Co-Fe-NBC: Enhanced Biochar for Hydrogen Production

• The paper introduces Co-Fe-NBC as a hybrid biochar integrating cobalt and iron redox couples within an N-doped carbon matrix to manage electron flux in photo-fermentative hydrogen production.
• It is synthesized by stepwise pyrolysis of corn straw with melamine and metal nitrates, creating a porous, defect-rich structure with abundant oxygen vacancies.
• The nano-capattery exhibits dual electrochemical behavior with 287.91 F/g capacitance and 38.3 mC/g charge storage, leading to a 367% increase in cumulative hydrogen output.

Cobalt-iron-nitrogen doped biochar (Co-Fe-NBC) is a hybrid biochar in which a nitrogen-doped carbon matrix is co-modified with cobalt and iron species, specifically mixed Fe$^{2+}$/Fe$^{3+}$ and Co$^{2+}$/Co$^{3+}$ states anchored in an N-doped carbon scaffold. In the reported photo fermentation hydrogen production (PFHP) system, Co-Fe-NBC functions as a “nano-capattery,” combining capacitor property, battery-like charge storage, and redox mediation to regulate electron flux, suppress competing metabolic pathways, and increase cumulative hydrogen production from 151.03 mL in the control to 589.54 mL, corresponding to a 367% increase (Shahzaib, 27 Nov 2025).

1. Material identity and functional concept

Co-Fe-NBC is presented as a biochar-based hybrid material designed to address electron flux diversion in PFHP. The central problem is that electrons are diverted from optimal hydrogen production pathways to competitive pathways, thereby lowering overall PFHP efficiency. The reported solution is a “nano-capattery” architecture in which the material simultaneously exhibits capacitor-like buffering and battery-like electron storage and release (Shahzaib, 27 Nov 2025).

The electrochemical basis of this behavior is the coexistence of the Fe$^{2+}$/Fe$^{3+}$ and Co$^{2+}$/Co$^{3+}$ redox couples:

$$\text{Fe}^{2+} \rightleftharpoons \text{Fe}^{3+} + e^-$$

$$\text{Co}^{2+} \rightleftharpoons \text{Co}^{3+} + e^-$$

These redox couples are reported to provide pseudo-battery behavior, storing and releasing electrons under small overpotentials. The paper’s usage of “nano-capattery” therefore does not denote a conventional electrical double-layer capacitor alone, nor a battery electrode alone, but a coupled storage modality in which fast buffering and redox reservoir functions coexist. A plausible implication is that the term is intended to capture time-dependent electron traffic control rather than only bulk charge storage.

The reported physicochemical basis for this functionality includes a highly porous surface, high defect density, and abundant oxygen vacancies (OVs). In the paper’s mechanistic framing, these features generate low-resistance electron highways and facilitate rapid interfacial electron exchange. The reported conclusion is that electron flux control, rather than adsorption alone, is the operative principle underlying the PFHP response (Shahzaib, 27 Nov 2025).

2. Synthesis pathway and precursor system

The precursor biomass for Co-Fe-NBC is corn straw (CS), which is air-dried and pulverized to 60-mesh. Melamine, used at 10 wt% relative to CS, provides nitrogen doping, while cobalt nitrate hexahydrate, Co(NO$^{3+}$0)$^{3+}$16H$^{3+}$2O, and ferric nitrate nonahydrate, Fe(NO$^{3+}$3)$^{3+}$49H$^{3+}$5O, serve as the cobalt and iron sources (Shahzaib, 27 Nov 2025).

The synthesis is stepwise. First, pristine biochar (PBC) is obtained by pyrolyzing 50 g CS at 900 $^{3+}$6C with a 10 $^{3+}$7C$^{3+}$8min$^{3+}$9 ramp for 4 h under vacuum, followed by cooling, milling, washing with water/ethanol, and drying at 60 $^{2+}$0C for 24 h. Second, nitrogen-doped biochar (NBC) is prepared by mixing 50 g CS with 10 wt% melamine in 100 mL water, stirring for 4 h, drying at 60 $^{2+}$1C for 24 h, and then pyrolyzing at 900 $^{2+}$2C for 4 h, followed by the same wash/dry treatment.

