Scalable electrosynthesis of commodity chemicals from biomass by suppressing non-Faradaic transformations

Biomass is an abundant and renewable carbon source that can replace fossil resources for the production of chemicals and fuels. Owing to its high oxygen content, biomass-derived platforms (e.g., sugars, HMF, lignin derivatives) are ideal for producing valuable oxygenates such as formic acid and FDCA. However, many biomass derivatives contain reactive oxygenated functionalities (aldehydes, ketones) that are prone to degradation under harsh conditions, particularly in alkaline media and at high concentrations. For example, sugars rearrange and degrade to organic acids in base via first-order reactions with respect to sugar concentration, and HMF can decompose and polymerize to humins, especially at elevated concentrations. While electrooxidation of biomass platforms is attractive due to mild conditions, no need for external oxidants, and pairing with green H2 evolution, most prior studies have been performed at dilute concentrations and small volumes, which limits scalability. A key, underexplored barrier is non-Faradaic degradation of substrates and intermediates in bulk alkaline electrolytes, which compromises selectivity and carbon efficiency, particularly under industrially relevant high-concentration, large-volume conditions. Here, the authors identify and address this non-Faradaic degradation challenge by engineering a single-pass continuous flow reactor (SPCFR) to suppress unwanted base-catalyzed side reactions, enabling selective and scalable production of oxygenates from concentrated biomass-derived feedstocks.

The paper surveys electrocatalytic upgrading of biomass-derived molecules (glycerol, glucose, HMF, lignin) highlighting advances in catalyst design that have achieved high conversion and selectivity, particularly for HMF oxidation to FDCA in dilute solutions (≤10 mM) and small volumes. These conditions, prevalent across many studies, hinder scalability. Strategies to suppress degradation, such as stabilizing reactive functional groups (e.g., protection of sugars and HMF), have been reported but add complexity and cost. Notable efforts to mitigate degradation in HMF electrooxidation include: (1) base-catalyzed Cannizzaro pre-conversion of HMF to more alkali-stable species (HMFCA, DHMF) enabling subsequent electro-oxidation (≈80% carbon balance at 250 mM feed); (2) high HMF concentration operation (10 wt.%) in neutral electrolyte with continuous NaOH addition in a circulated reactor achieving up to ~5 wt.% FDCA and 60–70% yields. Despite these advances, facile and selective electrooxidation at high concentrations remains challenging. The literature also indicates widespread use of strong alkaline electrolytes (typically 1 M KOH) to enhance current density, which accelerates degradation of typical feedstocks such as glucose and HMF. This underscores the need for system-level approaches to suppress non-Faradaic pathways during electrolysis under practical conditions.

Catalysts and electrodes: The anodic catalyst is γ-phase cobalt oxyhydroxide (CoOOH) nanosheet arrays electrodeposited on nickel foam (NF) to form CoOOH/NF. α-Co(OH)2 was deposited from 100 mM Co(NO3)2 at −80 mA for 300 s, then activated in 1 M KOH to yield γ-CoOOH/NF. Nickel foam served as cathode. For comparison, Ni2P/NF was also evaluated for HMF oxidation.

Electrochemical setups:

• Batch reactor: Undivided beaker cell, three-electrode configuration (CoOOH/NF 1 cm2 working, Ag/AgCl reference, Pt counter). Electrolyte typically 1 M KOH with substrates (e.g., 100 mM glucose or glycerol) at 1.5 V vs RHE. Product analysis by HPLC and 1H NMR.
• Single-pass continuous flow reactor (SPCFR): Custom two-electrode flow cell without ion-exchange membrane. Key features: high electrode-area/electrolyte-volume (~2.5 cm2 mL−1), short residence time, and separate feeding of substrate solution and KOH which are mixed just before entering the reactor. Typical single-module configuration used 30 cm2 CoOOH/NF anode and 30 cm2 NF cathode with ~12 mL internal electrolyte volume. Substrate solutions (e.g., 200 mM polyhydroxy compounds for single module, 300–800 mM HMF for stack) and 2 M KOH were delivered by pumps; constant current operation (e.g., 3–5 A single module). Flow rate and current were tuned to target high SPCE.
• Stacked SPCFR: Nine stacked modules providing total geometric electrode area of 270 cm2. Operation at higher currents (e.g., 15 A) and controlled flow rates enabled high productivity. The system supports linear scale-up by tandem design.

