n(b) Electrolyte ions in ECSA measurements can effectively access these confined regions, leading to a more\nrepresentative evaluation of real electrochemically active surface sites.1-3 ECSA is largely determined by the\nnumber of electrochemically active surface sites, and both materials may exhibit comparable densities of such\nsites.\n(c) The high electrical conductivity of p-rGO facilitates more efficient utilization of active sites during\nelectrochemical processes.4, 5 Furthermore, the interconnected porous network and enhanced charge-transfer\nproperties of p-rGO are the key contributors to its superior electrochemical performance, rather than its physical\nsurface area alone.5-7 [...]”\n5. I am confused by the product distribution. When the authors conducted the anodic decarboxylation,\nolefin products were generated. In these electrochemical experiments, I notice that graphite rod was used as cathode.\nHowever, the authors further declared that graphite rod can catalyze the olefin hydrogenation with considerable\nactivity (Figure 2c). So, how the author obtained the olefin products without hydrogenation by cathodic graphite\nrod?\nReply: We thank the reviewer for raising this insightful point. This concern touches on a key aspect of our system:\nthe conditions under which olefins, rather than alkanes, are obtained at the cathode when using graphite. We have\ncarefully thought about this comment and made a response as following:\n(a) On the interpretation of Figure 2c (now revised as Figure 2b): In this figure, the “alkane yield” reflects the\nhydrogenation of olefins in the presence of Pd/C catalysts dispersed in the electrolyte, not from graphite alone.\nThe observed differences between graphite and Pt cathodes arise from their distinct behavior in supplying reactive\nhydrogen species for Pd/C-assisted hydrogenation. Graphite generates adsorbed H species more steadily and with\nlower H₂ evolution, which can be effectively transferred to Pd/C for olefin hydrogenation. In contrast, Pt favors\nrapid H₂ gas evolution, which competes with and reduces hydrogenation efficiency.\n(b) How olefin products are obtained without hydrogenation at the graphite cathode: While graphite can generate\nreactive H species, its hydrogenation activity is limited in the absence of Pd/C. In experiments targeting olefin\nformation, we deliberately exclude Pd/C from the cathodic chamber, and finely control the applied voltage. Our\ndata indicate that: At low voltages (2.5–3.5 V), alkene formation dominates due to limited surface adsorption of\nalkenes and hydrogen species. Moderate voltages (4–6 V) yield a mixture of alkenes and alkanes. At higher\nvoltages, alkene selectivity increases again, as rapid H₂ evolution outcompetes hydrogenation pathways.\nThus, to obtain olefin products from anodic decarboxylation without hydrogenation, we adopt the following\nstrategies: 1) Use of graphite cathode without Pd/C, to avoid catalytic hydrogenation; 2) Tight control of the\napplied voltage within the range of 3–4 V, where the generation of reactive H species is moderate, and H₂\nevolution is not dominant. These carefully tuned conditions allow us to selectively isolate olefin intermediates\ndespite the use of a graphite cathode. We appreciate the reviewer’s thoughtful comment, which helped clarify this\nimportant mechanistic distinction.\n6. “The O-H bond of ethanol is cleaved on the cathode”. This statement is controversial. The dehydrogenation of\nalcohol though O-H bond cleavage should be an oxidation reaction.\nReply: We thank the reviewer for pointing out this issue. The cleavage of the O–H bond in ethanol is indeed often\nassociated with oxidative dehydrogenation. However, under electrochemical cathodic conditions, this cleavage\noccurs via proton reduction and electron transfer, leading to H₂ evolution, and thus does not constitute an\noxidation process. We acknowledge that our original wording was ambiguous. To avoid confusion, we have\nrevised the sentence in the revised Manuscript, Line 216 Page 9:\n“[...] On the other hand, for electrochemical cathodic reaction, EtOH can be reduced on the cathode through\nelectron supply and hydrogen ion acceptance, producing reductive hydrogen species [...]”.\n7. As H is supplied by ethanol, what does an it become after providing H? The authors should give a clear picture\nabout the charge and mass balance in the whole reactor.\nReply: We thank the reviewer for the insightful question regarding the fate of ethanol after hydrogen donation\nand the overall charge and mass balance in the system. In our electrochemical setup, ethanol serves as the\nhydrogen donor at the cathode, and its transformation occurs under basic conditions. The anodic and cathodic\nprocesses are summarized as follows:\n(a) Anodic half-reaction:\nUnder alkaline conditions, carboxylate anions (R–CH₂–COO⁻) are oxidized at the anode via decarboxylation and\ndehydrogenation. This reaction involves the participation of OH⁻ and generates alkenes, CO₂, and electrons:\nR–CH₂–COO⁻ + OH⁻ → R–CH=CH₂ + CO₂ + H₂O + 2e⁻\n(b) Cathodic half-reaction:\nAt the cathode, ethanol undergoes deprotonation under alkaline conditions to generate ethoxide anions (EtO⁻) and\nprotons (H⁺). These protons are reduced at the cathode to form reactive hydrogen species (H*), which participate\nin olefin hydrogenation on the Pd/C surface:\nEtOH ⇌ EtO⁻ + H⁺\nH⁺ + e⁻ → H*\nR–CH=CH₂ + 2H* → R–CH₂–CH₃\nThe byproduct, ethoxide anion (EtO⁻), is stable in basic media and does not accumulate indefinitely. Instead, it\ncan recombine with water (produced at the anode) to regenerate ethanol and hydroxide:\nEtO⁻ + H₂O ⇌ EtOH + OH⁻\nThis internal proton shuttle between ethanol, ethoxide and water ensures that both mass and charge are conserved\nin the overall system.\n(c) Overall reaction summary:\nCombining the anodic and cathodic processes, the overall reaction pathway converts fatty acids into alkanes with\nCO₂ as the main byproduct, while ethanol serves as a recyclable hydrogen donor. No net accumulation of ethoxide\nor loss of ethanol occurs.\nResponse to comments from Reviewer #4\nComments: Wei, Ma, Ouyang, Ren, and coworkers present the development of an electrochemical\ndecarboxylation methodology for the generation of long chain alkanes from waste fatty acids and fatty acid esters.\nTheir method utilizes a porous reduced graphene oxide (p-rGO) anode and graphite cathode to promote\ndecarboxylative carbocation formation followed but either nucleophilic attack, beta-hydride elimination, or\nbeta-scission to generate alkenes. These alkenes are then reduced to alkanes with the hydrogen atoms generated at\nthe cathode via reduction of the ethanol solvent with the help of a Pd/C catalyst. The authors were able to\ndemonstrate broad applicability not only by successfully generating alkanes from a large variety of fatty acid\nchain lengths, but also by performing this transformation with crude mixtures that include other additives such as\nsalts, amides, and phospholipids. Finally, the authors were able to demonstrate the scalability of this method by\nprocessing 40 g of fatty acids providing a 67% yield of long-chain olefins. Some notes are provided below to aid\nin clarification of points made by the authors.\nReply: We sincerely thank the reviewer for the positive and constructive feedback. We are pleased that the\nnovelty, practicality, broad substrate scope, crude-tolerance, and scalability of our electrochemical\ndecarboxylation strategy were recognized. In response to the reviewer’s comments, we have revised the\nmanuscript to clarify key points, including text modifications, improved figure legends, and added explanations in\nboth the main text and Supplementary Information.\n1. Optimization tables in the supplementary information would benefit from clear reaction schemes, reminding the\nreader of the variables that are not being changed as mentioned in lines 121-129 of the SI. Some of the figures\nalso contain too many subheadings and inserts. This should be broken up into figures specific to the discussion.\nReply: We thank the reviewer for the valuable suggestion. In response, we have added reaction schemes to Tables\nS1, S2, and S4 to clearly indicate the unchanged variables (Tables R2, R3, R4). In addition, to improve clarity\nand focus, the original Figure S4 has been separated into three individual figures—Figures R5, R6, and R7—in\nthe revised Supplementary Information (Figures S5, S6, and S7), each corresponding to specific aspects of the\ndiscussion. Meanwhile, we have separated the original Figure 2h and Figure 3d from the main text and moved\nthem to the Supplementary Information.\nTable R2 The optimization of the solvents.\nEntry Solvent Yield (%)\n3\n4 tetrahydrofuran n.d.\n2 2\n6 hexafluoroisopropanol 36\nNote: “n.d.” refers to not detected.\nTable R3 The optimization of the bases.\nEntry Base Yield (%)\n2 2,4,6-collidine 35\n2 3\n3\n5 triethylenediamine n.d.\n6 triethylamine n.d.\n8 potassium isopropoxide < 5\n9 pyridine n.d.\n3\n11 quinoline n.d.\n12 4-dimethylaminopyridine n.d.\n13 2,4,6-trimethylpiperidine n.d.\n14 n-Bu NOH 76\nTable R4 The optimization of the electrolytes.\nEntry Electrolyte Yield (%)\n1 n-Bu NPF 83\n4 6\n3 n-Bu NI n.d.\n4\n4 n-Bu NOAc 58\n4\n5 n-Bu NCl 47\n4\nNote: “n.d.” refers to not detected.\nFigure R5 TEM images of (a) rGO and (b) p-rGO.\nFigure R6 XRD patterns of G, GO, rGO and p-rGO.\nFigure R7 (a) N adsorption-desorption isotherm curves and (b) pore size distribution of p-rGO.\n2\n2. The reaction is fairly dilute with regards to the long chain fatty acid (~15 mM). In order to minimize solvent\nusage in a large scale process, what is the highest concentration this methodology can tolerate with regards to the\nfatty acid? Are there any hypotheses as to why the organic amine bases cannot be used? This seems highly\nimpractical for scale.