The metal-doping step begins with dispersing 10 g NBC in 100 mL water and adding the metal precursors. Three variants are described: Co-NBC with 10 wt% Co(NO$^{2+}$3)$^{2+}$46H$^{2+}$5O, Fe-NBC with 10 wt% Fe(NO$^{2+}$6)$^{2+}$79H$^{2+}$8O, and Co-Fe-NBC with 5 wt% Co(NO$^{2+}$9)$^{3+}$06H$^{3+}$1O plus 5 wt% Fe(NO$^{3+}$2)$^{3+}$39H$^{3+}$4O. The resulting dispersion is dried at 60 $^{3+}$5C for 24 h, pyrolyzed again at 900 $^{3+}$6C for 4 h, washed, milled, and stored.

This synthesis sequence establishes a comparative framework in which PBC, NBC, Co-NBC, Fe-NBC, and Co-Fe-NBC can be distinguished structurally and functionally. That comparative design is significant because the reported oxygen-vacancy concentration and electrochemical performance are highest for the co-doped material rather than for the single-metal analogues (Shahzaib, 27 Nov 2025).

3. Structural, textural, and spectroscopic characteristics

Nitrogen adsorption/desorption measurements identify a Type I + IV isotherm, indicating a combined micro- and mesoporous structure. For Co-Fe-NBC, the BET surface area is 291.81 m$^{3+}$7g$^{3+}$8, with mesopore volume 0.109 cm$^{3+}$9g$^{2+}$0 and micropore volume 0.152 cm$^{2+}$1g$^{2+}$2. The paper links this porous architecture to biofilm adhesion, local pH buffering, and mass transport during PFHP (Shahzaib, 27 Nov 2025).

Raman spectroscopy shows a D-band at approximately 1356 cm$^{2+}$3 and a G-band at approximately 1588 cm$^{2+}$4. For Co-Fe-NBC, $^{2+}$5, which is interpreted as indicating high defect density and abundant edge sites for catalysis. In the reported mechanistic picture, these defects are not incidental; they are part of the electron-exchange interface that supports Fe/Co redox activity.

Electron paramagnetic resonance reveals a single Lorentzian signal at $^{2+}$6, characteristic of unpaired electrons at OV sites. The relative intensity follows Co-Fe-NBC $^{2+}$7 Fe-NBC $^{2+}$8 Co-NBC, confirming the highest OV concentration in Co-Fe-NBC. This ranking is consistent with the broader claim that co-doping produces the most favorable electron-transfer microenvironment among the tested biochars.

X-ray photoelectron spectroscopy resolves the local chemical states. The C 1s spectrum contains C–C/C=C at approximately 284.8 eV, C–O at approximately 286.2 eV, and C–N at approximately 287.6 eV. The N 1s region includes pyridinic N at approximately 398.1 eV, pyrrolic N at approximately 399.4 eV, graphitic N at approximately 400.8 eV, and oxidized N at approximately 402.5 eV. The Fe 2p spectrum contains Fe(II) 2p$^{2+}$9 at approximately 711.3 eV, Fe(III) 2p$^{3+}$0 at approximately 713.8 eV, and Fe$^{3+}$1 features at approximately 720.4 eV, with Fe$^{3+}$2/Fe$^{3+}$3 ratio $^{3+}$4. The Co 2p spectrum contains Co(III) 2p$^{3+}$5 at approximately 780.7 eV and Co(II) 2p$^{3+}$6 at approximately 785.1 eV, with satellite peaks confirming Co–N coordination. The stated conclusion is that Co-Fe-NBC contains mixed Fe$^{3+}$7/Fe$^{3+}$8 and Co$^{3+}$9/Co$^{2+}$0 states anchored in an N-doped carbon matrix (Shahzaib, 27 Nov 2025).

4. Electrochemical properties and capattery behavior

The reported electrochemical signature of Co-Fe-NBC is explicitly dual-mode. In cyclic voltammetry over 0–2 V, the specific capacitance at 10 mV$^{2+}$1s$^{2+}$2 is

$^{2+}$3

with the capacitance calculated as

$^{2+}$4

The same material also exhibits battery-like charge capacity

$^{2+}$5

at 10 mV$^{2+}$6s$^{2+}$7 (Shahzaib, 27 Nov 2025).

The corresponding energy density is calculated using $^{2+}$8:

$^{2+}$9

The abstract identifies the capacitor property as 287.91 F/g and the battery-like charge storage as 38.3 mC/g, with the highest energy density of 159.95 mWh/g.

These values are attributed to the Fe$^{3+}$0/Fe$^{3+}$1 and Co$^{3+}$2/Co$^{3+}$3 redox cycle ability together with the highly porous surface, defects, and abundant OVs. The mechanistic interpretation given in the paper is that the capacitor-like component absorbs transient electron surges, whereas the battery-like redox component releases electrons during low-flux intervals. This distinction is important because it ties the electrochemistry directly to bioprocess dynamics rather than treating the material as a passive support (Shahzaib, 27 Nov 2025).