Operating conditions and measurements:

• Glucose electrooxidation in SPCFR (single module): 100 mM glucose in 1 M KOH conditions were mimicked by separate feeds (substrate and 2 M KOH mixed inline). At 3 A and 1.98 mL min−1, SPCE 80.2%, formate selectivity 83.8%, FE 89.6%.
• HMF oxidation in SPCFR (single module): 200 mM HMF feed; at 3 A and 0.79 mL min−1, SPCE 96.6%, FDCA selectivity 91.3% with minor intermediates (DFF, HMFCA, FFCA totaling 0.70%). Continuous operation at 5 A for >100 h yielded stable SPCE (~94%), cell voltage (~2.7 V), and FDCA selectivity (~95%).
• Stacked SPCFR (nine modules): Glucose-to-formate at 15 A achieved SPCE 81.8%, formate selectivity 76.5%, FE 91.7%, producing 562.8 mM formate with space-time-yield (STY) 256.6 mmol h−1 (11.8 g h−1) and concurrent H2 (>99.9% purity) at 279.8 mmol h−1 (0.56 g h−1). HMF-to-FDCA: Starting with 600 mM HMF, produced 556.9 mM FDCA with 96.9% selectivity and STY 76.2 mmol h−1 (11.9 g h−1); up to 602.7 mM FDCA achievable at lower selectivity (80.7%). Continuous production of 530–560 mM FDCA at SPCE >95% for >50 h.

Mechanistic studies:

• SPCFR coupled with immediate acid quenching and rapid HPLC to capture short-lived intermediates during glucose electrooxidation at high flow (11.4 mL min−1) and varied currents (0–7 A). Detected aldehydes (arabinose, erythrose, glycolaldehyde, formaldehyde) and glycolate in quenched effluents; without quenching, aldehydes rapidly degraded to acids (lactate, glycolate, glycerate) and fructose appeared.
• Batch reactor on-line DEMS detected formaldehyde, indicating aldehydes form but are consumed by non-Faradaic reactions in bulk alkaline electrolyte.
• 13C labeling experiments supported an aldose (aldehyde) route initiated at the C1–C2 position for glucose-to-formate, excluding aldonic acid intermediates.

Feedstock preparation and product isolation:

• Crude sugars: Birch wood fractionation via aqueous formic acid (FA) reflux (FA:water 7:3 v/v, 3 h), followed by cellulose pulp saccharification with cellulase (Cellic CTec2) at 50 °C to obtain glucose; used directly in stacked SPCFR.
• Crude glycerol: Produced via KOH-catalyzed transesterification of soybean oil with methanol at 65 °C; lower phase (glycerol/KOH/methanol) used as feedstock.
• KDF isolation: Effluents (~8 L) from crude sugars/glycerol electrooxidation acidified to pH 3 with formic acid, decolorized with activated charcoal, concentrated, mixed with ethanol, crystallized at −20 °C, filtered and dried to obtain potassium diformate (KDF) crystals.
• FDCA isolation: High-purity (>99%) FDCA collected continuously from HMF electrooxidation; kilogram-scale isolation achieved.

Analytical techniques: HPLC with UV and RI detectors; 1H NMR; XRD, SEM, TEM, Raman, XPS for materials; performance metrics include SPCE, selectivity, Faradaic efficiency (FE), STY, and carbon balance.