\nReply: We thank the reviewer for highlighting the concern regarding substrate concentration and solvent use.\nUsing this method, the highest concentration of fatty acids can reach up to 0.2 M. When it exceeds 0.2 M,\ninsoluble substrates appear in the reaction mixture. For fatty acid concentration of 0.10 M and 0.20 M in EtOH,\nboth afforded comparable conversion (70~80%) and selectivity (> 90 %) to our original 15 mM runs (Figure R8),\nconfirming that feedstock concentration can be increased by over an order of magnitude. Consequently, the\nrequired volume of EtOH is reduced by more than 80%, markedly improving process efficiency. Furthermore, we\nhave implemented a simple distillation recovery step, achieving 60–70% EtOH reclamation, which further\nminimizes solvent costs and waste. These results collectively demonstrate the feasibility of our electrochemical\ndecarboxylation at industry‐relevant concentrations and underline its practical application potential.\nIn addition, we have screened several organic bases and found that only tetrabutylammonium hydroxide could\nreplace KOH as the base (Table R3). However, compared to KOH, this organic base is much more expensive and\nmay not be suitable for practical applications. Most organic bases are too weakly basic to generate a sufficiently\nhigh concentration of fatty acid anions.\nFigure R8 Performance in 15 mmol/L, 100 mmol/L and 200 mmol/L stearic acid, with KOH (1.2 equivalents), n-\nBu NPF (0.06 M), EtOH (20 mL) and voltage of 4.5 V.\n4 6\nWe have now included a relevant discussion in the revised Manuscript\nLine 110 Page 6:\n“[...] can reach 83%. Additionally, to minimize solvent usage in a large-scale process, the concentration of fatty\nacid is increased to 0.1 M and 0.2 M, which both yield satisfactory results (Figure S4). The post-electrolysis\nanalysis [...]”\n3. Figure S13: The onset of oxidation is identical between the two fatty acids. The anodic peak is different, but\nthat difference is an effect of the mass transport kinetics rather than the thermodynamics of C vs C redox.\n11 18\nThis should be corrected in the discussion.\nReply: We thank the reviewer for the insightful comment. We have carefully re-examined the data presented in\nFigure S13 (now Figure S30 in the revised Supplementary Information) and agree that the differences in anodic\npeak potentials between C and C fatty acids are primarily attributed to mass transport kinetics, rather than\n11 18\nthermodynamic differences in their redox behaviors.\nAs shown in the cyclic voltammetry data, the identical onset potentials of C and C suggest that the initial\n11 18\nelectron transfer originates from the carboxyl group rather than from the alkyl chain. The shift in oxidation peak\npotentials reflects the impact of mass transport limitations. According to the Duclaux-Traube rule, the surface\nactivity of homologous saturated fatty acids at interfaces increases with the length of the alkyl chain. As such,\nlonger alkyl chains exhibit higher adsorption coefficients, which enhance molecular mass transport to the\nelectrode surface and reduce the required redox potentials. (Anal. Chem. 2018, 90, 11917-11924; Analyst 2014,\n139, 4673-4680) Moreover, C fatty acid exhibits a higher anodic current compared to C , because of the\n18 11\nimproved mass transport to result in increased interfacial accumulation of substrate. (J. Chem. Thermodyn. 2007,\n39, 1027-1037) The interplay between the stronger adsorption, improved mass transport and increased surface\nconcentration exemplifies how mass transport kinetics, rather than intrinsic thermodynamics, govern the\nelectrochemical behavior observed.\nWe have revised the discussion section accordingly to clarify that the observed peak potential differences stem\nfrom chain-length-dependent mass transport effects, In the Supporting Information, Page 43:\n“[...] We also found that the electrochemical behavior varies with different chain lengths. As shown in the cyclic\nvoltammetry data, the identical onset potentials of C and C suggest that the initial electron transfer originates\n11 18\nfrom the carboxyl group rather than from the alkyl chain. The shift in oxidation peak potentials reflects the impact\nof mass transport limitations. According to the Duclaux-Traube rule, the surface activity of homologous saturated\nfatty acids at interfaces increases with the length of the alkyl chain. As such, longer alkyl chains exhibit higher\nadsorption coefficients, which enhance molecular mass transport to the electrode surface and reduce the required\nredox potentials.53,54 Moreover, C fatty acid exhibits a higher anodic current compared to C , because of the\n18 11\nimproved mass transport to result in increased interfacial accumulation of substrate.55 The interplay between the\nstronger adsorption, improved mass transport and increased surface concentration exemplifies how mass transport\nkinetics, rather than intrinsic thermodynamics, govern the electrochemical behavior observed. [...]”\n4. There is no explanation as to how this process circumvents the formation of alkyl radicals, which are frequently\ninvoked in such electroxidative decarboxylation reactions.\nReply: We thank the reviewer for raising the key question of how our system circumvents the formation of alkyl\nradicals. In contrast to classical Kolbe decarboxylation, our observations and mechanistic considerations indicate\na carbocation‐mediated, heterolytic decarboxylation pathway on the carbon electrode surface, which inherently\navoids free‐radical intermediates:\n(a) Surface‐bound carboxylate activation and heterolysis. Under our applied potential window, the carboxylate\nanion undergoes a two‐electron oxidation at the electrode interface, leading directly to heterolytic C–C bond\ncleavage and formation of a surface‐stabilized carbocation, rather than homolytic decarboxylation to a free radical.\nThis concerted, two‐electron process is favored on graphitic carbon surfaces, as reported in similar non-Kolbe\nsystems. (Carbon Resour. Convers. 2023, 6, 287-297)\n(b) Electrode material effects. Planar graphite and our porous carbon electrodes provide high densities of edge and\ndefect sites that enhance adsorption coefficient and lower the barrier for heterolytic C–C bond cleavage.\n(ChemRxiv. 2025; doi: 10.26434/chemrxiv-2025-r8nkf; Electrochim. Acta 2011, 56, 2246-2253; J. Chem. Soc.,\nPerkin Trans. 1973, 1, 257-261)\n(c) Rapid interfacial charge transfer vs. radical diffusion. The porosity of our carbon not only enhances mass\ntransport but also accelerates charge transfer. (Joule 2018, 2, 76-93) Any transient radicals generated would be\nimmediately intercepted by the high local electric field and converted to carbocations, preventing their escape into\nthe bulk and subsequent dimerization. (Angew. Chem. Int. Ed. 2024, 136, e202407207; Green Chem. 2020, 22,\n286-301)\n(d) Lack of radical byproducts. Consistent with a non-radical mechanism, we do not observe symmetric alkane\ndimers by GC-MS. Moreover, the observed olefin rearrangement is associated with the carbocation intermediate\nbecause the lower rearrangement barrier compared with radical rearrangement (Acc. Chem. Res. 1977, 10, 9-15).\nTaken together, these points demonstrate that our electrooxidative decarboxylation proceeds via a heterolytic,\ncarbocation‐driven pathway at the electrode surface, thereby circumventing the formation—and recombination—\nof free alkyl radicals. We have now included a relevant discussion in the revised Manuscript\nLine 282 Page 12:\n“[...] the formation of alkyl radicals through employing porous carbon material, which are known to non-Kolbe,\nheterolytic decarboxylation pathways and minimize surface radical accumulation, thus preventing unwanted\nradical [...]”\n5. The DFT calculations require a proper discussion and summary in an energy landscape, rather than a series of\nsimple mol2 structures in the SI.\nReply: We thank the reviewer for this constructive suggestion. In the revised manuscript, we have now\nincorporated a comprehensive energy landscape (Figure R9) to better summarize and illustrate the DFT results.\nThis energy diagram provides a clear visualization of the key intermediates and transition states involved in the\nolefin formation process from different fatty acid substrates. The corresponding discussion has been added to the\nmain text as follows:\nWe performed DFT calculations to examine the dehydrogenation behavior of α- or β-carbocations derived from\nfatty acids of varying chain lengths. (a) Short-chains (e.g., C ): Geometry optimization failed to stabilize the α-\n10\ncarbocation, indicating its tendency to rearrange into a more stable β-carbocation. The β-carbocation favors OH\nquenching over high-barrier OH-assistant dehydrogenation, aligning with the lower olefin yield and selectivity\nobserved for C fatty acids. (b) Long-chains (e.g., C , C ): The α-carbocation is structurally stabilized, enabling\n11 15 17\nlow-barrier dehydrogenation to olefins without OH assistance, consistent with the higher olefin yield and\nselectivity from C and C fatty acids. (c) Longer-chains (e.g., C , C ): Besides olefin formation, the α-\n16 18 19 21\ncarbocation can undergo low-barrier βC–γC bond cleavage, generating a truncated species with two fewer carbons,\nexplaining the chain-shortened products observed.\nIn summary, short-chain fatty acids tend to form β-carbocations with high barriers to olefin formation, whereas\nlong-chain fatty acids preferentially generate α-carbocations that readily produce olefins, resulting in higher yield\nand selectivity.