The conversion of CO<inf>2</inf> into value-added chemicals is highly critical for sustainable development. Among the various strategies, the dicarboxylation of alkenes with CO<inf>2</inf> offers a highly attractive route to access synthetically valuable dicarboxylic acids, which serve as key intermediates in the production of polymers and pharmaceuticals. Photoelectrochemical (PEC) carboxylation represents an efficient and sustainable carboxylation strategy, offering distinct advantages including mild reaction conditions, cost-effectiveness, and environmental compatibility. In this study, an efficient PEC system is presented for the carboxylation of styrene using a Ni-modified p-type micro-pyramid silicon (Ni/p-Si) photocathode. The incorporation of the Ni catalyst significantly suppresses charge recombination and accelerates charge transfer at the electrode-electrolyte interface, thereby enhancing the overall photoelectrochemical performance. The optimized Ni/p-Si photocathode achieved 77.7% faradaic efficiency (FE) for phenylsuccinic acid at −2.4 V vs. Ag/AgCl, with a photocurrent density of −4.5 mA cm⁻². Moreover, this PEC platform demonstrates moderate FEs across a range of substituted styrenes, indicating good functional group tolerance. Mechanistic studies reveal that the reaction proceeds via single-electron reduction of styrene to generate radical anions, which undergo CO<inf>2</inf> addition followed by further reduction and subsequent attack on a second CO<inf>2</inf> molecule to yield succinic acid. These findings broaden the scope of CO<inf>2</inf> utilization through selective and sustainable C-C bond formation processes.