Capacitive mixing is a promising blue energy technology due to its membrane-free electricity generation and long electrode life cycle. However, because of limited performance, existing systems do not lend themselves to practical implementation. Although it is a crucial factor directly influencing electrode behavior, surface chemistry has largely been overlooked in capacitive mixing. Here, we show that manipulating surface functionalization alone can tune the responses of electrodes to produce a high voltage rise without altering the pore structure of the electrodes. Our findings reveal that the spontaneous electrode potential of a surface-modified carbon electrode shifts negatively proportional to the surface charge due to the surface groups, which explains why and how manipulating the surface chemistry can improve the power generation capacity. Using electrodes fabricated with identical activated carbon material but with different surface treatments, we have achieved a remarkably high power density of 166 mW/m2delivered to an electrical load under a 0.6 M to 0.01 M salinity gradient, with the total power generated of 225 mW/m2. The corresponding volumetric power densities were 0.88 kW/m3net and 1.17 kW/m3total. The volumetric power density of our prototype is comparable to or better than those of prevailing membrane technologies, such as pressure retarded osmosis and reverse electrolysis, whose volumetric power density values are 1.1 kW/m3and 0.16 kW/m3, respectively. In the seawater stage, the net power density reached 432 mW/m2or 2.3 kW/m3. Such performance far exceeds existing membrane-free systems, with the highest reported power density of 65 mW/m2under a 0.5 M to 0.02 M salinity gradient (121 mW/m2in this work). The device demonstrated unparalleled durability, maintaining 90% of the maximum energy capacity after 54,000 charge–discharge cycles.