\nWe have revised the figure in the revised Supplementary Information, Page 44 and the relevant discussion in\nthe revised Manuscript\nLine 265 Page 11:\n“[...] DFT calculations were performed to examine the dehydrogenation behaviors of α- or β-carbocations derived\nfrom fatty acids of varying chain lengths (Figure S31). (a) Short-chains (e.g., C ): Geometry optimization failed\n10\nto stabilize the α-carbocation, indicating its tendency to rearrange into a more stable β-carbocation. The β-\ncarbocation favors OH quenching over high-barrier dehydrogenation, aligning with the lower olefin yield and\nselectivity observed for C fatty acids. (b) Long-chains (e.g., C , C ): The α-carbocation is structurally\n11 15 17\nstabilized, enabling low-barrier dehydrogenation to olefins without OH assistance, consistent with the higher\nolefin yield and selectivity from C and C fatty acids. (c) Longer-chains (e.g., C , C ): Besides olefin\n16 18 19 21\nformation, the α-carbocation can undergo low-barrier βC–γC bond cleavage, generating a truncated species with\ntwo fewer carbons, explaining the chain-shortened products observed. [...]”\nFigure R9 (a) The energy change of OH binding with β-C+ for C β-carbocation. (b) The energy change of self-\n10\ndehydrogenation for C and C α-carbocations. (c) The energy change of β-scission for C and C α-\n15 17 19 21\ncarbocations.\nOverall, this is a nice report but has a number of shortcomings that ignored. A number of poorly justified\narguments are made regarding the selectivity of the reaction. The DFT is unclear, and the experimental\nmechanistic study is buried in the SI. Finally, the system is completely impractical as the reaction is performed at\n15 mM concentrations. This is a huge waste of EtOH solvent, which his likely the most expensive and valuable\ncomponent of the reaction. Given the precedent for such reactions/systems, this work is more suitable for a more\nspecialized journal.\nReply: We thank the reviewer for these constructive comments. We have carefully addressed each of the raised\nconcerns through significant revisions and additional experiments, as detailed below:\n(a) In response to this comment, we have made the following revisions: (1) We rationally clarify that the CV\ncurves reflect how the alkyl chain length of fatty acids determines mass transport kinetics, which leads to\ndifferences in oxidation potentials and currents. (2) Based on the chain-length-dependent selectivity tests, we\nfurther elaborate on the crucial role of alkyl chain length in influencing the reaction kinetics of carbocations.\n(3) We also reorganize the DFT calculation figures in the revised Supplementary Information and clarify the\ninterpretation in the main text to rationalize how different chain lengths affect the carbocation reaction kinetics,\nthereby impacting the overall yield and selectivity.\n(b) To demonstrate the practical viability of our protocol, we conducted experiments at higher fatty acid loadings:\n0.10 M and 0.20 M in EtOH (Figure R9). Both conditions afforded comparable conversion (70~80%) and\nselectivity (> 90 %) to our original 15 mM runs, confirming that feedstock concentration can be increased by over\nan order of magnitude. Consequently, the required volume of EtOH is reduced by more than 80%, markedly\nimproving process efficiency. Furthermore, we have implemented a simple distillation recovery step, achieving\n60–70% EtOH reclamation, which further minimizes solvent costs and waste. These results collectively\ndemonstrate the feasibility of our electrochemical decarboxylation at industry‐relevant concentrations and\nunderline its practical application potential.\nWe hope these revisions and new data effectively address the reviewer’s concerns and better demonstrate both the\nmechanistic foundation and practical relevance of our work. We sincerely appreciate the reviewer’s comments,\nwhich have helped us significantly strengthen the manuscript.\nResponse to comments from Reviewer #5\nComments: In this paper, Ma and co-workers describe an electrochemical method for upgrading fatty acids from\nwaste and crude oil, into sustainable fuels including long-chain olefins and alkanes. This method is advantageous\nrelative to previous methods in that it does not require exogenous H2, high temperature, or pressure, and is\neffective against crude mixtures. This work shows high potential for the green upcycling of waste feedstocks into\nfuels with high efficiency and scalability. The authors also include DFT analysis of alkene isomerization after the\ndecarboxylation process and begin to explain the selectivity of carbon chains of different lengths. I found the\nmethod to be useful and interesting, with high potential for impact, but found the manuscript somewhat difficult to\nfollow. Many experiments were shown, but their (i.e. DFT studies) importance was not well justified. Figures\nwere dense, and included many bar/pie charts that would be better suited as tables in the SI. The authors report\nthe use of a new electrode material (p-rGO) and do a good job characterizing and explaining its use. However,\nthey were less successful in demonstrating clearly their exact electrochemical set-up, including the composition of\ntheir custom reactor. These details are critical for replicability. There are also several examples through the text\nwhere the authors make claims about their method relative to others, but do not reference any citations, leading to\nskepticism in some places. Finally, the authors only show the scaled-up conversion of stearic acid using their\nmethod. I recommend also demonstrating application of the crude inputs to the full method, going from input to\nalkane mixture.\nDue to the reasons listed above, while I find this paper to be of potentially high importance and impact, I do not\nbelieve it rises to the level of rigor necessary for publication in Nature Communications, and recommend it be\npublished elsewhere. I recommend authors perform significant revision of their writing/organization, and\ndemonstrate a broader scope in their ultimate transformation.\nReply: We sincerely thank the reviewer for the thoughtful and constructive feedback. We greatly appreciate the\nrecognition of the potential impact of our electrochemical method for converting waste fatty acids into long-chain\nolefins and alkanes under mild and green conditions. We also thank the reviewer for pointing out areas where the\nmanuscript could be strengthened. In response to the concerns raised, we have performed a substantial revision of\nboth the Manuscript and the Supplementary Information. Please see our responses and revisions listed as below:\n1. 78-81: The discussion of these alternative electrocatalytic systems read as non-sequiturs, and details are given\nhere that have no point of comparison -why is this important?\nReply: We thank the reviewer for this valuable comment. We agree that the discussion of the BiO /TiO system is\nx 2\nnot necessary; therefore, we have now deleted the relevant discussion on the BiO /TiO system. Meanwhile, we\nx 2\nhave revised the relevant discussion to enhance the logical flow and clarity of the content.\nWe have now included a relevant discussion to clarify the purpose of including the example and how to relate to\nour study’s objective in the revised Manuscript\nLine 65 Page 3:\n“[...] water on the Pt/TiO catalyst.9 To fully eliminate the reliance on fossil fuel–derived H₂, an electrocatalytic\n2\nsystem can be employed to achieve both higher efficiency and an adequate supply of hydrogen species. In such a\nsystem, the hydrogen species generated at the cathode are expected to be directly utilized for olefin\nhydrogenation.21 However, the related reports on [...]”\n2. 81-83: this statement needs a reference\nReply: We appreciate the reviewer’s insightful suggestion. To support the statement regarding the substantial\nhydrogen consumption during the conventional conversion of waste oils to long-chain alkanes—especially in the\npresence of unsaturated fatty acids—we have now added appropriate references (ACS Catal. 2017, 7, 1158–1169;\nEnergy Fuels 2018, 32, 3519–3525; now listed as Ref. 22, 23 in the revised Manuscript).\n3. 81-85: The statement is given that alkane production from waste oils is “inefficient” and requires H . However,\n2\nno references are given. This comes across as obfuscating the relevant background to what this paper purports to\nshow. If production of alkanes from waste oils truly has never been done and this work is fully novel, the authors\nshould say that explicitly.\nReply: We thank the reviewer for raising this important point. First, we apologize for the lack of proper\nreferencing. In conventional processes, hydrodeoxygenation (HDO) of waste oils to alkanes typically requires\nhigh-pressure hydrogen (e.g., 4 MPa) and elevated temperatures (e.g., > 300 ℃), which results in high energy\nconsumption and operational complexity. These processes can also suffer from catalyst deactivation due to coke\nformation. We have now included relevant citations in the revised Manuscript to clarify this context (Ref. 22, 23\nin the revised Manuscript). Furthermore, while some methods for valorizing waste oils into hydrocarbons exist\n(ACS Catal. 2017, 7, 1158-1169), to the best of our knowledge, electrochemical conversion of waste fatty acids\ninto alkanes without external hydrogen or high temperature remains unexplored. We have revised the manuscript\nto more clearly highlight this novelty and state the distinction between our approach and conventional HDO routes.