• Non-Faradaic degradation is the primary barrier to scaling alkaline electrosynthesis of biomass platforms, causing severe carbon loss and low selectivity at high substrate concentrations and large electrolyte volumes in batch reactors (e.g., glucose formate selectivity drops to 30.4%; HMF FDCA selectivity to 37.8% at 200 mM HMF, 50 mL).
• SPCFR design suppresses non-Faradaic pathways via: high electrode area/electrolyte volume ratio (~2.5 cm2 mL−1), short residence time, and separate feeding of substrate and alkali.
• Single-module SPCFR results: • Glucose (100 mM) in alkaline conditions: SPCE 80.2%, formate selectivity 83.8%, FE 89.6% at 3 A, 1.98 mL min−1, far exceeding batch selectivity (30.4%). • HMF (200 mM): SPCE 96.6%, FDCA selectivity 91.3% at 3 A, 0.79 mL min−1; other products total 0.70%; carbon balance 92.3% (remaining attributed to reduced humins formation compared to batch). Stable continuous operation for >100 h at 5 A with SPCE ~94%, cell voltage ~2.7 V, FDCA selectivity ~95%.
• Stacked SPCFR (nine modules, 270 cm2): • Glucose-to-formate at 15 A: SPCE 81.8%, formate selectivity 76.5%, FE 91.7%; produced 562.8 mM formate with STY 256.6 mmol h−1 (11.8 g h−1) and co-produced H2 (>99.9% purity) at 279.8 mmol h−1 (0.56 g h−1). • HMF-to-FDCA at high concentration: From 600 mM HMF produced 556.9 mM FDCA with 96.9% selectivity; up to 602.7 mM FDCA attainable (80.7% selectivity). Continuous 50+ h production of 530–560 mM FDCA at SPCE >95%.
• Scalability demonstrations: • Produced 0.7 kg potassium diformate (KDF) from birch wood-derived sugars and crude glycerol (from soybean oil biodiesel). • Produced 1.17 kg FDCA from HMF.
• Mechanistic insight: Aldehydes (aldoses and small aldehydes) are the real intermediates in glucose-to-formate electrooxidation; non-Faradaic degradation in bulk alkaline media converts these to acids (e.g., aldonic acids), which previously may have been misidentified as intermediates. 13C labeling and detection of aldehydes support an aldose route initiated at the C1–C2 bond rather than an aldonic acid pathway.

The study identifies non-Faradaic degradation of substrates and intermediates in alkaline media as the dominant cause of carbon loss and low selectivity in conventional batch electrooxidation of biomass platforms at high concentrations and volumes. By engineering the SPCFR to minimize contact time between reactive species and base and to maximize electrode area per electrolyte volume with separate feeding, the authors steer reaction flux toward desired electrocatalytic pathways and away from base-catalyzed side reactions. This enables high SPCE, high selectivity, and high product concentration under industrially relevant conditions, demonstrating continuous and kilogram-scale syntheses of KDF and FDCA. The suppression of non-Faradaic products also clarifies mechanistic pathways: aldehyde intermediates govern glucose-to-formate conversion via a processive α-scission starting at the C1–C2 bond, contradicting the commonly assumed aldonic acid route. The SPCFR approach thereby addresses both scalability and mechanistic ambiguity, and proves compatible with crude, realistic feedstocks while co-generating high-purity hydrogen at the cathode.

This work demonstrates a system-engineering strategy—single-pass continuous flow reactor (SPCFR)—that suppresses non-Faradaic degradation and enables scalable electrosynthesis of commodity chemicals from concentrated biomass-derived feedstocks. The SPCFR and its nine-module stack achieve high SPCE and selectivity for glucose-to-formate and HMF-to-FDCA at high product concentrations, and support continuous, kilogram-scale production of KDF (0.7 kg) and FDCA (1.17 kg). Mechanistically, the elimination of degradation artifacts provides evidence that aldehydes, not aldonic acids, are the key intermediates for oxidative C–C bond cleavage of polyhydroxy compounds. Future work should focus on thermal management (to mitigate Joule-heating-accelerated degradation), reduction of cell ohmic resistance, integration of cooling systems, and incorporation of advanced in-line/on-line analytics to further resolve transient intermediates and optimize reactor performance for pilot- and industrial-scale deployment.

• Temperature rise at high currents due to Joule heating accelerates non-Faradaic degradation significantly (e.g., ~70 °C at 15 A; ~163-fold acceleration), lowering selectivity without temperature management. This calls for improved thermal management (reduced ohmic resistance, cooling integration).
• Residual carbon loss remains (e.g., humins formation during HMF oxidation), and humins are difficult to quantify precisely.
• While catalyst choice was not the main determinant of carbon loss under tested conditions, broader catalyst screening and optimization under SPCFR conditions may further enhance performance.
• Mechanistic detection of short-lived intermediates still requires rapid quenching and advanced analytics; more sophisticated in-line/on-line tools would improve temporal resolution and pathway elucidation.