Two Food Wastes, One Furnace Step — a Supercapacitor Electrode From Coffee and Oranges
Spent coffee grounds and orange peels are two of the most abundant food wastes on earth. Co-pyrolysed together in a single step, they become a porous carbon good enough for a high-performance supercapacitor.
Supercapacitors sit between batteries and conventional capacitors — fast to charge, long-lived, and increasingly wanted for grid buffering, regenerative braking and pulse power. Their workhorse electrode is porous carbon, and the field has spent years turning biomass waste into it. The usual recipe, though, needs a separate chemical activation step — soaking the char in KOH or similar — to blow open the porosity, which adds cost, corrosive reagents and washing. A 2026 study in the Journal of Environmental Chemical Engineering from Feng Chia University skips that step by letting one waste activate the other.
The Obstacle
A good supercapacitor carbon needs high, accessible surface area (a hierarchical mix of micro-, meso- and macropores), enough graphitic order for conductivity, and ideally some heteroatom doping for extra pseudocapacitance — all from a cheap precursor. Biomass gives you the carbon, but developing the porosity normally means a dedicated chemical activation with added reagents. The challenge is to get the pore structure and the conductivity in one green step, from waste, without the corrosive activation stage.
The Innovation
The precursor is spent coffee grounds (CF); the twist is co-pyrolysing them with orange peels (OP) as a natural activating agent. Orange peel is rich in potassium and volatile organics; during one-step co-pyrolysis, its in-situ potassium acts like a built-in activator, etching porosity into the coffee-derived carbon, while its decomposition promotes graphitic ordering. Systematic optimisation of pyrolysis temperature, CF:OP mass ratio, and coffee roast level landed on the best material — CFOP, made at 800 °C from a 1:1 mass ratio of light-roast coffee and orange peel. Trace retained potassium in the final carbon further boosts capacitance. (Chang & Chang; Journal of Environmental Chemical Engineering, 2026, 14, 120742; DOI 10.1016/j.jece.2026.120742.)
Key Results
Three results carry the story: the co-pyrolysed carbon beats either waste alone, Raman links the performance to its graphitic/defect structure, and it holds up over thousands of cycles.
275.3 F/g From Waste — and Better Than Either Waste Alone
The optimised CFOP electrode delivered a specific capacitance of 275.3 F/g at 1 A/g in 3 M KOH — far above the coffee-only carbon (141.8 F/g at 1 A/g), confirming that the orange-peel activation does real work.
Charge Is Stored by Diffusion-Driven Pseudocapacitance
Cyclic voltammetry across scan rates shows the CFOP electrode stores charge through a diffusion-dominated pseudocapacitive mechanism — not just double-layer capacitance — consistent with its redox-active, potassium-bearing porous structure.
Raman ties performance to structure. Recording Raman on an MRI-series micro-Raman (532 nm) — reported in the paper’s Supplementary Information (Figures S2 and S3) — the team tracked the carbon D-band (~1348 cm-1, defects) and G-band (~1591 cm-1, graphitic sp2 carbon). The I_D/I_G ratio rose systematically with pyrolysis temperature (CF-400/600/800 = 0.452 / 0.666 / 0.808; OP-400/600/800 = 0.471 / 0.608 / 0.762), mapping the balance between defect creation (which adds active sites) and graphitisation (which adds conductivity). The optimum at 800 °C reflects that trade-off, and the Raman trend is the structural evidence behind the electrochemical optimum. (XPS confirmed C, N, O and retained K.)
It Lasts — 63% Retention Over 5000 Cycles
Over long-term cycling the CFOP electrode retained 63.27% of its initial capacitance after 5000 charge–discharge cycles at 1 A/g with high coulombic efficiency — respectable durability for a waste-derived carbon, with retention actually exceeding 100% during the first ~200 cycles as the pore network wetted and activated.
Why ProTrusTech Is Featuring This Research
Carbon-electrode performance lives and dies by its graphitic order and defect chemistry, and the fastest, most direct way to read both is the D/G Raman fingerprint. Measured on an MRI-series micro-Raman (532 nm), the I_D/I_G trend across pyrolysis temperature is what connects the synthesis knob (temperature) to the property (capacitance) — exactly the kind of structure–performance link Raman was made for. It also opens a new Energy Storage & Batteries thread for us, alongside our in-situ battery-electrode Raman work.
Featured Instruments:
Why This Matters
Turning two everyday food wastes into a functional energy-storage material — in one green step, with no separate chemical activation — is a genuine circular-economy result: it lowers cost, removes corrosive reagents, and diverts waste. And it shows a general design idea — using one biowaste’s mineral content to activate another — that could travel to many precursor pairs. Raman is the quick structural check that keeps such carbons on-spec.
What’s Next
- Full-cell device figures (symmetric/asymmetric energy and power density), not just three-electrode capacitance.
- Generalising the “one waste activates another” idea to other biomass pairs.
- Pushing cycle-life retention above the current 63% with pore and heteroatom engineering.
Working on biomass-derived carbons, supercapacitor or battery electrodes — and need Raman to quantify graphitisation and defects? Get in touch with our team.
Key Takeaways
- One-step co-pyrolysis of spent coffee grounds + orange peels makes a hierarchical porous carbon — orange peel is a natural activator, no separate KOH step.
- Optimised CFOP (800 °C, 1:1 light-roast) reached 275.3 F/g at 1 A/g (vs 141.8 for coffee alone), with 63.27% retention after 5000 cycles.
- Raman (MRID, 532 nm) — the I_D/I_G trend across pyrolysis temperature — links graphitisation and defects to the capacitance optimum.