\nWe would like to emphasize the innovations in our current work: We report, for the first time, an electrochemical\ndecarboxylative coupled with hydrogenation strategy that entirely eliminates fossil fuel-derived H₂ input, while\ndelivering high product yields. Moreover, a comprehensive and systematic investigation was conducted to\nelucidate the underlying mechanism and improve the process.\nWe have now cited the literature and suggested this issue more explicitly in the revised Manuscript\nLine 68 Page 3:\n“[...] utilized for olefin hydrogenation. However, there is no report on an electrochemical decarboxylative coupled\nwith hydrogenation strategy for the transformation of waste oils into alkanes, which entirely eliminates fossil fuel-\nderived H₂ input, while delivering high product yields.22, 23 Therefore, developing methods for [...]”\n4. 87: Be more specific as to which contaminants are referred to here and are common in these feedstocks.\nReply: We thank the reviewer for raising this point. We have already specifically suggested which contaminants\nin the sentence in the revised Manuscript, Line 75 Page 4:\n“Here, we demonstrate a highly efficient electrochemically coupled approach that can proceed smoothly even\nwith elevated contaminant levels (like water, salt (mainly NaCl), lipid polymers, food particles/colloid) in the\nfeedstock, obviating the need for costly purification (Figure 1b).”\n5. 138-140: How are these yields determined? Constitutional isomers are notoriously difficult to differentiate, and\nno information is provided to prove this claim. The SI implicates GCMS, but shows few traces, and certainly not\nfor most of the reactions described.\nReply: Thank you very much for the reviewer’s suggestion. We agree that constitutional isomers are notoriously\ndifficult to differentiate. Below are the methods for determining the yield and product distribution:\n(a) Determining the yields: Firstly, upon completion of the reaction, the crude was purified by flash column\nchromatography to afford a mixture of olefins. Secondly, based on Yield = Actual mass / Theoretical mass ×\n100%, the yields can be calculated.\n(b) Determining olefin distribution: GC-MS is employed to analyze and quantify the purified olefin mixtures.\nAs shown in the chromatograms of products derived from C to C fatty acids, the mixture consists of 1-n-olefin,\n11 22\n2-n-olefin, 3-n-olefin, 4-n-olefin, and branched olefin (Figures R10~R21). Despite their similar boiling points,\nthe optimization of the GC-MS temperature program—specifically, a reduced heating rate and lower initial\ntemperature—enables separation of the isomers. Partial overlap can be observed between branched olefin and 1-n-\nolefin, and the proportion of branched olefin can be obtained by peak deconvolution using Origin software.\nTo describe clearly how to determine the yields, we have now included the GCMS parameters and experimental\ndiscussion in the revised Manuscript\nLine 418 Page 11:\n“[...] The description of specific parameters for GCMS: Column oven temperature: 25 ℃; injection port\ntemperature: 25 ℃; carrier gas: He; inlet pressure: 100 kPa; total flow rate: 50 mL/min; column flow rate: 1.93\nmL/min; linear velocity: 49.7 cm/s; purge flow rate: 3 mL/min; split ratio: -1. GC column: Rtx-5MS capillary\ncolumn (30 m × 0.25 mm i.d., film thickness: 0.25 μm). Ion source temperature: 230 ℃; interface temperature:\n280 ℃; mass scan range: m/z 50-550. [...]”\nLine 430 Page 18:\n“[...] Note that the moles of the products were determined by weighing actual mass of the purified product after\npurifying the reaction mixture through flash column chromatography. [...]”\nFigure R10 GCMS of C fatty acid decarboxylation products.\n22\n6. 203: should be “sacrificial”\nReply: Thanks for the reviewer’s suggestion. We have now corrected this in the revised Manuscript, Line 188\nPage 8: “[...] To investigate the cathodic proton reduction processes, we examined the electrochemically-assisted\nhydrogenation of 1-heptadecene via using ethanol oxidation as sacrificial anodic reaction. [...]”\n7. 221-222: This discussion might be improved by being more explicit about what Fig 2d is showing. I did not\nfollow this part, and do not understand Fig 2d.\nReply: We much thank the reviewer for pointing this out. We apologize that the explanation of Figure 2d in the\noriginal version was not sufficiently clear (now Figure 2c in the revised Manuscript). In fact, the figure is intended\nto show the barrier of olefin hydrogenation (y-axis) with an increase of H–H distance (x-axis), ranging from 0.74\nÅ (corresponding to a molecular H₂) to 1.32 Å (corresponding to two separated H atoms) (Figure R22).\nThe revised caption now reads:\n“c Hydrogenation barrier of olefins as a function of H–H distance, ranging from 0.74 Å (leftmost, H₂ molecule) to\n1.32 Å (rightmost, two separated H atoms).”\nWe have now depicted the figure more clearly and included the discussion in the revised Manuscript\nLine 203 Page 9:\n“[] used as the cathode. Density functional theory (DFT) calculations were employed to suggest barrierless olefin\nhydrogenation for separated H atoms. It is evident that when H species transition from H molecule to separated H\n2\natoms, the barrier of olefin hydrogenation decreases significantly (Figure 2c). [...]”\nFigure R22 Hydrogenation barrier of olefins as a function of H–H distance, ranging from 0.74 Å (leftmost, H₂\nmolecule) to 1.32 Å (rightmost, two separated H atoms).\n8. 244: Need references here.\nReply: Thanks for the reviewer’s suggestion. We have now added the literature references (Ref. 18, 42-52 in the\nrevised Manuscript) in the sentence in the revised Manuscript, Line 224 Page 10: “[...] the hydrogenation and\ndecarboxylation rate. Therefore, the productivity of long-chain alkanes notably exceeds that of relevant\nphotocatalytic and thermocatalytic studies (Figure S17, Table S5).18,42-52 [...]” Figure 2h has been placed in the\nrevised Supplementary Information (Figure S17).\n9. 266-274: I do not follow the rationale for the difference in CVs. I would not expect that significant a difference\nbetween fatty acids of these chain lengths – can this be explained another way?\nReply: We thank the reviewer for this thoughtful comment. We agree that, based on redox thermodynamics alone,\none would not expect a large difference in oxidation potentials between fatty acids of similar chain lengths. Upon\nre-examination of the data (original Figure S13, now updated as Figure S30 in the revised Supplementary\nInformation), we propose an alternative explanation: the observed variation in cyclic voltammetry (CV) behavior\nis primarily governed by mass transport kinetics rather than intrinsic redox potential differences. Specifically:\n1) The identical onset potentials for both C and C fatty acids suggest that the initial electron transfer occurs at\n11 18\nthe carboxylate group, which is the same in both molecules. This confirms that the redox process itself is not\nfundamentally altered by chain length.\n2) The shift in anodic peak potentials and the higher oxidation current observed for C are better explained by\n18\ndifferences in interfacial adsorption and diffusion behavior. According to the Duclaux-Traube rule, surface\nactivity of homologous fatty acids increases with chain length, which leads to stronger adsorption at the electrode\ninterface for longer chains.\n3) Stronger adsorption enhances local substrate concentration at the electrode surface and improves molecular\ndelivery to the reaction zone. As a result, C fatty acid exhibits faster mass transport and higher surface\n18\naccumulation, leading to a lower peak oxidation potential and higher current density compared to C (Anal.\nWe have revised the discussion section accordingly to clarify that the observed peak potential differences stem\nfrom chain-length-dependent mass transport effects in the revised Supplementary Information, Page 43:\n“[...] We also found that the electrochemical behavior varies with different chain lengths. As shown in the cyclic\nvoltammetry data, the identical onset potentials of C and C suggest that the initial electron transfer originates\n11 18\nfrom the carboxyl group rather than from the alkyl chain. The shift in oxidation peak potentials reflects the impact\nof mass transport limitations. According to the Duclaux-Traube rule, the surface activity of homologous saturated\nfatty acids at interfaces increases with the length of the alkyl chain. As such, longer alkyl chains exhibit higher\nadsorption coefficients, which enhance molecular mass transport to the electrode surface and reduce the required\nredox potentials.53,54 Moreover, C fatty acid exhibits a higher anodic current compared to C , because of the\n18 11\nimproved mass transport to result in increased interfacial accumulation of substrate.55 The interplay between the\nstronger adsorption, improved mass transport and increased surface concentration exemplifies how mass transport\nkinetics, rather than intrinsic thermodynamics, govern the electrochemical behavior observed. [...]”\n10. 275-300: The authors discuss carbocation rearrangements of fatty acids of different chain lengths. However,\nthe text is dense and difficult to follow. Fig 3b similarly attempts to draw conclusions about alkene selectivity,\nand subsequent discussion focuses on DFT calculations. I would like to understand why a detailed analysis of\ncarbocation/alkene rearrangements is necessary especially given that the authors emphasize the practicality of\nthe method (the alkenes should be reduced to the alkane anyway).\nReply: We sincerely thank the reviewer for the insightful comments and constructive criticism. We have revised\nthe manuscript to clarify the rationale behind our analysis and improve the readability of the relevant discussion.