5. Integration into photo-fermentative hydrogen production

Co-Fe-NBC was integrated into standard 250 mL PFHP reactors at a dose of 20 mg$^{3+}$4L$^{3+}$5, with the control defined as no additive. In this setting, the paper defines electron management efficiency (EME) using consumed electrons proportional to [AA] + [BA] and diverted electrons proportional to [PA] + [BE]. Co-Fe-NBC achieved EME = 65.3%, compared with 8.9% for the control (Shahzaib, 27 Nov 2025).

The metabolic readouts are consistent with suppression of competing pathways. Propionic acid was reduced by 85%, from 2.07 to 0.31 g L$^{3+}$6, thereby minimizing competitive pathways. The NAD$^{3+}$7/NADH ratio improved to 1.34 from a baseline of approximately 0.8, and dehydrogenase activity rose from 13.21 to 24.73 $^{3+}$8g$^{3+}$9mL$$\text{Fe}^{2+} \rightleftharpoons \text{Fe}^{3+} + e^-$$0, corresponding to $$\text{Fe}^{2+} \rightleftharpoons \text{Fe}^{3+} + e^-$$1. The abstract describes these observations as confirming the superior role of the nano-capattery in efficient regulation of the metabolic pathway and electron flux management.

The paper’s summary narrative associates these changes with stabilization of intracellular redox cycling and channeling of electrons to nitrogenase. Because the reported shifts include both extracellular indicators such as volatile fatty acid distribution and intracellular indicators such as NAD$$\text{Fe}^{2+} \rightleftharpoons \text{Fe}^{3+} + e^-$$2/NADH ratio and dehydrogenase activity, the intervention is framed as a systems-level redirection of fermentation biochemistry rather than a purely catalytic surface effect (Shahzaib, 27 Nov 2025).

6. Mechanistic interpretation, outcomes, and scope

The cumulative hydrogen output increased from 151.03 mL in the control to 589.54 mL with Co-Fe-NBC, which is reported both as a 367% increase and as a 3.7-fold enhancement. The mechanistic link proposed by the paper is multi-component: capacitor-like buffering at 287.9 F/g minimizes ORP spikes during metabolic electron surges; battery-like redox release at 38.3 mC/g sustains electron supply during low-flux intervals; and direct interfacial electron transfer with $$\text{Fe}^{2+} \rightleftharpoons \text{Fe}^{3+} + e^-$$3 short-circuits slower soluble shuttles, stabilizes intracellular Fd redox, and boosts NADH regeneration (Shahzaib, 27 Nov 2025).

The structural contributors are also specified. Surface OVs at $$\text{Fe}^{2+} \rightleftharpoons \text{Fe}^{3+} + e^-$$4 and high defect density at $$\text{Fe}^{2+} \rightleftharpoons \text{Fe}^{3+} + e^-$$5 provide abundant active sites for Fe/Co redox and enhance electron exchange kinetics. The porous architecture at 291.8 m$$\text{Fe}^{2+} \rightleftharpoons \text{Fe}^{3+} + e^-$$6g$$\text{Fe}^{2+} \rightleftharpoons \text{Fe}^{3+} + e^-$$7 is reported to support biofilm adhesion, local pH buffering at pH $$\text{Fe}^{2+} \rightleftharpoons \text{Fe}^{3+} + e^-$$8, and mass transport. The net result is described as precise electron flux control that reprograms microbial metabolism toward acetate/butyrate routes, suppresses propionate formation, elevates hydrogenase activity, and increases hydrogen output.

A recurrent misconception in reading such systems is to treat the observed hydrogen gain as a simple consequence of higher surface area or generic biochar conductivity. The reported mechanism is narrower and more specific: the paper attributes performance to the synergistic capacitor–battery behavior of Fe/Co–N$$\text{Fe}^{2+} \rightleftharpoons \text{Fe}^{3+} + e^-$$9-containing porous biochar, the presence of mixed-valence Fe and Co centers, and the OV- and defect-rich interfacial structure. This suggests that, within the reported framework, Co-Fe-NBC is defined less by biomass origin alone than by the coupling of hierarchical porosity, heteroatom doping, mixed-valence redox reservoirs, and metabolic electron management (Shahzaib, 27 Nov 2025).