\nOur detailed response is as follows:\n(a) On the necessity of carbocation/alkene rearrangement analysis:\nFirstly, while it is true that the ultimate goal is to produce alkanes, understanding the intermediate carbocation and\nalkene species is crucial for rationalizing the observed chain-length-dependent yield and selectivity.\nExperimental results (e.g., Figure 3b) indicate non-trivial differences in product distribution based on chain length,\nconsistent with the results of DFT calculations. Secondly, the chain-length-dependent reaction kinetics can\ninfluence the overall production of long-chain alkanes. For example, short-chain fatty acids tend to generate\nrearranged carbocations after decarboxylation, which face higher barriers for dehydrogenation to form alkenes\nand are more prone to be quenched by OH- ions. In contrast, long-chain fatty acids are more likely to retain the\nα-carbocation after decarboxylation, which can more readily undergo dehydrogenation to form alkenes. The\nkinetics of α-carbocation transformation partially determines the alkene production, thereby affecting the\nefficiency of final alkane. Therefore, investigating the alkene selectivity and carbocation kinetics provides\nvaluable theoretical guidance for improving production efficiency.\n(b) On the link between DFT and practicality:\nThe DFT analysis is intended to provide mechanistic insight into why long-chain fatty acids favor specific\ncarbocation stabilization and dehydrogenation pathways. This understanding allows for better control of\nselectivity and efficiency, especially when extending the method to more complex or mixed feedstocks in\npractical applications.\n(c) On clarity and revision of Figure 3b and related text:\nWe have revised the relevant section of the manuscript to improve clarity and flow. Specifically, we simplified the\ndescription of carbocation rearrangements and rephrased the discussion around Figure 3b to make the key\nconclusions more accessible to readers. The DFT results have also been reorganized to more directly support the\nexperimental trends (Figure R23).\nWe have now improved the relevant discussion in the revised Manuscript\nLine 246 Page 11:\n“[...] The cyclic voltametric results also indicate non-trivial differences in product distribution based on chain\nlength (Figures 3b, S30). For C to C fatty acids, 1-n-olefin accounts for only 30~40% of the olefins, with the\n11 13\nremainder being thermodynamically more stable olefins. Instead, for C to C fatty acids, the 1-n-olefin\n14 19\nproportion rises to around 50%, indicating that long-chain fatty acids are more likely to retain the α-carbocations\nafter decarboxylation. Intriguingly, with further chain elongation in C to C fatty acids, a two-carbon atom\n20 22\ndeparture in the carbocations follows –COOH group removal. Figure 3c illustrates the reaction pathways of\ncarbocations with different carbon lengths. Obviously, the kinetics of α-carbocation transformation partially\ndetermines the olefin production, thereby affecting the efficiency of final alkane. [...]”\n“[...] The chain-dependent kinetics of α-carbocation (R–CH +) is crucial for improving the yield and selectivity of\n2\nolefins. The initial intermediate species formed during fatty acid decarboxylation is the α-carbocation, and its\ntransformation pathways may be: 1) Directly converting into 1-n-olefins; or 2) Undergoing rearrangement to form\nother more stable carbocations, which then convert into other types of olefins. The higher proportion of 1-n-\nolefins is primarily attributed to α-carbocations, which are favored by reaction kinetics. In contrast, rearranged\nolefins originate from rearranged carbocations, such as β- and γ-carbocations, and their formation is driven by the\nthermodynamic stability of internal alkenes. Collectively, the longer the carbon chain, the more favorable the\nstabilization and dehydrogenation kinetics of the α-carbocation, thereby improving the yield and selectivity. [...]”\n“[...] DFT calculations were performed to examine the dehydrogenation behaviors of α- or β-carbocations derived\nfrom fatty acids of varying chain lengths (Figure S31). (a) Short-chains (e.g., C ): Geometry optimization failed\n10\nto stabilize the α-carbocation, indicating its tendency to rearrange into a more stable β-carbocation. The β-\ncarbocation favors OH quenching over high-barrier dehydrogenation, aligning with the lower olefin yield and\nselectivity observed for C fatty acids. (b) Long-chains (e.g., C , C ): The α-carbocation is structurally\n11 15 17\nstabilized, enabling low-barrier dehydrogenation to olefins without OH assistance, consistent with the higher\nolefin yield and selectivity from C and C fatty acids. (c) Longer-chains (e.g., C , C ): Besides olefin\n16 18 19 21\nformation, the α-carbocation can undergo low-barrier βC–γC bond cleavage, generating a truncated species with\ntwo fewer carbons, explaining the chain-shortened products observed. [...]”\nFigure R23 (a) The energy change of OH binding with β-C+ for C β-carbocation. (b) The energy change of self-\n10\ndehydrogenation for C and C α-carbocations. (c) The energy change of β-scission for C and C α-\n15 17 19 21\ncarbocations.\n11. 347-371: This is supposed to be the heart of the method – direct, coupled net conversion of fatty acids to long\nchain alkanes. However, few details are given as to what the input is in the bulk reactor! Is this study only\nperformed from stearic acid? What happens to waste oils on bulk scale? Ultimately, it would be very important to\nshow that the method is applicable to crude inputs, or else explain why it is not.\nReply: Thanks for the reviewer’s raising this important point. In response to the reviewer’s comment, we provide\nthe following reply:\nBesides scale-up stearic acid, we also applied this method to the scale-up conversion of waste oils to alkanes.\nA 10 g sample of waste oils was collected from the cafeteria of Central China Normal University, containing a\nsubstantial amount of H₂O, salts (mainly NaCl), lipid polymers, and food particles/colloids. The waste oil was\ndirectly used for the transformation (Table R3, Entry 4). Compared to stearic acid, the transformation of waste\noils involves two notable differences in the experimental input: the addition of more KOH and a small amount of\nH₂O, due to the high content of fatty acid esters present in waste oils. Eventually, approximately 3 g of products\ncan be obtained.\nTo demonstrate that the method is applicable to crude inputs, we have now included the results in the revised\nManuscript (Figure 3e) and included the experimental description in the revised Manuscript\nLine 441 Page 18:\n“The reactor (1 L) was loaded with waste catering oil (10 g), KOH (2 g), n-Bu NPF (0.06 M) and EtOH (450 mL)\n4 6\n+ H O (50 mL). A reactor cap, fitted with three pairs of anodes (working electrodes) and cathodes (graphite rods),\n2\nwas then installed into the reaction mixture. The mixture was stirred at 60 °C for 30 minutes without an electric\ncurrent. Then a palladium on carbon (Pd/C, 20 wt.% Pd, 20 mg) catalyst was added. The reaction system was then\nsubjected to electrolysis at a constant current of 40 mA at 60 °C for 24 hours. After the reaction, the solvent was\nremoved under vacuum, and the residue was purified by column chromatography using petroleum ether as the\neluent.”\nTable R3 The results of conversion of crude soybean oil, tall oil and waste catering oil. a The products are olefins;\nb The products are alkanes.\nSelectivity of C olefins/alkanes (%)\nn-1\nEntry Feedstock Conv. (%)\nC C C C C C\n14 15 16 17 18 19\n1a Crude soybean oil 83 5.3 1 8.8 45 13.9 26\n2a Crude tall oil 85 0 5 1 85 4 5\nWaste catering oil\n3a 78 0 19 8 73 0 0\n(10 g)\nWaste catering oil\n4b 73 0 17 5 78 0 0\n(10 g)\n12. 350: “custom built reactor” – details must be provided for replicability.\nReply: Thanks for the reviewer’s raising this important point. We have now demonstrated our reaction setup\n(Figure R24). Our experimental apparatus mainly consists of a single-compartment reactor (with a stirrer),\nelectrode clamp, graphite sheet substrate, and graphite counter electrode. The type of reaction setup delivers some\nadvantages: The setup is simple and inexpensive, easy to operate and maintain, and safe and stable, making it\nmore suitable for practical applications.\nWe have now provided the pictures and details of the custom built reactor in the revised Supplementary\nInformation, Page 51:\nFigure R24 The components of the custom-built reactor, including single-compartment electrolysis cell, constant\nvoltage/current power source, electrode clamp, high-purity conductive high-temperature-resistant graphite\nelectrode, high-purity graphite rod electrode.\n“Single-compartment electrolysis cell: (1) Reactor 1: Volume = 1000 mL, Inner diameter = 110 mm, Depth = 120\nmm; (2) Reactor 2: Volume = 100 mL, Inner diameter = 49 mm, Depth = 55 mm.\nHigh-purity conductive high-temperature-resistant graphite electrode: Electrical resistivity = 9 mΩ m, Length ×\nWidth × Thickness = 50 mm × 45 mm × 2 mm for Reactor 1 or Length × Width × Thickness = 25 mm × 25 mm ×\n2 mm for Reactor 2.\nHigh-purity graphite electrode: Diameter = 6 mm, Length = 85 mm.”\n13. 367-369: “surpass” should be elaborated on. The physical properties differ, but authors need to justify why\nthese are better.\nReply: We appreciate the reviewer’s comment and apologize for the ambiguity in our previous description.\n(1) We have revised this sentence to convey our intended meaning more clearly: “According to the comparison\npresented in Table S6, our product can serve as a viable substitute for diesel fuels. Furthermore, compared to\ncommercial gasoline, petro-diesel, biodiesel, and ethanol fuel, it exhibits a higher calorific value, which translates\nto greater energy output and improved fuel economy at a lower cost.”\n(2) We have justified why these are better as following compared with Petro-diesel and bio-diesel fuels:\nModerate Density: It can release more energy during combustion, providing more power, while also generating\nfewer pollutants, making it more environmentally friendly.\nModerate Boiling Point: It avoids incomplete combustion caused by too high a boiling point, which leads to\ncarbon deposits, increased fuel consumption, and more pollutants; and avoids poor start-up performance and\nengine air resistance problems caused by too low a boiling point.\nModerate Viscosity: It avoids flow issues due to high viscosity, which could cause insufficient fuel supply, and\nalso prevents incomplete combustion caused by low viscosity, leading to carbon buildup and increased fuel\nconsumption.\nHigher Calorific Value: It ensures higher energy output and better fuel economy, reducing fuel consumption.\nThe production cost of the fuel via our method is lower, therefore, there is no concern about cost. Additionally, it\ncan be blended with commercial diesel fuels for use.\nWe have now revised the sentence and included the relevant discussions\nIn the revised Manuscript, Line 334 Page 14:\n“[...] reducing the cost of products. According to the comparison presented in Table S6, our product can serve as a\nviable substitute for diesel fuels. Furthermore, compared to commercial gasoline, petro-diesel, biodiesel, and\nethanol fuels, it exhibits a higher calorific value, which translates to greater energy output and improved fuel\neconomy at a lower cost. Notably, the reactor design is [...]”\nIn the revised Supplementary Information, Page 52:\n“[...] Moderate Density: It can release more energy during combustion, providing more power, while also\ngenerating fewer pollutants, making it more environmentally friendly.\nModerate Boiling Point: It avoids incomplete combustion caused by too high a boiling point, which leads to\ncarbon deposits, increased fuel consumption, and more pollutants; and avoids poor start-up performance and\nengine air resistance problems caused by too low a boiling point.\nModerate Viscosity: It avoids flow issues due to high viscosity, which could cause insufficient fuel supply, and\nalso prevents incomplete combustion caused by low viscosity, leading to carbon buildup and increased fuel\nconsumption.\nHigher Calorific Value: It ensures higher energy output and better fuel economy, reducing fuel consumption.\nThe production cost of the fuel via our method is lower, therefore, there is no concern about cost. Additionally, it\ncan be blended with commercial diesel fuels for use. [...]”\n14. 383-385: where are these numbers from?\nReply: Thanks for the reviewer’s raising this important point. The consumption of fossil fuels and the emission of\ngreenhouse gases were calculated using the GREET software:\n(a) For the production of 1 kg of long-chain alkanes via using our method, the consumption of fossil fuels and the\nemission of greenhouse gases are 50.13 MJ and 2.281 kg, respectively (Figures 4c, 4d). Therefore, for the\nproduction of 100 kt of long-chain alkanes, the corresponding values are 50.13 MJ × 10⁸ = 5013 TJ and 2.281 kg\n× 10⁸ = 228.1 kt, respectively.\n(b) For the production of 1 kg of long-chain alkanes via using the default method in GREET, the consumption of\nfossil fuels and the emission of greenhouse gases are 72.05 MJ and 3.403 kg, respectively (Figure R25).\nTherefore, for the production of 100 kt of long-chain alkanes, the corresponding values are 72.05 MJ × 10⁸ = 7205\nTJ and 3.403 kg × 10⁸ = 340.3 kt, respectively.\n(c) By subtracting the values obtained from our method from those obtained from the common method, we can\nconclude that “implementing this system on an industrial scale for 100 kt of long-chain alkanes could save 2,192\nTJ of fossil energy and reduce greenhouse gas emissions by 112.2 kt.”\nWe have now included the figure (Figures 4c, 4d) on life cycle FED and GGE of long-chain alkanes production\nvia using the default method in GREET in China and supplemented the suggestion in the revised Manuscript\nLine 351 Page 14:\nSignificantly, compared with the default production method in GREET, implementing this system on an industrial\nscale for 100 kt of long-chain alkanes could save around 2,192 TJ of fossil energy and reduce GGEs by 112.2 kt,\noffering an overwhelming advantage over CD production.\nFigure R25 Life cycle (a) FED and (b) GGE of long-chain alkanes production.\n15. Fig 2h: This appears unnecessary to me – it is simply a comparison of the productivity of this method\ncompared to other methods. Should be kept in the SI and summarized in a sentence or two in the text.\nReply: We thank the reviewer for the helpful suggestion. We agree that the productivity comparison is better\nsuited for the Supplementary Information. Accordingly, Figure 2h has been moved to the revised Supplementary\nInformation (now Figure S17), and we have summarized the key point in the revised Manuscript\nLine 224 Page 10:\n“[...] and decarboxylation rate. Therefore, the productivity of long-chain alkanes notably exceeds that reported in\nrelevant photocatalytic and thermocatalytic studies (Figure S17, Table S5).18,42-52 [...]”\n16. Fig 3e: this figure shows pie charts that give the various components of different oils. This can be summarized\nin text or in the SI, but does not significantly enhance the story as a figure.\nReply: Thanks for the reviewer’s suggestion. We have now provided a brief component summary of different oils\nin the revised Supplementary Information, Page 48:\n“[..] In crude soybean oil, the mass fraction of fatty acids is 51.2%, which includes 2.3% of C fatty acids/esters,\n16\n30% of C fatty acids/esters, 11.4% of C fatty acids/esters, and 7.5% of other fatty acids/esters. In addition,\n18 20\nthere are 48.8% unidentified additives.\nIn crude tall oil, the mass fraction of fatty acids is 90%, which includes 5.5% of C fatty acids/esters, 79.8% of\n18 20\n10% unidentified additives. [..]”\n17. Fig 3f: this figure seems to show conversion of waste oils to different alkene components. Are they not then\nreduced further to the alkanes? There is ambiguity as to what the figure is showing.\nReply: Thanks for the reviewer’s raising this point. We apologize for not clearly presenting the transformation\nfrom waste oils to long-chain alkanes. The conversion in the original Figure 3f refers to the conversion of fatty\nacids to long-chain olefins. Meanwhile, we also employed our strategy to transform waste oils into long-chain\nalkanes (Table R3, Entry 4), and their conversion and selectivity have minor difference.\nWe have now clarified the illustration in the figure caption and added the part of waste oil-to-alkane (Entry 4):\n“Table R3 The results of conversion of crude soybean oil, tall oil and waste catering oil. a The products are\nolefins; b The products are alkanes.”\nSelectivity of olefins/alkanes (%)\nEntry Feedstock Conv. (%)\nC C C C C C\n14 15 16 17 18 19\n1a Crude soybean oil 83 5.3 1 8.8 45 13.9 26\n2a Crude tall oil 85 0 5 1 85 4 5\n3a Waste catering oil 78 0 19 8 73 0 0\n(10 g)\nWaste catering oil\n4b 73 0 17 5 78 0 0\n(10 g)\n18. This manuscript never shows a general reaction scheme that describes the main components of the reaction.\nThis would be beneficial in the main text.\nReply: We thank the reviewer for this helpful suggestion. We agree that a general reaction scheme would\nsignificantly aid reader understanding by summarizing the key components and transformation pathways in our\nelectrochemical system. The reaction components and pathways are described as follows:\n(a) Anodic half-reaction:\nUnder alkaline conditions, fatty acid anions (R–CH₂–COO⁻) undergo oxidative decarboxylation and\ndehydrogenation, with the participation of OH⁻. This generates long-chain alkene intermediates, CO₂, H₂O, and\nelectrons:\nR–CH₂–COO⁻ + OH⁻ → R–CH=CH₂ + CO₂ + H₂O + 2e⁻\n(b) Cathodic half-reaction:\nAt the cathode, ethanol (EtOH) is deprotonated under alkaline conditions to form ethoxide anion (EtO⁻) and\nproton (H⁺). The proton is reduced by the cathode to generate reactive hydrogen species (H)*, which then\nhydrogenate the alkenes to form alkanes on the Pd/C surface:\nEtOH ⇌ EtO⁻ + H⁺\nH⁺ + e⁻ → H*\nR–CH=CH₂ + 2H* → R–CH₂–CH₃\nThe ethoxide anion remains stable in the electrolyte and does not interfere with the overall redox cycle.\n(c) Overall reaction and system balance:\nElectrons released from the anodic oxidation are consumed in the cathodic hydrogenation, ensuring charge\nbalance. Ethanol acts as a sacrificial hydrogen donor, converting to ethoxide, while fatty acids are selectively\ntransformed into diesel-range alkanes. The schematic in Figure 1b reflects this paired redox process and outlines\nthe roles of all key species involved.\nWe sincerely thank the reviewer again for this valuable suggestion, which has helped improve the clarity and\ncompleteness of our manuscript.\n19. The manuscript further gives very few details into the construction of the electrochemical reactor that\npurports to convert stearic acid on the 40 g scale. Details must be provided to enable replicability.\nReply: Thanks for the reviewer’s raising this important point. We have now exhibited the construction of the\nelectrochemical setup (Figure R26). Our experimental apparatus mainly consists of a single-compartment reactor\n(with a stirrer), electrode clamp, graphite sheet substrate, and graphite rod counter electrode.\nWe have now provided the pictures of the custom-built reactor in the revised Supplementary Information\n(Figure S37), Page 51:\nFigure R26 The components of the custom built reactor, including single-compartment electrolysis cell, constant\nvoltage/current power source, electrode clamp, high-purity conductive high-temperature-resistant graphite\nelectrode, high-purity graphite rod electrode.\n“Single-compartment electrolysis cell: (1) Reactor 1: Volume = 1000 mL, Inner diameter = 110 mm, Depth = 120\nmm; (2) Reactor 2: Volume = 100 mL, Inner diameter = 49 mm, Depth = 55 mm.\nHigh-purity conductive high-temperature-resistant graphite electrode: Electrical resistivity = 9 mΩ m, Length ×\nWidth × Thickness = 50 mm × 45 mm × 2 mm for Reactor 1 or Length × Width × Thickness = 25 mm × 25 mm ×\n2 mm for Reactor 2.\nHigh-purity graphite electrode: Diameter = 6 mm, Length = 85 mm.”\n20. Many schemes in this manuscript rely on bar graphs, when a simple table would suffice. Greater care should\nbe taken in effective visual communication. (This applies to many parts of Fig. 2, for example).\nReply: We appreciate the reviewer’s constructive feedback regarding data presentation. In response, we have\ncarefully reviewed all relevant figures and replaced several bar graphs with concise tables where appropriate (e.g.,\nFig. 2b and 3f in the original Manuscript). These changes improve clarity and facilitate direct comparison of\nnumerical values, in line with effective visual communication. For datasets where graphical trends are important\nfor interpretation, bar graphs were retained but improved in formatting for better readability.\n21. What is “waste catering oil”? Is this the same as “cooking oil”? Need details as to where they source this\nmaterial and its composition.\nReply: Thank the reviewer for the important suggestion. The “waste catering oil” in our manuscript is the same as\n“waste cooking oil”. We sourced the waste catering oil from the slop bucket in the canteen of Central China\nNormal University. The waste oil could contain H O, salt (mainly NaCl), food residue, fatty acids/esters\n2\n(including C , C , C ), triglyceride, and amide. The mass fraction of fatty acids is 25.7%, which includes 4.5%\n16 17 18\nof C fatty acids/esters, 1% of C fatty acids/esters and 21.2% of C fatty acids/esters (Figure R27).\n16 17 18\nFigure R27 GC pattern of waste oils.\nWe have supplemented the source and composition of the waste catering oil in the revised Manuscript\nLine 449 Page 18:\n“We sourced the waste catering oil from the slop bucket in the canteen of Central China Normal University. The\nwaste oil could contain H O, salt (mainly NaCl), food residue, fatty acids/esters (including C , C , C ),\n2 16 17 18\ntriglyceride, and amide. The mass fraction of fatty acids is 25.7%, which includes 4.5% of C fatty acids/esters,\n16\n1% of C fatty acids/esters and 21.2% of C fatty acids/esters.”\n17 18\n22. In general, the manuscript relies on claims about the components of complex crude mixtures, but is not\nexplicit about how they are analyzing these mixtures. It appears, upon careful digging through the SI, that GCMS\nmay be indicated, but is it truly quantitative? Details are needed throughout.\nReply: Thanks for the reviewer’s precious suggestion. The crude soybean oil and crude tall oil obtained from the\nfactory not only contain a variety of fatty acids but also include other compounds that cannot be identified, as well\nas insoluble substances (Figure R28). Given the complexity of the components, with a few peaks overlapping and\nbeing difficult to separate, we can only obtain their approximate percentages. This is the inherent limitation of the\nquantitative analysis of crude oils and waste oils.\nBelow, we elaborate on the details of the quantitative analysis via using GCMS:\n(1) For major fatty acids with well-resolved and distinct peaks—such as peaks d, i, and j in Figure R28a, and\npeaks a and f in Figure R28b—the peak areas were directly integrated and concentrations calculated based on the\ncalibration curve.\n(2) For major fatty acids overlapping with other peaks—such as peak d* in Figure R28a—the peak areas were\nobtained through peak deconvolution by Origin software.\n(3) For minor peaks—such as peaks b, c, f, g and h in Figure R28a, and peaks b, c, d, e, g, h, i, j, k, and l in Figure\nR28b—the peak areas were integrated within defined boundaries and then quantified using the calibration\ncurve.\nFigure R28 GC of (a) crude soybean oil and (b) crude tall oil.\nWe have now elaborated on the details of the quantitative analysis for crude oils using GCMS in the revised\nSupplementary Information\nLine 173 Page 6:\n“[...] 1. The quantitative methods for crude oils were described as following:\nThe components of crude soybean oil and crude tall oil were analyzed by GC-MS to identify their fatty acid\ncontents. These industrial crude oils contain not only a variety of fatty acids, but also unidentified compounds and\ninsoluble substances. Due to the complexity of the mixtures—along with peak overlap and limited\nchromatographic resolution—it is only possible to obtain the approximate composition. This represents an\ninherent limitation in the quantitative analysis of crude feedstocks and waste oils.\nBelow, we provide further details on the quantitative analysis conducted using GC-MS:\n(a) For major fatty acids with well-resolved and distinct peaks—such as peaks d, i, and j in Figure S34a, and\npeaks a and f in Figure S34b—the peak areas were directly integrated and concentrations calculated based on the\ncalibration curve.\n(b) For major fatty acids overlapping with other peaks—such as peak d* in Figure S34a—the peak areas were\nobtained through peak deconvolution by Origin software.\n(c) For minor peaks—such as peaks b, c, f, g and h in Figure S34a, and peaks b, c, d, e, h, i, j, k, and l in Figure\nS34b—the peak areas were integrated within defined boundaries and then quantified using the calibration curve.\n[...]”\nResponse to the reviewers’ comments\nGeneral remark: We sincerely thank all referees for taking the time to carefully review our manuscript and\nprovide constructive feedback, which has greatly contributed to improving its clarity, completeness, accuracy, and\noverall quality. We have thoroughly addressed all the comments and revised the manuscript accordingly. The\nrevisions are highlighted in yellow. Point-by-point responses are provided below under “Reply”, with the referees’\noriginal comments presented in italics.\nResponse to comments from Reviewer #4\nComments: Overall, the authors have adequately addressed all points, including optimization of figure\npresentation formats, exploration of higher substrate concentrations, clarification of the applicability of organic\namine bases, analysis of CV differences across fatty-acid chain lengths, discussion of pathways that avoid alkyl-\nradical formation, and the addition of DFT energy-landscape diagrams showing barrier variations for key\nintermediates and transition states during decarboxylation of different chain lengths.\nReply: We sincerely thank Reviewer for the positive and encouraging evaluation on our manuscript. We have\ncarefully addressed all the concerns raised by the reviewer. Please see our point-by-point response below.\n1. In response to Question 2, the authors demonstrate that increasing the fatty-acid concentration to 0.20 M\nmaintains conversion and selectivity, indicating scalability potential. I recommend adding LCA results under this\ncondition (Fig. S39) to better support the case for practical production.\nReply: We sincerely thank Reviewer for this constructive suggestion. In response, we performed LCA analysis at\nthe higher substrate concentration of 0.20 M. The results show no significant difference compared with the 0.10 M\ncase (Figure R1), indicating that the overall environmental and energy efficiency trends are maintained under\nscalable conditions. To address the reviewer’s comment, we have now included the LCA results at 0.20 M (Fig.\nS39) and clarified this point in the revised Supplementary Information, Line 555 Page 54:\n“[...] In addition to the 0.015 M condition, LCA analysis was also carried out at 0.20 M concentration of fatty acid.\nThe results show tiny difference, confirming that the environmental and energy efficiencies are preserved at\nhigher substrate concentration. [...]”\nFigure R1 Life cycle (a) FED and (b) GGE of long-chain alkanes production under the fatty-acid concentration of\n0.015 M via our method in China, the USA and the European Union (EU). Life cycle (c) FED and (d) GGE of\nlong-chain alkanes production under the fatty-acid concentration of 0.2 M via our method in China, the USA and\nthe European Union (EU).\n2. In response to Question 3, the authors apply the Duclaux–Traube rule to interpret adsorption versus chain\nlength. This rule is most accurate in dilute solutions. Please specify the solution concentrations used for the CV\ntests in Fig. S13 (now Fig. S30) and confirm that the analysis remains valid at those concentrations, or qualify the\nconclusion accordingly.\nReply: We thank the reviewer for the constructive comment. In our CV tests shown in Fig. S30, the\nconcentrations of the fatty acids were 15 mmol L⁻¹. At these concentrations, the solutions remain in the dilute\nregime where the Duclaux–Traube rule is generally applicable. Thus, the qualitative trend we observed—stronger\nadsorption with increasing chain length—can still be reasonably interpreted by this rule under our experimental\nconditions.\nTo address the reviewer’s concern, we have revised the text to clarify the solution concentrations used and to note\nthat the rule provides a reasonable interpretation in the dilute concentration range in the revised Supplementary\nInformation, Line 465 Page 43:\n“ [...] The concentrations of the fatty acids are 15 mmol L-1. At the concentration, the solutions remain in the\ndilute regime where the Duclaux–Traube rule is generally applicable. [...]”\nResponse to comments from Reviewer #5\nComments: I thank the authors for satisfactorily addressing my concerns. After reviewing the revised manuscript\nI have only a few additional comments:\nReply: Thank you for your valuable comments. We have carefully considered your suggestions and address them\nas follows.\n1. In most cases, figures involving DFT-related structures are extremely small and cannot be read easily. i.e. S31,\nFig 3C, Fig R9, Fig R23.\nReply: We thank the reviewer for pointing out this important issue. In the revised manuscript, we have revised the\nmanuscript to enlarge the figures involving DFT-related structures and adjusted the resolution to ensure clarity\nand readability. All revised figures have now been updated in both the main text and the Supplementary\nInformation (Figures R2, R3).\nFigure R2 (a) Yield and (b) selectivity of decarboxylation into olefins using different carbon-chain length of\nfatty acids with acid (0.3 mmol), KOH (1.2 equivalents), n-Bu NPF (0.06 M), EtOH (20 mL) and current of\n4 6\n10 mA. (c) Scheme of the reaction pathways of different 1-n-carbocations from decarboxylation of fatty\nacids. (d) The components in crude soybean oil, crude tall oil and waste catering oil. (e) The results of\nconversion of crude soybean oil, crude tall oil and waste catering oil. a The products are olefins; b The\nproducts are alkanes. Entry 4 differed from Entry 3 solely by the addition of Pd/C catalyst, with all other reaction\nconditions held constant.\nFigure R3 (a) The energy change of OH binding with β-C+ for C β-carbocation. (b) The energy change of self-\n10\ndehydrogenation for C and C α-carbocations. (c) The energy change of β-scission for C and C α-\n15 17 19 21\ncarbocations.\n2. In Fig 3e: Thank you for differentiating entries 3 and 4. However, what is the difference in reaction conditions\nthat leads to one providing olefins and another providing alkanes? No difference is noted.\nReply: We thank Reviewer for pointing out this important clarification. In Fig. 3e, the difference between entries\n3 and 4 lies solely in the presence of Pd/C catalyst. Specifically, Entry 3 was conducted using waste oil (10 g),\nKOH (2 g), n-Bu₄NPF₆ (0.06 M), EtOH (450 mL) + H₂O (50 mL) at 40 mA and 60 °C. Entry 4 was performed\nunder the same conditions, but with the addition of Pd/C catalyst. We have revised the figure caption and the\nrelated discussion in the revised Manuscript, Line 234 Page 10:\n“[...] Entry 4 differed from Entry 3 solely by the addition of Pd/C catalyst, with all other reaction conditions held\nconstant. [...]”\n3. While in general the writing has been vastly improved, there are a few places where revision is still needed.\nFor example, the sentence on lines 298-300 is hard to understand.\nReply: We thank Reviewer for the positive feedback and for pointing out the unclear sentence. The sentence on\nlines 298–300 has been carefully revised for clarity. The revised version (Line 292 Page 12, in the revised\nManuscript) now reads as follows: “[...] electrochemical approach. For the production of “green” alkanes, it is\npreferable to use non-food-based feedstocks to avoid competition with the food supply.56,57 In this study, we\ndirectly employed crude soybean fatty acids (a by-product of soybean oil refining) and tall-oil fatty acids (a\nby-product of the pulp industry) as substrates. Soybean oil is predominantly [...]”\nResponse to the reviewers’ comments\nGeneral remark: We sincerely thank all referees for taking the time to carefully review our manuscript and\nprovide constructive feedback, which has greatly contributed to improving its clarity, completeness, accuracy, and\noverall quality. We have thoroughly addressed all the comments and revised the manuscript accordingly. The\nrevisions are highlighted in yellow. Point-by-point responses are provided below under “Reply”, with the referees’\noriginal comments presented in italics.\nResponse to comments from Reviewer #1\nComments: The authors have well addressed my concerns. It can be accepted now. Reviewer #1's feedback on\nresponses to reviewer #4's comments (Remarks to the Author): I have reviewed the authors' responses to referee\n#4 and the revised manuscript. Overall, the authors have adequately addressed all of referee #4’s points,\nincluding optimization of figure presentation formats, exploration of higher substrate concentrations, clarification\nof the applicability of organic amine bases, analysis of CV differences across fatty-acid chain lengths, discussion\nof pathways that avoid alkyl-radical formation, and the addition of DFT energy-landscape diagrams showing\nbarrier variations for key intermediates and transition states during decarboxylation of different chain lengths. I\nhave two minor suggestions to further strengthen the revision:\nReply: We sincerely thank Reviewer for the positive and encouraging evaluation on our manuscript. We have\ncarefully addressed the concerns. Please see our point-by-point response below.\n1. In response to Question 2, the authors demonstrate that increasing the fatty-acid concentration to 0.20 M\nmaintains conversion and selectivity, indicating scalability potential. I recommend adding LCA results under this\ncondition (Fig. S39) to better support the case for practical production.\nReply: We sincerely thank Reviewer for this constructive suggestion. In addition to the condition (0.015 M), we\nhave also performed LCA analysis at the higher substrate concentration of 0.20 M. The results show no significant\ndifference compared with the 0.015 M case (Figure R1), indicating that the overall environmental and energy\nefficiency trends maintain under scalable conditions. To address the Reviewer’s comment, we have now included\nthe LCA results at 0.20 M (Fig. S39) and clarified this point in the revised Supplementary Information, Line\n“[...] In addition to the 0.015 M condition, LCA analysis was also carried out at 0.20 M concentration of fatty acid.\nThe results show tiny difference, confirming that the environmental and energy efficiencies are preserved at\nhigher substrate concentration. [...]”\nFigure R1 Life cycle (a) FED and (b) GGE of long-chain alkanes production under the fatty-acid concentration of\n0.015 M via our method in China, the USA and the European Union (EU). Life cycle (c) FED and (d) GGE of\nlong-chain alkanes production under the fatty-acid concentration of 0.2 M via our method in China, the USA and\nthe European Union (EU).\n2. In response to Question 3, the authors apply the Duclaux–Traube rule to interpret adsorption versus chain\nlength. This rule is most accurate in dilute solutions. Please specify the solution concentrations used for the CV\ntests in Fig. S13 (now Fig. S30) and confirm that the analysis remains valid at those concentrations, or qualify the\nconclusion accordingly.\nReply: We thank Reviewer for the valuable suggestion. In the CV tests shown in Fig. S30, the concentrations of\nthe fatty acids were 15 mmol L⁻¹. At these concentrations, the solutions remain in the dilute regime where the\nDuclaux–Traube rule is generally applicable. Therefore, we believe that the qualitative trend of adsorption versus\nchain length derived from the Duclaux–Traube rule is still valid under our experimental conditions. We have\nrevised the text to clarify the solution concentrations used and to note that the rule provides a reasonable\ninterpretation in the dilute concentration range in the revised Supplementary Information, Line 465 Page 43:\n“ [...] The concentrations of the fatty acids are 15 mmol L-1. At the concentration, the solutions remain in the\ndilute regime where the Duclaux–Traube rule is generally applicable. [...]”"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have well addressed all my concerns, thus the manuscript can be accepted as it is.", "reviewer_2": "The authors have adequately addressed all prior concerns. The revisions have substantially enhanced the technical rigor and clarity of this work. Following a comprehensive evaluation of the revised manuscript, I am satisfied with the improvements made and recommend its acceptance for publication.", "reviewer_3": "In reviewing the revised manuscript, I think that all the comments have been well addressed. I thus recommend the publication of this work as is.", "reviewer_5": "I thank the authors for satisfactorily addressing my concerns. After reviewing the revised manuscript I have only a few additional comments:\n1. In most cases, figures involving DFT-related structures are extremely small and cannot be read easily. i.e. S31, Fig 3C, Fig R9, Fig R23.\n2. In Fig 3e: Thank you for differentiating entries 3 and 4. However, what is the difference in reaction conditions that leads to one providing olefins and another providing alkanes? No difference is noted.\n3. While in general the writing has been vastly improved, there are a few places where revision is still needed. For example, the sentence on lines 298-300 is hard to understand."}, "author_response": ""}, {"round": 3, "reviewer_comments": {"reviewer_1": "The authors have well addressed my concerns. It can be accepted now."}, "author_response": ""}, {"round": 4, "reviewer_comments": {"reviewer_1": "The authors have well addressed my concerns. The manuscript can be accepted."}, "author_response": ""}]

Decision letter